FLOOD01619

• Designation Number: 320
• County: Jefferson
• Community: Arvada
• Stream Name: Lower Ralston, Van Bibber, Leyden Creeks
• Title: Major Drainageway Planning
• Date: 2/1/1986
• Prepared For: Arvada
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION III
HYDROLOGICAL SUMMARY

II I-I

GENERAL
The summary of the flood hydrology for the Wright Water Engineers 1984
master drainageway planning of Lower Ralston Creek and Van Bibber Creek is
presented in this section.

is reliable when input is reliable. The program has gone through many up-
dates which have improved its rel iabil ity. The version used in the 1984
study is 9B. The version used in 1976 was 6B, Version 6B was no longer
available in 1983 and 1984. This was a handicap in that extensive work was
required to reestablish the 1976 basin model so as to attain internal re-
sult consistency.

This 1984 hydrologic study has had the advantage of past studies for com-
parison and checking. These past studies include:
U. S. Army Corps of Engi neers 1970
Frasier and Gingery 1974
Wright-McLaughlin Engineers 1976
Gingery and Associates 1983

RAINFALL
Patterns and amounts of rainfall were determined for the 2-, 5-, 10-, and
100-year frequencies, Three location point rainfall sets were developed
based on geographi c area. These were for:

The unit 100-year and 2-year frequency flood flows for Van Bi bber Creek
were plotted against drainage basin area for seven conditions to check for
internal consistency. Consistency was found to be satisfactory.

1, The eastern portion of the study area,
2, The central portion of the study area,
3. The western portion of the study area.

Areal corrections were made as deemed necessary for each basin or subbasin.

The methodology used for the flood hydrology studies is in accordance with
commonl y accepted procedures used by flood control engi neers in Colorado
for similar purposes. The methodology includes:

Under future development the peak floods at the lower end of Ralston Creek
woul d be a resul t of intense prec i pitat ion over the eastern 30 square mil es
of Ralston Creek, rather than a less intense storm over the entire basin.

1. Determination of design rainfall for selected frequencies.
2. Determination of basin runoff characteristics.
3. Construction of a rainfall-runoff model for the basins.
4. Determination of runoff at selected locations for frequencies de-
sired.

The eastern point rainfall was used for Leyden Creek and small tributaries
to Van Bibber Creek. Eastern and central area point rainfalls, adjusted
for 17 square mil e areal corrections, were used for Van Bi bber Creek at its
mouth.

The hydrologic program utilized was the Massachusetts Institute of Technol-
ogy Catchment program (MITCAT), the same as for the 1976 Wright-McLaughlin
study. This is a propriety program which has been carefully prepared and

Rainfall data is presented in Tables III-l and III-2 in this report and is
the same as that used in the 1976 study. As an example, the 3-hour,
100-year frequency, east area uncorrected rainfall totals 3.0 inches. When
corrected for 30 square miles the 3-hour rainfall is reduced to 2.65
inches.