Transcript Two-Dimensional Hydrodynamic Flood Modeling by Matt Wilson

Use of satellite altimeter data for
validating large scale hydraulic
models
Matt Wilson, University of Exeter
Doug Alsdorf, Ohio State University
Paul Bates, University of Bristol
Matt Horritt, University of Bristol
Hydraulic modelling of floodplains
• Flow on floodplains is
controlled by topography and
friction
• Leads to complex spatial
patterns of water depth and
velocity that are 2D in space
and dynamic in time
• Until recently modelling of
such flows has only been
possible for small river
reaches (10-50km)
New opportunities
• Large scale modelling has now been made
possible by:
– Simplified 2D hydraulic models
– Faster computers
– New satellite data sources e.g. SRTM, satellite
radars
LISFLOOD-FP
• Hybrid 1D/2D model
– Based on raster DEM
– 1D Kinematic or diffusion wave routing in channel
– Once bankfull depth is exceeded calculates a flux to floodplain
cells using Manning’s equation or 2D diffusive wave to route water
over complex floodplain topography
Model discretization of floodplain
and channel topography
In-channel flow routed using a 1D
wave equation
Once bankful depth is exceeded
water can flow laterally over
adjacent low lying floodplains
according to topography and free
surface gradient
LISFLOOD-FP
• Inundation is based on a simple continuity
equation:
dV
 Qup  Qdown  Qleft  Qright
dt
where:
V = cell volume
t = time
Qup, Qdown, Qleft and Qright = flow rates in each
direction into (positive Q) and out of (negative Q)
the cell
LISFLOOD-FP
i
j
• Flux between cells
calculated using the
Manning equation
Qi , j 
where:
Qi,j =
Ai,j =
Ri,j =
Si,j =
n =
Ai , j Ri2,j/ 3Si1,/j 2
n
flux between two cells i and j,
cross sectional area at the cell
interface,
hydraulic radius at cell interface,
water surface slope between cells,
Manning friction coefficient.
Simplified 2D models
• Advantages
– Floodplain flow is solved analytically rather than
numerically so very efficient
• 150-500k cells for full dynamic events should run in less than 1
day on a pc
– Can use large elements (e.g. 250m – 1000m grids)
– Intrinsically mass conservative treatment of floodplain
flow
• Disadvantages
– Simplified floodplain flow representation
– Wetting front propagation may be grid and time step
dependent
LISFLOOD-FP
• Successfully applied to reaches 10-50km in
UK and continental Europe
• We are now applying LISFLOOD-FP to
100-500km reaches
Solimões
inflow
Purus inflow
100 km
Amazon model implementation
• 200x280km model domain, Purus and
Solimões as 1D channels
– 90m SRTM DEM averaged to 270m,
processing for vegetation
– Around ~800,000 cells
– Simplified channel information
• 4 year simulation, each annual hydrograph
is the average of 20 years of gauged data
Comparison with JERS-1 imagery
Topex/POSEIDON altimetry in Amazon region
Altimetry data for study site
Model vs. altimetry
Model vs. altimetry
Model vs. altimetry
Floodplain flow, mid-rising
Some conclusions
• Comparison of inundation extent using
JERS image is useful but uncertain.
• Altimetry data are a welcome addition, but
miss the spatial complexity floodplain flows.
• To capture these complexities, a twodimensional approach is required.