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BASIN DESCRIPTION II. I I I structure at 20th Avenue was recently constructed. The new structure, a 5-cell reinforced concrete box culvert, provides enough capacity for the 100-year flow without overtopping 20th A venue. In addition to the box culvert, a floodwall along the east side of the charmel upstream of 20th Avenue was also constructed. These facilities should eliminate much of the property damage which has historically occurred in this reach Basin Characteristics The Upper Lena Gulch basin above Maple Grove Reservoir consists of approximately 6,230 acres (9.73 square miles) with a wide range of topographic features. At the upper end of the basin, Apex Gulch is primarily undeveloped land, Steep mountainous slopes make up Apex Gulch and much of the area is covered by coniferous timber. I I I Several improvements have been made in this reach which have served to stabilize the channel and slightly increase conveyance capacity. The charmel crossings at Alkire Street, Alkire Court, and Arbutus Street have been removed and the charmel section enlarged. Several portions of this reach have rcceived maintenance level improvements such as minor channel excavation and riprap placement to protect against erosion. In some portions of the reach, timber retaining walls have been installed to protect against bank sloughing without the need for acquiring additional right-of- way. TIle improvements in this reach have primarily stabilized the channel. Capacity ofthe channel is not adequate for more than about the 2-year flow. Frequent flooding problems sti remain in this reach. West. Denver Y oungfield Street to Lower L-2 Above its confluence with Apex Gulch, the mainstem of Lena Gulch, also. referred to as Jackson Gulch, flows through upland areas characterized by mostly treeless open space. This upper portion of the Lena Gulch basin has also been developed in some places, Specifically, a rock quarry owned by Mobile Premix, mc, and the Heritage Square Amusement Park have increased the amount of impervious area in the upper basin I Slopes in the upper basin exceed 400 feet per mile compared with slopes in the lower portion of the basin which average approximately 80 feet per mile, At its widest point the Upper Lena Gulch basin is approximately 3.6 miles wide with an overall length of approximately 7 miles. I The developed areas in the lower portion of the basin are primarily residential and conunercial. Some areas of light-industrial land use are also present An average of approximately 32 percent of the lower basin is impervious areas. I West Improvements in this reach since the previous Master Plan are many. The entire course and character of the drainageway has been changed by the development of the Denver West Office Park. In the lower portion of this reach, a series of three small detention ponds, intended primarily to control local site runoff, have been constructed, These ponds collect a considerable amount of sediment and offer very little actual flood control benefit downstream. Upstream ofI-70 the natural chalmel aliglilllent has been moved and the existing channel enlarged. Several erosion control check structures have been installed for charmel stabilization. The channel in Denver West to Upper Denver Lowcr L-3 Downstream of 1-70, the Denver West Office Park development has contributed to increased runoff estimates from the conditions which existed at the time of the previous Master Plan. Below Denver West the residential developments immediately adjacent to the mainstem of Lena Gulch have historically experienced frequent flooding problems. The basin in these lower reaches between 1-70 and Maple Grove Reservoir is narrow and near ful development according to current land use maps I I I thIS reach has Plan had in the to have adequate capacity for the I OO-year flow, The previous Master a detention basin upstream of the 1-70 crossing. That basin was not included been enlarged recommended development Improvements Several improvements have been made to the drainageway since the previous Master Plan was produced. Some were consistent with the recommendations made in that Master Plan. Others were made in an effort to solve a relatively localized problem without regard to their impacts on the rest of the basin. Existing Drainageway I I tributary. Master Plan. The Street, was constructed during enough capacity for the 100- TIle drainageway in this reach consists of a relatively The channel passes through mostly undevclopcd land properties also locatcd adjaccnt to the drainageway the confluence with the Pleasant View One improvemcnt has been constructed in this reach since the previous improvement, which involved enlargement to the crossing at Isabell this study. The new triple 12.5' x 8' concrete box culvert provides year flow without overtopping Isabell Street shallow natural chalUlCl that is undersized with a few residential West to Denver Upper L-4 Master Plan since the earlier which have occurred a reach-by-reach description of the changes tion Reach Descri IS The following Reach I to South Golden Road this rcach is locatcd on the grounds of Canlp Gcorge West The drainagcway througl West has remained mostly unchanged and consists of a well-incised natural channe Above Camp George West. the existing natural channel is unimproved and is tributary View low-density Conflucnce with the Pleasant Much Camp that is of George undersized L-5 4 Maple Grove Reservoir inlet to the outlet of the Youngfield Street culvert The most notable improvement constructed in this reach is the enlarged culvert crossing under 20th Avenue. The original structure was undersized and frequently became blocked with debris during high flows. The structure which replaced it was a 103-foot long, 8.5-foot by 6.0-foot corrugated metal pipe arch culvert. Grouted riprap was placed at the inlet and outlet for erosion protection. This structure was constructed in an effort to eliminate the flood damagcs occurring due to debris blockage, however it did not have capacity for even the IO-year flo\\'. A sccond replaccmcnt L-I I I I I