Assessing the impact of pumped-storage operation on lake stratification

The practice of impounding water from mountain streams for anthropogenic (e.g., hydropower) uses has been shown to significantly affect riverine ecosystem, mainly by reducing the biodiversity. The biogeomorphological basis responsible for such an effect has been related to the establishment of minimum flow requirements from river intakes and/or reservoirs (Arthington et al., 2006). To minimize this ecological impact, it is imperative to assess the extent of ecological disturbance resulted from changing the natural flow regime. The environmental suitability of flow release rules is often assessed for different species by modeling software such as CASiMir and PHABSIM (Maddock, 1999, Milhous et al., 1989). These softwares are commonly used to model Weighted Usable Area (WUA) curves for the fishes. Although they can model the WUA curves corresponding to the riverine ecosystem in large scales, they are not able to capture the hydrodynamic impacts resulted from smaller scales such as the effect of macroroughnesses (i.e., boulders). Due to the limitation of computational cost, mesh scale used in the numerical software is larger than the macroroughness size. As a result, the presence of macroroughnesses is filtered and their effect cannot be seen in the simulated WUA curves. However, the presence of stones may positively affect the riverine ecosystem by inducing sheltered zones (i.e., the wake), which are typically temporary stationary points for many fish species. Fishes minimize energy expenditure by resting in these low velocity regions, but can quickly move to nearby fast water to feed (Hayes and Jowett, 1994). By quantifying these wake areas we will be able to assess how the physical properties and number of such zones may change in response to the changing hydrologic regime. Up to a given flow rate that covers such stones, the total wake area in the river reach being monitored is therefore a function of the streamflow, Q, and it is an actual Usable Area for fishes that can be used to correct the one computed by classic software such as PHABSIM or CASIMIR at each flow rate. Quantifying Weighted Usable Area Produced by Macro Roughness Elements in River Flow and Habitat Simulations Holly J. Oldroyd1, Amin Niayifar1 and Paolo Perona2 D. Future Work • Perform controlled laboratory experiments to quantify uncertainties in using the Negtretti et al. (2006) solution for more realistic macro roughness elements of varying aspect ratio and shape • Obtain field data: Ariel photogrammetry and discharge for a test river • Test and compare our method to traditional habitat simulations, i.e., test our hypothesis. B. Results for a Synthetic River with 10 Boulders References 1. ARTHINGTON, A H, BUNN, S E, POFF, N L & NAIMAN, R J (2006). The challenge of providing environmental flow rules to sustain river ecosystems. Ecological Applications, 16, 1311-1318. 2. HAYES, J & JOWETT, I (1994). Microhabitat models of large driftfeeding brown trout in three New Zealand rivers. North American Journal of Fisheries Management, 14, 710-725. 3. LIAO, J C (2007). Review of fish swimming mechanics and behaviour in altered flows. Phil Trans R Soc B. 362, 1973-1993. 4. MADDOCK, I (1999). The importance of physical habitat assessment for evaluating river health. Freshwater Biology, 41, 373-391. 5. MILHOUS, R T, UPDIKE, M A & SCHNEIDER, D M (1989). Physical Habitat Simulation System Reference Manual: Version II. US Fish and Wildlife Service. 6. NEGRETTI, M E, VIGNOLI, G, TUBINO, M & BRONCCHINI, M (2006). On shallow-water wakes: An analytical study. J Fluid Mech, 567, 457-475. A. Project Methods 1. Survey a river using high-resolution aerial photography: Geomorphology, distribution and size of boulders 2. Use a hydrodynamic model to 2D flow conditions on a coarse grid: u∞ and h∞ upstream of boulders 3. Estimate the wake area behind stones: • Use the analytical solution of the shallow water equations by Negretti et al. (2006) for wakes behind cylinders in shallow flow S=ratio of bed friction to lateral shear effects on TKE Typical river flow regime Adapted from Negretti et al. (2006) For S > 0.5, obtain wake dimensions: Then integrate to estimate wake area Adapted from Liao (2007) Sheltering and resting C. Implementation to Correct Traditional WUA curves Directly incorporating the wake areas generated by boulders into WUA estimates will: • change the curvature of the WUA estimate (from red to green lines), • shift the threshold point (yellow dot) to the right, • will enable lower minimum flow requirements, and result in higher potential for hydropower production. 4. Calculate an improved UWA estimate: Combine wake area into UWA obtained from a river habitat simulation ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE 1 2 • Uniform river flow: Q from 0 to 2.5 m3/s Manning coefficient: n = 1/30 • 10 boulders with random D and h: 0.1 m < D < 0.4 m 0.05 m < h < 0.25 m Stream diversion for hydropower Hypothetical WUA curves: Note: Minimum flow requirements are typically set by a variety standards according to the UWA curves (e.g., the threshold [yellow dot] or break point [blue line]).