II. GENERATION OF WATER MOTION AND DIRECTION OF FLOW 



1. General . 



The motion o£ water in a lake is the result of many complex processes 

 involving the interaction of the hydrosphere, atmosphere, and lithosphere. 

 Present knowledge of water motion is a result of both theoretical and 

 eii5)irical investigations. Essential results of the physical theory along 

 with a discussion of currents are described here in a qualitative manner. 



Most of the energy expended by moving water in the earth's lakes and 

 oceans originates from extraterrestrial sources, imparted either directly 

 or indirectly to the water. Examples of direct application of extra- 

 terrestrial forces to the earth's waters include: solar and lunar gravi- 

 tational attraction (which produces tides) , the uneven heating of the 

 water by solar radiation (creating density differences that in turn 

 generate density currents) , and meteorological phenomena (atmospheric 

 pressure differences and winds). Meteorological forces, in turn, act 

 directly on lakes and oceans to produce currents, waves, and storm surge 

 or wind setup. Meteorological forces are by far the most efficient 

 natural mechanisms for generating currents, waves, and storm surge; in 

 most cases their effects greatly overshadow the direct effects of solar 

 heating and solar and lunar gravitational attraction. 



Water motion in the form of either waves or currents, entrains and 

 transports sedimentary materials that comprise lake and ocean bottoms 

 and shorelines. Bathymetric and shoreline configuration of lake and ocean 

 basins reflect a response to the fluid forces exerted on them. The bottom 

 and shoreline respond in proportion to their ability to withstand the 

 fluid forces; loose, unconsolidated materials such as sand, respond 

 quickly to changes in flow conditions, while consolidated materials such 

 as granite rock respond slowly. A wide range of intermediate degrees of 

 erodibility exist depending on the characteristics of the bottom and 

 shoreline sediments. 



To understand erosion, transport, and deposition processes in the 

 ocean or lake environment, it is first necessary to understand the 

 phenomena that generate waves and currents and their relative importance 

 at various locations in the lake or ocean. This brief summarization 

 describes the generation of waves and currents; their direction and 

 characteristics; the influence of water levels on the effectiveness of 

 waves and currents in eroding shorelines; the effect of waves and currents 

 in entraining and transporting sediments; and the effect of manmade 

 structures in modifying the flow and thereby modifying natural conditions. 



Two basic physical principles govern the flow of all fluids : 

 conservation of mass and conservation of momentum. In addition, infor- 

 mation on the physical state of the fluid is required (i.e., is the fluid 

 a gas or liquid, and how will the fluid density respond to changes in 

 pressure and temperature) . The first principle states that mass is 



