19: Flood Analysis

Question 1

Hydrology and spatial analysis

Answer 1

GIS, remote sensing and numerical modelling allow:

• detailed spatial analysis of flood risk and surface water
• spatially-distributed measurements and predictions of hydrodynamics
• assessment of potential flood impact and vulnerability, incorporating socioeconomic spatial data

Question 2

Flood modelling inputs and outputs

Answer 2

Inputs: topography (DEM), hydropgraph, land cover, river location, flood distribution, settlement distribution, etc.

Outputs: flood extends/depths, assessment accuracy, hazard/risk map

Question 3

Three levels of integration of dynamic models and GIS

Answer 3

more efficient computation outside GIS using code software (Python)

• loose coupling (software is completely separate - outputs can be read by the GIS)
• tight coupling
• embedded coupling (run code within GIS)

Question 4

Reduced complexity modelling

Answer 4

Advances in computing capabilities and reduced complexity hydraulic models allows simulation of:

• larger more complex river reaches with more spatial detail
• longer flood events
• multiple scenarios

Question 5

LISFLOOD-FP Hydraulic model

Answer 5

• raster based model
• requires DEM plus friction (from land cover)
• 2D diffusive wave on the floodplain
• outputs: dynamic map of inundation extent and depth

Question 6

how does the flood modelling work?

Answer 6

• add water to the channel from rainfall
• incrementally predict water level throughout time (time step)
• use calculation of water flow in and out of each cell knowing elevation of the adjacent cells and in particular the slope, which corresponds to flow rate
• over time produce many images of how the water level changes and spills out onto the flood plain because slope affects where water flows
• these images can be used to create an animation

Question 7

Remote sensing data

Answer 7

Topography from LiDAR w/o surface features, convert to raster DEM
Friction coefficients from land cover classification of Landsat imagery

Question 8

Remote sensing for model validation

Answer 8

Mapping inundation extent with SAR (synthetic aperture radar) imagery
Water elevations from satellite altimetry

Question 9

Accuracy assessment

Answer 9

Look at the prediction and observation and calculate the percentage accuracy

correctly predicted areas divided by the correct + overprediction + underprediction

Question 10

How do we get the observation for accuracy assessment?

Answer 10

use SAR imagery (synthetic aperture radar) which uses microwaves instead of visible light

• works in all weather, incl. clouds
• flood water appears dark because water surface is smooth and reflects beam away from sensor

Question 11

Why test accuracy?

Answer 11

Check the accuracy of the model against an existing event so we can use it to make other predictions based including potential changes to the environment or for simulating different size flood events to understand probability and flood risk assessment

Question 12

mapping flood risk damage

Answer 12

use statistical relationship between water depth and damage to bring together flood likelihood and assessment of economic cost

Question 13

water source tracing

Answer 13

• can also look at where water comes from
• hydro-ecological studies (sources of nutrients)
• tracing of point source pollution in floodwater