Conceptual approach for a holistic low-flow risk analysis

The recent drought events (e.g. 2022) highlighted the impacts caused by hydrological drought and low-flow events to society and ecosystems. The consequences of low-flow events in recent years emphasizes the urgent need for a structured low-flow risk management. The DryRivers project aims to develop a software-based tool for an effective support of low-flow risk management. The low-flow risk analysis is the core of the supporting tool and will be described in detail in this work.

In the field of flood risk applications, scenario-based calculations are often performed. Due to a relative short duration of flood events between a few days to a few weeks and in general negligible hydrological interaction between temporal distant flood events, a clear distinction of such events is quite simple. However, for low-flow risk modelling, the definition of scenarios is considerably more complex due to their long-term development and occurrence. Thus, hydrological conditions from previous years can be essential for the development of a low-flow event. Due to this, the use of long-term continuous time series seems to be more suitable for low-flow risk modelling than a scenario-based approach. This continuous approach has been used in flood risk analysis by Sairam et. al. (2021). In this work it is adapted and extended for low-flow risk analysis.

The approach to low-flow risk analysis consists of four basic analyses. These include the meteorological-hydrological analysis, which generates synthetic long-term weather data - using, e.g., a stochastic weather generator - and transforms these weather data into long-term runoff time series. Therefore, a rainfall-runoff model is applied, considering the catchment-specific characteristics. The hydrodynamic analysis quantifies water levels, water temperatures and flow velocities along the river. Core of the analysis is a numerical 1D-river model, which calculates the hydraulic values using runoff time series and river characteristics (e.g., cross sections). The influence of the near-surface groundwater on the river by in-/exfiltration is considered via a bidirectionally coupled 2D-groundwater model. Water temperature is determined in a unidirectionally coupled temperature model. Weather data and hydraulic values are transformed into water temperature within the river. Based on the time series of the hydraulic values the consequences of low-flow are quantified as sum over the considered period within the analysis of consequences. Different categories of low-flow consequences are considered: socioeconomic consequences, e.g., for shipping or industrial water use, as well as ecological consequences for fish and macrozoobenthos. Threshold approaches for quantifying impacts are generally applied in both categories. Finally, in the risk analysis, the low-flow risk is calculated by dividing the damage sums per consequence category with the number of simulated years. This results in an annual low-flow risk, e.g., in €/a. The calculated low-flow risk is an essential basis for a transparent and objective decision support in low-flow risk management.

This research is funded within the research framework of WaX (Wasser-Extremereignisse) by the Federal Ministry of Education and Research of Germany.

 

Sairam, N., Brill, F., Sieg, T., Farrag, M., Kellermann, P. and Nguyen, V. D. (2021), Process‐Based Flood Risk Assessment for Germany, Earth's Future 9 (10), DOI: 10.1029/2021EF002259.