Water and sediment transport processes have been changed by the operation of larger scale reservoirs, which can result in the adjustments of river topography and water levels. The polynomial fitting method was applied to analyze the variation characteristics of the water levels under different characteristic water discharges in the Jingjiang reach of the Yangtze River during 1991-2016. The segregation variable method was used to estimate the contributions of the varied riverbed evaluation, the downstream controlled water level and the comprehensive roughness on the altered water level at the same flow. The results obtained are as follows: (1)The low water level in the Jingjiang reach of the Yangtze River during 1991-2016 presented a significant downward trend, and this trend intensified since 2009; riverbed scouring was the dominant factor for the reduced low water level, while the increased roughness alleviated this reduction. (2) During 1991-2016, the high water level decreased firstly and then increased. The variation characteristic of "high flood discharge with high water level" before 2003 transformed into "average flood discharge with high water level" since 2009. The increased comprehensive roughness was the main reason for the increased high water level, while the river scouring alleviated this increase. For navigation conditions and flood control, the intensifying riverbed scouring of the sandy reaches downstream dams enhanced the effects of the downstream water level on the upstream water level. (3) This led to the insufficient water depth in the reaches below dams, which should arouse great attention. The variation characteristics of high water level increased the flood pressure in the middle reaches of the Yangtze River.

Chongqing reach is located in the fluctuating backwater zone of the Three Gorges Reservoir (TGR). Sediment deposition and erosion of Chongqing reach is related to flood control, shipping and wharf operation. It has become one of the key problems the TGR faces. In this paper, based on the prototype observation data, the sediment scouring and silting law of Chongqing reach before and after the 175 m experimental impoundment of the TGR was investigated for the first time. In addition, combined with the analysis of river bed composition and the one-dimensional mathematical model, the scouring conditions of suspended sediment in Chongqing reach were calculated and analyzed. Furthermore, the basic conditions of sediment transport in the study reach were applied to the practice of sediment reduction of the TGR. The following conclusions can be drawn as follows: (1) From 2008 to 2012, when the water storage level of TGR reached 175 m, the erosion intensity of the river bed in Chongqing reach started to decline, and the main souring period of suspended load was postponed from the water-impoundment stage to the pre-flood stage. After 2013, Xiluodu and Xiangjiaba hydropower stations in the lower reaches of Jinsha River was completed and put into operation. The sediment of Jinsha River was intercepted and the sediment entering the Three Gorges was greatly reduced, which led to another increase of the river bed scouring intensity of Chongqing reach. (2) When the inflow in front of the Three Gorges Dam (TGD) is greater than 4000 m ³/s and the water level is lower than 167 m, the suspended load on the river bed in Chongqing reach begins to scour. When the inflow exceeds 5000 m ³/s, the water level in front of the TGD drops to 163 m, and the scouring intensity of suspended load increases obviously. Increasing the drawdown rate of water level from 163 m before the TGD to 30 cm per day can avoid the cumulative sedimentation in the Chongqing reach.

One of the main tasks for the river regulation in the Lower Yellow River (LYR) is to improve sediment transport capacity of the braided reach, which is influenced by incoming flow and sediment regime as well as riverbed boundary condition. Variations in the reach-scale geomorphic coefficient and incoming flow-sediment regime (incoming sediment coefficient and fluvial erosion intensity) were investigated, using the hydrological data and cross-sectional profiles surveyed in the braided reach between Huayuankou and Gaocun over the period from 1971 to 2016. Empirical relationships were then established to quantitatively demonstrate the integrated effects of flow-sediment regime and the post-flood channel geometry in the previous year on the values of sediment delivery ratio (SDR) during flood seasons and flood events respectively in the study reach. Results indicate that: (1) the values of SDR of flood seasons and flood events increased with a smaller incoming sediment coefficient or a larger fluvial erosion intensity, and the incoming sediment coefficient was equal to 0.012 kg·s/m ⁶ when the value of flood-season SDR approached 1. The correlation coefficient is 0.76 for the relationship between flood-event SDR, incoming sediment coefficient, and the ratio of the water volume at the inlet section to the one at the outlet section in the study reach. (2) The SDR of the braided reach decreased with wider and shallower channel geometry, and it was less than 1 when the geomorphic coefficient was greater than 15 /m ^0.5. (3) The determination coefficient is 0.82 for the empirical function established among flood-season SDR, incoming sediment coefficient and geomorphic coefficient, and the weight of the channel geometry is greater than the incoming sediment coefficient in the function of flood-event SDR. The established functions can reflect the sediment-transport characteristics of the braided reach, which is helpful to quantify the influence of channel geometry and flow-sediment regime on sediment transport capacity.

