Hydrologic, Hydraulic and Geomorphic Technical Memorandum

Transcription

1 Appendix A Hydrologic, Hydraulic and Geomorphic Technical Memorandum 01054/ /14/Rohner_Alt_Analysis_Report Rohner Creek Flood Control, Habitat and Seismic Improvement Project Alternatives Analysis Report 2

2 May 30, 2013 To: Kevin Carter, City of Fortuna From: GHD Inc: Greg Garrison Michael Love & Associates, Inc.: Rachel Shea, Michael Love Tel: Reviewed By: Jeremy Svehla, Merritt Perry Subject: Rohner Creek Flood Control and Habitat Improvement Project Hydrologic, Hydraulic and Geomorphic Technical Memorandum 1. Purpose of Memorandum This Technical Memorandum (TM) summarizes methods and results for development and analysis of flood reduction alternatives for Rohner Creek in the City of Fortuna, California. The purpose of this TM is to support technical findings and discussion provided within the Alternatives Analysis report dated May 22, Structure of Technical Memorandum This TM presents the hydrologic and hydraulic analyses that were performed to identify existing condition flood capacity and potential geomorphic instability problems within the project area. The results of these analyses were used to identify design alternatives to address these issues (Sections 3 to 5). Sections 6 and 7 present the design methodologies used for designing the various components of each of the four alternatives. The results of the design process were then incorporated into the proposed condition hydraulic models to evaluate flood reduction benefits (Section 7). The results of the proposed condition modeling were also used to assess design condition channel stability and sediment transport, for developing channel and bank stabilization approaches and identifying fisheries habitat opportunities (Section 8). All elevations referenced in this TM are based on the NAVD88 vertical datum. 3. Hydrologic Model Development 3.1 HEC-HMS Model As part of the Rohner Creek Phase I and II studies completed by GHD, Version 3.3 of the USACE Hydrologic Engineering Center s Hydrologic Modeling System software (HEC-HMS) was utilized to compute peak flows at specific locations within the Rohner Creek watershed. HEC-HMS simulates precipitation-runoff and routing processes and allows the generation of storm hydrographs.

3 The commonly used Soil Conservation Service (SCS) empirical curve number method was utilized in HEC-HMS to estimate total excess precipitation. The curve number (CN) represents the soil cover, land cover and antecedent moisture conditions of a watershed. The CN method determines runoff using the amount of precipitation and the infiltration parameters associated with soil type, soil moisture, preceding rainfall, and surface retention. The amount of rainfall is converted to runoff using the CN. The CN ranges from 0 to 100, where a value of 100 represents zero losses or an impermeable surface (USDA, 1986). The SCS Unit Hydrograph (UH) model was used in HEC-HMS as the direct-runoff transform method which describes the parameters associated with the transition from rainfall to overland flow. The model is based upon averages of UH derived from gauged rainfall and runoff for a large number of small agricultural watersheds throughout the United States. The SCS UH model uses a dimensionless, singlepeaked unit hydrograph. Utilizing the UH method in HEC-HMS requires the lag time for each subbasin. The lag time is defined as the difference in time between the center of mass of effective rainfall and the center of mass of runoff produced (Viessman, 1995). The Standard Muskingum Cunge routing method was used to model the hydrograph routing within each reach. Utilizing the 5-foot contour photogrammetry data as well as the existing HEC-2 model, the length, slope, and typical channel cross-sectional shape were inputted to the HEC-HMS model. Model calibration was completed as part of Phase II. The purpose of model calibration was to increase model accuracy using actual measured data thereby improving the reliability of model results. In the absence of calibration, model results can vary significantly. The calibration of the HEC-HMS model required two pieces of information; one being the streamflow data acquired from the streamflow monitoring conducted by Graham Matthews and Associates (GMA) and the other being the precipitation data collected by the City. The advantage of using streamflow monitoring data for the calibration of a hydrologic model is that the calibration method adjusts the runoff characteristics of the sub-basins to result in total streamflow that matches the historical streamflow recorded. The resultant calibrated model has the ability to take any storm rainfall data and output a streamflow hydrograph for the project reach. The peak flow events for the 10, 50 and 100-year storm events for the non-calibrated HEC-HMS model, the calibrated HEC-HMS model, and the FEMA FIS study are summarized in Table 1. A discussion of the peak flows can be found in the Phase II Study Report (GHD 2012). The calibrated hydrographs developed in the HEC-HMS model were imported into the MIKE 11 1D open channel model of Rohner Creek. The Rohner Creek model domain included the portion of the creek from just west of US 101 to the culvert crossing of Main Street. Hillside Creek was added to the ECM model to better understand the impact of the storm hydrograph from the Hillside Creek drainage and to capture floodplain flow contributing from Hillside Creek when its banks are overtopped. The Hillside Creek hydrologic model was not calibrated, as no streamgage data was available. The FEMA FIS Study mentioned previously did not provide peak flow estimates for Hillside Creek, only cumulative Rohner Creek flows downstream of Hillside Creek confluence.

4 Table 1. Calibrated HEC-HMS flows for the Rohner Creek Watershed. Rohner Creek Peak Discharge (cfs) Return Period Location HEC-HMS NON-CALIBRATED HEC-HMS CALIBRATED FEMA FIS STUDY FLOWS Corporate Limits 10-year year Location Downstream of Carson Woods Crossing 10-year year Location Main Street Crossing 10-year year Location Upstream of Confluence with Hillside 10-year year Location Downstream of Confluence with Hillside 10-year year Location 12 th Street (Dinsmore Drive) 10-year year Location Hillside Creek 10-year 122 N/A N/A 100-year 249 N/A N/A 3.2 Frequent Flows Peak flows for the 2-, and 5- and 10-year storm events were computed in Rohner Creek at the Main Street Crossing and downstream of where Hillside Creek enters Rohner Creek at Fortuna Boulevard

5 using regional regression equations in USGS (2012) using a rainfall depth of 50 inches per year (PRISM, 2007) and the drainage area to each location (Table 2). Flows with year return period less than the 2- years were logarithmically extrapolated from the computed values. Drainage areas were obtained from the Phase I Study Report (GHD, 2011). The 5- and 10-year flow predicted by the USGS regression equations are higher than flows of the same return period predicted by the calibrated HEC-HMS. However, these flows were only used for extrapolation between and 1.5-year flows, which span the range of likely bankfull flows with the channel. Bankfull flows have been found to commonly have a return period between 1.2- and 1.5-years and serve as the dominant channel forming flow, which shapes the active channel of a stream (Leopold et al., 1964). Table 2. Flows in Rohner Creek and Hillside Creek computed using USGS, Location Drainage Area 1-Year* 1.5-Year* 2-Year 5-Year 10-Year Rohner Creek at Main Street Rohner Creek Downstream of Hillside Creek Hillside Creek at Rohner Creek 3.9 mi cfs 214 cfs 289 cfs 552 cfs 774 cfs 4.3 mi cfs 235 cfs 316 cfs 602 cfs 808 cfs 0.6 mi 2 15 cfs 37 cfs 54 cfs 106 cfs 144 cfs *Extrapolated 3.3 Fish Passage Flows Fish passage design flows for salmonids are intended to define the primary range of flows that a fish is expected to migrate upstream. These flows are defined for specific lifestages by both the California Department of Fish and Wildlife (CDFG, 2001) and National Marine Fisheries (NMFS, 2000). They are based on daily average exceedance flows. For the project reach, these flows were obtained from a regional flow duration curve developed for Humboldt County as part of the Caltrans District 1 fish passage assessment (Lang, 2005). Results are provided in Table 3. Additional information is available in the Fish Passage Conditions for Rohner Creek at 12 th Street and Main Street Crossings Technical Memorandum (Alternatives Analysis Report, Appendix D).

6 Table 3. High and low passage flows computed using Caltrans District 1 fish passage assessment regional flow duration curve (Lang, 2005) and (PRISM, 2007). Salmonid Lifestage 12 th Street Main Street Low Fish Passage Flow Adult Anadromous Adult Resident Juvenile High Fish Passage Flow Adult Anadromous Adult Resident Juvenile 3 cfs 2 cfs 1 cfs 95 cfs 40 cfs 22 cfs 3 cfs 2 cfs 1 cfs 86 cfs 36 cfs 20 cfs 4. Hydraulic Model Development 4.1 MIKE 11 MIKE 11 (M11) is an implicit, finite difference model developed by DHI Water and Environment used for modeling networks of one-dimensional channels with respect to both hydrodynamics and water quality transport. It is an unsteady flow model based on the Saint Venant equations and thus capable of modeling one-dimensional channel flows over time and space. M11 has the ability to be dynamically coupled with MIKE 21 (M21) to simulate interactions between the one-dimensional channels and a twodimensional floodplain, which is described in the following section. When M11 is coupled with M21, it is also known as MIKE FLOOD (MF). The basic hydrodynamic (HD) model within M11 was utilized for simulating Rohner Creek hydraulics. The HD model allows for calculation of water level, velocity, and discharge throughout the model domain over the simulation period. Simulation periods of 24 hours were utilized to match the 24 hour storms simulated for the 10-year and 100-year return period precipitation events. Input data used in constructing the M11 model for Rohner Creek included: Network data defining the spatial alignment of the channels Topographical data from surveys and FEMA HEC-2 data to define structural elements within the channels including bridges and culverts Topographical data to define channel cross sections derived from field survey information and photogrammetry data Hydrodynamic parameters such as Manning s n values determined from field survey information Initial conditions including water level at the downstream boundary of the model domain and base flow at the upstream boundary of the model Boundary conditions including water level at the downstream boundary of the model and HEC- HMS runoff hydrographs mentioned previously

7 The spatial alignment of the channels was derived from a combination of thalweg survey data collected at cross sections and digitizing based on aerial topographic mapping provided by the City. Manning s n values used in the model were also obtained from the FEMA HEC-2 models. It is important to note that contrary to the FEMA HEC-2 models, M11 cross sections extend only from left top of bank to right top of bank rather than including the floodplain within the cross sections. Therefore, MIKE 11 models only the active channel, while M21 simulates floodplain hydraulics two dimensionally for a better representation of floodplain and street flooding. The development of the M21 portion of the model is discussed in the following section. 4.2 MIKE 21 M21 is a 2-Dimensional (2-D), unsteady hydrodynamic model capable of simulating complex floodplain and street flooding. The M11 model, described above, simulates only the active channel portion of Rohner Creek; the M21 model routes flow 2-D once flows from the M11 model exceed the active channel carrying capacity. The two models are dynamically coupled at the banks of the channel through lateral weir connections. The M21 model utilizes photogrammetry data provided by the City for the floodplain topography. The photogrammetry data consists of 2 ft contours within the City limits and 4 ft contours for the surrounding watershed area outside of the City limits. The photogrammetry data was obtained by the City in November of The M21 model was dynamically coupled to the M11 model simulating one-dimensional active channel hydraulics for Rohner Creek. The coupling of these models is described in Section below. 4.3 MIKE FLOOD MIKE FLOOD (MF) represents the model developed by dynamically coupling the M11 1D channel hydraulics and the M21 2D floodplain hydraulics. The ability to accurately simulate these dynamics ultimately allows for accurate prediction of baseline conditions and the ability to better determine the benefit from flood reduction alternatives. 4.4 Boundary Conditions As discussed in the Phase II Study, a downstream boundary condition for each flow event was obtained from the Eel River HEC-RAS Existing Condition Model (ECM) at the confluence of Strongs Creek. The Eel River water surface elevation (WSE) for the 10 and 100-year storm events were obtained from the FEMA FIS study and are described in further detail in the Phase II Study. The use of the Eel River WSE as a downstream boundary condition assumes that the Eel River is reaching its peak WSE for a storm event concurrently with Rohner Creek. To test the sensitivity of the model to the downstream boundary condition, the model was also run with normal depth as the downstream boundary condition for the Rohner Creek 100-year event. The normal depth boundary condition assumes that the channel flows under uniform flow conditions and the model back-calculates the WSE using the Manning s equation and slope of the channel thalweg. A total of three model runs were completed for the existing conditions; as summarized in Table 4 below.

