Kem C. Kadavy

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...

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).

Physical Model Study of a RCC Stepped Spillway for Renwick Dam, North Dakota

The North Dakota NRCS requested a specific model study of a RCC stepped spillway proposed for the rehabilitation of Tongue River Dam M-4, also known as Renwick Dam. The USDA- ARS Hydraulic Engineering Research Unit (HERU) constructed a two-dimensional, 1:8 scale physical model to evaluate the energy dissipation of a section of the structure. The proposed spillway entrance consists of a broad crested weir, with flow continuing down a 4(H):1(V) stepped chute. Prototype step heights ranging from 0.3 m (1 ft) to 0.61 m (2 ft) will be tested to compare the influence on energy dissipation. Stepped spillways provide a significant amount of energy dissipation compared to relatively smooth spillways. Additionally, stepped spillways require shorter stilling basins than smooth spillways, and the rough surface created by the step height influences the stilling basin design. The stepped spillway modeled with the larger step heights is expected to create more energy dissipation; thereby, creating the need for a shorter stilling basin and allowing for some cost savings. This research is expected to impact the development of design guidelines for stepped spillways planned on relatively flat slopes (θ ≤ 22°).

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.

ROCK CHUTE OUTLET STABILITY

Rock chutes, rock riprap on steep slopes, are used as grade control structures to safely conduct a water flow to a lower elevation. Previous studies have reported relationships to predict the highest stable unit discharge on the sloping face as a function of the material D50 and the bed slope. However, the studies do not report the riprap size required for stability at the toe of the chute. The riprap size required for outlet stability was examined in two separate flumes and two field-scale structures. For all tests, the D50 size predicted for stability on the sloping bed was also stable at the toe of the chute. The tailwater elevation resulting from the outlet reach and downstream channel resistance was sufficient to prevent movement of the riprap in the outlet reach.

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...

Pressure Forces in a Fractured Matrix

Substantial pressure forces can be transmitted to an overfall boundary as water flows over a headcut. These pressure forces, acting with the hydraulic shear stress, cause soil erosion, scour, and headcut instability. Fracture patterns or cracks can naturally occur in earth materials (soil and rock), and these cracks can accelerate soil erosion and headcut advance. This project examines how pressure forces are transmitted through a fractured matrix in the impingement area. Two crack spacings and two overfall heights were tested over a range of flow rates. Pressure forces were measured below a fixed block matrix, and the ability of these forces to remove material was examined. Pressure magnitude and variance was observed to be greatest near the point of nappe impingement. Piezometers placed below two layers of fractured block material indicated that the fractured matrix dramatically dampened the boundary pressure magnitude but spread the pressure over a wider area. This paper describes the magnitude and location of pressure forces downstream of an overfall with and without blocks. This research should enhance our understanding of how pressure forces and fracture patterns can influence soil and rock erosion.

Model Study of a Riffle-Pool Rock Chute

A model study was performed to evaluate a new in-stream grade-control structure. A rock chute, a loose riprap structure, was modified to include three riffle-pool sequences. The structure is designed as an in-stream grade-stabilization measure that allows fish passage and biological enhancement. This paper summarizes the results of a three phase model study that examined the seepage characteristics, rock stability, flow velocities, and overall hydraulic performance of this structure. The models suggest that the structure will perform well and provide a viable new treatment technique for in- stream grade stabilization.