ATTACHMENT 5 8, 2013, CESPK-ED-HD

Transcription

1 ATTACHMENT 5 October 8, 2013, CESPK-ED-HD Memorandum for Record, HEC-RAS Modeling of the Cumulative Impact of the Corp s Tentatively Selected Project (TSP) and the Proposed SouthEast Connector, Truckee Meadows Flood Control Project, NV

2 CESPK-ED-HD 8 October 2013 MEMORANDUM FOR RECORD SUBJECT: HEC-RAS Modeling of the Cumulative Impact of the Corps Tentatively Selected Project (TSP) and the Proposed SouthEast Connector, Truckee Meadows Flood Control Project, NV 1. Purpose. A previous MFR titled Cumulative Impact of the Corps Tentatively Selected Project and the Proposed SouthEast Connector, Truckee Meadows Flood Control Project, NV and dated 8 July 2013 documented the undersigned s review of the draft plans, hydraulic documentation, and other miscellaneous material for the SouthEast Connector (SEC) proposed for the Reno/Sparks area. Of concern was the potential for cumulative impacts for both the Corps proposed TSP project and the SEC on water surface elevations in the Truckee Meadows. The MFR concluded that quantifying the impacts with any certainty would require numerical modeling. 2. This memo documents follow-on work wherein the Corps with-project HEC-RAS model was modified to include the proposed SEC. Additionally, the memo presents the resulting cumulative impacts to water surface elevations, or depths, throughout the ponding area of the Truckee Meadows for the 1% (1/100) annual chance exceedance (ACE) flood event. 3. Background. The Regional Transportation Commission of Washoe County (RTC) has proposed to construct the SouthEast Connector. As illustrated in Attachment No. 1, this is a 6-lane roadway connecting the intersection of Greg Street and Sparks Boulevard (City of Sparks) with the intersection of Meadows and Veteran s Parkways located south of the Huffaker Narrows. The proposed roadway will be about 5-1/2 miles long in total length. The entire project should be completed approximately in April The SEC will be constructed in two phases. Phase I extends between the intersection of Sparks Boulevard and Greg Street going south over the Truckee River, over Clean Water Way (without connector), and will terminate about 200 feet south of Clean Water Way. A Corps issued regulatory permit was not required for the Truckee River crossing since the proposed bridge will clear-span the river and all of the piers will be located above the ordinary high water line the channel. Phase I is currently under construction. 5. Phase 2 will extend between the southern limit of Phase 1 and run southward over the existing Yori Drain (bridge), an at-grade intersection with Pembroke Drive, over Boynton Slough (bridge), over a drainage channel in the golf course (bridge), and an at grade intersection with Mira Loma Drive. The SEC will continue to run southward through the Huffaker Narrows and connect with Meadows Parkway on the southern end. Portions of the existing Steamboat Creek upstream of Mira Loma Drive may be impacted and/or relocated due to the SEC alignment. 1

3 6. The roadway will be designed to allow for one travel lane in each direction to remain above the pool elevation corresponding to the 0.85% (1/117) ACE event. In addition, the design was required to not impact the water surface in the Truckee Meadows. There are several culverts included under/through the SEC to ensure equalization of WSEL s on both sides of the SEC. Furthermore, the design includes volume mitigation borrow holes/pits to ensure that the volume-stage relationship of the Truckee Meadows is not impacted by construction of the SEC. A no-rise certificate was issued for Phase 1 of the SEC project by the City of Sparks. 7. It appears that Stantec Consulting Services Inc. performed the hydraulic analysis of the Phase I portion of the project. CH2M Hill is currently completing the hydraulic analysis of the Phase II portion of the project. For both Phases, the without-project HEC-RAS model developed by WEST Consultants was used as the initial base model which focused on the 0.85% ACE event. Given the ill defined situation wherein it is not clear which flood damage reduction (FDR) project will be constructed, the consulting firms have proceeded by simply comparing the with-sec condition to the without-sec condition (a.k.a. baseline condition). There is no FDR component to either of these conditions. 8. As discussed above, it has been demonstrated that the Phase I portion of the SEC project does not induce an increase in the water surface elevation for the 0.85% ACE event throughout the Truckee Meadows. This conclusion appears reasonable given that although most of the Phase I segment (i.e., with a total length of about 4400 feet) is to be constructed above grade, there is a wide opening under the proposed Truckee River bridge (i. e., the bridge spans about 1400 feet) and there is another opening over Clean Water Way. Additionally, there are two sets of equalization culverts located through the earthen road embankment which are designed to equalize the water surface elevation between the eastern and western sides of the roadway. There are two volumetric mitigation basins which provide volume to offset the volume of the roadway s earthen embankments. Furthermore, this segment is located in the deepest part of the Truckee Meadows floodplain where flood flows emanating from the Truckee River, Steamboat Creek, and Boynton Slough are fully comingled. Nonetheless, it is unknown if there would be a cumulative impact on the water surface elevations if the Phase I portion was combined with the Corps TSP project. 9. Phase II of the SEC project is currently undergoing final hydraulic analysis and tweaking of the final design which results in a no increase in water surface elevation condition. A review of the Phase II plans indicates the following: a. Most of the roadway will be located on top of an earthen embankment which is required to ensure that at least two lanes will be above the 0.85% ACE water surface elevation. This requires that the Mira Loma and Pembroke Drive intersections will be elevated compared to their current configuration. b. A series of bridges are included as follows: i. Huffaker Narrows Bridge (about 510 feet long), ii. Mira Loma Bridge (located south of Mira Loma Drive and about 400 feet long), iii. Rosewood Lakes Bridge (about 170 feet long), 2

