Soil Erosion:Simulations with Soil Erosion
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 are:
1 - Engelund-Hansen 2 - Kilinc Richardson 3 - Stream power 4 - Effective stream power 5 - Unit stream power 6 - Shear stress
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 talbe. 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 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 - Stream power, 4 - Effective stream power, 5 - Unit stream power, 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 = (KI)(CG)(Ci)
10.4.2.4.1 KI
KI 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:
| Soil Texture | Raindrop Erodibility Factor KI, J-1 Meyer and Harmon (1984) |
Raindrop Erodibility Factor KI, J-1 Morgan (1985) |
Raindrop Erodibility Factor KI, J-1 Bradford et al. (1987 a,b) |
Raindrop Erodibility Factor KI, J-1 Verhaegen (1987) |
|---|---|---|---|---|
| Clay | 19.0 | 73.5 | ||
| Silty Clay | 18.2 | |||
| Silty Clay Loam | 16.2 | 22.2 | ||
| Silt | 29.8 | |||
| Silt loam | 39.8 | 25.7 | 24.7 | |
| Loam | 28.2 | 30.0 | 37.6 | 23.4 |
| Sandy Loam | 32.0 | 34.4 | 30.0 | |
| Sand | 62.4 |
10.4.2.4.2 CG
CG 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 Ci
Ci 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, τcr) 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, τcr
τcr 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.
| Soil Texture | Erodibility Factor K |
|---|---|
| Sand | 0.05 |
| Loamy sand | 0.12 |
| Sandy loam | 0.27 |
| Loam | 0.38 |
| Silt loam | 0.48 |
| Sandy clay loam | 0.27 |
| Silt | 0.60 |
| Clay loam | 0.28 |
| Silty clay loam | 0.37 |
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.
| Cover | Cropping Factor C |
|---|---|
| None (fallow) | 1.00 |
| Native vegetation | 0.01 |
| Crops | 0.08 |
| Pasture | 0.01 |
| Forest | 0.005 |
| Urban | 0.01 |
| Other | 1.0 |
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.
| General land use | Control Practice Factor P |
|---|---|
| Crop | 0.5 |
| Pasture | 1.0 |
| Forest | 1.0 |
| Urban | 1.0 |
| Other | 1.3 |
Table 19 – General landuse erosion control factors (P), Wanielista (1978)
10.4.3.2 Stream Power Coefficients, a, b, and ωc - no longer used
Prior to verion 5.7 the stream power transport formulations require three parameters, a, b, and ωc, that are described in Equation 110. In this equation, a is a transport coefficient, b is an exponent, and ωc 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). The stream power approach is no longer used in current versions of GSSHA. These are only used for verions before 5.7.
10.4.3.2.1 Transport Coefficient, a - no longer used
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 used
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, ωc - no longer used
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 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.
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
