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<br />k lot z f~. ass 0 cia t e s <br /> <br />design flow (of 100-year frequency; see discussion of Section 3 regarding design criteria), <br />the evaluation of extreme event flows was used as the guideline to assess the likelihood <br />of channel overflow as a significant problem source. For FEMA channels, the 100-year <br />flow was evaluated to determine in- or out-of-bank conditions, while non-FEMA <br />channels were examined for the 100-year flow and less severe flood levels as well. <br /> <br />Four basic options were considered in addressing flooding due to channel overflow: <br /> <br />. <br /> <br />Channel widening, selected as the most feasible and desirable channel <br />modification technique if channel modification is to be used to increase channel <br />capacity. The channel reach length where widening was proposed could be <br />limited in channel length to areas where actually needed. For planning purposes, <br />the widening was assumed, when based upon FEMA model analysis, to generally <br />extend the length of the channel reach between FEMA model sections in the <br />vicinity of where the current floodplain was out of banks and structures were <br />being adversely impacted by flooding, such that the reaches to be improved were <br />contiguous. For non-FEMA model analysis dealing with tributaries, the widening <br />was assumed to occur in the channel reaches between model sections (with <br />section locations based upon field survey) where the downstream end of a reach <br />did not have adequate existing capacity to carry the 100-year discharge. The <br />upstream end of non-FEMA model channel improvements were, based upon <br />professional judgment, sometimes not included because improvements in the <br />excluded reaches were judged not to have significant potential for reduction in <br />structural flooding. <br /> <br />In addition, the widening for which cost estimates were made was done so as to <br />approximately 1) maintain the widened channel within its current top width and 2) <br />using side slopes 'which approximated existing side slopes. In many instances, the <br />existing slide slopes were steeper than a 3: 1 horizontal to vertical side slope. The <br /> <br />4-2 <br /> <br />Klotz Associates Project No. 0127.008.000 <br />January 2009 <br /> <br />La Porte Citywide Drainage Study <br />City of La Porte <br />