Historic, Archive Document Do assume not content reflects current scientific knowledge, policies, or practices. aTC529 .R47 FED STATES DEPARTMENT OT AGRICULTURE 02S Agricultural Research Service National Sedimentation Laboratory Channel and Watershed Processes Research Unit Oxford, Mississippi Acoustic Imaging of Sediment Impounded Within USDA-NRCS Flood Control Dams, Wisconsin 200kHz Dam 735800 Easting(m) By John A. Dunbar, Paul D. Higley, and Sean J. Bennetts Research Report No. 30 August 2002 Executive Summary Since 1948, the USDA-NRCS has constructed nearly 11,000 upstream flood control dams in 2000 watersheds in 47 states, most with a design life of50 years. The watershed projects, which represent a $14 billion infrastructure, have provided flood control, municipal water supply, recreation, and wildlife habitat enhancement. Because of population growth and land use changes through time, sediment pools are filling, some structural components have deteriorated, safety regulations are stricter, and the hazard classification for some dams has changed. Presently, 42 dams in Wisconsin are in need ofimmediate rehabilitation at an estimated cost (1999 dollars) of$3 million. Before any rehabilitation strategy can be designed and implemented, the sediment impounded by these dams must be assessed in terms of the structure’s efficiency to regulate floodwaters and the potential hazard the sediment may pose ifreintroduced into the environment. At the direct request ofthe USDA-NRCS in Wisconsin, two reservoirs, White Mound Lake located in Sauk County and Twin Valley Lake located in Iowa County, were chosen as part of their statewide assessment program. To this end, this study was undertaken to determine the amount and distribution of post-impoundment sediment contained within these selected flood control reservoirs using an acoustic profiling system. The acoustic profiling system used in the surveys was developed in collaboration with Specialty Devices, Inc. of Plano, TX. The complete system consists of one, suitcase- sized, 10 kg, water resistant control module, a 50 cm long, 15 kg acoustic source array, GPS and differential correction antennas, and associated cables. The system images the bottom and sub-bottom sediments with acoustic signals produced at 200, 48, and 24 kHz, deployed from a single Johnboat. The results from the two surveys were quite similar. The acoustic response ofsediments onlapping the dam face shows three distinct acoustic layers. The upper-most layer, which appears white on the multifrequency displays, corresponds to fme-grain, water-rich sediments. The layer beneath the first, which appears light blue in the display, preferentially fills the lows and channel axes, and thins and becomes more restricted upstream. The bottom-most layer deposited on the dam face, which appears dark blue in the display, is relatively thin in both reservoirs, but reaches considerable thickness in the channel axes. At Twin Valley Lake, the sediment isopach map indicates a maximum thickness of 3.1 m, in a small area along the main channel axis, and a total volume ofpost-impoundment sediment of 458,659 m (371.8 acre-ft). At White Mound Lake, the sediment isopach map indicates a maximum thickness of 1.4 m (4.6 ft) and a total volume of post- impoundment sediment of 203,205 m3 (164.7 acre-ft). These values represent conservative estimates of sediment accumulation without corroboration with ancillary data. 2 Table of Contents Executive Summary 2 Acknowledgments 6 1. Introduction 7 1.1 Federal Program for Flood Control 7 1.2 Current Status ofSmall Watershed Program 8 1.3 USDA-NRCS Flood Control Structures in Wisconsin 8 1.4 Problem Statement 8 2. Field Sites 10 2.1 White Mound Lake 10 2.2 Twin Valley Lake 12 3. Acoustic Survey System 15 3.1 Acoustic Profiling System 15 3.2 Survey Procedure 17 3.3 Digital Processing and Interpretation ofCollected Acoustic Data 17 4. Results 20 4.1 Acoustic Survey ofTwin Valley Lake 20 4.2 Acoustic Survey ofWhite Mound Lake 26 4.3 Discussion 31 5. Conclusions 32 6. References 34 3 1 List ofFigures Figure 2-1. Map ofWhite Mound Lake Park and environs (from Wisconsin Department ofNatural Resources) 10 Figure 2-2. Orthographic composite photographic ofWhite Mound Lake, WI. Coordinates are in meters (UTM Zone 15) 1 Figure 2-3. The photograph ofWhite Mound Lake taken from the boat ramp area and m facing south. Areas ofthe lake with 4 ofwater depth or less contain aquatic vegetation that extends from the bottom to the lake surface as shown in distance. 12 . Figure 2-4. 1967 base map ofTwin Valley Lake (from Wisconsin Department ofNatural Resources) 13 Figure 2-5. Orthographic composite photographic ofTwin Valley Lake, WI. Coordinates are in meters (UTM Zone 15) 14 Figure 3-1. Photograph ofacoustic profiler control module. All acoustic profiling and DGPS navigation electronics are contained in a compact, water-resistant control module 16 Figure 3-2. Picture ofacoustic profiler transducer array. The profiling system produces acoustic signals with three widely separated frequencies (200, 48, and 24 kHz) in rapid succession as the profiles are traversed 17 Figure 3-3. Picture ofaquatic vegetation. Both White Mound Lake and Twin Valley Lake contain vegetation that extends from the water bottom to the surface in areas of m 4 water depth and less. The vegetation scatters the 200 kHz acoustic signals. ... 19 Figure 4-1. Profile map ofTwin Valley Lake acoustic survey. The survey contains 17 km ofprofile lines. Profiles 1 1 and 52, highlighted in red, are shown as example profiles in accompanying figures. Coordinates are in meters (UTM Zone 15) 21 Figure 4-2. Southeast end ofTwin Valley Lake axial profile, Line 52 (see Figure 4-1 for line location). Three acoustic intervals are seen within the sub-bottom in Twin Valley Lake. All three onlap onto the dam face. Hence, all three must be part ofthe post-impoundment sediment fill package 22 Figure 4-3. Processed Line 1 1 with interpretation (see Figure 4-1 for line location). The multifrequency acoustic profiles show three distinct sediment layers within the post- impoundment fill 23 Figure 4-4. Contour map ofwater depth in Twin Valley Lake, WI. Depth is measured m from the water surface at an elevation of277.4 (910 ft). Coordinates are in meters (UTM Zone 15) 24 Figure 4-5. Contour map ofsediment thickness in Twin Valley Lake, WI. Sediment thickness is measured from the water bottom to the interpreted base ofpost- impoundment fill. Coordinates are in meters (UTM Zone 15) 25 Figure 4-6. Profile map ofWhite Mound Lake acoustic survey. Profiles 25 and 50, highlighted in red, are shown as example profiles in accompanying figures. Coordinates are in meters (UTM Zone 15) 26 Figure 4-7. Southeast end ofWhite Mound Lake axial profile, Line 50 (see Figure 4-6 for line location). Three acoustic intervals are seen within the sub-bottom in White Mound Lake. All three onlap onto the dam face. Hence, all three must be part of the post-impoundment sediment fill package 27 4