Numerical analysis of rapid drawdown: applications in real cases

In this study, rapid drawdown scenarios were analyzed by means of numerical examples as well as modeling of real cases with in situ measurements. The aim of the study was to evaluate different approaches available for calculating pore water pressure distributions during and after a drawdown. To do that, a single slope subjected to a drawdown was first analyzed under different calculation alternatives, and numerical results were discussed. Simple methods, such as undrained analysis and pure flow analysis, implicitly assuming a rigid soil skeleton, lead to significant errors in pore water pressure distributions when compared with coupled flow-deformation analysis. A similar analysis was performed for the upstream slope of the Glen Shira Dam, Scotland, and numerical results were compared with field measurements during a controlled drawdown. Field records indicate that classical undrained calculations are conservative but unrealistic. Then, a recent case of a major landslide triggered by a rapid drawdown in a reservoir was interpreted. A key aspect of the case was the correct characterization of permeability of a representative soil profile. This was achieved by combining laboratory test results and a back analysis of pore water pressure time records during a period of reservoir water level fluctuations. The results highlight the difficulty of predicting whether the pore water pressure is overestimated or underestimated when using simplified approaches, and it is concluded that predicting the pore water pressure distribution in a slope after a rapid drawdown requires a coupled flow-deformation analysis in saturated and unsaturated porous media.Peer ReviewedPostprint (published version

SALMON RECOVERY IN THE COLUMBIA RIVER BASIN: ANALYSIS OF MEASURES AFFECTING AGRICULTURE

The effects of salmon recovery measures on the Northwest agricultural sector are evaluated. Relevant recovery measures, such as: modified timing for dam releases, reservoir drawdown, and flow augmentation in the Columbia River basin, on the regional agricultural sector are evaluated. Combined, these measures would increase power rates, grain transportation costs, and irrigation water costs and reduce the supply of water to irrigators. We quantify these input cost and quantity changes and combine them into seven recovery scenarios for analysis. Results suggest that drawdown and/or minor reductions in irrigation water diversions would reduce producers' profits by less than 1% of baseline levels. However, the most extreme scenario-a long drawdown period combined with a large reduction in irrigation diversions-would reduce producers' profits by $35 million (2.5$35 million decline in annual profits, more than $27 million occur in southern Idaho and eastern Oregon. The federal government would bear these costs if it acquires water via voluntary transactions.Agricultural and Food Policy, Resource /Energy Economics and Policy,

THE EFFECT OF RESERVOIR WATER LEVEL FLUCTUATION TO THE SEEPAGE ON EARTH DAM

The modeling of earth dam was carried out in a drainage and seepage tank to analyze the seepage resulting from water level fluctuation in the upstream of the dam. The dam models were made of the mixture of Mt. Merapi sand deposit with the soil of sandy-silt from Wonosari area. The variations of sand content in the mixture were 100%; 90% and 80% and the upstream slope inclinations were 1:1; 1:1.5 and 1:2. The result showed that the dams with more sandy-silt in the mixture have smaller seepage and the dams with steeper upstream slope have greater seepage. During rapid rising of water level, the dams with steeper upstream slope have a high rising rate of upstream water level and higher height of downstream slope failure. Moreover, during rapid drawdown, the dams with gentler upstream slope have a smaller rate of upstream drawdown and lower height of upstream slope failure. The dams with more sandy-silt in the mixture have a higher value of rising rate and drawdown of upstream water level but lower height of downstream and upstream slope failure. In the dam management, continuous monitoring of the seepage resulting from reservoir water level fluctuation is required to avoid dam failure. Keywords: Earth dam, rapid rising, rapid drawdown, seepage, slope failure.

