Drainage Report. New Braunfels Municipal Airport. Master Plan Update 2005

Transcription

1 Drainage Report Master Plan Update 2005 General The Texas Department of Transportation (TxDOT), Aviation Division, retained the consulting engineering team of Parkhill, Smith & Cooper, Inc. (PSC), to prepare a drainage plan as part of the overall (NBMA) master planning process. This report details the approach, results, and recommendations for that drainage plan. As identified in the consultant s agreement with the TxDOT Aviation Division, the scope of work includes: 1. Evaluation of the existing airport drainage patterns. 2. Evaluation of the condition of the airport s culverts, storm sewers, ditches, and waterways. 3. Determine the existing capacity and adequacy of the airport s drainage features to carry design storm flows. 4. Present an overview of the existing drainage system and brief recommendations on necessary improvements. PSC was also asked to provide finish floor elevation recommendations for future hangar installations. These recommendations have been provided as those hangar areas develop, and are not presented in this report. Drainage Area A hydrologic evaluation was first performed to determine the extent of the airport s watershed. The airport s drainage patterns primarily contribute runoff to three different watersheds. Most of the airport property lies in the drainage area of Alligator Creek, in Guadalupe County. Alligator Creek itself lies northeast of Runway 13-31, and eventually feeds downstream into Geronimo Creek. A second portion of the airport area drains southeastward to Geronimo Creek. The head of Geronimo Creek lies to the southeast of the terminal building area, and this flow is carried under FM 758 by state highway department culverts. Geronimo Creek eventually drains into the Guadalupe River near Seguin, Texas. A third portion of the airport drainage contributes to an unnamed tributary south of the airport. This tributary flows through, or near, Clear Spring in Guadalupe County. This tributary drains into the Guadalupe River upstream of Lake McQueeney. The detailed study area for NBMA encompasses 462 acres of drainage area. Watersheds determined not to contribute to the airport drainage system were not included in the analysis. Maps The consultant used two-foot interval contoured survey data for the airport property, proper. Where on-the-ground topographic survey data was lacking, five-foot interval USGS contour information was used to verify the overall airport drainage area. New Airport Master Plan Page I-1 May 2008

2 Flood Zones The northeast boundary of the airport property is affected by the floodplain for Alligator Creek. The effective Federal Emergency Management Agency (FEMA) map for 2006 of this reach of Alligator Creek was compared to the previous 1986 effective flood zone map. The 2006 effective mapping was undertaken by FEMA to upgrade Comal County to the Digital Flood Insurance Rate Map (DFIRM) version that FEMA is migrating to. The flood zone boundaries between the effective map versions were compared and considered essentially identical for all practical purposes. A formal map revision for Alligator Creek adjacent to the airport was not completed by FEMA for the most recent effective flood zone maps. The mapped flood zone falls under the Federal Emergency Management Agency s regulations and the City of New Braunfels Code of Ordinances for floodplain management. The flood zone, as shown in the FEMA effective map, encroaches onto the airport property to near the existing row of hangars northeast of Runway 13-31, on the northeast side of the airport. The consultant contacted the City of New Braunfels Floodplain Manager, the FEMA contract consultant for the federal flood zone programs administration, and the contract consultant doing the DFIRM upgrade for Comal County. No detailed hydrologic and hydraulic analyses for Alligator Creek are available. The effective FEMA Special Flood Hazard Areas (the floodplain) was superimposed onto the airport aerial photography coverage. This is shown in Exhibit I-1. Criteria FAA Advisory Circular 150/5320-5B requires that drainage structures be designed to drain a 20 percent probability (5-year) precipitation event in 48 hours or less. Also, ponding areas edges are to be kept a minimum of 75 feet distance from edges of aircraft-use pavement. Inlets at these ponding areas are to be at least 75 feet from pavement edge for airports with scheduled air carrier service operations. If general aviation only is using the airport, then the inlets may be placed as close as 25 feet to the pavement edge. However, with ponding, the inlets may actually have to be located at least 75 feet from the pavement edges to meet the ponding edge requirement. Also affecting the design for hydrology and hydraulics are the City of New Braunfels requirements contained in the City of New Braunfels Drainage and Erosion Control Design Manual (Rev. September 2000). Within that manual, the criteria relates to post-developed versus pre-developed precipitation peak discharges. As demonstrated by the May 2006 control tower and hangar construction, the City of New Braufels intent is to apply the Drainage and Erosion Control Design Manual criteria to the NBMA proposed developments and improvements. The criteria from the New Braunfels manual, in summary, is: Airport developments and improvements are likely to fall under defined Type 3 development. Post-developed peak discharge not greater than pre-developed discharge for the 10 percent (10-year) and one percent (100-year) rainfall events. Detention areas must be emptied within 36 hours unless designed as a permanent water body. New Airport Master Plan Page I-2 May 2008

