Channel evolution under changing hydrological regimes in anabranching reaches downstream of the Three Gorges Dam

Elucidating the influence of dams on fluvial processes can benefit river protection and basin management. Based on hydrological and topographical data, we analyzed channel evolution in anabranching reaches under changing hydrological regimes influenced by the Three Gorges Dam. The main conclusions are as follows： 1） the channels of specific anabranching reaches were defined as flood trend channels or low-flow trend channels according to the distribution of their flow characteristics. The anabranching reaches were classified as T1 or T2. The former is characterized by the correspondence between the flood trend and branch channels, and the latter is characterized by the correspondence between the flood trend and main channels; 2） on the basis of the new classification, the discrepant patterns of channel evolution seen in anabranching reaches were unified into a pattem that showed flood trend channels shrinking and low-flow trend channels expanding; 3） flood abatement and the increased duration of moderate flow discharges are the main factors that affect channel adjustments in anabranching reaches after dam construction; and 4） in the next few decades, the pattern of channel evolution will remain the same as that of the Three Gorges Dam operation. That is, the morphology will fully adapt to a flow with a low coefficient of variation. Our results are of interest in the management of the Yangtze River and other rivers influenced by dams.

Responses of flood peaks to land use and landscape patterns under extreme rainstorms in small catchments-A case study of the rainstorm of Typhoon Lekima in Shandong,China

Investigations of the formation mechanisms of flood peaks in small catchments facilitate flood prediction and disaster prevention under extreme rainstorms.However,there have been few studies on the re-sponses of flood peaks to land use landscape patterns using field surveys during extreme rainstorm events.Based on field data from 17 small catchments near the rainstorm center of Typhoon Lekima,7 landscape indices were chosen.The flood peak and its sensitivity to the land use landscape were investigated by combining remote sensing interpretation and related analysis.The conclusions are as follows:(1)The peak discharge of the small catchment was 2.36e56.50 m3/s,the peak modulus was 8.00 e48.89 m3/(s$km2),and the flood index K ranged from 3.61 to 4.55.(2)Under similar rainfall conditions,the flood peak modulus,K and the proportion of sloping cropland had significantly positive correlations(p<0.05).The flood peak modulus was significantly negatively correlated with the proportion of forest-grassland and terrace(p<0.05),and K and the proportion of forest-grassland and terraced land exhibited a negative correlation.(3)The flood peak modulus and K were positively correlated with the landscape fragmentation.(4)The sensitivities of small catchments to floods were evaluated to be moderate compared to K values from other studies.The ability of small catchments to cope with extreme rain-storms can be improved by increasing the areas of forest-grassland,and terraces and reducing landscape fragmentation.Our results could be applied to provide a basis for land use planning and support for the response against disasters caused by extreme floods.

Changes of Water stage in the middle Yangtze River influenced by human activities in the past 70 years

Water stages play a critical role on flood control,water supply,navigation,and ecology in rivers.Investigation of water stages provides better understanding of riverbed evolution processes and river management.Based on the hydrological observation in past 70 years,the changes of low-flow and flood stages were investigated by a combination of Mann-Kendall test,moving t-test,and wavelet analysis.1)In accordance with the location,the middle Yangtze River was divided into upper reach,middle reach,and lower reach.Water stages in the upper reach show a decreasing trend,while that in the middle reach present an increasing trend,and the lower reach are mainly dominated by natural evolution.2)The mutation year of water stages in the upper reach was around 1985,indicating that the Gezhouba Dam facilitated the reduction of water stages.The trend mutation in the middle reach was in 1969,which was consistent with the implementation of Jingjiang Cutoff.3)Human activities aggravated the change of water stages,leading the primary period of water stage time series to exceed 20 years.4)In the upper reach,the reductions of water stages were attributed to the riverbed erosion induced by human activities.While in the middle reach,the recent falling effects of riverbed erosion can hardly offset the rising effects of the channel resistance on water stages.5)In the future,the increasing trend in the middle reach may be arrested due to the riverbed erosion induced by the Three Gorges Dam.Long-term observation of the flood stage must be conducted in the middle Yangtze River.