River Runoff in Flood Risk Models

Local Community Engagement, 1, 2, 2a, 3, 4, 5, 6, 7, 8, 9, 10 & 11

DEFRA modelled flood risk from river and sea. Having just mapped coastal inundation, can we not derive flood risk models from river runoff alone? Let’s do this for both sea level rise (SLR) models on Ordnance Survey DEM and Coastal DEM1.1: 1, 3 and 5 m. SLR being the high, medium and low probability range, let’s match these up with the same for DEFRA’s flood risk models previously posted. Note that this is a qualitative measure, as we’re mixing probability data types. It nonetheless helps sort the risk of landward runoff from that of coastal inundation. From the legend in the swipe maps below:

To examine the contribution of river and sea vs. and coastal SLR, lets subtract the three respective levels: (river + sea) – sea = river contribution. This is a qualitative assessment, as we’re mixing data types:

+1: river + sea contribution from DEFRA exceeds sea level rise contribution from OS Open DEM
0: river + sea contribution from DEFRA equals sea level rise contribution from OS Open DEM
-1: sea level rise contribution from OS Open DEM exceeds river + sea contribution from DEFRA

In other words positive values in greys model the river flooding contribution.

click to enlarge or go to map

click to enlarge or go to map

Click on each, recenter the map if needed, and swipe the slider back&forth to see where the derived high river flood risk is, compared to DEFRA’s original model from sea and river. One good place to contrast the effects are the rivers themselves:

inland winding rivers, the mid-to-dark greys show the influence is terrestrial runoff, as sea level rise will never reach there
in large straight ‘levels’, the mid grey-to-white shows the influence is coastal inundation, as sea level rise will inundate them first

Again, the point here is to show how relatively simple GIS operations – raster classification and calculation here – help you the citizen scientists further study government-provided open data.

This information was first published on http://blog.zolnai.ca/2019/12/river-runoff-in-flood-risk-models.html