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APPENDIX A AIRPORT DR~INAGE STUDY AIRPORT MASTER PLAN SAN MARCOS, T~~S The purpose of airport drainage is to dispose of storm water which may hinder any activity necessary to the sa=e and efficient operation of the airport. The drainage sys- tem should collect and remove surface water runoff from all operational areas of the airport. The most dangerous consequences of an inadequate or poorly functioning drain- age system involve saturation of the subgrade and subbase with subsequent loss of load-bearing capacity of the paved surfaces and excessive ponding promoting unsafe operation of the airport. Several "trouble areas" -- where storm water was draining poorly and was ponding -- were pointed out to the consul- tant by o==icials of the City and the Airport COQffiittee. A drainage system capacity analysis, therefore, was initi- ated to determine the efficiency of the system design. Although the inlet and pipe system design appears adequate to drain a five year storml, the grade differentials between the paved areas and the inlet tops are extremely small, cre- ating slow runoff patterns. In many areas plow patterns generated by agricultural tilling have virtually eliminated natural runoff characteristics. In the course of the inves- tigation, however, we found no evidence of pipe or inlet obstructions, failures, high or perched water tables, or any other similar condition detrimental to storm water floH. We, therefore, offer the following recommendations to improve runoff capacity and to reduce the chance of further subbase deterioration and/or ponding. Locations of areas referenced in this section can be found in Figure 1 on the following page. 1. hIe recommend discontinuing land-use for agricultural purposes on those interior land parcels surrounded by airfield paving. Since the airfield area is relatively flat, 1 A maximum rainfall expected once in 5 years is generally recommended for estimating runoff for airports. The damage or inconvenience which may be caused by greater storms is insufficient to warrant the increased cost of a drainage system large enough to accommodate a storm expected once in a period longer than five years.