HEC-15 Section 3.1 & HDM Index 864.2
Shear Stress Equation
The maximum shear stress is given by:
τ = γ⋅Hn⋅S
where:
- τ = shear stress in channel at maximum depth (N/m2 or psf)
- γ = unit weight of water
- Hn = depth of flow in channel (ft or m)
- S = channel bottom slope
| Boundary Category | Boundary Type | Permissible Shear Stress (lb/ft2) | Permissible Velocity (ft/s) |
|---|---|---|---|
| Soils(1) | Fine colloidal sand | 0.03 | 1.5 |
| Sandy loam (noncolloidal) | 0.04 | 1.75 | |
| Clayey sands (cohesive, PI ≥ 10) | 0.095 | 2.6 | |
| Inorganic silts (cohesive, PI ≥ 10) | 0.11 | 2.7 | |
| Silty Sands (cohesive, PI ≥ 10) | 0.072 | 2.4 | |
| Alluvial silt (noncolloidal) | 0.05 | 2 | |
| Silty loam (noncolloidal) | 0.05 | 2.25 | |
| Finer than course sand – D75 < 0.05 in. (non-cohesive) | 0.02 | 1.3 | |
| Firm loam | 0.075 | 2.5 | |
| Fine gravels | 0.075 | 2.5 | |
| Fine gravel (non-cohesive, D75 = 0.3 in, PI<10) | 0.12 | 2.8 | |
| Gravel (D75 = 0.6 in) (non-cohesive, D75 = 0.6 in, PI<10) | 0.24 | 3.7 | |
| Inorganic clays (cohesive, PI ≥ 20) | 0.14 | 2.9 | |
| Stiff clay | 0.25 | 4.5 | |
| Alluvial silt (colloidal) | 0.25 | 3.75 | |
| Graded loam to cobbles | 0.38 | 3.75 | |
| Graded silts to cobbles | 0.43 | 4 | |
| Shales and hardpan | 0.67 | 6 | |
| Vegetation | Class A turf (Table 4.1, HEC No. 15) | 3.7 | 8 |
| Class B turf (Table 4.1, HEC No. 15) | 2.1 | 7 | |
| Class C turf (Table 4.1, HEC No. 15) | 1 | 3.5 | |
| Long native grasses | 1.7 | 6 | |
| Short native and bunch grass | 0.95 | 4 | |
| Rolled Erosion Control Products (RECPs) | |||
| Temporary Degradable Erosion Control Blankets (ECBs) | Single net straw | 1.65 | 3 |
| Double net coconut/straw blend | 1.75 | 6 | |
| Double net shredded wood | 1.75 | 6 | |
| Open Weave Textile (OWT) | Jute | 0.45 | 2.5 |
| Coconut fiber | 2.25 | 4 | |
| Vegetated coconut fiber | 8 | 9.5 | |
| Straw with net | 1.65 | 3 | |
| Non Degradable Turf Reinforcement Mats (TRMs) | Unvegetated | 3 | 7 |
| Partially established | 6 | 12 | |
| Fully vegetated | 8 | 12 | |
| Rock Slope Protection, Cellular Confinement and Concrete | |||
| Rock Slope Protection | Small-Rock Slope Protection (4-inch Thick Layer) |
0.8 | 6 |
| Small-Rock Slope Protection (7-inch Thick Layer) |
2 | 8 | |
| No. 2 | 2.5 | 10 | |
| Facing | 5 | 12 | |
| Gabions | Gabions | 6.3 | 12 |
| Cellular Confinement: Vegetated infill |
71 in2 cell and TRM | 11.6 | 12 |
| Cellular Confinement: Aggregate Infill | 1.14 – in. D50 (45 in2 cell) | 6.9 | 12 |
| 3.5” D50 (45 in2 cell) | 15.1 | 11.5 | |
| 1.14” D50 (71 in2 cell) | 13.2 | 12 | |
| 3.5” D50 (71 in2 cell) | 18 | 11.7 | |
| 1.14” D50 (187 in2 cell) | 10.92 | 12 | |
| 3.5” D50 (187 in2 cell) | 10.55 | 12 | |
| Cellular Confinement: Concrete Infill |
(71 in2 cell) | 2 | 12 |
| Hard Surfacing | Concrete | 12.5 | 12 |
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