those waters. A good example is San Francisco Bay, mostly rimmed with a wide shelf of soft 

 bay silts and mud, the surface of which lies but a few feet below mean sea level. Any harbor 

 constructed along the margin of this bay must have an access channel to deep water through 

 this shelf. Wind waves generated within the bay frequently stir this muddy bottom material 

 into suspension and move it long distances; most of the material is redeposited in navigation 

 channel and harbor entrances. As a result, the channel characteristics are continually 

 changing and the channels must be marked with buoys and redredged frequently to 

 maintain navigation. 



Rivers meander from the natural process of flood plain and delta aggradation. River flow 

 continuously erodes material from concave bends where the currents concentrate; the 

 material is then deposited on convex bars farther downstream where the current is slower. 

 Examples of this meandering are prevalent throughout the Mississippi Valley and other 

 major river systems. The harbor designer must study the proposed site carefully to 

 determine if this natural channel migration and its related movements of bottom sediments 

 pose a severe threat, or if their effect can be overcome by maintenance efforts within the 

 financial capabilities of harbor operations. If the problem is serious, he should seek the 

 advice of a river-hydraulics specialist familiar with the particular area involved. 

 4. Geological Factors. 



a. Basin Excavation. On rare occasions, the harbor site provides basins that are already 

 the correct size and shape and have adequate depth throughout. However, some excavation 

 is usuaUy required, and a knowledge of the characteristics of the substrata to be removed 

 must be obtained to determine the best method of removal. This is usually done by taking 

 borings and core samples. The core samples are analyzed to determine if the materials can be 

 moved by normal dredging or excavating equipment, or if ripping or blasting is required. 



In areas where the material is granular and unconsohdated, wash-borings may be 

 satisfactory for determining the nature of the materials and can be done at less cost than 

 core-borings. However, wash-borings tend to segregate the fine materials from the coarse 

 materials and give an unreliable indication of substrata characteristics. Borings should be 

 done only by personnel familiar with the local geology and with wash -boring techniques; 

 they must be able to distinguish the different strata by their relative resistance to probing 

 with the jetting rod, by the color of tlie effluent brought to the tops of the casting, and by 

 the feel and appearance of the sediments entrained in the effluent. 



Occasionally, the subsurface characteristics of the area are so well known or uniform that 

 it is only necessary to know the depth of bedrock below the overlying strata of 

 unconsolidated materials. Sonic probings can make this determination at less cost than any 

 boring procedure. Sonic probings can complement the knowledge gained by a few 

 strategically placed core-borings; geologic interpretations are possible over a large area, and 

 thus reduces the overall cost of the subsurface exploration. In some instances, a satisfactory 

 delineation of the mud-sand or the sand-rock interface may be obtained simply by probing 

 with a long steel rod. 



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