Impact of a large tropical reservoir on riverine transport of sediment, carbon, and nutrients to downstream wetlands

Large dams can have major ecological and biogeochemical impacts on downstream ecosystems such as wetlands and riparian habitats. We examined sediment removal and carbon (C), nitrogen (N), and phosphorus (P) cycling in Itezhi-Tezhi Reservoir (ITT; area = 364 km(2), hydraulic residence time = 0.7 yr), which is located directly upstream of a high ecological value floodplain ecosystem (Kafue Flats) in the Zambezi River Basin. Field investigations (sediment cores, sediment traps, water column samples), mass balance estimates, and a numerical biogeochemical reservoir model were combined to estimate N, P, C, and sediment removal, organic C mineralization, primary production, and N fixation. Since dam completion in 1978, 330 x 10(3) tons (t) of sediment and 16 x 10(3), 1.5 x 10(3), 200 t of C, N, and P, respectively, have accumulated annually in ITT sediments. Approximately 50% of N inputs and 60% of P inputs are removed by the reservoir, illustrating its potential in decreasing nutrients to the downstream Kafue Flats floodplain. The biogeochemical model predicted substantial primary production in ITT (similar to 280 g C m(-2) yr(-1)), and significant N-fixation (similar to 30% for the total primary production) was required to support primary production due to marginal inputs of inorganic N. Model simulations indicate that future hydropower development in the reservoir, involving the installation of turbines driven by hypolimnetic water, will likely result in the delivery of low-oxygen waters to downstream ecosystems and increased outputs of dissolved inorganic N and P by a factor of similar to 4 and similar to 2 compared to current dam management, respectively.

Carbon and nitrogen dynamics in a soil profile: Model insights and application to a restored Swiss riparian area

David Andrew Barry, Jordi Batlle-Aguilar, Alessandro Brovelli, Pascal Niklaus

The key environmental importance of natural, healthy ecosystems has been progressively recognized and restoration of degraded lands towards their former natural state has become an area of active research worldwide. During restoration, environmental conditions (such as vegetation type and water availability) are manipulated to create ecological conditions suitable for the successful establishment of a target composition of species. Often, ecological restoration induces changes to adjacent ecosystems. This is the case of riparian ecosystems, and their restoration to their original undisturbed situation is likely to cause changes in nutrient cycles. For example, following the restoration of a riparian zone, microbial communities adapted to one set of environmental conditions have to acclimatize to another, and the subsequent changes in the composition of the biomass populations might induce changes in soil organic matter mineralization and soil respiration rates. Since the biogeochemical cycles are tightly interconnected, these changes can trigger nutrient storing or release, therefore inducing changes in nutrient cycles of adjacent ecosystems. Overall, the effects of the restoration activities on the hydrologic regime, soil properties and vegetation are still largely unknown and poorly understood. Within the RECORD project (http://www.cces.ethz.ch/projects/nature/Record), a large collaborative research effort undertaken to monitor and understand the changes in ecosystem functioning in riparian areas undergoing restoration, a numerical model has been developed to simulate the vertical transport of the mobile C and N components in a soil profile (model development discussed in the companion submitted abstract Batlle-Aguilar et al.). In the model, microbial decomposition of the soil organic matter drives biogeochemical transformations of C and N, while the activity of the soil biota is primarily controlled by the soil moisture content. The temporal evolution of the soil properties measured at one location of the RECORD experimental site, in a mixed forest dominated by ash and maple characteristic for the transition from riparian to upland forest, was used to validate the model and to gain insights into the key factors controlling the nutrient turnover. The site is located next to the Thur River where a revitalization project involving removal of levees has been implemented to create more natural conditions in the riparian zone. Soil water content and temperature at several depths were monitored continuously between October 2008 and October 2009. In October 2008, January 2009 and in biweekly frequency between April and October 2009, topsoil and soil solution at several depths were sampled. The soil solution samples were analysed for major carbon and nitrogen species, and the soil samples for denitrification enzyme activity, potential nitrification and related properties. In addition, soil respiration and N2O emissions were measured at each sampling event. Preliminary modelling results are shown, together with a discussion of the most influential parameters and processes controlling C and N turnover in riparian soils.

2010

Hybrid modeling of sediment management during drawdown of Räterichsboden reservoir

Milad Daneshvari, Giovanni De Cesare

The Swiss utility Kraftwerke Oberhasli AG (KWO) currently implements "KWO plus", an upgrading program involving a large number of economical and ecological improvements of their hydropower schemes. A major project, the modification of the Handeck 2 power plant intake, requires total emptying of the Räterichsboden reservoir having an active volume of 25 million m3. In order to define the most efficient and ecological emptying process for these circumstances, KWO commissioned a hybrid modelling program of the sediment remobilisation processes during the planned reservoir drawdown. Hydraulic model tests were conducted at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) by means of a 1:35 scaled Froude model. Numerical simulations of sediment transport were performed by the Laboratory of Hydraulic Constructions (LCH) using the software FLOW-3D. A good agreement between both models was found for total values of the sediment concentration and its variation in time. Both the physical and the numerical model were validated by prototype data obtained during an annual functional test of the bottom outlet. These measurements were conducted at three downstream positions in the Aare River. The involved solution procedures provide the (1) sediment flux during pressure flushing, (2) influence of outlet gate opening height and opening velocity on sediment concentration, and (3) time-dependent distribution of sediment concentration along the Aare River.

