Sediment Gradation

Sediment transport rates are computed for the prescribed hydraulic and sediment parameters for each representative grain size. Transport capacity is determined for each grain size as if that particular grain size made up 100% of the bed material. The transport capacity for that size group is then multiplied by the fraction of the total sediment that that size represents. The fractional transport capacities for all sizes are summed for the total sediment transport capacity.

1)	$\begin{array}{l}\displaystyle \displaystyle g_s = \sum_{i=1}^n g_{si}p_i\end{array}$

Symbol	Description	Units
$\begin{array}{l}g_s\end{array}$	Total sediment transport
$\begin{array}{l}g_{si}\end{array}$	Sediment transport for size class i
$\begin{array}{l}p_i\end{array}$	Fraction of size class i in the sediment
$\begin{array}{l}n\end{array}$	Number of size classes represented in the gradation

Figure 12-10 Adjustment Factor for Concentration of Fine Sediment (Colby, 1964)

The user enters gradation information as particle sizes with an associated percentage value that indicates the amount of material within the sediment mixture that is finer by volume (percent finer). HEC-RAS then interpolates logarithmically to determine a representative percent finer for the standard grade class sizes. The standard grade class sizes are based on the American Geophysical Union (AGU) classification scale shown in Table 12-6.

If a maximum particle diameter is not entered (i.e. d100), HEC-RAS will automatically assign the 100% finer value to the next greater standard grain size from the largest particle diameter established by the user. For example, if the largest particle diameter is entered as 1.6 mm with a percent finer value of 84%, then the maximum grain size will be automatically assigned to 2.0 mm with 100% of the particles finer than that. On the low end, if the user does not establish a zero percent finer particle diameter (i.e. d0), then the smallest standard grain size range (0.002 – 0.004 mm) is assigned zero percent. Because the ultra-fine sized sediment has a tendency to produce inaccurate results for certain transport functions, it is important that the user realize the extrapolation used in this instance. To avoid the automatic extrapolation on the fine-side of the gradation curve, simply enter in a particle diameter with an associated "percent finer" value of zero.

Table 12-7 Grain Size Classification of Sediment Material American Geophysical Union

Sediment Material	Grain Diameter Range(mm)	Geometric Mean Diameter (mm)
Clay	0.002-0.004	0.003
Very Fine Silt	0.004-0.008	0.006
Fine Silt	0.008-0.016	0.011
Medium Silt	0.016-0.032	0.023
Coarse Silt	0.032-0.0625	0.045
Very Fine Sand	0.0625-0.125	0.088
Fine Sand	0.125-0.250	0.177
Medium Sand	0.250-0.5	0.354
Coarse Sand	0.5-1.0	0.707
Very Coarse Sand	1-2	1.41
Very Fine Gravel	2-4	2.83
Fine Gravel	4-8	5.66
Medium Gravel	8-16	11.3
Coarse Gravel	16-32	22.6
Very Coarse Gravel	32-64	45.3
Small Cobbles	64-128	90.5
Large Cobbles	128-256	181
Small Boulders	256-512	362
Medium Boulders	512-1024	724
Large Boulders	1024-2048	1448

If the user enters in one or more particle sizes that are less than the smallest standard grain size diameter (0.002 mm), HEC-RAS will automatically lump all of that sediment into the smallest standard grain size range (Clay, 0.002 to 0.004 mm). This is done so that all of the sediment in the gradation curve will be accounted for volumetrically.

The rate of transport is extremely sensitive to the grain size distribution, particularly on the finer side, and should be chosen carefully. The application of grain size particles smaller than the designated range of applicability for a given function can lead to extremely high, and unreasonable sediment transport rates. For this reason, RAS provides an option to not compute sediment transport rates for grain sizes outside the range of applicability on the low end. This is done by going to the options menu and selecting "No" under the menu item "Compute for Small Grains Outside Applicable Range". Still, the user should check unreasonable results for all given parameter ranges (Table 12-7). (Note: the low end of applicable grain size for Laursen was chosen as that used in the field research.) The selection of a representative sediment sampling is described in EM 1110-2-4000.