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Floods are amongst the most frequent and widespread disaster worldwide, posing enormous development challenges. Also in West Africa, flood risk still needs proper addressing. Ecosystem-based disaster risk reduction (Eco-DRR) approaches are increasingly recognised as cost-effective part of the solution, providing ecosystem services that reduce all three components of risk, namely hazard, exposure and vulnerability. Indeed, Eco-DRR, such as floodplain restoration or agroforestry, can affect hydrological processes, altering the flood hazard and exposure, and provide ecosystem services that reduce people’s vulnerability to floods and/or enhance their adaptive capacities. To fully understand the impact of Eco-DRR on the three components of risk, it is thus important to take an interdisciplinary approach to the assessment of Eco-DRR. Yet, there remains a substantial gap in the comprehensive evaluation of Eco-DRR effects, including guidance on how to depict the flow of ecosystem services and their benefits to people, which undermines the effective use of Eco-DRR measures in flood-prone environments.Using the case study of flood risk in the Ouémé River Basin, this contribution will share advances in the comprehensive evaluation of Eco-DRR measures. After defining locally-relevant Eco-DRR measures based on administrative plans, the scientific literature, and expert surveys, a systematic literature review has been conducted to understand the impact of selected Eco-DRR measures on both hydrological processes and ecosystem services provisioning, so as to evaluate the Eco-DRR measure against its effects on all risk dimensions. For the hazard, the hydrological model SWAT is used, comparing the flood hazard under different land use and land cover change scenarios. Preliminary results, with a focus on the Eco-DRR measure of agroforestry, and lessons learnt will be presented in the session.

West Africa has a complex climate regime. It affects hydrological predictivity in the region where the majority population depends on agriculture. With a warming planet, the challenge is further exacerbated by frequent hydroclimatic extremes. To achieve secured livelihoods and resilience, hydrological understanding is a key. However, there is an elemental challenge of missing measured weather data. Weather variables that are drivers of the water and energy balance are necessary for the setup of robust hydrological models. We focus on the Ouémé River Basin in Benin, which lacks spatially representative in-situ temperature observations. To fill this gap, the study evaluates global earth datasets in the form of reanalysis products that are emerging useful for hydrological modeling. We perform an intercomparison of five temperature reanalysis datasets for the basin using the hydrological model Soil and Water Assessment Tool (SWAT). These datasets are CFSR, CPC, ERA5, EWEMBI, and PGFv3, available at a daily temporal resolution. We test their performance on the simulation of hydrological processes in the Ouémé basin.To evaluate each temperature data, a multi-site calibration is performed in SWAT using daily discharge time series. Validation is carried out as a two-fold process. The first is point validation performed using discharge data at five gauge sites and the second is spatial validation on the sub-catchment level conducted using satellite-derived actual evapotranspiration (AET) data from GLEAM v3.5b. This multi-gauge and multi-variable approach is used to minimize uncertainties associated with the application of SWAT.This study is one of a kind for the basin, testing the datasets for their hydrological performance and overcoming a major gap toward achieving robust models. Temperature reanalysis products provide high temporal resolution, long time series, and spatially representative datasets. However, the response to input data errors can vary significantly given the non-linear interaction of parameters in a hydrological model. Therefore, hydrological evaluation is an important step before reanalysis data can be used for modeling and decision-making. We also demonstrate the significance of testing multiple water fluxes to assess the performance of climate datasets. A higher variation in performance for temperature datasets is observed for AET than for the streamflow component. It is an important outcome to determine the most suitable temperature product for the Ouémé basin.

