Artificial floods in residual flow reaches – How do they affect hydraulic habitats

Floodplains in hydropower-controlled rivers, where a constant residual flow discharge is imposed, often lack sediment supply and periodic inundation due to the absence of natural flood events. In this study we compare two residual flow systems. The first river is the Spöl, located in the Swiss National Park close to the boarder with Italy. Since the year 2000, artificial floods are released from the Ova Spin Dam on a yearly bases, with the aim to restore river dynamics and habitats. The second river, is the Sarine in Western Switzerland where an artificial flood was released from the Rossens Dam for the first time. At one location this flood event was combined with sediment replenishment. In this study, an analysis of the two different systems before and after the event using the Hydro-Morphological Index of Diversity (HMID) is presented. The index is based on the variability of flow depth and flow velocity and represents the habitat diversity on a reach scale. Results reveal that a flood pulse can significantly increase hydraulic habitat diversity in a river reach and that sediment availability is crucial. Nevertheless, the change in habitats of a single flood event has a minor effect if artificial floods are periodically released. Under such conditions, the river has already reached an equilibrium condition considering habitat diversity.

Hydraulic-morphological habitat assessment in rivers considering residual flow, sediment replenishment and artificial flood events

Severin Stähly

Dams store water and trap sediment in their reservoirs. Downstream river segments, hence, are affected by a limited sediment dynamics. This becomes obvious by river bed incision, limited geomorphological variability and depletion of hydraulic habitats for in-stream ecological organisms. Replenishment is a measure to counteract the lack of sediment. In segments dams such a sediment replenishment can be combined with the release of an artificial flood, by activating spillways and bottom outlets. Recent laboratory studies revealed the potential of multi-deposit replenishment configuration towards an increase of geomorphological richness in the river. However, this has not yet been tested in the field. The hydro-morphological index of diversity (HMID), is based on the spatial variability flow depth and velocity in a river segment, allowing the quantification of the change in morphology such a measure. Its ease in computation and its objectivity makes the index a useful tool for a large variety of users. However, knowledge concerning data sufficiency and a standardized sampling procedure does not exist. Further, the HMID does not account for temporal variability. This research aims at closing these gaps of knowledge for a sustainable management of complex floodplains downstream of dams. Flow depth and velocity data from 19 river segments were analyzed with the HMID. Structured sub-sampling and analysis of the data revealed different data sufficiency depending on the complexity of the river geomorphology. A representative HMID value is computed from at least 100 datapoints distributed over eight cross-sections, in braided rivers more datapoints are required. The geomorphological changes by an artificial flood event in the Sarine were analyzed with the HMID. In the impact perimeter of the multi-deposit sediment replenishment, the HMID increased by 36 % in the Sarine, in the rest of the river 18 %. A similar study was investigated at the Spöl, pointing out that sediment availability is crucial for the hydraulic habitat diversity. With the help of 489 tracer stones, the movement of the replenished sediment was investigated in detail. The deposition pattern results in repeating sediment clusters on the riverbed, as it was observed in the laboratory experiments. For the two characteristic diameters, the tracers allowed the estimation of transport velocities of 19 cm/min (dm = 57 mm) and 28 cm/min (d90 = 113 mm) during the flood event and consequently the erosion efficiency of the different parts of the flood hydrograph. Macroinvertebrates were less affected by the artificial flood event in the hydropeaking river segment of the Sarine compared with the residual flow segment. The species diversity indices decreased significantly due to the artificial flood event. A direct link between flow regime and EPT richness was observed. A river with a natural flow regime hosts the highest richness of macroinvertebrates followed the residual and finally the hydropeaking flow regime. For other ecological indicators, additional physical river parameters for example the frequency duration curve or the grain size distribution need to be considered as well.

EPFL2019

Sediment replenishment as a measure to enhance river habitats in a residual flow reach downstream of a dam

Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss, Severin Stähly

Floodplains downstream of a dam, where the natural flow regime is replaced by a constant residual flow discharge, often lack sediment supply and periodic inundation due to the absence of natural flood events. In this study, a flood with a one-year return period was released from an upstream reservoir and combined with sediment replenishment. The aim was to enhance hydraulic habitat conditions in the Sarine river downstream of Rossens dam in Western Switzerland. A special configuration of sediment replenishment was applied. It con- sisted of four sediment deposits distributed as alternate bars along the river banks, a solution which was previously tested in the laboratory. The morphological evolution of the replenish- ment and of the downstream riverbed were surveyed including pre- and post-flood topog- raphy. The hydro-morphological index of diversity (HMID) was used to evaluate the quality of riverine habitats in the analyzed reach. It is based on the variability of flow depth and flow velocity. The combination of the artificial flood with sediment replenishment proved to be a robust measure to enhance sediment dynamics.

