变水位静水法在渠道水利用系数计算中的应用

静水法是测验渠道渗漏水平、计算渠道水利用系数的试验方法之一,其测验精度高于动水法。尤其对于衬砌渠道,其测验精度更能得到体现。而变水位静水法是分析变动流量渠道水利用系数的适用方法。该文以水量平衡为依据,分析了渠道水利用系数的计算过程,以及当渠道水深变化时渠道水利用系数计算中相关因素的变化规律,并将渠道水利用系数计算过程概化为6步。以石津灌区四干三分干南四支混凝土衬砌渠道0+054~0+475渠段的变水位静水法和流量观测为实据,展示了衬砌渠道水利用系数的计算过程,计算水深介于0.56~0.99 m之间,渠道水利用系数为0.988~0.991之间。实例分析结果表明:随着渠道水深的增加,虽流量和渗漏强度均增加,但渠道水利用系数随水深呈增加趋势,该实例在水深大于0.7 m时,渠道水利用系数变化趋势渐趋平缓。可见,当渠道流量不稳定时,渠道水利用系数宜采用变水位静水法结合流量进行计算。

Application of dropping head ponding test on calculation of water use efficiency in concrete lined canal

Abstract: Canal efficiency calculation by using ponding test shall be assisted with canal discharge. Ponding test is one of the tests to observe seepage quantity. Furthermore, to calculate canal efficiency, it is efficient to test low seepage canals such as concrete lined canal. The observation object in ponding test is a length of canal water statically. The test results could reflect seepage quantity influence factors, such as canal material, section form and depth of canal water. Canal efficiency calculation with inflow-outflow test is based on flow difference between upstream section and downstream section, which could reflect seepage process dynamically. According to canal discharge design formulas, four factors were concluded in canal discharge calculation: discharge section area, Chezy coefficient, hydraulic radius (or wetted width and longitudinal slope. Ponding test has two measurements according to the range of water level, one is constant head and the other is dropping head. It was assumed that as canal cross-section is designed, its discharge section area and R(X) could be calculated by depth of canal water, and different depth of canal water will lead to different discharge section area, R(X) and canal efficiency undoubtedly. If the canal discharge varies during the period of irrigation, the dropping head ponding test was fit for analyzing the relations between h and canal efficiency. The water balance theory was used to describe the process of canal efficiency calculation, and the relative parameters regulation was analyzed when depth of canal water was increased. Based on those, we set the canal discharge of unit time as the water volume, which could reflect canal discharge characteristic. It was assumed that W flowed along canal by v, if the length of canal was 1 km, canal efficiency calculation was described as six basic steps: 1) Applying ponding test to representative canal, and establishing the relationship between depth of canal water and seepage quantity; 2) Observing canal discharge to establish relationship between depth of canal water and canal discharge; 3) Letting time equal 1 second, and calculating the length of water volume; 4) Calculation of water seepage time; 5) Calculation of water seepage volume; and 6) Calculation of canal efficiency. Ponding test and canal discharge observation were both applied on the south 4th branch canal of main canal in Shijin Irrigation District, and dropping head method was adopted in ponding test, depth of canal water ranged from 0.294 to 0.940 m. Establishment process of relationship between depth of canal water and seepage quantity can be described as seven steps: 1) Making a scatter plot with X-axis as time and Y-axis as depth of canal water, and R2 of the regressing the function was 0.99; 2) Discretizing depth of canal water interval 0.294-0.940] into intervals of length 0.001 m, calculate ti for hi. 3) Calculating water volume for each interval; 4) Calculating seepage time corresponding to each water volume; 5) Calculating seepage quantity in each interval; 6) Calculating the percentage of evaporation in total lost water, and correct the seepage quantity in each interval; and 7) Making a scatter plot with X-axis as depth of canal water of each interval and Y-axis as corrected seepage quantity of each interval, R2 of the regression function was 0.99998. The sample size of canal discharge was 92, and corresponding depth of canal water fell in between 0.56 and 0.99 m. Through canal efficiency calculation, the results showed that canal efficiency was increased with depth of canal water and ranged from 0.988 to 0.991, and seepage quantity met the criterion of "Standard for engineering technique of seepage prevention on canal". While depth of canal water >0.7 m, the trend of canal efficiency was increasing smoothly and lightly. Hence, it was necessary to regard canal efficiency calculation by dropping head ponding test as a routine work for those canals which were working on different canal discharge.