Difference between revisions of "Soil Erosion:Simulations with Soil Erosion"

Latest revision as of 17:39, 15 June 2015

10.4.1 General

A detailed report on the soil erosion methods, estimated parameter values, and testing is available here Sediment Tech Note. Soil erosion simulations are specified with the SOIL_EROSION project card. The SOIL_EROSION card is also used to select the transport equation to be used, with an integer value of 1-6. The six transport capacity options currently are:

1 - Engelund-Hansen
2 - Kilinc Richardson
3 - Unit stream power - replaces generalized stream method formulation, new for version 5.7 and beyond
4 - Effective stream power, new for v5.7
5 - Slope and unit discharge, new for v5.7
6 - Shear stress, new for v5.7

It should be noted that methods 3,4,5, and 6 replace a generalized "stream power" method, no longer in the model. The parameters used in the generalized "stream power" method are no longer input for versions 5.7 and beyond. The following information is annotated for this change in the model.

Soil and erosion properties are assigned with the use of the Mapping Table, Chapter 11. For GSSHA versions 5.7 and up, the first three variables are not used and should not be included in the table. The eight variables are input in the following order:

 
Transport Capacity Coefficient - no longer used
Transport Capacity Exponent - no longer used
Transport Capacity Critical Value - no longer used
Rain Splash Detachment Erosion Coefficient
Overland Detachment Coefficint
Overland Detachment Exponent
Overland Detachment Critical Value
Overland Transport Capacity Erosion Coefficient

Additional optional inputs and outputs are also available. These are described in detail in Section 3.10. If channel routing is not specified in the project file only overland soil erosion calculations are performed. Sediment transport can be simulated in either event or long-term simulation mode.

There are three general categories of parameters to enter into your mapping tables when running a sediment transport simulation. These categories of parameters are explained as follows:

10.4.1.1 Overland Sediment Particle Detachment Parameters

In the formulation used in GSSHA, overland sediment particle detachment is caused in two ways: by rainfall impact detachment and by surface runoff detachment. The following input parameters are used to control how much sediment is detached by these two methods:

Rainfall Impact Detachment

Rain splash detachment coefficient (SPLASH_K)

Surface Runoff (Rill) Detachment

Overland detachment coeff (DETACH_ERODE)

Overland detachment exponent (DETACH_EXP)

Overland detachment critical shear (DETACH_CRIT)

10.4.1.2 Overland Sediment Transport Capacity Parameters

GSSHA has several equations that can be used to simulate sediment transport capacity. You can select the appropriate equation for your model based on the information in this manual and in this Sediment Technical Note. You must define and calibrate correct input parameters based on the equation you select with the SOIL_EROSION project card. The following input parameters are used to control the sediment transport equations in each of the six methods:

1 - Engelund-Hansen and 2 - Kilinc Richardson

Overland Transport Capacity Erosion Coefficient (SED_K)

3 - Unit stream power, 4 - Effective stream power, 5 - Slope and unit discharge, and 6 - Shear stress

For the current formulation of these methods, there are no adjustable parameters. Parameters previously assigned by the user, see list of parameters above, are assigned to literature values for the various methods.

10.4.1.3 Sediment Fractions

For each mapping table index, a fraction between 0.0 and 1.0 must be defined for each type of sediment defined in the simulation. The total of these sediment fractions should equal 1.0 so all sediments are included in the particle detachment and sediment transport equations.

10.4.1.4 Overview

Methods for obtaining initial values for the sediment particle detachment parameters and the sediment transport parameters are described in the following sections of this document.

10.4.2 Assignment of Overland Sediment Particle Detachment Parameter Values

The rainfall impact detachment and surface runoff detachment parameters must be initialized to correct values to get reasonable results for the detachment of sediment from the soil surface. For rainfall impact detachment, a rain splash coefficient (SPLASH_K) must be defined. For surface runoff detachment, two parameters--a runoff detachment coefficient (DETACH_ERODE) and a runoff detachment index (DETACH_CRIT) must be defined.

10.4.2.4 Rainfall Impact Detachment Parameter Values

The rain splash coefficient (SPLASH_K) entered in GSSHA is the multiplication of three factors used in Equation 98, which determines rainfall impact detachment. SPLASH_K = (K_I)(C_G)(C_i)

10.4.2.4.1 K_I

K_I is the soil erodibility factor for detachment by raindrop impact. The units for this factor are J^-1 (1/Joules) and the value is approximated using the table 2 from Wicks and Bathurst [1996], shown below:

10.4.2.4.2 C_G

C_G is the ground cover factor and is a value between 0.0 and 1.0. This factor = 1.0 - Fraction of ground covered by ground cover. This factor is unitless and can be obtained from a land use map, a ground cover map, or an aerial photograph.

10.4.2.4.3 C_i

C_i is the cover-management factor and is a value between 0.0 and 1.0. This factor = 1.0 - Fraction of ground covered by canopy cover. This factor is unitless and can be obtained from a land use or a canopy/vegetation cover map or a recent aerial photo of the area being modeled.

