Hypsographic, Hydro-hypsographic and Hydrological Analysis of Coastal Bay Environments, Great Machipongo Bay, Virginia
Keywords:Hypsography, coastal lagoon, flushing, benthic environments
The Delmarva Peninsula is located on the mid Atlantic Coast Plain of the United States. The Atlantic coast of the Delmarva Peninsula is a typical coastal compartment with five landscape elements described by OERTEL and KRAFT (1994). Behind each element are coastal lagoons and bays with rich benthic and aquatic ecosystems. Environmental quality of these systems is strongly dependent on the exchange of water over benthic subenvironments of the bay floors. Hypsometric analysis of drainage basins is an organizational tool that may be used for quantifying the relative distribution of drainage-surface areas at different elevations.
Hydro-hypsographic curves may be used to show the relative distribution of benthic-surface area at different depths. Depth variations among subenvironments are linked to the complex hypsometric characteristics that have resulted from the transgressive reworking of relict antecedent surfaces. Very shallow areas provide optimal sites for wetland colonization. Hypsographic curves of the Great Machipongo lagoon show that about 30% of the basin is colonized by marsh. Almost 50% of the benthic-surface area has potential for colonization by submerged aquatic vegetation (SAV). However, turbidity caused by currents and bay waves limits light for primary production. Since basin topography is quite variable along the mid Atlantic coast, sea-level rise and hydro-hypsography may have a stronger influence on wetland decline ("health") than anthropogenic stresses. The exchange of water between bays and the coastal ocean is related to the hypsographic characteristics of the tidal prism and the whole basin prism (volume hypsography). The volume-hypsograph of the Great Machipongo lagoon illustrates that the tidal prism accounts for 54% of the lagoonal prism. The relatively rapid hydraulic turn-over time for the lagoon may be an end-member for a suite of hypsographic curves from well-flushed lagoons to poorly flushed lagoons.