FLOOD07908

• Designation Number: 260
• County: Jefferson
• Community: Unincorporated Jefferson County
• Basin: South Platte
• Title: Major Drainageway Planning - Sanderson Gulch/Weir Gulch Volume I
• Date: 8/1/1972
• Designation Date: 12/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTIO}l III

HYDROLOGY

GENERAL

As required by the contract, hydrologic studies were completed
for both the Sanderson and Weir Basins. Through the hydrologic
studies the magnitude of the peak rate of storm runoff and volume
of storm runoff were determined for the six required storm
frequencies. The frequencies analyzed are the 2, 5, la, 25,
50 and 100 year return periods. The 10 year and 100 year
frequencies were selected for a basis of final design. The
determination of the peak flow rates and volumes was done for
two different degrees of basin development. Land use under
existing conditions was analyzed to provide an indication of
the adequacy or inadequacy, as the case might be, of the existing
system. An estimate of future land use was then projected, with
the aid of local planning officials, so as to establish the
requirements of the total drainage system for the ultimate
development in the basins. With the requirement of analyzing
six storm frequencies for two different conditions of basin
development, the need was evident for a computer hydrograph
model. Therefore, a program was developed by Frasier &
Gingery which is designed to generate the storm hydrograph
for each sub-basin with a minimum of input. The program,
referred to as HYDRO, was built around the Colorado Urban
Hydrograph Procedure as outlined in the Urban Storm Drainage
Criteria Manual. A discussion of the program input and opera-
t~on ~s given in the HYDRO Section.

Upon the completion of the development of HYDRO, the model was
tested in a number of areas where runoff had been recorded or
estimated through earlier studies. One particular instance
where the model satisfactorily duplicated the recorded run-
off was the June e, 1969 storm on Harvard Gulch. With minor
manipulation of the rainfall storage and loss parameters, the
recorded hydrogr~ph at Logan Street was duplicated well within
~ydrological accuracies. Other basin analyses tested also
showed very good comparison.

With the unusually high number of reservoirs within the Sanderson
and Weir GUlch Basins, it was evident that hydrographs would
have ~o be routed through the reservoirs to reflect the actual
overflow conditions. A program was developed which followed
the simple reservoir routing procedure outlined in most hydrology
texts. The location of the reservoirs made it difficult to
totalize the tributary basin into a larger single basin at each
design point, so a computerized methOd of hydrograph lagging and
adding was also developed. Both the routing and lagging and
adding programs were tied into the HYDRO Program.

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All computer work wag compieted on the company !HM 1130 Computer.
The computer has an BK storage capacity which can be increased
to 12K with the use of disc storage units. This added capacity
made operation of the total model quite simple. The program
offers an optional amount of computer output. All hydrographs
were available in storage at all times and could be recalled for
any operation.

All storm hydrographs were computed on a two minute time increment.
This had a number of advantages. First of all, it provided for
a more accurate determination of the peak flow rate and time to
peak, as well as the computation of the total hydrograph volume.
Also, it provided for greater accuracy in the routing program
where the determination of required storage is quite important.
Thirdly, the two minute increment provided the ability to lag
hydrographs to the nearest two minutes of flow time. In urban
drainage, where normal storm hydrographs characteristically
have steep narrow peaks, the flow time increment becomes very
important. For most conditions, the flow time varied from 0
to 16 minutes.

Lagging and adding the total hydrograph in the downstream direc-
tion provides a fairly realistic representation of the actual
flow condition. Ideally, the hydrograph should be attenuated
by recognizing channel storage. In the case of Sanderson and
Weir GUlches, it was felt that the existing reservoirs provide
storage detention to a much larger degree than channel storage
and the effect of channel storage would be negligible in
comparison. The lagging effect, however, keeps the time frame
of the hydrograph in the proper perspective.

Overall, utilization of the computer model as an analytical
tool permitted the calculation of nearly 1,300 hydrographs,
as required by the contract, in a comparatively short period
of time. Also, the computer model offered a much greater
degree of flexibility.

HYDRO DESCRIPTION

As mentioned earlier, the HYDRO model directly follows the
Colorado Urban Hydrograph (CUHP) Procedure as outlined in Part
4 of the Runoff Section of the Urban Storm Drainage Criteria
Manual. A quick review of the CUHP ~nd~cates that th~s pro-
cedure is based On the derivation of a ~ynth~tic unit hydro-
graph and statistically determined rainfall information.
The unit hydrograph approach to storm runoff detc~ination,
when historical recorded information is not available, was
originally developed by Sherman in 1932 and expanded upon by
Snyder in 1938. The unit hydrograph reflects the integrated
effects of basin characteristics such as area, shape, slope

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