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Property damage from gredt floods in developed areas can Flooded Areas andf'lood Damage. The study areas subject to flooding by the 100- and 500~year floods are shown On Plate 4 through B. Plate 3 is an index map showing the relative location of the flooded area plates in the study area. Limits of the over- flow areas may vary from those shown on the Floodad Area plates because of variation between map contours pres..nted herein and actual locations of ground contours. Depths of flow for any location along a stream can be estimated from the flood profiles shown on Plates 10 through 16 and from the flood elevatIon lines shown on the flooded area plates. Floodwater elevations determined in any particular area are not always indicative of the depths actually attained, often flood- water becomes isolated and trapped in overbank areaS resulting in depths greater than those shown on the various plates and tables. Obstructions. Maintaining the channel and overbank capac- ity _of a stream to convey floodwater through the community without causing serious flood damage should be of major concern. Rcstrict~ ing the floodway by bridges and their approach embank.'Ilents, rOildb<.--ds, levees, or otherwise changing channel and overbank ca:paciHes will impede floodflow$ and cause greater backwater depths with conse- q~ent increase in flooded areas. The growth of vegetation, obntruc- tive new developments or land filling in a stream's floodway can be ~ major factor in ~ultiplying the adverse effects described. For study purposes herein, it has basn assumed that limited clogging ot bridges and cuiverts wUI occur and that they would ..tand lntdct during the 100~ and 500~~ar frequency floods. Significant changes in this premise could alter the flood crests and flood limits shawn in relat~d plate~ and tahleR. Ysidrio Avenue bridge has several wooden piers that are closc together and may be subject to severe clogging CAusing in~ creased backwater that may inundate some residences currently not in the lOO~year flood plains. The Ysidrio Avenuc bridge is very percent chAnce of being equaled or exceeded in any year. Perhaps more significantly, it has about a 25 percent chance of occurrence during a 30-year mortgage period. Hazards of Larqe Floods The hazards to life and extent of damage caused by any flood depend on the topography of the area flooded, depth and duration of flooding, velocity of flow, rate of rise, and devclop~ ments on the flood plaing. Velocities greAter than 3 feet per second combined with depths of 3 feet or more are generally considered haza~dous to life as well as to property. water flowing in excess of 4 feet per second is capable of transporting ~ediment and causing sevore erosion of streambanks and fill around bridge abutments. Where veloclties drop below 2 feet per second, debris and silt deposits can build up, ex~ tending the flood damage~ and creating adverse health conditions. The saturation effects of long-duration floodiny can weaken bridge abutments, levees, and other embankment works that may fail as the f.loodwaters recede. be ovel.w;J<l1mlllg. The entire co=,,,,,.,i';.j' ~uffe:-s wh"n str"""", bridg,,~, sewers, and other public utilities are destroyed or otherwise made inoperative. The delineation of what will occur during a flood cannot be totally measured in dollars. The po~sibl~ loss of life, h<<alth, and general well-baing of a community are it...rns that cannot be measured and must be considered. A great flood Can unleaSh ill- neSS and deadly epidemic disease. Damaged or destroyed sanitary sewage facilities can pollute the floodwaters and create a health hazard. Newspapers sometimes report a "lighter side of the n"ws" in picturing children at play in the mud and standing water left byareOOdingflood.onfortun"tely,however,th"contamillation threat to domestic water supplies cannot be illustrated with similar ease. " u