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I I I I I I I I I I I I I I I I I I I General ARVADA LITTLE DRY CREEK (Adco) SELECTED PHASE B PLAN August 1, 1978 1. Recommend enforcement of flood plain regulations of the 100-year f1 ood plai n. 2. Discuss in the plan the need for regulating the residual 100- year fl?od plain.as shown on the preliminary design drawings for dralnageway lmprovements designed to convey less than lOO-year. 3. Any storm sewer inlets to be provided in conjunction with the rec?TI1lIended and/or selected iJ1llrovements should be "Type R" as des~gnated ~y the Colorado Department of Highways and should be deslgned uS1ng the Standard Curb-Opening Inlet Chart in the June, 1977 issue of "Flood Hazard News." - 2 - 1. Provide a preliminary design per Phase A report recommendations. 2. Provide for a strengthened downstream flow control structure on the main channel to insure the peak flows are forced into the side channel storage at Pamona Lakes 2 and 3. Existing pedestrian bridge may not be structurally adequate to serve such a purpose. 3. Provide facilities to convey a 100-year flood along reaches of Tributaries Band C added to the project by Addendum Number 1. WESTMINSTER 4. The plan should encourage purchase of flood insurance. 5. Provide f1 ow separati on structures to all i rri gati on di tches . 6. Provide appropriate maintenance access to all improvements and open channel reaches. 1. For Little DrY Creek, Provide a 10-year grass lined channel. 2. Porvide facilities for 100-year protection along the Shaw Heights Tributary. The preliminary design shall generally follow the the historic thalweg of this drainageway. 3. Provide a lo-year storm drainage system east of the existing detention pond on the Shaw Heights Tributary. Also, investigate the possibility of using a detention pond east of Lowell Blvd. capable of storing the entire 100-year runoff from upstream lands yet to be developed. Such a pond has a potential for reducing 10-year storm sewer pipe Slzes downstream and may even reduce the enlargement of the existing pond. ADAMS COUNTY 7. All road crossings shall be designed in accordance with the Urban Storm Drainage Criteria Manual recommendations. - 8. Provide velocity control structures where excessive velocities and/or sustained low flows would cause channel side slopes to erode and/or the stream bed to degrade. 9. All open channels shall have adequate freeboard at the deSign flow. It is recommended the freeboard above the water surface be equal to one velocity head plus one foot. If site conditions so indicate, the freeboard may be reduced, but under no circumstances should it be less than one foot. 1. Provide 25-year open channels and related structures for Reaches 1 and 2A generally consisting of a grass lined channel. The channel along Reach 1 is to be located east of the railroad track. Investigate a concrete lined' (or concrete walled) channel along portions of Reach 1, such as 64th Avenue to Clear Creek where a potential for cost reduction of multiple street and/o} railroad crossings would justify its cost and the loss of a greenbelt. 2. Provide for an enlargement to the existing detention pond on the Shaw Heights Tributary to a 100-year capacity. This improve- ment is needed to remedy the flooding potential through Westminster and all cOsts of enlarging the existing pond will have to be Westminster's responsibility. 3. Provide a lo-year storm sewer upstream of the existing detention pond On Shaw Heights Tributary. JEFFERSON COUNTY 1. Provide a preliminary design based on Phase A recommendations.