Analysis of Diversion Capacity of Yellow River Diversion Sluices in Irrigation Areas of Lower Yellow River

: This paper analyzes the influence on the diversion capacity of the Yellow River diversion sluices of factors such as the diversion water level change of the Yellow River diversion sluices in the irrigation areas of the lower Yellow River, the scouring of river channels and the siltation change in the diversion channels. It is suggested that the main reason for the decrease in diversion capacity of the Yellow River diversion sluices in different inflow levels is the obvious decrease in the water level of the same discharge due to the continuous scouring of river channels. At the end of this paper, targeted countermeasures and suggestions are put forward to solve the problem of the water diversion capacity decline of Yellow River diversion sluices.


1.Introduction
Since the construction and operation of Xiaolangdi Reservoir, engineering facilities and dispatch means have been used to change the water and sediment inflow conditions of the irrigation areas in the lower reaches of the Yellow River through the water and sediment transfer of the reservoir. Through the impact of the water flow, the sediment of the reservoir and that of the river bed are sent to the sea, and the main river channel in the lower reaches has been scoured in the whole line and the shape of the river channel has been adjusted. Meanwhile, due to the joint influence of the riverbed undercutting, river regime change and other factors, the diversion conditions of the downstream Yellow River diversion sluices have changed compared with the designed conditions, resulting in water diversion difficulties in some river channel sluices [1] . This paper analyzes the change in the diversion capacity of the Yellow River diversion sluices in the irrigation areas of the lower Yellow River.

Change of water diversion capacity of Yellow River diversion sluices in irrigation areas of lower Yellow River
The designed water diversion capacity of Yellow River diversion sluices in the lower Yellow River irrigation areas is 3929.7 m 3 /s, among which the designed water diversion capacity of Henan reaches is 1506.4 m 3 /s, and that of Shandong reaches is 2423.3 m 3 /s. In 2016, when the incoming water from the Yellow River reached 300 m 3 /s, 500 m 3 /s, 700 m 3 /s and 900 m 3 /s, the actual water diversion capacities of Yellow River diversion sluices in the lower Yellow River irrigation areas were only 13.63%, 23.6%, 31.64% and 39.43% of the designed diversion capacity respectively. The details are shown in Table 1. The water diversion capacity of Yellow River diversion sluices has decreased obviously.

3.1.1Design of diversion levels
Since the operation of Xiaolangdi Reservoir, sediment blocking of the reservoir and water and sediment transfer have caused the river bed of the lower Yellow River irrigation areas to be continuously scoured and cut down, and the water level of the same discharge to be reduced, which has an adverse effect on the water diversion capacity of the Yellow River diversion sluices [2] . According to statistics, the water level changes corresponding to Yellow River discharge relevant to the water diversion designed by Huayuankou, Jiahetan, Gaocun, Sunkou, Aishan, Luokou and Lijin hydrologic stations in the lower Yellow River irrigation areas in 2000 and 2015 are shown in Table 2. As can be seen from the  [3] .

Actual change in diversion levels
In 2016, under the condition of the Yellow River inflow discharge being 500m 3 /s, there were 14 sluices whose decrease in pre-sluice water level reached 1.5m compared with the pre-sluice water level in 2002, among which the pre-sluice water level decreases of the sluices with the same flow level with Hongqi, Liuyuankou, Yudian, Madu, Liulou and Wangji sluices reached 2m [4] [5] . After the operation of Xiaolangdi Reservoir, great changes have taken place in the inflow of water and sediment from the lower Yellow River irrigation areas. Owing to the continuous scouring, the downstream river has significantly deeper channels with an obvious sandflushing effect, and the water level of the same discharge has decreased significantly in recent years. Although presluice water levels were all higher than the elevations of culverts and sluice gates, there were still many sluices with the same flow level below the designed diversion level, and the water level difference of diversion with the same discharge decreases, which affects the water diversion capacity of the sluices.