4 iv. Boynton Slough Bridge (about 170 feet long), and v. Yori Drain Bridge (about 140 feet long). c. Approximately six sets of equalization culverts through the roadway embankment. d. A number of additional structures which will act as equalization culverts during a flood event (e.g., a golf cart underpass through the embankment, etc.) e. Several large areas graded to provide volumetric mitigation to offset the volume of the roadway embankment. These volumetric mitigation basins were not identified or completely illustrated on the plans provided by CH2M Hill. f. Hydraulic modeling for either Phase I or Phase II was not provided for the initial review. g. Documentation of the hydraulic modeling for Phase II was not provided for the initial review. 10. CH2M Hill updated the without-project (baseline) version of the Truckee Meadows HEC-RAS model geometry by integrating new 1-foot topographic strip data into Washoe County s 2007 and foot topographic data (i.e., the strip data was collected along the SEC alignment). 1 Specifically, CH2M Hill reported that they merged the strip topo into the 2007/08 topo and then merged the Corps 2006 bathymetry (i.e., from the Ayres 2006 hydro-survey) into the terrain data. The impacted HEC-RAS geometric elements were then extracted from the new composite terrain data. 11. For the with-sec project condition HEC-RAS model, CH2M Hill modified the updated withoutproject terrain data to include the proposed topographic features (e.g., road embankment, volume mitigation ponds, etc.) and then re-extracted the impacted HEC-RAS geometric elements. 12. Qualitative Assessment. Given the alignment of the SEC, it is obvious that the earthen embankment will act as a levee or a dam which will interrupt movement of overbank floodwaters in the Truckee Meadows from moving eastward to drain through Steamboat Creek to the Truckee River. As discussed above, there are a number of design features passing through the embankment to minimize the damming effect. There is no mention of the impact of the SEC project (without a FDR project) on the flows going past Vista towards Wadsworth. 13. CH2M Hill reported that the hydraulic modeling indicated that greater than 1:1 volumetric mitigation was required in order to not increase the water surface of the 0.85% ACE event. This result makes sense if the mitigation volume is too low in the ponding area of the Truckee Meadows. That is, the added volume can be used up prior the peak conditions (i.e. peak WSEL) and not be as effective as desired. Furthermore, as mentioned above, CH2M Hill is currently working through several tweaks in the final design in order to arrive at a no impact condition. 1 Similarly, the Corps HEC-RAS modeling was built upon a combination of Washoe County s 2007 and foot topo, located along the channels and critical overbanks, and 2-foot topo dated 2004, 2006, and 2007 located in outlying areas of the Truckee Meadows study area. 3