8 Table 4. Storm Event and Corresponding Boundary Conditions for the Rohner Creek MIKE Model Runs. Rohner Creek Storm Event Downstream Boundary Condition WSE (ft) or Channel Slope (ft/ft) 10-year Eel River 10-year Backwater ft (NAVD 88) 100-year Eel River 100-year Backwater 45.77ft (NAVD 88) 100-year Normal Depth (no Eel River Backwater) 0.01 ft/ft 5. Existing Conditions Modelling Results and Geomorphology 5.1 Rohner Creek Channel Capacity The M11 model predicts the limiting capacity of the Rohner Creek channel to be approximately 290 cfs, which is less than the 10-year event (411 cfs). The limiting capacity cross-section occurs between Station and along Stillman Way. There are a number of bridges that span the Rohner Creek channel between Main Street and 12 th Street. A number of the bridges have soffit elevations that are below the top of bank elevations of the channel and contribute to overland flooding along the channel. These results are consistent with previous study efforts. In general, the conveyance capacity of Rohner Creek increases as you move downstream. However, there are sections of the channel below the Forbusco property that do not convey the 100-year storm event. See Attachment A for the WSE profiles of the Rohner and Hillside Creek channels. Figure 3 presents peak flow conveyed within the Rohner Creek channel for existing condition 1.3-, 10-, and 100-year storm events. A peak flow with a 1.3-year return period was selected to represent the bankfull flow event (See Section 3.2). At the 1.3-year peak flow all the flow is conveyed within the channel. During a 10-year peak flow, water flows out of the channel onto the floodplain, resulting in a slight loss in the amount of flow conveyed within the stream channel. Flow resistance on the floodplain forces the main channel to convey a large portion of the total flow. During a 100-year event, 960 cfs enters the project reach at Main Street, but out-of bank-flows reduce the peak flow conveyed by the channel to approximately 450 cfs, slightly larger than a 10-year flow event. Channel flows increase at the discharge point of Hillside Creek. Water on the floodplain does not re-enter the channel until downstream of 12 th Street. As a consequence, Rohner Creek, as it currently functions, does not experience flows much greater than those associated with a 10-year event Floodplain Inundation Figure 1 shows the updated topography of the Rohner Creek floodplain and the floodplain flow path. Any overbank flow occurring on the east bank of the channel and downstream of Main Street would flow in a south-easterly direction along Fortuna Blvd. until re-entering the channel near the Hillside Creek confluence. Any overbank flow occurring west of the channel would sheet flow in a south-westerly direction towards the Fortuna High School and the 12 th Street crossing of Rohner Creek. There currently exists three drainage inlets that would collect some overbank flow and direct it into a 32 inch and 48 inch diameter stormdrain pipes which discharge to Rohner Creek immediately downstream of the 12 th Street crossing. The drainage inlets are located along the western edge of the field as shown in Figure 1. The

9 existing stormwater infrastructure (both stormdrain pipes) has limited capacity (72 cfs) that would be quickly exceeded by the contributing overland flow during a large storm event. It is important to note that the existing stormdrain infrastructure described above was not included in the MIKE model development. As such, the severity of inundation in the field west of the Rohner Creek channel may be altered by the stormdrain s ability to convey floodplain flow back into the Rohner Creek channel at the 12 th St crossing. Figure 1. Rohner Creek Existing Channel Capacity and Floodplain Flow Direction Exhibits A, B, and C in Attachment B display the maximum extent and depths of floodplain inundation for the 10- and 100-year storm events. The MIKE model predicts the 10-year storm event overtops the right bank of the Rohner Creek channel just upstream and downstream of the Ash Street Bridge and along Stillman Way. Once the flow leaves the Rohner channel, it flows in a south-westerly direction onto the field and existing drainage ditches to the east of the High School. The overland flow concentrates to the east of 12 th Street where existing depressions in the topography collect the floodwaters. The flow path and overtopping location for the 10-year event is consistent with field observations and analysis of updated topography. In addition to overland flow occurring from Rohner Creek flooding, Hillside Creek contributes to floodplain inundation with flooding occurring at each culvert in Hillside Creek. The culverts do not have the capacity to convey the 10-year event.

10 Exhibit B displays the results of the Eel River backwater boundary condition and Exhibit C displays the results of the normal depth boundary condition. For the 100-year, 24-hour storm event model runs, the extent and depth of floodplain inundation is similar. The different boundary conditions resulted in different floodplain inundation sequencing. The Eel River 100-year water surface elevation backwaters the existing 12 th Street storm drain infrastructure and inundates the field prior to flow overtopping the Rohner Creek channel. Extensive bank overtopping occurs along Stillman Way down to the Hillside Creek confluence. In addition, the undersized Hillside Creek culvert under Fortuna Boulevard backwaters and floods the neighborhoods adjoining Fortuna Boulevard. This is consistent with the 2005 Storm Drain Master Plan which identified the Hillside Creek culvert as being undersized for the 100-year, 24-hour storm event Geomorphology General Description Rohner Creek is a tributary to the Eel River. Geologic mapping of the project area indicates that Rohner Creek is underlain by recent alluvial sediments and late-pleistocene-aged Eel River floodplain sediments (Blackburn Consulting, 2013), but the stream channel is no longer a part of the active floodplain due to its elevation. The upper portion of the Rohner Creek watershed is predominately comprised of second and third-growth redwood forest. A large portion of the upper watershed is currently owned by a resource management company that maintains active Timber Harvest Plans for the area. The middle portion of the watershed consists of rural residential area, and the lower portion of the watershed is comprised of a mix of residential, commercial, and industrial land uses. From Main Street to its confluence with the Eel River, the stream is in a mostly urbanized reach and appears to have been straightened and realigned in places. The existing Rohner Creek stream channel is narrow and incised, and has been highly encroached upon by adjacent land use along most of its length. The channel is armored in numerous places by riprap, concrete, and retaining walls. Numerous structures are located within close proximity of the channel banks. Urban impacts to the stream include hastened stormwater runoff response and increases in magnitudes of flow associated with impervious surfaces. The stormwater runoff discharged into the channel consists of clear water discharge and is not conveying the natural load of sediments that the channel historically received. Historically, Rohner Creek likely flowed through an extensive riparian forest comprised of large conifers. These trees would have provided large wood to the channel that would have provided channel bed controls. The clearing of historical riparian forest, removal of large wood from the channel, increased peak discharges and changes in sediment loads has likely resulted in the existing channel conditions. These include downcutting and bed scour, development of head-cuts or knickpoints, and channel bank erosion. Channel Description The channel is generally trapezoidal in shape with a top width generally ranging between 30 and 50 feet and a depth of up to 10 feet. In general, the channel consists mostly of runs and deep scour pools formed in clays, with only occasional areas of gravel deposition forming low riffles. The channel lacks inset benches or floodplains, with the exception of a short reach immediately upstream of the 12 th Street crossing. Sand and silt deposition was evident on the benches and on some areas of streambanks, indicating that the channel conveys sand and silt as part of its sediment load. Figure 2 presents the results of channel thalweg survey obtained during the project topographic survey. Dashed lines represent the overall slope of three discrete channel reaches. Though each reach has a similar slope averaging 0.46%, they are separated by two knickpoints (discontinuities within the profile),

11 Elevation (ft) where the channel drops steeply within a short distance. The downstream-most knickpoint is located approximately 500 feet upstream of the Main Street culvert, and consists of an approximately 1-foot drop in the channel bed. The location is coincidental to the upstream end of the historical channel realignment that was conducted as part of relocating the 12 th Street crossing. This location contains riprap along the channel bed and banks that appears to have temporarily arrested headwater migration of the knickpoint, although the riprap armoring appears to be slowly failing. Downstream of the knickpoint is a deep scour pool ft Knickpoint Hillside Creek Outlet 2.8 ft Knickpoint Forbusco Bridges Alder Drive Culvert 30 12th St. Culvert Inv. 28 RC 0+00 RC 5+00 RC RC RC RC RC RC RC RC RC RC Rohner Creek (RC) Station (ft) Driveway Bridge Stillman Way Bridge Beech Street Bridge Main Street Culvert Figure 2. Existing condition thalweg Profile of Rohner Creek between Main Street and 12 th street. Overall channel slope trends are shown as dotted lines and labeling. The upstream knickpoint is located begins immediately downstream of where Hillside Creek enters Rohner Creek. It consists of an over-steepened channel segment that has approximately 2.8 feet of drop over a distance of 300 feet. The average slope of this segment is 0.93%, which is double the slope of the upstream and downstream reaches. The headwater migration rate of this knickpoint is unknown; however, the localized drop and steepness of the stream channel in the knickpoint area is not expected to remain stable over the long-term. Rather, if left unchecked it will likely to continue to migrate upstream, causing upstream channel incision. This can result in over-steepening of streambanks and has the potential to cause geomorphic channel instabilities. Both the Main Street and 12th Street culverts have experienced substantial scour upstream of the culvert inlets, as indicted by the substantially lower stream channel reaches upstream of the crossings. Channel Velocities and Stability Analysis A channel stability analysis was prepared to compute allowable velocity using methods presented in USDA (2007a). The allowable velocity method is used to evaluate whether the bed and banks of an

12 earthen channel will erode or remain stable during a given flow event. The method assumes that the channel is a threshold channel. A threshold channel is, when stable, a channel that experiences little adjustment of its bed and banks but can convey a substantial sediment load primarily as suspended and wash load. The analysis is based on frequency of design flow, channel depth, relative channel sediment load, the plasticity index and void ratio of the materials forming the channel, and planform geometry. The allowable velocity method recommends that to maintain a stable channel, water velocities for any return period flow event do not exceed allowable velocities computed for a 10-year return period event. Therefore, the allowable velocity for Rohner Creek was computed for a 10-year event. Flow depths within the existing condition channel during a 10-year event are approximately 6 feet deep. The plasticity indices of samples collected at a 10-foot depth from soil borings performed at multiple locations adjacent to the channel range from 14% to 24%, and averaged 18.3%. This indicates the channel banks are composed of lean clays. A void ratio of 80% was used, representing fairly compact clay (USDA, 2007). A straight channel was assumed for the computations. An allowable velocity for Rohner Creek of 6.3 feet per second was computed. Figure 3 presents existing 1.3-, 10-, and 100-year average velocities at riffles obtained from the hydraulic modeling, and compares them to the computed allowable velocity. Generally, water velocities are lowest downstream of Main Street and adjacent to Stillman Way, but increase at the knickpoint upstream of Hillside Creek and remain high until they pass through the knickpoint upstream of 12 th Street. Flow velocities decrease between the 12th Street crossing and the knickpoint as a result of the elevated invert of the 12 th Street culvert crossing.

13 a. b. Figure 3. Existing condition 1.3-, 10- and 100-year (a) peak flow and (b) water velocities within the Rohner Creek channel. Water velocities for the 1.3-year flow event generally fall below the allowable velocity, except in areas associated either with channel confinement formed by structure encroachment or related to the knickpoints. The 10-year water velocities are typically slightly lower than the allowable velocity, and rise above the allowable velocity in the same locations where 1.3-year velocities are excessive. Water velocities for the 100-year event follow a similar pattern as the 10-year flows, which was expected because out-of-bank flows result in Rohner Creek only conveying slightly more than the 10-year peak flow during the 100-year event.

