harmf ulness and assigning them scores from 1 , for 

 10 percent effectiveness, to 9, for 90 percent. The 

 score 10, indicating 100 percent destructiveness, is 

 omitted because no oyster population can exist 

 under such a condition. Zero score means 

 complete absence of a negative factor. 



SEDIMENTATION 



Rapid settling of suspended material may be 

 highly destructive to an oyster community. All 

 coastal waters contain a certain amount of solids 

 in suspension of either organic or inorganic origin. 

 The particles settle on the bottom, depending on 

 their weight and shape, chemical composition, 

 temperature, viscosity of water, and character of 

 water movements. The velocity of the fall of a 

 particle through a liquid is a function of the radius 

 of the particle governed by Stoke's law: 



_2{Po-P)9r' 



gn 



where v is the velocity of sinking, ?„ and P are the 

 densities of the particle and of the liquid respec- 

 tively, r the radius of the particle, n the absolute 

 viscosity of the liquid, and g the acceleration of 

 gravity. It is assumed in the equation that the 

 particles are spherical, that they are small enough 

 so that the viscosity of the water is the only 

 resistance to their fall, and that their sinking is not 

 impeded by adjacent particles. The equation is 

 appHcable to spheres varying in size from 0.2 to 

 200ju and suspended in quiet water. Obxdously 

 such conditions represent the "ideal" situation 

 which cannot be found in an estuarine environ- 

 ment. Here the water is in almost constant 

 motion with rapid changes in the du-ection and 

 velocity, and carries sediments consisting of parti- 

 cles of differing sizes, shapes, and densities. The 

 discussion of the physical aspects of sedimentation 

 problems is beyond the scope of this book. The 

 reader interested in these problems is referred to 

 comprehensive textbooks on the subject 

 (Twenhofel, 1961 ; Linsley, Kohler, and Paulhus, 

 1949). For an understanding of the ecological 

 effects of sedimentation it is enough to describe 

 in general terms the conditions under which silt 

 particles are transported and deposited on es- 

 tuarine bottoms. 



Observations made on waterflow in a tube show 

 that at low velocities the particles of a liquid move 



in parallel lines and the resistance to their motion 

 is due to viscosity. This condition is called 

 streamline or laminar flow. High velocity of the 

 water and roughness in the walls of the tube make 

 the stream break into a turbulent flow charac- 

 terized by iiTegular, eddying, and rolling move- 

 ments. The formation of the eddies counteracts 

 the gravitational settling of particles, and more 

 material is moved upward than sinks toward the 

 bottom. When the amount of sediment picked 

 up by a turbulent flow exceeds the amount 

 deposited, the liottom is eroded. If the gravita- 

 tional force predominates, more material is being 

 deposited than carried away and the bottom is 

 rapidly covered by the sediment. In an estuary 

 both processes alternate following the rhythmic 

 changes in the direction and velocity of tidal 

 cm-rents. In many instances an equilibrium is 

 established and continues for a long period of time 

 unless the balance between the two forces is upset 

 by \'iolent water movements from severe storms 

 or floods. Depending on the configuration of the 

 bottom, certain areas of an estuary are scoured, 

 while vast quantities of sediments are deposited 

 on others. This is typical on oyster grounds in 

 many tidal streams. 



The material suspended in coastal water is a 

 very complex mixture of particles of differing size, 

 shape, specific gravity, and mineralogical compo- 

 sition. The particles are sorted out as they are 

 moved by the water. According to Trask (1950, 

 p. 10), slowly moving water seems to make for 

 poorer sorting than a fast moving current. To a 

 sedimentation geologist the resulting pattern of 





SILT FLOCCULATION / , A /• / 



l9%o  ^ ^I0%« 



SILT RUNOFF 



FiGi'RE 370. — Diagrammatic representation of the sorting 

 out and deposition of coarse sand, fine sand, and silt in 

 an estuary. The lower part of the diagram indicates 

 the complex system of current and counter current. 

 From Rochford, 1951. 



410 



FISH AND WILDLIFE SERVICE 



