was of great importance and could not be determined by direct measurements 

 or historic records. 



99. Contributions by Bluff Erosion. - Eroding bluffs are the last 

 major source of beach material. Along the Great Lakes this is a major 

 source, whereas, along most of the sea coasts it is of comparative un- 

 importance. As long as a beach berm is maintained between the bluff and 

 the action of the waves, the bluff contributes negligibly to the littoral 

 supply. At some locations, littoral drift has been interrupted by 

 artificial barriers and the ocean has turned to the upland for its supply, 

 causing serious recessions of the coast line. The amount of such contribu- 

 tion can only be estimated through comparative surveys, sub-division plot 

 maps, property surveys, and statements of long-time residents of an area. 

 The bluffs frequently contain much material too fine to remain on the 

 beach. The proportion of beach material supplied out of total material 

 eroded may be determined by mechanical analysis of a composite sample. 

 Each stratum should be represented in proportion to its thickness. In 



the Great Lakes area, rises in lake level may allow waves to attack the 

 bluffs, which are generally of a very friable material. This causes 

 recession of the shore line and contributes to the supply of beach ma- 

 terial. Where bluff erosion is important, a geological study may be re- 

 quired. The extent of field work and investigation will depend on the 

 importance of bluff erosion as a source of littoral material, 



100. Natural Losses of Littoral Material . - Principal avenues of 

 loss of littoral material from a specific beach area include (a) drift 

 of material laterally out of the ?.rea; (b) movement of material offshore 

 into water of sufficient depth that it is lost to the littoral supply; 

 (c) loss of material into submarine canyons; and (d) loss of material 

 inland. Loss of material by abrasion of sand has been foujid of slitht 

 importance (Mason 19h'^) » 



101. Losses by Littoral Drift . - The drift of materials out oi the 

 area is measured by the net rate of drift at the dotmdrift end of the 

 beach segment under study. It may be that this loss can be measured 

 directly as outlined under rates of littoral drift. If it cannot be 

 measured directly at a particular location, it may be possible to esti- 

 mate it by considering the rates of drift at the two closest known 

 points above and below where the rate has been established or can be 

 measured directly. At best this is a rough estimate as the unknown 

 factors of added supply and losses throughout tlie area must also be 

 taken into consideration. 



102. Movement of Material Offshore . - The quantity of material 

 lost to the offshore depths cannot in itself be determined in the 

 light of present knowledge. It is possible that as information on 

 material sorting with respect to slope and wave characteristics is de- 

 veloped, equations may be evolved by which this iiriportant avenue of 

 material loss may be evaluated. At present it can only be assumed as 



61; 



