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<br /> <br />Velocitie$, Rates of Rise, and D~~atio~ <br /> <br />The Occu~~ence of the Intermediate Regional Or Standar~ <br />Project Flood would result in the flood characteristics Shown in <br />Table 3. These data reflect the ave~a<:ed maximuCl COnditions "..ith <br />heights Of ~ise, rates of ~ise, and duratioo$ of flondino measured <br />above bankfull staqe. <br />Floodflow velocities depend upon t~s osoeral waterway <br />configuration, and ths size, gha~e and slope of the st~eambed and <br />overbank flo~ areas. Average velocities may range from the maximums <br />appearing in Table 3 to less than fifty percent of these values. <br /> <br />TABLE 3 <br />FLOOD CHARACTERISTICS OF SIXMILE CREEK <br /> <br />IweraqcdMaxi:num Conditions Above <br /> <br />BankfullStaqe <br />l-!agnitudeofOccurrence <br /> <br />Standard <br /> <br />Project <br /> <br />Characteristic <br />Peak L1i,'c.~arge (cubic feet pe~ second) <br />HeiqhtofRise (feet) <br />Rate of Rise (fcct:;>erhour) <br />Duration of Flooding (hours) <br />Main Char.nel Velocity {fe"t per second) <br />CN"rbankVe10city(feetp"rsecond) <br /> <br />Flood <br />35,655 <br />15.5 <br />,., <br />22.0 <br />13.8 <br />,,' <br /> <br />Obst~uctions <br /> <br />!ntennediate <br />Regional <br />""'" <br />19,379 <br />12.7 <br />U <br />22.0 <br /> <br />12.3 <br /> <br />,., <br /> <br />Br"shanddebriswlIShinqdownstrea.-:tduring!loodsoftcn <br />collect against hri.dges nr other re~trictcd flow arc~~, reducing the <br />waterway openinas and otherwise impeding flood flow. The brush and <br />d",briscr"ateadaWl"lingeff"ctalld,dependinqonth"deqreeofclog_ <br />gino, cause greater back~ater depths with resultant increased overbank <br />!looding. W.1teroressurebuildingupagainstembankmentsandbridg(>s <br /> <br />" <br /> <br />. <br /> <br />can result in stress damage, or total destruction of the structure <br />inVOlved. A pronounced increase in flow velocities usually occurs <br />downstream from an obst~uction, thus inc~easing the flood's damage <br />potential. Ohst~uctiv" new development and future land fills in o~ <br />near the stream's flood~lIY Can be a ~ajo~ factor in ~~ltiplyinq the <br />adverse effects described abov~. <br />Esti~ated future flood crests and pertiner.t elevations <br />at b~idge c~ossings and culv"rts within t"" ~tudy area are listed in <br />Table 4. For purposes of this study, it has been assuned that limited <br />cl099inq can be expecte~ and that all bridge structures and culverts <br />~~uld remain intact. Significant char~es in this premise, iM~osed <br />by differing co~ditions of a future flood, could alter the estimated <br />flood crest~ And flood li~it~ ~hown in th~ ~~blp. ~nd rp.1At~d olates. <br /> <br />. <br /> <br />Future Flood Beiqhts <br />ThrouQhout the study reach, average water surface eleva- <br />tions for the Standard Project Flood exceed those of the !nter~ediate <br />RegioO;>.l Flood by 2.8 f",et. Fi"u.es 3 and 4 sho~' n.e height th.1.t <br />would be ~..ached by futu~e flooos o~ faci.litie~ pre~e"tly existing <br />within the flood plain considered in ~he study. <br /> <br />. <br /> <br />n <br />