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<br />be washed out during these floods present a significant hazard to <br />downstream bridges. The bridge debris may restrict not only water <br />passage efficiency of any downstream crossing, but may also act as a <br />batteri ng ram to collapse or wea ken other st ruct ur'es. Consequent ly, <br />any relatively weak bridges or large potential debris in the upstream <br />reaches constitute a significant hazard to each structure below it. <br />D. Interface With Canal System <br />The general assumption in this study is that the various <br />irrigation ditches and canals are flowing full, such that surface <br />runoff flows across unimpeded. Typically, all streams are provided <br />crossings of the two major canals in the study area: the Loutsenhizer <br />Canal and the South Canal. As discussed 'n the Hydrology Addendum, <br />three sub-basins (M-3, 8, 18) are directly tributary to the South <br />Canal. Runoff hydrographs peak flows of these three sub-basins are not <br />additive, due to sub,basin location and characteristics, and the South <br />Cana 1 is judged to have adequate capac ity to convey thi s dra i nage out <br />of the study area for the 100,year storm. During a "SOO-year" storm <br />the canal may breach, adding approximately 1,000 cfs to the stream flow <br />quantities. It was decided not to consider this excess flow for pur- <br />poses of the backwate.r ana 1 ys is, however', as breachi ng potent i a 1 and <br />location are uncertain. In addition, the time and location of a <br />possible breach will significantly affect quantity and location of the <br />excess flow, and these aspects cannot be reliably estimated within the <br />realm of probabilities for a SOO,year fr~quency event. The South Canal <br />was thus considered to remain intact within the study area dur'ing all <br />storm frequencies examined for purposes of the backwater analysis. <br /> <br />3 <br />