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
Table 865.2 Permissible Shear and Velocity for Selected Lining Materials
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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