Drainage_and_Flooding_Report-dated_03-10-08

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Mr. Ron Bottoms March 10, 2008 Page 6 of 15 k I o t z associates residential areas are drained by roadside ditch drainage systems with slab elevations generally at or above the natural ground elevations. The large rural -like residential lot neighborhoods are generally north of Spencer Highway and are drained by road side ditches. Floor slab elevations in these neighborhoods arc typically at or above to natural ground levels or road elevation. Some areas have slab elevations below adjacent top of road elevations. The majority of the drainage east of State Highway 146 is a mixture of roadside ditch and storm sewers. Generally, the site inspection identified apparent problems of limited curb inlet capacity and limited or inadequate drainage pathways suitable for effectively relieving excessive accumulations of runoff during severe rainfall events. Conditions also suggested likely inadequate storm sewer capacity. Developments in the older portions of town were typically built without the requirements of detention facilities. New developments are being built to include detention to mitigate the effects of the new development. Short Term Drainage Projects The site visit also confirmed conditions conducive to severe flooding problems in the five short term drainage problems identified by the City. The conditions in and the evaluation of these five critical areas are as follows and are shown on Exhibit 5A: Short Term Drainage Project 1.) Catlett Lane Pavement Replacement Catlett Lane is located off Roseberry Drive and outfalls into Big Island Slough (HCFCD B 106-00-00; see Exhibit 6). The site visit revealed that dead end street of Catlett Lane at Big Island Slough appears to act like a flow restriction to the adjacent neighborhood, which is aggravated by the flat topography of the area. When _the capacity of the limited storm sewer drainage in the area is exceeded it results in backup of runoff waters on surface. Accumulated waters seek the path of least resistance and drain to the lowest intersection of Catlett Land and Roseberry Drive. We understand that City crews recently identified some problems with a drain pipe to some of the existing inlets in the local area that would restrict the drainage capacity of the area. We also know that the inlets to the storm sewer system are small and reduce the inflow into the storm sewer system. An analysis of the complete storm sewer system for this area has not been prepared but our experience is that the capacity of inlets were usually matched to the capacity of the storm sewer system so increasing the throat inlet size may not provide much change in the overall capacity of the system. However the smaller inlets do clog easier due to surface debris. Recently there have been constructed