Average Velocity and Bank Shear Stress Calculations for Bank Stabilization of Huron Creek
Average Velocity and Bank Shear Stress Calculations for Bank Stabilization of Huron Creek
Houghton Waterfront Park
Average velocity of Huron Creek in the waterfront park:
Assuming for 25-yr storm: base width = 10 ft, bank height (water depth) of ~4 ft in flow zone, bank angle in flow zone = 2H:1V Area = (10 ft + 26 ft)/2 * 4 ft = 72 ft2
Estimate of Average Velocity based on HydroCAD modeling (Post-development 25-yr storm)
Q = 329 cfs
Assume x-sectional area from above of 72 ft2 Velocity = (329 ft3/s) / (72 ft2) = 4.6 ft/s
Assuming for 100-yr storm: base width =10 ft, bank height (water depth) of ~6 ft in flow zone, bank angle in flow zone = 2H:1V Area = (10 ft + 38 ft)/2 * 6 ft = 144 ft2
Estimate of Average Velocity based on HydroCAD modeling (Post-development 100-yr storm):
Q = 578 cfs
Assume x-sectional area from above of 144 ft2, Velocity = (578 ft3/s) / (144 ft2) = 4.0 ft/s
Shear stress on bank of creek in waterfront park:
USACE Gabion guidance document SR-22 provides the formula for shear stress on the bed as:
where γw = Unit weight of water, S = slope of bed or water surface and d = local flow depth for designated velocity
Note: Slope of the creek in the park is much less steep than the creek reaches immediately upstream of the waterfront park. Therefore, separate calculations are provided for a local slope (25 & 100-yr storms) and averaged slope considering upstream areas (25 & 100-yr storms).
Local Slope of creek water surface/bed surface in park (between M-26 and mouth of creek) = (20 ft/530ft) = 0.037. Use 25-yr storm depth.
τb = (62.4 lb/ft3)*(0.037)*(4 ft) = 9.24 lb/ft2
USACE SR-22 Bank shear τm = 0.75(τb) = 6.93 lb/ft2
Averaged Slope of creek water surface/bed surface considering upstream portions of the creek (between Sharon Ave. and M-26) = ((795 ft – 620 ft)/3598 ft) = 0.048. Average this with slope in park = (0.037+0.048)/2 = 0.042. Use 25-yr storm depth.
τb = (62.4 lb/ft3)*(0.042)*(4 ft) = 10.48 lb/ft2
USACE SR-22 Bank shear τm = 0.75(τb) = 7.86 lb/ft2
Local Slope of creek water surface/bed surface in park (between M-26 and mouth of creek) = (20 ft/530ft) = 0.037. Use 100-yr storm depth.
τb = (62.4 lb/ft3)*(0.037)*(6 ft) = 13.85 lb/ft2
USACE SR-22 Bank shear τm = 0.75(τb) = 10.38 lb/ft2
Averaged Slope of creek water surface/bed surface considering upstream portions of the creek (between Sharon Ave. and M-26) = ((795 ft – 620 ft)/3598 ft) = 0.048. Average this with slope in park = (0.037+0.048)/2 = 0.042. Use 100-yr storm depth.
τb = (62.4 lb/ft3)*(0.042)*(6 ft) = 15.72 lb/ft2
USACE SR-22 Bank shear τm = 0.75(τb) = 11.79 lb/ft2
CONCLUSIONS:
The following conclusions are based on comparison of the calculated values with the permissible velocities and shear stresses for gabions presented in the USACE guidance document SR-29 Stability Thresholds for Stream Restoration Materials[1].
The calculated average velocities are well below the maximum listed for gabions in the SR-29 document, which is 14-19 ft/s. Therefore, going by average velocity, the gabions are more than adequate for stabilization.
The bank shear stresses calculated for the estimated 25-yr, 24-hour storm depth (4 ft) are well below the maximum listed for gabions in SR-29, which is 10 lb/ft2. Also, these values are at the upper threshold for the most stable “soft” surface control measure, vegetated coir logs, which can withstand 4-8 lb/ft2.
The bank shear stresses calculated for the estimated 100-yr, 24-hour storm depth (6 ft) are just at or slightly above the 10 lb/ft2 bank shear stress threshold. These values can be considered conservative estimates due to:
- The slope value 0.037 is likely higher than the actual slope within the waterfront park as it is based off of measurements from a topographic map (cross-section and slope measurements have not yet been completed in the waterfront park for the geomorphology survey).
- Using the upstream-averaged steeper slope is not likely exactly representative of conditions in the park, but it allows us to take the extreme upstream slopes into consideration for an absolute “worst-case” scenario.
- The culverts under M-26 have notched concrete rings inside them that provide a certain amount of energy dissipation to the water prior to it’s entry to the waterfront park area.
In summary, the gabions should be more than adequate for bank protection into the future, given the scenarios ranging from the smallest storm to 100-year storms.
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