using hec-1 for subdivision runoff detention pond design stacie kato april 26, 2004
TRANSCRIPT
Project Background A detention pond is needed for a developing
subdivision in the town of Cicero located in the Jackson Township in Indiana
The site is currently a farm field bordered by a fire station, wastewater treatment plant, park, church and residential lands.
There is an existing storm sewer that is located north of the site that the detention ponds will be routed to
The subdivision will have an average lot size of ¼ acres
Project Goal To design two detention ponds on the west side
of the site (one in north portion, the other in the south portion)
To meet the maximum release rate into the existing storm sewer set by the Town of Cicero
To design a storm sewer system to route runoff to the detention ponds
To design an emergency spillway channel that will be routed to Wareham’s Pond (northwest of site)
Model Description HEC-1 generates hydrographs from rainfall
data and routes them to reaches and reservoirs
Uses the continuity equation (inflow – outflow = change of storage rate), inflow information from design storm hydrograph ordinates, and storage-outflow relationship to calculate flood flow routing
Methods Determined watershed area from a survey
map located from the Hamilton county website
Determined basin areas from the slope of site Determined curve number for a residential
area with an average lot size of ¼ acres Calculated the time of concentration for each
sub basin using a figure from SCS National Engineering Handbook
Derived a rainfall intensity equation for the project area and calculated intensities for each sub basin and for the total watershed
Calculated the peak flow for each sub basin to determine the pipe diameter for the storm sewer
Varied detention pond area and outlet pipe diameter to obtain the best design to meet the maximum release rate into the storm sewer
Designed an emergency spillway using the maximum probable storm
Results South Detention pond size 0.5
acres and north detention pond size 0.75 acres, both having a 9” diameter outlet pipe
SUB1
SUB2
COM1
SUB3
SPond
NWSUB
COM2NPond
NESUB
South storm sewer system pipe diameter 36” to 42” to 48”, North storm sewer system pipe diameter 36” to 42”
36” pipe42” pipe
42” pipe36” pipe
48” pipe
Sub basin 1 & 2 runoff
Sub basin 1, 2 & 3 runoff
North East and West Sub basin runoff
North East Sub basin runoff
Sub basin 1 runoff
To pond
To pond
Emergency spillway from south detention pond to north detention pond at least 6.4 feet deep, from north detention pond to Wareham’s Pond at least 10.1 feet deep
North Detention pond spillway rectangle channel
South Detention pond spillway rectangle channel
10 ft 10 ft10.1 ft 6.4 ft
Sensitivity Analysis Most parameters were highly sensitive.
Lag time, pond depth, and spillway characteristics were not sensitive.
Parameters were not as sensitive for maximum stage output as they were for the peak flow
Parameter Sensitivity for Peak Flow
Sensitivity for Max Stage
Curve Number 1.146556 0.0180791Lag T ime -0.03583 -0.000583Basin Area 0.60823 0.0081895Total Storm 0.831915 0.0101903Pond Area -0.44546 -0.007437Pond depth 0 0Outlet elevation -103.544 0.3301666Outlet total area 0.86951 -0.003165Outlet discharge coefficient 0.858846 -0.00237Outlet exponent 0.763155 -0.001446Spillway elevation 1.33403 0Spillway length 0 0Spillway discharge coefficient 0 0Spillway exponent 0 0Initial pond elevation 19.0373 0.3005341
Recommendation for HEC-1 Use HEC-1 is easy to use Model parameters are sensitive to peak
flow output Good model to use for designing detention
ponds
Conclusion Two detention ponds were designed using
HEC-1 A storm sewer system was designed An emergency spillway was designed
using HEC-1 and FlowMaster