The channel widening/narrowing is accomplished by coupling a stream tube computer model

with a decision-making algorithm using rate of energy dissipation or total stream power

minimization. The first component, the fixed-width streamtube computer model, simulates the

scouring/deposition process taking place in the vertical direction across the channel. The user

can select from one of seven sediment transport functions including one general function

including velocity depth, discharge, energy slope, and grain size. The second component, the

total stream power minimization algorithm, determines what takes place in the lateral or vertical

direction. It is this component that allows the lateral changes in channel geometries. Finally,

the bridge component allows the computation of the hydraulic flow variables and the resulting

scour due to highway encroachments.

HEC-6. The HEC-6 program (Scour and Deposition in Rivers and Reservoirs) was developed

at the Hydrologic Engineering Center of the U.S. Army Corps of Engineers (HEC 1993). The

quasi-steady backwater equation is used to compute water-flow conditions uncoupled from the

sediment-continuity equation, and is intended for long-term river response simulation.

The sediment-continuity equation is solved using an explicit finite-difference scheme, with

sediment-transport capacities determined from water-flow conditions previously determined in

the uncoupled backwater computation. The entire movable bed portion of the channel is

assumed to aggrade or degrade uniformly. Sediments are routed by individual size fraction,

which makes possible a detailed accounting of hydraulic sorting and development of an

armored layer. The user can select from thirteen sediment transport functions including one

general function including depth and slope. Bank lines are assumed to be stable and fixed in

the HEC-6 computation.

HEC-6 is strictly a one-dimensional model with no provision for simulating the development of

meanders or specifying a lateral distribution of sediment transport rate across the section. The

model is not suitable for rapidly changing flow conditions but can be applied to predict reservoir

sedimentation, degradation of the streambed downstream from a dam, and long-term trends of

scour or deposition in a stream channel, including the effects of dredging. It is anticipated that

future versions of HEC-RAS will incorporate sediment transport and replace HEC-6.

FLUVIAL-12. This uncoupled model was developed at San Diego State University by Chang

(1998) to simulate one dimensional, unsteady, gradually varied, water and sediment flows for

channels. The model also includes the capacity to simulate bank erosion and is described as

an erodible-boundary model as opposed to an erodible bed model by Chang. FLUVIAL-12 first

solves the unsteady, flow-continuity and flow-momentum equations in one time step by

neglecting storage effects due to unsteady flow.

The model uses an implicit,

central-difference, numerical scheme in solving for the two unknown variables of water

discharge and cross-sectional area. The flow information is then used to compute the bed

sediment discharge at each section using one of six user selected sediment transport

functions.

Next, the net change in cross-sectional area is obtained by solving the sediment-continuity

equation using a backward-difference scheme for space and a forward-difference scheme for

time. The computed cross-sectional area change is then adjusted for the effects of channel

migration. Width adjustments are made in such a manner that the spatial variation in power

expenditure per unit channel length is reduced along the reach by a trial and error technique.

Further adjustment of cross-sectional area is made to reduce the spatial variation in power

expenditure along the channel. The effect of lateral channel migration is determined by solving

the sediment-continuity equation in the transverse direction, which incorporates the effect of

radius of curvature of the river bend into the transverse component of the sediment transport

rate.

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