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

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or greater), but are separated for the frequent floods, A control structure

discharges the combined flows back into the drainageway at the downstream side

of the ditch,

Phase I, Drainagew~s A, A-I, A-2, B, and C

The basins tributary to these drainageways are substantially developed. The

flooding problems vary from local nuisance flooding along South Boulder Road ("A-I")

to frequent flooding at Elm Street and Front Street ("B"). Drainageway A downstream

of U, S. Highway 42 lacks adequat:e capacity for even the frequent floods with the

upper basin area substantially developed.

C, Outfall System Description

The recommended Outfall System consists of grassed and rock lined channels

with check drops, storm sewers, culverts/bridges, improvements to existing deten-

tion facilities, and ansite detention. Onsite detention is required for all new

developments to reduce the developed flood peaks to the historic flood peaks for

the 10- and the IOO-year floods. The onsite detention is crucial to the Outfall

System since the facilities are sized based upon the existing (1982) development.

The onsite detention must reduce the future development flood peaks to the existing

development levels. The onsite detention also extends the flood protection provided

by the 2-year storm sewer, which was sized based upon fully developed flood peaks

without the benefits of onsite detention,

Phase II, Drainageways C-I, D, D-I, G, H-I, and I

The basins tributary to these drainageways are under development or are platted,

Enforcement of the drainage policies and adh~:rence to the Outfall System plan for

these drainageways will result in adequate drainage facilities for the new develop-

ment. Presently, these drainageways have only minor problems, but the development

underway will aggravate the problems if the policies and thc Outfall System plan are

not enforced. In the interium, the City should define the nuisance areas as they

Occur and provide temporary or permanent solutions.

Phase III, Drainageways E, F, G-I, H, and J

The basins tributary to these bilsins are essentially undeveloped and without

substantial major drainage problems. Adherence to the policies and the Outfall

System plan will result in adequate drainage facilities for future development,

The 2-year recurrence interval storm was selected as the initial design storm

frequency for all land use categories to achieve uniformity in the analysis (Ref-

erence-I), In addition, the effect of the proposed onsite detention is to extend

the protection of the initial storm drainage system by reducing the flood peaks

for lesser frequency floods. Therefore, the 2-year design frequency was considered

to provide adequate protection from regular recurring storm events.

E. Operations and Maintenance

One of the major considerations given to the design of the improvements was

the operation and maintenance aspect of the improvements. Several factors were

considered during the selection and design of the selected improvements, such as

erosion control, debris removal, structural repair, channel shoaling, grass reseedingl

bank stabilization, and drainageway access.

D. Construction Sequence

To determine the priority of the construction improvements, each drainageway

was reviewed for status of development, severity of existing and potential flooding

problems, capacity of existing facilities to accommodate frequent and rare event

floods, and local requirements. Since the improvements for a particular drainageway

are essentially independent, the priorities are also independent, The suggested

construction phases are presented belo~.

The operations and maintenance a.spects of the major elements of the drainage-
way improvements are discussed below.

I. Open Channels

The grass-lined channel improvement consists of extensive modification

to the drainageway with check drops and grass seeding to reduce flow velocities,

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