湘西大龙洞地下河流域水土流失特征及其对水库工程的影响

Soil loss and its effect on the planned reservoir project in the Dalongdong underground river catchment,Xiangxi,Hunan

用人工断面法和流量堰法测流,高密度采集水样,在室内用称重法测得水体含沙量,对大龙洞地下河出口及其邻近地表河、地下河上游地表河段进行同步输沙特征监测,以为拟建的大龙洞水库泥沙淤积评价提供依据。两个水文年的研究结果均显示,地下河与地表河输沙率呈同步变化特征,平均悬移质输沙模数分别为37.26～58.33t/km2·a和56.82～76.80t/km2·a,最大日平均输沙率分别为540mg/L和890mg/L,说明地下河空间以大型岩溶管道为主,连通性好,系统水力坡度大,对泥沙输出和减轻地下空间淤积有利。最大日平均输沙率均与最大流量峰值对应,说明水土流失主要发生在暴雨期间。选择流域内三种典型生态环境类型,进行原位水土流失观测,获得场雨产流过程和坡面流输沙率变化情况。结果显示,三种类型的平均悬移质输沙模数为65.35～884.78t/km2·a,暴雨期间准森林类最大瞬时输沙率为2926mg/L。在此基础上,估算得到在建库条件下50年总淤积量不超过地下库容的10%,说明水土流失造成的水库淤积程度并不严重。

In order to forecast sediment accretion in the planned Dalongdong underground reservoir, the following methods were undertaken： measuring the discharges by means of profile-current meter and discharge weir, water sampling at small intervals and the sand content analyzing in laboratory by weighting. The discharge measurement at the outlet of the underground river and the neighboring surface rivers as well as the upper segment of the underground river, synchronous monitoring of sand transportation are carried out to provide the evidence for assessment on siltation to the reservoir. The results obtained in two hydrological years show that the rates of sand transportation in the underground river and surface river change synchronously with average suspended load conveyance modules 37. 26-58.33 t/km2·a and 56. 82-76.80 t/km^2·a, and maximum diurnal average load 540 mg/L and 890 mg/L respectively. That means that the underground river is dominated by large karst passages with well connection and large hydraulic gradient which are favorable for sand output and reducing underground siltation. The maximum diurnal average load is consistent with the peak of discharge, indicating that water and soil losses occur largely during the rainstorms. Three monitoring stations in three typical ecological environments in catchment areas are built for monitoring local soil erosion in order to obtain the slop flow generation processes and the variation of sand transportation rates. The results show that the average suspended load for three types is 65.35-884.78 t/km^2·a, and the maximum instantaneous load is 2 926 mg/L which occurred in forest-land during rainstorm. Based on the monitoring data,the estimated calculation of the total siltation within the future 50 years will not exceed 10% of the underground capacity under artificial adjustment ,which demonstrates the reservoir siltation caused by soil erosion is not so serious.