FLOOD06025

• County: Boulder
• Community: Louisville
• Stream Name: Coal Creek, Rock Creek
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 11/1/1982
• Prepared For: Boulder County
• Prepared By: WRC Engineering, Inc.
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I

I
I

E. Results

D. Flood Routing

To evaluate the existing detention ponds in the study area, a comparison

was made between the runoff volumes from the CURP computer runs, the existing pond

volumes, and the volume required by the criteria (Reference-I). Where the ponds had

sufficient volume to detain the various design storms, the historic condition hydro-

graphs from the area were used as the resultant discharge. For example, where a

detention pond volume was equal to or greater than the volume required for the 10-

year runoff, the outflow hydrograph from the pond was taken as the 10-year historic

runoff. Where the volume provided was less than the volume required by the criteria,

but more than 50% of the runoff volume, then the pond was assumed to be effective in

reducing the peak flows to existing condition levels. Each pond was reviewed sep-

arately and modifications to the basic procedure were made as required.

The results of the hydrologic analysis are presented as flood peaks (Drawing-

S and -6) and hydrographs (Drawing-?) for the sub-basins and design points shown

on Drawing-2. The flood peaks for the 10- and 100-year storms take into account the

effect of sonsite detention for future development by assuming the peaks are equal

to the existing development flood peaks, The 2-year storm peak flows assume the

onsite detention will not effect the peaks and represent runoff peaks for fully

developed basins. The complete calculations for the hydrological analysis are

presented in Reference-S.

I
I
I
I
I
I
I

The final step in the hydrologic analysis was to combine and route the sub-

basin flood hydrographs to obtain flood hydrographs at the selected design points,

The hydrographs were routed by determining the time required for the peak flow to

travel downstream to the next selected design point. The maximum travel time velo-

city utilized was 6-feet per second. The hydrograph was then lagged by this amount

and the ordinates of the contributing hydrographs summed.

I
I
I
I
I

I
I
I

The City of Louisville policy (Reference-I) is to require onsite detention for

the 10- and IOO-year floods for all new development. The effect of the detention

is that the runoff peaks from future development are assumed to be equal to the

existing development runoff peaks for the 10- and 100-year floods. The 2-year flood

peaks are assumed to be unaffected by the detention and will increase with future

development, The routing of the hydrographs takes into account the onsite deten-

tion by using lIexisting" conditions hydrographs for the "futurell conditions, where

applicable. The increase in runoff volume for the future conditions was not

accounted for since the outfall system design is dependent upon flood peaks only.

These assumptions were considered valid for planning and outfall design purposes,

Also, the routed flood peaks takes into account the proposed outfall system align-

ment, which combines or diverts existing drainageways in some instances.

-12-

-11-