Figure 5.6 Conceptual Sketch of CASC2D Overland Flow

Routing (from Ogden, 1998)

The grid elevations shown in Figure 5.6 represent water surface elevations, rather than land-surface

elevations because of the diffusive wave formulation.

Overland soil erosion is calculated in each CASC2D model grid using the Kilinc (1972) approach

as implemented by Johnson (1997). This method calculates the sediment discharge qs in tons per second

per meter width of overland flow plane using:

q s ' 25500 qo2.035 So .664

1

(5.2)

where: qo = overland flow discharge per unit width (m2/s) in x and y directions, respectively; and

So = land-surface slope in x and y directions, respectively.

The Kilinc (1972) method is empirical and based on plot-scale data collection using a bare, sandy-soil.

In CASC2D, three different size-classes of material are considered; sand, silt, and clay. Sediment transport

is calculated using Eq. (5.2), and the net erosion/deposition of each size class is assumed proportional to

the percentage of soil in each size class. The amount of sediment transport predicted using Eq. (5.2) is

multiplied by an erosivity factor, and a land-use management factor to consider both of these effects. Both

of these factors have values between 0 and 1.

Once overland sediment reaches the CASC2D channel network, silt and clay size fractions are

routed as suspended or "wash" load. The sand size fraction is assumed to be deposited on the bed of the

channel and routed as bed-load using Yang's (1973) method. Channel bed elevations are allowed to erode

and aggrade in accordance with the rates of sediment influx and outflow.

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