Drainage_and_Flooding_Report-dated_10_03_2008

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Mr. Ron Bottoms, City Manager k I o t z 'L4 0 a s s o c 'i a t e s October 1, 2008 Page 7 of 31 2.4.5 Capacity Improvements in Storm Sewer Systems A significant limitation to storm sewer capacity, as discussed in regard to sources of drainage problems, is believed to derive from inadequate number or size of storm sewer inlets in sewered areas. The City's drainage criteria spees that maximum water travel distance in the street to a curb inlet shall not exceed 400 feet in residential areas. The common recurrence of residential street flooding in sewered-areas suggests that the 400-feet criteria may be inadequate. A reasonable sense of the potential problems arising from inadequate storm sewer inlet capacity because of too few inlets can be obtained by computing the average length between inlets along storm sewered-streets (obtained by dividing the total length of streets in sewered-areas by the number of street inlets in the same area). Representative examples of such calculation are shown in Exhibit 12. The subdivisions were arbitrarily chosen as representative examples and predate the current criteria of 400 ft. Values commonly appear to exceed 400 feet in existing residential areas, suggesting that inlets are typically spaced too far apart along streets. A detailed analysis for the storm sewer would have to be made to determine if there are any benefits for adding inlets for current storm sewer systems. However, consideration should be given to requiring that maximum inlet spacing be 300-ft for new residential developments, unless it can be shown by engineering calculation that the maximum ponding depth in the gutter upstream of the inlet will not exceed the lesser of the top of street crown or the top of curb along the drainage path leading to the inlet from the farthest point draining to the proposed inlet. Also to be avoided are the long stretches of no street inlets at the upstream end of a sewer line run. Slight changes in street slope due to construction procedures may reduce the planned gutter flow capacity toward the last upstream inlet in a line of street inlets. It would be recommended that engineering for new developments be required to assess inlet spacing and location in light of the realities of very flat street slopes and inaccuracies in construction, and identify an appropriate maximum distance of gutter runs without sewer inlets which would recognize inaccuracies in slope estimate and construction. Capacity limits can also be significant at outfalls if outfalls are too small. Small outfalls (i.e., small diameter outfall pipe) produce large energy losses. In addition, small outfalls are quite susceptible to blockage by debris, either from upstream or backup from receiving waters. It would be recommended that the minnnum outfall size be 24-inches in diameter (or equivalent diameter).