10.4.2.5 Surface Runoff Detachment Parameter Values

GSSHA uses three surface runoff detachment parameter values that must be entered in your GSSHA model. The runoff detachment coefficient (DETACH_ERODE, a), the runoff detachment index (DETACH_INDEX, b), and the runoff detachment critical shear (DETACH_CRIT, τ_cr) are used in Equation 101, which determines surface runoff detachment.

10.4.2.5.1 Runoff Detachment Coefficient, a

According to the Water Erosion Prediction Project (WEPP) model documentation, a value of 0.004 for rangeland and a value of 0.05 for cropland should be used for the runoff detachment coefficient.

10.4.2.5.2 Runoff Detachment Index, b

According to the Water Erosion Prediction Project (WEPP) model documentation, a value of 1.0 should be used for the runoff detachment index.

10.4.2.5.3 Runoff Detachment Critical Shear, τ_cr

τ_cr is the runoff detachment critical shear. The units for this factor are Pascals and research is still being conducted on how to obtain this value. We recommend using the default value of 3.5 Pa for this value.

10.4.3 Assignment of Overland Sediment Transport Parameter Values

The Kilinc Richardson and Engelund Hansen transport capacity formulations in GSSHA (Equations 108 and 109) require an erodibility coefficient (SED_K, or K) that varies between zero and unity. As originally formulated for CASC2D, the erodibility factor for the Kilinc Richardson equation contained three factors K, C, and P, that were combined in the equation to describe the overall erodibility of the soil, including the effects of soil texture, vegetation coverage, and management practices. Lower values indicate less erodibility. Although only one factor is currently used in GSSHA, this single factor represents the combined effects of the three original factors contained in the original Kilinc Richardson formulation, as well as other factors. The combined erodibility can thus be assigned based on three original factors as described.

Downer et al (2010) states that the Engelund-Hansen equation is applicable for soils in which <math>\sqrt{D_{75}/D_{25}} < 1.6</math> and for sand-size sediments coarser than 0.15 mm.

The other stream power transport formulations require three parameters that are described in Equation 110 and below.

Downer et al (2010) states that a comparison of the three sediment transport relations in Equations 108, 109, and 110 (Kilinc Richardson, Engelund Hansen, and Stream Power) was performed by running the GSSHA model with each equation. Surprisingly, the result of this comparison showed that for sediments with S=2.65, there is very little difference between them. Therefore, the user is advised to use the Kilinc-Richardson method because it has the smallest number of parameters. However, for simulations involving sediments with specific gravities different from 2.65, the use of the Engelund-Hansen equation is required.

10.4.3.1 Erodibility Coefficient, K

The general erodibility, K, is based on the composition of the soil in each cell. This erodibility can be reduced by vegetative or other coverage with the C factor, and can be further reduced, or increased, by the management practice factor, P. The erodibility coefficient, SED_K = (K)(C)(P).

Soil erosion parameters and soil erosion factors may be estimated from land use, vegetation, and soil texture indices. These values are entered with in the Mapping Table File.

10.4.3.1.1 Soil Erodibility (K)

Soil erodibility describes the susceptibility of the soil to detachment and transport by rainfall impact and overland flow. Soil erodibility is generally a function of soil texture, soil structure, organic content, and permeability. In general, larger particles are harder to erode, as are undisturbed soils. Organic soils are less susceptible to erosion, and have lower K values. The values in Table 17, from Wanielista (1978), are for undisturbed, inorganic soils.

Table 17 – Soil erodibility factor (K) of the modified Kilinc Richardson Equation, Wanielista (1978)

10.4.3.1.2 Cropping Management Factor (C)

The cropping management factor (C) describes the effect of land coverage on reducing the erodibility of bare soils. In general, covered lands are less susceptible to erosion, and have lower C values. With no cover on the soil, the full erodibility (K) can be achieved, and C=1.0. Table 18 lists values for general land coverage types.

Table 18 – Cropping management factors (C), Wanielista (1978) and Goldman et al. (1986)

10.4.3.1.3 Conservation Practice Factor (P)

The conservation practice factor reflects efforts specifically intended to reduce erodibility of the soil. This factor is generally associated with practices used by farmers to conserve the soil, such as no-till and contour farming. This factor may also be important for certain types of construction practices, such as coverage with geotextiles, which tend to reduce erodibility. Other construction practices, such as smoothing and compacting the soil, actually increase erodibility and can result in the value of P being greater than 1.0. Table 19 lists recommended values of P for general land use.