14 In general, the results of the velocity analysis indicate that Rohner Creek is near or above the threshold of channel stability along most of its length between the knick points. The presence of the knickpoints, localized bank erosion, minimal sediment deposition, and the deeply scoured nature of the channel bed support this conclusion. Sediment Transport Analysis The volume and particle sizes that Rohner Creek transports are unknown because sediment sampling was not possible during the project timeline due to lack of a rainfall event sufficient to mobilize sediment within Rohner Creek. A sediment transport competence analysis (channel shear stress sufficient to mobilize a sediment particle of given size) was performed to evaluate the channel bed stability. The sediment transport analysis at riffle cross sections was prepared using the results of average channel shear stress from the existingcondition hydraulic modelling. The analysis was prepared for a 1.3-year peak flow (165 cfs at Main Street and 190 cfs downstream of the Hillside Creek outfall), based on the assumption that in alluvial channels, the bankfull discharge mobilizes the streambed and cumulatively moves the most sediment compared to other flows and is responsible for creation and maintenance of bedforms within a stream channel (Knighton, 1998; Wolman and Miller, 1960, Leopold et al., 1964). The analysis was prepared assuming equal mobility theory, which stipulates that the entire streambed becomes mobilized once the D 50 mobilizes (Parker et al., 1982). A Shields parameter of was used to reflect the loosely packed nature of the sands and gravels on the riffles (Gordon, et al., 1992). The results of the sediment transport analysis (Figure 4) indicate the channel transport competence generally follows the same pattern as water velocities within the channel. In areas of confined channel and at the knickpoints where channel velocities are accelerated, a median grain size (D 50 ) of approximately 150 to 280 mm is predicted to be mobilized. Within the remaining channel reaches, where water velocities and shear stresses are lower, a D 50 of 50 to 100 mm is predicted to be mobilized at the 1.3-year peak flow. A pebble count at a gravel riffle downstream of the Stillman Way Bridge found it to be composed of medium to coarse gravels with a median grain size (D 50 ) of 9.3 mm and a D 84 size of 19.2 (Attachment C). Though not observed on the riffle where the pebble count was conducted, rare particles between 40 to 100 mm were observed in isolated depositional areas along the study reach. Pebble counts in the Rohner Creek channel upstream of the project area (See Section 0) indicated that the channel bed consisted of a an average D 50 of 15.4 mm and an average D 84 of 47 mm, and represent the typical sediment sizes delivered to the project reach (Figure 4 and (Attachment C). The sediment transport analysis indicates that the channel has the competence to transport much larger D 50 and D 84 particle sizes than are delivered to the project reach. The lack of depositional features within the channel, frequent exposures of clay within the channel bed, and deep bed scour support a conclusion that the stream channel may be supply-limited for gravels, which would result in the observed channel bed erosion. Though the channel appears to be supply limited for gravels, the channel does convey a sand and silt suspended sediment load. Because this load is unknown, it is not possible to predict the amount of energy the channel uses to transport this load, which would reduce the energy available to transport gravels on the channel bed. Sediment sampling would be necessary to accurately compute difference the actual sediment load within the channel and the potential load it can transport, which would provide be informative to assess channel stability.

15 Figure 4. Results of sediment transport competence analysis for a 1.3-year return period event for the Rohner Creek project reach. Average D 50 and D 84 particle sizes measured upstream of the project area represent the typical sediment sizes delivered to the project reach. 5.2 Hillside Creek Hydraulic Modelling Results Hillside Creek contributes to floodplain inundation with flooding occurring at each culvert in Hillside Creek. The culverts do not have the capacity to convey the 10-year event (Attachment A, Figure 38). The undersized culvert along Hillside Creek, which lies under Fortuna Blvd., backwaters and floods the neighborhoods around the Fortuna Boulevard and Hillside Creek intersection. This is consistent with previous stormdrain infrastructure analysis completed by Winzler & Kelly in the 2005 Stormdrain Master Plan, which identified the Hillside Creek culvert as being undersized for the 100-year storm event. The existing crossing under Fortuna Boulevard is perched where it outfalls at Rohner Creek, likely creating a barrier for fish access into Hillside. Fish passage conditions within the existing culvert were not assessed Geomorphology A geomorphic assessment was not conducted along Hillside Creek and the stability of the channel is unknown. Hillside Creek within the project area is a highly disturbed. Hillside Creek is piped for a distance of 270 feet under Fortuna Boulevard to the confluence with Rohner Creek. Upstream of Fortuna Boulevard, the channel has been straightened and is confined to a narrow corridor between buildings and parking lots. There is substantial sediment aggradation in the channel resulting from the undersized culverts. Further upstream, an in-stream sediment basin was constructed by the City in 199X and has since filled in with sediment and is beginning to support riparian vegetation.

16 6. Design Development Four design alternatives were developed to improve channel conveyance in Rohner Creek and reduce flooding. The alternatives were based on the flood capacity and floodplain inundation identified in the existing condition models of the 10-year and 100-year storm events. The four alternatives address the project objectives of enlarging the channel to provide capacity for at least the 10-year storm events while creating a stable, self-maintaining stream channel, avoiding impacts to existing structures, and improving in-stream habitat for anadromous fish. All alternatives include improving flood capacity of lower Hillside Creek. The four alternatives are comprised of combinations of in-stream improvements for Rohner and Hillside Creeks using natural channel design methods, where feasible, and two methods of flood bypass routing. Detailed descriptions of each Alternative are presented in the Alternatives Analysis Report. 6.1 Channel Design Process All alternatives include enlarging portions of Rohner Creek to increase flood conveyance. Stream improvements along Rohner Creek and Hillside Creek were designed, where feasible, using the natural channel design methodology (USDA, 2007b; USFS, 2008). When applying this methodology, the improved channel would be designed to have similar form and function as an adjacent geomorphically stable reference channel reach. Replacement culvert crossings for Hillside Creek were designed using the Stream Simulation Methodology (CDFG, 2009), which provides 100-year flood conveyance and fish passage conditions similar to reference reach conditions. The objectives of the natural channel design process were to create a geomorphically stable, selfmaintaining stream channel, where feasible, with an increased channel capacity and provide spawning and rearing habitat for anadromous salmonids while minimizing impacts to existing structures. The natural channel design process was applied for all alternatives to the channel reach between Main Street and Alder Drive, the new inset floodplain between 12 th street and Forbusco Lumber, the realigned channel reach for Alternative 2, and the channel improvements on Hillside Creek (Figure 5 and Figure 6). The results of the natural channel design were then incorporated into the proposed condition hydraulic models to evaluate flood reduction benefits, provide detailed channel hydraulics used for channel stability and sediment transport analysis, and provide design parameters for developing bank stabilization approaches and identifying fisheries habitat opportunities. The existing channel reach from Hillside Creek confluence to the Forbusco Vehicle Bridge, referred to here as the CalFire-Forbusco reach, is restricted to widening due to the existence of structures adjacent to the top of bank. This reach and adjacent structures are susceptible to geologic hazards related to oversteepened streambanks and continuing scour and erosion from streamflows. These factors limited the application of a natural channel design approach within this reach.

17 Figure 5. Alternative 1: Rohner Creek and Hillside Creek Improvement areas to achieve 10-year storm event conveyance. Figure 6. Alternative 2: New Rohner Creek Alignment with Rohner and Hillside Creek Improvements to achieve conveyance capacity for the 100-year storm event.

18 6.1.1 Reference Reach Characterization The natural channel design methodology is based on the use of stable reference reaches that have similar geology, flow, sediment, and slope characteristics as the desired channel. Rohner Creek in Rohner Park experiences the same flows and sediment supply as the project area; but is less encroached upon by adjacent land use. The reference reaches were selected based on their stability and similarity to the project reach of Rohner Creek, including similar channel slope, gravel riffles, moderately stable channel banks with riparian areas, and ability to convey larger flow events. Though not an optimal stable natural channel, the reference reach reflects hydraulic geometry characteristics that were used as baseline dimensions for which to base the channel design in the project reach. Rohner Creek in the park consisted of entrenched though relatively stable, fairly straight channel reaches containing several gravel riffles and scoured clay pools. Similar to the project reach, there are long stretches of channel in Rohner Park where channel consists mostly of runs and deep scour pools, with only occasional areas of gravel deposition forming low riffles, making it difficult to identify suitable reference reaches. Two reference reaches within Rohner Creek were identified and evaluated for use in the design of a stable natural stream channel within the project area. Both the upstream and downstream reference reach locations were located within Rohner Park (Attachment C). The downstream reference reach was located approximately 750 feet upstream of the Main Street crossing, and extends upstream of the pedestrian bridge. The upstream reference reach was located approximately 1,500 feet upstream of the Main Street crossing. The reference reaches were surveyed on 2/26/13. The reference reach surveys consisted of auto level surveys of approximate 100 feet of channel thalweg at each reference reach location, survey and tape measurements of riffle cross sectional dimensions, and pebble counts of the streambed material. A bulk sample of the bed material was obtained on a riffle at the downstream reference reach location. A summary of the reference reach surveys is presented in Attachment C Channel Hydraulic Geometry and Profile Characteristics The upstream reference reach had discontinuous sets of benches at varying elevations along the channel that appear be historical silty depositional features through which the channel is currently cutting. There was substantial silt deposition on the benches, likely a result of a large storm event on December 1, 2012, which local stream gages recorded peak flows with return periods ranging between 5- and 10-years. Bankfull indicators were located coincidental with the higher depositional surfaces present within the channel but below the December 2012 high flow indicators. The overall water surface slope within the 85-foot long upstream reference reach was 0.3%, though localized water surface slopes were measured across riffles of up to 1.25%. The overall water surface slope at the 114-foot long downstream reference reach was 0.8%, though localized water surface slopes were measured across riffles of 1.6 to 3.6% (Attachment C). Bankfull flows and channel hydraulic geometry were computed for each measured cross section using Manning s equation in WinXSPro (USFS, 2005) and using either the local water surface slope or the overall slope, and the slope of the field-surveyed bankfull elevations (Attachment E). Computed bankfull flows ranged from 142 to 234 cfs, averaging 196 cfs. A flow of 196 cfs has an approximately 1.4 year return period (Section 3.2). Table 5 summarizes the measured hydraulic geometries of the two reference reach sites.

