Mixed Grain Size Bed Material
Overview
Real riverbed material is rarely a single grain size. Coarse particles are harder to move and tend to remain on the bed surface, causing armoring, while fine particles are selectively washed away. As a result, the grain size composition of the bed changes over time and space depending on the flow conditions. Cabernet2D can account for this evolving grain size composition in the bed variation calculation, configured in the “Bed Material / Mixed Grain Size” tab of the calculation conditions.
GUI option |
Name |
Description |
|---|---|---|
Uniform grain size |
The simplest setting: the bed material is treated as a single representative grain size. |
|
Mixed grain size |
The bed material is treated as a mixture of several grain size classes, and the fraction of each class in the exchange layer evolves over time. |
The exchange layer concept
In the mixed grain size calculation, the bed is divided into an “exchange layer” (a surface layer that exchanges sediment with the flow) and the material below it. The exchange layer keeps track of the fraction \(p_k\) of each grain size class. As erosion and deposition proceed, the grain size composition of the exchange layer is updated, which in turn affects the per-class transport calculation (critical shear stress and bedload formula evaluation for each grain size class).
In the GUI, the thickness of this layer is set with “Exchange layer thickness (m)”.
Entering the grain size distribution
The grain size composition is entered in the “Grain size distribution” table of the GUI. The input format is selected with “Grain size input type”.
Input format |
Description |
|---|---|
|
Each row gives the percentage of that grain size class in the total bed material. All values must sum to 100% (auto-normalized if not). Example: d=0.15mm: 10%, d=0.30mm: 20%, d=0.43mm: 40%, d=0.61mm: 20%, d=0.86mm: 10% |
|
Each row gives the percentage of material finer than that grain size (sieve analysis format). Values must be monotonically increasing (grain sizes are automatically sorted in ascending order). Example: d=0.15mm: 10%, d=0.30mm: 30%, d=0.43mm: 70%, d=0.61mm: 90%, d=0.86mm: 100% |
Both formats are internally converted to fractions summing to 1.0 before computation.
Friction parameters
The frictional properties of the bed material are set with the following parameters.
Parameter |
Description |
|---|---|
Static friction coefficient |
Default value 1.0. |
Kinematic friction coefficient |
Default value 0.5. |
Note
Reducing these values increases the gravitational effect of the bedload, smoothing out irregularities of the riverbed. Be cautious: making them too small may cause the desired microtopography to disappear.
Output
When the mixed grain size calculation is enabled, the following representative grain sizes are output as results.
Mix mean grain size dm (mm): the weighted mean grain size of the exchange layer
Mix grain size d10 (mm) / d50 (mm) / d90 (mm): the grain sizes corresponding to 10%, 50% and 90% cumulative passing
These outputs let you track how the bed surface coarsens or fines as the calculation proceeds.