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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Two typical cross sections were used for the Outfall System; Type A with

maintenance road, and Type A with existing road used for maintenance access as

shown on Drawing-36. The side slope for grassed channels is 4 horizontal

(minimum) to 1 vertical and for rock channels is 2 horizontal (minimum) to

vertical. The maximum turbulence factor (Froude Number) of 0.8 was used for

all open channels,

the cost estimates. In accition, where local inlets would not be capable of

intercepting the design flows, local interceptor sewers are calLed for on the

plans. However, since these interceptors are not part of the Outfall System,

the cost of the interceptors was not included.

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2, Check Drops

Check drops are required for all open channels where the natural longitud-

inal ground slope is steeper than the design slope of the channel, The proposed

Outfall System consists of sloping 3-foot drops constructed with riprap and a

concrete breast wall as shown on Drawing-36. All riprap is class "H" as

described in the USDCM section "Major Drainage - Riprap" (Reference-2) and

includes a sand and gravel filter blanket. Other check drops configurations

in accordance with the requirements of References-I and -2 may be used.

4. Culverts/Bridges

Street culverts were sized for the IO-year flood peaks as a minimum design

flood and include headwalls/wingwalls at the inlet and outlet. Where the Out-

fall System consists of a storm sewer beginning at or crossings a street or

railroad, the culvert design frequency was the same as the storm sewer design

frequency. The culverts were sized for the headwater condition where the road

would first be overtopped, In situations where the headwater would flood local

properties, the culverts were sized using a lower headwater depth.

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The storm sewer portion of the Outfall System was designed for the 2-year

recurrence interval flood peaks, except for the reach in Drainageway A-] along

South Boulder Road. The sewer in this reach was designed for the 10-year flood

peaks due to the minimal increase in sewer size from the 2-year flood peaks.

Included as part of the Outfall System planning was the preliminary design

of the C&S.R.R. crossing of Coal Creek, The bridge was sized based upon the

flows presented in Reference-3, The size of the bridge was based upon a head-

water that would minimize the flooding to the dwellings upstream of the

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3. Storm Sewers

crosslng.

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5. Detention Ponds

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The basis for the selection of the 2-year flood peaks for storm sewer

design was the allowable street capacity (Reference-I) compared to the 100-year

flood peak. For each of the reaches where a storm sewer was considered a viable

alternative, the allowable street capacity was estiamted. By providing a storm

sewer with a 2-year capacity, the excess flood peaks during the 100-year flood

was found to be carried by the street within the allowable capacity limitations,

This design capacity however can only be achieved if the onsite detention policy

for all new development (Reference-I) 1S regidly inforced,

Whereas local onsile detention for all new development is part of the Out-
fall System, regional larger volume detention is not part of the system. However
the regional detention facilities that existed at the time of this study were
evaluated and included 1n the Outfall System were applicable. These facilities
include the detention for Hillsborough West, Mesa Point, Parkwood, Colorado
Technical Center, and Centennial Valley, Each of these facilities were evaluated
as to hydrologic impact and conformance with the hydraulic design criteria,
Where required, the Outfall System plan recommends specific improvements to
the facilities as shown on the plan drawings.

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The storm sewer system was designed using an n-value of 0.012. Manholes

were spaced a maximum distance of SOD-feet. Local inlets were included in

sufficient quantities to intercept the design sewer flow. The inlets are shown

on the plan drawings (Drawings-8 to -24) within the existing development areas.

Inlets in undeveloped areas are not shown on the drawings, but are included in

6. Irrigation Ditch Crossings
The proposed Outfall System crosses several irrigation ditches throughout
the study area. Where practical, the storm flows and irrigation flows are
spearated by a 100-year ditch crossing, Where the site constraints are prohibi-
tive, the ditch and flood flows are combined for the rare floods (i.e., 10-year

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