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United States Department of Agriculture

Agricultural Research Service

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Location: Hydraulic Engineering Research

2009 Annual Report

1a. Objectives (from AD-416)
Improve methods of predicting earthen embankment erosion and failure, and develop generalized hydraulic guidelines and tools for roller compacted concrete spillways used to protect earthen structures from erosion and increase discharge capacity. Improving methods of predicting earthen embankment erosion and failure will include sub-objectives of quantification and erosion measurement of embankment materials, quantification of protective capabilities of vegetation, development of algorithms and computational models that can be used by the profession to predict earthen embankment erosion and failure causing downstream flooding. The development of generalized hydraulic guidelines and tools for roller compacted concrete spillways will include sub-objectives of development of preliminary guidelines for dimensioning converging sidewalls as well as understanding air entrainment, flow bulking and energy dissipation leading to generalized equations for dimensioning stepped spillways, downstream basins and rip-rap protection that will be used by the engineering profession to design spillways.

1b. Approach (from AD-416)
Small-scale erosion tests and large-scale physical models will be used to develop knowledge of erosion resistance of embankment materials and to develop key relationships related to earthen embankment erosion. Small-scale and large-scale physical models will also be used for water control studies. Data and relationships from physical models and case studies from the literature will be used in the development of predictive and design tools for embankment erosion and spillway design. This will include determination of allowable overtopping and erosion processes associated with overtopping and internal erosion. Other ARS, government, university, international scientists and consultants will collaborate with the USDA-ARS-HERU in carrying out these objectives. The results from this research will be incorporated into evaluation tools, software, design criteria, and management practices that will allow the continued service and increased benefit of the nation's agricultural flood control infrastructure.

3. Progress Report
Bench erodibility tests at a wide range of compaction effort and water contents have been analyzed and compared to large-scale erosion tests on headcut and embankment breach tests. A series of Jet erosion tests and Hole erosion tests were completed. The tests were conducted and analyzed in collaboration with the United States Bureau of Reclamation (USBR), United States Army Corps of Engineers (USACE), and Electricity DeFrance (EDF). The two erosion test techniques have been evaluated to provide essential information for prediction of processes and rates in computer embankment breach models being developed. Evaluated and validated predictive rate process algorithms using laboratory results from physical models, flume tests, and case histories for homogenous earthen embankments. Model evaluation was also conducted in collaboration with an international team coordinated by the Canadian Electrical Association Technical Interest, Dam Safety Interest Group (CEATI-DSIG). This team has virtually completed evaluating state-of-the-art embankment breach computational models including technology developed by the USDA-ARS-HERU as well as models developed in Canada and in the UK. The WinDAM (Windows Dam Analysis Modules) development continued with WinDAMa+ and WinDAMb. A beta test version of WinDAMa+ was produced that links embankment overtopping and spillway erosion technology in a single computational model. An alpha test version of WinDAMb was produced that is a process-based breach erosion prediction model for homogeneous earthen embankment breach to be used by the engineering profession. Developmental work also continued on investigation of the potential use of algorithms for evaluation of aerial and satellite images to determine earthen spillway vegetal maintenance condition. Final testing was completed on a generalized model study of the RCC (roller compacted concrete) 4(H):1(V) spillway based on the specific Renwick Dam stepped spillway model. Generalized lab tests at multiple scales were conducted to study air entrainment, energy dissipation, and flow bulking on RCC spillways. Observations were noted through photography and data, including measurement of water surface profiles, percent air entrainment, and velocities in the physical model. The results from this work were analyzed and resulted in generalized equations that will be validated in a planned large outdoor flume study. Construction was completed on a large-scale generalized model of a RCC 3(H):1(V) spillway. Generalized lab tests at multiple scales will be conducted to study air entrainment, energy dissipation, and flow bulking on RCC spillways. Tests were completed on sloped sidewalls for converging RCC stepped spillways. Generalized theoretical and empirical relationships were developed for vertical and sloped training walls on converging RCC spillway chutes. Additional testing is expected to validate the relationships developed for both vertical and sloped training walls.

4. Accomplishments
1. Dimensioning of Converging Vertical and Sloped Training Walls: Increasing the flow capacity of small watershed flood control dams is a growing issue for rehabilitation and safety of these structures. Roller compacted concrete (RCC) stepped spillways are being applied by NRCS engineers to increase capacity and protect the earthen dams from potential erosion. One important design issue is determining dimensions of the vertical and sloped training walls that would be required in instances where the spillway chute is required to be narrower at the bottom of the chute than at the top. RCC spillways cost several million dollars to build and therefore this research provides optimal design criteria. Generalized model studies were completed, and as a result theoretical and empirical relationships for dimensioning converging vertical and sloped sidewalls were developed and reported.

2. Engineering Guidance for Roller Compacted Concrete Spillway: The majority of research studies conducted on roller compacted concrete (RCC) spillways are based on steep sloped (greater than 2(H):1(V)) applications. This research is intended to address RCC spillways applied to embankment dams that have slopes flatter than 2(H):1(V), so a generalized model study based on the original Renwick Dam 4(H):1(V) spillway was completed. This study was conducted across multiple scales to investigate the affect step height has on air entrainment, energy dissipation, and flow bulking. Hubert Chanson developed an inception point relationship for steep sloped stepped spillways. From the study described herein, Chanson's relationship was optimized for flatter sloped stepped spillways for F*, Froude surface roughness, ranging from 1 to 100. The inception point is an important parameter for determining energy dissipation within the spillway chute, and the energy dissipation is a parameter used in the design of the stilling basin located at the downstream toe of the spillway chute. Generalized relationships regarding energy dissipation have been developed using the optimized relationship for inception point. Downstream of the inception point, the flow is highly aerated. As a result, flow bulking, an increase in flow depth as a result of air present in the flow, is created. The optimized inception point relationship shows promise in developing generalized relationships for air concentrations and equivalent clear water flow depth with in the spillway chute. This information is important with regards to the design of the training walls for stepped spillways. This study will contribute significantly to future applications of planned RCC spillways for rehabilitation applications.

3. Alpha Version of a Predictive Overtopping Breach Erosion Model Completed: The alpha test version of the computer engineering application tool WinDAMb, as a cooperative effort between the USDA-ARS-Hydraulic Engineering Research Laboratory, Natural Resources Conservation Service, and Kansas State University, has been completed. The enhancements to the application software WINDAM were completed that allow users to evaluate embankment overtopping erosion, breach timing, breach location, breach failure, and flood discharge for homogeneous earthen embankments. This computer tool will be important in evaluating existing structures, with potential for determining hazard classification, emergency action plans, and reducing costs associated with rehabilitation.

Review Publications
Constantine, C.R., Dunne, T., Hanson, G.J. 2009. Examining the physical meaning of the bank erosion coefficient used in meander migration modeling. Geomorphology. 106(3-4):242-252.

Morris, M., Hanson, G.J., Hassan, M. 2008. Improving the accuracy of breach modelling: Why are we not progressing faster? Journal of Flood Risk Management. 1(3):150-161.

Hanson, G.J., Temple, D.M. 2009. Final report of coordination and cooperation with the European Union on embankment failure analysis. Washington, DC: The National Dam Safety Program FEMA 602 Report. 168 p.

Last Modified: 2/23/2016
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