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Construction exposed to rainfall could cause erosion and sedimentation in the stream, as well as water pollution. Therefore, calculation of soil loss is significant in order to estimate the mitigation approach. 

Universal Soil Loss Equation (USLE) and Modified Universal Soil Loss Equation (MUSLE) are to be introduced in this section to assess the site erosion risk and the load of sediments. 

The approaches are implemented in MiTS 2, according to MSMA 2nd Edition, Chapter 12.

i.  USLE

USLE is developed by W. Wischmeier and D. Smith to assess soil losses caused by sheet and rill erosion. The equation is given as,

 

            A = R.K.LS.C.P (Equation 12.1)

 

Where,

A = Annual soil loss, in tonnes/ha/year

R = Rainfall erosivity factor, an erosion index for the given storm period in MJ.mm/ha/h

K = Soil erodibility factor, the erosion rate for a specific soil in continuous fallow condition on a 9% slope having a length of 22.1 m in tonnes/ha/(MJ.mm/ha/h)

LS = Topographic factor, which represents the slope length and slope steepness. It is the ratio of soil loss from a specific site to that from a unit site having the 9% slope with a slope length of 22.1 m when other parameters are held constant.

C = Cover Management factor, which represents the protective coverage  of canopy and organic material in direct contact with the ground. It is measured as the ratio of soil loss from land cropped under specific conditions to the corresponding loss fro tilled land under clean-tilled continuous fallow )bare soil conditions. This is the factor that indicates the effect of erosion control facilities in an ESCP

P = Support practise factor, which represents the soil conversation operations or other measures that control erosion, such as contour farming, terraces and strip cropping. It is expressed as the ratio of soil loss with a specific support practise to the corresponding loss with up and-down slope culture. This is also a factor that indicates the effect of sedimentation control facilities in an ESCP. 

We also carried out examples to benchmark the calculation with MiTS, using the example project file

 

 

ii.   MUSLE

MUSLE is used for estimation of sediment yield on a storm basis. The estimated sediment yield volume is then used for sediment basin design. The equation is given as;

 

                  𝛄 = 89.6(VQp)0.56(K.LS.C.P) (Equation 12.4)

 

where,

Y = Sediment yield per storm event (tonnes)

V = Runoff volume (m3)

Qp = Peak discharge (m3/s)

K, LS, C, P = USLE factors

An example calculation is carried out to benchmark with our software. 

 

iii. Sediment Basin (BMP)

Estimation of sediment yield is used to estimate the sediment basin sizing for Best Management Practices (BMPs) control measure. The benchmark example is carried out to be compared with our software.