Come Rain or Shine.

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July 27, 2018 | Sarah Netherclift

One of the (many) joys of working at Innovyze is its location. Since joining the company a couple of months ago, I’ve enjoyed a walk along the river most mornings before work. It’s not without its hazards. Bear in mind that this is Oxfordshire and therefore Inspector Morse country. I half expect to find a dead body floating among the reeds. Hasn’t happened yet. In fact, this morning I saw some live bodies as two ladies enjoyed a swim.

That’s pretty unusual. Ducks, coots, Canada Geese certainly, even the occasional spaniel, but relatively few people swim in the Thames. Anyone who’s been watching the BBC documentary, The Five Billion Pound Super Sewer, will understand why.

It got me thinking – what’s going to happen when it does finally rain? Thankfully, at Innovyze there’s no shortage of clever people on hand to tell me.

Here’s what they said.

Spoiler – it’s not pleasant.

A high rate of blockages is expected with the first rainfall event after a such a long dry spell. This is because deposited material that has settled, starts to move and cause problems. If this is near a Combined Sewer Overflow (CSO) the continuation pipe could get blocked, resulting in spills. To be fair, studies by some water companies haven’t found this to be as prevalent as you might expect. However, no CSO spill is a good thing.

If CSOs do spill, because of either a downstream blockage or simple lack of capacity because of intense rainfall, the spill is likely to be highly polluted. Particularly where there are no storm tanks to capture the stormwater.

It’s not for nothing that a spill after a dry spell is often called the ‘first foul flush’.

In rural landscapes the ground at present is very dry and hard, so flash runoff is highly likely. That’s especially the case as the grass is non-existent, so there’s less resistance to slow down the flow. This will have an effect on the water quality in rivers, and may affect water quality in reservoirs.

That’s just in rural areas. Urban areas aren’t let off easily either.

In towns and cities, and the roads that connect them, surface water runoff could be high in heavy metals from roads. Over the past weeks, we’ve had an extended period of pollutants like hydrocarbons and heavy metals building up on impervious surfaces. So, the diffuse pollution coming with the ‘first flush’ will have higher concentrations. That means highways and car parking areas, particularly at motorway service areas could generate the most diffuse pollution.

There’s always a case for sustainable drainage and live monitoring of drains – especially CSOs. It’s something our software handles expertly for customers around the world.

Although we typically connect drainage design, integrated catchment modelling, live monitoring and data analytics, with rainfall – dry spells as much as extreme rainfall can call for intelligent drainage solutions.

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Tags: combined sewer overflow, runoff, sustainable drainage

About the Authors

Sarah Netherclift

Sarah Netherclift

Marketing Content Manager, EMEAI


Sarah is a writer and editor for Innovyze, based in the UK.