Exchange between a freshwater embayment and a large lake through a long, shallow channel

Abstract

We analyze the exchange between a weakly forced lacustrine embayment and a large lake through a long, shallow channel. Exchange in the channel is the result of a multiple and subtle balance in which spatial thermal variations (baroclinic forcing), oscillations in the water level (barotropic forcing), bottom friction, diffusion, wind forcing, and the effects of unsteadiness are all important. Temperature gradients across the channel result from differences in thermal inertia of the embayment and lake at seasonal time scales and differences in the wind-driven internal dynamics of the lake and the embayment at diurnal to synoptic time scales. These gradients are, in general, weak and barotropic forcing dominates the channel momentum balance; however, episodic upwelling in the lake can shift the balance in favor of baroclinic dominance. A combination of scaling, analysis of observational data, and threedimensional simulations are used to demonstrate that bed stress, vertical turbulent diffusion, wind stress, and unsteadiness effect exchange relative to the predictions of internal hydraulic theory-the quasisteady inviscid theory that describes the motion resulting from a purely barotropic/baroclinic force balance.The performance of aquatic systems, often characterized as biological and chemical reactors or chemostats, is, to a great extent, dependent on hydrodynamic processes. Hydrodynamic processes determine the environmental conditions that affect the biogeochemistry, and importantly, the length of time water and its constituents remain in the system. This time scale, generally known as the hydraulic residence time, has been proposed in the literature to explain a range of water-quality phenomena, such as the variability in lake eutrophication processes, thermal stratification, isotopic composition, alkalinity, dissolved organic carbon concentration, elemental ratios of heavy metals and nutrients, mineralization rates of organic matter, and primary production (see Monsen et al. 2002 for a list of references). The experiments of Fussman et al. (2000) go further, suggesting that residence time scales control the structure of aquatic ecosystems and the extent that these systems are self-organized or dominated by exogenous influences.In semienclosed basins separated by topographic constrictions (sills, contractions, or their combinations) from adjoining oceans, their residence time scale, and hence, their biogeochemical behavior are determined by the rate at which

Summer water circulation in Frenchman's Bay, a shallow coastal embayment connected to Lake Ontario

“…For instance Hamblin and He (2003) showed that in a field experiment in the very long 828 m canal that connects Lake Ontario to Hamilton Harbour that only 23% of the observed flows driven by seiche motions had an excursion length greater than that of the channel and so could actually produce flushing of the 20 m deep waters of the Hamilton harbour. Similar remarks were also made by Rueda and Cowen (2005) about the inability of small water level fluctuations to actual drive water excursions in the 480-m-long shipping canal in the 10 m deep Little Sodus Bay. In contrast the Frenchman's Bay channel is only 130 m long, and as the mean depth of the bay is 1.5 m the small amplitude water level fluctuations can lead to flushing on a timescale of weeks.…”

“…4. The cold upwelling events occur much more frequently at Frenchman's Bay than were reported in the study site of Rueda and Cowen (2005), located in Little Sodus Bay on the southeast side of Lake Ontario. As the prevailing winds are from the west, the warm epilimnion is forced to the south by the Coriolis forces with an upwelling of cold water in the region near Toronto.…”

“…Such temperature gradients can arise due to the differential heating and cooling of shallow water bodies (Monismith et al, 1990;Andradóttir and Nepf, 2001;Wells and Sherman, 2001). In studies of deep coastal embayments of Lake Ontario, these temperature driven exchange flows have been found to be the dominant water exchange process during the summer (Lawrence et al, 2004;Rueda and Cowen, 2005), rather than the flows due to short term water level fluctuations. The temperature difference between a coastal embayment and the lake varies on a variety of timescales in (a) Shaded relief bathymetric profile of Frenchman's Bay completed using differential GPS and an echo sounder (van Hengstum et al, 2003).…”

“…The water circulation in coastal wetlands has characteristics that are typical of an estuary, in that the water is a mixture from incoming streams or rivers and water from the receiving lake (Mortimer, 1987;Bedford, 1992;Herdendorf, 1992;Keough et al, 1999). In such a fresh water estuary, there are two main mechanisms that can lead to exchange and inflow of water from a confined bay with the surrounding lake waters; namely (1) the response to water level fluctuations, where the short term rise and fall of the lakes level due to seiches, storm surges and tides drives water in and out of a confined bay in a manner similar to "tidal pumping" (Fischer et al, 1979;Trebitz et al, 2002;Trebitz, 2006) and (2) the response due to horizontal temperature differences, where the difference in density between warm and cold water can lead to currents where colder denser lake water flows into the base of the confined bay, and warmer lighter water flows out of the bay at the surface (Herdendorf, 1990;Lawrence et al, 2004;Rueda and Cowen, 2005). The circulation and flushing caused by these two processes is illustrated in Fig.…”

Temporal and spatial variations in heat content of a French pond

“…Most of the studies, especially unpublished papers of the French authorities, are focused on the temperature continuum stream-pond-stream, without knowing the heat content behaviour inside the pond. In lake's study, the horizontal scale getting more important and some studies took spatial measurements about great lakes (Xing et al, 2012;Wang et al, 2014), the embayment of lakes, especially about lake Ontario (Rueda & Cowen, 2005;Murphy et al, 2012) or also about a big mire system (Ramos, 2012), without taking into consideration the ponds, where the application should be the most achievable. It is a very important point regarding the ponds, where it is possible to choose the depth of the outflow (monk, weir…) or to change the localization of the outfall in order to decrease the temperature's impact on downstreams.…”