Drainage_and_Flooding_Report-dated_10_03_2008

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Mr. Ron Bottoms, City Manager October 1, 2008 Page 6 of 31 k I o t z associates �4 I Consideration should be given by the City to requiring that for new development or re dr ina� ,qe relief of excess 3ondin_4 during extreme storm events be provided if significant localized ponding (as determined by engineering evaluation) is to be expected in view of the topography of the drainage area. Such relief would be required to be provided by sheet flow paths or equivalently effective alternate means. If the drainage relief were provided by a sheet flow path, the development or redevelopment would be required to identify and provide drainage pathways and pathway easement(s) which would not adversely affect nearby structures. INIF11111 !111 111111111 I -Ki I 0_7 The limits on construction in floodways should be reconsidered. Infringement on floodways has a cumulative effect that is difficult to identify on an individual the cumulative effect of Nadraulic blockingiiLD-um alters the energy losses -in a channel and thus eventually impacts floodway capacity). In addition, some construction in floodways, such as construction on piers in wide, shallow floodways, while not increasing flood levels, still poses significant hazard to the constructed structure because of the force of flowing flood waters and the large debris that may be carried in floodway waters. Thus it is generally preferable and is recommended that new construction in floodways not be allowed. However, if the proposed construction can be shown with ay-p".ai9Ae.vo*mo,* nisomlmw�anger to other structures and facilities, the City can make a decision on a case by case basis as to whether such construction could be allowed. In those cases where the floodway or those portions of a floodway are very wide and shallow, the City Engineer should have the option, based upon appropriate and engineering evaluation, to allow variance to limits on construction in a floodway, provided such variance is consistent with the City's floodplain management ordinances and would not adversely impact the City's compliance to the Federal Flood Insurance Program. A submittal to the Federal Emergency Management Agency (FEMA) may be required as part of the approval for such a variance. The City should consider clarifying the requirement that there should be a minimum 18-inch clearance of the low chord of a bridge above the 100-year flood level when 1) changes in design flood levels are proposed, 2) when new bridges are built, or 3) existing bridges are rehabilitated.