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Table 14 North Avenue H Overflow Rates Proposed Conditions <br />Study <br />Overflow Rate (cfs) <br />Point <br />100-yr <br />50-yr <br />25-yr <br />10-yr <br />5-yr <br />2-yr <br />1 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />2 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />3 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />4 <br />36.67 <br />0.15 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />Total: <br />36.67 <br />0.15 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />Table 15 North Avenue H Overflow Rates Difference (PR EX) <br />Study <br />Overflow Rate (cfs) <br />Point <br />100-yr <br />50-yr <br />25-yr <br />10-yr <br />5-yr <br />2-yr <br />1 <br />-11.63 <br />-9.84 <br />-7.87 <br />-5.81 <br />-4.36 <br />-1.08 <br />2 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />3 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />0.00 <br />4 <br />-37.23 <br />-46.45 <br />-28.25 <br />-10.01 <br />-2.23 <br />-3.01 <br />Net: <br />-48.86 <br />-56.29 <br />-36.12 <br />-15.83 <br />-6.59 <br />-4.09 <br />With the proposed drainage improvements to the airport property, the overflow rate into the North <br />Avenue H system was decreased by approximately 49 cfs for the 100-year storm event and <br />decreased completely (approximately 7 cfs) for the 5-year storm event. Table 15 presents the net <br />change in the overflow rate for all storm events studied. The largest decrease in overflow rate <br />occurred at Study Point 4, where the two northern ditches converge into one drainage ditch. The <br />runoff is contained within the channel for 25-year storm event and the flooding depth is limited to <br />0.08 ft. during the 50-year storm event, and 0.51 ft. during the 100-year storm event. <br />m Conclusions and Recommendations <br />Given the current conditions, runoff overflows from the La Porte Municipal Airport property into <br />the bordering western and northern neighborhoods generally at the 5-year storm event. With the <br />proposed improvements currently recommended to the TxDOT Aviation Division for the airport <br />drainage system, there will be a significant reduction in storm water overflow to the neighborhoods. <br />The improvements to the drainage ditches along the west and north sides of the airport property <br />will help contain the airport runoff and convey it to the outfall system. The runoff will generally <br />contain in a channel for storm event up to 25-year return period. <br />It is the opinion of HT&J that the proposed improvements to the La Porte Municipal Airport <br />drainage system will be beneficial to the City of La Porte. The improvements will minimize the <br />impacts of excess floodwaters from the airport property and keep the neighborhood drainage <br />systems from becoming overwhelmed during large storm events. The peak runoff in a 100-year <br />storm event will be reduced by approximately 29 cfs along Farrington Drive and by <br />approximately 49 cfs along North Avenue H. <br />La Porte Municipal Airport - Offsite Drainage Analysis <br />HT&J, LLC, January 2018 Page 21 <br />