Continuous channel erosion in the irrigation areas of the lower Yellow River
Since the operation of water storing and sediment containment of Xiaolangdi Reservoir, the downstream channels have experienced continuous scouring. The average erosion thickness of Huayuankou-Yijiahetan, Jiahetan-kaocun, Gaocun-Sunkou, Sunkou-Aishan, Aishan-Luokou and Luokou-Lijin channels are 3.88 m, 3.06 m, 1.84 m, 1.88 m, 1.88m and 2.02 m respectively. Therefore, the erosion thickness of the Sunkou-Gaocun channel is the smallest. The analysis of the scouring of hydrological sections in the lower Yellow River irrigation areas is as follows [6] .
(1)Huayuankou section The pre-flood large section data of Huayuankou of the Yellow River in 2002, 2008 and 2014 are selected for the set drawing, as is shown in Figure 1. It can be seen that the left and right banks of the river channel have been scoured [7] .
According  [8] .  (Figure 2), it can be seen that the section was in a state of being continuously scoured, and that the main channel has been significantly scoured and slightly widened, and that the thalweg point has moved about 550m to the right bank [9] .    (Figure 4), it can be seen that the section was in a state of being continuously scoured and that the main channel has been significantly scoured without transverse swing. The elevation of the thalweg point of the main channel in the Luokou section changed most obviously.

Sediment accumulation in the diversion and conveyance channels before and after the sluices
Since the Yellow River carries heavy sediment, diverting water with sand is a prominent feature of the Yellow River diversion supply. The Yellow River diversion sluices in the irrigation areas of the lower Yellow River continue to divert water with a small flow and a long time, which aggravates the channel siltation to a certain extent [10] . Overdue dredging has resulted in the serious siltation of diversion channels and the sharply declining flow capacity of the channel, which makes it impossible to meet the water supply demands of the Yellow River diversion sluices. It is also one of the main reasons for the decline of the water supply capacity of the diversion sluices. Therefore, channel siltation is one of the reasons for the inadequacy of the diversion capacity of the Yellow River diversion sluices in the irrigation areas of the lower Yellow River. Since the operation of Xiaolangdi Reservoir, the main channel of the Yellow River was cut down year by year, and the diversion water level of the Yellow River with the same discharge decreased, which changed the boundary conditions of the originally designed diversion of the Yellow River diversion irrigation areas, and made the diversion continuously in the state of "small discharge, low velocity and long time". In recent years, after the transformation of the first sluice in the canal, the longitudinal gradient of water transmission channels in some irrigation areas has become slower, leading to a sharp reduction of sand carrying capacity. Therefore, diversion channels, trunk channels and branch channels undertake the tasks of both transferring water and accommodating sediment, which is the main cause of the current siltation of diversion channels, trunk channels and branch channels in the Yellow River irrigation areas.

Countermeasures and suggestions to improve the water diversion capacity of Yellow River diversion sluices
(1) Establish a linkage response mechanism between water and sediment transfer and downstream Yellow River diversion sluice water supply The Yellow River diversion irrigation areas of the lower reaches of the Yellow River have two peak water consumption periods each year, respectively in spring and autumn. Therefore, Xiaolangdi Reservoir should meet the water consumption needs of the two peak agricultural irrigation periods in spring and autumn. The suggestion is to increase the base flow of the mainstream of the Yellow River from 500 m 3 /s to 600～700 m 3 /s to meet the daily water demand for Yellow River diversion. In addition, the irrigation systems of crops in different areas along the Yellow River are different, and the irrigation period of crops is not exactly the same. The irrigation period becomes longer after the periods of all regions are superimposed. Therefore, the drainage time of the reservoir should be extended appropriately during the peak water consumption periods in spring and autumn.
(2) Adopt temporary or permanent engineering measures to actively promote the reconstruction or demolition of the sluices to ensure the safety of water diversion For the Yellow River diversion sluices with other water supply tasks except for irrigation areas, temporary or permanent engineering measures should be taken in time to solve the problem of water diversion difficulties, and the role of regulation and storage engineering should be given full play to improve the water diversion capacity of the sluices.
(3) Establish long-term dredging mechanism for Yellow River diversion projects The irrigation areas are mainly responsible for the dredging of diversion channels, and the cost of dredging becomes the bottleneck restricting the dredging and excavation of diversion channels, resulting in the overdue dredging and excavation of diversion channels, or even perennial failure of dredging, which seriously affects the irrigation water of diversion channels. The practice has proved that unified organization and arrangement can make the dredging of the Yellow River diversion irrigation area be carried out orderly, and the effect is more obvious than the discrete activities of the beneficiary areas. The administrative units of the Yellow River diversion irrigation area should formulate the dredging implementation plan, issue the corresponding dredging management measures, and dredge the diversion channels and the channels behind the sluices in time to improve the water diversion capacity of the Yellow River diversion sluices.