Nature-based retention measures are an essential part of a sustainable and integrated flood protection strategy and can contribute to a holistic flood mitigation approach. Thereby river restoration measures such as channel widening, or flow path extension to increase the channel meandering represent successfully used components. Coupled with flood plain measures, retarding and retention effects of flood events are possible. These effects are commonly computed applying two-dimensional hydrodynamic modelling approaches. However, these developments rely on high spatial and temporal resolutions which are generally characterized by a high computational demand and are hence time and cost expensive. Thus, the evaluation and derivation of flood routing parameters to reproduce the resulting hydrodynamical processes in hydrological models can provide an effective and fast computation of river restoration scenarios. The objective in the present study is the derivation and application of flood routing parameters which can account for the effects of river restoration and flood plain measures in hydrological models. Further, this study aims to determine if the catchment and scale specific outcomes and parameter sets are also applicable to a broader range of catchments.For this purpose, commonly applied flood routing approaches and the associated parameters used in hydrological models (e.g. the kinematic wave approach in the WaSiM model) are investigated for catchments of different scales in Bavaria (Germany) and for flood events of varying characteristics (e.g. return period, flood volume). To determine the effects of channel restoration and flood plain measures, two-dimensional hydrodynamic models (HYDRO_AS-2D) are set up to simulate the current state as well as restoration scenarios. Based on the simulation results of the hydrodynamic models, the parameters of the flood routing approaches are calibrated to match the catchment specific restoration effects for a first set of river sections. Catchment and scale dependent parameter sets (dominating valley type, flood plain slopes) are then derived to reproduce the specific river restoration. First results of the calibration of the parameter sets show a satisfying fit of the hydrological model to different restoration scenarios of the hydrodynamic model. For the validation of the derived parameter sets of the flood routing methods in the hydrological model additional river sections of the hydrodynamic models are subsequently investigated.The implementation of the new flood routing parametrization of the hydrological models is finally examined as an alternative resource efficient way of calculating the effects of river restoration scenarios. Moreover, the applicability of the outcomes as a cost-efficient alternative compared to hydrodynamic models in land use planning and risk assessment is assessed and discussed within the frame of river restorations as flood mitigation measures.

Nature-based solutions are an important component of integrated flood mitigation strategies for improving both the protection against hazardous flood events and the ecological conditions of river-floodplain systems. In order to be able to take these types of measures into account in upcoming flood management decisions, it must be possible to reliably estimate their effects on flood events. Therefore, this study focuses on a more general view on the catchment dependent contribution of combined river and floodplain restoration measures to the strengthening of river retention and flood protection. Furthermore, the importance of considering site-specific circumstances (e.g., the superposition of the flood waves of the main river and its tributaries), is evaluated.The study is based on five investigation areas in Bavaria (Germany) with various topographic properties and different spatial scales (~ 90 – 560 km2). For each catchment, a physically based hydrological model (WaSiM) was coupled with the two-dimensional hydrodynamic model HYDRO_AS-2D by means of direct and diffuse inflow boundary conditions. Five flood events with various rainfall characteristics (advective/convective) and different return periods (5, 20 and 100 years) were generated with WaSiM. The holistic restoration scenarios are implemented by catchment dependent modifications of river channels and floodplains. As the aim of this study is to analyze the maximum possible efficiency of the restoration scenarios, it is assumed that almost the entire floodplain is available for the implementation of these measures. Highly restricted areas (e.g., settlement & industrial areas, important infrastructure) are excluded from this assumption. First results show that the peak discharge attenuations resulting from the restoration measures are exemplarily dependent on the characteristics of the floodplains (e.g., slope and extent) and the volumes of the flood events. It could be shown that the largest peak discharge attenuations (up to 28 %) and retardation (up to 8 h) occur in catchments with relatively flat and wide floodplains in combination with comparatively small flood volumes. Furthermore, the effectiveness of these measures can be considerably affected by local superposition effects with incoming tributaries. These effects can have site and event specific positive or negative impacts on the peak discharges and may not be neglected when planning restoration measures.Based on these investigations, it is possible to evaluate if catchments are likely to be suitable for river and floodplain restoration in the course of flood management decisions. However, the effectiveness of the measures is always influenced by a combination of many area-specific factors that can only be predicted to a limited extent and therefore requires the modelling of an area.