Population characteristics and habitat selection of muskrats (Ondatra zibethicus) in response to water level management at the Summerberry Marsh Complex, The Pas, Manitoba, Canada

Hydroelectric developments have significantly altered the hydrology and the historical wetland cycle in the Saskatchewan River Delta (SRD) by reducing both long term and within year flood frequency. This research details the responses of SRD wetlands to water level manipulation and links it to the habitats selected by muskrats thus highlighting the conditions that should be the focus of wetland management. Following the partial drawdown (PD) in the fall of 2007, muskrat densities derived from mark recapture surveys did not differ between PD and full supply level (FSL) wetlands. On a per flooded area basis, PD wetlands supported residual muskrat population at similar densities as FSL wetlands during the years of the drawdown. The partial drawdown resulted in increased amounts of senescent vegetation in PD wetlands in 2008 and 2009, mainly affecting Carex and Typha vegetation classes. The result of habitat selection modeling was generally consistent with other studies of muskrats, although it was complicated by the habitat structure of these northern wetlands. Muskrats selected for rooted Typha with greater frequency than any other habitat, followed by rooted Equisetum. Ducks Unlimited Canada\u27s records from 1979 to 1990 show that water level drawdowns were successful at increasing muskrat house densities in SRD wetlands. In the years after a drawdown muskrat house densities generally increased and peaked three years after a drawdown, however the ten year densities were unaffected. Low muskrat densities, and low recruitment in SMC wetlands compared to other northern deltas are likely due to degenerating wetland habitat conditions created by prolonged water level stabilization. Small scale water level manipulation efforts by various managers, most notably Ducks Unlimited Canada, have produced increases in muskrat populations. Although expensive and logistically difficult the results I have presented suggest that a large scale drawdown and refill would stimulate muskrat populations in the SRD

INVESTIGATING THE EFFECTS OF WINTER DRAWDOWNS ON THE ECOLOGICAL CHARACTER OF LITTORAL ZONES IN MASSACHUSETTS LAKES

Anthropogenic alteration of water levels in lakes is a major stressor to the ecological integrity of littoral zones, which provide critical heterogenous resources that support diverse biological communities. Annual winter drawdowns have been practiced in Massachusetts (MA) for several decades; however, few studies have estimated impacts to littoral zone habitat and biological communities, particularly at relatively mild magnitudes (i.e., m) and in lakes that co-occur with other anthropogenic pressures (e.g., lakeshore development) as seen in MA lakes. My dissertation reviewed the winter drawdown literature and collected empirical data in MA lakes to characterize winter drawdown hydrological regimes and estimate responses of physical habitat (macrophytes, sediment texture, coarse wood), macroinvertebrate assemblages, and mussel assemblages to variable levels of drawdown magnitude. Through a stratified random selection approach, I selected 21 MA lakes (18 drawdown, 3 non-drawdown) based on drawdown information from an email survey to local conservation commissions and lake and pond associations. I continuously monitored water levels for 3–4 years within these lakes that represented a drawdown magnitude gradient. Drawdown regimes displayed considerable inter- and intra-lake variability in the timing and duration of annual drawdown events. The majority of winter drawdown events were incongruous to MA state issued timing guidelines, particularly for April 1st refill dates. In the same set of lakes, I found increased drawdown magnitude was correlated with coarser substrates and reduced silt, reductions in macrophyte biomass and biovolume, and proportional increases of macrophyte taxa with annual longevity strategy and amphibious growth form. During normal water levels, I found markedly lower freshwater mussel densities at drawdown-exposed depths compared to the same depths in non-drawdown lakes. I also found drawdown magnitude significantly structured macroinvertebrate taxonomic and functional composition with evidence that suggests several drawdown-sensitive taxa (e.g., Amnicola) and traits (e.g., semivoltinism). To minimize losses to lake ecological integrity, winter drawdown management should consider the extent of lakebed and littoral zone area exposed during drawdowns, incorporate depth-specific monitoring efforts for susceptible biota (e.g., mussels), and anticipate water level responses to climate change

Estudio experimental y numérico de la estabilidad de la pendiente aguas arriba en un embalse de presas de tierra en condiciones de extracción rápida

