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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Rocky Mountain Consultants, Inc. <br /> <br />Mr. Larry F. Lang <br />May 14, 1991 <br /> <br />2. Since the approval of gambling, the market value of land between <br />Lawrence and Gregory Streets east of Spring Street (i.e. Gregory Gulch) <br />has become extremely valuable. Requiring an open channel section would <br />essentially require land purchase by the City. Proposed instead is a <br />closed conduit capable of conveying the 100 year flow (oversized to <br />accommodate future increases attendant with development), with the <br />balance of flow conveyed by Lawrence and Gregory Streets. Inasmuch as <br />these streets are over 7% in grade, conveyance capacity is high. <br />Additionally, periodic street drains with large inlets will convey <br />additional surface flow to the closed conduit. <br /> <br />An open channel section must be provided, however, just upstream from <br />Black Hawk. A direct connection would not be advisable due to the <br />1 imited capacity of the Black Hawk storm drainage. <br /> <br />3. The parking lot upstream of the Opera House where the recommended <br />detention pond would be located is owned by the Opera House Association. <br />Stage prop trucks use the parking lot to back across "A" Street to load <br />and unload props for performances. Raising "A" Street and/or walling <br />off the lower end of the parking lot for detention purposes would <br />disrupt this practice, and perhaps for minimal benefit. The maximum <br />practical ponded depth would be 3.0 feet, allowing a total detention of <br />0.12 acre feet. Theoretically, only 2 cfs in the 100 year storm event <br />will contribute to the parking lot, but the concern expressed in the <br />field was the possibility of other systems failing and that the parking <br />lot would receive more flow. Were that the case, 0.12 acre feet (or <br />even double that) would have negligible impact. Furthermore, due to <br />past flooding which has occurred without improvements which are <br />proposed, the Opera House has been "flood proofed" on the upstream side <br />with a concrete wall. Water ponded in this area would eventually <br />gravity overflow to Eureka Street if the flume capacity was exceeded or <br />inlets failed, so a surface outlet does exist for floodwater. <br /> <br /> <br />4. Flow velocities of 20 feet per second, even for 100 year events, would <br />normally be avoided. However, consideration of several factors has <br />caused us to feel that this condition is acceptable. Reference is made <br />to the LOMR application report, Section VI-B. <br /> <br />We are available to discuss these issues more if desired. <br /> <br />Sincerely, <br /> <br />ROCKY MOUNTAIN CONSULIANTS, INC. <br />~/.U~ ) <br />Gerald R. Williams, P.E. ~ <br />Project Engineer <br />cc: Jack Hidahl I CITY OF CENTRAL ADMINISTRATOR <br />