Gregory J. Hanson

WINDAM – Analysis of Overtopped Earth Embankment Dams

Windows Dam Analysis Modules (WINDAM) is a modular software application which is being ndeveloped for the analysis of overtopped earth embankments. The development is being carried out in stages nwith the initial computational model addressing the routing of the flood through the reservoir with dam novertopping and evaluation of the potential for vegetation or riprap to delay or prevent failure of the nembankment. That model, WINDAMa, is in alpha testing with availability of a beta test version expected in the nfall of 2006. Work has begun on expanding the model, WINDAMb, to include erosional failure of a nhomogeneous embankment through drainage of stored water. The approach taken in developing this failure nmodel has been to first develop a research oriented model for use in analyzing data from embankment breach ntests conducted in the United States and Europe. This SIMplified Breach Analysis (SIMBA) model is designed nto allow application of different computational procedures to data analysis for the purpose of increasing nunderstanding of the process and determining the approach best suited to field application in the WINDAM nmodel. Analysis of the available data with SIMBA is continuing with simultaneous development of the structure nof the WINDAM program to allow incorporation of the computational routines found to best represent the data. nLater expansion of the models to include analysis of non-homogeneous embankments is anticipated.

Earth Dam Overtopping and Breach Outflow

Over the past half-century, the USDA has assisted in the design and construction of approximately 11,000 flood control and multipurpose dams. As these dams approach the end of their planned service life, sediment pools fill and sediment deposition in the flood pool reduces flood storage. Simultaneously, development in the watershed may increase runoff while development downstream of the dam increases the consequences of failure. Therefore, there is an increased need for improved procedures for predicting performance of overtopped earth embankment dams. Field experience and laboratory experiments indicate that the erosion process for overtopped earth embankment dams can be divided into sequential stages or phases for computational purposes. These stages include: 1) an initial stage prior to the failure of the vegetal cover or other protection on the downstream slope, 2) a breach development stage during which the resulting headcut or overfall moves from the point of formation through the dam crest, 3) a breach stage during which the headcut advances into the reservoir, and 4) a drawdown stage during which the breach continues to widen as the stored water in the reservoir is released. The flow geometry and the relations describing the discharge and erosion processes vary with stage. This report discusses the dominant characteristics of the identified stages of the progressive breach process and the impact of the resulting geometry on discharge prediction. Results of an idealized study of breach outflow during headcut entry into the reservoir are briefly discussed.

Evolution of Vegetated Waterways Design

In 1990, the USDA-ARS Hydraulic Engineering Research Unit (HERU) was recognized as a National Historic Landmark by ASABE for its groundbreaking work and development of vegetated waterways design procedures. In 2000, ASABE acknowledged the vegetated waterway design criteria as an Outstanding Achievement of Agricultural Engineering in the 20th Century. Current design procedures have evolved, but its roots are in the early conservation programs of the 1930s when former ASABE Fellow and one of the pioneering researchers of the vegetated waterways design criteria, William O. Ree, began studies on the subject in South Carolina while working for the Soil Conservation Service (now NRCS). He completed this work at HERU near Stillwater, OK in the 1940s. During this time, permissible velocities for vegetated channels and n-VR curves were developed as valuable design tools. The design criteria developed by Ree have been used worldwide for more than 60 years. In 1987, the design method evolved to a stress-based procedure. Today, NRCS is leading an effort to update current design procedures in their Engineering Field Handbook and developing computer aided design software for vegetated waterways with the assistance of the USDA-ARS HERU. Researchers at HERU have continued to study vegetation in high stress flow environments that have played an important part in the development of the computational models, SITES and WINDAM, for evaluating vegetated auxiliary spillways and earthen embankments. SITES is used by NRCS and engineers worldwide for the design and analysis of watershed flood control dams. The evolution of the design procedures for grass-lined channels has had a tremendous impact on agricultural engineering.

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.

Sediment Control and Erosion Research: Then and Now

Over the past 65 years, scientists at the USDA-ARS Hydraulic Engineering Research Unit n(HERU) in Stillwater, Oklahoma have made great strides in the design and technological ndevelopment of hydraulic structures and vegetated channels. From its inception, the laboratory has ngained notoriety for its accomplishments in vegetated channel design and development of design ncriteria for many hydraulic structures including trash racks, low-drop grade-control structures, and nriprap design for rock chutes and stilling basins. Thousands of these structures and over a halfmillion nmiles of grassed waterways based on design criteria developed by the research scientists at nHERU have shaped the landscape of the world into what it is today. In addition, research at the nlaboratory has provided field engineers with new techniques in measuring flow, soil erodibility, and ncontrolling sediment release. Today, scientists at the laboratory remain dedicated in their study of nhydraulic structures, channels, issues associated with the rehabilitation of aging watershed floodcontrol nand multi-purpose reservoirs, and sediment control structures. The purpose of the research nhas been, and still is, to assist engineers in the design of safe, efficient, and economical hydraulic nstructures and channels.