Sherry L. Hunt

Inception Point for Embankment Dam Stepped Spillways

AbstractRetrofitting embankment dams with stepped spillways has become a common design practice, particularly for those dams that change hazard classification from low to high. For embankment dams retrofitted with stepped spillways, the chute length is often insufficient for developing aerated flow or an inception point. The inception point is a key spillway design parameter used in energy dissipation, flow depth, and air entrainment prediction relationships. Original research for developing an inception-point relationship for stepped spillways was based on primarily gravity (θ≥26.6°) stepped spillways, with the majority having an ogee crest control section. The resulting, inception-point relationship tends to overestimate the inception-point location for broad-crested weir stepped spillways (θ≤26.6°) when the Froude surface roughness (F*) is less than 10. Consequently, research on broad-crested weir stepped spillways retrofitted for embankment dams has been conducted to provide an optimized inception-poi...

Development and Characterization of Soil Material Parameters for Embankment Breach

Despite the widespread use of earthen dams, the processes by which earthen embankments erode and fail when overtopped during extreme events are only imperfectly understood. Aging of these dams and the associated recent focus on dam rehabilitation has increased the need for new technology and tools for predicting the performance of earthen embankments during overtopping. Windows Dam Analysis Modules (WinDAM B) is a modular software application being developed to address this need for homogeneous earthen embankments. This software is being developed through joint efforts of the Agricultural Research Service, the Natural Resources Conservation Service, and Kansas State University. The computational model being incorporated into this software is the result of research including embankment overtopping tests conducted in the outdoor laboratory. The WinDAM B model is a simplified representation of the observed process of progressive erosion leading to embankment breach to be useful for field application the model has been developed to utilize soil parameters that may be reasonably obtained for the conditions to which the model is applied. This article presents the background for the material parameter input requirements for overtopping erosion and breach analysis including: 1) the detachment rate coefficient and critical stress parameters of the excess stress equation, 2) the total unit weight of soil, 3) the undrained shear strength, and 4) the headcut migration coefficient.

Converging Stepped Spillways: Simplified Momentum Analysis Approach

AbstractRoller compacted concrete (RCC) stepped spillways are growing in popularity for providing overtopping protection for aging watershed dams with inadequate auxiliary spillway capacity and for the construction of new dams. Site conditions, such as limited right-of-way, topography, and geological formations, often dictate that these spillways converge. Convergence increases the flow depth near the training walls and alters the stilling basin design requirements as compared with traditional straight spillways. A simplified control volume momentum analysis is presented for predicting the minimum vertical training wall height necessary to prevent wall overtopping in converging stepped spillways. An expression is developed to predict vertical training wall height as a function of centerline flow depth, centerline velocity, chute slope, and convergence angle. A three-dimensional 3(H):1(V) sloping stepped spillway model with an ogee crest and convergence ranging from 0–70° was constructed to verify this rel...

Velocities and Energy Dissipation on a Flat-sloped Stepped Spillway

In recent years, hazard classifications for many existing embankment dams have changed because hydrologic conditions have been altered. Consequently, many of these dams require increase spillway capacity in order to meet state and federal dam safety regulations. Stepped spillways have become a popular choice for providing increased spillway capacities to existing embankment dams. Stepped spillways in these applications are typically placed over the existing embankment. Consequently, the chute slope has the same slope as the downstream embankment face or in some cases have the same slope as the existing auxiliary spillway. Typical slopes for existing embankments range from 2(H):1(V) and flatter. Design guidelines and literature in general for these stepped spillways are very limited, so further research on these stepped spillways is warranted.

Inception Point Relationship for Flat-Sloped Stepped Spillways

Many small earthen embankments are faced with hazard classification changes due to urban encroachment. As a result, some embankments have inadequate spillway capacity. To bring the dam into compliance with state and federal dam safety laws, rehabilitation of the dam is often required. RCC stepped spillways are becoming a popular choice for addressing these issues. However, design guidelines for RCC stepped spillways applied to small earthen dams are scarce, especially for spillways with slopes flatter than 2(H):1(V).

Model Study of RCC Stepped Spillways with Sloped Converging Training Walls

Approximately half of the over 11,000 small watershed dams designed and constructed under the supervision of the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) will reach the end of their planned service life within the next 10 years. Many of these dams have inadequate spillway capacity due to changes in hazard classification and revised dam safety laws. Urbanization of surrounding areas limits the rehabilitation options of these dams. Roller compacted concrete (RCC) stepped spillways provide an effective solution to this problem. Recent years have seen a growth in the research and application of RCC, but there are no readily available generalized guidelines. Research has been performed on converging spillway chutes with vertical training walls. Public safety, aesthetics, and construction efficiency suggest sloped training walls are often a more desirable option.

Lessons Learned using Laboratory Jet Test Method to Measure Soil Erodibility of Compacted Soils

Overtopping, internal erosion, and piping are the main causes of accidents and failures of embankments for dams, lagoons, and levees. A key parameter in the failure of these structures is the erodibility of the soil materials used in construction of the embankments. The soil materials are typically constructed based on compaction specifications. The jet erosion test (JET) has been developed to study the erosion characteristics of soil materials. A laboratory version of this apparatus is described in this paper and is used in this study to investigate the effects of compaction on erodibility. Two approaches for determining compaction specifications are also compared and discussed. Soil samples, 944 cm3, were prepared at different compaction water contents and compaction efforts, and tested using the jet erosion test (JET) method. Erodibility was observed to vary by several orders of magnitude dependent on the soil gradation and plasticity, and the compaction effort and water content. The findings indicate the resistance to erosion for a given soil can be improved considering the following: 1. Compaction near optimum creates a structure most resistant to erosion, 2. Higher compaction effort at a given water content increases erosion resistance, and 3. Soil properties including, texture and plasticity, influence erosion resistance as much or more than compaction factors. The findings also indicate the usefulness for erodibility testing of embankment materials to aid in determining the optimal compaction specifications for a given soil.

Physical Model Study of a Proposed Converging RCC Stepped Spillway

Nearly half of the 10,000 small watershed dams constructed in the U.S. with the assistance of the NRCS will reach the end of their proposed service life in the next 10 years. While many of these structures were originally built to protect agricultural land from floods, urbanization around some of them has; thereby, changed the hazard classification of these dams. To meet current federal and state dam safety standards, rehabilitation of these structures are necessary. If ignored, these dams could place life and property at risk. The Georgia NRCS requested a specific model study for a proposed design of a converging roller compacted concrete (RCC) stepped spillway for the rehabilitation of Big Haynes Creek Watershed Dam Number 3 (H-3) in Gwinnett County, Georgia. The USDA-ARS Hydraulic Engineering Research Unit conducted a three-dimensional, 1:22 scale physical model study to evaluate the hydraulic performance of this structure. The results of the study indicated that the proposed height of the spillway training walls will be overtopped by the maximum expected discharge. Additionally, the extension of the stilling basin downstream will provide better protection. The design recommendations resulting from this model study are discussed herein.

Estimated Splash and Training Wall Height Requirements for Stepped Chutes Applied to Embankment Dams

AbstractAging embankment dams are commonly plagued with insufficient spillway capacity. To provide increased spillway capacity, stepped chutes are frequently applied as an overtopping protection sy...