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and possible in succe~sive years. The lOO~year frequency of occur~ rence can be ~xpre~sed in terms of percenta~e to avoid the po~sibl~ inference of regul~rity of occurrence. Thus, a flood with a lOO~ :tear reCUrrence Interv~l woeJ,ld have" one percant of ooin9 equall"d <>r ""c",,,,ded in ~ny year. Perh~I''' more significantly, it has ~bout '" 2S percent ch.ul(;e of o<':"\lrrcncc d\lring d :JQ~year mortgage period. , I I I I I I , The areas that ~ill be inundated by the 100~ and 500~year floods ~re shown on Plates 5 through 8. Plate 4 is an index ~ ~ho~jng t."" T"lativ<' loc"tion of th.. pl"to." ~it,hin th.. study "T~". SlM,ll ..rea~ ",ithln tM flOO<led dreaS may be <ibove the flood eleva~ H~~ards of Large Floods Thc hazard~ to life and extent of damage caused by any flOOd depend on the topography of the .arc.a flooded, depth and dur- ation of floodinq, velocity of flow, rate of rise, and developments on the flood plains. Rapidly rising and swiftly flowing water may trap people in their homos or vehicles that may be submerged or dc~ strey~d. Wat~r and s.anitary s~wage lines can be ruptured by depo5~ its of debris or the forc~ of floodwater. flooded sewage treatment facilities and damaged scwagc lines could result in the release of pollutants into floodwators, creating healtr. hazards. Velocities greater tn...n three teet per ~eeOn(l combinLod with d~pthH of three feet OT more are generally ~nn5idered hazard~ oUS to life, ~s well as property. Water flo~inq in excess of four f~et per "",;{md could easily swe~p adult people off their f"et. whore velocitiss drop below two feet per second, debris a"d silt deposits can build up, e"tending tha flood ddI't\a'le and creatin9 ad~ verse health conditions. tions and thereiore not subject to flooding. Depth~ of flow in the channels can be estimated from the flood profiles shown On Plates 10 through 20. Depths of floodwater can also be estimated fromthelOO-year flood elevation reference line shown on the flooded are~ plates. Floodwater elevations de- terrnincd in any particular area are net always indicative of the depths actually attained. Often floodwaters become isolated and entrapped in overbank areas resulting in depths greater than those shown on the plates and tables. Obstructions. Maintanan"e of channel and overbank capac- ity of a stream to convey flood~atBr ~ithout causing serious flood damage should be alll<\)or "<m",,rnof the c=unity. Reducingtbe passageway for floodwater by obstructive naw developments, road- beds, bridges and their approach embankments or land fills ~ill im~ pede floodfloW8 and CaU30 greater bdckwater depths with consequent increase in flood..d ar..as. Most of the bridges in the study area are obstructive to the 100-year flood ~nd even more obstructive to the SOO~year fiood. Estimated future flood crests and pertinent elevations at bridges Flooded Areas and Flood :Jdmage. Property damage trom great floods in developed areas can be overwheiminq. The e"tire community suffers when streets, bridyes, utili~ies and public facilities are damaged or destroyed. The delineation of w~~t will oeeur during a flood cannot be totally measured. The possible 1055 of lite, healf:h, and general W<lll-being to a community are items that ""nnot he measured and must be considered. within the study areu "rc listed in Table 4. Por study purposes herein, it has been assumed that limited clogging of all bridges will oceur and that they would rCOklin inta"t during future floods. significant changes in this promise "ould alter the estimated flood cr"$U, anJ. fl~ limit", in t.h" t.ilbt"'5 and r..lat-...d plat.",.~. velocities of Flo~. Water velocities depend larqely on the size and shape of the Cross section~, toughness conditions of U,C stre~~ which tcnd to rctard the flow, and tho bed slope, all of ~hich vary at differ"nt locations on the stream. DUrinq the lOO~ycdr " "