19 Table 5. Measured reference reach characteristics in Rohner Creek at two sites in Rohner Park. Measured Feature Range Average Channel Bottom Width 8.0 fee to 11.7 feet 9.8 feet Bankfull Width 11.6 fee to 18.0 feet 14.3 feet Maximum Bankfull Depth 3.0 feet to 4.4 feet 3.4 feet Bankfull Width/Average Depth Ratio 3.2 to Bed Material Characterization Two pebble counts were conducted on riffles in the upstream reference reach, and one pebble count and one bulk sample were collected in the downstream reference reach. The bulk sample was separated into surface and subsurface samples, which were sieved separately. Table 6 summarizes the results of the pebble count and bulk sample grain size analyses. Plots of the pebble counts and bulk sampling are presented in Attachment C. Table 6. Results of pebble counts and bulk sampling in the Rohner Creek reference reaches. Sample D 50 (Median) D 84 Pebble Counts mm 42.1 to 53.3 Bulk Sample (Surface) 17.6 mm 31.9 mm Bulk Sample Sub-Surface 8.5 mm Channel Design: Main Street to Alder Objectives Design objectives in this reach for Alternative 1 included enlarging the channel within site constraints to convey the 10-year flow event without overtopping the channel banks. The increased flow capacity in the channel was then considered when developing the flood bypass structures for Alternatives 3 and 4 to determine the amount of flow to be bypassed around Rohner Creek to maintain in-bank flow in Rohner Creek. Where feasible, design objectives included construction of variable width floodplains on one or both sides of the stream channel to further increase flood conveyance, reduce erosional stresses within the channel and create areas where natural gravel recruitment can occur that would increase fisheries habitat. Design Constraints Rohner Creek between Main Street and Alder is an entrenched channel that is highly encroached upon by adjacent land use. Property lines, building corners, retaining walls, parking lots, utilities and other manmade features are in close proximity to the stream channel. Geotechnical recommendations indicated

20 that the top of the channel bank can be no closer than 10 feet from an existing structure and that the maximum slope for channel banks should be no steeper than 1.5:1 (H:V). A stream corridor in which channel improvements can occur was identified based on property ownership, location of structures and utilities, and geotechnical recommendations. The stream corridor is of variable width ranging from approximately 35 feet to 75 feet. The channel enlargement would follow the existing Rohner Creek alignment, but the actual centerline of the creek would vary within the stream corridor to accommodate a wider channel and to maximize floodplain widths. Though not a design constraint, the presence of existing, visibly stable retaining walls were considered when develop channel grading. Location and species of large trees were also considered when developing the channel grading. Design Profile The channel enlargement design was based on the existing channel profile that has an average slope of 0.48%. Lowering the channel elevation to increase flood capacity and eliminate the existing 2.8-foot knickpoint between Alder Drive and Hillside Creek was considered. However, it was determined that further deepening of the channel would not fit within the allowable stream corridor without substantially narrowing the channel or use of retaining walls. Additionally, narrowing the channel would not be conducive to maintaining a geomorphically stable stream channel. Construction of retaining walls through this reach was determined to be cost prohibitive. Design Cross Sections Three different typical channel cross sections where applied in this reach. Application depended on the width of the stream corridor. Where the allowable corridor is narrow, a simple trapezoidal cross section was used that enlarges the existing channel (Figure 7). Where the corridor is wider, floodplains will be constructed on one or both sides of the enlarged stream channel. These would become inundated at flows greater than the bankfull discharge (Figure 8 and Figure 9). A design bankfull flow with a return period of 1.3-years was selected, which would be slightly more frequent than the bankfull flow return period of 1.4-years in the reference reaches. This would allow inundation of the channel floodplains more frequently, increasing overall flood capacity of the channel and reduce channel velocities and shear stresses to facilitate gravel recruitment and vegetation growth. The hydraulic geometry for the design of the bankfull channel portion of the cross sections was based in part on the reference reach (Table 7). For all cross sections, the typical bankfull channel bottom width will be 12 feet wide, which is at the upper range of the bottom widths measured in the reference reaches. A wider bottom width was selected to increase overall channel conveyance and to reduce channel velocities and shear stresses to promote channel stability and encourage gravel recruitment. Bankfull channel banks would be constructed at a 1.5:1 (H:V) side slopes. The design bankfull flow and cross sectional hydraulic geometry yields bankfull channel width of 20 feet, bankfull depth of 3 feet and a bankfull width to average depth (W/d) ratio of 8.9. This value is slightly higher than maximum measured W/d ratio of 7.9 measured in the reference reach and is a product of the larger design channel bottom width and more frequent design bankfull discharge. Where the stream corridor is wider, variable width floodplains will be constructed on one or both sides of the bankfull channel.

21 Table 7. Hydraulic Geometry for design cross sections between Main Street and Alder Drive. The overall slope of the channel would be 0.48%. Design Hydraulic Geometry Bankfull Flow Channel Bottom Width Bankfull Width Maximum Bankfull Depth Value 160 cfs 12 feet 20 feet 3 feet Bankfull Width/Average Depth Ratio 8.9 Floodplain Width 0 to 25 feet Figure 7. Typical trapezoidal channel cross section applied to channel sections where the stream corridor is constricted.

22 Figure 8. Typical cross section of stream channel applied to channel sections where the allowable space would be available for inclusion of an inset floodplain on one side of the channel. Figure 9. Typical cross section of stream channel applied to channel sections where the allowable space would be available for inclusion of an inset floodplain on both sides of the channel Channel Design: Calfire and Forbusco Retaining Wall Reach The reach of the existing channel downstream of the Hillside confluence to the Forbusco Vehicle Bridge is restricted to widening due to the existence of structures adjacent to the top of bank. This reach and adjacent structures are susceptible to geologic hazards related to the over-steepened banks and channel bed and bank scour. Structural improvements along this reach consist of soldier pile walls with concrete lagging on both sides of the channel (Figure 10). The height of the soldier pile flood control walls are generally dictated by the existing grade and structural needs associated with conforming to existing ground and not dictated by flood water elevations. Therefore, flood control walls associated with various alternatives are all the same height. Additional bank treatments include boulder bankline rock on both sides and riparian planting along the top of bank where feasible.

23 Figure 10. Vertical retaining walls (soldier pile walls) are to be installed to accommodate widening of the existing channel and maintain structural requirements for adjacent structures between Hillside Creek confluence and Forbusco Vehicle Bridge. Bank line boulders and riparian planting are to be installed for geomorphic stability and habitat enhancements Floodplain: Forbusco Lumber to 12 th Street Objectives The design objectives for the channel reach between 12 th Street and Forbusco Lumber were to create a new inset floodplain on the north side of the channel that would allow conveyance of the 100-year flow without inundating the adjacent field. One-foot of freeboard between the 100-year water level and existing ground surface beyond the new inset floodplain was used as a design criteria. The design involves grading only the upper portion of the northern channel bank above the bankfull elevation. The existing channel and left bank would remain undisturbed. Because the property where the floodplain would be constructed is privately owned, the width of the floodplain was minimized as much as feasible while meeting the 100-year conveyance objective. Design Constraints The proximity of private property and structures on the south bank of Rohner Creek along this reach precluded construction of a floodplain on the south side of the channel. To minimize disturbance to the existing channel and riparian area on the south side of the stream, the new floodplain will be constructed only on the north side of the channel. No structures, infrastructure or utilities are present along the north streambank.

24 Design Cross Section The new floodplain was designed to become active when flows exceed the design bankfull flow of 190 cfs. The new floodplain elevation and width were designed using a single typical cross section within this reach (Figure 11). The design was verified with hydraulic modeling to ensure that it met the 100-year conveyance objectives. To convey 100-year flow with 1 foot of freeboard, a 100-foot wide floodplain would be necessary. The new floodplain will slope gently towards the stream channel. The back of the floodplain will meet existing grade at a gentle 3:1 (H:V) slope. The stream-wise slope of the floodplain was set to match the overall channel slope within this reach of approximately 0.4%. Figure 11. Typical cross section of inset floodplain applied to the north side of Rohner Creek between 12 th Street and Forbusco Lumber Profile Control Design Knickpoints The existing channel profile exhibits two knickpoints where the channel stability is currently unknown (Figure 2). It is recommended that the two knickpoints along the channel profile be stabilized with grade control structures to prevent potential knickpoint migration and avoid further channel downcutting that can lead to bank instabilities. To facilitate upstream passage of juvenile salmonids, the grade controls should be placed so that the water surface drop between grade controls across the range of fish passage flows does not exceed 6 inches (NFMS, 2001). The existing knickpoint in Rohner Creek upstream of Hillside Creek consists of an approximately 2.8-foot drop. Therefore, a minimum of 6 grade control structures should be installed through the knickpoint to allow a maximum 6 inch drop between structures. The existing knickpoint in Rohner Creek upstream of 12 th Street consists of an approximately 1-foot drop. Therefore, a minimum of 2 grade control structures should be installed through the knickpoint to allow a maximum 6 inch drop between structures. Grade control structures can consist of boulder, log or concrete weirs (Figure 12). It is recommended that vertical cutoff walls made of sheet pile or concrete be installed below the channel bottom to prevent potential seepage and piping through the weirs. Boulders could be embedded and anchored to concrete

25 placed around the sheetpile, or anchored to a concrete-formed sheetpile cap to create a more natural appearance and provide more hydraulic diversity. Boulder weirs without cutoff walls are not recommended within the project reach due to the lack of bed material to seal them for stability and control to control seepage. Figure 12. Boulder weirs with subsurface cut-off walls are to be installed for grade control and geomorphic stability. Main Street to Alder Drive Grade controls are also recommended within the enlarged channel reaches between Main Street and Alder Drive in locations where the narrow stream corridor prevents the construction of floodplains on one or both sides of the stream channel. The grade controls would maintain the channel profile and cross sectional shape within the confined reaches of the channel and may also encourage recruitment of gravel recruit gravel that would be beneficial for aquatic habitat. The grade controls in this reach should be similar in construction to those that would be placed at the knickpoints. To facilitate upstream passage of juvenile salmonids, the grade controls should be placed so that the drop between grade controls does not exceed 6 inches. A total of up to 12 grade control structure may be necessary New Rohner Creek Channel Alignment Design (Alternative 2) Objectives Design objectives for Alternative 2 include constructing a new realigned channel that conveys the 100- year flow within the project corridor. Where feasible, design objectives included construction of variable width floodplain on one or both sides of the stream channel to increase flood conveyance, reduce

26 erosional stresses within the channel, and create areas where natural sedimentation can occur that would increase fisheries habitat. Design Constraints A stream corridor was identified for Alternative 2 channel based on property ownership, location of structures, utilities, and coordination with local landowners. The upstream origin of the Alternative 2 channel was limited by the presence of a driveway bridge off of Stillman Way. In the upstream reach of the new channel the channel corridor is confined to an approximately 60-foot width by the presence of several residential structures and a driveway. Approximately 500 feet downstream from the origin of the new channel, the stream corridor is located between multiple parcels on 16th Street and a single parcel off of Beech Street, where a stream corridor of approximately 45 feet is available. Due to the narrowness of the stream corridor in this area, it would be necessary to convey the channel through a culvert or between retaining walls. Downstream of the culverted reach, the new channel would pass through several parcels, including Fortuna High School, before re-joining existing Rohner Creek downstream of Forbusco Lumber. A channel corridor width of approximately 100 feet was defined for this reach. Though present within the stream corridor, utilities were not considered to be a design constraint. Design Profile The design profile with a slope of 0.7% for the new channel was determined based on the length of the available stream corridor and channel elevations in Rohner Creek at the upstream and downstream limits of the new channel. Design Cross Sections Two different channel cross sections would be applied in this reach depending on location within the new channel stream corridor. A trapezoidal channel would be implemented within the confined portions of the channel corridor, which would extend from where it departs from the existing Rohner Creek channel to approximately 75 feet downstream of the culvert that would be installed on the realigned channel. Where the corridor is wider, floodplains with widths of 25 feet would be constructed on both sides of the stream channel. These floodplains would become inundated at flows greater than the bankfull event (Figure 14). A design bankfull flow with a return period of 1.3 years was used, which is the same as the design bankfull flow for the channel enlargement of Rohner Creek for Alternative 1. Table 8 summarizes the design hydraulic geometry used for the cross sections. The trapezoidal channel would have a 30-foot bottom width, a 36.4-foot bankfull width, a total depth of 1.6 feet, and a W/d ratio of Channel banks would be constructed at 1.5:1 (H:V) side slopes, which would limit the overall width of the channel within the narrow stream corridor. The hydraulic geometry of this cross section diverges substantially from reference reach conditions. A cross section of this size would be necessary to maintain flows within the channel upstream of the proposed culvert, which creates a substantial backwater during a 100-year flow event due to the restricted corridor width where it would be constructed (See next section). Where the stream corridor is wider, floodplains on both sides of a bankfull channel would be used to increase the channel conveyance at flows higher than a bankfull event. The bankfull channel would consist of a channel bottom width of 18 feet and a bankfull width of 26.4 feet, a total depth of 2.1 feet, and a W/d ratio of The floodplains would become inundated at flows larger than a bankfull event. The

