Drainage_and_Flooding_Report-dated_10_03_2008

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Mr. Ron Bottoms, City Manager October 1, 2008 Page 16 of 31 k I o t z associates problem rank) against the number of family units in the general area • where the flooding occurs which are estimated to benefit from remedy of the flooding problem. As a surrogate for the number of family units, we use the number of lots in the area estimated to be beneficially impacted; see Table 1. In identifying the number of lots beneficially impacted, professional judgment is used to identify the boundary of the area where the benefits would be significant in light of where the Egli concentration of flooding reports occurs. For convenience, the identified areas of benefit are referenced ► the subdivision in which they occur. The most critical and most beneficial problem areas to select for remedy are those which have high problem intensity (corresponding to a high rank number) and frugh numbers of people benefiting. By examining the pattern of points in the plot of problem rank and people benefiting, we can identify those subdivisions or local areas which have high •!• rank and high •` For the ranked subdivisions and local problem areas, Figure I plats the number of lots in the subdivisions or local area against the flooding problem rank. Based upon review of these plots, the recommended priority (all other factors, e.g., budget limitations, being equal) for implementing the long term drainage and flooding problem solutions are given in • 2. - I . UL '171111TJ Unclies-ITTU U1.70-ULICIS ALIU S111a11U1 ULURCS �SCC F-iMMUIL if Yor ynufary ditches and channels). When the capacity of a major channel ("drainage ditch") is insufficient to carry the flow delivered to it, two basic problems can arise: 1) flood waters can rise above the banks and overflow adjacent properties, and 2) flood waters can back up local runoff attempting to drain to the channel and contribute to local flooding problems These latter problems have been prioritized above. Hydraulic models can be used to assess channel overflow and flow obstructions (e.g., low bridges, small culverts). In terms of modeling, the major drainage ditches in the City fall into one of the following categories: • A drainage ditch for which a hydraulic model has been previously developed by FEMA or others; such models referred to as FEMA models. • A drainage ditch for which field survey has been made to define channel cross section at a limited number of locations. • A drainage ditch for which no model exists and no type of model is to be developed as part of this CWDS.