Controlled Reservoir Drawdown—Challenges for Sediment Management and Integrative Monitoring: An Austrian Case Study—Part A: Reach Scale

Abstract

For Europe, a reduction of 80% of the potential storage volume due to reservoir sedimentation is predicted by 2080. Sedimentation processes trigger the decrease of the storage volume and a related restriction in hydropower production. Further, the artificial downstream flushing of deposited fines has manifold effects on the aquatic ecology, including changes in morphology and sediment quality, as well as increased turbidity and subsequent stress for aquatic species. However, it is common to lower the water surface of reservoirs for technical inspections, which is not comparable to reservoir flushing operations. The presented case study deals with such a controlled drawdown beyond the operational level of the Gepatsch reservoir (Tyrol, Austria). Based on the awareness of possible ecological consequences, an advanced set of measures and an integrative monitoring design, consisting of a detailed event-based quantification of suspended sediments, changes in the morphology, especially with respect to fine sediments, and analyses of the biological quality element fish on the reach scale along the Inn River have been developed.

Controlled Reservoir Drawdown—Challenges for Sediment Management and Integrative Monitoring: An Austrian Case Study—Part B: Local Scale

“…The monitoring during the controlled drawdown of the Gepatsch reservoir [15,31] was established based on the two defined research questions and in order to fulfil the governmental requests (i.e., clauses related to the permit). The monitoring was set up as (i) a reach scale-monitoring network for continuous turbidity measurements, fine sediment deposits on gravel bars and tributary connectivity [15], and as (ii) local scale monitoring presented in this paper of cross-sectional variability of turbidity as well as habitat-related turbidity and sedimentological/ecological analyses by freeze-core sampling to obtain FSI. Moreover, fish egg development (salmonid incubation) was analyzed during the event and the effects on macroinvertebrates were analyzed with pre-and post-event monitoring.…”

“…For evaluating changes of the abiotic environment, measurements of fine sediment infiltration (FSI) and the documentation of the fine sediment infiltration rate are state-of-the-art. A frequently applied assessment tool is freeze-coring (FC), developed to gain information of the Fine Sediment Infiltration (FSI) not only close to the surface but also in the subsurface layer; compare to [13][14][15]. Moreover, suspended sediment concentration (SSC) have, besides the already above mentioned negative influences, been highlighted as one of the central parameters, where the environmental assessment shows clear responses of aquatic organisms [16].…”

“…Here, macroinvertebrates are an important indicator, which are frequently analyzed to evaluate flushing of reservoirs; e.g., [7,9,27]. On a higher trophic level, fish are much more flexible in habitat selection in case of disturbances [28,29], therefore, we consider them as reach scale indicators; compare to [15]. Especially earlier live stages are more sensitive as they are not that mobile compared to older ones; e.g., [30].…”

“…Thus, the aim of this part of the study is to investigate abiotic and biotic elements on a local scale in order to test their indicative power in the framework of an environmental assessment related to the drawdown of the Gepatsch reservoir. The monitoring on reach scale was previously defined in [15]. This contributes to (i) an improved process understanding concerning the fine sediment dynamics in an Alpine river at various scales, as well as (ii) monitoring strategies for future projects based on the detection of possible harmful impacts, e.g., increased turbidity/fines in the river system, as a result of anthropogenic influences.…”

“…The focus in this local scale study is on determining FSI by freeze-core sampling, the analysis of cross sectional variability and habitat-related turbidity, as well as biotic components like macroinvertebrate communities and fish egg development. Important to distinguish but lacking in terms of definitions is the differentiation of (i) reservoir flushing from (ii) a controlled drawdown of a reservoir for operational, inspection or maintenance purposes [15,31].…”