Chapter 4 Water, Salt and Heat Balance of Coastal Lagoons

Abstract

Water, salt and heat balance studies are surveyed and summarized within the context of coastal lagoons. The most important geomorphological factors influencing these physical processes are inlet configuration and dimensions, lagoon size and orientation with respect to prevailing winds, and water depth. Results of past water balance studies show that advective transport dominates gains and losses by rainfall, evaporation, surface runoff and groundwater seepage. All terms, however, show a distinct seasonality in response to seasonally changing winds, wet and dry periods and higher evaporative losses during summer months. Current measurements are decomposed into steady and non-steady components to show that in coastal lagoons the time varying, low frequency fluctuations are often dominant. When lagoon-shelf exchanges are restricted, tidal currents are often important only in the vicinity of inlets, and advection is primarily in response to local wind forcing. The wind driven circulation includes a downwind drift that produces a surface slope and drives a near-bottom return flow. A density driven current can be significant even in well mixed lagoons. Residual tidal flow can be important in the long term, but over shorter time intervals the net flow in response to local wind forcing is usually dominant. Salt balance is discussed in terms of transport through a transverse cross section, and in terms of local gains and losses at a given location. Transport occurs by advection and diffusion. Long term balances maintain brackish conditions in the lagoon; short term imbalances can change the salinity distribution significantly. Results from previous studies in lagoons and well mixed estuaries indicate that salt transport is primarily in response to advection associated with freshwater outflow or the slow draining and filling forced by changes in coastal sea level, tidal diffusion, and to shear effects arising from spatial correlations of vertical and especially transverse deviations in salinity and current speed. Heat balance, like water balance, is a response to local processes and advection. Heat energy gains and losses occur primarily over annual and diurnal periodicities, and over time scales on the order of 1-2 weeks in response to synoptic scale weather patterns. Previous studies indicate that heating is dominated by incoming solar radiation; cooling is primarily in response to outgoing long-wave radiation and latent heat fluxes. Advective heat fluxes are especially difficult to quantify because of strong spatial gradients in both temperature and velocity. Advection is of particular importance in the vicinity of inlets, because of lagoon-shelf differences in the rates of warming and cooling.