Drawdown Event Research Articles

Case Study on Analyses of Slope Riverbank Failure

A slope riverbank failure is a natural event that occurs globally on each riverbank, and a drawdown event usually causes slope riverbank failure. This case study is aimed at analysing slope riverbank failures by evaluating the seepage and slope stability of the riverbank under slow and rapid drawdown. The riverbank in this case study is located at KM 3.49, Jalan Pantai Luagan in the Sipitang district (N 4° 59 ′ 12.9 ″ E 115° 31 ′ 13.3 ″ ). A literature review was conducted to view the current study pattern and retrieve a methodology based on the current study pattern. GeoStudio is a commercial finite element software. The data obtained from the borehole log report and online resources were utilised to create the riverbank model in software. The phreatic line shows a slow change over time, indicating that the riverbank takes a long time to stabilise after the drawdown. The FOS value decreases during the drawdown occurrence and slowly increases after the drawdown has ended. In conclusion, the drawdown event can cause slope riverbank failure, and the seepage and stability analysis using GeoStudio can show the condition of the riverbank during the drawdown event.

Annual winter water level drawdowns limit shallow‐water mussel densities in small lakes

Abstract Regulated water level fluctuations alter the physical, chemical, and biological environments in lakes. However, few studies have measured the effects of repeated annual winter drawdowns on freshwater mussel populations (Bivalvia: Unionida), and it is unknown whether drawdowns permanently constrain mussel populations to deeper depths or are resilient to the annual disturbance. We quantified mussel densities during normal water levels and their mortality after the initiation of drawdown in lakes with annual winter (December–March) water level drawdown regimes that have existed for several decades. We used systematic quadrat sampling and visual snorkel surveys to estimate the number of live mussels at the surface and buried, and shell length in six drawdown and three control lakes at exposed (0.5‐m) and unexposed (1‐m) depths during winter drawdown. We also estimated mortality of mussels along the exposed lakebed for nine drawdown lakes. Study lakes were dominated by Elliptio complanata and Pyganodon cataracta. During normal water levels, mussel densities were much lower in drawdown lakes than control lakes at 0.5‐m depths. When mussels were present in drawdown lakes at 0.5‐m depths, they were often burrowed in the substrate and smaller than buried mussels in control lakes. At unexposed 1‐m depths, mussel densities were primarily explained by geographic region (western versus central Massachusetts), and found higher densities in drawdown lakes compared to control lakes. Mussel mortality after a single drawdown event in nine drawdown lakes was not correlated with drawdown magnitude or rate. Concordance of shell length‐frequency distributions between dead and buried living mussels in drawdown lakes suggests annual mortality of colonising individuals, which are typically small and presumably of young cohorts. Annual winter drawdowns still limit densities of E. complanata and P. cataracta in shallow‐water habitats in the following autumn (September–October) after 4–5 months of potential recovery, and hence constrain their distribution in lakes exposed to drawdowns. The development of sustainable water level management practices will be essential to minimise impacts to native mussel populations.

Summer Redox Dynamics in a Eutrophic Reservoir and Sensitivity to a Summer’s End Drawdown Event

In eutrophic lakes and reservoirs, reduced mixing during stratified conditions limits oxygen (O2) supply to the hypolimnion (that is, bottom waters). In the absence of an O2 supply, microbial decomposers consume alternative electron acceptors, generally in order of their thermodynamic favorability, releasing soluble, reduced manganese (Mn), iron (Fe) and methane (CH4) to the water column with implications for reservoir water quality and greenhouse gas dynamics. Still, there are very few studies that quantify intra- and inter-annual controls on lake and reservoir redox chemistry, especially in managed systems. To address this knowledge gap, we examined redox-sensitive water column chemistry before and during four summer’s end water-level drawdown events (~ 2 m in magnitude) in a eutrophic reservoir. We observed lower dissolved Fe and CH4 concentrations in years with higher hypolimnion O2 and NO3−, suggesting that water column oxidant availability controls the extent of the redox cascade. During drawdowns, dissolved CH4, Mn and Fe concentrations increased in the hypolimnion (on average by 50%, 40% and 175%, respectively) concomitant with order of magnitude increases in methane bubbling rates (that is, ebullition). To our knowledge, this is the first in situ evidence for enhanced flushing of sediment pore water into a reservoir during water-level drawdowns. Furthermore, the mass of CH4, Mn and Fe released varied as a function of summertime redox conditions. Thus, the timing of reservoir water-level drawdowns may determine the degree to which reduced solutes enter the water column during drawdown, with longer pre-drawdown periods of hypolimnion hypoxia leading to higher solute fluxes.

