07-09-18 Drainage and Flooding Committee Meeting

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IML seal coat surfacing of aprons is also removed from the recommended improvements. These reductions reduce the overall project cost and is discussed in detail below in Section 8. 6.2. DRAINAGE IMPROVEMENTS FAA requires no encroachment of runoff on runway and taxiway pavements for the 5-year storm event. The center 50-percent of runways and taxiways is to be kept free of ponding resulting from the 10-year storm event. This report takes 10-year storm event as a design storm event and provides recommendations to keep the runways and taxiways free from the runoff encroachment for storm events up to and including the 10-year storm event for the reasons as noted in Section 2.4. Additionally, we have evaluated the cost impact of selecting 10-year storm event as a design event. We conclude the cost impact is very minimal. All the structural improvements recommended for 10-year event are necessary even for 5-year storm event. The selecting 5-year event as design event would result in approximately 2 ac-ft. of less detention volume, equivalent of about $26,000. It is recommended that the airport pursue the proposed enhancements to the drainage system in order to eliminate ponding over the runways for storm events less frequent than the 10-year event. Additionally, the proposed improvements will lower the impact to airport operations during the more severe storm events. The recommended improvements consist of (See Exhibit 8): 1) Modification of 7,600 LF of drainage ditches including 1,600 LF of linear detention 2) Upsizing of 12 drainage structures totaling 860 LF The anticipated construction cost of all improvements for drainage is approximately $612,000 dollars. Of the total, modification of open channels and linear detention would cost about 45% and modification to structures would cost approximately 55% of the total estimate. The cost estimate includes a 20% contingency. Due to the limitations of funding, it is recommended that approximately'/4 of the total design volume of detention be constructed in the project. This will bring the recommended drainage improvements total cost to approximately 351,800, which is much closer to the programmed $330,000 budget level. 6.3. ALTERNATES The former apron pavement near the end of Runway 30 is currently not scoped in the base project for pavement rehabilitation. This apron serves the self -serve fueling on the airport and should be preserved, even though it was not identified in the project. This pavement can be placed into the construction project as an additive alternate. Based on available budget and local interest, this pavement could also be seal coated with the other construction on the airport. Page 133