SW1190701_Shear Stress Calculation_11/3/2019

Existing DrainagO $wales
Two existing drainage swales serve the site. These swales are present within the agricultural area of the
site and appear to have been in place for many years. While the swales are not corn iletely uniform
throughout their length, their general characteristics have been utilized below to allow the calculation of
expected flow velocities and resulting tractive forces in each swale.
Velocity
HydroCAD modeling software was utilized to determine the anticipated velocities
drainage ditches during the 10-year storm event.
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floes in the existing

Shear Stress
t = rDSw
Where:
T0= Shear Stress (N/mz, ❑❑)
I
y❑= Weight Density of Water (N/m', Ib/ft }
D = Average water depth (m, ft)
Sw = Water Surface slope (m/m, ft/ft)
Therefore: 62.4*.08*,02 = .O.Ipsf for Existing Drainage Ditch 1
& 62.4*.09*.02 = .I 1psf for Existing Drainage Ditch I
Soil types in the area of the existing drainage ditches are Rosman loam with a typica profile of fine
sandy loam and loam and Codorus loam with a typical profile of loam and clay loam Table 4.6 —
Allowable Tractive forces Channel Linings (NCDOT Level lil Design of Erosion and Se iment Control Plans
Mannual) indicates that soils of there types can withstand tractive forces of 0.1 Ibs/ 3 to 0.25 Ibs/ft3.
Thus, the in -situ soils are expected to be readily able to withstand the tractive force developed during
the 10-year storm event.