| Stepped spillway is not only a kind of drainage structure but also energy dissipator,widely used in practical engineering. The hydraulic characteristics of stepped spillway have been research hot spot for many scholars. At present, study on the characteristics of step pressure, aeration and energy dissipation is in majority. The energy dissipation effect is one of the focus in engineering application. Energy dissipation ratio, a relative dimensionless parameter, has always been used to reflect energy dissipation effect. From another perspective,absolute flow energy on stepped spillway was studied in this paper, the relationship between specific energy and its factors, such as the step size, unit discharge and slope, was analyzed and the results show that there exist good rules. Based on the energy rules, new calculation methods about energy dissipation ratio and water surface profile were put forward. Specific content is in the following respects:(1) Specific energy is a foremost hydraulic parameter of stepped spillways, a measure of kinematic energy of moving water. In order to study the energy characteristic of water flow on stepped spillway, model test of stepped spillway with slope of 38.7° and different step heights was conducted. The results indicate that gross head of stepped spillway decreased dramatically along the flow path, specific energy on stepped spillways can first increase and then remain constant; The change of specific energy determines the gross head,low stable specific energy can reflect better energy dissipation. Two dimensionless parameters about stable specific energy and its influence factors were got through dimensional analysis,and a good power function law was found between the two dimensionless parameters based the test data of 30.0°~63.4° slope, the correlation coefficient is 0.9949~0.9972.An empirical formula to calculate stable specific energy and dissipation rate was put forward, providing references for related design.(2) Stepped spillway is developed from smooth spillway. Specific energy of stepped spillway and smooth spillway were compared in this paper and a new concept——relative specific energy describing the difference of head loss between the two kinds of spillway wasintroduced. Model tests on skimming flows of 21 sets were conducted using a spillway slope range of 26.6°~48.0° and a step height range of 0.5~2.0 m, focusing on the variations of specific energy and relative specific energy with the length of flow path, unit discharge, step height, and bottom slope. The results indicate that the specific energy varies with these parameters in a manner too complicated to apply practically while the relative specific energy that is independent of discharge, varies linearly with the length of flow path. Step height has a great influence on the relative specific energy that increases with it and spillway slope. The results prove the necessity of stepped spillway analysis adopting relative specific energy and its advantage in practical applications. An empirical formula for calculation of water head loss is given.(3) Based on the research achievements, a calculation formula of energy dissipation ratio and new calculation method of water surface profile were put forward. The current formulas of energy dissipation ratio was summarized, and the applicable conditions of each formula were also analyzed and evaluated in this paper. It showed that the formula in this paper has a higher calculation precision. Based on the linear relationship between specific energy and the length of flow path, a new water surface profile calculation method for stepped spillway was introduced, and the calculation results agree with the experimental data. |
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