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, mouth. This redu~ed peak discharge is due primarily to flood- waters lost to valley storage. These discharges compare statis- tically with the recorded maximum fioods whiCh are listed in Tab1e6. . freouency lt is imprectical to assign a frequency to the Standard Project flood. Tho occurrence of such a flood would be 8 rare event; however, it could occur in any year. Possible Leroer floods floods larger than the Standard Project flood are possible; however, the combination of factors necessary to pro- duce such floods would seldom occur. Ths consideration of floods of this magnitude is more import~nt in Some areas then in others, but they should be considered in any study. Ha2ards of Great floods Tho amount and oxtent of da~agc caused by any flood de~~nds in general upon how mvcn area is flooded, tha height of flooding, the velocity of flow, tho rate of rise, and the dursA tion of flooding. Areas Flooded and Heiohts of floodino The arSaS along the St. Charles River that would be flooded by the Standerd Project flood and the Intermediate Re- gional flood are shown on Plates 4 through 10. Depths of flow Can bs estimated from the high water profiles shown on Pl~tes 11 through 20. , The profiles for tho stream were computed by using stream characteristics, topographic maps, and valley cross sec- tions which were surveyed in August 1969. Tha overflow areas end elevations shewn on the referenced plates have been determined with en accuracy consistent with the purposes of this study, the accuracy of the basic data, and within the limitations imposed . " by the map scale. Theselimit""ayalsovary,astimepassas,beA Cause future urbanization or building_over of the watershed can increase runoff, flood heights, and the size of overflow areaS. Ths profiles of tha Standard Project flood and the In_ termediate Regional Flood depSnd in part upon the degree of struc- tural obstruction end cl09gin9 that may Occur at the various bridges during the flood. These factors cannot ba accurately forijcast, and some clogging is to be expected which may change estimated flood limits. It was assumed that the bridge structures ~ould stand in- tecto The Standard Project Flood profile for the St. Charles River averages 3.8, 5.6, and 5.2 feet higher than the Intermediate Regional Flood in the upper, middle, and lower reaohes, respec~ tively. figures 4 end 5 show the heights that would be reached by the Standerd Project flood and the Intermediate Regionel flood on facilities presently existing within the St. Charles River flood plains considered in the study. Plates 21 through 24 show selected Cross s"'ction~ that at.. typical of the 28 sections curvcyed in the St. Charlos River study reach. The lo~ations of all sections are shown on the profile plates, as wall ~s 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 Si20, shape, and slope of the 8tra~mbed, end the condition of the chennel. These factors vary on differ~nt streams and at differ- ent locations and times on the sams stroam. T~ble 7 shcws the pSllk discharges, heights of rise, max_ imum rlltes of rise, durations of flooding "bove bankfull stags, ",nd the maximum velocities th~t would occur in the main channel and overl""nkareasoftheSt.CharlesRivsrduringthelntermediata Reqional Flood "nd Standard Project Flood. As given in the table, "