FLOOD09485

• Designation Number: 206
• County: Larimer
• Community: Fort Collins
• Stream Name: Fossil Creek Drainage Basin
• Basin: South Platte
• Title: Master Drainageway Planning - Fossil Creek Drainage Basin, Fort Collins, Larimer County, Colorado
• Date: 8/1/1982
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

I

I

I II. HYDROLOGIC AND HYDRAULIC SUMMARY

Flooding in the Fossil Creek basin is often a result of general

rainstorms and cloudburst storms during the late spring and summer months.

Flood hydro graphs resulting from heavy general rainfall over the basin are

characterized by high peak flow of moderate volume and duration. Flood

hydrographs resulting from cloudburst type storms are characterized by high

peak flow of short duration and a small volume of runoff. Runoff from

snowmelt occurs and is characteristized by hydrographs with moderate peak of

long duration.

I

I

I

I

I

3.1 Hydrologic Analysis

A hydrologic analysis was conducted to determine runoff hydrographs from

one-hour rainfall depths for the 2-, 5-, 10-, 50-, 100-, and 500-year

recurrence interval. A rainfall depth of a given return period was assumed to

generate a flood hydrograph of the same return period. The "Storm Water

Management Model" (SWMM) as developed by the Environmental Protection Agency

and later modified by the Missouri River Division of the U.S. Army Corps of

Engineers (ref. 19) was used to estimate peak flows and runoff volumes. Flood

flows and runoff volumes were determined for existing basin conditions, deve-

loped basin conditions and developed basin conditions without ponding behind

roadway embankments. The parameters used in the analysis were established by

the Omaha District, COE (ref. 10) and are consistent with those used during

previous studies in the area (ref. 2, 4, 17).

The Fossil Creek drainage basin delineation is shown in Figure 3. The

area marked with slashes represent~ noncontributing areaj water bodies are

also marked. The tributary area upstream of Fossil Creek Reservoir is 20.9

square miles. The drainage basin was divided into 150 subcatchments and 135

channel units. The configuration for the subcatchment and channel units is

also shown in Figure 3. Definition of tributary subcatchments assumed all

irrigation ditches flowing full and not intercepting overland flow.

I
I

I

I

I

I

I

I

I

3.2 Rainfall Data

Rainfall depths for the l-hour, 5-, 10-, 50-, and 100-year recurrence

intervals were obtained from the "precipitation Frequency Atlas of the Western

United States, Atlas 2, Volume Ill, Colorado," (ref. 7). The 500-year rain-

fall depth was estimated by extrapolation of one-hour depths for the other

I
I

I

recurrence intervals on semi logarithmic paper. Resource Consultants Inc. of

Fort Collins, Colorado, provided a two-year, one-hour rainfall depth that was

obtained from a frequency analysis of 30 years of precipitation records at

Fort Collins. This is 0.13 inch higher than the value given by the two-year

rainfall intensity duration curve. The Colorado Urban Hydrograph Procedure

was used as a guide in distributing the incremental rainfall values.

The one-hour distribution is shown in Table 1. The point rainfall depths

for each recurrence interval along with an 85 percent areal reduction are

shown in Table 2. Table 3 contains the rainfall hyetographs used for the dif-

ferent recurrence interval storms for the Fossil Creek basin.

3.3 Drainage Basin Parameters

The parameters required to determine runoff peaks and volumes from SWMM

are subcatchment area, width, percent of impervious area, infiltration rate,

depression and detention storage, overland roughness coefficient and storage

discharge curves for ponded areas. The percent of impervious area was deter-

mined for existing and developed conditions. For existing conditions, the

percent of impervious area was determined by the COE (ref. 9) and checked with

U.S. Geological Survey topographic maps (ref. 20). The percent of impervious

area for developed conditions was based on land use as defined by the Zoning

Map for the Urban Growth Area in Figure 4 (ref. 6). Percent of impervious

area by land use is in Table 4.

The hydrologic analysis for the 2-, 5-, 10-, 50-, 100- and 500-year

recurrence intervals was performed for two cases in the Fossil Creek

watershed. Existing and developed conditions with all storage behind existing

roadways were two cases. A third case was for developed conditions without

existing storage behind roadways to evaluate the effectiveness of detention

storage behind roadways. This was for the 10-, 50- and 100-year floods.

Infiltration rates for pervious areas are based on soil types obtained

from the "Larimer County Soil Survey," (ref. 23). Table 5 shows the range of

infiltration rates used for each soil type. The Heldt-Renohill-Kim asso-

ciation was predominant. Depression and detention storage used was 0.3 inch

for pervious areas and 0.2 inch for impervious areas. Subcatchment roughness

characteristics were consistently 0.013 for impervious areas and 0.12 for per-

vious areas. Channel units, lengths, slopes and geometries were defined from

field surveys, 1" = 200 feet topographic maps (ref. 1, 7) or USGS 1" = 2000

8