27 hydraulic geometry of this cross section also diverges substantially from reference reach conditions, but was necessary to maintain the 100-year flow within the channel banks. Table 8. Hydraulic Geometry for Design Cross Sections in the Alternative 2 realigned channel. The overall slope of the channel would be 0.7%. Design Hydraulic Geometry Bankfull Flow Channel Bottom Width Bankfull Width Maximum Bankfull Depth Bankfull Width/Average Depth Ratio Floodplain Width Value 160 cfs 30 feet (Trapezoid) 18 feet (With Floodplains) 36.4 feet (Trapezoid) 26.5 feet (With Floodplains) 1.6 feet (Trapezoid) 2.1 feet (With Floodplains) 24.8 feet (Trapezoid) 14.9 feet (With Floodplains) None (Trapezoid) 0 to 50 feet (With Floodplains)

28 Figure 13. Typical cross section of a trapezoidal channel cross section applied to the upstream reaches of the realigned channel in Alternative 2. Figure 14. Typical cross section of a floodplain channel applied to the lower reaches of the realigned channel in Alternative 2. Culvert Design Methods: The Stream-Simulation Design method specified that the crossing width encompass the bankfull channel. The method also specified that the soffit of the crossing be located above the 100-year water surface elevation to allow passage of debris and to eliminate channel scour due to pressure flow (CDFG, 2009). Stream simulation guidelines require that the stream channel through a crossing consist of streambed material having similar bed gradation as the adjacent natural channel. The thickness of the stream simulation material should be a minimum of 20% of the culvert rise. Culvert capacity was evaluated using the FHWA HY-8 culvert analysis program with a design 100-year flow of 971 cfs (Section 3.2). A tailwater elevation for the culvert was determined using a normal depth computation of the trapezoidal channel that would be located downstream of the culvert outfall (Attachment D). The culvert would consist of a 35-foot wide by 8 feet high concrete box culvert that spans the bankfull channel for the design floodplain cross section (Attachment D). A three-foot depth of stream-simulation bed material would form the channel within the crossing. To minimize the potential for structure failure due to scour, a slab bottom for the box culvert is recommended. Due to stream corridor confinement, the

29 tailwater elevation downstream of the culvert submerges the outlet soffit of the culvert at the 100-year peak flow. At this flow the headwater would be at the inlet soffit of the culvert. Nearly vertical retaining walls could be used instead of a culvert Hillside Creek Design Objectives Design objectives for Hillside Creek included sizing replacement culverts using Stream Simulation Design Methodology for the three undersized crossings improvements to convey the 100-year peak flow and provide anadromous fish passage. The channel capacity would be improved with the removal of deposited sediment and construction of a self-sustaining natural stream channel with riparian area that would also provide habitat for anadromous salmonids. Design Constraints Design constraints for the Hillside Creek improvements included existing parking lots and building structures limiting the potential channel corridor width to a total width of approximately 35 feet. Additional constraints included meeting existing grade both upstream and downstream of the area of improvements, cover requirements for the crossings, and consideration of the effects of the 100-year water surface elevation in Rohner Creek on channel and culvert capacity. Design Profile The design profile for Hillside Creek extends for a length of 832 feet at a slope of 0.9% from the existing channel invert elevation of Rohner Creek at the outfall of the Fortuna Boulevard culvert to the lower portion of the sedimentation basin. The profile was designed to lower the existing channel bottom elevation nearly two feet near Fortuna Boulevard and nearly one foot in the sediment basin. It is expected that after construction, the channel profile would self-adjust upstream of the constructed profile, removing approximately 1-foot of aggraded sediment over time. Design Cross Sections Site constraints limited the cross section of the design channel to a trapezoidal shape with 1.5:1 to 2:1 (H:V) side slopes. No reference reach surveys were performed in Hillside Creek, but scaled reference reach hydraulic geometry from Rohner Creek was deemed to be suitable for a preliminary design. Similar to Rohner Creek, a flow in Hillside Creek with a return period flow of 1.3-years was selected as the bankfull flow, resulting in a bankfull flow of 30 cfs in Hillside Creek. A channel bottom width of 5 feet was selected, resulting in a bankfull width of 9.4 feet, depth of 1.5 feet and a W/d ratio of 8.4 (Figure 15). Similar to Rohner Creek, the design W/d ratio is slightly larger than identified in the reference reach. As part of final design, reference reach hydraulic geometry should be obtained in Hillside Creek and the channel design refined, as applicable.

30 Figure 15. Typical trapezoidal channel cross section applied to Hillside Creek. Culvert Design Methods: The culvert design was performed using the same stream simulation method presented for the proposed culvert for Alternative 2. Culvert capacity was evaluated using the FHWA HY-8 culvert analysis program with a design 100-year flow of 249 cfs (Section 3.2). A tailwater elevation in Rohner Creek of 55.0 feet was used, which simulated the water surface at the 100-year flow event in Rohner Creek. The 100-year tailwater elevation from Rohner Creek backwater into Hillside Creek and into the lower portion of the sediment basin. Fortuna Boulevard: A 270-foot long, 10-foot wide by 8-foot high concrete box culvert embedded 2-feet below the finished channel grade was selected for the replacement crossing under Fortuna Boulevard (Attachment E). Due to the high tailwater elevation in Rohner Creek, the outlet of the culvert would be fully submerged and the culvert would be in outlet control. The soffit of the culvert would be not submerged, meeting stream simulation design criteria. Cover requirements over the crossing precluded further increasing the height of the crossing. A two-foot depth of stream-simulation bed material would form the channel bed within the crossing. To minimize the potential for structure failure due to scour, a closed bottom for the box culvert is recommended. Private Crossings: An HY-8 analysis was prepared for the private crossing immediately upstream of Fortuna Boulevard. The preliminary design for this crossing was also applied to the next upstream crossing. The analysis indicated that either a 10-foot wide by 7-foot high concrete box culvert of a 128 by 83 CMPA (10.7 feet by 6.9 feet) would meet stream simulation requirements for the two private crossings (Attachment E). Due to the tailwater effects from Rohner Creek, the culverts are in outlet control but are not fully submerged at the inlet or outlet. A two-foot depth of stream-simulation bed material would form the channel bed within the crossing. To minimize the potential for structure failure due to scour, closed bottom culvert is recommended.

31 6.2 Flood Bypass Routing Design Alternatives 3 and 4 are designed to convey the 100-year storm event with the addition of bypass structures. The bypass structures are designed to convey the remaining flow that cannot be conveyed by the improved channel design. The following sections describe Alternatives 3 and 4 in more detail. For both Alternatives 3 and 4, the improvements to both Rohner Creek and Hillside Creek would occur. The improvements to Rohner Creek and Hillside Creek are discussed in more detail in Section Alternative 3: Rohner and Hillside Creek Improvements with Field Bypass Culvert, 100-year flood conveyance Alternative 3 consists of in-stream channel improvements to Rohner Creek and Hillside Creek, installation of a labyrinth flow control weir, bypass culvert, and alcove. Improvements achieve a 100-year flood capacity (Figure 16). Figure 16. Alternative 3: Rohner and Hillside Creek Improvements with Field Bypass Culvert, 100-year flood conveyance. As stated above, Alternative 3 is designed to convey the 100-year storm event. The improved Rohner Creek channel conveys the 10-year storm event while the bypass structure diverts flows at approximately the 2-year storm event and conveys them to a bypass culvert which discharges back into Rohner Creek downstream of channel constrictions. In conjunction with bypass improvements included in this alternative, the channel hydraulic grade line (HGL) is lowered enough to allow routing 100-year flows. Alternative 3 also consists of improvements to Hillside Creek which are discussed in the following section. The hydraulic model of Alternative 3 demonstrated the conveyance of the 100-year storm event with no

32 floodplain inundation from either Rohner or Hillside Creek. See Attachment A, for WSE profiles for Alternative 3. Field Bypass Labyrinth Weir, Culvert and Alcove The field bypass is designed to convey the flows exceeding the 1-year storm event up to the 100-year storm event. Improvements include the installation of a labyrinth weir, culvert and alcove (Figure 17). Figure 17 Flood waters are diverted through a labyrinth weir to a bypass culvert, returning to Rohner Creek through an alcove. Labyrinth Weir A number of different weir options were considered for the bypass structure. A radial gate weir, a straight lateral weir and a labyrinth weir were considered. See Attachment F for sketches of the different weir options. The level control gate bypass structure was dismissed due to debris jam possibilities and the need for increased maintenance by the City. Both the labyrinth weir and the straight lateral weir require less maintenance and are less susceptible to debris jams than the level control gate option. The labyrinth weir option was favorable over the straight lateral weir option due to the reduced footprint required for the desired flow diversion, resulting in minimized impacts to adjacent landowners. Hydraulic analysis of the weir structures was conducted to determine the flow characteristics both over the weir and in the Rohner Channel immediately upstream and downstream of the weir. The labyrinth weir and grading in the vicinity is designed to direct excess flows in the Rohner Creek channel to a culvert, bypassing the existing Rohner Creek channel. The labyrinth weir is approximately 39 feet to 55 feet wide, 50 feet long, and 7 feet tall (Figure 18).

33 Figure 18 A labyrinth weir is to be installed to control and direct flow into the bypass culvert. Field Bypass Culvert The field bypass culvert carries excess flow from Rohner Creek approximately 2,250 feet downstream to a new alcove adjacent to Rohner Creek. The culvert bypasses approximately 2,510 feet of the existing channel, routing the flood waters to and area with more capacity. The culvert is 4 feet by 12 feet buried with a minimum of 2 feet cover (Figure 19). Hydraulic analysis of the bypass culvert was completed and demonstrates that the culvert conveys the desired flow diverted by the labyrinth weir to the downstream discharge location.

34 Figure 19 A 12' x 4' culvert is to be installed to carry overflow from Rohner Creek. Field Bypass Alcove The alcove is to be constructed adjacent to the existing Rohner Creek channel, set inside the proposed inset floodplain at the outfall of the field bypass culvert. The alcove is designed to reduce velocities and creates off channel habitat. The alcove contains large wood habitat structures, log sills, log bank deflectors, and a willow mattress along the banks (Figure 20). Figure 20. New alcove to be installed at field bypass outfall.

35 6.2.2 Alternative 4: Rohner and Hillside Creek Improvements with Fortuna Boulevard Bypass Culvert, 100-year flood conveyance Alternative 4 consists of in-stream channel improvements to Rohner and Hillside Creeks, installation of a labyrinth flow control weir and bypass culvert. Improvements achieve a 100-year flood capacity (Figure 21). Figure 21. Alternative 4: Rohner and Hillside Creek Improvements with Fortuna Boulevard Bypass Culvert, 100-year flood conveyance. Alternative 4 As stated above, Alternative 4 is designed to convey the 100-year storm event. The improved Rohner Creek channel conveys the 10-year storm event while the bypass structure diverts flows exceeding the 10-year event and conveys them to a bypass culvert which discharges back into Rohner Creek downstream of channel constrictions. In conjunction with bypass improvements included in this alternative, the channel hydraulic grade line (HGL) is lowered enough to allow routing 100-year flows. Alternative 4 also consists of improvements to Hillside Creek which are discussed in the following section. The hydraulic model of Alternative 4 demonstrated the conveyance of the 100-year storm event with no floodplain inundation from either Rohner or Hillside Creek.

