Technical Studies


Role of aquatic vegetation distribution on natural convection (thermal-driven flow) over a sloping bottom

During no or weak wind and no river flow conditions, natural convection or so-called thermal-driven flow generated from non-uniform heat distributions plays an important role on the exchange of nutrients, pollutants and chemical substances in littoral zones of wetlands and lakes, or the sidearms of reservoirs. The scale of the magnitude of the thermal-driven flow is much larger than that from the molecular diffusion alone in water body. The thermal-driven flow due to differential surface heating or cooling mainly arises from topographic variations or aquatic vegetation shading effects. For example, during daytime, the same amount of solar radiation entering into the water body leads to warmer water in the shallow region and cooler water in the deeper region. The temperature differences lead to variation of water densities, and subsequently drive thermal convective circulation. As a result, induced circulation flowing from shallow water to deep water along the water surface and underflow uphill over the sloping bottom toward nearshore is generated. The process is reverse during nighttime. Similarly, for vegetation shading areas covered by emergent or floating vegetation are cooler than open water or less vegetation areas during daytime. The horizontal temperature differences can also result in thermal-driven flow. However, the emergent and floating vegetation can shade incoming solar radiation, and generate a horizontal temperature gradient, which may interfere or reinforce the temperature gradient from a sloping bottom. Some questions are raised about how changes of vegetation distribution can influence the thermal-driven circulations, and if the thermal-driven circulation is increased under certain vegetation distribution. The species and density of vegetation may affect the patterns and magnitudes of the thermal-driven flow as well as the transport of the nutrients. Ultimately, the nutrient budget in the littoral ecosystem may be changed. In this study, we propose laboratory experiments to study the effects of vegetation distribution on the thermal-driven exchange flow, and expect to obtain an empirical equation to estimate the exchange flowrates. The results of this study can be a very fundamental basis for water and ecological administrators to manage the shallow water region especially when planting vegetation is needed.