Gensen, Matthijs R.A.

Hulscher, Suzanne J.M.H.

Huthoff, Fredrik

Warmink, Jord Jurriaan

At river bifurcations, discharge is distributed over downstream branches, thereby also distributing flood risk.An accurate estimate of the discharge distribution is essential for flood risk management. In our research, weanalyze the water balance and discharge distribution at two major bifurcations of the Rhine river in the Netherlands. The Dutch Rhine branches are highly engineered, and human intervention will continue in the future.For each of these branches, a 30-year record of discharge and water level measurements is available. Currentlyoperational rating curves show non-closing water balances at the bifurcations, with errors of up to 10%. Thisindicates large uncertainties in these rating curves. In our study, we construct multi-stage rating curves using Bayesian inference and Markov chain Monte Carlo simulations. We explore two methods to increase ratingcurve accuracy. First, we will take explicitly into account the constraints of the river bifurcations, namely a closing water balance, in the construction of the Bayesian rating curves. Secondly, we will consider water levelsalong downstream branches as an indicator for discharge measurement inaccuracy. The results show that bothmethods are able to strongly reduce the water balance error, particularly at very high discharges. However,this comes at the expense of a wider uncertainty range in the rating curves. Still, the new rating curves may bemore accurate due to the reduced water balance error. Using the rating curves, we quantify the 90% confidenceinterval of the discharge distribution at both bifurcations at around 10%, a value which is fairly independentof upstream discharge. At design conditions, this amount of uncertainty could result in uncertainties in waterlevels of up to 0.8m. We conclude that it is essential to regard the entire river system with its bifurcations forflood risk management and for future planning of human interventions in the system

Analysis of water balance and discharge distribution at river bifurcations using Bayesian rating curves

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