Sediment reduction analysis method
This section presents the methods used to calculate sediment reduction caused by the major contributors, i.e., reservoir works, water diversion works, soil and water conservation works, and rainfall.
Sediment reduction by reservoir works
Reservoir works reduce sediment by impounding and retaining the sediment. Recent variations in sediment reduction due to reservoir works were analysed according to the variations in the average annual sediment deposition in the reservoirs of the basin during different periods.
The average annual sediment reduction by various reservoirs can be calculated by dividing the accumulated sediment in each reservoir during a certain period by the number of years:
$$ WS_{r} = sumlimits_{1}^{n} {D_{i} /N} , $$
(1)
where (D_{i}) is the accumulated sediment (100,000,000 t) in a reservoir during a certain period, N is the number of years in the period, and WSr is the average annual sediment reduction (100,000,000 t) in the period by all reservoirs in the basin.
The Hydrological Bureau under the Yellow River Water Conservancy Commission annually measures and calculates the deposition of sediment in all reservoirs in the Yellow River Basin. Two methods can be used, namely, the topographic method and the section method. In the topographic method, the area enclosed by contour lines on the topographic map of the reservoir area is measured to calculate the reservoir volume. The cumulative deposition of sediment during a specific period is the difference between the current and the previous reservoir volume. The topographic method requires closed contour lines on the map. In reality, however, the contour lines cannot be closed due to the presence of farmland, houses, and other artificial structures in the reservoir area, resulting in measurement errors. Therefore, the section method is mainly used at present. Here, M test sections were deployed in the reservoir area, and the test section data were used to calculate the total storage capacity of the reservoir in sections by period, as follows:
$$ V_{i} = sumlimits_{m = 1}^{M – 1} {V_{i,m} } . $$
(2)
The difference in the storage capacity measured twice is the cumulative deposition of sediment in reservoir (D_{i}):
$$ D_{i} = V_{i – 1} – V_{i} , $$
(3)
where Vi is the storage capacity measured at the end of period i and Vi,m is the storage capacity measured in section m – 1.
Sediment reduction by water diversion works
During water diversion, a certain amount of sediment is diverted, along with water, and is deposited in irrigation areas, resulting in a decrease in the volume of the sediment in the river channel. The average annual sediment reduction by water diversion works can be calculated by multiplying the average annual water diversion in different periods in the Jinghe River Basin by the average annual sediment concentration in the water diversion period, as follows:
$$ WS_{d} = sumlimits_{1}^{n} {W_{di} /N times overline{S}} /{1}000, $$
(4)
where (W_{di}) is the cumulative water diversion (100,000,000 m3) in the basin in the water diversion period, N is the number of years in the period, (overline{S}) is the average annual sediment concentration in the period (kg/m3), and (WS_{d}) is the average annual sediment reduction (100,000,000 t) in the basin during the period. Recent variations in sediment discharge caused by water diversion works were analysed according to the variations in the average annual water diversion in the basin in different periods.
Sediment reduction by soil and water conservation works
A commonly used method to compute the sediment reduction by soil and water conservation works is to multiply the area subject to the soil and water conservation works, such as terracing, forestation, grassing, creating enclosures, and constructing silt-arrester dams, by the sediment reduction by each measure per unit area, followed by their summation, as follows:
$$ WS_{SC} = sumlimits_{1}^{n} {F_{i} times S_{j} /10^{8} ,} $$
(5)
where Sj is the sediment reduction due to each soil and water conservation measure (t/hm2), published by the soil and water conservation monitoring institutions in each basin based on the analysis of the long-term observation data, Fi is the area subjected to each measure (hm2), and WSSC is the comprehensive sediment concentration for each measure (100,000,000 t). The variations in sediment reduction by soil and water conservation works were analysed based on the variations in the soil and water conservation areas in the basin during different periods.
Analysis of rainfall-induced sediment yield
The deduction method was adopted to analyse the rainfall-induced variations in the sediment yield. Recent variations in sediment reduction attributable to reservoirs, water diversion, and soil and water conservation works were computed and deducted from the measured sediment reduction in recent years (2000–2015):
$$ Delta WS_{p} = Delta WS_{t} – Delta WS_{r} – Delta WS_{d} – Delta WS_{sc} , $$
(6)
where (Delta WS_{t}) is the recently measured sediment reduction (100,000,000 t), (Delta WS_{r}) is the recent variation in the sediment reduction (100,000,000 t) caused by variations in the sediment retention due to reservoir works, (Delta WS_{d}) is the recent variation in sediment reduction (100,000,000 t) caused by variations in water diversion, (Delta {text{WS}}_{{{text{SC}}}}) is the recent variation in sediment reduction (100,000,000 t) caused by variations in the soil and water conservation area, and (Delta WS_{p}) is the recent variation in the rainfall-induced sediment yield caused by variations in rainfall.