Table 19 – General landuse erosion control factors (P), Wanielista (1978)

10.4.3.2 Stream Power Coefficients, a, b, and ω_c - no longer assigned

The stream power transport formulations require three parameters, a, b, and ω_c, that are described in Equation 110. In this equation, a is a transport coefficient, b is an exponent, and ω_c is a critical stream power. In GSSHA, these are called, respectively, the transport coefficient (TC_COEFF), the transport index (TC_INDEX), and the critical transport capacity (TC_CRIT). In GSSHA version 5.7 and beyond, these parameters no longer assigned by the user. The values are set internally to literature values. These paramters should not be included in the sediment mapping table when using GSSHA versions 5.7 and beyond.

10.4.3.2.1 Transport Coefficient, a - no longer assigned

The Transport Coefficient (TC_COEFF) is used in the stream power equation. No advice is available for determining this parameter from watershed data, so it is recommended that you use Kilinc-Richardson equation (Equation 108) and use the default value of 0.285 for this coefficient.

10.4.3.2.2 Transport Index, b - no longer assigned

The Transport Index (TC_INDEX) is used in the stream power equation. No advice is available for determining this parameter from watershed data, so it is recommended that you use Kilinc-Richardson equation (Equation 108) and use the default value of 1.3 for this coefficient.

10.4.3.2.3 Critical Transport Capacity, ω_c - no longer assigned

The Critical Transport Capacity (TC_CRIT) is used in the stream power equation. No advice is available for determining this parameter from watershed data, so it is recommended that you use Kilinc-Richardson equation (Equation 108) and use the default value of 0.0002 for this coefficient.

10.4.4 Adjusting Overland Elevations during Sediment Simulations=

Unless specified with the ADJUST_ELEV card, changes in elevation due to erosion and deposition are not tracked during the simulation. If the user wishes to include the ongoing effect of erosion and deposition on hydrologic processes during the simulation, the ADJUST_ELEV project card is used to specify this option and to specify the name of a file that will contain the final elevations (in a GRASS ASCII map) at the end of the simulation. The volumetric change in each cell, positive for deposition and negative for erosion can be output using the NET_SED_VOLUME card regardless of whether the ADJUST_ELEV card is used or not so that the user need not include the ADJUST_ELEV card just to get information about the net gain/loss of sediment over the duration of the simulation. The primary purpose of the ADJUST_ELEV card is to include the effects of erosion and deposition on the elevations used during the simulation.

10.4.5 Channel Routing of Sediments

If explicit channel routing is specified in the project file with the CHAN_EXPLIC or DIFFUSIVE_WAVE project cards along with the SOIL_EROSION project card, sediment routing in channels will also be performed. In the CHAN_INPUT file, the user specifies the initial cross-section of each channel link. For each erodible link the maximum depth of erosion is specified with the MAX_EROSION card. Proper construction of a channel input file is given in Section 5 of this manual.

10.4.6 Output

When simulating sediment routing the volumetric flux of sediment at the outlet (cms) is output with the project card OUTLET_SED_FLUX followed with the name of the file for the output. This is a REQUIRED card. The outlet sediment flux will be output at the channel outlet, if channel routing is specified, or at the specified overland flow outlet cell. The format of the outlet sediment flux is a time stamp followed by a column of flux (cms) for each sediment particle specified in the mapping table file. Wash load particles are listed first, in the order specified in the mapping table file, followed by the sand and larger size particles, also in the order specified in the mapping table file. In addition to the specified sand size particles there is one additional column for sand or larger sizes that represents the bed load originally in the channel, so that even if no sand size particles are specified in the mapping table file, the user will still see one column representing bed load for the initial channel sediments. If channel routing is being conducted, then the same information can be output at interior link/node locations using the IN_SED_LOC card, to specify the desired sedograph link node pairs, and the OUT_SED_LOC card which specifies the name of the file that will contain sedographs at the desired locations. The information in the OUT_SED_LOC file will be same as in the OUTLET_SED_FLUX file, except there will be a series of data for each chosen output link/node pair. Data from the link/node pairs in the OUT_SED_LOC file will be listed in the order they were specified in the IN_SED_LOC file. The format of the IN_SED_LOC file is exactly the same as the format for the IN_HYD_LOCATION file, and if the same points are desired for sediment flux as flow, then the same file can be used for both. Additionally, the concentration (mg/L) of total suspended sediments (TSS) at the outlet can be output with the OUTLET_SED_TSS card which is used to specify the file for the outlet TSS information. TSS at interior points can be specfied using the OUT_TSS_LOC card. The link node pairs for the OUT_TSS_LOC file are specified in the file named by the IN_SED_LOC card. Additional information about the output available for sediment processes can be found in Section 3.13.3 of the User's Manual.

Maps of the maximum sediment flux (cms) and the volumetric change (m3) can be output using the MAX_SED_FLUX and NET_SED_VOLUME cards, respectively. Additional information about the output available for sediment processes can be found in Section 3.13.3 of the User's Manual.

GSSHA User's Manual

10 Soil Erosion

10.1     Overland Erosion Formulation

10.2     Channel Sediment Transport Formulation

10.3     Sediment in Lake

10.4     Applicability of the Sediment Routing Methods

10.5     Simulations with Soil Erosion