36 Fortuna Boulevard Bypass Labyrinth Weir and Culvert The Fortuna Boulevard bypass is designed to convey the flows exceeding the 1-year event and up to the 100-year storm event. Improvements include the installation of a labyrinth weir and culvert (Figure 22). Figure 22. Flood waters are diverted through a labyrinth weir to a bypass culvert under Fortuna Boulevard, returning to Rohner Creek just upstream of the Hillside Creek outfall. Labyrinth Weir The labyrinth weir and grading in the vicinity is designed to direct excess flows in the Rohner Creek channel to a culvert, bypassing the existing Rohner Creek channel. The labyrinth weir is approximately 47 feet to 55.5 feet wide, 59 feet long, and 10 feet tall (Figure 23). A more detailed discussion of the labyrinth weir option can be found in Section

37 Figure 23. A labyrinth weir is to be installed to control and direct flow in excess of the 10-year flow into the bypass culvert. Fortuna Boulevard Bypass Culvert The street bypass culvert carries excess flow from Rohner Creek approximately 2,690 feet, underneath the existing median and roadway, downstream to the confluence of Rohner Creek and Hillside Creek. The culvert bypasses approximately 2,250 feet of the existing channel. The culvert is 4 feet by 12 feet buried with a minimum of 2 feet cover (Figure 24). Hydraulic analysis of the bypass culvert was completed and demonstrates that the culvert conveys the desired flow diverted by the labyrinth weir to the downstream discharge location.

38 Figure 24. A 12' x 4' culvert is to be installed to carry overflow from Rohner Creek. 7. Results of Proposed Condition Hydraulic Analyses 7.1 Alternative 1 As discussed in previous sections, the objective of Alternative 1 was the conveyance of the 10-year storm event (411 cfs) in the improved channel. The ECM MIKE model was modified with the channel widening and terracing discussed above and ran with the 10-year storm event. The Rohner Creek channel conveyed approximately 650 cfs (larger than the 10-year storm event) throughout the entire reach from Main Street to 12 th Street with a minimum freeboard of 1.3 ft (Attachment A). In addition, the MIKE model was used to determine the extent of floodplain inundation for Alternative 1 during the 100-year storm event (Attachment A). It is important to note when analyzing the WSE profiles in Attachment A, that if flooding occurs, the flow that leaves the channel does not contribute to the WSE downstream of the overtopping location. The hydraulic model of Alternative 1 demonstrated reduced, but continued floodplain inundation from Rohner Creek during the 100-year storm event. Conveyance of the 100-year storm event with no floodplain inundation was achieved in the Hillside Creek vicinity (Attachment B, Exhibit D). 7.2 Alternative 2 The two design cross sections for the realigned channel were evaluated using the WinXSPro software (USFS, 2005), which computes channel hydraulics at a single cross section assuming normal depth. Modeling cross sections were selected in the upstream portions of the channel reach, where freeboard would be most limited. Table 9 presents a summary of channel hydraulics modeled for the proposed trapezoidal channel. Though the normal-depth freeboard within the trapezoidal channel reach is predicted to be approximately 3.2 feet from the top of the stream bank, the proposed culvert near the downstream end of the trapezoidal channel reach creates a backwater that reduces the freeboard to approximately 0.5 feet. MIKE11 was not used to model the realigned channel proposed for Alternative 2. If this alternative is pursued, the proposed channel should be modelled using MIKE11, which will provide more hydraulic information along the entire channel profile rather than at typical cross sections.

39 Table 9. Summary of channel hydraulics for the typical trapezoidal channel and typical channel with inset floodplain for the proposed Alternative 2 realignment of Rohner Creek. Flow Event Average Velocity Channel Flow Floodplain Flow (Total) Channel Shear Stress Average Shear Stress Freeboard Bankfull (165 cfs) Trapezoidal 3.0 fps 165 cfs 0 cfs 0.6 psf 0.6 psf 6 feet Floodplain 3.5 fps 165 cfs 0 cfs 0.7 psf 0.6 psf 5.5 feet 10-Year (414 cfs) Trapezoidal 4.4 fps 414 cfs 0 cfs 1.1 psf 1.0 psf 4.9 feet Floodplain 3.5 fps 361 cfs 53 cfs 1.3 psf 0.6 psf 4.4 feet 100-Year (971 cfs) Trapezoidal 5.9 fps 971 cfs 0 cfs 1.8 psf 1.4 psf 3.3 feet* Floodplain 4.2 fps 700 cfs 271 cfs 2.1 psf 1.2 psf 3.0 feet *Freeboard reduced to 0.5 feet due to backwater of proposed culvert on the realigned channel. 7.3 Alternative 3 The improved Rohner Creek channel conveys the 10-year storm event while the bypass structure diverts flows exceeding the 1-year event and conveys them to a bypass culvert which discharges back into Rohner Creek downstream of channel constrictions. In conjunction with bypass improvements included in this alternative, the channel hydraulic grade line (HGL) is lowered enough to allow routing of 100-year flows (Attachment A). Alternative 3 also consists of improvements to Hillside Creek which are discussed in the following section. The hydraulic model of Alternative 3 demonstrated the conveyance of the 100- year storm event with no floodplain inundation from either Rohner or Hillside Creek. 7.4 Alternative 4 The improved Rohner Creek channel conveys the 10-year storm event while the bypass structure diverts flows exceeding the 1-year event and conveys them to a bypass culvert which discharges back into Rohner Creek downstream of channel constrictions. In conjunction with bypass improvements included in this alternative, (Attachment A) Alternative 4 also consists of improvements to Hillside Creek which are discussed in the following section. The hydraulic model of Alternative 4 demonstrated the conveyance of the 100-year storm event with no floodplain inundation from either Rohner or Hillside Creek.

40 8. Proposed Condition Geomorphic Analysis, Stabilization Methods and Habitat Recommendations 8.1 Geomorphic Analysis Each alternative changes the amount of streamflow conveyed within the existing channel during larger flow events. Both increases and decreases in the amount of flow conveyed in the channel could result in geomorphic channel instabilities. Significant decreases in the amount of flow conveyed within a channel may decrease the bedload carrying capacity of the channel (sediment competence), potentially causing excessive sediment deposition and overall channel aggradation. This can result in loss of channel capacity and, in the case of Rohner Creek, could eventually cause channel avulsion (rapid abandonment of stream channel and formation of a new channel). Primary channel changes due to increases in flows could include scour and incision of the channel bed and coarsening of substrate in channels with non-cohesive bed material. Channel incision commonly leads to over-steepened streambanks that result in bank failures and a widening of the channel. In urban areas where infrastructure encroaches into the stream corridor, these affects are often addressed through channel bank and bed hardening. One of the project objectives is to achieve a geomorphically stable channel, where feasible, to avoid the need for costly channel hardening or ongoing maintenance of sedimentation, which also has detrimental effects on aquatic and riparian habitats. Therefore, this section focuses on a preliminary evaluation of the potential for geomorphic instabilities associated with each alternative. The evaluations focus on assessing the changes in the magnitude and frequency of flows above bankfull discharge, channel velocities, and sediment transport capabilities. Evaluations were then prepared to identify suitable planting and bank protection measures for the project area Design Condition Channel Conveyance The peak flow rate conveyed within the channel and along inset floodplains through the project reach for 1.3-, 10- and 100-year storm events are provide in Figure 25 for existing conditions and Alternatives 1, 3, and 4. Figure 26 shows the flow reduction in Rohner Creek downstream of the proposed bypass structures for Alternatives 3 and 4 and compares these results with existing conditions. Alternative 2 is not plotted because the realigned channel corridor would convey all streamflow associated the 100-year storm event and the remanent Rohner Creek channel would be used to convey local storm drainage. The intake structures for the 4 bypass systems become active at a flow less than the 1.3-year flow events, reducing flows in Rohner Creek by approximately 5%. During a 2-year event, the bypass structures reduce in Rohner Creek by less than 5% for Alternative 3 and approximately 25% for Alternative 4. At the 10-year peak flow, there is a slight reduction in streamflow conveyed by the existing channel due to overbank flooding between Main Street and Hillside Creek confluence. The Alternative 1 improvements result in the channel conveying all of the in-channel flows to the downstream project reaches. The bypass

41 structures divert flow out of the channel for Alternative 3 and 4, reducing the flow within Rohner Creek by approximately 38% and 18%, respectively to below the existing condition 10-year flow. For Alternative 3 the flow returns immediately downstream of Forbusco Lumber (Figure 25b). For Alternative 4, the flow returns at the Hillside Creek Confluence. The discharge point for Alternative 4 results in an increase in flow, compared to existing conditions, within the CalFire reach. At the peak flow during the 100-year storm event, Alternative 1 improvements result in a small amount of flow spilling from the channel, which is conveyed across the floodplain and returns to the channel downstream of 12 th Street (Figure 25c). However, the new inset floodplain upstream of 12 th Street allows the channel to convey substantially more flow than under existing conditions between the bypass point of return and the 12 th Street crossing for Alternatives 3 and 4. The high-flow bypass structures in Alternative 3 and 4 reduce the flow within the channel to levels close to existing condition between Main Street and Hillside Creek.

42 a. b. c. Figure 25. Comparison of existing and proposed condition flows in Rohner Creek for the (a) bankfull, (b)10-year and (c) 100-year flow events.

43 Figure 26. Precent reduction in Rohner Creek flows downstream of the Alternative 3 and 4 floodplain bypass structure intakes compared to existing conditions Allowable Velocity Analysis To assess channel stability for the design alternatives, proposed-condition channel velocities at riffles crests were obtained from the hydraulic modelling and compared to the allowable velocity of 6.3 ft/s computed in Section The allowable velocity method recommends that to maintain a stable channel, water velocities for any return period flow event do not exceed allowable velocities computed for a 10- year return period event. Figure 27 presents model-computed average channel velocities at riffles for Alternatives 1, 3 and 4 for the 1.3-year, 10-year and 100-year peak flows. Existing condition channel velocities are shown for comparison. The proposed channel improvements between Main Street and Alder Drive associated with Alternatives 1, 3 and 4 reduce channel velocities only slightly from existing conditions for the 1.3-year and 10-year flow events. However, existing condition velocities in much of this reach are currently lower than the allowable velocity, except at discrete locations within the channel where channel confinement formed by structure encroachment created localized spikes in velocities. The removal of these confinements and channel enlargement in these areas would result in decreased velocities in these areas. At the 100-year peak flow, proposed condition velocities between Main Street and Alder Drive for all alternatives are lower than existing conditions. They are also substantially lower than the allowable velocity threshold, indicating that the proposed channel improvements would increase channel stability within these areas. The predicted decrease in velocities for proposed conditions is a result of the channel enlargement, presence of floodplains, and overall reduction in channel flows associated with the bypass structures in Alternatives 3 and 4.

44 Between Hillside Creek and the knickpoint, flow velocities for flows greater than a 10-year event are increased for Alternative 1 as a result of additional flow being retained within the channel as a result of the channel enlargement, and exceed the allowable velocity. This indicates the need for stream bed stabilization within this reach. Downstream of Hillside Creek, proposed condition velocities would be nearly unchanged from existing conditions during a 1.3-year event. However, the flow reductions from the Alternative 3 and 4 bypasses, the proposed floodwalls, and the proposed floodplain would substantially reduce 10 and 100-year flow velocities to well below the allowable velocity, indicating that the proposed channel improvements would increase channel stability within these areas. Alternative 1 channel velocities during the 10 and 100-year events would increase where floodwalls are proposed on the CalFire property. In general, channel velocities would increase at and upstream of the two knickpoints for all alternatives, indicating that grade control and bank stabilization would be necessary at these locations. Table 10 presents the channel velocities computed for the Alternative 2 c realigned channel. Proposed condition velocities do not exceed the allowable velocity for any flow event, indicating that the proposed channel will be stable. To ensure channel stability, a more detailed hydraulic analysis should be prepared a part of final design to evaluate channel velocities at flow transitions between cross sections at the proposed culvert inlet and outlet.