Does artificial drawdown affect zooplankton structure in shallow lakes? A short-term study in a tropical reservoir

Water level fluctuation by artificial drawdown is one management activity that has the potential to control macrophyte growth, but there is little knowledge of how this operational procedure affects other biotic components of the ecosystem. This study investigated zooplankton dynamics in response to artificial drawdown over a short timeframe (13 days) in a Brazilian reservoir, by examining the impact on zooplankton communities in two shallow lakes (Lake Pedra Branca-LPB, and Guarita-LG) connected to a reservoir (Salto Grande) that undergo sudden and remarkable fluctuations in their water levels. Zooplankton communities were sampled in both lakes before (pre-drawdown), during (low water), and after (post-drawdown) the artificial drawdown procedure. In LPB, drawdown resulted in an increase in zooplankton density, and temporarily changed the community in association with an increase in water conductivity and presence of non-planktonic organisms during the low water phase. In LG, drawdown had no significant effect on zooplankton community between the phases before and during the drawdown event. The results from this study suggest that artificial drawdown over a short timeframe in reservoir systems do not negatively affect the overall density, richness, and diversity of zooplankton communities in marginal shallow lakes.

Climate forcing for dynamics of dissolved inorganic nutrients at Palmer Station, Antarctica: An interdecadal (1993–2013) analysis

AbstractWe analyzed 20 years (1993–2013) of observations of dissolved inorganic macronutrients (nitrate, N; phosphate, P; and silicate, Si) and chlorophyll a (Chl) at Palmer Station, Antarctica (64.8°S, 64.1°W) to elucidate how large‐scale climate and local physical forcing affect the interannual variability in the seasonal phytoplankton bloom and associated drawdown of nutrients. The leading modes of nutrients (N, P, and Si empirical orthogonal functions 1, EOF1) represent overall negative anomalies throughout growing seasons, showing a mixed signal of variability in the initial levels and drawdown thereafter (low‐frequency dynamics). The second most common seasonal patterns of nitrate and phosphate (N and P EOF2) capture prolonged drawdown events during December–March, which are correlated to Chl EOF1. Si EOF2 captures a drawdown event during November–December, which is correlated to Chl EOF2. These different drawdown patterns are shaped by different sets of physical and climate forcing mechanisms. N and P drawdown events during December–March are influenced by the winter and spring Southern Annular Mode (SAM) phase, where nutrient utilization is enhanced in a stabilized upper water column as a consequence of SAM‐driven winter sea ice and spring wind dynamics. Si drawdown during November–December is influenced by early sea ice retreat, where ice breakup may induce abrupt water column stratification and a subsequent diatom bloom or release of diatom cells from within the sea ice. Our findings underscore that seasonal nutrient dynamics in the coastal WAP are coupled to large‐scale climate forcing and related physics, understanding of which may enable improved projections of biogeochemical responses to climate change.

