Culvert Design for Low and High Gradient Streams in the Midwest Dale Higgins, Hydrologist Chequamegon-Nicolet National Forest
Overview Culvert Design Considerations Hydraulic Terms Culvert Impacts Low vs High Gradient Design Low Gradient Culvert Design High Gradient Culvert Design
Culvert Hydraulics Terms Plunge pool Invert, Headwater (HW), Tailwater (TW) Headwater/Depth Ratio (HW/D): HW / pipe depth Supercritical Flow: high velocity, shallow water Subcritical Flow: low velocity, deep water
Aquatic Organism Passage Culvert Impacts Depth, Velocity and Exhaustion Barriers Jump Barriers
Sediment Sources Culvert Impacts Road Surface Erosion Frequent Failures Scour Embankment Erosion
Upstream Ponding Culvert Impacts on Channel Morphology Caused by culverts set too high or sediment deposits from frequent failures Result in stagnant water and upstream sediment deposition of muck, silt and sand Can increase water temperature
Culvert Design Methods Low Gradient: No Slope, Tailwater Control High Gradient: Stream Simulation
Low vs High Gradient Low Gradient = Tailwater Control Key Considerations Channel slope and bedform lengths Culvert length (fill ht, skew, side slope) Headcut potential (slope, channel type/material) Tailwater control Approximate Slope Breaks Low gradient < 0.3 percent High gradient > 1.0 percent
Water Surface Slope ~ 0.1% Low Gradient Low vs High Gradient
Water Surface Slope ~ 1.4% High Gradient Low vs High Gradient
All Good Culvert Designs Require: Complete Field Survey Stream profile Stream x-sec (2 up, 2 down) Road profile (and x-sec) Plan view sketch and/or topographic survey with total station Hydrologic Analysis Design flood flow (100-yr with HW/D<1, check 500-yr) Frequent flood flow (1.5 or 2-yr to check model accuracy) Fish passage flows (1 and 99% duration, ave daily flows) By-Pass flows during construction (ave monthly, median)
Culvert Size and Bankfull Width Low Gradient Culvert Design Bankfull Width Straight segment Narrow Unaffected by road crossing Bankfull Width - Riley Cr Example Min = 7.0 feet Mean = 9.2 feet Range = 7.0-11.5 feet, n=6 Select Culvert Width > BF Width Compare H&H Width to BF Width
Low Gradient Culvert Design Field Survey Hydrologic Analysis Select Invert Elevation (and Skew) Hydraulic Analysis - Model Flows Finalize Design
Culvert Skew Low Gradient Culvert Design Determine culvert skew Affects culvert length May affect stream profile
Culvert Elevation Low Gradient Culvert Design Set flat (on low gradient streams) Check profile for channel morphology impacts Consider channel adjustment and restoration Consider plunge pool and tailwater longevity Ensure tailwater will provide good depth and low velocity in culvert
Elevation (ft, local datum) Specify Invert Elevation Low Gradient Culvert Design 106 105 104 103 102 101 100 99 98 97 96 95 Riley Cr at FR 2161 Stream Thalweg Water Surface Road Surface Existing Culvert Invert Existing Culvert Top New Culvert 94-250 -200-150 -100-50 0 50 100 150 200 250 300 Distance (ft)
Elevation (ft) Specify Culvert Elevation Low Gradient Culvert Design 96 95 94 93 92 91 90 89 Stream Water Road Culvert Invert New Culvert Invert 88-150 -100-50 0 50 100 150 200 Distance (ft)
Low Gradient Culvert Design Field Survey Hydrologic Analysis Select Invert Elevation (and Skew) Hydraulic Analysis - Model Flows Finalize Design
42 x29 CMP Profile Low Gradient Culvert Design Riley Cr at FR 2161 Example
87 x63 CMP Profile Low Gradient Culvert Design Riley Cr at FR 2161 Example Culvert 7.25 ft wide BF Widths: Min=7.0 ft Mean=9.2 ft Max=11.5 ft N=6
72 x54 CMP, Culvert Table Low Gradient Culvert Design Riley Cr at FR 2161 Example
Low Gradient Culvert Design Field Survey Hydrologic Analysis Select Invert Elevation (and Skew) Hydraulic Analysis - Model Flows Finalize Design
Finalize Design Low Gradient Culvert Design Culvert Elevation, Size, Type, Shape, Length and Skew Flood and fish passage flows Channel morphology Channel restoration Fill height and side slopes Stream-road alignment Road Construction Surfacing, low point, ditches, side slopes Erosion control: riprap, silt fence, by-pass, re-vegetation
Stream Simulation High Gradient Culvert Design Premise: provide a channel through the structure that will present no more of a challenge to organisms than the natural channel. (i.e., pass water, organic matter, sediment and aquatic organisms) How? Use a reference reach to guide design of a simulated channel through the structure 24
Stream simulation does not provide: Riparian functions especially bank vegetation Light Lateral channel and floodplain processes Passage of some aquatic, semi-aquatic or terrestrial organisms
Stream Simulation Design Process Assess stream simulation feasibility Examine profile and alignment Find reference reach Unstable channel Determine bed shape and material Define structure width, elevation, details Mobility / stability Design profile control Aggrading or alluvial fan
Suitable for stream simulation most sites in the midwest
Stream Simulation Design Process Assess stream simulation feasibility Examine profile and alignment ID reference reach Determine bed shape and material Define structure width, elevation, details Evaluate mobility/stability Design profile controls
