The discusser wishes to congratulate the authors for their authoritative paper. He believes that the work is a necessary addition to the topic of stepped spillway flow. The paper presents a true systematic study of skimming flow. Despite the outstanding insights of the study, the discusser wishes to point two weaknesses: an inadequate analysis of the air entrainment process and some limitations of the velocity probe measurements.

https://espace.library.uq.edu.au/view/UQ:9291/DIS_Cham.pdf

Characteristics of Skimming Flow over Stepped Spillways

Ohtsu et al. must be congratulated for their systematic study of skimming flows on stepped channels with different slopes and step heights. Their experimental data provide a solid database that might lead to better and improved design criteria. Important outcomes include some estimate of friction factors for skimming flows and a design flow chart for stepped canals. Despite these outstanding results, the writers would like to comment constructively on further design criteria for skimming flows, and to provide additional flow resistance results and a discussion on the physical processes. It is hoped that the present contribution will add to safer stepped chute design. The writers would like to point out that the energy dissipation performances of stepped canals with moderate slopes are far from being totally understood and further experimental research is needed.

Flow Characteristics of Skimming Flows in Stepped Channels

Gabions are a common construction material in the African Sahel and elsewhere, especially for spillways. Stepped gabion spillways are structurally stable, resistant to water loads, and efficient energy dissipators, but there are no clear design rules to date. One‐fifth‐scale model tests representing 3–5 m high standard prototypes discharging up to 3m3⋅s-1⋅m-1 enabled us to establish quantified energy‐dissipation criteria and efficient stilling‐basin design rules, saving 10–30% on basin length as compared with current methods. The different types of flow over the stepped face are also described and analyzed: isolated nappe flow, partial nappe flow, and skimming flow. From a technical point of view, stepped gabion spillways can withstand flows up to 3m3⋅s-1⋅m-1 without great damage, if setting of gabions complies faithfully with the code of practice. Some advice is given on preventing gabion deformation.

Flow and Energy Dissipation Over Stepped Gabion Weirs

The stepped spillway has increasingly become and effective energy dissipator. When the hydraulic performance of the overflow is clearly known, the energy dissipation could be increased. However, the study of stepped spillway overflow has been based only on model tests until now. In this paper, the \ik-ϵ turbulence model is used to simulate the complex turbulence overflow. The unstructured grid is used to fit the irregular boundaries and the volume of fluid method is introduced to solve the complex free-surface problem. The free surface, velocities, and pressures on the stepped spillway are obtained by the turbulence numerical simulation. Furthermore, the simulation results compare well with measured data. The study indicates that the turbulence numerical simulation is an efficient and useful method for the complex stepped spillway overflow.

Volume of Fluid Model for Turbulence Numerical Simulation of Stepped Spillway Overflow

Stepped chutes have become recently a popular method for discharging flood waters. The steps increase significantly the rate of energy dissipation taking place on the spillway face and reduce the size of the required downstream energy dissipation basin. This paper reviews the hydraulic characteristics of skimming flows. The onset of skimming flows is discussed. New results are presented to estimate the flow resistance along stepped chutes. The study indicates some major difference between the flow patterns on steep and flat stepped chutes. An analogy with flow over large roughness is developed. Then the calculations of the start of air entrainment are detailed. The results indicate that free surface aeration occurs much more upstream than on smooth spillways. The effects of flow aeration are later discussed.

https://espace.library.uq.edu.au/view/UQ:9384/jhr94_3.pdf

Hydraulics of skimming flows over stepped channels and spillways

A physical hydraulic model investigation was conducted to evaluate the performance of a stepped overflow spillway. The spillway has a standard ogee profile with continuous steps cut into the spillway face from just below the crest, to the toe. The steps significantly increase the rate of energy dissipation on the spillway face, thus eliminating or greatly reducing the need for a large energy dissipation basin at the spillway toe. Primary objectives of the investigation were to evaluate the effectiveness of the flow transition from the smooth crest profile to the steps, to quantify the energy dissipation on the spillway face, and to define the flow characteristics on the steps. The investigation demonstrated that this stepped spillway is quite effective at dissipating energy and that smooth flow transition from the spillway crest to the stepped face is easily achieved.

Stepped Spillway Hydraulic Model Investigation

Maximum ship-generated wave heights and the related wave half-periods were measured (with a step-resistance wave gage) at various distances from the sailing line for several vessels operating at a range of speeds in the Oakland Estuary. The methods and results of these measurements are reported and evaluated together with the pertinent background information on the nature of ship-generated waves.

Investigation of Ship-Generated Waves

Elements of stereophotogrammetric theory and technique pertinent to the present study are analyzed. This study involved the stereophotogrammetric analysis of the form and variation of the surface waves generated by a ship model. The specific application of stereophotogrammetry used in the study is discussed in detail along with some of the advantages and disadvantages of stereo-analysis. Suggested improvements in the instrumentation and technique are used.

Robert M. Sorensen

Papers

Stepped Spillway Hydraulic Model Investigation

Investigation of Ship-Generated Waves

Stereophotogrammetric Analysis of Wave Surfaces