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Table 3 Harris County Hydrologic Region 3 Rainfall (inches) <br />Storm Frequency <br />Duration <br />2- r <br />5- yr <br />10- yr <br />25- yr <br />50- yr <br />100- r <br />250- r <br />500- yr <br />5 Minutes <br />0.7 <br />0.8 <br />0.9 <br />1.0 <br />1.1 <br />1.2 <br />1.3 <br />1.4 <br />15 Minutes <br />1.1 <br />1.4 <br />1.5 <br />1.7 <br />1.9 <br />2.1 <br />2.3 <br />2.5 <br />30 Minutes <br />1.5 <br />1.9 <br />2.1 <br />2.4 <br />2.7 <br />3.0 <br />3.4 <br />3.7 <br />60 Minutes <br />2.0 <br />2.5 <br />2.9 <br />3.4 <br />3.8 <br />4.3 <br />5.0 <br />5.5 <br />2 Hours <br />2.4 <br />3.1 <br />3.7 <br />4.4 <br />5.0 <br />5.7 <br />6.8 <br />7.7 <br />3 Hours <br />2.7 <br />3.5 <br />4.2 <br />5.1 <br />5.9 <br />6.8 <br />8.2 <br />9.4 <br />6 Hours <br />3.2 <br />4.4 <br />5.3 <br />6.6 <br />7.7 <br />9.1 <br />11.2 <br />13.1 <br />12 Hours <br />3.8 <br />5.3 <br />6.4 <br />8.0 <br />9.5 <br />11.1 <br />13.6 <br />15.9 <br />24 Hours <br />4.5 <br />6.4 <br />7.8 <br />9.8 <br />11.6 <br />13.5 <br />16.6 <br />19.3 <br />2 Days <br />5.3 <br />7.5 <br />9.0 <br />11.2 <br />13.1 <br />15.1 <br />18.1 <br />20.7 <br />4 Days <br />6.2 <br />8.7 <br />10.5 <br />12.9 <br />14.8 <br />16.9 <br />19.8 <br />22.3 <br />The Rational Method Runoff Coefficient for each drainage area was calculated with land cover <br />imperviousness data downloaded from the National Land Cover Database and verified with the <br />latest aerial photographs. The average percent imperviousness was converted to a runoff <br />coefficient (C) value decimal with 100 percent impervious equal to a 0.9 C-value and zero percent <br />impervious equal to 0.2 C-value. <br />In order to fully integrate the system and define the overall drainage impacts, XPSWMM models <br />were set up for the airport site and northern bordering neighborhood with links and nodes <br />representing the drainage areas and drainage structures. <br />The Green-Ampt Method was used to calculate infiltration losses, and Harris County Region 3 <br />loss parameters from HCFCD design manual were used. Table 4 presents the parameters. <br />Table 4 Harris County Hydrologic Region 3 Loss Parameters <br />Parameter <br />Value <br />Initial Loss (in) <br />0.100 <br />Moisture Deficit <br />0.385 <br />Suction (in) <br />12.45 <br />Conductivity (in/hr) <br />0.024 <br />For each individual drainage area, the Clark Method was used to develop a hydrograph. Time of <br />Concentration (TC) was calculated using the Kerby-Kirpich Method, per the TxDOT Hydraulic <br />Design Manual Chapter 4 Section 11. <br />Where: <br />Tc = Toro + Tch <br />Toro = overland f low time (min) = K(L x N)o.467S-o.235 <br />La Porte Municipal Airport - Offsite Drainage Analysis <br />HT&J, LLC, January 2018 Page 7 <br />