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Table 2 Rainfall Intensity Coefficients <br />Rain Event Frequency <br />b <br />d <br />e <br />2-year <br />68 <br />7.9 <br />0.800 <br />5-year <br />70 <br />7.7 <br />0.749 <br />10-year <br />81 <br />7.7 <br />0.753 <br />25-year <br />81 <br />7.7 <br />0.724 <br />50-year <br />91 <br />7.7 <br />0.728 <br />100-year <br />91 <br />7.9 <br />0.706 <br />The Kerby-Kirpich Method was used in adherence with the TxDOT Hydraulic Design Manual <br />Chapter 4 Section 11 to calculate the Time of Concentration for each basin. The peak flow values <br />were used to determine the storage coefficient for each individual drainage area on site. <br />City of La Porte Design. Criteria <br />The City of La Porte Public Improvement Criteria Manual (PICM) Chapter 5, Storm Water Design <br />Criteria, was reviewed. The design event for storm sewer systems is the 5-year storm event, with <br />requirements to keep the hydraulic grade line below the gutter elevation. <br />Hydrologic Analysis <br />Hydrologic calculations were performed in accordance with the TxDOT Hydraulic Design Manual, <br />in order to be consistent with on -site and off -site analysis. The Rational Method was used to <br />calculate peak flows using the TxDOT rainfall intensity coefficients in Table 2. Contributing <br />drainage area boundaries were established based on the LiDAR data from HGAC and topographic <br />survey and drainage culverts detail data provided by the Gessner Engineering survey. Rainfall <br />totals were obtained from the Harris County Flood Control District H&H Manual for the purpose <br />of developing hydrograph and model simulation, and can be found in Table 3. <br />La Porte Municipal Airport — Offsite Drainage Analysis <br />HT&J, LLC, January 2018 Page 6 <br />