Importance of Designing for Exceedance in Urban Drainage

Water is a temperamental element – a fact we here at Inspect-a-Drain know very well. It can cause problems when it is both limited and also when it is in abundance, as such planning for such occasions is often needed in urban areas. After all, recent years has proven that water exceedance can have a devastating impact on urban areas. As such, the need to plan for such occurrences has become even more evident.

So, what is exceedance and why is it important to keep in mind when considering urban drainage?

What is Water Exceedance

Water exceedance is a larger rainfall than predicted in any giving period. There is usually an annual exceedance probability, but sometimes this is surpassed further than it is possible to be predicted because of storms or torrential weather. Sometimes the exceedance is negligible, other times it is damaging on a massive scale to the local environment and even urban areas.

cars in heavy rain
Without proper planning, people may lose their homes and businesses can be closed because of flooding. Proper exceedance planning is, therefore, essential in order to prevent these issues from occurring.

Designing Urban Drainage

But, how do you design an urban area to deal with flooding? It can be difficult, especially in areas located closer to rivers and other high-risk areas. Understanding the issues is important, i.e. the areas that can experience water exceedance and not be affected in the long term compared to areas that need to be protected as a matter of urgency.

Surface flooding of large flat areas such as parks and carparks are expected and over a short period should not be damaging. However, drainage should be provided to route water away from anything vulnerable and emergency exits to prevent costly damage or health and safety violations. Of course, excess flooding may block the drainage used for this purpose and as such long-term flooding may occur. In which case, plans need to be made in order to block these areas off.

However, as car parks and recreational parks are not habitable the risk of flooding because of exceedance is not high. Precautions should be made, but not at the expense of residential areas.

Big Ben and Westminster Bridge, seen with a crystal ball
So, in residential areas, it is important to ensure that water flows away and that it is also not allowed to pool. The slowing down of the flow can have serious repercussions for residential areas. Lowering pavement height only 10cm can prevent homes from being flooded in times of water exceedance, a small and cost-effective change.

Also, raised residential areas should be designed with a flood flow in mind. Determining where water will flow in the case of exceedance, considering the topography of the area, can help create a non-damaging path for the water to take. Pavements should also be linked together in some way, creating a link that the water can cross and not accidentally creating a flooding water torrent instead.

Overall, pre-planning is essential in order to prevent urban areas from experiencing devastating damage as a result of water exceedance. The cost saved should far exceed the initial starting cost of such a process.

For expert discussion about your drainage, and how to ensure your water flow stays healthy, get in contact with us.

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