Drainage_and_Flooding_Report-dated_10_03_2008

Browse Search

Mr. Ron Bottoms, City Manager October 1, 2008 Page 19 of 31 k I o t z t associates Major Drainage Ditch Assessment for Ditches without FEMA Models For major drainage ditches without a base FEMA model, a different approach for using the limited information on channel configuration was used to assess the capacity of the channel to contain flows of different frequencies. Since these drainage ditches, conveniently termed tributary channels (though in fact two of these channels drain directly to Galveston Bay) are generally short and drain to a channel (or in the two instances, Galveston Bay) for which a FEMA model has previously been developed, the tailwater level (either at the receiving channel or the bay) will dominate and largely determine the water level in the tributary channel. The potential out -of -bank conditions can be thus assessed for planning purposes by comparing tailwater levels (i.e., water level in the receiving channel for the storm frequency of interest) to the bank elevation at the available cross sections along the tributary channel. Table 3 summarizes whether or not the tailwater level is or is not above the top -of -bank at the available sections along the tributary. Based upon a review of these levels, an assessment can be made as to whether or not, for the storm event frequency of interest, the tributary size is likely sufficient to keep the flow within banks. These assessments are provided in Table bank overflow can be expected to be frequent, the tributary should be considered for improvement. If frequent out -of -bank conditions occur in the close vicinity of reported flooding, tributary capacity is likely a significant contributor to the flooding problems, and capacity improvements axe of high priority. Flooding problem reports have been described in section 3.2. The evaluation of drainage channel and ditch overflow discussed in sections 4.1.1. and 4.1.2 above can assist in identifying likely significant sources or causes of reported flooding problems. There are different likely significant sources or causes of reported flooding that can be reasonably distinguished by the frequency of channel overflow when there is a drainage charmel in the vicinity of an area of numerous flooding reports (i.e., near flooding "hot spot" areas). These sources or causes are categorized as follows: Insufficient Channel Capacity: The frequency at which channel overflow is so large (in a relatively sense) that it can be expected that an overflowing channel will be a ma or cause of the reported flooding if the area of flooding is reasonably close to the channel. For evaluation purposes, it is assumed if a channel is estimated to overflow its banks for 1 0-year and larger storm events that insufficient channel capacity is the primary or certainly a prime cause of the reported flooding. Note that the insufficient channel capacity could be due