Corps of Engineers are usually current but are generally limited to the 

 immediate vicinity of a Corps of Engineers' project such as a navigation 

 channel. If the available surveys are several years old, and there is 

 doubt as to their accuracy, cross-channel profiles should be obtained at 

 about 1/4-mile intervals to verify old surveys. In areas where the bed 

 is subject to rapid change, hydrographic surveys should be scheduled to 

 essentially coincide with the velocity and salinity surveys. The hydro- 

 graphic surveys must include the intertidal zone between mean low water 

 (MLW) and mean high water (MHW) . Recent aerial photos, especially of the 

 bank line, are helpful in confirming the location of structures along the 

 shore. 



(2) Topographic Surveys . Topographic surveys are required to 

 determine the overbank slopes immediately adjacent to the MHW line. 

 These surveys should extend to about 10 feet above MHW. If the model 

 is used for hurricane-surge protection studies, the topographic surveys 

 must cover all areas which may be subject to inundation during a surge 

 for existing or proposed conditions. In a large estuary model (i.e., 

 Chesapeake Bay or Delaware Bay) the topography can be obtained from U.S. 

 Geological Survey (USGS) quadrangle sheets and supplemented with field 

 surveys as required. 



(3) Tidal Observations . Tide records must be obtained along the 

 length of the estuary and major tributaries. Depending on the complexity 

 of the system, tide records should usually be obtained at intervals of 10 

 to 20 percent of the length included in the model. Examples of tide gage 

 layouts for estuaries of various sizes and complexities are shown in 

 Figures 3-4 to 3-8. If possible, a gage should be located in the ocean, 

 even if the datum of the gage cannot be accurately established. The data 

 from this gage are valuable in determining whether a significant choking 

 of the tide occurs through the estuary mouth. The datum of each gage 

 should be determined to an accuracy of ±0.1 foot, and all the gages should 

 be referenced to a common horizontal datum, such as mean sea level. The 

 gages should be put into operation about 3 months before the velocity and 

 salinity surveys and operated continuously throughout these surveys and 

 for an additional 2 months. 



(4) Current Velocity and Direction . Velocity metering stations 

 must be established on several ranges across the estuary and major tribu- 

 taries. These current ranges should be spaced (as for the tidal stations) 

 at intervals of 10 to 20 percent of the length of that part of the estuary 

 in which model studies are conducted. It is usually unnecessary to obtain 

 velocity data upstream from the problem area, since that part of the model 

 can usually be satisfactorily adjusted using only tidal data. If the 

 problem area is confined strictly to the entrance area, three or four 

 velocity ranges may suffice. Depending on the width and shape of the 

 cross-sectional area of the estuary at each range, one to five velocity 

 stations should be located on each range. As many as 11 stations on a 

 single range have been required on ranges across wide, deep estuaries 

 such as Chesapeake Bay (Fig. 3-4) . For channels in the upstream reaches 

 of the estuaries or tributaries, a single station may be sufficient 



66 



