Use of satellite altimeter data for Use of satellite altimeter data for validating large scale hydraulic validating large scale hydraulic

models models

Matt WilsonMatt Wilson, University of Exeter, University of ExeterDoug AlsdorfDoug Alsdorf, Ohio State University, Ohio State UniversityPaul BatesPaul Bates, University of Bristol, University of BristolMatt HorrittMatt Horritt, University of Bristol, University of Bristol

Hydraulic modelling of floodplainsHydraulic 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 opportunitiesNew 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-FPLISFLOOD-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

rightleftdownup QQQQdt

dV

where:V = cell volumet = timeQup, Qdown, Qleft and Qright = flow rates in each direction into (positive Q) and out of (negative Q) the cell

LISFLOOD-FPLISFLOOD-FP

• Inundation is based on a simple continuity equation:

where: Qi,j = flux between two cells i and j, Ai,j = cross sectional area at the cell

interface, Ri,j = hydraulic radius at cell interface, Si,j = water surface slope between cells, n = Manning friction coefficient.

n

SRAQ jijiji

ji

2/1,

3/2,,

,

LISFLOOD-FPLISFLOOD-FP

• Flux between cells calculated using the Manning equation

i j

Simplified 2D modelsSimplified 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-FPLISFLOOD-FP

• Successfully applied to reaches 10-50km in UK and continental Europe

• We are now applying LISFLOOD-FP to 100-500km reaches

SolimSolimõõesesinflowinflow

Purus inflowPurus inflow 100 km

Amazon model implementationAmazon 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 imageryComparison with JERS-1 imagery

Topex/POSEIDON altimetry in Amazon regionTopex/POSEIDON altimetry in Amazon region

Altimetry data for study siteAltimetry data for study site

Model vs. altimetryModel vs. altimetry

Model vs. altimetryModel vs. altimetry

Model vs. altimetryModel vs. altimetry

Floodplain flow, mid-rising

Some conclusionsSome 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 two-dimensional approach is required.