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. mouth. This redu~ed peak discharge is due primarily to flood- waters lost to valley storage. These discharges compars statis- tically with the recorded maximum floods whiCh are listed in TablB6. . freQuency It is impractical to assign a frequency to the Standard Project flood. Tho occurrence of such a flood would be 8 fafe event; however, it could occur in any year. Possible Laroer floods floods larger than the Standard Project flOOd are possible; however, the combination of fectors necessary to pro- duce such floods would seldom occur. The consideration of floods of this magnitude is mOfe Import~nt in Some areas then in others, but they should be considered in any study. Ha2ards of Great floods The amount end extent of da~agc caused by any flood de~~nds in general upon how much area is flooded, tha haight of flooding, the velocity of flow, tho rate of rise, and the duraA tion of flooding. Areas Flooded and Heiohts of floodino The areaS along the St. Chorles River that would be flooded by the Standard Project FlOOd and the Intermediate Re- gional flood are shown on Plates 4 through 10. Depths of flow Cen be estimated from tho high water profiles shown on Plates 11 through 20. , The profiles for tho stream were computed by using stream characteristics, topographic maps, and volley cross sec- tions which were surveyed in August 1969. Tho overflow atOes and elevations shown on the referenced piatoe have been determined with sn accuracy consistent with the purposes of this study, the accuracy of the basic data, and within the limitations imposed . " by the map scale. Theselimitsmayalsovary,astimepassas,beA Cause future urbanization or building_over of the watershed ~an in~reass runoff, flood heights, and the size of overflow areaS. The profiles of the Standard Project flood and the In_ termediate Regional Flood depend in part upon the degree of struc- tural obstruction and clogging that may Occur at the various bridges during the flood. These factors cannot be accurately fo,ijcast, and some clogging is to be expected which may change estimated flood limits. It was assumed that the bridge structures ~ould etand in- teet. The Standard Project Flood profile for the St. Charles River overages 3.8, 5.6, and 5.2 feet higher than the Intermediate Regional Flood in the upper, middle, and lewer ,oaohss, respec~ tively. figures 4 end 5 show the heighte that would be reached by the Standard Project flood and the Intermediate Regional flood on facilities presently existin9 within the St. Charles River flood plains considered in the atudy. Plates 21 through 24 show selected cross s"'ction~ that ar.. typical of the 28 sections curvcYlld in the St. CharI 05 River study reach. The locations of all sections are shown on the profile plates, as well as the elevations of the Stand~ ard Project Flood and Intermediate Regional Flood. Velocities. Rates of Rise. and Duration Water velocities during floods depend largely upon the sizo, shape, and slope of the streambed, and the condition of the ~hannel. These factors vary on different streams and at differ- ent locations and times on the sams stream. Table 7ehowe the peak discharges, heights of rise, max_ imu::l rates of rise, durations of flooding above bllnkfull atage, and the maximum velocities that would occur in the main Channel and overbankareasoftheSt.CharlesRivs<duringthajntermediata Reqional Flood ~nd Standard Project flood. As given in tho table, "