Final Baseline Hydrology Report - October 2012 - Urban Drainage ...

Sanderson Gulch Major Drainageway Plan
Baseline Hydrology Report October 2012
2.0 STUDY AREA DESCRIPTION
2.1 Project Area
The project study area consists of the Sanderson Gulch watershed including the tributary North Sanderson
Gulch. A map of the watershed is provided in the interactive Hydrology Map, Figure B-1 of Appendix B.
Sanderson Gulch is a west bank tributary to the South Platte River in Denver. The confluence with the
South Platte River is immediately downstream of Florida Avenue. The confluence and lower portion of the
Sanderson Gulch watershed are located within the City and County of Denver. The watershed extends
west from the South Platte River approximately 7 miles to S. Union Boulevard in the City of Lakewood and
Jefferson County. The jurisdictions within the Sanderson Gulch watershed are shown on Figure 2-1. The
northern boundary of the watershed varies from Exposition Avenue on the east to Florida Avenue on the
west. The southern boundary of the watershed varies from approximately Harvard Avenue on the east to
Morrison Road on the west.
The Sanderson Gulch watershed has a drainage area of 7.6 square miles. NRCS hydrologic soil types
within the basin are predominantly Type C. A soils map is provided in the interactive Hydrology Map, Figure
B-1 of Appendix B. The watershed is essentially fully developed. The 152-acre Green Gables Country
Club located near the southeast corner of the intersection of Wadsworth and Jewell in Jefferson County is
the only significant undeveloped parcel and it is currently under redevelopment as mixed-use commercial
and residential. Since development is approved and construction is imminent, this study assumes
development of this site under the future conditions model. Watershed elevations range from 5230 feet at
the confluence with the South Platte River to 5805 feet at the western limit of the watershed near S. Union
Boulevard.
Figure 2-2 identifies the parcels within the Sanderson Gulch that are city-owned. Within the City and County
of Denver, Sanderson Gulch was one of the first drainageways to preserve the floodplain as open space
and the gulch is almost entirely an open channel system contained within linear park and open space areas.
Sanderson Gulch is not designated a wildlife corridor by Denver Parks and Recreation, but there are
portions of the gulch that are designated as “Park” which means there are some use restrictions for those
designated areas. The middle and upper reaches of Sanderson Gulch and the entirety of its tributary, North
Sanderson Gulch, are located within Jefferson County and the City of Lakewood. Upper Sanderson Gulch
and North Sanderson Gulch are also generally open channel systems, but the number of adjacent parks or
open space areas is much more limited. Significant portions of the upper gulches are contained in narrow
constructed drainageways. Figure 2-3 depicts the parks and open spaces within the Sanderson Gulch
watershed.
Map, Figure B-1 of Appendix B. Residential land uses make up a majority of the watershed. Limited
commercial and industrial development exists along major arterial streets primarily Federal Boulevard,
Sheridan Boulevard, Wadsworth Boulevard and the western part of Jewell Avenue. Land use within the
watershed is summarized in Table 2-1.
Table 2-1 Watershed Land Use Summary
Land Use
Percentage of Watershed
Residential 75.7%
Commercial/Industrial 11.4%
Parks/Open Space/Ponds 12.9%
Total 100.0%
Watershed Area-Weighted Imperviousness 49.6%
2.3 Reach Descriptions
For the purposes of the evaluation of alternative improvements, Sanderson Gulch has been divided into 12
reaches and North Sanderson Gulch into 4 reaches. Descriptions of the reaches are provided in the
sections below and reach limits are illustrated in Figure 2-5. Major crossings are shown on Figure 2-6 and
listed in Table 2-2. An inventory of existing drop structures is shown in Table 2-3. There are currently 35
drop structures on Sanderson Gulch and 16 on North Sanderson Gulch as shown on Figure 2-7. In
addition to these existing structures, the previous report Sanderson Gulch Drop Structure Inventory and
Bank Stabilization (Icon Engineering, 2002) recommended three new drop structures within the City and
County of Denver.
Sanderson Gulch
Reach 1 – South Platte River to Lipan Culvert Entrance (Denver)
There are several irrigation ditches and water storage reservoirs within the Sanderson Gulch watershed.
Figure 2-4 shows the ditch locations for the Agricultural Ditch, Welch Ditch, and the Salisbury Lateral Ditch.
Smith Reservoir, Ward Reservoir No. 1, Ward Reservoir No. 2 and Ward Reservoir No. 5 within the
Sanderson Gulch watershed are privately owned and used for irrigation water storage. The operation of
these ditches is described in Section 2-4.
2.2 Land Use
Land use and zoning data was provided by the City and County of Denver and the City of Lakewood. A
summary of how impervious area values were determined for hydrologic modeling is included in Section
3.3.2 Watershed Imperviousness. Land use in the watershed is illustrated on the interactive Hydrology
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