The drainage of Lake Ojibway in glaciolacustrine sediments of northern Ontario and Quebec, Canada

Physical evidence for the drainage of glacial lakes remains relatively rare in depositional records, giving rise to much debate on the location of outlets and discharge pathways, as well as on the climate impact of the attendant meltwater forcing. Lake Ojibway developed following the withdrawal of the Laurentide Ice Sheet in northern Ontario and Quebec, Canada. The late‐stage evolution of this large ice‐dammed lake was influenced by the complex dynamics of the retreating ice margin, which highly complicates the identification of the termination of Lake Ojibway in glaciolacustrine sediment records. Here, we document the composition of sections of rhythmites that contain in their upper part an anomalously thick and whitish bed (10–15 cm) that is in turn overlain by ∼1 m of faintly bedded rhythmites. Grain‐size analyses showed that the thick whitish bed consists primarily of fine to coarse silt (2–63 μm), contrasting with the lower and upper rhythmites that are largely dominated by clay (<2 μm). The detrital carbonate content of the thick silt bed is characterized by consistently high values (2.5 to 2.8%), whereas the bounding rhythmites show lower and highly variable values. Oxygen isotope measurements further show a marked change going from typical glacial meltwater values (∼ −29.6 to −27.7‰; VSMOW) for the lower rhythmites and the silt bed to modern‐like meteoric values (−18.4 to −14.6‰) for the uppermost rhythmites. These data suggest that this marker bed may be associated with a major drawdown event that possibly corresponds to the final drainage of Lake Ojibway. AMS radiocarbon dating of ostracods extracted from the drainage bed also documents an important hardwater effect within the Ojibway basin.

An analysis of the late glacial lake levels within the western Lake Superior basin based on digital elevation models

This study establishes a detailed lake-level history for the Lake Superior basin by mapping strandlines from 10-m and 3-m digital elevation models. There are 24 levels above the mid-Holocene Nipissing level, and elevations increase along a direction of 23.1° due to post-glacial rebound. The highest level, the Epi-Duluth, is steeper than subsequent levels and may pre-date the Lake View ice advance into the western Lake Superior basin at the end of the Younger Dryas stade. The most prominent level is the Duluth, ca. 10,800calyrBP. Ice retreat exposed successively lower outlets, routing overflow to the Lake Michigan and Huron basins. By 10,600calyrBP, lake levels in the western Superior basin had dropped almost 200m. This transformative period is complicated by multiple basin-wide events: the influx of glacial Lake Agassiz overflow, the creation of three sub-aqueous moraines, and a red to gray color transition in basin sediments. A later drawdown event has been hypothesized to have initiated the 9300calyrBP cooling event, but this flood was much smaller than estimated previously. If freshwater triggered the 9300calyrBP event, the source of the water must have been Lake Agassiz, not Lake Superior.

Resilience of phreatophytic vegetation to groundwater drawdown: is recovery possible under a drying climate?

AbstractBanksia (Proteaceae) woodlands are one of a number of groundwater‐dependent ecosystems in southwestern Western Australia that are threatened by groundwater abstraction. In addition to this threat is an ongoing decline in regional water tables due to a drying climate. We used ecological resilience theory to analyse and interpret a long‐term vegetation monitoring dataset from a site that has experienced an abstraction‐induced acute groundwater drawdown in the late 1980s and early 1990s. Despite reduced plant abundance, all dominant over‐ and understorey species were still found on all transect plots in which they were recorded pre‐drawdown. This suggests a notional resilience and a strong likelihood of recovery, in the event that pre‐drawdown ecohydrological habitat conditions were to return. However, since the drawdown event, the regional water table continued to decline, with the vegetation responding through progressive and uni‐directional change in abundance and composition. The change in composition was primarily manifested as a shift towards non‐woody, shallow‐rooted species not dependent on specific hydrological conditions. This slow, progressive change in hydrology associated with reduced rainfall and land use changes has continued to force a transition in the floristics towards an alternative ecohydrological state. Despite the absence of an acute drawdown event, the same progressive floristic response was also observed at two reference sites that were not under the immediate influence of production bores. The challenge for adaptive water resource management will be to enhance the capacity for resilience in these groundwater‐dependent ecosystems in a drying environment through appropriate regulation of groundwater abstraction. Copyright © 2010 John Wiley & Sons, Ltd.

