河流生态需水量计算的湿周法拐点斜率取值的改进

Modification of wetted perimeter method for determining the ecological flow requirement

现行计算生态需水的湿周法在相对湿周-相对流量曲线上判定拐点时,常以斜率为1的点为拐点。在应用于标准抛物线形河道断面计算河段的生态需水时发现,在采用该拐点确定生态需水流量时,不同形状断面的需水流量比例都非常接近,没有反映出河段特征的差异。在对其原因分析后建议将拐点的斜率取为多年平均流量与相应湿周长比值的平方根。并以此进行计算,结果表明:河道断面越宽浅生态需水量占多年平均流量的比例越大;河流多年平均流量越大生态需水量所占的比例越小。反映出河流特征的差异,其变化趋势符合对河流生态需水规律的一般性认识。同时在计算得出的需水流量下,断面水力参数除湿周外均能达到水力学方法R2CROSS法提出的水力参数取值范围,计算结果较为合理。

The ecological flow requirement obtained from the current method is determined by the inflection point on relationship between relative flow and relative perimeter with slope equal to 1. But this method is not suitable to rivers with standard parabolic cross section, since the ratios of flow requirement for cross sections with different curvature are approximately the same. The characteristics of river section can not be reflected. Based on the analysis on the causes inducing this problem, a new method adopting the square root of the ratio of mean annual flow discharge to corresponding wet perimeter as the slope of curve deflection point is suggested. The application example shows that the proportions of ecological flow requirement to mean annual flow increase with the increase of river section＇s width-to-depth ratios, whereas the proportion decreases with the increase of mean annual flow. The hydraulic parameters obtained by the proposed method approximately agree with that obtained from R2CROSS method.