Section II. HYDROGRAPHY AND GEOLOGY OF THE STUDY AREA 



1. Hydrography 



The Chesapeake Bay Entrance study area encompasses shallow portions 

 of lower Chesapeake Bay and adjacent inshore sand flats in and around 

 the bay entrance (Fig. 1). Most of this area lies under less than 35 

 feet of water. Deeper waters occur in the channels and closed depres- 

 sion in the lower Bay, and on the shelf seaward of the inshore flats 

 and shoals. 



Figure 3 shows the gross morphology of the bottom, (distribution 

 of shallow bay and inshore terraces and deeper waters). The division 

 between the main flats and deeper waters (which generally falls at 

 around -30 to -36 feet MLW) is drawn at the top of the slope. Locally 

 the slope is so gentle that the break is indefinite; here the dividing 

 line was arbitrarily drawn at -33 feet MLW. 



Although open water extends across Chesapeake Bay from Fisherman 

 Island off Cape Charles to Cape Henry 10 nautical miles southward, the 

 main inlet channel is less than 2 nautical miles wide. This channel is 

 roughly in the form of a curved rectilinear depression with maximum 

 depths of around 90 feet at MLW, and it is partially closed at both 

 ends where the depth decreases to about 45 feet. Maximum depths in this 

 channel occur NNE of Cape Henry. From this point the channel curves 

 southeastward (and ends) about 5 nautical miles southeast of the Cape. 

 West of Cape Henry , the main inlet channel terminates off Lynnhaven 

 where it subdivides into three smaller and shallower channels: Thimble 

 Shoals Channel leading westward to Hampton and Norfolk; Chesapeake 

 Channel leading northward into middle and upper Chesapeake Bay; and a 

 small channel leading a short distance WSW into Lynnhaven Roads. 



Cape Henry is steep-to and closely borders the deep water in the 

 main inlet channel. Southward from Cape Henry the Virginia shore is 

 bordered by a terrace-like flat at about -25 to -30 feet MLW. West of 

 Cape Henry within the study limits the south Bay shore is fronted by a 

 gently sloping bottom of sand, silt and sandy silt, extending north to 

 Thimble Shoals Channel. A sandy flat called Tail of the Horseshoe lies 

 between Thimble Shoals Channel and Chesapeake Channel. This flat-topped 

 shoal is triangular with the apex at the confluence of the two flanking 

 channels with the main inlet channel. 



The most extensive sand terrace of the study area borders Cape 

 Charles. On the Bay side this terrace extends west and south as far as 

 Chesapeake Channel; on the ocean side, inshore flats extend up to 6 nauti- 

 cal miles seaward and are prolonged further over the shelf by linear 

 northeast-trending "finger" shoals. 



South of Cape Charles the bay and ocean flats extend to within 2 

 miles of Cape Henry. Here a lobe of the flats projects along the flank 

 southeast for about 7 nautical miles. 



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