Influence of extreme drawdown on water quality downstream of a large water storage reservoir

AbstractThe Serial Discontinuity Concept (SDC) proposes that dams have the potential to affect the downstream ecological condition of rivers. While the SDC was developed principally around changes to physical habitat or temperature, reservoirs also have the potential to impact on downstream water quality, including algal community structure. In the current study we examined the impacts of an extreme drawdown event on nutrient loads and algal community structure downstream of a large water storage reservoir in south‐eastern Australia—Lake Hume. The lake was a net exporter of carbon, nitrogen, phosphorus and iron during the study period and was a net sink for manganese. Most of the carbon, nitrogen and phosphorus exported from the lake was in the form of algal biomass. Processes in the lake also influenced the downstream algal community structure. Upstream of the reservoir green algae were the most dominant species; within and downstream of the reservoir cyanobacteria dominated. Much of the algal biomass found at the downstream sites appeared to originate in Lake Hume and was physically transported downstream. Copyright © 2009 John Wiley & Sons, Ltd.

Assessing the impacts and potential control of terrestrial plant colonization of a reservoir bed during an extreme drawdown event

AbstractExtended periods of drawdown of reservoirs can result in the ‘terrestrialization’ of the lake bottom sediments, which describes the process whereby the exposed dry sediment takes on soil‐like qualities. Opportunistic colonization of lake bed by terrestrial plants can occur under such conditions. A survey of plant communities of a large reservoir (Lake Hume) following its extreme drawdown, resulted in significant quantities of carbon, nitrogen and phosphorus being bound up in the terrestrial vegetation that colonized the lake bed. The phosphorus uptake from the sediment into plant material particularly could represent a significant pathway for phosphorus re‐mobilization back into the lake. The potential of using stock grazing as a control measure for mitigating the effects of terrestrial plant colonization is also discussed.

Exploring the ‘Birch effect’ in reservoir sediments: influence of inundation history on aerobic nutrient release

The release of nutrients on wetting (‘the Birch effect’) is well documented for terrestrial soils. The effect for aquatic sediments has received much less attention. An extreme drawdown event in a large reservoir during a period of drought presented an opportunity to test whether or not reflooding dried sediments would be a source of nutrients to the overlying water column. Wetting and drying history influences the release of ammonium, but not oxides of nitrogen, organic nitrogen, filterable reactive phosphorus or total phosphorus, from dried sediments following re-wetting. Dried reservoir sediments that have never, or have only rarely, been desiccated since the 1920s produced a pulse of ammonium on re-inundation. Conversely, dried sediments from parts of the reservoir that undergo frequent wetting and drying cycles do not release substantial amounts of ammonium on rewetting.

Effects of Reservoir Operations on Hatchery Coastal Rainbow Trout in Lake Roosevelt, Washington

AbstractIt is suspected that reservoir operations limit the mitigation fishery for coastal rainbow trout Oncorhynchus mykiss irideus in Lake Roosevelt, Washington. Entrainment through Grand Coulee Dam has been suggested as the primary impact of reservoir operations. However, the causal mechanisms and effects of entrainment have been difficult to quantify. A logistic regression model was used to examine the effect of reservoir operations on the coastal rainbow trout tag return probability. Eight independent variables were studied: release location, presence–absence of a tag reward, minimum reservoir elevation (m above mean sea level (msl)) during a sample year, reservoir elevation on the fish release date, mean daily water retention time (d) 2 weeks postrelease, mean daily retention time 4 weeks postrelease, mean annual water retention time, and water year type (associated with shallow, average, or deep drawdown events). Models were selected on the basis of likelihood ratio tests, odds ratio confidence intervals (CIs), and predictive ability. Analyses indicated that the significant independent variables were release location, water year, release elevation, and mean water retention time at 4 weeks postrelease. Angler return of tagged fish was 1.86 times more likely after a shallow drawdown event than after a deep drawdown event (95% profile likelihood CI = 1.15–3.03). The effect of mean water retention time was dependent on reservoir elevation at the time of release. When release elevation was low (366–381 m above msl), increasing mean water retention time increased the probability of tag return. However, when release elevation was high (381–393 m above msl), increasing mean water retention time did not increase the probability of tag return. The logistic regression model can be used by fisheries managers to predict the impacts of hydropower operations on the coastal rainbow trout fishery in Lake Roosevelt, thereby allowing for annual adjustments to release strategies that would maximize harvest potential.

