Mapping Table:Sediment Erosion Mapping Tables

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Sediment transport on the oveland flow plane requires two mapping tables. In the first mapping table, SEDIMENTS, the number of soil particles and the physical properties of those soil particles must first be specified. Then the erosive properties of the soil are specfied in a separate table, SOIL_EROSION_PROPS. These tables are part of the grid mapping table file (.cmt) but are discussed separately because the format is different from the other mapping tables.

Sediment Mapping Table

The SEDIMENTS mapping table is used to define the sediments to be transported. The user must specify the number of sediments and the specific gravity and particle diamter (mm) and an output filename of each sediment to be simulated. The number of sediments is defined for the entire simulation. There is no linkage to an index map for this table. The size fractions are linked to the overland cell in the SOIL_EROSION_PROPS table. The sediments specified may or may not be in all the cells but only sediments specified in the SEDIMENT table can be included in the simulation. The table header information consists of the table card and the NUM_SED card.


This is followed by the table data, which contains one line for each specified sediment in the following format.

Text line...
[ID]    [DESC...]           ... [specific gravity] [particle diameter]  [output filename]
[ID]    [DESC...]           ... [specific gravity] [particle diameter]  [output filename]
[ID]    [DESC...]           ... [specific gravity] [particle diameter]  [output filename]

For the most common case, three sediments sizes (sand, silt, clay) all made of quartz, the SEDIMENTS table would look like this.

Sediment Description                        Spec. Grav  Part. Dia   Output Filename
Sand                                        2.650000    0.250000    sand
Silt                                        2.650000    0.016000    silt
Clay                                        2.650000    0.001000    clay

Soil Erosion Properties Mapping Table

After defining the sediment particles to be transported in the SEDIMENT table, the SOIL_EROSION_PROPS table is used to specify the soil erosion properties for each index type. The SOIL_EROSION_PROPS in the table are:

Table Name # Values Parameter Units
SEDIMENT 6+ Soil erodibility due to rainfall, Equation (98) J-1
Runoff detachment coefficient "a", Equation (101) dimensionless
Runoff detachment index "b", Equation (101) dimensionless
Critical shear, Equation (101) Pa
Erosivity, Equations 108 and 109 dimensionless (0-1)
 % Sediments fraction

The equations referred to in the table are described in Section 10.1 of the manual.

The format is: table name, SOIL_EROSION_PROPS, followed by the name of the index map on the first line; NUM_IDS followed by the number of IDs on the second line; NUM_SED with the number of sediments in the table on line 3.

SOIL_EROSION_PROPS "index map name"
NUM_IDS #ids
NUM_SED #sediments

This is followed by the table data with one line of five erosion properties followed by the fraction of sediments for each NUM_SED size classes, listed in the order of the particles sizes in the SEDIMENTS table, for each NUM_ID. For our three sediment sizes described in the SEDIMENTS table, a SOIL_EROSION_PROPS table for an index map based on land use and soil type with 4 categories might look like.

101 Grass   Coarse   0.50000  0.050000    1.0000       3.1000      0.0002  0.9000 0.1000 0.0000
102 Res     Coarse   0.50000  0.000100    1.0000       3.1000      0.0005  0.9000 0.1000 0.0000
103 Grass Sandy Loam 5.00000  0.010000    1.0000       3.1000      0.0150  0.6500 0.2000 0.1500
104 Grass Loam       10.0000  0.004000    1.0000       3.1000      0.0050  0.4500 0.3500 0.2000

For this case cells with LUST index 101, defined as coarse textured soils with grass land use, are 90% sand, 10% silt, and no clay. Loam soils with a grass land use, LUST index 104, are 45% sand, 35% silt, and 20% clay. If one or more of these LUST types includes particles sizes different from the three specified, that soil particle will have to be added to the SEDIMENTS table before a fractional amount can be added to the SOIL_EROSION_PROPS table.

GSSHA User's Manual

12 Mapping Table
12.1     File Description
12.2     Index Maps
12.3     Grid-based Mapping Tables
12.4     ID Line Format
12.5     Example Mapping Table File
12.6     Stream Mapping Tables
12.7     Sediment Erosion Mapping Tables
12.7     Constituent Mapping Tables