Different types of flooding

This course has mainly focused on rainfall in cities. Rainfall is however only one source of flooding that may affect cities. This article will briefly describe the different types of flooding that cities may be exposed to.

Rainfall-induced flooding

When it rains in a city and the drainage system is not capable of removing this rain some water will drain along the surface or be left standing on the surface. Because rainfall can vary strongly across an area, not all parts of a city may see flooding at the same time. In some cases, the local topography can create depressions that have no natural drainage and these can flood frequently if no additional measures are taken. In cities, it is also important to consider how buildings change the paths water can take on the surface. Buildings can act as a barrier creating an area from where water cannot drain along the surface. If there is no sufficient pipe network to remove this water, there is an increased chance of flooding in these areas that is almost guaranteed to affect the buildings that caused the flooding.

This rainfall-induced flooding is also known as pluvial flooding. Pluvial flooding can occur in almost any city, as even very dry environments will see rain occasionally. Weather forecasts can help us predict if pluvial flooding is possible in the near future, while 2D hydraulic models can help us assess the vulnerability of different parts of the city to this type of flooding. If this type of flooding does happen it usually lasts for a relatively short time, e.g. some hours, although some areas may take longer to become dry again.

River-induced flooding

Many cities are built alongside rivers. If the flow in the river is particularly high the river may flood its banks and cause flooding. The extent of this flooding depends on the local geography. Cities can be particularly at risk of flooding, since many cities have been built on the natural flood plains of the river. If the river does not have anywhere else to go, it is inevitable that the city will flood occasionally.

This type of flooding is called fluvial flooding. The time-scale of flooding caused by rivers and streams depends on the river itself: for large rivers the rain that falls far upstream can take days or weeks to reach a city, so there can be advanced warning of the flooding risk. This also means that such flooding can last much longer than pluvial flooding, as it can take days for the peak in the river flow to pass the city. Assessing vulnerability to fluvial flooding requires information about the local geography (including both the city and the river course) as well as information about what flow rates can be expected from the river. This can be based on historical data (similar to IDF curves for rainfall) as well as weather forecasts and simple models of the upstream area.

Coastal flooding

For coastal cities, flooding can also be caused by storm surges. High water levels can be caused by a combination of tides and high winds. If local flood defenses are not adequate this can lead to flooding of coastal areas and cities. Storm surges can also contribute to erosion which can pose risks to people and buildings over time and worsen the effect of future storms.

Climate change is also expected to lead to rising sea levels in the future. If weather and tides cause high water levels they can last for several hours, while rising sea levels caused by climate changes are not likely to drop in the near future. Assessing coastal flooding is done using models that consider the probability of tides and high winds combining to create storm surges, as well as the local geography. Climate models and scenarios can provide forecasts of changes in sea level that also have to be included in these studies.