Location: Hydraulic Engineering Research2012 Annual Report
1a. Objectives (from AD-416):
The long-term objective of this project is to develop stand-alone and integrated tools for engineers to evaluate allowable embankment overtopping; predict dam failure from overtopping and internal erosion for homogeneous and zoned embankments with simple and complex downstream embankment slope geometries; design alternative surface protection methods, including vegetation, riprap, and concrete blocks; and develop generalized design criteria for increased spillway capacity for small dams (i.e., less than 20 m high). Specifically, the objectives of this project are: Objective 1: Enhance the WinDAM model to predict erosion and breach of complex earthen embankment geometries and materials. Subobjective 1A: Quantify the impact of complex embankment geometries on erosion processes during overtopping, including: convergence zones at the intersection of the earthen embankment and valley walls, and embankment berms and toes. Subobjective 1B: Quantify the impact of changes in soil materials on erosion processes and rates of earthen embankment erosion and breach. Objective 2: Develop engineering tools for design of earthen embankment protection alternatives and increasing the discharge capacity of small dams. Subobjective 2A: Develop guidelines for dimensioning stilling basins and downstream channel protection for non-converging RCC stepped spillways constructed over existing earth dams. Subobjective 2B: Develop guidelines for dimensioning stilling basins and downstream channel protection for converging RCC stepped spillways constructed over existing earth dams.
1b. Approach (from AD-416):
Large-scale physical models will be used to develop knowledge on the protective capability of vegetation and/or riprap on embankment slopes with convergences and berms. Large and small-scale models will be used to develop knowledge of erosion resistance of zoned embankment materials and to develop key relationships related to earthen embankment erosion. Small-scale and large-scale physical models will also be used to develop knowledge on the impact of discharge, energy dissipation, flow depth, velocities, and downstream tailwater depth on stilling basin and downstream channel protection design for stepped spillways. Data and relationships developed from these physical models coupled with in-depth literature review will be used in the development of predictive and design tools for embankment erosion and spillway and stilling basin design. USDA-ARS HERU scientists will collaborate with other ARS, government, university, international scientists, and consultants to carry-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:
Dam safety is of high importance worldwide, and although dams have excellent safety records, dams do fail, creating a potential risk to life and property downstream. ARS scientists at Stillwater, OK, along with cooperators from Kansas State University and the Natural Resources Conservation Service (NRCS) have for several years been developing the WinDAM (Windows Dam Analysis Modules) software for prediction of earthen embankment erosion and failure. To expand on the development of WinDAM, outdoor physical models for conducting convergent and bench flow testing have been designed, constructed, and configured; vegetation has been placed for testing of Bermuda grass and will be allowed to mature for 12 months prior to testing; and a water delivery system is in the process of being modified to handle the necessary flow. Laboratory bench testing for characterizing erodibility of the soil materials to be used in field testing is complete. The outdoor facilities for conducting zoned embankment tests have been substantially reconfigured to conduct tests. Construction of the first test section is nearly ready to begin. Aging embankment dams are vulnerable to changes in hazard classification due to changing demographics in the vicinity of the dam. As a result, many of these dams have insufficient spillway capacities due to changes in hazard classification requiring more stringent dam safety regulations to be met. ARS scientists at Stillwater, OK, over the past several years have developed design criteria for the cost-effective application of roller compacted concrete (RCC) stepped spillways to provide increased spillway capacity and overtopping protection of embankment dams. Air entrainment inception point relationships were optimized for stepped spillways having a broad-crested weir using data collected from a 3(H):1(V) stepped spillway physical model. The relationships were further validated from data available from literature. Data collection of velocities, flow depths, and air concentrations from a 2(H):1(V) stepped spillway model are substantially complete. Analyses of these data are in the preliminary stages to further enhance relationships developed for air entrainment inception point, flow depth, energy dissipation, and air concentrations from a 4(H):1(V) and 3(H):1(V) stepped spillway models. Limited data from literature will provide independent validation for these relationships. Flow depth and energy dissipation are parameters for designing the stilling basin and riprap size for downstream channel protection. These relationships with additional testing will be used to optimize the design of the stilling basin and riprap size for downstream channel protection for stepped spillways.
1. Improving methods for predicting earthen embankment erosion and failure. Although dams have excellent safety records, dams do fail, placing life and property at risk downstream. ARS researchers at Stillwater, Oklahoma, along with cooperators from Kansas State University and the Natural Resources Conservation Service (NRCS) are continuing to enhance the WinDAM (Windows Dam Analysis Modules) software to provide a tool for predicting earthen embankment erosion and failure processes and impacts. Soil parameters are essential for improved model development. Laboratory bench testing for characterizing erodibility of the soil materials to be used in field testing was completed. Physical models to evaluate convergent and bench flow on embankment dams have been constructed, so WinDAM can be further enhanced to evaluate erosion prone areas of embankment dams. Increased knowledge of embankment breach erosion and failure processes due to overtopping and internal erosion, resulting in enhanced prediction tools, has great potential to improve: risk assessments, ranking systems for prioritization of rehabilitation, emergency action plans, inundation maps, and flood warning systems.
2. Generalized hydraulic guidelines and tools developed for roller compacted concrete (RCC) spillways. Inadequate spillway capacity is a common deficiency for aging embankment dams. Researchers at the Hydraulic Engineering Research Unit, Stillwater, Oklahoma, are continually developing and enhancing generalized design criteria for RCC stepped spillways for overtopping protection and to increase spillway capacity for the rehabilitation of aging embankment dams. Flow depth, energy dissipation, air concentrations, and air entrainment relationships developed from 4(H):1(V) and 3(H):1(V) sloped RCC stepped spillway physical models are complete. Data collection from a 2(H):1(V) slope stepped spillway has substantially been completed, and preliminary analysis of the data is underway. These data will enhance generalized design relationships for inception point, flow, depth, energy dissipation, and air concentrations. These relationships will provide quantifiable design guidance for engineers to use in designing RCC stepped spillways. Additionally, these relationships will provide engineers economic justification on selecting particular design parameters (i.e., step height) while also keeping in mind the safety of the general public. Approximately, 1100 embankment dams constructed under the Small Watershed Program administered through Natural Resources Conservation Service (NRCS) are expected to utilize this technology. Increased knowledge of air entrainment properties within stepped spillways has great potential to extend the design life of thousands of embankment dams worldwide.
Hunt, S.L., Temple, D.M., Abt, S.R., Kadavy, K.C., Hanson, G.J. 2012. Converging stepped spillways: Simplified momentum analysis approach. Journal of Hydraulic Engineering. 138(9):796-802.