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A. Drainage areas, including areas draining from off-site onto or adjoining the project <br />B. Design storm runoff, based on the type of facility and storm frequency listed in Section <br />5.4. <br />C. 100 – year runoff sheet flow pattern. <br />D. Route of overland flow including the overflow to a drainage channel or detention facility <br />E. Water surface profiles for the 25-year and/or 100-year storms in the outfall channel. All <br />available information will be considered when making this determination. <br />F. Flow per inlet <br />G. Maximum ponding elevation <br />H. For bridge designs the 100-year WSEL must be shown on the plans and the low chord of <br />the bridge must be a minimum of eighteen inches above the 100-year WSEL. <br /> <br />5.2.2 Detailed drainage calculations shall be submitted with the construction plans. <br /> <br />5.2.3 A lot grading plan should demonstrate that the finished grading plan will drain to approved <br />collection and discharge points and that the overland flow of water from adjacent properties will <br />not be impacted. In no instance will plans be approved without specific detail as to how the 100- <br />year event will sheet flow from the development to the outfall without adversely impacting <br />structures. <br /> <br />5.2.4 The hydraulic gradient for the design storm may be shown on the construction drawings. <br />Calculations for the elevation for the hydraulic gradient shall be provided with the design storm <br />drainage calculations. The hydraulic gradient must be below the gutter line for the design storm. <br />The tailwater elevations based on a 25-year frequency in the outfall channel shall be used for <br />calculations of the hydraulic gradient. If the 25-year WSEL is not available then use 80% of the <br />ultimate channel height. In areas which are tidally influenced the mean high tide elevation will be <br />used unless it is lower than the 25-year WSEL, in which case the higher value will be used. In <br />instances where the 25-year WSEL in the receiving channel is lower than the top of pipe then the <br />top of pipe shall be used as the starting point for HGL calculations. <br /> <br />5.3 USE OF PREVIOUSLY DESIGNED AND INSTALLED INFRASTRUCTURE <br /> <br /> Situations where previously installed infrastructure is in place but not yet utilized to its design <br />capacity will be considered on a case by case basis. The developers engineer shall after <br />consultation with the City, prepare a report that: <br /> <br /> a. outline the original design criteria <br />b. evaluates the impact of the original design on the receiving stream, adjoining <br />properties and/or the 100 year Floodplain. <br /> <br />Based on the report, the City Engineer may allow full or partial use of the previously installed <br />infrastructure and may require it to be supplemented with detention or other facilities. <br /> <br />5.4 STORM FREQUENCY, RUNOFF AND DATUM <br /> <br />A. Storm Frequency <br /> <br />All drainage improvements shall be designed for the following storm frequencies based <br />on Atlas 14 rainfall data for Harris County Region 3. <br /> <br /> Type of Facility <br /> Road Side Ditches and Culverts 5 year <br /> Storm Sewers 5 year <br /> Ditches & Culverts Drainage 100 acres and more 25 year <br /> Bridges 100 year <br /> Creeks 100 year <br />Stormwater Criteria Page 2 of 10 <br /> <br />