Source: BAYLOR UNIV submitted to NRP
VARIABLE FREQUENCY ACOUSTIC PROFILING FOR SEDIMENT SURVEYS OF FLOOD CONTROL RESERVOIRS
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
COMPLETE
Funding Source
NRI COMPETITIVE GRANT
Reporting Frequency
Annual
Accession No.
0192811
Grant No.
2002-35102-12383
Cumulative Award Amt.
(N/A)
Proposal No.
2002-00593
Multistate No.
(N/A)
Project Start Date
Sep 1, 2002
Project End Date
Aug 31, 2006
Grant Year
2002
Program Code
[26.0]- (N/A)
Recipient Organization
BAYLOR UNIV
ONE BEAR PLACE
WACO,TX 76798
Performing Department
(N/A)
Non Technical Summary
Over the next 10 years 1,800 of the nearly 11,000 flood control reservoirs built in the U.S. by the USDA-NRCS since 1948 will reach their design life and will require some form of rehabilitation. Prioritizing these reservoirs for rehabilitation and choosing between rehabilitation strategies will require assessment of the amount of sediment they contain, as well as the physical and the chemical properties of the sediment. However, conducting these assessments will pose a difficult surveying problem that will require special tools to solve, because of the sheer number of flood control reservoirs and their limited water depth. The purpose of this project is to develop a new acoustic profiling system that is optimized for sediment surveys of flood control reservoirs. We will test different acoustic techniques for determining sediment thickness and sediment texture in flood control reservoirs using multiple discrete frequencies. The goal will be to produce an integrated acoustic system that can be deployed quickly in small lakes without normal boat access. Surveys should require no more than a few hours to complete, allowing multiple reservoirs to be surveyed in one day. For determining the sediment volume, the system will need to clearly image the water bottom and the base of sediment through up to 10 m of sediment fill. Special processing software will be written to make the post-survey analysis efficient. We will test the new system by conducting sediment surveys of flood control reservoirs in a range of physiographic settings throughout the U.S.
Animal Health Component
100%
Research Effort Categories
Basic
(N/A)
Applied
100%
Developmental
(N/A)
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
11203992020100%
Knowledge Area
112 - Watershed Protection and Management;

Subject Of Investigation
0399 - Watersheds and river basins, general;

Field Of Science
2020 - Engineering;
Goals / Objectives
The objective of the proposed research is to develop an optimized acoustic surveying system for characterizing impounded sediments typical of USDA-NRCS flood control reservoirs. Since 1948, local communities, assisted by the USDA-NRCS, have constructed nearly 11,000 small flood control dams on tributary streams in 47 states. At present, more than half of these dams are older than 34 years and more than 1,800 will reach their 50-year design life within the next 10 years. A rapid survey conducted in April 1999 by the USDA-NRCS revealed more than 2,200 dams are in need of immediate rehabilitation. However, before any rehabilitation strategy can be designed and implemented, the USDA-NRCS needs to determine the volume of sediment deposited within these reservoirs and the quality of the sediment with respect to agrichemicals and contaminants. Because of the sheer number of flood control reservoirs, their small size (ca. 35 acres), and limited water depth (ca. 1 to 2 m), they pose a difficult surveying problem that will require special tools to solve. The conventional approach to estimating sediment volume in reservoirs is to measure the change in water storage capacity over time. This is done by repeating bathymetric surveys along profiles established shortly after initial impoundment using acoustic fathometers and differential GPS navigation. However, in the case of small flood control reservoirs, initial surveys were seldom done and complete records from the surveys that were done are, in many cases, no longer available. Hence, new surveys of aging flood control reservoirs will need to determine sediment volumes directly, without reference to previous surveys. Achieving acoustical discrimination between post-impoundment sediments and the underlying pre-impound alluvium is key to accomplishing this goal. The surveying system developed by this project will be one that is compact and that can be deployed quickly in small water bodies without previsions for boat access. Surveys made with the system will require no more than a few hours to complete, allowing multiple reservoirs to be surveyed in one day. To provide adequate spatial coverage in shallow reservoirs, the system will produce usable data in a few centimeters of water as well as in water many meters deep. To measure sediment volume, the system will to clearly image the water bottom and the base of sediment fill. Finally, as an aid in mapping potential contaminants in the sediments between core samples, the system will be able to image the internal stratigraphy within the sediment fill.
Project Methods
The approach taken in this study will be to develop a sub-bottom profiling system that combines multiple, widely spaced acoustic frequencies into a single optimum image of the water and sediment column. This approach is based on the phenomenon of frequency-dependent volume scatter exhibited by unconsolidated sediments. Volume scattering of acoustic signals occurs as the signals encounter inhomogeneities within the sediment column. These inhomogeneities may be caused by local differences in compaction or grain size. Scatter levels are small for signal wavelengths that are much larger than the spatial dimensions of the inhomogeneities and reach a peak at signal wavelengths that match the dimensions of the inhomogeneities. Prior experience in water supply reservoirs indicates that low levels of volume scatter occur in reservoir fill sediments at signal frequencies of a few kilohertz, intermediate levels occur at frequencies of a few tens of kilohertz, and high levels of scatter occur at signal frequencies of a few hundred kilohertz. In contrast, high scatter levels occur at all these signal frequencies within the more compacted pre-impoundment materials. The best combination of frequencies to use to discriminate between pre- and post-impoundment materials depends on the texture of the reservoir sediment and the nature of the pre-impoundment surface. Given the broad range of conditions found in flood control reservoirs across the U.S., it is unlikely that a fixed set of signal frequency will work in all cases. To address this problem a new variable-frequency survey system will be built that will make simultaneous recordings in different, selectable, signal bands. A specially designed composite acoustic transducer, made of many small transducers imbedded in an elastomer, will be developed that will generate both high- and low-frequency signals in shallow water. Data in the different signal bands will be color-coded as red, green and blue intensities in multi-frequency color displays. The profiler electronics, plus a differential GPS navigation system will be integrated into one briefcase-size box. Special processing and interpretation software will be developed to make post-survey analysis efficient. Field tests will be conduced over the range of U.S. physiographic and geologic provinces that contain flood control reservoirs. The goal of these tests will be to select a set of signal bands and extracted data characteristics that optimally define the water bottom, base of reservoir sediments and the internal stratigraphy of flood control reservoir sediments. An extensive program of vibracoring will be conducted along with the acoustic tests to validate the results.