Sediment yield calculation method
Figure 6 depicts the computational process for the sediment calculation. First, a reduction calculation of the natural runoff was performed as follows:
$$ W_{0} = W_{m} + W_{cum} + W_{s} + W_{e} + W_{SC} , $$
(7)
where W0 is the natural runoff, Wm is the measured runoff, Wcuw is the industrial water consumption in the basin, Ws is the water retention by reservoirs, We is the water evaporation and seepage losses, Wsc is the water reduction by soil and water conservation, and W0 is the natural water volume in the basin. All these terms are in 100,000,000 m3.
Second, the runoff-sediment relationship in the natural state was established based on the measured runoff and sediment data in periods with negligible human activity, as well as when the underlying surface was in a nearly natural state. Natural sediment discharge was calculated using the relationship between runoff and sediment discharge. According to the observation data from the basin for the past 35 years, runoff was closely related to sediment discharge. Given China’s climatic conditions and economic growth, the basin was nearly in a natural state up to 1960 because human activity had a minor impact on runoff and sediment discharge. Based on the runoff and sediment discharge measurements at Zhangjiashan Station from 1932 to 1960, the relationship between the natural runoff and sediment discharge was established as WS0 = f(W0). Natural sediment discharge in the basin was calculated considering the restored natural runoff.
Third, the natural sediment discharge was calculated using the natural runoff results and the runoff-sediment relationship. Based on the major contributors to sediment reduction in the basin, the future sustainable sediment reduction was calculated as the sum of sediment reduction due to reservoirs, water diversion, and soil and water conservation measures. Sediment reduction caused by variations in rainfall was limited to certain periods. For example, recent reduced heavy rainfall has led to a decreased rainfall-induced sediment yield and consequently a decreased sediment discharge. However, according to forecasts by the Intergovernmental Panel on Climate Change (2014)50, extreme weather and heavy rainfall events are likely to increase in the future. The reduction in sediment due to variations in rainfall was calculated as follows:
$$ WS_{d} = WS_{r} + WS_{d} + W_{SC} , $$
(8)
where WSr is the future sediment reduction caused by reservoir works, i.e., the sum of the sediment retention potential of the remaining capacity of the existing reservoirs and that of planned future reservoirs; WSd is the sediment reduction caused by future water diversion works, which can be obtained by multiplying the water diversion in the basin forecasted according to the social and economic development by the average sediment concentration in the water diversion period; WSsc is the future sediment reduction caused by soil and water conservation, obtained from areas subject to existing and planned soil and water conservation works and the corresponding sediment reduction rates; and WSd is the forecasted value of sediment reduction in the basin. All these terms are in 100,000,000 t.
Fourth, the sustainable sediment reduction in the basin was calculated considering variations in the contributions to sediment reduction in a future period and their effect. Future sediment discharge in the basin is the difference between the natural and future sediment reduction, as follows:
$$ WS_{f} = WS_{0} – WS_{d} , $$
(9)
where WS0 is the natural sediment discharge in the basin, WSd is the forecasted sediment reduction in the basin, and WSf is the forecasted sediment discharge in the basin. All these terms are in 100,000,000 t.
Finally, future river sediment discharge was obtained by subtracting the future sustainable sediment reduction from the natural sediment discharge.
Data acquisition
Hydrological data
A total of 28 hydrometric stations and 190 rainfall stations are located along the main stream and tributaries of the Jinghe River to effectively monitor rainfall, runoff, and sediment in the basin.
Zhangjiashan Station, located at the outlet of the Jinghe River Basin, has a catchment area of 432,160,000 km2, covering 95% of the total area of the basin. Few hydrometric and rainfall stations were operational in this basin before 1956, and hence incomplete data were collected. Analyses in this study were based on data from the Zhangjiashan Station from 1956–2015. At this station, the cross-sections in the main stream and Jinghui Canal (a water diversion canal) were hydrologically measured to determine the discharge, sediment transport rate, and sediment concentration.
Engineering data
Data on sediment reduction due to reservoir works and terraces, forests, grasslands, enclosures, and dams in the basin were based on the results of the National Water Resources Census and official data collated by the Upper and Middle Yellow River Bureau of the Yellow River Conservancy Commission. These data are thus accurate and reliable.
For data collection and erosion–deposition calculations, DL/T 5089–1999 “Specification for Sediment Design of Hydropower and Water Conservancy Projects” provided that “The calculated results of erosion and deposition should be compared with the measured data for several years of operation. If the amount and location of sedimentation are 70% consistent, and the elevation of sedimentation in the reservoir differs by 1 to 2 m, then the calculated results are deemed reliable. For erosion–deposition calculation results, only reliability is considered”.
Relevant data from the stations were systematically verified and collated by the Hydrological Bureau of the Yellow River Conservancy Commission and are therefore accurate and reliable.
Source: Ecology - nature.com
