19 - Drainage Presentation - Sandy Creek Upper Tributaries 4 and 6

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Regarding #3: There are opportunities where concentrated efforts could improve certain areas of the floodplain. For example, the large open area on the western edge of Wade Park, which used to be the location of the wading pool, could be used for overflow storage in a flood event. It would be designed to fill up when needed, and then slowly empty as the flood event wanes, reducing the intensity of flood events downstream. Exhibits showing this location as well as a location immediately north of the Paris Family Dental are provided below. Regarding #4: When possible, given the City has existing crews in the budget, we would recommend utilizing these crews to perform some of this work as the workload allows. Regarding # 5: As properties develop in this and any drainage area around the city, I recommend each developer perform an engineering analysis of pre -development and post -development flows. This is normally done on a 25 year recurrence interval (4% chance of this storm happening in any given year). The water leaving the site needs to be metered to pre -development levels after the build. In the case of large impervious surface area increase (i.e. parking lot, added roof space, patios, etc.), this could involve vaults, storm water catchment areas, swales, and/or ponds. Flow control devices such as weirs, orifice plates, baffles, and other methods of metering are very effective for 25 year and lower storms. When storms exceed the 25 year recurrence interval, the devices have overflow openings to allow free flow of water downstream. The current ordinances relating to stormwater improvements during development do not currently require what is discussed in the previous paragraph. The current drainage requirements located in Exhibit 9A Subdivision Ordinance, Section IX. Design of Improvements, Subsection E: Drainage reads as follows: "E. Draingge. Storm sewers, bridges, culverts, inlets, and drainage channels or ditches of a permanent design adequate to carry off such rainfall shall be installed by the subdivider throughout the entire length of the drainage area where the surface drainage traverses the subdivision. The subdivider may construct in lieu of storm sewers suitable drainage channels which shall be of sufficient width, depth and shape to carry off the surface drainage and allow for maintenance operations in keeping such ditches open. All drainage ditches so constructed shall have their sides sloped so as to retard erosions; the flow line of said ditches shall be permanently established by small low flow concrete sections, and the banks of all curves or bends shall be permanently protected to prevent erosion. No open drainage channels shall be constructed within the areas dedicated as public streets and alleys in said subdivision provided however, that in cases where utilization o the land ol the im_ roved development warrants otherwise, such ditches ma ,v be located in the center :, ortion o ' the street_ (except at street intersections where culverts or bridges of a permanent design shall be constructed) where a right-of-way not less than eighty (80) feet in width, plus width, for drainage channel, is provided. Computations to determine whether such lots or land will be flooded by rainfall and the size of drainage facilities adequate to prevent flooding shall be based upon the "Rational Method" using the Texas State Highway Department's Frequency of Rainfall for District 1, and a five-year cycle 19