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I 3 I Flows based on 100-year frequency rainfall were determined for both existing and future development using the existing drainage system in order to assess the effect of future basin development. As a result of this determination, together with other information that was gathered, existing and potential problem areas were identified and evaluated. SECTION II I BASIN DESCRIPTION I I Computer modeling indicated that additional future urban development, without upstream storage would cause an increase in storm runoff, both in rate and volume. On that basis, it is evident that there would be further deteriora- tion in the natural character of the stream channel. The South Lakewood Gulch drainage basin is located in Lakewood, Colorado, in Township 4 South, Range 69 West of the 6th P.M. The basin has been divided into five subbasins for the purpose of this study, as shown on Drawing No.1. Each subbasin is discussed in detail below. I Floodplain delineation is important toward the management of a drainage basin and is essential to the planning process. For this report, flood peak water surface profiles were computed and plotted to define the flood elevations along the present channel for future development discharges. The basic pro- cedure in determining the floodplain was the HEC-2 backwater program developed by the U.S. Army Corps of Engineer. SUBBASI::I (Headwaters to Addenbrooke Pond) I I THE STUDY AREA Subbasin 1 is characterized by single family residential development up- stream of Kipl ing Street, accounting for about 80 percent of the total subbasin area. The subbasin is about 1.2 miles long, and about 0.2 miles wide. Overall imperviousness is rresented in Table 11-1 for present and future conditions. The average slope is about 4 percent. A well-defined drainage channel does not exist in this subbasin, except for a small swale just upstream of Kipling. Below Kipling, a portion of the Agri- cultural Ditch system crosses the drainageway, which is a very wide, flat depression from this point to the farm pond at the lower end of the subbasin. The culvert at Kipl ing Street is a 72 x 44-inch pipe arch corrugated metal pipe (CMP) structure. A rainfall/runoff gauging station is located just upstream of Kipling to measure runoff from the intensely developed area upstream. I I South Lakewood Gulch is the drainage outfall for a 1,040 acre basin in Lake- wood, Colorado. The basin originates at a point about one-half mile south of the Denver Federal Center, and extends approximately 4 miles in a north- easterly direction, entering Lakewood Gulch near the intersection of West Sixth Avenue and Harlan Street. The basin is long and thin in shape, with a length to width ratio of about 8. In some areas, the gulch has been entirely contained within an underground pipe. Floodplain encroachment is evident in the lower half of the basin. The basin is approximately 70 percent developed, with mostly single family residential and some commercial development. Sixth Avenue effectively cuts off approximately 120 acres of residential development which would otherwise be tributary to the lower portion of the basin. Due to the fact that South Lakewood Gulch is a rather small tributary to Lakewood Gulch, there are nO major subtributaries, other than small natural depressions and residential streets. Because of the intense urban development, under existing conditions, much of the local runoff is carried by the streets. The lower end of this subbasin (downstream of Kipling) is open farmland which is to be eventually developed into a park. The farm pond located at the intersection of the main Agricultural Ditch and the gulch is en- envisioned by the city as being used for water supply and/or flood control purposes in the future. I I 1 I I There appears to be no major problems at the present time in this sub- basin, except for the lack of a well-defined channel in the upper end. This would cause major drainage runoff flows to occur in the streets, which might result in some temporary traffic blockage during more ex- treme events. The right-angle turns of the defined channel just up- stream of Kipling would most likely result in some flooding in this area. This could be worsened in the future by building homes on the vacant area just upstream of Kipling without a properly defined and regulated channel. The farm pond at the lower end of this subbasin might create a problem further downstream by overtopping and washing out during flood- ing events. I SUBBASIN 2 (Addenbrooke Pond to Rockmont Ponds) I Approximately one-half of this subbasin is presently undeveloped with residential development interspersed with rur.;l farmland. Overall imper- viousness is presented in Table 11-1. Although the subbasin itself is about 1.3 miles long, the major drainageway is only about 0.8 miles long. The upper third of the channel is a continuation of the wide, flat 1 I.