trolled by five main groups of factors: (a) precipitation and runoff 

 characteristics; (b) soil resistance; (c) basin topography; (d) vegetation cover; 

 and (e) land use. Of these, most consider mean annual precipitation or runoff 

 to be the most important variable. Although studies show great disparity, the 

 greatest sediment yields occur in semi-arid regions and very humid regions 

 (Knighton 1984). Only a small proportion of the sediment mobilized from in- 

 land areas on a given occasion will reach the coast, while most is stored 

 temporarily in the basin. There is large geographical variability in concentra- 

 tion and distribution of sediment carried from sources to the coast by running 

 water. 



In a fluid, the entrainment, transport, and deposition of sediment particles 

 are often defined as functions of sediment diameter and mean fluid velocity. 

 In flowing water, the Hjulstrom curve indicates that the threshold velocity is 

 at a minimum for medium size quartz sand particles, and that higher velocities 

 are necessary to entrain both finer and coarser sediments (Figure 10), for 

 reasons to be discussed shortly. Greater velocities are required to transport 

 materials of similar size in wind as opposed to water (Bagnold 1941). 

 Velocity alone does not control the capacity of a fluid to entrain particles. 

 Other characteristics include the viscosity of the fluid, the nature of the fluid 

 motion, the character and shape of the bed materials, and the impact of 

 saltating or bouncing grains. 



\ 



£ 



>^ 



'o 

 _o 



> 



10 



1 



0.1 



0.01 



0.001 















/' 





^ 





Non-cohesi 





5/ocity ___,^- 











~l 





Transp 



ortation 



/ 



Sedime 



ntation 









^Fall 

 Velocity 









1 I 1 1 ' 1 ' 







0.( 



301 0.01 0.1 1.0 10 100 



Size (mm) 



1C 



00 



Figure 10. Conditions for erosion, transportation, and deposition of particles in water 

 according to Hjulstrom (1935) 



28 



Chapter 2 Relevant Processes and Factors 



