Computation of backwater effects in low lying (marshland) catchments – a re-usable and efficient method in an open source hydrological model

An efficient method to solve a significant weakness in hydrological modelling to compute backwater effects in low lying catchments is presented. The re-usable and transferable method is implemented in the open source software KalypsoNA (KalypsoHydrology) and validated with results of a tidal influenced low lying catchment study. 
Especially in low lying (marshy) catchments, the pressure on current storm water drainage systems raises due to combined impacts of enlarged urbanisation on the one hand and mean sea level rise and heavy storm events on the other hand. Models are applied to analyse and assess the resulting consequences by these impacts on the flood routing along a stream using different hydrological approaches: (i) pure black box (namely empirical, lumped), (ii) hydrological conceptual or (iii) hydrodynamic-numerical approaches. The computation of flow depths, velocities and backwater effects in streams as well as on forelands are not yet modelled with hydrological approaches, but using simplified hydrodynamic-numerical approaches. A requirement for accurate hydrodynamic-numerical modelling is high resolution data of the topography of the main channel and the flood plain in case of bank overflow. Hence, the availability of suitable detailed profile data from measurements is crucial for hydrodynamic-numerical modelling. The comparatively long computing time for hydrodynamic-numerical model simulations is no limitation for answering special research questions, but it poses a limitation in real-time operational application and for meso to regional scale catchment modelling (>100 km²). 
To resolve the shortcomings in hydrological approaches to model water depths and backwater effects, new concepts are required which are applicable for catchments with scarce data availability, efficient for real-time operational model application, open for further model developments and re-useable for other hydrological model implementations.
This contribution presents the development, implementation and evaluation of a method for modelling backwater effects based on a hydrological flood routing approach and a backwater volume routing according to the water level slope. The developed method computes the backwater effects in two steps. First, the inflow from sub-catchments and the non-backwater affected flood routing processes are computed. Secondly, the afflux conditions are calculated which cause backwater effects in upstream direction. Afflux conditions occur mainly at tributary inlets or control structures (for example, tide gates, weirs, retention ponds or sluices). The input parameters comprise simplified or complex geometrical data per stream segment. Therefore, the model is applicable for catchments with a good or scarce availability of data. Computation time is in the range of max 3 minutes even for large catchments (> 150 km² with several sub- and sub-sub-catchments) using a time step size of 15 minutes for a 14 days simulation and is therefore applicable for real-time operational simulations in flood forecasting. 
The proposed method is re-useable and transferable to other hydrological numerical models which use conceptual hydrological flood routing approaches (e.g. Muskingum-Cunge or Kalinin-Miljukov). The open source software model KalypsoHydrology and the calculation core KalypsoNA are available at https://sourceforge.net/projects/kalypso/ and http://kalypso.wb.tu-harburg.de/downloads/. Open access for developments and user application is supported by an online accessible commitment management via SourceForge and a wiki as an online manual.