5 14. The hydraulic modeling performed to date for the Phase II segment, and previously for the Phase I segment, did not include any impacts from the Corps TSP project. This situation is of great concern since the Corps project will displace large volumes of flood waters from the Sparks commercial area and from the Reno airport area into the Truckee Meadows. The Corps has already determined that this displacement of volume results in increases in depths of 6 to 8 inches in the Truckee Meadows for the 1% ACE event and an increase in flow going downstream past Vista. 15. Quantitative Assessment. The Corps with-project HEC-RAS model was modified to include the model geometric elements affected by the SEC roadway. Specifically, CH2MHill provided the Corps a working copy of their HEC-RAS model 2. The model modification process proceeded as follows: 1) review the Corps and CH2M Hill HEC-RAS models and identify all of the model geometric elements impacted by the SEC, 2) record the impacted geometric elements in a checklist as shown on Attachment No. 2, 3) import/replace the geometric elements in the Corps model with elements from the CH2M Hill model using the check-list to insure that all of the elements were imported and to facilitate quality control. The specific geometric elements impacted by the SEC are highlighted in yellow on Attachment No. 2 and included storage areas, storage area connections, lateral structures, and channel cross sections along Boynton Slough and Steamboat Creek. 16. An alternative and more independent approach could have consisted of using GIS to layout and extract the geometric elements from the Corps terrain model and then import these elements into the Corps modeling. However, there was very little funding and time available for conducting this quantitative assessment and the approach used was considered reasonable. Additionally, it was assumed that the model geometry developed by CH2M Hill was basically error-free having a sufficient level of detail such that the results from the composite model would be reliable. 17. The specific Corps plan which was modified to include the SEC configuration was labeled 100- yr 50yr_FIP_NVB&NERB_PDC_11 3 (file = *.p063). This baseline plan included the recommended plan with the 1% (1/100) ACE flood event. The HEC-RAS plan that includes the SEC modifications is titled 100yronTSPNoPump_11_w-SEC_Draft (*.p089). 18. The HEC-RAS model with the modified plan was run and debugged to ensure reasonable results. One item which required refinement to better define the SEC configuration was the geographic extents of storage areas Area5 and Area5B as well as the connection between the storage areas. It is important to note that at this area in the model, the SEC consists of a bridge over the Truckee River and a causeway leading southward for about 1100 feet before the roadway sits atop an 2 The CH2M Hill models included a copy of their updated without-project condition (i.e., no Corps or SEC projects a.k.a. baseline condition) and two versions of the with-sec condition. The specific plans included: baseline , Proposed V1, and Proposed V1. The latter plan was assumed to represent the latest version of CH2M Hill s with-sec plan and was used as the source of the impacted geometric elements. 3 The plan name infers the following: the 100-yr flood event on the Federal Interest Plan (FIP) sized for approximately the 50-yr level of protection with no benching at Vista (NVB), no ecosystem benching (NERB) in the Truckee Meadows, and the junctions of the Peoples Drain Capped (PDC). The specific plan was version 11 of this configuration. 4

6 earthen embankment. As illustrated in Attachment No. 3, the footprints for the two storage areas were re-cut to align with the SEC alignment and the storage area connection was modified to fall on the SEC alignment. Additionally, the volume-elevation curves for Area5 and Area5B were adjusted for the change in areal extent 4. Also depicted in Attachment No. 3 is the profile view of the updated storage area connection which now includes a portion of the SEC embankment located along the southern end of the connection. Due to limitations of the HEC-RAS program, the causeway is not included in the storage area connection (e.g., the bridge deck and piers). It was learned that inclusion of the road embankment has a noticeable impact on the computed depths/water surface elevations in the UNR Farms ponding area. 19. In general, the computed water surface elevations for the 1% (1/100) ACE event are similar to the with-project elevations. A comparison of computed depths between the with- and without-sec conditions is shown on Attachment No. 4. Both of these plans include the Corps recommended plan. 20. A review of Attachment No. 4 indicates increases in depth throughout the ponding area of the Truckee Meadows to the west of the SEC alignment. Most of the increase in depth ranges between 0.02 and 0.1 feet (1/4 to 1-1/4 inches). In storage area Area5, increases in depth of 0.1 to 0.2 feet (1-1/4 to 2-1/2 inches) and 0.4 to 0.5 feet (4-3/4 to 6 inches) are computed. The 0.4 to 0.5 feet increase is partly due to model construct issues. Specifically, this area is part of storage area Area5B in the TSP modeling (as well as in the without-project modeling); whereas, this area is part of storage area Area5 in the updated SEC modeling. A portion of the 0.4 to 0.5 feet increase (i.e., about 0.37 feet) is due to the difference in water surface elevations between storage areas Area5 and Area5B for just the TSP modeling. 21. Furthermore, the SEC could decrease depths along the east side of the SEC alignment (refer to the green shading) ranging between 0.02 and 0.5 feet (1/4 to 6 inches). Additional areas of green shading, on the west side of the SEC alignment, also indicate locations where the depths may be decreased by up to 6 inches due to the SEC. 22. On the upper reach of Steamboat Creek, there is an apparent increase in depth for a number of cross sections. The increase in depth ranges between 0.02 and 0.4 feet. It is unclear if this increase in depth is intentional or not given that portions of the overbanks appear to have been reshaped and deeper depths may have been desired. Furthermore, the computed increase of 0.4 feet in the water surface elevation in this area may only induce minimal damages since there are no structures and the land appears to be used for grazing. 4 Specifically, the volume-elevation data was adjusted as follows: the volume data of Area5B(original) was prorated by the relative areas of Area5B(new) and the temporary SA Area5A. Where Area5B(new) is the portion of Area5B located to the east of the SEC roadway and Area5A is the portion of Area5B located on the west side of the SEC roadway. The new volumes for Area5B(new) were a direct ratio of the original volumes of Area5B(original). The prorated volume-elevation data of Area5A was added to that of Area5(original) to develop the new data for Area5(new). 5

