A large lake plays an important role in mitigating flood disasters in its nearby regions during the flooding period; however, the effect is limited, because most of its storage capacity becomes dead storage prior to the arrival of the flood wave. In the current study, an innovative flood control scheme (IFCS) is applied to Dongting Lake (the second largest freshwater lake in China) to alleviate flood disasters. MIKE 21 FM was used to examine its feasibility to mitigate flood disasters. One of the largest floods in the 20th century, the 1998-type flood, was modelled and the maximum water levels with/without IFCS were compared. The result shows that the effective flood control storage could be at least doubled when compared with the natural condition once IFCS was applied. The peak flood level in the Dongting Lake could be lowered by at least 0.32 m at the Chenglingji station in the same flood passage of Dongting Lake. The case study reveals that, after applying IFCS, the hydraulic gates play a very important role in floodwater regulation and further study should be conducted to find the optimized operation for each gate in the flood control scheme system.

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This study examines an innovative Coastal Reservoir (CR) technique as a feasible solution for flood adaptation and mitigation in the Brisbane River Estuary (BRE), Australia, which is vulnerable to coastal flooding. The study analysed the operation of a CR by using the MIKE 21 hydrodynamic modelling package. The 2D hydrodynamic model was calibrated and validated for the 2013 and 2011 flood events respectively, with a Nash-Sutcliffe coefficient (Ens) between 0.87 to 0.97 at all gauges. River right branch widening and dredging produced a 0.16 m reduction in water level at the Brisbane city gauge. The results show that by suitable gate operation of CR, the 2011 flood normal observed level of 4.46 m, with reference to the Australian Height Datum (AHD) at Brisbane city, could have been reduced to 3.88 m AHD, while under the improved management operation of the Wivenhoe Dam, the flood level could be lowered to 4 m AHD at Brisbane city, which could have been reduced with CR to 2.87 m AHD with an overall water level reduction below the maximum flood level. The results demonstrated that the innovative use of a CR could considerably decrease the overall flood peak and lessen flood severity in the coastal city of Brisbane.

Tidal bores are sudden discontinuities in water depth, and can be classified as undular and breaking bores when tidal waves propagate upstream in a river mouth and on beaches. The impact of different types of bores on sediment transport is not clear; hence, specially designed experiments were conducted to clarify the mechanism of sediment initiation. Here, a laser Doppler anemometer, a force sensor, and ultrasonic displacement meters accompanied by video recordings were used to investigate simultaneously the incipient motion of sediment under tidal bores. No sediment motion was observed during the initial steady flows. It was observed that only a few particles changed their initial position during the undular bore; however, a group of particles were seen moving upstream during the breaking bore. The inception of sediment motion was closely related to the passage of the breaking roller, that is, the forces that caused the inception of particle motion were not only due to velocity but also to the sudden discontinuity in free water surface. According to the experimental results, the upward vertical force due to swelling in free water surface before the roller toe was the main force in destabilizing the particles. Later, a large upstream longitudinal force was found to be the dominant cause promoting upstream particle motion during the breaking roller passage. Furthermore, phases of both horizontal and vertical forces play an important role in tidal bore propagation. It is interesting to note that by using the measured force, the observed phenomenon can be well explained by the Shield threshold criterion. RIAZ ET AL.

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