The implementation of the Water-Sediment Regulation Scheme (WSRS) since 2002 has changed the water and sediment entering the Yellow River Estuary, which may lead to adjustment of the estuarine channel. Based on the elevation data of 41 transects from 2002 to 2017 in the estuarine channel downstream from Lijin, we established a digital elevation model (DEM) based on curved orthogonal grids to investigate the spatiotemporal characteristics of channel erosion and deposition, and further analyzed its controlling factors combining the morphological changes of river mouth and water-sediment data. The results show that the Yellow River estuarine channel has undergone significant erosion with a total sediment volume of 62.4×10 ⁶ m ³ since 2002. The channel evolution process can be divided into three stages according to the rates of erosion or deposition, namely: rapid erosion (2002-2005), slow erosion (2006-2014), and slow deposition (2015-2017). Approximately 80% of the total eroded sediment was depleted in the first stage. The erosion flux has shown a significant decreasing trend after 2006, and the overall characteristic of the channel transferred from erosion into deposition afterwards. On a seasonal scale, the estuarine channel was eroded in the flood seasons and deposited in the non-flood seasons; from the spatial perspective, the erosion intensity decreased in the downstream direction. The operation of the WSRS changed the distribution of water and sediment entering into the sea during a year, most of which was delivered in a short time (no more than 20 days), thus caused the continuous erosion of channel as well as the morphological adjustments of the estuarine channel. As a result of the intensive erosion in the first stage, the estuarine channel riverbed has lowered so much, and accompanied by the impact of the extension of river mouth, the erosion efficiency decreased continuously. Besides, the channel reach near the mouth has undergone significant siltation after the first stage causing the decline of riverbed longitudinal slope, which increased the instability of the estuarine channel.

Practices in developed countries reveal that low impact development based on rainfall flooding control measures can effectively alleviate rainfall flooding. Focusing on the relevance of new and old towns in the development of sponge cities in China, we try to put forward a stormwater management path of "present situation appraisal - decomposition of low impact development indicators - simulation and verification of construction result". A case study is unfolded on Fenghuang County, Hunan Province. On the basis of urban waterlogging model, the rainfall flooding risk evaluation and renewal feasibility evaluation is conducted on the new and old towns; based on the evaluation of the present situation, a low impact development control indicator decomposition system is established accordingly to implement low impact development conceptually and technically from macro control strategy to detailed planning, and the validity of this path is verified through simulation of the water accumulating volume of the stagnant point. The results indicate that the old towns of Fenghuang County have a higher rainfall flooding risk and an evidently lower current annual runoff volume control rate than the new towns, however, the old towns have a much lower transformation feasibility than the new towns due to large historical and cultural preservation areas and a lower terrain. From the perspective of the integrity of new and old towns, more runoff discharge and peak runoff can be reduced by establishing a three-level (Proper-Block-Plot) indicator decomposition system so as to lighten the pressure of flood disaster in the lower old towns. However, under the circumstance of heavy rainstorm, it is still very difficult to solve the problem of waterlogging only by low impact development facilities, and grey infrastructures based on water accumulating volume calculation are also needed in order to efficiently control water logging caused by short intense rainfall.