Progress 09/01/02 to 08/31/06

Outputs
The goal of this project was to develop a new acoustic profiling system for conducting surveys of flood control reservoirs. Nearly 11,000 small flood control reservoirs have been built in the U.S. by the USDA-NRCS since 1948. In the next 10 years 1,800 of these structures will reach their design life and will need to be surveyed to determine the amount of sediment they contain. The survey system we developed addresses the practical problem of conducting sediment surveys in hundreds of small, shallow, and highly vegetated reservoirs in a time efficient and cost effective manner. Four subtasks were identified in the project: (1) designing and constructing the survey system itself, (2) developing a suitable survey vessel, (3) developing special-purpose software for post-survey data analysis, and (4) validation of the system and methods in flood control reservoirs from around the U.S. In the following sections we describe progress made in each of the tasks during this project. The development of the acoustic survey system was done in collaboration with Specialty Devices Inc. of Wylie Texas (SDI). During the project three prototype sub-bottom profiling systems were developed and tested. The final prototype uses signals at 12, 25, 50, 125, and 200 kilohertz to image the water bottom and base of sediment. The sub-bottom profiling system and integrated navigation system are housed in a single, suite-case size box that can be deployed in small the boats needed to survey flood control reservoirs. Now that development is complete, additional copies of the system are available from SDI. To survey flood control reservoirs it is necessary to collect profiles in water as shallow as 15 cm and to collect sediment cores from the same vessel, without significant change over time. We tested three prototype survey vessels. The final configuration is based on an extra wide 14 ft Jon boat, with portable roller ramps, and a fold-down coring gantry. This boat is trailored to the water's edge, and then rolled down the ramps and into the water. It can be deployed and retrieved rapidly and has sufficient buoyancy for coring operations. The acoustic surveys of flood control reservoirs generate massive amounts of digital data that must be processed to determine water and sediment volumes. The processing needs to be time efficient and accurate. To facilitate survey data processing, we developed special purpose interpretation and mapping programs that together allow a user to analyze the data from a survey in about the same amount of time it takes to collect the data. Typical surveys can be collected and processed in one work day, excluding travel time. The completed flood control reservoir survey system consists of a compact sub-bottom profiling unit, a light-weight vibracoring system, a special survey vessel, and software to process the resulting data. Over the course of the project we tested different prototype configurations of this system by conducting complete surveys of USDA flood control reservoirs. To date we have surveyed 23 flood control reservoirs and 3 water supply reservoirs in Texas, Oklahoma, Arkansas, and Wisconsin.

Impacts
In this project we developed a compact acoustic surveying system that can be used to directly measure the volume of trapped sediment in USDA-NRCS PL-566 flood control reservoirs in one survey, requiring one day or less to complete. This means that it is now practical to survey large numbers of PL-566 reservoirs and to routinely monitor the status of their sediment pools. The new survey system is commercially available and in active use by a number of Federal and State agencies and private contracting firms. The USGS, for example, has used the system for the past three years to conduct surveys of PL-566 reservoirs in Iowa and Michigan. We have surveyed 23 PL-566 reservoirs in Texas, Arkansas, Oklahoma and Wisconsin and continue to conduct surveys for the State USDA-NRCS offices in Texas and Arkansas as well as city governments in both states. Together, these surveys are providing much needed data on the functional status of PL-566 flood control reservoirs and the rates at which they are filling with sediment.

Publications

  • Dunbar, J. A., P. M. Allen, Long-term Sediment Yield from the Blackland Prairie from Sedimentation in Large Water Supply Reservoirs and SCS Flood Control Structures, (abstract) Presented at the annual meeting of the Texas River and Reservoir Management Society, Austin, Texas, May18-19, 2006.
  • Dunbar, J. A., P. M. Allen, Long-term Sediment Yield and the Trap Efficiency of SCS Flood Control Reservoirs in the Texas Blackland Prairie, (abstract) Programs with Abstracts for the USDA-CSREES National Water Quality Conference, San Antonio, Texas, Feb. 7-9, 2006.
  • Dunbar, J. A., P. M. Allen, Acoustic profiling system for sediment surveys of flood control reservoirs, (abstract) Programs with Abstracts for the USDA-CSREES National Water Quality Conference, Clearwater Florida, Jan. 11-14, 2004.
  • Lindemann, E. T., J. A. Dunbar, and P. M. Allen, Sedimentation survey methodology utilizing a multi-frequency acoustic profiling system and an integrated differential GPS navigation system, National Watershed Conference, Council Bluffs, Iowa, June 8-11, 2003.