FLOOD01374

• Designation Number: 521
• County: Weld
• Community: Erie, Firestone, Dacono
• Stream Name: Tri-town drainageway, Godding Hollow Basin
• Title: South Weld I-25 Corridor Master Drainage Plan
• Date: 9/8/1999
• Prepared For: Weld County
• Prepared By: Anderson Consulting Engineers, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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IV. HYDROLOGIC ANALYSES AND MODELING

4.1 Formulation of Hydrologic Model

The primary objective of the hydrologic analyses and modeling efforts was the development
of peak discharge and hydro graph data at various locations throughout the Godding Hollow and
Iri-Town Basins, This information combined with the capacity of the existing drainage facilities
provided insight to existing and future flooding problems and promoted the development of
stormwater management plans for the basins. The hydrologic analYSeS for the Godding Hollow
and Tri-Town Basins were conducted for the 2-,5-, 10-, 25-,50-, and 100-year return periods,
Three scenarios were simulated by the model: (a) existing condition with existing facilities; (b)
future development condition with existing facilities; and (c) future development conditions with
on-site detention ahd proposed improvements.

4.1.1 Model Description

The modeling approach chosen to simulate the runoff generated within and routed through
the Godding Hollow and Tri-Town Basins involved the application of two computer models, the
Colorado Urban Hydrograph Procedure (CUHP) and the Urban Drainage Stormwater Management
Model (UDSWM2-PC), The CUHP model is a hydrologic simulation program developed in 1978
for the Urban Drainage and Flood Control District; it is used to generate storm hydrographs for
a number of basin subcatchments. The program requires input of the design storm as a 3-hour
hyetograph (5 minute increments) distribution from which a total precipitation depth is computed.
The methodology used in developing the design storm is outlined in the Urban Storm Drainage
Criteria Manual (USDCM, 1978). Storm hydro graphs were generated by the CUHP model for
the 2-, 5-, 10-, 25-, 50-, and lOO-year return periods; these hydrographs were written to an input
file to be used by the UDSWM2-PC model.
The UDSWM2-PC model is essentially the runoff block of me Environmental Protection
Agency's Stormwater Management Model (SWMM), This model simulates the quantity of runoff
from an urban drainage basin and the routing of flow through the basin. The hydrologic analyses
and modeling for the Godding Hollow and Tri-Town Basins utilized this model to develop flood
hydro graphs at various locations throughout the basin. Storm flow hydro graphs generated by the
CUHP model are entered into the UDSWM2-PC model and routed through the drainage network
to depict the actual network of detention ponds and open channels that exist within the basin,

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4,1

September 8, 1999