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Flooding: Grills/Grates over stream culvert on new build estate

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bakedbeans
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Flooding: Grills/Grates over stream culvert on new build estate

Post by bakedbeans »

Hi All

I thought I'd post here to get your thoughts about a 'potential' flooding issue on our new build estate, and how the management company has installed a grate which twice now has nearly caused a flood.

Basically a stream flows through the centre, and a road bridge goes over the stream. in the summer (heatwave) the stream is dry, and the recent storms turn it into quite a torrent of water. The tunnel for the stream is approx 1.3mtr in diameter. If that tunnel was blocked, the water would rise until it goes over the road bridge, potentially flooding two houses (maybe more) at the same time.

In the planning permission there is a very detailed flooding report which shows that the 1/1000 year risk is minimal and wouldn't flood. During the recent storm, the water (although a lot of it) only rises half way up the tunnel.

However, during a 'risk assesment' it was decided a grate needs to be installed over the tunnel entrance and exit to 'stop unaurothised access'. To be fair, in the summer the kids did play in the stream/tunnel. Now a grate has been added, the debis blocks up against the grate causing a flood.

Before christmas a flood was averted by the Environment Agency, and the grates were tied open. This weekend the debis which pilled up against the open grate (it can't fully open due to the square grate over a circular tunnel) pulled the top of the bridge off (which it was tied to) and shut, causing a flood. The water level was probably >3 meters and within a foot of bursting the banks.

My issue, and question to you guys, the grates imo have created a greater risk with deeper water and being pinned against the grate. Even with the grates tied open, the chances of being caught up if swept through the tunnel is greater due to the not-fully-open grates.

The solutions proposed is
1) tie the grates open during winter - personally this is the worst of all options as the risk is with deep water
2) remove the grate and add a fence with signs of deep water keep out - the risk accessor has said a grate needs to be installed.
3) change the grate for a different design

However, I struggle to see how any grate design would not cause flooding!

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WillPool
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Re: Flooding: Grills/Grates over stream culvert on new build estate

Post by WillPool »

For me personally:

1. By adding the grate you have added new risks/hazards
2. By changing the grate design/style, I dont see much significant change unless it operates like a port cullis on a castle where it can be removed during high flow periods.
3. By removing the grate you are not interfering with nature.
4. By adding the fence and signage as you say, the river can do its own thing and hopefully people will not climb the fence into the stream during high flows.

There maybe other options but the fence to me seems the most practical approach.

Will .salut
It is better to be careful 100 times than to get killed once.
Mark Twain (1835 - 1910)

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WillPool
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Re: Flooding: Grills/Grates over stream culvert on new build estate

Post by WillPool »

p.s. Welcome to the forums :wave:

hsfbaddict

Will .salut
It is better to be careful 100 times than to get killed once.
Mark Twain (1835 - 1910)

bakedbeans
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Re: Flooding: Grills/Grates over stream culvert on new build estate

Post by bakedbeans »

WillPool wrote:
Tue Feb 11, 2020 1:06 pm

There maybe other options but the fence to me seems the most practical approach.
Hi! :wave:

I agree, but trying to get the managment company to agree is impossible. At least I have now forced them to get approval from the EA over the grate, and they can decide what is suitable.

I just find it strange as I could find lots of streams where I could get through the tunnel with no grate.

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