Canadian Dam Association2019

Operation of Complex Hydropower Schemes and its Impact on the Flow Regime in the Downstream River System under Changing Scenarios

Martin Peter Bieri

Hydropower is the world’s most important renewable electricity source. More than 40% of European hydroelectric energy is produced in Alpine countries. High-head storage hydropower plants (HPP) contribute significantly to peak energy production as well as electricity grid regulation. Future plant management is faced with several challenges concerning modified availability of water resources due to climate change as well as new economic constraints associated with legal, political and electricity market issues. HPP operation results in unsteady water release to the downstream river system. Hydropeaking is the primary factor of flow regime alteration, impacting the river ecosystem. Even when the biological response to hydropeaking is not fully understood, the recently adapted law on water protection prescribes its mitigation in Switzerland. In this research project, a novel integrative approach to model and assess the impact of the operation of a complex hydropower scheme on the downstream river system is developed. It contains (1) a precipitation-runoff model extended for long-term simulations of glacierized Alpine catchment areas, (2) an operation tool for high-head storage HPP, (3) flow regime generation with cost estimation of hydropeaking mitigation measures and (4) a habitat model of reference river morphologies for a target species. The upper Aare River (Hasliaare) in Switzerland is an Alpine stream, affected by hydropeaking from a complex hydropower scheme with several storage volumes and power houses. Since the 1930s, seasonal water transfer from summer to winter and the amplitude and frequency of daily peak discharge have been continuously increased. Furthermore, the dynamic braided river network with various mesohabitats gave way to a mainly monotonous channel. Although diversity of species and biomass of aquatic biota have drastically decreased, the potential of redevelopment remains. Investigations to improve the river morphology and the flow regime are under discussion. The upper Aare River catchment is therefore an appropriate case study for analysis of the interactions between climatic, hydrological, hydraulic, economic as well as ecological parameters. The simulation of runoff in Alpine catchment areas is essential for optimal hydropower exploitation under normal flow conditions, but also for the analysis of flood events. The semi-distributed conceptual modeling approach Routing System contains a reservoir-based precipitation-runoff transformation model (GSM-SOCONT), extended by dynamic glacier simulation tool. Spatial precipitation and temperature distributions are taken into account for simulating the relevant hydrological processes, such as glacier melt, snowpack constitution and melt, soil infiltration and runoff. The model development, calibration and validation are illustrated for the 2005 flood event, where the flood reduction capacity of the HPP is discussed, as well as future long-term runoff estimations. Climate change scenarios, based on a reference climate period, take into account intra-annual temperature and precipitation variations as well as their long-term tendencies. Runoff series of daily resolution are produced by hourly updating of the meteorological, glaciological and hydrological parameters. An almost complete deglacierization of the upper Aare River basin is simulated for the late 21st century. The resulting reduction of glacier melt in summer and earlier snowmelt in spring change the runoff regime from glacio-nival to nival. The implemented heuristic hydropower modeling tool in Routing System allows simulation of the operating mode of complex HPP. Within the case study of the upper Aare River catchment and despite the complexity of the HPP network, the influence of climate change, electricity market issues, plant enhancements as well as hydropeaking constraints is simulated and assessed. Despite the reduction of future runoff, increased flexibility due to new turbine and pumped-storage capacities allows compensation, especially in the case of volatile electricity prices, and could even partially restore the natural flow regime. Several operational and construction measures to reduce hydropeaking are implemented in the model. Resulting flow regimes as well as the related costs are defined. Operational constraints, such as limitation of turbine discharge, increase of residual flow or limited drawdown range, generate relatively high costs compared to their environmental effectiveness. Better ecological and economic response is achieved by construction measures, such as flow deviation systems or compensation basins installed downstream of the power house outflow where the water is temporarily stored and then released to the river by a guided system. The simulated flow regimes are rated by a river specific habitat model for representative morphologies and three life stages of the target species brown trout (Salmo trutta fario). This is based on results from a 2D hydrodynamic model and in situ investigations undertaken in the framework of a joint project of EAWAG. Steady and dynamic indicators quantify fish habitat suitability and allow comparison through economic indices of the implemented mitigation measures. For the Hasliaare River, investments for mitigation of hydropeaking are only justified by morphological improvements. The developed approach is useful for the enhancement of complex storage hydropower schemes regarding mitigation of altered flow regimes. Despite several uncertainties, it allows operators, authorities and researchers to define and rate the impact of HPP operation on the river network, to ecologically and economically assess mitigation measures and thus to address hydropeaking in a straightforward manner.