Location: Hydraulic Engineering Research2008 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 were conducted on a broad range of soils at standard compaction effort. Test results have been analyzed and require evaluation and comparison to large-scale erosion tests on headcut and embankment breach tests. A series of Jet erosion tests and Hole erosion tests were conducted on saturated and unsaturated soil samples in order to determine preferred application of tests for erosion prediction. These tests have been conducted in collaboration with the U.S. Bureau of Reclamation, U.S. Army Corps of Engineers, and Electricite DeFrance. Erosion test techniques are being evaluated to provide essential information for prediction of processes and rates in computer embankment breach models being developed. Initiated work on development of a mini-jet in order to increase convenience, rate, and efficiency of in-situ and laboratory erosion testing. Set up and conducted large-scale internal erosion embankment failure test. This test is the fourth in a series of large-scale physical model internal erosion tests that provide key computer model validation data for the mathematical relations that have been tentatively developed for describing the embankment failure due to internal erosion. Continued to evaluate and validate predictive rate process algorithms using laboratory results from physical models, flume tests, and case histories for homogenous earthen embankments. This work is being carried out by the staff at the USDA-ARS-HERU Laboratory. Model evaluation is also being conducted in collaboration with an international team coordinated by the Canadian Electrical Association Technical Interest, Dam Safety Interest Group. This team is evaluating state of the art embankment breach computational models, including technology developed by the HERU as well as models developed in Canada and in the UK. 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. The WinDAM (Windows Dam Analysis Modules) development continued with WinDAMa+ and WinDAMb. An Alpha test version of WinDAMa+ was produced that links embankment overtopping and spillway erosion technology in a single computational model. Work on integration of SIMBA (SIMplified Breach Analysis) algorithms into WinDAMb in order to develop a homogeneous earthen embankment breach model to be used by the profession continued. Final testing was completed on the specific model study of the RCC (roller compacted concrete) spillway proposed at Renwick Dam in North Dakota. Generalized lab tests at multiples scales were also 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 models. The results from this work will be analyzed and resulting generalized equations will be validated in a planned large outdoor flume study. (NP 211; Component 4).
1. Dimensioning of Converging Vertical Training Walls: Increasing the flow capacity of small watershed flood control dams is a growing issue for rehabilitation and safety of these structures. RCC spillways placed over the existing embankment is one solution being utilized by the profession because of the cost savings it provides as compared to other rehabilitation designs, but there are design issues that need resolution regarding converging RCC spillways. An increase in flow depth near the training walls occurs when converging a spillway, so determining the height of the training walls to retain the flow in the spillway chute is required. A generalized model study was completed, and as a result an equation for dimensioning converging vertical sidewalls was developed and reported. The development of this training wall height equation for converging spillways will provide design guidance so that the spillway adequately protects the remainder of the embankment. (NP211; Component 4)
2. Engineering Guidance for RCC Spillway: A specific hydraulic model study and final design guidance for a proposed $5.42 M RCC spillway for Renwick Dam in North Dakota, was completed. The design engineers were unsure of the spillway and downstream stilling basin dimensions and the stable riprap required for downstream protection of the spillway. This model study provided viable design solutions for these questions. This study was conducted using a 1 to 8 scale model in a large flume at the HERU. This study will contribute significantly to the planned design of this specific project and to future applications of planned RCC spillways for rehabilitation applications. (NP211; Component 4)
3. Alpha Version of an Enhanced Limited Overtopping Computation Model Completed: The alpha test version of the computer engineering application tool WINDAMa+, as a cooperative effort between the HERU, Natural Resources Conservation Service, and Kansas State University, has been completed. Enhancements to the application software WINDAM were completed that allow users to evaluate overtopping limits of vegetated embankments without failing the vegetation and to evaluate the integrity of the earthen spillway in one computational package. The software will determine the amount of flow through the principal spillway(s), auxiliary spillway(s), over the vegetated or rock rip-rap protected earthen embankment and erosion of the auxiliary spillway. This computer tool will be important in evaluating existing structures, with potential of reducing costs associated with rehabilitation. (NP211; Component 4)
Hanson, G.J., Caldwell, L., Lobrecht, M., McCook, D., Hunt, S.L., Temple, D.M. 2007. A look at the engineering challenges of the USDA Small Watershed Program. Transactions of the ASABE. 50(5):1677-1682.