3 Nonerosive discharge velocities at outlets, or install erosion prevention or energy dissipation measures. Maintain natural discharge areas or points. Existing Storm Drainage System The existing storm drainage pipeline network is, for the most part, the same installation that was constructed in the 1940 s for the airfield to conduct military pilot training. The overall construction reflects the practices of that era while under a wartime emergency footing. The storm drainage pipeline network consists of three separate systems, each with its own outlet near the northeast property boundary. Each pipeline system appears to be precast concrete pipe. It is unknown what joint materials were used to seal the joints or under what standards, if any, the pipe was manufactured, or whether it contains any steel reinforcement. The appearance of the pipe interior at each outlet is in good shape given the pipe age and length of service. The pipe grades are such that for the upstream reaches of each system the pipeline has minimal cover below the ground surface. This severely limits the continued use of the drainage systems since it is unlikely the existing pipeline grades and locations will be compatible with new proposed airport improvements. However, from the pipeline interiors observed, the existing pipeline grades appear adequate to minimize siltation in the pipe inverts. The existing inlets are primarily drop inlets with the surface grating being comprised of precast concrete inserts that, once installed, leave rectangular openings for the entry of water and the exclusion of larger debris. It was evident from the consultant s site visit that the inlets were more or less sized to discharge runoff over time rather than carry the peak discharge as it occurred. The existing inlets showed evidence of catching and retaining a significant amount of farm crop residue, such as corn and sorghum plant stalks carried in the runoff reaching the inlets. For areas not served by the existing storm drainage pipeline network, the runoff is carried via overland flow. Northeast of the existing hangars, the surface runoff flows directly to Alligator Creek. In the existing terminal area, the surface runoff flow is southeastward toward FM 758 and some highway department culverts. South of the runways, there is overland flow toward an unnamed tributary that lies south of the airport. Existing System Analysis In order to analyze the drainage conditions for the entire airport, computer model simulations were required. A dynamic model software package known as Interconnected Channel and Pond Routing (ICPR) was used for the analyses. Models were defined to simulate existing conditions. ICPR is a FEMA-accepted hydrologic and hydraulic software package distributed by Streamline Technologies of Florida. Precipitation values associated with return intervals of 20 percent, 10 percent and one percent (5-, 10-, and 100-year events respectively), 24-hour durations, were obtained from the City of New Braunfels Drainage and Erosion Control Manual and were applied to the models. The hydrologic simulations and resultant runoff discharges were calculated using the NRCS Unit Hydrograph method. The simulated runoff discharges were then routed through the hydraulic parameters of the storm sewer pipeline systems. The detailed study area for NBMA encompasses a total of 462 acres of local drainage area, or watershed, which was then subdivided into 37 sub-watersheds. These sub-watersheds are New Airport Master Plan Page I-3 May 2008

4 primarily created by the inlet locations for the existing storm drainage pipeline and inlet systems. Watersheds that were determined as not contributing to the airport drainage system were not included in the storm sewer pipeline analyses. Upon hydrologic and hydraulic analysis, it was determined that many of the airport s hydraulic structures were designed to convey only the 20 percent (5-year) probability precipitation event. Under war-time emergency, drainage systems were often standardized as much as possible for speed and economy of construction. Also, military flying is often undertaken in conditions that would not be conducive to civilian flights. Therefore, the concept that the military probably just wanted a drying out drainage system lends credence to the capacity limitations of the existing storm drainage pipeline systems. The military training flights would have been only temporarily suspended while runoff waters were allowed to recede. Although the results are compatible with the FAA Advisory Circular criteria (5-year capacity), the function of the existing storm drainage system does not meet the City of New Braunfels criteria for 10-year and 100-year post-development detention. With additional development and infrastructure improvements anticipated, the post-development runoff detention criteria for the City of New Braunfels have to be accounted. In addition, the locations of several of the existing systems inlets are in conflict with either proposed runway extensions or additional taxiways. Thus, some of these features will have to be replaced due to physical conflict with other improvements. The two additional events, the 10 percent probability (10-year) and one percent probability (100- year) rainfall events, were examined to provide insight into flooding resulting from more intense, less frequent storms. The 100-year rainfall event results were useful to validate model results. For calculated peak water surface elevations (WSEL) refer to the exhibits included with this report. Peak discharges in excess of what either the inlets or the pipelines can accommodate simply overflow those locations and proceed via surface flow to the property boundaries. Often this means that these overflows overtop some of the runways and taxiways, and flow out-ofbank where channels and ditches exist. During the analysis of the existing system it was discovered that the military in the 1940 s may have directed additional runoff, via the existing storm sewers, to Alligator Creek. The current contours indicate that the additional runoff actually belongs to a watershed to the south. This needs to be corrected as part of the planned airport improvements. Proposed Improvements Proposed improvements will be identified and developed as the overall airport Master Plan develops. Some of the items and concepts being evaluated as part of the general airport planning process are: Infield area between runways are not proposed for development probably little hydrologic change there unless new taxiways or apron pavements. Runway ends require clear zones, therefore little hydrologic change anticipated there. City requirement for 36-hour emptying is compatible with FAA AC for 48-hour maximum emptying of ponded areas. Look at detention in upstream reaches where first costs are potentially lowest. Right now it seems that detention parallel to the proposed taxiway for Runway might be feasible, but storm drain replacement will be necessary. (Conflict with taxiway location). Grades will have to be evaluated. New Airport Master Plan Page I-4 May 2008

5 Depending on development, might need open channels to the southeast and south to serve as linear detention and provide a place for development to grade to for drainage. Look at a quick overall hydrograph for Alligator Creek, and see if detention north of Runway hangars can be avoided (hydrograph timing). Development in this area severely restricted because of floodplain regulations. Detention facilities located northeast of hangars liable to impact floodplain, and facilities might get rather expensive this close to the bottom of the airport sub-watersheds. Look at overall hydrograph to point of discharge on unnamed tributary to the south. Results might indicate minimal detention. Detention to the south, and possibly the southeast, might require pumps to empty detention basins until such time as a roadway drainage system is constructed. Items in favor, analytically speaking: o Hydrologic group of existing soils not too far removed from pavement runoff characteristics. o AMC III NRCS runoff condition indicates a smaller infiltration to runoff ratio, thus bringing pre-developed discharges somewhat closer to post-developed discharges. Time of concentration changes between pre-developed and postdeveloped conditions is one of the biggest contributors to discharge differences. o City s pre-developed versus post-developed discharge policy should help mitigate Alligator Creek flood zone from getting too much worse. Layout of taxiways, areas proposed for development still need better definition. These affect available grades and potential locations of drainage facilities and detention zones. New Airport Master Plan Page I-5 May 2008