7 23. Qualification of Results. This modeling effort focused on determining the impacts to computed depths corresponding to the condition where the SEC project is added to the Corps TSP or Recommended Plan for the 1% (1/100) ACE flood event. Given the geometric detail imported from the CH2M Hill HEC-RAS model, it should be expected that the new model can be reasonably used for events more frequent than the 1% event (e.g., the 20%, 10%, 5%, and 2% ACE events). However, four of the bridges located along the SEC alignment are not explicitly included in the geometry and the 0.5% (1/200) and 0.2% (1/500) ACE events, which overtop the SEC embankment, may produce spurious results. The specific bridges include the Truckee River Bridge, the Yori Drain Bridge (i.e., UNR Farms drainage channel), the Rosewood Lakes Bridge (located over the drainage channel in Area 25 in the northern portion of the golf course ), and the Mira Loma Bridge. The bridges decks, and in some cases the piers and abutments, were not included in the CH2M Hill modeling due to limitations of the HEC-RAS program. The bridge openings were included in the modeling. 24. One final note regarding the results presented herein. The floodplains used to compare depths between the with- and without-sec conditions were based on the 5-foot raster ( meadowsraster ) which was developed from the original without-project TIN ( meadows a ) and is similar to the gridded data used by WEST Consultants to extract the basic HEC-RAS geometry. Thus, the topographic features corresponding to the SEC project (i.e., road embankments, volume mitigation ponds, etc.) and of the new strip topo are not included in development of the 1% ACE floodplains. Depths for the with-tsp floodplain were then subtracted from depths for the with-tsp and SEC floodplain to generate the impact plot shown on Attachment No. 4. There may be inaccuracies in the areas where the proposed SEC topographic features are located. 25. Cumulative Impacts. As discussed above, the modeling focused on quantifying the impacts of the SEC when added to the Corps TSP. This section presents the combined cumulative impacts of both projects compared to the without-any project conditions. As a starting point, the impacts of the Corps TSP on depths compared to the without-project condition (a.k.a. the baseline condition) are illustrated in Attachment No. 5. A review of this plot indicates the areas, and the depth differentials, where the Corps project reduces or eliminates flooding (i.e., green colors) for the 1% (1/100) ACE flood event and where the project increases flood depths (i.e., orange-red colors). 26. One quick note regarding induced flooding by the Corps project. Although negative impacts to the flood depths and extents of the Corps TSP project on the FEMA base flood elevations are not at a depth, duration, or frequency to justify hydraulic mitigation as a project feature based on Corps policy, increases in the base flood elevation will require some form of mitigation by TRFMA to satisfy National Flood Insurance Program (NFIP) requirements. For the purposes of developing reasonable costs for the Corps economic analysis, the mitigation was assumed to consist of the following measures: 1) in the Truckee Meadows ponding area located south of the Truckee River, residences will be raised above the Corps with-project 1% (1/100) ACE water surface elevation and commercial structures would be flood-proofed, and 2) along the North Truckee Drain corridor, pumping capacity of 400 cfs would be included to reduce flooding to pre-project elevations or less. It needs to be noted that these measures were included in the 6

