obstructions, but the potential exists for bed scour at the base of the 

 obstruction, erosion of the obstruction, and ice jamming at the obstruction. 

 Erosion of a dike at Skeetercake Creek increased siltation as discussed in 

 the following section. 



SEDIMENTATION 



Sedimentation includes the processes of erosion, transportation, and 

 deposition of sediment. These are complex processes related to sediment 

 and water flow properties. Attempts to quantify these processes provide, 

 at best, estimates of the quantity. A very brief discussion of sediment 

 size distribution, channel erosion, and sediment transport are given in 

 the following section. Changes to these sedimentation characteristics due 

 to gravel removal are then briefly discussed. 



Undisturbed Condition 



Sediment Size Distribution. An important factor influencing most sedi- 

 mentation problems is the size distribution of the sediments. The typical 

 descriptors of the size distribution of sediment are the median diameter and 

 graduation coefficient of the material. Natural sediment distribution tends 

 to be log-normal, which is a two parameter distribution. The median diameter 

 of a distribution has 50 percent of the material sma I ler by weight and 50 

 percent of the material larger by weight. The second parameter, the grada- 

 tion coefficient, gives the slope of the straight line resulting from plot- 

 ting the distribution on log-probability paper. It is defined as 



(9) 



where a is the gradation coefficient and D is the particle diameter for 

 which X percent of the material is finer. The gradation coefficient is 

 related to the standard deviation of the material. The material can be 

 described as uniform if its gradation is less than 1.3 or graded if its 

 gradation Is greater than 1.5. 



115 



