APPENDIX F: HYDROCAD STORM POND SIZING NOTES/ASSUMPTIONS
Pre-Development Scenario
CC Mall Catchment (1S):
Area = 91452 m2 x (1 ac / 4046.85 m^2) = 22.6 ac
Ground Cover: Pasture, grassland or range – GOOD
*Taken from historical aerial maps
Soil Types:
*Taken from Soil_Types.pdf
53721 m2 x (1 ac/ 4046.85 m^2) = 13.3 ac, Type A, CN = 39
32,539 m2 x (1 ac/ 4046.85 m^2) = 8.05 ac, Type C (some type D in this unit not considered), CN = 74
5,056 m2 x (1 ac/ 4046.85 m^2) = 1.25 ac, Type B, CN = 61
Total acres = 22.6
Tc :
Lag/CN Method
*From DEQ Stormwater Management
Hydraulic Length = 1,360 ft
Guidebook
Ground Slope = 0-8% from soils maps – going with 8% to be conservative = .08
Designated an elevation of the head of shopping cart creek at 938 ft. (outlet of ditch at 940)
Festival Plus Catchment (2S):
Area = 18,600 m2 x ( 1 ac/ 4046.85 m2) = 4.60 ac
Ground Cover: Pasture, grassland or range- GOOD
Soil Types:
5,389 m2 x ( 1 ac/ 4046.85 m^2) = 1.34 ac Type B
13,210 m2 x ( 1 ac/ 4046.85 m^2) = 3.26 ac Type A
Total acres = 4.6
*Taken from Historical Aerial Maps
*Taken from Soil_Types.pdf
Tc :
Lag/CN Method
*From DEQ Stormwater Management
Hydraulic Length = 490 ft
Guidebook
Ground Slope = 0-8% from soils maps – going with 8% to be conservative = .08
Storm Data:
SCS TR-20 Runoff Method
Reach-routing method – stor/ind+trans
Pond routing – stor/ind
Type II 24-hr Storm Type
Depth from MI Stormwater Manual = 4.75 in.
100-yr rainfall event
From HydroCAD hydrograph, Q = 10.75 cfs
*From DEQ Stormwater Management
Guidebook
Post-Development Scenario
CC Mall Catchment (1S):
Area = 22.6 ac paved parking lots, roofs, CN = 98
Tc :
Lag/CN Method
*From DEQ Stormwater Management
Hydraulic Length = 1,360 ft
Guidebook
Ground Slope = 0-2% – going with 2% to be conservative*From Map Unit Descriptions
Designated an elevation of the head of shopping cart creek at 938 ft. (outlet of ditch at 940)
Festival Plus Catchment (2S):
Area = 4.6 ac urban districts commercial and business, 85% impervious, CN = 89
Tc :
Lag/CN Method
*From DEQ Stormwater Management
Hydraulic Length = 490 ft
Guidebook
Ground Slope = 0-2% – going with 2% to be conservative*From Map Unit Descriptions
Culverts (4R):
*Combined both culverts into one node because of 5-node limit
24” x 18” rectangular culvert
*From previous measurements
Length = 159 ft total
*From GIS
Manning’s n = 0.017 concrete, unfinished, typical
Inlet invert = 959.5 ft (known), assume slope of 0.005 to obtain outlet invert of 958.71 ft
Ditches (5R):
*Combined both ditches into one node because of 5-node limit
Parabolic channel
Top width assumed to be 10 ft
*Too snowy to take actual measurements
Channel depth assumed to be 5 ft
*Too snowy to take actual measurements
Length = 874 ft
Manning’s n = 0.03 earth grassed and winding open channel flow
Outlet invert = 940 ft
Inlet invert = 958.71 ft
Reach 6 (3R):
Adjustment elevation = 938 ft
Storm Data:
SCS TR-20 Runoff Method
Reach-routing method – stor/ind+trans
Pond routing – stor/ind
Type II 24-hr Storm Type
Depth from MI Stormwater Manual = 4.75 in.
100-yr rainfall event
*From DEQ Stormwater Management
Guidebook
From HydroCAD hydrograph, Q = 140.2 cfs
Post-Development with Detention Pond
To find the volume of the detention pond, the pre-development and post-development hydrographs
were superimposed on one another and the difference in area inside the curves was found (blue shaded
area below).
It was assumed that the detention pond collects from the mall and the ditches/culverts connected to
Festival Foods Plus.
The depth of the detention pond is 10 ft (from GIS, elevation difference from the ditch outlet and Reach
6 inlet = 10.25 ft). It was assumed that the pond dimensions were 200 ft x 250 ft with 2 ft/ft slopes.
CC Mall Catchment (1S):
Area = 22.6 ac paved parking lots, roofs, CN = 98
Tc :
Lag/CN Method
*From DEQ Stormwater Management
Hydraulic Length = 1,360 ft
Guidebook
Ground Slope = 0-2% – going with 2% to be conservative*From Map Unit Descriptions
Designated an elevation of the head of shopping cart creek at 938 ft. (outlet of ditch at 940)
Festival Plus Catchment (2S):
Area = 4.6 ac urban districts commercial and business, 85% impervious, CN = 89
Tc :
Lag/CN Method
Hydraulic Length = 490 ft
*From DEQ Stormwater Management
Guidebook
Ground Slope = 0-2% – going with 2% to be conservative*From Map Unit Descriptions
Ditches/Culverts (5R):
Combined Ditches/Culverts from Post-Development for 5-node limit.
Length = 1033 ft
Manning’s n: 0.027999
*Weighted average of n for ditch and culvert
Inlet invert = 959.5 ft
Outlet invert = 940 ft (slope*281ft)
For discharge culvert (from pond to Reach 6):
Inlet = 938.25 ft
Outlet = 938 ft
Length = 25 ft
Diameter = 12 in
Manning’s n = 0.025, corrugated metal, typical
Entrance = CMP, square edge headwall
Assume free discharge