Runoff from different urban catchments

In the previous step we looked at the concept of hydrographs, that is graphs showing how the runoff from a catchment (or the flow rate ata specific point) varies over time. In this step, we will have a look at how different levels off urbanization affect the runoff from a catchment.

In this case, we assume that the only difference between the catchments we are looking at is the land use, i.e., how heavily urbanized the catchment is. There are also other factors which affect runoff, and we will describe those in the next step.

Pre-development

In a natural catchment that consists mainly of vegetated areas, most of the precipitation that falls will be able to infiltrate into the soil and be used by the plants. Especially for small rain events there will often be no to very little surface runoff. Typically, 0 – 10% of the rainfall volume will become runoff. In addition, the surface runoff that does take place will travel quite a long time through the catchment before it reaches the outlet, so the runoff peak will be much lower and later than the rainfall.

We will now have a look at catchments with different levels or urbanization and how they typically affect runoff. Note that the aerial images of each type of catchment are at approximately the same scale and show a catchment area of around 8 ha.

Suburban housing area

© Lantmäteriet

In this suburban housing area with mainly detached housing we can see that there are still quite a lot of green areas between the buildings. In addition, it is common that building roofs in such catchments also drain onto green areas, rather than directly into the sewer. This further reduces the percentage of effective impervious area, that is the area that will generate high runoff with only a small delay. In areas like these, somewhere between 30-50% of the precipitation volume will become runoff.

Industrial catchment

© Lantmäteriet

In this light industrial catchment, there are more impervious areas than in the suburban housing catchment. However, there are usually also some green areas left. Some of the areas used by the industries may also be (partly impermeable) gravel surfaces and these are not usually connected directly to the stormwater sewer.

For catchments like these, it is typical that around 50-70% of the rainfall volume will become runoff. Because there are more impervious areas overall and more areas connected directly via storm drains to the sewer network, the delay imparted on the runoff is also smaller. This means that we get both higher peak flow rates from this catchment and a larger runoff volume.

City centre

© Lantmäteriet

In a typical city centre, most of the green areas have been converted to impervious surfaces. This leads to a quite high percentage of rainfall becoming runoff, around 70-90%. Roads and roofs will almost always be connected directly to the sewer network, so runoff flows through the area relatively quickly. This leads to high flow rates and a sharp, early peak in the runoff:

Airport

© Lantmäteriet

As an extreme example, an airport consists almost exclusively of impervious areas. These will also usually be connected to a sewer network as the airport is keen to avoid water on the surface. As a result, as much as 80-90% of the precipitation volume can become runoff. The hydrograph for such an area will feature a sharp and high peak:

Comparing all catchments

We have now seen a number of different catchments and how a typical hydrograph for each catchment may look. If we compare the hydrographs for all catchments we can clearly see that, as the percentage of impervious areas increases, both the runoff peak becomes higher, and the total runoff volume. Further, the lag time between the rain and the peak flow will be shorter, i.e. the rise of the hydrograph is steeper. Thus, we can see that the time when the peak flow occurs is different for the various catchments. The larger the role of impervious areas and sewer pipes, the earlier the peak will be. For catchments with more green areas the peak will be later and can even occur after the rainfall has already ceased.

Remember that the total area under a hydrograph shows the total runoff volume.

If we calculate the runoff volumes and find the peak flow rates for all catchments (as well as the rainfall volume and the peak rainfall rate), there are clear differences between the different types of catchments. Note that, as with the graphs above, these are just typical examples and in reality there are many other factors that will affect these values.

Conclusion

In this step we have seen that the changes we make to a catchment when we are building cities can have a large effect on the runoff from these catchments. Higher flow rates and runoff volumes lead to flooding occurring more and more often. One way to reduce the risk of flooding is to try to use more green areas and fewer stormwater sewers in urban areas. Next week, we will have a look at some solutions that can be used for this.