FLOOD05231

• Designation Number: 518
• County: Douglas
• Community: Unincorporated Douglas County
• Stream Name: Oak Gulch and Lemon Gulch
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Oak Gulch and Lemon Gulch
• Date: 2/1/2000
• Designation Date: 5/1/2000
• Floodplain - Doc Type: Floodplain Report/Masterplan

Section 1.0
Introduction

The Lemon Gulch watershed is approximately 6 miles long and 7.5 square miles in area.
Watershed topography shifts from relatively steep, rugged terrain in the middle portions of
the watershed to moderate rolling terrain along the valley of Lemon Gulch,

1.1 Authorization

2.2 Soils

This report documents the analysis for the Oak Gulch and Lemon Gulch Flood Hazard Area
Delineation (FHAD) study. This study was authorized by the Urban Drainage and Flood
Control District (UDFCD) under UDFCD Agreement No. 97,12.03 dated December 31,
1997.

Hydrologic soil types within the watershed study area are shown in Appendix B.
Hydrologic soil types were identified based on soil surveys produced by the U.S. Soil
Conservation Service for the Castle Rock Area (SCS, 1974).

1.2 Purpose

Hydrologic soil types B, C, and D are predominant in the watershed. Type B soils have
moderate infiltration rates and type C and D soils have lower infiltration potential. The main
channel beds of Lemon Gulch and Oak Gulch consist primarily of type A soils, which have
high infiltration characteristics.

The purpose of the analysis was to establish peak discharges in the study area for the 10"
50" 100" and 500,year storm events and to delineate 100,year floodplain and floodway
limits for future watershed development conditions,

2.3 Surface Water Resources

Section 2.0
Study Area Description

2.3.1 Stream Characteristics

2.1 Topography and Physiography

All three of the tributaries in the study area are ephemeral, flowing intermittently in response
to significant rainfall or snowmelt events or high groundwater inflow.

The study area consists of three Cherry Creek tributaries: Oak Gulch, an unnamed tributary,
and Lemon Gulch. All three tributaries are left bank tributaries that drain from the west into
Cherry Creek The tributaries are located in Douglas County, Colorado, between the towns
of Castle Rock and Parker. Portions ofthe watershed are located within the two town limits.

The Oak Gulch channel is incised with a sandy bottom, 10 to 30 feet wide, and steep side
slopes. The depth of the incised channel varies from 4 to 10 feet. The channel exhibits a
meandering channel form and the banks and bottom are vegetated with short grasses.
Reaches in the upper portion of the watershed are vegetated with occasional shrubs. The Oak
Gulch tributaries are generally vegetated only with short grasses.

The watershed for the three tributaries is shown in Appendix A. The total watershed area is
approximately 10.4 square miles. The upstream watershed limit is located approximately two
miles north of Castle Rock The downstream limit is located a mile within the Town of
Parker corporate limits.

The Unnamed Tributary does not have a pronounced channel. The channel side slopes are
generally mild and vegetated with short grasses.

The watershed is located in the far southeastern portion of the UDFCD. The study area is
under the jurisdiction of Douglas County and the Town of Parker.

The Lemon Gulch channel is wide, deeply incised, and meandering. The bottom is sandy
and is vegetated with scattered grass, trees, and shrubs. The bottom width varies from 65 to
l"9&feetandthe--cIepth varies from 10 to 15 feet. The banks are generally steep and eroded.
Occasional shrubs and trees vegetate the banks. The tributaries of Lemon Gulch are steep
and are moderately vegetated with short grasses and shrubs.

The highest point in the watershed is 6620 feet above mean sea level (MSL). The lowest
point in the study area, at the valley of Cherry Creek, is at elevation 5870 MSL.

The Oak Gulch watershed is approximately 35 miles long and 1.8 square miles in area.
Watershed topography consists of moderate rolling terrain.

Oak Gulch has an average channel gradient of 1.8 percent and the Unnamed Tributary has an
average channel gradient of 2.0 percent. Lemon Gulch has an average gradient of 1.6
percent. The tributaries to these main channels have generally steeper gradients, ranging as
high as 4.4 percent.

The Unnamed Tributary watershed is approximately 2 miles long and 1.0 square miles in
area, Watershed topography consists of moderate rolling terrain,

I

Oak Gulch and Lemon Gulch FHAD.DOC