45 a. b. c. Figure 27. Channel velocities for existing conditions and for Alternatives 1, 3 and 4 for the (a) 1.3- year, (b) 10-year, and (c) 100-year flow events. Allowable velocity of 6.3 ft/s for the lean-clay comprising much of the existing channel bed is shown for comparison.

46 Table 10. Channel velocities for Alternative 2 for the 1.3-year, 10-year, and 100-year flow events. Flow Event Channel Velocity Trapezoidal Channel Channel Velocity Bankfull Channel with Floodplains 1.3-Year 3.0 fps 3.5 fps 10-Year 4.4 fps 3.5 fps 100-Year 5.9 fps 4.2 fps Sediment Transport Analysis A sediment transport competence analysis, the channel s ability to mobilize a sediment particle of a given size, was performed for all analyses to evaluate changes in sediment transport associated with the proposed Alternatives that could result in potential channel bed scour or aggradation. The analysis was prepared for the 1.3, 2 and 10-year flow events using average shear stresses from the hydraulic modeling. The results of the sediment transport analysis (Figure 28 a through c) indicate the channel sediment competence would decrease slightly under 1.3 and 2-year flows for Alternatives 1,3 and 4 between Main Street and Alder Drive, where the channel would be enlarged. The proposed floodwalls and associated channel widening downstream of Hillside Creek also slightly reduce channel sediment transport competence. During a 10-year flow event, the Alternative 3 and Alternate 4 bypass structures would divert 38% and 18% of flow from the channel, respectively (Figure 26). However, the flow reductions associated with the bypass structures reduce flow to less than what the channel is currently experiencing during a 10-year event (Figure 25b), and sediment competence between Main Street and Alder Drive is would remain relative unchanged. The proposed floodwalls and associated channel widening downstream of Hillside Creek also slightly reduce channel sediment transport competence. Table 11 presents the results of the sediment transport competence analysis for the Alternative 2 realigned channel. During a 1.3 and 2-year flow event, the trapezoidal channel will transport a similar size D 50 than in Rohner Creek where it enters the realigned channel, maintaining sediment continuity from upstream. During a 10-year event, the trapezoidal channel will have more sediment transport competence than in Rohner Creek, indicating that the channel bed would require stabilization in this area. During a 1.3-year event, the bankfull channel with floodplains will transport a similar size D 50 than in Rohner Creek where it enters the realigned channel, maintaining sediment continuity from upstream. At flows greater than the 1.3-year flows, the floodplain channel, by design, will have less sediment transport competence than in Rohner Creek, which will facilitate gravel recruitment along its length. To ensure channel bed stability along the Alternative 2 realigned channel, a more detailed hydraulic analysis should be prepared a part of final design to evaluate sediment transport competence at flow transitions between cross sections at the proposed culvert inlet and outlet. Similar to the sediment transport discussion for existing conditions, the sediment transport analysis for all design Alternatives indicates that the channel has the competence to transport larger D 50 and D 84 particle sizes than are delivered to the project reach. Because the channel appears to be supply limited for gravels, the flow conditions in the stream channel for all Alternatives may continue to limit or prevent the recruitment of gravels desirable for fisheries habitat.

47 a. b. c. Figure 28. Results of sediment transport competence analysis for (a) 1.3-year, (b) 2-year, and (c) 10-year return period event for existing and proposed conditions. Average D 50 and D 84 particle sizes measured upstream of the project area represent the typical sediment sizes delivered to the project reach.

48 Table 11. Results of sediment transport competence analysis for 1.3-year, 2-year, and 10-year return period event for the Alternative 2 Realigned Channel. Flow Event D 50 Moved Trapezoidal Channel D 50 Moved Bankfull Channel with Floodplains 1.3-Year 50 mm 60 mm 2-Year 68 mm 40 mm 10-Year 81 mm 53 mm 8.2 Proposed Condition Channel Stabilization Streambank and Bed Stabilization Structures The corridor for the proposed channel enlargements was determined to allow a minimum of a 10-foot distance between the proposed top of bank and an adjacent structure. In these locations, the geotechnical engineer recommended that Rock Slope Protection (RSP) be placed from the toe of the streambank to the top (is this in report or personal communication?). Additionally, where the channel flows through relatively tight bends, RSP placement to the top of the streambank is recommended on the outside of the bend, especially where an existing structure is nearby. The proposed-condition allowable velocity analysis (Section 8.1.1) indicated that channel velocities will be close to or above the allowable velocity at, and upstream of, the two knickpoints. In these locations it is recommended that a rock toe be constructed along both sides of the channel. Additionally, the boulder weirs with cut-off walls described in Section would be necessary to ensure that the knickpoints remain stable in their current location and not migrate headward. The allowable velocity analysis also indicated that Alternatives 1, 3 and 4 channel velocities will increase where new floodwalls are proposed on the Calfire property. This indicated that the channel bed should be protected from scour using an engineered streambed material that will remain stable at the 100-year flow. The proposed condition sediment transport competence analysis (Section 8.1.2) indicated that for all Alternatives, the stream channel would remain supply-limited for gravels, which may limit or prevent the recruitment of gravels desirable for fisheries habitat. Localized recruitment could potentially be facilitated with the placement of large wood structures and boulder weirs to create flow-field divergences Stream Channel and Floodplain Plantings Schiechtl and Stern (1994) and USDA (2007c) present maximum permissible shear stresses for various planting methods and bioengineering techniques for channel banks for pre-establishment construction and after 3 to 4 years (Table 12). These values were used to select the appropriate type of planting along the improved reaches of the Rohner Creek channel based on average shear stresses for each Alternative. Figure 29 presents model-computed average shear stresses at riffles for Alternatives 1, 3 and 4 for the 1.3-year, 10-year and 100-year flow events. Existing condition shear stresses are shown for comparison. Table 13 presents shear stresses computed for the Alternative 2 channel cross sections.

49 Except in the vicinity of the two knickpoints, proposed-condition shear stresses in Rohner Creek for all Alternatives typically would remain below 2.5 pounds per square foot (psf) under all flows up to the 100- year flow event. Therefore, streambank plantings consisting of trees and shrubs would be appropriate along most of channel reaches. Native grasses would be suitable within the enlarged channel between Main Street and Alder Drive, the constructed floodplain between Forbusco Lumber and 12 th Street, and along the realigned. Because shear stress would exceed the permissible shear stress for turf, it is recommended that coir matting be placed along the channel banks where rock toes or RSP would not installed. The matting would provide erosion resistance to the streambanks for the first 3 to 5 years, until the roots of the planted vegetation become stable. Because shear stresses will be low on the wide inset floodplain between Forbusco Lumber and 12 th Street, coir matting will likely only be necessary for a 20-foot width behind the bankfull channel bank. Live willow staking is the most cost effective way to stabilize the lower portions of streambanks. However, it is only effective in permeable soils. The heavy clay nature of the streambanks are likely not suitable for use of live staking. However, during construction the materials forming the excavation banks can be evaluated and live staking could possibly be substituted for a portion of the streambanks. Shear stress increases dramatically at the knickpoint upstream of Hillside Creek and exceed the stable shear stresses for planting alone. Where shear stresses exceed 2.5 psf, it is recommended that the streambank be stabilized with a rock toe to bankfull elevation and the voids between the rocks filled with soil and planted. Selection of plant species will be performed during final design. However, within constrained reach of the stream channel where debris jams may still have a chance of forming, lower growing, more flexible plant species will be selected to minimize vegetation encroachment into the channel. Because of the seasonal rainfall patterns on the North Coast, summer irrigation for the first 3 years after construction may be required for container plantings and higher elevation live stakes.

50 Table 12. Selection of bioengineering treatments for streambanks applicable under a range of channel shear stresses in pounds per square foot (psf) (Adapted from NRCS, 2007c and Schiechtl and Stern, 1994). Bioengineering Treatment Allowable Pre-Establishment Shear Stress Allowable Shear Stress after 3-4 Year of Establishment Turf (no matting) 0.2 psf psf Turf with coir matting* 4.5 psf NA Long Native Grasses NA psf Tree and Shrub Planting 0.41 psf 2.46 pfs Live Staking 0.5 to 2 psf 2 to 5 psf Willow Mattress 0.4 to 4.1 psf 3.8 to 8.2 psf Brush Mattress 0.4 to 4.2 psf 2.8 to 8 psf Live Staking in Riprap 3+ psf 6 to 8+ psf *based on Coir Mat 700 (ACF Environmental)

51 a. b. c. Figure 29. Shear stresses for existing conditions and Alternatives 1, 3 and 4 for the (a) 1.3-year, (b) 10-year, and (c) 100-year flow events.

52 Table 13. Shear stresses for Alternative 2 for the 1.3-year, 10-year, and 100- year flow events. Flow Event Shear Stress Trapezoidal Channel Shear Stress Bankfull Channel with Floodplains 1.3-Year 0.8 psf 0.5 psf 10-Year 1.0 psf 1.3 psf 100-Year 1.4 psf 2.1 psf 9. Fisheries Passage and Protection Culvert Modifications for Fish Passage 12 th Street With all of the alternatives, modifications to the 12 th Street culvert are proposed. The culvert retrofit is intended to improve fish access to allow both adult and juvenile salmonids the ability to freely move upstream and downstream. This will allow adult salmonids access to spawning habitat upstream of the project area, as well as access to any newly formed spawning habitat as a result of the project. The improvements will also allow juvenile salmonids access to Rohner Creek. Given Rohner Creek s proximity to the lower Eel River, the creek could serve as valuable habitat for non-natal rearing by coho salmon and other juvenile salmonids. Main Street All of the alternatives include construction of rock banklines within the Main Street culvert, which already maintains a natural substrate bottom. These banklines are intended to create hydraulic diversity, including slower water velocities along the margins suitable for use by fish during higher flows. The banklines also provide a pathway for terrestrial animals that tend to migrate along the riparian corridor, but are reluctant to walk within the wetted channel which currently spans the entire culvert width. Terrestrial pathways can reduce the need for these animals to go up to the road, thus reducing the risk injury or depth from traffic and improve public safety. Fortuna Boulevard at Hillside Creek All of the alternatives include replacement of the Hilliside Creek culvert at Fortuna Boulevard to provide capacity to convey the 100-year storm event. As currently proposed, this culvert would be designed based on stream simulation methodology and would provide both adult and juvenile salmonids access to upstream habitat. There may be potential for the existing sediment basis to be used by juvenile salmonids as rearing habitat in the future.

53 9.1.2 Fish Passage at Profile Control Weirs All of the alternatives include use of boulder weirs to control the channel profile. These weirs will be designed to provide for passage of adult and juvenile salmonids. In addition, they may be used to encourage gravel recruitment and formation of pool habitat Flow Bypasses for Alternatives 3 and 4 Alternatives 3 and 4 include bypass structures designed to divert high flows out of the channel, and then returning the flow to the channel at a designated downstream location. For both alternatives, the bypass flows are conveyed through one or more buried culverts. As designed, the bypass structure for Alternative 3 begins to divert streamflow into the bypass culvert slightly below the 2-year flow, and diverts approximately 18% of the streamflow at a 10-year flow (Figure 26). Alternative 4 begins diverting streamflow into the bypass culvert at a lower flow. At the bankfull discharge (1.3-year flow), approximately 5% of the streamflow is divert out of the channel, and at a 10-year flow event nearly 38% of the streamflow is diverted into the bypass culvert. Because these bypasses convey a substantial amount of the streamflow during large flows, there is potential for fish to be entrained into them and conveyed through the bypass culvert. The culvert would return the fish back to the stream channel. To prevent stranding within the culvert, they should have a positive slope throughout. If multiple culverts are used to convey the bypass flow, it may be desirable to focus lower bypass flows into a single culvert to provide sufficient depth. To ensure there is adequate depth for the fish within the bypass culvert(s) to avoid injury it may be necessary to include baffles and/or a u-shaped bottom if a flat-bottom culvert shape is used. High-flow, low-velocity fish habitat should be provided near the discharge point of the bypass culvert to ensure fish delivered to this location are not swept further downstream. These areas should also be shaped to avoid stranding as flows rapidly recede once the bypass becomes inactive. There is the potential that an adult salmonid could attempt to migrate into the discharging bypass culvert. This is particularly the case for Alternative 4 because the bypass becomes active at streamflows below bankfull flow. If a fish was able to swim through the bypass culvert, they would likely not be able to pass over the labyrinth weir bypass structure and into Rohner Creek. Therefore, efforts should be made to design the bypass culvert outlets to dissuade fish from attempting to swim up them. This can include creating a combined depth and velocity barriers at the outlet, similar to barriers created to exclude non-native fish from upstream habitats.

