Virginia Capes Range Complex Final Environmental Impact Statement

VACAPES Range Complex FEIS/OEISChapter 3 Affected Environment andEnvironmental Consequences3.3 – Water ResourcesThe physical oceanography of the Study Area can be characterized in terms of its bathymetry, or bottomtopography, and its circulation. Sediment transport and deposition and bottom composition also areelements of physical oceanography. Bathymetry and bottom composition are addressed in Section 3.1,Bathymetry and Sediments. Water characteristics, sediment transport, deposition and circulation arediscussed below, along with marine water quality. Fate and transport of expended materials in the marineenvironment is discussed in Section 3.2.3.3.2.1 Marine Water QualityThe VACAPES Study Area is located in the coastal and offshore waters of the western North Atlanticocean adjacent to the States of Delaware, Maryland, Virginia, and North Carolina and extends seawardinto waters more than 4,000 m deep (see Figures 1.1-1 and 2.1-1). Cape Hatteras, North Carolina isgenerally considered to be a transition zone between the warm, tropical waters found to the south and thecool, temperate waters to the north. Cape Hatteras separates the oceanic provinces of the South-AtlanticBight (SAB) from those of the Mid-Atlantic Bight (MAB). The SAB encompasses the area from theFlorida Straights to Cape Hatteras, while the MAB extends from Cape Hatteras to the southwestern flankof Georges Bank (DoN, 2001a; DoN, 2008). The majority of the VACAPES Study Area is located in theMAB, but the southernmost section of the OPAREA is located in the northernmost limit of the SABprovince. Thus, both oceanic provinces influence the physical environment of the OPAREA.Water quality in the marine environment is determined by a complex set of interactions between chemicaland physical processes operating continuously in the ocean system. This dynamic equilibrium isexpressed by a variety of indicators, including temperature, salinity, dissolved oxygen, and nutrient levels.Water pollutants alter the basic chemistry of sea water in various ways. The following discussioncharacterizes in general terms the major determinants of marine water quality in the VACAPES StudyArea.CurrentsPrevailing winds and centripetal force cause surface waters to move in a gyre or circular fashion in oceanbasins. In the North Atlantic Ocean, this gyre system is composed of the Gulf Stream, North Atlantic,Canary, and Equatorial Currents. Additional surface water masses found in the VACAPES Study Areaare Chesapeake Bay plume water, Delaware Bay plume water, and mid-Atlantic shelf water (or VirginiaCoastal Water) (DoN, 2008).The Gulf Stream exerts a considerable influence on the oceanographic conditions in the VACAPES StudyArea. In general, the Gulf Stream flows roughly parallel to the coastline from the Florida Straits to CapeHatteras, where it is deflected from the North American continent and flows northeastward past the GrandBanks (Figure 3.3-1). After the Gulf Stream separates from the east coast in North Carolina, the currentpasses through the southeastern portion of the VACAPES Study Area (DoN, 2008). In this area, the GulfStream is approximately 27 nm wide and 3,281 feet deep. Surface velocity ranges from two to five knotsand temperature from 77 to 82 o F.Relatively fresh or brackish water from the Chesapeake and Delaware Bays flows out of these estuaries inthe form of plume water. The Coriolis force causes this less dense (because it is lower in salinity) waterto turn south, resulting in southward-flowing, coastally trapped currents. An increase in river flow andebbing tides force more water out of the respective bays; thus, the seaward front of the plume extendsacross the shelf. During the summer months predominant southwesterly winds cause a seawardexpansion of the plume over the continental shelf, creating a well-stratified, two-layer system. The warmsurface waters are replaced by deeper, more saline, nutrient-rich water (DoN, 2008).3-39 March 2009