Fort Bragg Cantonment Area

1. Watersheddelineation at Fort Bragg, North Carolina using LIDAR data Beth M. Wrege,U.S. Geological Survey, Water Resources Division andMichelle Cienek, WRRI, 3916 Sunset Ridge Road, Raleigh, NC 27607; tel. (919) 571-4091; email: bmwrege@usgs.gov • Background • The USGS is working with the U.S. Army at Fort Bragg to develop a Storm Water Pollution Prevention Plan (SWP3). The Plan will provide data for analysis of stormwater runoff at Fort Bragg. • This part of the study concentrates on delineating the source of surface-water runoff to one water-quality testing site, Outfall 53. The process can be repeated for the other 13 water-quality testing sites. • The data then can be used to develop a plan that will help Fort Bragg Military Reserve develop best management practices for the reduction of surface water pollutants from Fort Bragg into the Little River and Cape Fear Watersheds. • Objective • To study the effectiveness of delineating stormwater runoff using LIDAR data Study Area ABSTRACT   In order to develop best management practices for Industrial Areas and to effectively manage their watersheds, Fort Bragg requires digital information on elevations, drainage basins, stormwater conveyance systems, streamflow, and existing manmade and natural features. This information will be used to develop a stormwater drainage basin runoff model. The stormwater basin model will serve many applications, including the evaluation of areas of high load erosion events, tracking non-point source pollution, and planning for future development on base. This pilot study focuses on the effectiveness of using LIDAR (Light Interferometric Distance and Ranging) data to develop an urban stormwater drainage basin model. LIDAR data are acquired with aircraft-mounted lasers. Aerial topographic surveys produced from the LIDAR data provide high-resolution land surface elevations. GIS data, supplemented with aerial photography and LIDAR data, are used to model the areas flowing into industrial outfall 53, a water quality sampling site. The pollutants sampled in the runoff at outfall 53 can be linked to the associated Industrial Area, Area 3-3. The LIDAR data consists of points spaced at five-meter intervals with a longitudinal, latitudinal and vertical accuracy within a measured 35 centimeters in all three dimensions. These data will allow for the creation of a digital elevation model (DEM) with a five-meter cell size that can be utilized to predict the flow accumulation and flow direction within each cell. For the pilot study, the elevation of the road and curb were raised slightly to create a DEM and a flowdirection model that account for the influence of manmade structures. Next, a map linking all known stormdrains and outfalls was digitized and the stormdrains were adjusted to fit within the cell of highest flow accumulation. The area upgradient of the stormdrains is delineated to create a predictive model of the overland flow to outfall 53. The estimated area flowing into outfall 53 by the computer model is 22.3 acres. A USGS hydrologist delineated a drainage area of 24 acres flowing into outfall 53 during field visits to the site. Close examination of the final flow map shows a few areas between the stormdrains that are not captured in the model. This relates to the flatness of the area at these points. The model does not have the sensitivity to account for this lack of elevation difference. Slight variances in elevation could not be discriminated using the LIDAR data. The next step in this pilot study will be to include the influence of pervious/impervious land cover. In conclusion, when working with very small areas with moderate to little elevation change, LIDAR data with five-meter spacing can provide a useful tool in predicting urban runoff and watershed delineation, but the resolution still may be too large to predict small variances in the landscape.   • LIDAR (LIGHT INTERFEROMETRIC DISTANCE AND RANGING) • LIDAR commonly is referred to as Light Detection and Radar • LIDAR data is collected by aerial topographic surveys that measure elevation using lasers • LIDAR is an active sensor that works by shooting lasers at the earth and measuring the pulse’s return time • The vertical precision of this LIDAR data is within 35 centimeters of reality with a 5 meter point spacing • LIDAR data currently is being flown for the entire state of North Carolina Cape Fear River Basin Fayetteville North Carolina Fort Bragg Cantonment Area Outfall 53 in meters Methods • A DEM (Digital Elevation Model), a filled DEM and tagged elevation contour lines of various intervals were created from the LIDAR point data • The elevation of the road and curb downstream of Industrial Area 3-3 were raised in the DEM to account for manmade drainage • Flow direction and flow accumulation grids were developed using this DEM • Existing storm drains and outfalls were digitized from field data • The placement of the storm drain network was modified to be consistent with the flow accumulation map created from LIDAR data • All areas flowing into the storm drain system and the water-quality testing site were delineated to produce the final storm drain map • Results • From this model, Outfall 53 drains approximately 22.3 acres. • Field inspections estimated a 24 acre drainage area. • The 5 m resolution of this LIDAR data may be too large to capture areas with small elevation changes at this scale. • Conclusions and Continuing Study • This process works well for computer modeling and approximating a drainage area. • Field inspections are still necessary to insure the accuracy of the results. • LIDAR data is a rapid means of collecting digital elevation data that can be used for hydrologic modeling. • In the future, the land cover of Outfall 53’s drainage area can be classified using aerial photography for pervious and impervious surfaces.