Other factors that affect runoff

In the previous step we had a look at how urbanization and the increase in impervious areas and sewers affects the runoff from catchments. There are also several other factors that affect how much runoff will be generated.

These are related to:

• the catchment’s natural characteristics
• the ambient conditions
• the characteristics of the rainfall event.

In this article we will go over the most important factors for urban catchments.

Catchment characteristics

Depression storage

Real-life urban catchments do not drain perfectly. There are (almost) always some places where water will gather on the surface without having anywhere to go. You can see this clearly when there are puddles on the street after it rains. How much water is stored in these small depressions depends on the local topography, how flat the area is, and what type of surface we are dealing with. Grass areas will normally have more detention storage than paved areas, while inclined roofs may have no depression storage at all. The wetting of the surface also provides a small amount of storage volume for water. Some rain will fall on trees and plants which can store some of the water. This is called interception storage.

Slope

Water will run off from steep areas more quickly than from flatter areas. This also means that water has less time to infiltrate into green areas. Cities follow the existing topography, so there is usually not much we can do to affect the slope. When studying urban catchments, we therefore need to keep in mind that steeper areas will generate higher peak flows and larger runoff volumes.

Steeper areas will also lead to higher flow velocities. This can cause erosion in green areas or e.g., road embankments. When designing drainage in steeper areas we need to consider if velocity-reducing measures such as check dams are needed.

Soil characteristics

The soil characteristics affect how much the rainfall will infiltrate in the green areas. For instance, relatively coarse sandy soils will infiltrate a lot of water given their high infiltration capacity while clay soils will generate runoff also from non-paved surfaces. Rocky areas will generate runoff even under natural conditions.

Flow path length

Not all catchments are the same shape. Consider the two examples in the image below. In the catchment on the right, water has a relatively short path to follow before it reaches the outlet of the catchment. In the image on the left, water needs to travel further to reach the outlet. This also means the water is slowed down more by the contact with the surfaces it runs over. Overall, this means that the hydrograph for the catchment on the right will have a lower and later peak than the catchment on the left. The flow may also spend more time on green areas if the flow path is longer, which can increase infiltration and reduce the total runoff volume.

Ambient conditions

Conditions prior to rainfall

Above we discussed how catchment characteristics can reduce the runoff volume. However, if it e.g. has rained only a few hours ago the depressions may already be filled with water, and they will not have an effect on the runoff from the later rain.

Infiltration into green areas will also be affected by the prior conditions. If it has rained a lot recently then green areas will already be saturated and will not be able to infiltrate as much water as if it has been dry. How quickly the infiltration capacity is recovered depends on the type of soil, the weather conditions, and how much it rained previously. Sandy soils will drain more quickly than clay soils and recover their infiltration capacity more quickly.

Season / temperature

In warm weather, evapotranspiration will be higher. This helps remove moisture from the soil and recover the infiltration capacity. Further, from small rains a significant amount of the water may evaporate from dark surfaces such as roofs and streets.

In contrast, in winter the soil can be frozen. This has an impact on infiltration: frozen soils can often not infiltrate much water at all (since pore water is frozen as well).

Also the vegetation is affected by seasonal changes. Leave trees can provide surface storage on their leaves (interception) only during summer.

Rain characteristics

Finally, the rain itself has an impact on the percentage of the total rain volume which will run off on surface. Small rains may for example not use all the available volume for water storage in the catchment or evaporate from dark surfaces and thus not generate any runoff at all. For intense rains, the percentage of surface runoff is thus in general higher.