Channel Adjustment Downstream Dams And Its Protective Measures

With dense population in basin, rivers are important components of environment. When reservoirs are built on rivers for certain purposes, downstream channel adjustment will be apparent, which will inevitably affect the sustainable development of society and economic in watershed scale. Negative impact aroused by these projects even will overtake their benefits. Thus, on the one hand water resource exploitation are necessary for energy deficit, which will consequently alter water and sediment delivery to downstream reach, on the other hand human have tried to harness the power of channel and create more stable system to protect surrounding communities. So, it's not only important to understand the natural variability and dynamics of river system, but also crucial to have the ability to predict its adjustment under disturbance. Base on greater understanding of river processes, human can create the beat possible scenario of river management and promote successful controlling measures. To nowadays, related researches are most aimed in some local river reaches, orienting at natural evolution process. Few of previous researches are about the mechanism of riverbed adjustment downstream dams, and few about corresponding harness works.In this paper, researches are focused on channel responses under altered water and sediment regime downstream dams, based on which the associated protection measures are also presented. Several key problems are emphasized, including the longitudinal and lateral adjustment process, stabilization of protection works under post-dam environment. Primary improvement and solution of river system harness are put forward.Some main improvements are described as following:(1) Analysis about the new characteristics of channel dynamics and the challenges on river harness in new environment. It is point out the good understanding of channel evolution is basis of river control works, when flow and sediment regime change.(2) Based on systematically analysis about the worldwide survey data of several reservoirs, the general features of water and sediment transportation is summarized. Some key conclusion listed below: Channel response rapidly to disturbances on flow regime and sediment amount, the change process shows nonlinear trend. The magnitude of adjustment is most strong at the beginning, and then it decreases gradually till it adjusts to a new balance. At downstream reaches, the phenomenon of the channel evolvement consists of longitudinal and lateral adjustment. With the change extent of water and sediment, the form of channel adjusts periodically. Based on these laws, the channel evolvement downstream Danjiangkou reservoir is analyzed and compared. It can be seen that the conclusion of this paper can give some guidance in practice.(3) According to the variation analysis of power station daily regulation wave (DRW), it is found that the DRW have different effect on the riverbed evolution with the difference of reservoir regulation modes and channel characteristics. Based on the daily regulation data of Three Gorge station, the process of DRW is simulated and analyzed and the result show that the DRW aggravate the negative effect of local gradient on navigation.(4) In this paper, the character of riverbed longitudinal deformation (RLD) isconcluded and the change character of RLD with flow and sediment regime variation is analyzed. Based on the comparison of evaluation approaches which predict the depth of RLD with flow and sediment regime variation, taking example for the sand-pebble riverbed downstream Three Gorge Dam, the process of the riverbed longitudinal erosion is simulated by using the numerical simulation. Then, according to the character of the riverbed longitudinal deformation, the effect, applicability, form and stability of different longitudinal protective measures are discussed and analyzed. It is found that the accurate estimation of maximal erosion depth around protective engineering and consolidating the dislocation which is caused by riverbed further erosion post-dam is the base of protective engineering improvement. In the paper, the different evaluation approaches of maximal erosion depth around protective engineering are compared and some formulas are improved based on the field data.(5) In general, the characteristics of lateral adjustment of channel are summarized. At the same time, the mechanism about the channel responses in condition of variance of water and sediment is analyzed. Then we get conclusions like these: channel adjustments downstream the dam is complicate. The first reason is because of the variance of the streamwise water and sediment condition. Second is on account of the variance of the capability of riversides and riverbed to resist erosion with time. Near the dam, the discharge becomes even and fewer, as well the quantity of sediment reduction. At the same time, little far from the dam, quantity of sediment recovered and increases gradually. Because riverbed becomes coarser, the length in which sediment can recover becomes far and far from the dam. Of course that will make the capability of riverbed stronger and the capability of bank weaker. All these factors may bring on diverse phenomenon when bank and riverbed adjust. So, when understanding and estimating the metamorphose of the downstream reach of reservoir, these dynamic features are needed to take care. From viewpoint of large spatial and temporal scale, the lateral activity of the channel has weakened and the river width is narrowed. But, for quite long time, the under-saturation has caused erosion, during which process, the under saturation has taken place of the discharge reduction to become the main factor. For that reason, at sections where bank resistance is weak, river widen is apparent.(6) Local distortion-bankfailure is analyzed. It is concluded that in post-dam period the intension of bankfailure is weakened because of flow process flattened, but its scope will increase. The point of curvature will shift downstream, and bankfailure will happen because of bed scouring. Then, according to the character of the riverbed lateral deformation, the effect, applicability, form and stability of different lateral protective measures are discussed and analyzed. It is found that the accurate estimation of maximal erosion depth around protective engineering and consolidating the dislocation which is caused by riverbed further erosion post-dam is the base of protective engineering improvement. It can use jackstone in advance and increasing roughness to strengthen the stability of engineering measures.(7) Conclusion of the characteristics of riverbed evolution of downstream of the Yangtze River in natural condition is used to estimate the channel response after TGP built. Every reach will appear different adjustment tendency because of the differences of boundary condition, river channel pattern and riverbed scouring process of thedownstream of dam. Based on these theoretical analyses, the engineering measures and its planned layout in downstream of Changjiang River are studied. Takeing Zhoutian Reach as an example, primary advice is put forward for practice.(8) Aimed at reducing the new phenomena such as bed degradation and water level decrement after dam building, a new work is introduced by improvement of existing engineering measures. The new work not only increase roughness of channel bed, but also protect riverbed from erosion to achieve the propose of maintaining water depth. The effectiveness of this measure is closely related to the diameter of the roughness material to be used. The stability of material, increase of water depth, variations of hydraulic gradient and velocity distribution in navigation channel after increasing of roughness must be comprehensively studied to prevent from harmful influences to navigation. This engineering measure is applied to solve the problem of insufficient water depth of navigation channel probably when the Three Gorges Project is put into operation. Through analyzing its theory and numerical simulation, the place, time, effect and the influence of daily-regulation wave of the engineering measure are capered, it concludes that increasing roughness can be used to maintain or lift Yichang's water level.