54 10. References GHD Inc. Rohner Creek Phase I Study Report. June GHD Inc. Rohner Creek Phase II Study Report. March Gordon, N.D., T. McMahon, and B. Finlayson Stream Hydrology: an Introduction for Hydrologists. John Wiley & Sons. 526 pp. Knighton. D Fluvial Forms and Processes: A New Perspective. Hodder Arnold Publication. 400 pp. NMFS Guidelines for salmonid passage at stream crossings, NOAA Fisheries, NMFS SW Region. Parker, G., Klingeman, P.C., and McLean, D.L Bedload and size distribution in paved gravel-bed streams: American Society of Civil Engineers, Proceedings, Journal of the Hydraulics Division, v. 108, p PRISM Parameter-elevation Regressions on Independent Slopes Model. Oregon State University. Schiechtl, H.M. and R. Stern Water Bioengineering Techniques for Watercourses, Bank and Shoreline Protection. Blackwell Science Ltd., Cambridge, Massachusetts, 86 pages. USDA. 2007a. Chapter 8: Threshold Channel Design. Part 654 Stream Restoration Handbook, National Engineering Handbook. USDA. 2007b. Chapter 9: Alluvial Channel. Part 654 Stream Restoration Handbook, National Engineering Handbook. USDA. 2007c. Technical Supplement 14I: Streambank Soil Bioengineering. Part 654 Stream Restoration Handbook, National Engineering Handbook. USFS Stream Simulation: An ecological approach to providing passage for aquatic organisms at road-strem crossings. Forest Service Stream Simulation Working Group, San Dimas, CA. USFS WinXSPRO, A Channel Cross Section Analyzer, User's Manual, Version 3.0, Gen. Tech. Rep. RMRS-GTR-147. U.S. Department of Agriculture, U.S. Forest Service, Rocky Mountain Research Station, Fort Collins, CO. Wolman, M.G and J.P. Miller Magnitude and frequency of forces in geomorphic processes. Journal of Geology. 68:

55

56 Attachment A Rohner and Hillside Creek WSE Profiles Figure 30 Rohner Creek WSE Profile, ECM 10-year, 24-hour storm event, Eel River BW Boundary Condition

57 Figure 31 Rohner Creek WSE Profile, ECM 100-year, 24-hour Storm Event, Eel River BW Boundary Condition Figure 32 Rohner Creek WSE Profile, ECM 100-year, 24-hour Storm Event, Normal Depth Boundary Condition

58 Figure 33 Rohner Creek WSE Profile, Alternative 1 10-year, 24-hour Storm Event, Eel River BW Boundary Condition Figure 34 Rohner Creek WSE Profile, Alternative year, 24-hour Storm Event, Normal Depth Boundary Condition

59 Figure 35 Rohner Creek WSE Profile, Alternative year, 24-hour Storm Event, Normal Depth Boundary Condition Figure 36 Rohner Creek WSE Profile, Alternative year, 24-hour Storm Event, Normal Depth Boundary Condition

60 Figure 37 Hillside Creek WSE Profile, ECM 10-year, 24-hour Storm Event, Eel River Backwater Boundary Condition Figure 38 Hillside Creek WSE Profile, ECM 100-year, 24-hour Storm Event, Eel River Backwater Boundary Condition

61

62 Attachment B Exhibits

63 # # Modeled Maximum Inundation (ft) Existing Main St. Box Culvert # Main St. Greater than 2 FEMA Flood Zone 100 year Stillman Stillman Way Way 12th St. Fortuna High School ` ` ` Existing Drainage Ditch Fortuna Blvd. Existing Drainage Ditch ` ` ` Hiillllsiide Creek Existing Stormdrain Pipes 32" and 48" Diameter ` ` Existing Stormdrain Inlets Existing Drainage Ditch ` ` ` ` R o hner Creek Flow # Existing Hillside Existing Hillside Culvert (72" x 44") # Existing 12th St. Box Culvert Paper Size 11" x 17" (ANSI B) Feet Map Projection: Lambert Conformal Conic Horizontal Datum: North American 1983 Grid: NAD 1983 StatePlane California I FIPS 0401 o G:\01054 City of Fortuna\ RohnerCrkFloodCntrlAltAnalysis\08-GIS\Maps\Figures\Alternatives_Analysis\Exhibit_A-ECM10yrBW.mxd While every care has been taken to prepare this map, GHD (and DATA CUSTODIAN) make no representations or warranties about its accuracy, reliability, completeness or suitability for any particular purpose and cannot accept liability and responsibility of any kind (whether in contract, tort or otherwise) for any expenses, losses, damages and/or costs (including indirect or consequential damage) which are or may be incurred by any party as a result of the map being inaccurate, incomplete or unsuitable in any way and for any reason. Data source: Data Custodian, Data Set Name/Title, Version/Date. Created by:bvivyan Drainage Ditch Stormdrain Pipe Hillside Culvert Hillside Creek Rohner Creek City of Fortuna Rohner Creek Flood Control Alternatives Analysis Existing Conditions 10 Year Eel River Backwater Job Number Revision Date A 14 May 2013 Exhibit A 718 Third Street Eureka CA USA T F E eureka@ghd.com W

64 # # Modeled Maximum Inundation (ft) Existing Main St. Box Culvert # Main St. Greater than 2 FEMA Flood Zone 100 year Stillman Stillman Way Way 12th St. Fortuna High School ` ` ` Existing Drainage Ditch Fortuna Blvd. Existing Drainage Ditch ` ` ` Hiillllsiide Creek Existing Stormdrain Pipes 32" and 48" Diameter ` ` Existing Stormdrain Inlets Existing Drainage Ditch ` ` ` ` R o hner Creek Flow # Existing Hillside Existing Hillside Culvert (72" x 44") # Existing 12th St. Box Culvert Paper Size 11" x 17" (ANSI B) Feet Map Projection: Lambert Conformal Conic Horizontal Datum: North American 1983 Grid: NAD 1983 StatePlane California I FIPS 0401 o G:\01054 City of Fortuna\ RohnerCrkFloodCntrlAltAnalysis\08-GIS\Maps\Figures\Alternatives_Analysis\Exhibit_B-ECM100yrBW.mxd While every care has been taken to prepare this map, GHD (and DATA CUSTODIAN) make no representations or warranties about its accuracy, reliability, completeness or suitability for any particular purpose and cannot accept liability and responsibility of any kind (whether in contract, tort or otherwise) for any expenses, losses, damages and/or costs (including indirect or consequential damage) which are or may be incurred by any party as a result of the map being inaccurate, incomplete or unsuitable in any way and for any reason. Data source: Data Custodian, Data Set Name/Title, Version/Date. Created by:bvivyan Drainage Ditch Stormdrain Pipe Hillside Culvert Hillside Creek Rohner Creek City of Fortuna Rohner Creek Flood Control Alternatives Analysis Existing Conditions 100 Year Eel River Backwater Job Number Revision Date A 14 May 2013 Exhibit B 718 Third Street Eureka CA USA T F E eureka@ghd.com W

65 # # Modeled Maximum Inundation (ft) Existing Main St. Box Culvert # Main St. Greater than 2 FEMA Flood Zone 100 year Stillman Stillman Way Way 12th St. Fortuna High School ` ` ` Existing Drainage Ditch Fortuna Blvd. Existing Drainage Ditch ` ` ` Hiillllsiide Creek Existing Stormdrain Pipes 32" and 48" Diameter ` ` Existing Stormdrain Inlets Existing Drainage Ditch ` ` ` ` R o hner Creek Flow # Existing Hillside Existing Hillside Culvert (72" x 44") # Existing 12th St. Box Culvert Paper Size 11" x 17" (ANSI B) Feet Map Projection: Lambert Conformal Conic Horizontal Datum: North American 1983 Grid: NAD 1983 StatePlane California I FIPS 0401 o G:\01054 City of Fortuna\ RohnerCrkFloodCntrlAltAnalysis\08-GIS\Maps\Figures\Alternatives_Analysis\Exhibit_C-ECM10yrND.mxd While every care has been taken to prepare this map, GHD (and DATA CUSTODIAN) make no representations or warranties about its accuracy, reliability, completeness or suitability for any particular purpose and cannot accept liability and responsibility of any kind (whether in contract, tort or otherwise) for any expenses, losses, damages and/or costs (including indirect or consequential damage) which are or may be incurred by any party as a result of the map being inaccurate, incomplete or unsuitable in any way and for any reason. Data source: Data Custodian, Data Set Name/Title, Version/Date. Created by:bvivyan Drainage Ditch Stormdrain Pipe Hillside Culvert Hillside Creek Rohner Creek City of Fortuna Rohner Creek Flood Control Alternatives Analysis Existing Conditions 100 Year Eel River Normal Depth Job Number Revision Date A 14 May 2013 Exhibit C 718 Third Street Eureka CA USA T F E eureka@ghd.com W

66 # # Modeled Maximum Inundation (ft) Existing Main St. Box Culvert # Main St. Greater than 2 FEMA Flood Zone 100 year Stillman Stillman Way Way 12th St. Fortuna High School ` ` ` Existing Drainage Ditch Fortuna Blvd. Existing Drainage Ditch ` ` ` Hiillllsiide Creek Existing Stormdrain Pipes 32" and 48" Diameter ` ` Existing Stormdrain Inlets Existing Drainage Ditch ` ` ` ` R o hner Creek Flow # Existing Hillside Existing Hillside Culvert (72" x 44") # Existing 12th St. Box Culvert Paper Size 11" x 17" (ANSI B) Feet Map Projection: Lambert Conformal Conic Horizontal Datum: North American 1983 Grid: NAD 1983 StatePlane California I FIPS 0401 o G:\01054 City of Fortuna\ RohnerCrkFloodCntrlAltAnalysis\08-GIS\Maps\Figures\Alternatives_Analysis\Exhibit_D-ChnlWide100yr.mxd While every care has been taken to prepare this map, GHD (and DATA CUSTODIAN) make no representations or warranties about its accuracy, reliability, completeness or suitability for any particular purpose and cannot accept liability and responsibility of any kind (whether in contract, tort or otherwise) for any expenses, losses, damages and/or costs (including indirect or consequential damage) which are or may be incurred by any party as a result of the map being inaccurate, incomplete or unsuitable in any way and for any reason. Data source: Data Custodian, Data Set Name/Title, Version/Date. Created by:bvivyan Drainage Ditch Stormdrain Pipe Hillside Culvert Hillside Creek Rohner Creek City of Fortuna Rohner Creek Flood Control Alternatives Analysis Channel Widening 100 Year Eel River Normal Depth Job Number Revision Date A 14 May 2013 Exhibit D 718 Third Street Eureka CA USA T F E eureka@ghd.com W

67 Attachment C Existing Condition Geomorphology

68

69 Locations of Reference Reaches on Rohner Creek