Inclusion of flood diversion canal operation in the H08 hydrological model with a case study from the Chao Phraya River Basin – Part 1: Model development and validation

Water diversion systems play crucial roles in assuaging flood risk by diverting and redistributing water within and among basins. For flood and drought assessments, including investigations of the effects of diversion systems on river discharge worldwide, the explicit inclusion of these systems into global hydrological models (GHMs) is essential. However, such representation remains in the pioneering stage because of complex canal operations and insufficient data. Therefore, we developed a regionalized canal operation scheme and implemented it in the H08 GHM for flood diversion in the Chao Phraya River Basin (CPRB), Thailand, which is a complex river network with several natural and man-made diversion canals and has been subject to severe flooding in the past, including recent years. Region-specific validation results revealed that the enhanced H08 model with the regionalized diversion scheme could effectively simulate the observed flood diversion pattern in the CPRB. Diverted water comprises approximately 49 % of the annual average river discharge in the CPRB. The simulations further confirmed that the presented canal scheme had the potential to reduce flood risk in the basin by significantly reducing the number of flooding days. A generalized canal scheme with simple input data settings was also constructed for future global applications, providing insights into the maximum level of discharge reduction achievable with diversion of nearly 57 % of the annual average river discharge of the CPRB. Overall, the enhanced H08 model with canal schemes can be adapted and applied to different contexts and regions, accounting for the characteristics of each river network by maintaining the basic principles unaltered.

Implementation of flood diversion canals and retention ponds to the H08 global hydrological model for flood management

<p>Flood diversion canals play a crucial role in assuaging the flood risk by diverting water from the main channel to the nearby rivers, downstream of the same river, or the ocean. For the impact assessment of such canal systems on river discharge worldwide, their explicit inclusion into the global hydrological models (GHMs) is necessary. Despite this fact, such representation is limited due to their complex operations and lack of data. Therefore, we aim to propose a generalized scheme for the flood water diversion in the H08 GHM that ideally requires the universal parameters only. In this scheme, if the discharge exceeds the channel capacity, an amount equivalent to canal capacity is diverted to the canal, which will then flow to the retention ponds, and finally to the destination once the retention ponds get full. A regionalized scheme with site-specific parameters was also considered to evaluate the validity of the simulations.</p><p>The proposed scheme was tested in the upper Chao Phraya River basin, which is characterized by four tributaries of Ping, Wang, Yom, and Nan. The government has implemented Yom-Nan canal system to divert water from Yom to Nan River since 2014 to alleviate flooding in the lower Yom basin. The effect of this canal system was analyzed from 1980-2004 using the H08 model with the generalized scheme as well as the regionalized scheme. The simulations showed that the total flood water diverted from the Yom River was around 1.00 km<sup>3</sup>/year and 1.64 km<sup>3</sup>/year under the generalized and regionalized schemes, respectively, over the 25 years. This constitutes about 2.62% and 4.29% of the river discharge in the Yom River at the diversion point. In both simulations, nearly 30% of the water has been diverted to the Nan River and the remaining 70% was stored in the retention ponds. To assess the validity of the simulations, we compared the simulation results of the wet water-year 1994 with the observed canal operation data of the wet water-year 2017. The total flood water diverted was around 0.47 km<sup>3</sup>/year during the year 2017, whereas the same for 1994 was about 0.17 km<sup>3</sup>/year and 0.48 km<sup>3</sup>/year under the generalized and regionalized schemes, respectively. This shows that the regionalized simulations are close to the observations, while the generalized simulations reproduced nearly half of the diverted canal flow. The generalized simulations can be further improved by parameterizations.</p>

Modeling and Solving of Joint Flood Control Operation of Large-Scale Reservoirs: A Case Study in the Middle and Upper Yangtze River in China

Flood disasters are the most frequent and most severe natural disasters in most countries around the world. Reservoir flood operation is an important method to reduce flood losses. When there are multiple reservoirs and flood control points in the basin, it is difficult to use reservoirs separately to fully realize their flood control potential. However, the multi-reservoir joint flood control operation is a multi-objective, multi-constrained, multi-dimensional, nonlinear, and strong-transition feature decision-making problem, and these characteristics make modeling and solving very difficult. Therefore, a large-scale reservoirs flood control operation modeling method is innovatively proposed, and Dynamic Programming (DP) combined with the Progressive Optimality Algorithm (POA) and Particle Swarm Optimization (PSO) methods, DP-POA-PSO, are designed to efficiently solve the optimal operation model. The middle and upper Yangtze River was chosen as a case study. Six key reservoirs in the basin were considered, including Xiluodu (XLD), Xiangjiaba (XJB), Pubugou (PBG), Tingzikou (TZK), Goupitan (GPT), and Three Gorges (TG). Studies have shown that DP-POA-PSO can effectively solve the optimal operation model. Compared with the current operation method, the joint flood control optimal operation makes the flood control point reach the flood control standard, moreover, in the event of the flood with a return period of 1000 years, Jingjiang, the most critical flood control point of the Yangtze River, does not require flood diversion, and the volume of flood diversion in Chenglingji is also greatly reduced.

Urban flood modelling in Qiqihar city based on MIKE flood

Qiqihar is a significant city on the Nen River in China, which is the main stream of the Songhua River basin. The length of the return period of Qiqihar's flood control design standard is fifty years. If a 100-year flood event happened, Qiqihar would face the risk of a burst levee. To quantitatively evaluate flood risk to the city from a burst levee or proactive flood diversion, a model for analysing flood submergence from a burst levee in the City of Qiqihar is established based on MIKE Flood. The model integrates one- and two-dimensional hydrodynamic models to implement coupled simulation. The terrain data are from city elevation data on a scale of 1:10 000. Following local modifications made based on survey data, such as on levees, roads, and buildings, a 20 m × 20 m grid of terrain data was formed as the terrain input of the model. The model simulates the water level of Nen River and the flood path, submerged time/depth/area, and duration in floodplain under three scenarios: baseline, proactive downstream flood diversion, and an upstream levee burst under a flood with a one hundred-year return period. Proactive downstream flood diversion can reduce the maximum water level by 0.068 m and correspondingly decrease peak flood flow by 1120 m3 s−1. These results provide basic information to support urban flood risk analysis and flood dispatching and management across the whole river basin.