Slope Area Index (SAI) of North Carolina

Slope-Area Index (SAI) is used to predict erosion along a stream channel. It is a function of channel slope, and drainage area upstream raised to the exponent used in equations for flood frequency of 2-percent exceedance floods. The guidelines for use of the coefficient from 2-percent exceedance came from meetings with cooperators describing bankfull discharge as a 2-percent exceedance.
The Slope-Area Index, as defined by Cartwright and Diehl, 2017, is calculated as:
SAI = S * A^b (1)
where
SAI is the slope-area index,
S is the channel slope,
A is the drainage area (the number of cells draining into the target cell), and b is a user-specified exponent.
The flood frequency report for NC (Weaver and others, 2009) defines the regional regression equations for exceedance flows for rural basins in the Southeast. The 2-percent chance exceedance flow raises the Drainage Area uses a 0.60 coefficient for all the physiographic provinces other than Small Urban basins in the Piedmont.
The Urban equations drainage area coefficients for 2-percent exceedance flow (Feaster and others, 2014) were used for Piedmont (HLR2) urban basins. Urban basins were defined as catchments which were greater than 10% developed (urban).
For piedmont (HLR2, small urban le 3 dasqmi and pcturb gt 10%) SAI = slope*da^0.8
For piedmont (HLR2, small urban dasqmi gt 3 and le 436 and pcturb ge 10%) SAI = slope * da^.5