stormwater.mang.2004.pg3 CALCULATIONS
The attached Drainage Area Plan shows the individual subcatchments, the drywells and the
shallow stormwater basins.
- All computations SCS TR-20 HydroCAD.
- 50 year storm event—Type 11 rainfall=4.9"
- Soil conditions— sandy loams and fine and coarse sands — SCS soil group A
- Existing stormwater runoff assumed to be zero due to permeable soils & good
groundcover.
- Design percolation rate= 1 inch in 1 minute (Safety Factor of 2 used in design rate.)
In-place percolation rate = 1 inch in 30 seconds at a depth of 24".
- Bottom of drywells not utilized in calculating infiltration capacity.
Runoff rate and volume computations for the 50 year storm event have been completed for
developed conditions. The drywells and detention basins have been sized to maintain adequate
freeboard during a 50 year design storm.
See attached HydroCAD analysis for all computations.
CONCLUSIONS
Based upon the attached HydroCAD analysis, the drywells and detention basins have sufficient
capacity to manage all runoff generated by the proposed site development during a 50 year storm
event and post development runoff will be maintained at or below pre-developed level.
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