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IAVIA MILLER. ERNSTSEN AND ASSOC.. INC. CIVIL AND LAND DEVELOPMENT ENGINEERING LAND SURVEYING 7'50 EAST PRENTICE AVE. #202 770-201 !5 ENGLEWOOD, COLORADO 801' 1 Cherry Vale Meadows Drainage Study Pa ge 2 with cross-sections taken from the map included in that study. The results from that run were in good agreement with the official flood pla in, thus verifying the roughness coefficients input into the program. ' During the course of field survey work for the proposed bridge, it became apparent that the existing topography as shown on the official map was in error by as much as four feet vertically. When this was discovered, the en- tire site and properties adjoining Little Dry Creek were field surveyed and a new topographmc map of the site and surrounding area was prepared. The information from the field survey was then input into the HEC-2 program, using the same roughness coefficeints as in the first run. The major change re- sulting from this recomputation is that the two existing homes on the east side of Cherry Vale Drive closest to the creek are no longer in the 100 year flood plain due to the fact that they sit about four feet higher than shown on the Little Dry Creek study. In addition, the flood plain on the proposed Cherry Vale Meadows Subdivision changes slightly. The third computer run was executed using data based on the proposed bridge and channel improvements, with a bridge consisting of 8-9 foot span by 6 fbot rise box culverts, excavation of the channel upstream and downstream of the bridge, and a limited amount of fill upstream of the bridge as shown on the attached plan and cross-sections. This set of computations iddicates that the proposed bridge, channel improvements and limited fill upstream meet all the required criteria in that the flood plain elevation is not raised, channel velocities remain the same or less, the channel volume is increased slightly over the existing capacity, and the bridge is not overtopped. Concern has been expressed at previous Planning Commission meetings on the sketch plat for this project on the effect of debris pl ugging a part of the bridge, so a fourth calculation was made with the bridge opening area reduced by 25% to simulate this condition. If this condition were to occur, the max- imum increase in the flood plain elevation would be IS" at cross-section 3. This WOuld correspond to an elevation of 5401.6 U.S.G.S. and would represent a flow of approximately 550 cfs over the top of,the boxes and adjacent low lying roadway with the remaining 2450 cfs passing through the bridge. Depth of flow would average about 0.6 feet over a length of 100 feet, with an ap- proximate velocity of 9 fps. This increased flood plain elevatJon would taper back to the historic elevation within a distance of 100' - upstream and within a distance of 10-15 feet downstream of the crossing. "