Hydraulic Properties of Stepped Spillway
سلام محمد علي جاسم, نجم عبيد سالم الغزالي
كلية الهندسة-جامعة بابل · العراق
Stepped spillways have been used since antiquity. In the last few decades, many stepped spillways have been constructed all over the world. This popularity is due to the compatibility of stepped spillways with new innovative construction techniques, especially RCC (roller-compacted concrete) and gabions. Using these techniques leads to time and cost savings compared with conventional concrete spillways. Previous studies defined three flow regimes on stepped spillways: nappe, skimming, and transition, which lies between the former two. Many studies recommended avoiding the transition flow regime in the design of stepped spillways. If a stepped spillway is designed for the skimming flow regime, more cost savings are obtained than for the nappe flow regime. The design value of the critical flow depth per step height (yc/h) is recommended to be about 20% higher or lower than the values of yc/h at the onset of skimming or transition flow regime, respectively. Due to the increasing use of stepped spillways, many studies have been conducted to identify their hydraulic properties and develop design guidelines. Most of the experiments were carried out on flat steps. In the last decade, a few studies introduced new designs to increase the efficiency of stepped spillways and improve their hydraulic properties. This study aims to contribute to the development of new designs for stepped spillways, as well as to study previous designs. The studied designs are: RCC-gabion (hybrid) stepped spillways, steps with non-porous end sills, and steps with porous end sills, in addition to the conventional non-stepped and flat-step spillways. Hydraulic Properties of Stepped Spillways Hydraulic model experiments were conducted using twelve stepped spillway models as well as three non-stepped models for comparison purposes. These models were designed with three downstream slope angles, which are 25°, 35°, and 45°, with five models for each slope angle. Stepped models were manufactured with flat steps, which were modified to the mentioned designs after conducting the experiments on the flat cases. The results revealed better flow behavior for gabion steps than flat and pooled steps. The flow is more stable in the gabion stepped models, and the transition flow regime shows fewer instabilities. The energy dissipation rate is maximum for pooled steps filled with gabions for relatively low discharges. At high flow rates, little differences between the tested configurations were observed, with the highest dissipation rate for pooled steps with and without porosity. The end sills highly affect the flow regime type. Using end sills increases the range of the transition flow regime (by increasing the lower limit of skimming flow) as well as the instabilities that occur in this flow regime. Due to these instabilities, energy dissipation in pooled steps is higher than in flat steps. Porous end sills provide better flow behavior and higher energy dissipation in the nappe flow regime than non-porous end sills. Their effects decrease as the discharge increases. New empirical equations for calculating the limits of flow regime occurrence, energy dissipation rate, location of inception of air entrainment, and hydraulic jump subsequent depths relationship in two-phase flow have been developed.