Drainage_and_Flooding_Report-dated_10_03_2008

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Mr. Ron Bottoms, City Manager k 1 0 t Z a s s o c i a t e s October 1, 2008 Page 8 of 31 In new development situations or where drainage ditches are being improved in existing or infill areas, open ditches should be designed to handle the design storm flow. In addition, design criteria should require the following: • Minimum bottom width of ditch (recommend 2 feet) • Maximum side slope of ditch (recommend 4:1 slope) • Minimum bottom slope of ditch (recommend 0.05% slope) • Minimum freeboard under design flow conditions (recommend I foot) Engineering evaluations for new development and redevelopment, if roadside ditches are to be used, should account for energy loses at culverts, including driveway culverts. Discharges to open ditches from adjacent properties should not exceed the capacity of the ditch if 1) the ditch is adequate for generally accepted areas of localized runoff (for the design storm) adjacent to roadways, or 2) specific inclusion of existing large, off -roadway drainage areas are recognized in the design of the original ditch when the natural (i.e., not altered by man) topography drains the area toward the roadway. Generally accepted areas of localized runoff to roadways and the ditches adjacent to them consists of 150-ft wide strips adjacent to the roadway right-of-way on each side of the road. Drainage from beyond this commonly accepted width zone is to be discouraged and should be directed to areas and/or drainage pathways to which natural drainage occur. If such drainage cannot be safely directed to riatural drainage pathways, on -site detention should be required to limit the peak cates of discharge to an acceptable level for discharge either to the roadway or friatural drainage paths. Allowable street ponding depth of storm waters poses one of the most difficult choices for establishing drainage design criteria, particularly 'in coastal or near - coastal areas where slopes are quite low and natural drainage is poor. Because of the naturally poor drainage conditions, use of streets as part of the storm water drainage, system is common in coastal Texas. However, such use sometimes poses inconveniences to residents. Such use may also prevent access by emergency vehicles or ordinary citizen vehicles dealing with an emergency. The City currently allows street ponding to depths reaching the top of curb or street crown, with the proviso that one clear lane (of 12-feet width) be maintained �n collector streets and two clear lanes (of total 24-feet width) be maintained in