Computation of backwater effects in surface waters of tidal lowland catchments including control structures – An efficient and re-usable method implemented in the hydrological open source model Kalypso-NA (4.0)

Abstract

Abstract. Backwater effects in surface water streams as well as on adjacent lowland areas caused by mostly complex drainage structures are not directly computed with hydrological approaches, yet. A solution of this weakness in hydrological modelling is presented in this article. The developed method enables to transfer discharges into water levels and to calculate backwater volume routing along streams and adjacent lowland areas by balancing water level slopes. The implemented and evaluated method extends the application of hydrological models for rainfall-runoff simulations of backwater affected catchments with the advantages of (1) modelling complex drainage systems in tidal backwater affected lowlands, (2) less effort to parameterise river streams, (3) directly defined input factors of driving forces (climate change and urbanisation) and (4) runtime reduction of one to two orders of magnitude in comparison to coupled hydrodynamic models. The developed method is implemented in the open source rainfall-runoff model Kalypso-NA (4.0). Evaluation results show the applicability of the model for modelling rainfall-runoff regimes and backwater effects in an exemplary lowland catchment (Hamburg, Germany) with a complex drainage system and where the drainage is influenced by a tidal range of about 4 m. The proposed method is applicable to answer a wide scope of hydrological and water management questions, e.g. water balances, flood forecasts and effectiveness of flood mitigation measures. It is re-usable to other hydrological numerical models, which apply conceptual hydrological flood routing approaches (e.g. Muskingum-Cunge or Kalinin-Miljukov).