Watershed Scale Physically Based Water Flow, Sediment and Nutrient Dynamic Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 143

S fy ¼ S 0y @ h
@ y

ð 8 : 2b Þ

where S fx , S fy = friction slope (energy grade line) in the x- or y-direction, S 0x ,
S 0y = ground surface slope in the x- or y-direction.

8.2.1.2 Channel Flow Routing

Channel flow routing in GSSHA employs the 1D diffusive wave equation. The 1D
(laterally and vertically integrated) continuity equation for gradually-varied flow
along a channel is (Julien et al. 1995 ):

@ A

@ t þ @ Q @ x ¼ q l

ð 8 : 3 Þ

where A = cross sectional area of channel flow [L 2 ], Q = total discharge [L 3 /T],
and q l = lateral flow into or out of the channel [L 2 /T].

8.2.2 Sediment Simulation

Sediment erosion and transport are potentially very important processes in water
quality modeling. Excess sediment affects water quality directly by itself. Sedi-
ment transport also influences chemical transport and fate. Suspended sediments
act as a carrier of chemicals in the watershed flow. Many chemicals sorb strongly
to sediment and thus undergo settling, scour, and sedimentation. Sorption also
affects a chemical's transfer and transformation rates. The amount of chemicals
transported by the sediments depends on the suspended sediment concentration
and the sorption coefficient. Both sediment transport rates and concentrations must
be estimated in most toxic modeling studies. The sediment algorithm is included as
a sub-model in GSSHA and invoked only when sediment simulation is required.
The sediment sub-model is designed for estimating sediment delivery and channel
transport in watersheds. It consists of four primary components: (1) sediment
transport; (2)

erosion; (3) deposition; and (4) bed processes (bed elevation

dynamics).

8.2.2.1 Sediment Transport

The sediment transport model is based on the suspended sediment mass conser-
vation equation (advection-diffusion equation with the sink-source term describing
sedimentation resuspension rate) and the equation of bottom deformation. For the
overland plane in 2D, the concentration of particles in a flow is governed by
conservation of mass (sediment continuity) (Julien 1998 ):