FLOOD10335 - Laserfiche WebLink

Home Browse Search

. ( " ..; "l c ... ( ." . - 28 - Flood routings were made on: (1) present condition, (2) future condition with land.treatment only, and (3) future condition Wit~ land treatment and structures. .Each city block in the 'floodplain was 'considered as an evaluation reach. Four peak rates of flow were associated with a dollar-damage for each section along the floodplain and this infor.mation was then used to construct discharge-damage curves for evaluation. The routing reaches in these studies were based upon the known pattern of in-channel and overbank flow that occurred in the June 1958 flood. Beginning at the High Line Canal, the floods were routed downstream to each successive channel restriction, collecting enroute the appropriate local inflow. All flows within the watershed were thus accounted for and routed through the project area for each flood condition. The reservoiring effect of the High Line Canal was determined by calculating its capacity over and above the normal flow of 600 c.f.s. of irrigation water by water surface profiles. Flood routing then determined the frequency at which overtopping would occur. An additional reservoiring effect will be gained when the new Interstate Highway that will cross the watershed parallel to and just north of the High Line Canal is completed. . Benefits fram the proposed land treatment measures were evaluated to the extent in which floodwater and sediment storage could be reduced. The emergency spillway of structure TIv-l.was designed to pass the freeboard hydxograph (7.Su of point precipitation = 5.6" of runoff) without overtopping the dam. For comparison, the maximum probable amount of point precipitation for Indian Wash as determined by use of hydrologic data developed by the U. S. Weather Bureau for the U. S. Bureau of Reclamation's nearby Blue l1esa Project was 7.2 inches. The IW-l reservoir will fully contain the volume of the spillway hydrograph (3.0" of point precipitation = 1.71" of runoff) without the emergency spillway operating. Sedimentation Investigations No previous records of sediment production were available for the area included in the Indian \'lash Watershed. Sedimentation rates at the proposed floodwater-retarding reservoir site were based on rates that have been measured by the U. S. Geological Survey in the nearby experimental Ba~ger Wash Watershed and on the results of a reservoir sediment survey made at an old reservoir in the upper part of the Indian Wash drainage area. .