Download LISFLOOD-FP User Manual

LISFLOOD-FP User Manual
Code release 5.9.6
thus far only been tested on a limited number of scenarios and may not be as robust as the other
more commonly used solvers.
1.3 Channel flow solvers
The most simple of the channel flow models is a 1D kinematic wave approximation of the shallow
water equations, which assumes all terms except the friction and bed gradient are negligible
(“kinematic” solver). The bed gradient is a simplification of the water slope term which takes into
account the effect of changes in bed height with distance, but not changes in the water free
surface height. In contrast, the “diffusive” solver uses the 1D diffusive wave equation which
includes the water slope term and thus is able to predict backwater effects. Using the 1D channel
solvers, once channel water depth reaches bankfull height, water is routed onto adjacent
floodplain cells to be distributed as per the chosen floodplain solver. Note: there is no transfer of
momentum between the channel and floodplain, only mass.
The most recently developed method for representing rivers is as sub-grid channels, embedded
with the 2D domain. Flow between channel segments is calculated based on the friction and
water slopes, and local water acceleration (i.e. using the ‘acceleration’ model equations). Only
convective acceleration is assumed negligible. For any cell containing a sub-grid channel
segment, the solver calculates the combined flow of water within the cell, contained both within
the channel located in that cell and across the adjacent floodplain. The model is designed to
operate over large data sparse areas where limited channel section data are available.
1.4 Model assumptions and key limitations
The code is limited to situations where there is sufficient information to accurately
characterise the model boundary conditions, specifically mass flux with time at all inflow
points. In addition, for fluvial flows at least some basic information on channel geometry
must also be available.
The model uses standard SI units for length (metres), time (seconds), flux (volume per time in
m3s-1) etc.
The solvers assume flow to be gradually varied (the routing solver is the exception for this
and can be used for cases of very shallow flow over steeps gradients or discontinuities, the
Roe solver may also handle flows that vary rapidly in time).
1.4.1 Channel flow solvers
The 1D kinematic and diffusive solvers assume that the in-channel flow component can be
represented using a kinematic or diffusive 1D wave equation with the channel geometry
simplified to a rectangle (1D kinematic and diffusive solvers only).
The 1D kinematic and diffusive solvers assume the channel to be wide and shallow, so the
wetted perimeter is approximated by the channel width such that lateral friction is neglected.
1.4.2 Floodplain flow solvers
For out-of-bank flow we assume that flow can be treated using a series of storage cells
discretised as a raster grid with flow in Cartesian coordinate directions only.
There is no exchange of momentum between 1D channel solvers and floodplain flows, only
mass.
During floodplain flow lateral friction is assumed negligible and is neglected.
The flow limited solver underestimates wave propagation speeds and can be a poor
representation of flow dynamics, and is left as an option for comparative experimentation
only.
Due to high computation cost the adaptive solver is rarely suitable for high resolution
simulations.
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