Changes in sediment microbial community structure within a large water-storage reservoir during an extreme drawdown event

Although drought and drying of waters occur globally, the effect of drying on sediment microbial communities underpinning aquatic biogeochemical processes is poorly understood. We used the molecular method of terminal-restriction fragment length polymorphism (T-RFLP) to assess changes in the microbial community structure of sediments undergoing different levels of inundation and drying within a reservoir during drawdown in a drought. Sediments with three hydrological conditions were investigated: dry sediments (no overlying water), littoral sediments (covered with 1–2 mm water) and inundated sediments (covered with >1 m water). Sampling was done in winter 2006 (August) and summer 2007 (January) in Lake Hume, Australia. The microbial communities differed significantly between the different levels of inundation at each sampling time. Community structure also changed significantly within each site between winter 2006 and summer 2007, possibly influenced by the change of season or protracted drying. Sites that were ‘littoral’ in winter 2006 became ‘dry’ in summer 2007, and became more similar to communities that were ‘dry’ at both sampling times. This suggested that the hydrological history of specific sites did not heavily influence the response of microbial communities to severe drying, and all communities undergoing ‘dry’ conditions within the summer 2007 sampling responded similarly.

Response of invasive macrophyte species to drawdown: The case of Elodea sp.

The response to drawdown of vegetative fragments (whole plants, shoot fragments and turions) of two invasive macrophyte species, Elodea canadensis and Elodea nuttallii, was studied through laboratory experiments. In addition, field observations were made on the colonisation of a wetland by E. nuttallii before and after a natural drawdown. The survival and the growth of vegetative fragments of E. nuttallii were higher than those of E. canadensis after an artificial drawdown of several days. In the field the recolonisation by E. nuttallii of a wetland that was drained for 10 weeks during a summer drawdown was very rapid, the abundance of this macrophyte species being not affected by the drawdown event. We conclude that E. nuttallii possesses a high resilience to desiccation and that a summer drawdown would not be efficient in the control of this invasive species.

Sulfidic materials in dryland river wetlands

Due to a combination of river regulation, dryland salinity and irrigation return, lower River Murray floodplains (Australia) and associated wetlands are undergoing salinisation. It was hypothesised that salinisation would provide suitable conditions for the accumulation of sulfidic materials (soils and sediments enriched in sulfides, such as pyrite) in these wetlands. A survey of nine floodplain wetlands representing a salinity gradient from fresh to hypersaline determined that surface sediment sulfide concentrations varied from <0.05% to ~1%. Saline and permanently flooded wetlands tended to have greater sulfide concentrations than freshwater ones or those with more regular wetting–drying regimes. The acidification risk associated with the sulfidic materials was evaluated using field peroxide oxidations tests and laboratory measurements of net acid generation potential. Although sulfide concentration was elevated in many wetlands, the acidification risk was low because of elevated carbonate concentration (up to 30% as CaCO3) in the sediments. One exception was Bottle Bend Lagoon (New South Wales), which had acidified during a draw-down event in 2002 and was found to have both actual and potential acid sulfate soils at the time of the survey (2003). Potential acid sulfate soils also occurred locally in the hypersaline Loveday Disposal Basin. The other environmental risks associated with sulfidic materials could not be reliably evaluated because no guideline exists to assess them. These include the deoxygenation risk following sediment resuspension and the generation of foul odours during drying events. The remediation of wetland salinity in the Murray–Darling Basin will require that the risks associated with disturbing sulfidic materials during management actions be evaluated.