The rapid ‎drawdown of the dam reservoir is one of the most common situations occurring in the lifetime of a dam. For this reason, one of the main factors in the design of the upstream slope is the rapid drainage of the reservoir. In this case, the upstream slope is in a critical condition and the slope may be unstable. When the water surface in the reservoir is drawdown suddenly, the water level in the dam body does not decrease at the same time as the reservoir water level. The analysis of seepage from the earth dam body and calculation of the water loss play an important role in calculating the amount of pore water pressure, and, consequently, the stability analysis of the dam body. In addition, any seepage analysis is dependent on the hydraulic properties of the dam materials. In order to investigate the effect of hydraulic conductivity on the rapid drawdown of water level and the seepage, an experimental model was constructed of an earth dam. By accurate measurement of hydraulic parameters of the materials in saturated and unsaturated media, the flow through this model was modeled using a disk penetrometer by seep/w software. The results were then compared with the observed data.The rapid ‎drawdown of the dam reservoir is one of the most common situations occurring in the lifetime of a dam. For this reason, one of the main factors in the design of the upstream slope is the rapid drainage of the reservoir. In this case, the upstream slope is in a critical condition and the slope may be unstable. When the water surface in the reservoir is drawdown suddenly, the water level in the dam body does not decrease at the same time as the reservoir water level. The analysis of seepage from the earth dam body and calculation of the water loss play an important role in calculating the amount of pore water pressure, and, consequently, the stability analysis of the dam body. In addition, any seepage analysis is dependent on the hydraulic properties of the dam materials. In order to investigate the effect of hydraulic conductivity on the rapid drawdown of water level and the seepage, an experimental model was constructed of an earth dam. By accurate measurement of hydraulic parameters of the materials in saturated and unsaturated media, the flow through this model was modeled using a disk penetrometer by seep/w software. The results were then compared with the observed data

Aquifer Depletion and the Cost of Water Conservation: The Southern High Plains of Texas Case

Irrigated agriculture has played a vital role in the development and growth of the Great Plains Region of the United States. The primary source of water for irrigation in this region is the Ogallala Aquifer. The Southern portion of the Ogallala Aquifer is considered exhaustible due to the low level of recharge relative to the quantities of water pumped. Analysis and evaluation of water conservation policies which could extend the economic life of the Ogallala Aquifer in the Southern High Plains of Texas and Eastern New Mexico, and which could contribute to maintaining the viability of the regional economy is important. This study evaluates the impacts of water conservation policies which limit drawdown of the Ogallala Aquifer. County level dynamic optimization models maximizing net present value of net returns to land, management, groundwater, and irrigation systems over a sixty year planning horizon were formulated to evaluate three aquifer drawdown restrictions. The results of this study indicate that because of the differences in hydrologic characteristics and current irrigation levels across counties in the study area, blanket water conservation policies for the region as a whole are likely to be inefficient. This study concludes that for this region, water conservation policies that focus on counties that would deplete the aquifer to less than 30 ft. of saturated thickness possess the lowest implicit cost of conserving saturated thickness.water conservation, water policy evaluation, aquifer management, dynamic optimization, Resource /Energy Economics and Policy,

Assessment of the Upstream Slope Stability of Darbandikhan Rockfill Dam during Drawdown

Earth and rockfill dams face a variety of loading conditions during lifetime. One of the most critical loading conditions is the rapid drawdown of water level after steady state conditions. Rapid drawdown may cause instability of upstream slope of the dam. The present work examines the stability of a rockfill dam under different drawdown rates in terms of factor of safety for the upstream slope of the dam. For this purpose, a computer software named GeoStudio 2012 SEEP/W and SLOPE/W has been used for the numerical analysis. The results showed that the drawdown rate has a significant effect on stability of rockfill dam in which increasing the drawdown rate from 1 m/day to 10 m/day decreases the stability of the dam by 33%. Based on the outcomes, for the studied case the drawdown rate (1 m/day) can be recommended