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gabion mat li;'\ing. Installation of concrete lining wO\.lld, how- ever, create exceedingly high velocity in thc channel requiring the use of clear span crossings and special cncrgy dissipation measures at Pierce St. The use of grouted riprap would represent another acceptable alternate. One advantage of the grouted rip- rap is that it has a relatively high ":')" value and would main- tain velocities within the limit recommended on the Master Plan. an existing 10 foot e<l!;€ment. It will be necessary to construct a vertical walled COncrete channel in this area. In Florida Ave. a large graded inlet extending totally across the street is designated to intercept accumulated flows on the surface of Florida Ave. The graded inlet and channel system below it are designed to accommodate essentially the 10 year storm runoff. From Kalahan AVe. to Wadsworth Blvd., the channel flows along an existing 36 foot drainage easement. Improvements have been made along this channcl reach by Jefferson County and consists of in- stallation of drop structures am:! check dGlms. Very minor revisions to this area are recommended. They are the installation of a 6 foot wide paved invert, placing of bluegrass sod On the side slopes and improvement of the existing drops and checks. The existing lush growth adjaccnt to the channel indicates that there is sufficient ground water and runoff from irrigation of adjacent lawns to maintain the bluegrass on the side slopes. The use of gabion mat lining, concrcte lining, or grouted riprap through this section would be an acceptable alternative, although significantly inoreasing the cost of the improvements. A reinforced concrete storm sewer is indicated in Florida Ave. to collect local drainage and serve as an outfall for low flows from the detention pond designed further to the west. The existing Wadsworth crossing should be replaced with a rein- forced concrete box culvert capable of passing the lOO year storm runoff. Drawing No. NS-4 The storm sewer in Florida Ave. extends to the proposed detention pond in Green Gables Park. The combined capacity of the street curb and gutter system and the storm sewer are sufficient to carry the design storm within the street right-of-way. During the 100 year storm, water would encroach onto front yards of houses but nO property damage is anticipated. The proposed detention pond in Green Gables Park utilizes the existing recreational pool with only minimal improvements. A spillway is to be constructed together with a morning glory type outlet structure. The design of the detention pond is such that the 10 year storm will be totally stored in the deten- tion pond and released via the storm sewer in Florida Ave. Drawing No. NS-3 Between Wadsworth Blvd. and Allison St. t~e gulch flows through an existing 36 foot easement. The channel should be regraded to the design invert profile and a paved invert installed. Side slopes of the channel would be 2:1 with bluegrass sodding. As with other portions of Sanderson Gulch, the lush growth existing along the channel indicatcs that sufficient water exists to ~aintain the bluegrass without additional irrigation. In the areas with 2:1 side slopes and bluegrass sodding, it will be necessary to mow the side slopes with hand mowcrs. The grass could be allowed to reach a height of 4 to 6" between mowing to reduce maintenance costs. It is recommended, however, that the grass be mowed several times during a summer. From the detention pond to Garrison St. the gulch will be con- tained in a grass lined channcl with 4:1 side slopes. The study arca terminates adjacent to Garrison St. where 2-48" reinforced COncrete pipes are proposed to cross the new Garrison St. align- ment. The existing as it passes culvert the 10 beneath Allison St. year design storm. may be left in place Drawing No. W-l On Weir Gulch, the existing railroad bridge adjacent to the South Platte River has a sufficiently wide span to allow passage of the gulch beneath the structure in a concrete lined, vertical walled channel. The vertical walled channel is also necessary to clear channel constriction immediately above the railroad tracks. From Station la+co to Bryant st., the channel widens nut into a grasslined channel with 3:1 side slopes. Between Allison St. and Station 80+00, the channcl flows in at existing 24 foot drainage and utility easement. It will be necess- ary to use a very narrow channel cross section employing gab ion side slope protection and a paved invert through this reach. From Station 80+00 to Florida Ave. the channel must pass through The Bryant St. crossing consists of a new bridge to replace the existing structure. The bridge will adequately pass the 10 year storm runoff without street overtopping, but will create local flooding from the 100 year storm occurrence. -102- -103-