Impact of groundwater abstraction on a Banksia woodland, Swan Coastal Plain, Western Australia

Summary The Gnangara Groundwater Mound, centred 38 km north of Perth, Western Australia, is a large, shallow unconfined aquifer that is currently under abstraction as part of the public metropolitan water supply. To investigate the impact of lowering groundwater levels on a Banksia woodland on the Mound, vegetation monitoring near a groundwater abstraction bore (known as P50) began 1 year before becoming operational. In February 1991, 2 years after abstraction commenced, extensive death of the Banksia overstorey was observed within close proximity of the bore, following a short period of high summer temperatures. The site was subsequently revisited and the understorey floristic composition, abundance and vigour of overstorey species resurveyed, and compared with data collected from a site under long‐term monitoring and not currently influenced by abstraction. A lowering of groundwater level by 2.2 m at P50 between the summers of 1990 and 1991, resulting from the cumulative effects of abstraction and below average annual rainfall (low groundwater recharge), coincided with a loss of between 20 and 80% of adults of overstorey species and up to 64% of adults of understorey species within 200 m of the bore. Over a similar time period no significant decreases in the abundance of overstorey or understorey species were recorded in the monitored site not influenced by groundwater abstraction. Of the overstorey species, Holly‐leaf Banksia (Banksia ilicifolia) displayed the greatest susceptibility and lowest net recovery following the abstraction event at P50. The negative impact of groundwater drawdown on Holly‐leaf Banksia populations makes this overstorey species an important indicator of decreasing groundwater levels on the Gnangara Groundwater Mound. Water stress may have been the primary cause of vegetation death in close proximity to the P50 bore, although this would have been exacerbated by extreme summer temperatures (> 45°C) recorded during February 1991. The P50 scenario represents a localized response to an acute drawdown event, in association with other environmental factors, and provides invaluable information on the assessment of groundwater abstraction and poor groundwater recharge events on a Banksia woodland community. However, there are limitations in using the community response at P50 to manage the impact of drawdown events on other plant communities occurring on sandy, shallow aquifers.

A Rapid Drawdown Event Related to Quarry Operations in a Dolomite Aquifer

AbstractTwo active quarries are mining stone from the Silurian dolomite aquifer in Waukesha County in southeastern Wisconsin. The village in which the quarries are located uses local zoning to control the depth of mining and to institute a long‐term water level monitoring program and well guarantee/one with the quarry owners. Water levels dropped as much as 40 feet in at least 24 residential wells surrounding the quarries over a period of a few hours to days. The rapid decline in head was caused by a single boring drilled lo a depth of 75 feel he low the floor of one quarry. The borehole penetrated a localized fracture zone under confined artesian head. Water levels recovered to previous static levels within nine days after grouting the borehole. The rapid drawdown event demonstrates the potential impact of mining in fractured aquifers. The apparent complete recovery of the aquifer demonstrates that quick response can sometimes restore an aquifer. However, the potential for blasting into a similar zone illustrates the need for a well‐thought‐out aquifer monitoring program and emergency response plan. The experience of the village is a good example of managing conflicting uses of a finite resource and collecting baseline data needed to make informed decisions.

Computer Solutions for the Stability of Reservoir Slopes

Prediction of the stability of slopes under various water conditions and for a wide range of loading and soil conditions can be made with the help of a digital computer. Reservoir slopes may reach a very critical state immediately after very rapid drawdown. At present, this condition is analyzed assuming an “instantaneous” drawdown event. This paper presents a method for obtaining the slope stability under a drawdown condition with a defined time-elevation history. The analyses are based on the simplified form of Bishop’s model with circular failure surface and vertical slices. Two options are available for determining the pore pressure. In the time-dependent drawdown analysis, a differential equation equates the volume of water that flows out of the slope in a given time with the product of the velocity of flow, area of flow, and time interval. The solution of this equation yields the position of the phreatic surface with time. This information helps determine the pressures along a trial circular sliding surface, and the variation of the factor of safety with time. The factor of safety decreases rapidly with the lowering of the reservoir level, followed by a slow and nonlinear increase to the steady state value.