Fate of turbid glacial inflows in a hydroelectric reservoir

Turbidity from glacial meltwater limits light penetration with potential ecological consequences. Using profiles of temperature, conductivity, and turbidity, we examine the physical processes driving changes in the epilimnetic turbidity of Carpenter Reservoir, a long and narrow, glacier-fed reservoir in southwest British Columbia, Canada. Following the onset of permanent summer stratification, the relatively dense inflows plunged into the hypolimnion, and despite the high glacial load entering the reservoir, the epilimnion cleared due to particle settling. Using a one-dimensional (longitudinal) diffusion equation for a decaying substance to describe the variation in epilimnetic turbidity, we obtain two nondimensional parameters: the epilimnetic inflow parameter, I , a measure of the turbidity flux into the epilimnion; and the dispersion parameter, D , a measure of longitudinal dispersion. In the case of Carpenter Reservoir: I ≪ 1 , indicating that turbidity declines over the summer; and D ≪ 1 , indicating a strong gradient in turbidity along the epilimnion. Using our theoretical formulation of epilimnetic turbidity variations in conjunction with monthly field surveys, we compute the particle settling velocity ( ∼ 0.25 m d - 1 ), the longitudinal dispersion coefficient (50-70  m 2 s - 1 ), and the flux of turbid water into the epilimnion ( ∼ 1 % of the total inflow). Our approach is applicable to other reservoirs and can be used to investigate changes in turbidity in response to changes in I and D .

Involving citizens in hydrodynamic research: A combined local knowledge - numerical experiment on Lake Garda, Italy

Local knowledge on surface currents and transport patterns in Lake Garda is acquired through interviews among wind-surfers, sailors, fishermen, ferry boat drivers, firefighters nautical rescue team, and officers from the environmental protection agency. Data are collected by means of individual interviews and focus groups, analyzed for internal consistency and summarized in qualitative maps. Three-dimensional numerical simulations are performed using a one-way coupled atmospheric-hydrodynamic model and the results are compared with the observations of the interviewees. Through this combined effort, currents that were not evident to the scientific community, but are well-known to sailors and surfers, can now be recognized and physically understood, like the 'Corif' that flows along the eastern shore in summertime between late morning and afternoon, when wind blows from the south. The transport patterns are also identified, like the predominant east-to-west surface transport experienced by fishermen under storm events and floods, that is confirmed for northerly wind, and the west-to-east transport for southerly wind. Moreover, the trajectory of a drifting capsized boat is reproduced by the model and the dynamics of the accident (location and timing) are reconstructed in collaboration with the firefighters nautical rescue team of Trento and based on information from local newspapers and witnesses. This exercise demonstrates that the joint effort of the scientific community and local experts can produce advances in the understanding of large-scale hydrodynamic processes in lakes.

Estimation of water quality profiles in deep lakes based on easily measurable constituents at the water surface using artificial neural networks coupled with stationary wavelet transform

This study proposes a novel framework to accurately estimate water quality profiles in deep lakes based on parameters measured at the water surface, considering Boulder Basin of Lake Mead as a case study. Hourly-measured meteorological data were used to compute heat exchange between lake and atmosphere. Heat fluxes combined with every 6-hour measured water temperature, conductivity, and dissolved oxygen (DO) profiles, from the water surface to a depth of 100 m over a 48-month period, were used to train seven different artificial neural network-based methods for estimating water quality profiles. Effects of different factors influencing lake water quality, including lake-atmosphere interactions, wind-induced mixing, thermocline depth, winter turnover, oxygen depletion and other factors were investigated in different methods. A method employing stationary wavelet transform with a depth-progressive estimation of temperature, conductivity, and DO generated the smallest average relative errors of 0.52%, 0.22%, and 0.62%, respectively in the water column over a 48-month period. Abrupt changes in temperature, conductivity, and DO profiles due to thermal stratification, winter turnover, and oxygen hypoxia increased estimation errors. The largest errors occurred near the interface between the epilimnion and metalimnion, where vertical mixing intensity significantly decreased.

The wind-driven formation of cross-shelf sediment plumes in a large lake

Wind-driven turbidity plumes frequently occur in the western arm of Lake Superior and may represent a significant cross shelf transport mechanism for sediment, nutrient and biota. Here we characterize a plume that formed in late April 2016 using observations from in situ sensors and remote sensing imagery, and estimate the volume of cross shelf transport using both the observations and an idealized analytical model of plume formation. The steady-state, barotropic model is used to determine a relationship between the intensity and duration of a wind event and the volume of water transported from nearshore to offshore during the event. The model transport is the result of nearshore flow in the direction of the wind and a pressure-gradient-driven counter flow in the deeper offshore waters, consistent with observations. The volume of offshore transport associated with the 2016 plume is estimated by both methods to have been on the order of 10¹⁰ m³. Analysis of similar events from 2008-2016 shows a strong relationship between specific wind impulse and plume volume. Differences in the intensity and duration of individual events as well as ice cover, which prevents plume formation, lead to interannual variability of offshore transport ranging over an order of magnitude and illustrates how wind-driven processes may contribute to interannual variability of ecosystem functioning.