FLOOD03719 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I adverse condition of forces exerted by flood flows and debris. As with the proposed North Washington Street bridge, the 136th Avenue bridge was raised to pass the lOO-year flood with a minimal 1.0 foot of clearance at the low chord. The upstream water surface within the golf course area was essentially unchanged by this revision when compared to the results of the outfall study improvements. The proposed bridge at 136th Avenue will include widening the channel immediately upstream and a drop structure, all located within the golf course area. The profiles of the 2-, 10-, and 100-year events (existing and future) are shown on Drawing Nos. 5 and 6. Note that although the proposed 100-year water surface is 1.7 feet higher than the existing water surface at Section 100+50 (50 feet downstream of 1- 25), the water surface upstream of 1-25 is the same at 168.1 feet (167.3 corrected to Mean Sea Level). The upstream water surface is controlled by the inlet opening at 1-25. SupercriticaI conditions exist through the bridge. Supercritical conditions generally result in extremely high velocities, standing waves, and high scour and bank erosion potential. Supercritical flow is independent of conditions downstream. However, the higher tail water of the proposed condition actually reduces the length of supercritical conditions downstream of 1-25 and provides more energy dissipation than for existing conditions. The proposed golf course grading will restrict the extent of overbank flooding compared to existing conditions, resulting in higher velocities and water surfaces for extreme events. To reduce potential fairway damage and expensive channel protection, the driving range will be utilized as an emergency overflow route. The overflow will occur in two areas, one just downstream and the other just upstream of the 17th green. The side spillway weir option of HEC-2 allowed for about 3,600 cfs to spill out of the main channel (out of 11,190 cfs total) and onto the driving (practice) range during the UDFCD 100-year event. A HEC-2 run modeled the driving (practice) range with the overflow, and separate water surface elevations are shown on the plan view sheets. About 590 cfs will spill onto the driving range during the lO-year event, and about 2,130 cfs will spill during the FEMA 100-year flow of 8,839 cfs. The separate driving range computer run begins at Section 963+20 at the 100-year water surface elevation. Because the 2-year flow is not contained within Big Dry Creek's streambanks, some flooding in the overbanks will occur. All greens, except for the 16th and 18th, are out of the 100-year floodplain. Five fairways, however, will experience flooding by the 2- IIl-7