8 Corps economic analysis to demonstrate that there are feasible measures to mitigate induced flooding, in response to NFIP requirements. The actual mitigation measures may consist of other features or approaches and will be selected and implemented by TRFMA. 26. As previously discussed, the changes to depths due to the addition of the SEC project to the Corps project are shown in Exhibit No. 4. The cumulative impacts to depths in the Truckee Meadows due to both the TSP and SEC project, compared to the without-project condition, are shown in Exhibit No. 6. This plot is similar to that shown in Exhibit No. 5. However, there are subtle differences where some depths are decreased by addition of the SEC project and other depths are increased by addition of the SEC project. Also, there are several areas where the SEC does not impact flood depths at all compared to just the TSP project condition. These areas are depicted on Attachment No. 4 where no color is present. 27. A review of Attachment No. 6 indicates that there are noticeable, cumulative impacts of both projects on the flood depths, water surface elevations, and extents of flooding f or the 1% (1/100) ACE flood event. Most of the induced flooding in the Truckee Meadows ponding area is due to the Corps TSP project. Addition of the SEC project has mixed impacts; flood depths are increased a bit (e.g., 0.02 to 0.2 feet) in some locations and in others, flood depths are decreased between 0.02 and 0.5 feet. The depth decreases due to addition of the SEC project occur mostly along the Steamboat Creek corridor. Additionally, a review of the computed discharge hydrographs at the upstream UPRR bridge (station 6650) for flow leaving the Truckee Meadows to the lower Truckee River indicates that the peak discharge is about the same for both the TSP project (by itself) and for the combined TSP and SEC projects. Both of these with-project conditions increase flow to the lower river by about 1520 cfs for the 1% (1/100) ACE flood event. 28. Questions and comments should be directed to the undersigned at (916) or scott.e.stonestreet@usace.army.mil. Scott E. Stonestreet, PE Hydraulic Design Section Attachments 7

9 Extents of the SouthEast Connector Attachment No. 1

10 DIFFERENCES BETWEEN THE PROPOSED SEC MODEL (W/O TSP) AND THE WITH TSP PLAN RAS MODEL STORAGE AREAS, STORAGE AREA CONNECTIONS, AND LATERAL STRUCTURES Storage Area Lateral SA Remarks SA Conn Remarks Lat Strctr Remarks Storage Area Connections Weirs Area 5 TR TR TR Mod Shp & E V Area5B Con5Bto5 Con5Bto5C TR TR TR TR TR Mod E V & Reversed SC SC 4570 SC SC 3080 Profile different, L diff Area5C ARea5Cto5 SC from Area5 Area6 Area6to5 Con7to6 Mod Shp & E V Area6A Con6ato6 Con6ato5C SC from Area6 Attachment No. 2

11 Area10 Con10to7 Con9to10 Con10to10a Con10to6 Mod Shp & E V Area10a Con10ato6a from Area10 SC Replaces SC BS 789 Replaces 625 Area11 Con11to10 Con12to11 Mod Shp & E V BS 3659 BS 2569 BS 1286 Replaces 1475 Area25 Mod Shp & E V BS 3660 BS 3116 BS 2570 BS 2225 Replaces 2226 Area25a from Area25a Area24ato25 Mislabeled Re labeled to Con25ato25 BS 1274 SC SC SC Area24 Con24To25 Con23To24 BS 5810 BS 4560 Mod Shp & E V Attachment No. 2

12 Area24a Con24ato24 Con24ato25A SC SC from Area24 Area53 Con53to52 Con53to24 Mod Shp & E V SC SC Replaces 22104&21890 Area52 Con23to52 Con52to24 Con52to35 SC Mod E V TR = Truckee River SC = Steamboat Creek BS = Boynton Slough Attachment No. 2

13 DIFFERENCES BETWEEN THE PROPOSED SEC MODEL (W/O TSP) AND THE WITH TSP PLAN RAS MODEL RIVERS, REACHES, AND CROSS SECTIONS Truckee River Upper Channel portions are the same; left overbanks are different due to with TSP vs. without project conditions Middle Channel portions are the same; left overbanks are different due to with TSP vs. without project conditions Lower Channel portions are the same; left overbanks are different due to with TSP vs. without project conditions Steamboat Creek Lower Some xsecs are the same,many have subtle differences, some have significant changes, some are just shifted to start at station 0 Recommend replacing all xsecs with ones from SEC proposed model Upper A lot of changes since the SEC embankment sits in the model of the floodplain/xsec Recommend replacing all xsecs with ones from SEC proposed model Boynton Slough Tributary Several changes between downstream end of station Recommend replacing all xsecs between d/s end and Bridge at sta 1625 (BS) is missing debris on one pier, why? Attachment No. 2

14 Updated configuration of Area5 and Area5B and the SA connection between the two storage areas Truckee With Project Cond Floodplain Plan: 100yronTSPNoPump_11_w-SEC_Draft Geom: yronTSPNoPump_11_w-SEC_Draft2 Flow: Legend Spillway 4400 Elevation (ft) Station (ft) Updated SA Connection (Con5to5B) with the SEC embankment included (Refer to light blue line in above figure) Attachment No. 3

15 Impacts of the SEC on depths computed for the Recommended Plan Attachment No. 4

16 Impacts of the Corps Recommended Plan compared to the without project condition Attachment No. 5

17 Cumulative impacts of the Corps Recommended Plan and SEC roadway project compared to the without project condition Attachment No. 6