Estimation of river discharge using Monte Carlo simulations and a 1D hydraulic model based on artificial multi-segmented rating curves at the confluence of two rivers

During extreme floods caused by climate change, reliable flow discharge data are essential 4 for successful reservoir operation to mitigate downstream flood damage. Generally, the flow discharge 5 is computed using the rating curve (RC) established from the relationship between the flow rate and 6 water stage level. Determining the parameters of rating curves is subject to uncertainties related to the 7 difficulties and limitations of flow monitoring in covering a wide range of flow variations. Especially 8 at river confluences, the uncertainties are pronounced when floods occur owing to several factors such 9 as roughness change, backwaters, and levee overflow. The Seomjin River Basin in Korea suffered from 10 flood inundation that occurred at the tributary confluence during an extreme flood in 2020. To identify 11 a reliable flow rate of the main stream and tributary, this study proposes an indirect flow assessment 12 scheme using a 1D hydrodynamic simulation model to find the best simulated water level in an iterative 13 manner based on Monte Carlo (MC) simulations. With a large amount of discharge data generated from 14 random-number combinations, it is possible to obtain the best results automatically by specifying the 15 reliability limitation considering the uncertainty of the predetermined RC parameters associated with 16 the roughness coefficient. Nash Sutcliffe Efficiency (NSE) was incorporated to evaluate the reproduced 17 water level to meet the threshold specified for NSE ≥ 0.75. The simulated flowrates computed from the 18 revised RC and roughness coefficients revealed an error range of 8–36.6% compared with the design 19 flood. The approach proposed in this study is applicable for determining the valid parameters necessary 20 to create a revised RC at an existing water level gauge station, where the uncertainties of the RC are 21 pronounced, particularly in the vicinity of the channel confluence.