ENGR10006 Chap.11 Flood Modelling and Temporary Flood Protection
Flood Modelling and Temporary Flood Protection
Workshops 4 to 6 model three kinds of flood in QGIS and then test temporary protection. This chapter explains the rainfall script built with the raster calculator, the r.lake tool that fills outward from a seed for burst-pipe and coastal floods, and why flood map cells hold water depth.
It shows how sandbag walls are modelled as a separate layer of wall heights added to the DEM, where walls work best, and how protection is tested by raising the water level. It works through flood volume from pixel depths, pyramid sandbag counts and labour planning from Workshop 7, and the flood tank practical with its 1:500 horizontal and 1:250 vertical scales and its pseudo-surface model.
The worked example sizes a pyramid wall and its crew, and practice items cover inundation area and scaled surge heights for the tank model.
What this chapter covers
- 01
Rainfall, burst-pipe and coastal scenarios
- 02
Raster calculator flood script
- 03
Seeds and water levels in r.lake
- 04
Flood depth and inundation area
- 05
Sandbag layer added to the DEM
- 06
Flood volume from pixel depths
- 07
Pyramid sandbag counts and crews
- 08
Flood tank scales and pseudo-DSM
Worked example · free
Sizing a sandbag wall on a ridge against a surge level
- +1The crest must reach at least 1.8 + 0.2 = 2.0 m, so the wall must add 2.0 - 1.6 = 0.4 m.
- +1At about 0.1 m per layer, that needs 0.4 / 0.1 = 4 layers.
- +1A four-layer pyramid has 4 + 3 + 2 + 1 = 10 courses, and each course along 6 m needs 6 / 0.6 = 10 bags.
- +1Total = 10 × 10 = 100 bags, giving a crest of 2.0 m that keeps the 0.9 m hollow dry for a 1.8 m surge.
Key terms
- r.lake
- A GRASS tool in QGIS that fills a terrain model with water up to a level from seed cells.
- Seed
- The starting cells for an r.lake flood, such as a rasterised manhole or coastline.
- Inundation area
- The area covered by water in a flood map, found by vector or raster methods.
- Sandbagged DEM
- The terrain model with the sandbag wall heights added cell by cell.
- Pseudo-DSM
- The tank practical's terrain model with buildings raised to 4 m.
- Vertical exaggeration
- A model with a larger vertical scale than horizontal scale, as in the flood tank.
- Raster calculator
- The QGIS tool that computes a new raster cell by cell from an expression, used for the rainfall script and the sandbagged DEM.
Flood Modelling and Temporary Flood Protection FAQ
Why use r.lake rather than the rainfall expression for a burst pipe?
The rainfall expression marks every pixel below a level as flooded, whether or not water could reach it. r.lake fills outward from seed cells, such as the rasterised manhole, so only connected low ground floods, which is how a local burst pipe or a coastal surge behaves.
How are sandbags represented in QGIS?
Workshop 6 creates a separate layer that is zero everywhere except at wall cells, which hold the wall height in metres. Adding it to the DEM with the raster calculator gives a sandbagged DEM, and r.lake is run on that. The original DEM stays untouched for comparing placements.
How do I find the volume of a flood?
Because each flood map cell holds a water depth, the volume is the sum of the cell depths multiplied by the area of one cell. With 5 m pixels each cell covers 25 square metres, so depths summing to 9 m represent 225 cubic metres.
What does the flooding project not model?
The project overview lists waves, flow and turbulence, destruction of obstacles, accumulation of material, and drains, sewers and tunnels as outside the model. These are natural limitations to state in the report discussion when comparing modelled and real floods.
Assessment move
Run each flood script yourself on the Port Melbourne data and save the outputs with meaningful names, then write one sentence on what each scenario assumes about where water starts. Practise reading a flood map as depths: pick a few cells, add their depths and convert to volume.
Experiment with sandbag placements at narrowing points, raising the water level in 0.1 m steps until each wall is overtopped, and record the level each placement holds. Do the Workshop 7 arithmetic for your own walls, stating the laying assumption, and repeat the crew calculation for the 200-person and 40-person scenarios so you can explain which walls to keep.
Before the tank practical, compute the scaled water heights you expect from the 1:250 vertical scale. Keep every map and calculation, because the Workshop 8 assessment and the final report both rely on them. Before the tank practical, also note which walls you would sacrifice first if the crew were halved, since that reasoning belongs in the report discussion.
Name each output file after its scenario and water level so maps can be matched to results later.