Elevation (ft). Longitudinal Profile Phase 2: Pre-emption at FR 377 102.00 Preemption Cr at FR 377 Chequamegon-Nicolet NF 101.00 100.00 99.00 98.00 Existing Culvert 5.0' x 3.0' 8 9 10 11 12 13 14 97.00 96.00 95.00 5 6 7 Bend 1.0% Bend w/ Some Wood (# 5) 94.00 93.00 92.00 3 4 2.3% Woody Debris (# 4) 91.00 90.00 89.00 88.00 87.00 86.00 1 Debris Jam (# 1) 2 2.4% Boulder Step and Log (# 2) Step at Base of Cobble/Boulder Cascade (# 3) Culvert Plunge Pool Stream Thalweg Water Surface Road Existing Culvert Invert Existing Culvert Top X-Section Locations 85.00-325 -300-275 -250-225 -200-175 -150-125 -100-75 -50-25 0 25 50 75 100 125 150 175 200 225 250 275 300 Distance (ft)
Elevation (ft). Design Slope and Vertical Adjustment Potential Phase 3: Pre-emption at FR 377 102.00 Preemption Cr at FR 377 Chequamegon-Nicolet NF 101.00 100.00 99.00 98.00 Existing Culvert 5.0' x 3.0' 8 9 10 11 12 13 14 97.00 96.00 95.00 5 6 7 Bend Bend w/ Some Wood (# 5) 94.00 93.00 92.00 3 4 Woody Debris (# 4) Ave Slope = 2.25% 91.00 90.00 2 Culvert Plunge Pool 89.00 88.00 87.00 86.00 1 Debris Jam (# 1) Boulder Step and Log (# 2) Step at Base of Cobble/Boulder Cascade (# 3) Stream Thalweg Water Surface Road Existing Culvert Invert Existing Culvert Top X-Section Locations 85.00-325 -300-275 -250-225 -200-175 -150-125 -100-75 -50-25 0 25 50 75 100 125 150 175 200 225 250 275 300 Distance (ft)
Design profile control, transitions Stream Simulation Design Process Assess stream simulation feasibility Examine profile and alignment Find reference reach Determine bed shape and material Define structure width, elevation, details Mobility / stability
Selection of reference reach Represents stream type through crossing Gradient Width/depth Channel materials pebbles count and key pieces Bedforms Provides design parameters for stream simulation Out of influence of existing crossing Try to avoid very complex channels
Channel Cross-Sections 5-3 Phase 2: Pre-emption at FR 377
Design profile control, transitions Stream Simulation Design Process Assess stream simulation feasibility Examine profile and alignment Find reference reach Determine bed shape & material Define structure width, elevation, details Mobility / stability
Stream Bed Particle Sizes Duck Creek at Hwy 139 Particle size distribution from pebble count Perform pebble count Measure 10 key (largest) pieces Streambed mix from particle size distribution
Design profile control, transitions Stream Simulation Design Process Assess stream simulation feasibility Examine profile and alignment Find reference reach Determine bed shape & material Define structure width, elevation, details Mobility / stability
Structure type and width (width >BF to allow for bank rocks and floodplain) Bridge Box Pipe Arch Bottomless Arch Embedded Round
Structure Selection Phase 3: Pre-emption at FR 377 Options: 12 x8 5 Ellipse 9 x12 Concrete Box 12 3 Aluminum Box Invert Elevations: Up = 91.2 Center = 90.5 Down = 89.8 Bed Elevations: Up = 94.2 Center = 93.5 Down = 92.8 Fill Over Pipe = 3 ft
Stream Simulation Design Process Assess stream simulation feasibility Examine profile and alignment Find reference reach Determine bed shape & material Define structure width, elevation, details Sediment mobility/stability Design profile control, transitions
Sediment Mobility and Stability Hydraulic modeling (HEC-RAS) and modified critical sheer stress equation Primary Considerations: Key pieces must be stable: bank rocks, grade controls Simulation and reference bed mobilize at same flow If upstream replacement unlikely, need most of bed stable 40
Stream Simulation Design Process Assess stream simulation feasibility Examine profile and alignment Find reference reach Determine bed shape & material Define structure width, elevation, details Mobility / stability Design profile control, transitions
Profile control options grade controls rock bands Riffle or riffle/step structures Constructed with a band of primarily larger (D84-D100) size unsorted rocks. They help form channel cross-section and profile shape.
Elevation (ft). Design Profile: Culvert and Bed Elevations Phase 3: Pre-emption at FR 377 102.00 Preemption Cr at FR 377 Chequamegon-Nicolet NF 101.00 100.00 99.00 98.00 Existing Culvert 5.0' x 3.0' 8 9 10 11 12 13 14 97.00 96.00 95.00 Streambed 5 6 7 Bend Bend w/ Some Wood (# 5) 94.00 93.00 92.00 91.00 90.00 89.00 88.00 87.00 86.00 1 Debris Jam (# 1) 2 3 Boulder Step and Log (# 2) 4 Step at Base of Cobble/Boulder Cascade (# 3) Culvert Plunge Pool Culvert Woody Debris (# 4) Ave Slope = 2.25% Stream Thalweg Water Surface Road Existing Culvert Invert Existing Culvert Top X-Section Locations 85.00-325 -300-275 -250-225 -200-175 -150-125 -100-75 -50-25 0 25 50 75 100 125 150 175 200 225 250 275 300 Distance (ft)
Project Construction Duck Creek at Hwy 139 Painted bed and bank lines Walk behind front-end loader Streambed and bank rocks
Culvert Design Methods Low gradient: Set it low and let it flow! (No Slope -Tailwater Control) High gradient: Simulate the stream! (Stream Simulation)