CRC Press, Taylor & Francis Group2010

Influence of in- and outflow sequences on flow patterns and suspended sediment behavior in reservoirs

Michael Müller

Reservoir sedimentation and the resulting storage losses impact reliability, efficiency, safety and thus the sustainability of the hydropower schemes. Beside traditional storage hydropower plants, also pumped-storage facilities are affected. Flexible turbine and pump operations between two reservoirs allow demand-depending electricity production and absorption. Thus, they play a dominant role in peak load energy production as well as grid regulation. The storage volumes are influenced by continuous in- and outflow cycles. Their impact on sediment settling processes has not been addressed in research yet. Inspired by the cyclic, bidirectional water exchange of pumped-storage plants, a novel method for sediment removal from reservoirs was investigated in the present study. After high sediment yield into the reservoir, fine sediment should remain in suspension in front of the water intake due to the pumped-storage induced turbulence. Thus, the settling process may be delayed and sediment be evacuated by the power or flushing tunnels. In prototype measurements and laboratory experiments the influence of pumped-storage operations on flow patterns as well as the settling rates and sediment balance in two interconnected reservoirs has been studied. The two approaches were completed by numerical simulations. In the lower reservoir of a pumped-storage hydropower plant in the Swiss Alps, flow velocities in front of the intake/outlet were measured by Acoustic Doppler Current Profilers (ADCP). The devices with independent energy supply were implemented on the reservoir bottom and sampled 2D flow velocity profiles over several weeks. The measurements showed only local influence of pumping (outflowing water) near the intake, whereas turbine operations (inflowing water) induce large eddy flow fields in the reservoir. Depending on lake topography, patterns with backflow can appear. A frequency analysis of the discharge and flow velocity series indicated a corresponding main period between the flow velocity profiles and the in- and outflow cycles of 1 day. The ANSYS-CFD simulated flow fields corresponded to the in situ flow patterns. The computed turbulent kinetic energy input due to turbine operation was some 25 times higher than the natural input by wind-forcing. In the upstream part of the power shaft of the same hydropower scheme, a turbidity probe was installed for monitoring reasons. Over a period of eight months, sediment concentration of the operated water was continuously measured. An autonomous remote data acquisition and transfer system may be helpful for real-time monitoring by the hydropower operators. The measurements showed seasonal change of sediment concentration in the power system. In winter, high reservoir levels and ice-cover reduced sediment content, whereas in spring, snowmelt and low reservoir levels increased sediment yield. Short-term variations of sediment concentration up to 16% correlated with the in- and outflow cycles, especially for low reservoir filling. During the sampling period, about 45’000 t of fine sediment were moved between the two lakes. However, the sediment balance due to pumped-storage operation remained equilibrated. Fine sediment was kept in suspension in front of the intake/outlet structure by the operation induced flow patterns. Pumped-storage facilities would only increase sedimentation in case of unequal sediment concentrations in the two reservoirs. The impact of magnitude and frequency of the in- and outflow cycles on flow field and fine sediment settling behavior was studied in laboratory experiments. Between two interconnected rectangular basins, sediment-laden water was pumped back and forth according to regular in- and outflow cycles. In both volumes, turbidity was continuously measured. In the main basin, Ultrasonic Velocity Profilers (UVP) allowed 2D flow velocity measurements and the analysis of the developing inflowing jet. In a numerical model, time for achieving steady state conditions in the reservoir was defined. This discharge dependent duration indicated the frequency of the initial in- and outflow cycles. A reference test in stagnant water conditions defined the sediment settling curve without operation, corresponding to a basis of comparison for the investigated test configurations. During five in- and outflow cycles of various magnitude and frequency, the evolution of suspended sediment ratio SSR was measured and compared to the reference case. Sediment influx and release (influx sediment rate ISR and evacuated sediment rate ESR) and thus the sediment balance SB of the system could be defined. In a first stage, 60% of the initially suspended sediment settled independently from magnitude of the in- and outflow cycles. High cycle frequency applied during this phase led to considerably higher SSR by the end of the test sequence. Low discharges increased particle suspension by 10 to 40% compared to conditions without operation, whereas high discharges increased concentration by 50 to 80%, depending on cycle frequency. With an intake/outlet structure close to the bottom or to the reservoir surface, 20% higher sediment concentrations could be achieved than in case of intermediate position. Sediment balance is only marginally impacted by in- and outflow cycles. In situ as well as laboratory investigations highlight the fact that pumped-storage operations influence the flow patterns in reservoirs, especially in the near intake area. The induced turbulence keeps fine sediment in suspension, leading to an extensive sediment exchange between the storage volumes. The sediment balance remains unaffected, as long as the sediment concentrations in the reservoirs are similar.