Using HEC-1 for Subdivision Runoff Detention Pond Design

Stacie Kato

April 26, 2004

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

Questions?