FLOOD01357

• Designation Number: 107
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Green Mountain Area
• Title: FHAD - Green Mountain Area
• Date: 4/1/1978
• Designation Date: 7/11/1978
• Floodplain - Doc Type: Floodplain Report/Masterplan

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CHAPTER I II
HYDROLOGIC ANALYSIS

Therefore, the total rainfall tabulated for both areas was reviewed,
When the rainfall data from Reference 12 was compared to the isohyets in
the Criteria Manual (Reference 5), the data for T4S, R69W was in close

peak flows and volumes is to subdivide the basin into smaller parcels and
determine the hydrological characteristics. The runoff can then be computed
and the information used to access the existing drainage facilities and poten-
t i a 1 flood damages.

agreement and, therefore was used for this analysis.
The design rainfall intensity patterns were determined based upon
the methodology presented in the Criteria Manual. The total rainfall was
divided into five minute incremental values. The recorded rainfall data
for the Denver metropolitan area, indicate the peak incremental values for the
higher intensity storms (i.e., 25, 50, and 100 year frequency) tend to occur
somewhat later than the peak incremental values for the lower intensity storms.
Therefore, the peak increment for the 10 year frequency storm was placed
between 25 and 30 minutes, whereas the peak increment for the 100 year
frequency storm was placed between 30 and 35 minutes. The resulting design
rainfall data is presented in Table 4.

A. Genera 1

The purpose of the hydrologic analysis in a Flood Hazard Area Delineation
is to define the storm runoff peak flows and volumes for the hydrologic points
of interest along the drainageways. The first step in determining the runoff

B. Sub-basin Characteristics

The overall study was divided into 104 sub-basins to adequately define

the runoff peak flows and volumes using the current U,D. & F.C.D. procedures.

C. Design Rainfall
The total rainfall values for the 10 and 100 year return period
storms were obtained from the Rainfall/Runoff Information report (Reference 12)
The study area lies in two townships, T4S, R69W and T4S, R70W.

D. Discharge Computations
1. Storm Runoff and Hydrographs
The peak runoff rate and runoff volumes are dependent upon various factors,
which include total precipitation, incremental precipitation pattern, infiltra-
tion rates, antecedent soil moisture conditions, and basin characteristics.
The infiltration rates of the soil below 6200 feet vary from 0,2 to 0.6 inches
per hour, depending upon soil moisture condition and soil cover complexes.
These are soils of the Denver/Kutch and Englewood series which are c1ay-10ams
(Reference 14). The soils on the steep, upper slopes of Green Mountain (above
elevation 6200 feet) are the Neder1and series, which is cobb1y, coarse sandy

The hydrological parameters were determined for each of the sub-basins

based on the future land use conditions (The determination of future
development conditions is defined in Chapter II). The basin peak discharge
and concentration time parameters (Cp & Ct), required for the Colorado
Urban Hydrograph Procedure (CUHP), were determined in accordance with the
latest U.D, & F.C,D. guidelines. These parameters and other hydrologic
characteristics are tabu1arized in Table 3.

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