FLOOD01635

• Designation Number: 558
• County: Douglas
• Community: Douglas County Unincorporated
• Title: Flood Hazard Area Delineation - Plum Creek Watershed
• Date: 11/1/2001
• Designation Date: 1/2/2002
• Prepared By: UDFCD, WRC Engineering
• Floodplain - Doc Type: Floodplain Report/Masterplan

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III. HYDROLOGIC ANALYSIS

A. GENERAL

The hydrologic analysis for the study area was performed to define the storm runoff peak flows and volumes for
the various sub-watersheds of the Plum Creek Watershed. The runoff information is subsequently used to evaluate the
existing drainage facilities, potential drainage problems, alternative drainage improvements, and in the development of
the Flood Hazard Area Delineation (FHAD) and Preliminary Design (Phase -B) studies and reports.

Due to the large size of the study area and the limited number of routing elements that SWMM accepts, the study

area was divided into three sections; the East, Southeast, and West The East Section was further divided into 175 sub-

watersheds; the West Section was divided into 151 sub-watersheds; and the Southeast Section was divided into 121 sub-

watersheds. The maximum size of the sub-watersheds within the study area was about 130 acres, and the average sub-

watershed area was 71 acres. Peak flows at each sub-watershed outlet point were calculated for the 10-, 50-, 100-, and

500-year recurrence interval using the future and existing land use conditions. The study area map is shown in Figure

I-I and the sub-watershed maps are shown in Figures III-I through 111-6.

The off-site area was divided into 41 sub-watersheds. The maximum sub.watershed size for the National Forest
Service area (areas that are less likely to be developed) was about 10 square miles, and the maximum sub-watershed size
for rangeland areas was about 5 square miles. Figure 11I-7 is the off-site boundary map showing the offsite sub-watershed
boundaries.

B. DESIGN RAINFALL

The design storm distributions used in the study were as recommended by the Urban Storm Drainage Criteria
Manual (Reference-13) and the Douglas County Storm Drainage Design and Technical Criteria (Referenee-3). The
rainfall distribution that is used depends on the size of the drainage area being studied and the location of the sub-
watershed within Douglas County. As the size of the watershed increases, the depth of rainfall decreases for a given
storm return period. An area-reduction factor must be applied to the point rainfall estimates provided by the NOAA
Atlas (Reference-4) to account for this depth reduction.

Most of Douglas County lies in the plains west of the mountains, but the western portion of Douglas County lies

in the foothills of the Rocky Mountains. The rainfall depth for a given storm return period is significantly different in the

foothills than it is on the plains. To account for this difference, Douglas County was divided into 3 rainfall zones (Zone

I, Zone II, and Zone III). Zone I represents the average distribution of rainfall that will fall on the eastern portion (the
plains) of Douglas County. Zone III represents the average distribution of rainfall that will occur in the western foothills.
Zone II represents the average rainfall distribution in the transition between the foothills and the plains.

The study area is entirely within Zone I, but the off-site area is in both Zone I and Zone II. The precipitation zone
boundaries for the off-site area are shown in Figure III-7. A sub-watershed is classified as being in Zone I if more than
50% of the sub-watershed is within Zone I and as being in Zone II ifmore than 50% of the sub-watershed is within Zone
II. Table III-I through Table I1I-6 provides the 10-year, 50-year, and I OO-year rainfall distributions at five-minute time
increments. The 2-hour, Zone I, unadjusted storm event is provided in Table I1I-L The 3-hour, Zone I, storm event for
drainage areas between 10 square miles and 20 square miles is provided in Table IIl-2. The 6-hour, Zone I, storm event
for drainage areas between 50 square miles and 75 square miles is provided in Table 111-3. The 2-hour, Zone II, unadjusted
storm event is provided in Table IlIA. The 3-hour, Zone 1I, storm event for drainage areas between 10 square miles and
20 square miles is provided in Table 111-5. The 6-hour, Zone II, storm event for drainage areas between 50 square miles
and 75 square miles is provided in Table I1I-6. Because of the watershed areas being analyzed, storm events in the range
between 5 and 10 square miles and between 20 and 50 square miles were not needed.

These rainfall distributions were used in the Colorado Urban Hydrograph Procedure (CUHP) model to generate
hydrographs from each sub-watershed.

C. COLORADO URBAN HYDROGRAPH PROCEDURE PARAMETERS

The CU liP computer program was used to compute storm runoff hydrographs for the various sub-watersheds.
The version used for the study was CUHP98. The sub-watershed input parameters required for CUHP are: sub-watershed
identification number, drainage area, length and centroidallength, drainageway slope, impervious percentage, detention
and retention storage losses, and infiltration loss rates. A summary of the CUHP input parameters used for the analysis
are presented in Tables I1I-7 through I1I-10. Table 111-7 is the CUHP input parameters for the west section of the study
area. Table I1I-8 is the CUHP input parameters for the southeast section of the study area. Table I1I-9 is the CUHP input
parameters for the east section of the study area. Table III-I 0 is the CUHP input parameters for the off-site watersheds.

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