HEM Impoundment—A Numerical Prediction Tool for the Water Framework Directive Assessment of Impounded River Reaches

Michael Tritthart,Christoph Hauer,Helmut Habersack,Peter Flödl

doi:10.3390/w12041045

Michael Tritthart, Christoph Hauer + Show 2 more

Open Access

https://doi.org/10.3390/w12041045

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Abstract

A novel prediction tool is presented as a component of the Habitat Evaluation Model (HEM), which allows the assessment of the ecological status of impounded water bodies based on environmental factors that were shown in literature to correlate with the abundance of benthic macro-invertebrates. Main model parameters are the observed grain sizes and depth-averaged flow velocities obtained from a hydrodynamic simulation. The tool was tested in three Austrian river reaches. It was found that the river lengths predicted to be ecologically affected by the impoundments were substantially shorter for mean flow conditions than previously assessed when employing a physical mapping approach. The differences disappeared for low discharge conditions. The numerical prediction tool allows us to perform a status assessment for discharge conditions, which are potentially more representative of the annual discharge spectrum than those within the in-situ observable range. This property, thus, bears the potential to facilitate the recommendation of sediment management strategies in impounded river reaches in the future.

Highlights

The abundance and biomass of aquatic biota, such as fish and benthic macro-invertebrates, reflects the hydro-morphological status of a water body [1]
The presented novel prediction tool Habitat Evaluation Model (HEM) Impoundment aggregates observed grain sizes and depth-averaged flow velocities resulting from a hydrodynamic simulation to perform an assessment of the ecological status of impounded river reaches
Since this approach allows a detailed analysis of the entire wetted area discretized by computation nodes, river lengths as well as areas and area fractions are obtained, which are predicted to be ecologically affected by the impoundment

Summary

Introduction

The abundance and biomass of aquatic biota, such as fish and benthic macro-invertebrates, reflects the hydro-morphological status of a water body [1]. Water quality and quantity of flow as well as bed sediments, such as the actual sediment composition, are both key elements and limiters of the abundance of benthic macro-invertebrates [2]. Lack of gravel and gravel dynamics are known to yield negative consequences for aquatic biota [3]. This is the case in impounded river reaches, where fine sediment in suspension settles due to low flow velocities. Various processes have been identified to play a role in habitat degradation of benthic macro-invertebrates related to fine sediment. The clogging of pore space based on very fine material, mainly clay substrate, has been frequently documented [4] and defined as siltation to describe the degradation of macro-invertebrate habitats [5]. An overall sediment deficit in free-flowing sections was shown to result in a depauperate

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