Extreme rainfall exceeding sewers and sewage works capacity

[N_Cook]

Google no help with this, perhaps someone here knows what the situation is called.

With old build housing it is very common for ,some at least, of the house rainfall roof run-off to go into the sewer system, called "combined sewers" in those situations. It sounds absurd with overwhelmed sewage works these days ( getting worse/more common, with global warming presumably) diverting the resultant excess rainfall plus the majority of any sewage (only coarse riddling) into the sea or local rivers, as permitted "emergency overflow". What is the technical term for this domestic roof run-off process and why are the water companies or councils not demanding simple and cheap mitigation/work-arounds? The sewer choke points are fine for normal rain , but extreme rain in the sewers rapidly back-up, upstream of the choke-point , coming out/lifting drain covers and out of house toilet pans etc.

Every time there is rainfall flooding in an area , the outcry is not so much about rainwater flooding of houses because there tends to be a "living memory" of previous floods , but sewage contaminated flooding. Then the local council does an investigation and someone declares its once in 50 years event or whatever, and thats it. eg a 30M pdf entitled "Winchester section 19 investigation", published june 2021 about previous floodings, has half a sentence noting roof-run off into sewers and nothing more.

[Andy Burns]

"Surcharging"

[Tricky Dicky]

Ours is a 1975 built bungalow and all surface water and grey water go down the same sewer. Soakaways are not feasible as we sit on clay literary inches below ground level. I have noticed on some recent new builds around here that builders seem to be putting in large sumps to store water during down pours. One placed one under a roadway into a development others have picked low points on the development. It must annoy some developers as they often have to sacrifice two or three houses to provide the space for the sump. We fortunately live in the Pennines in a steep valley near the top of a hill so barring blockages no real chance of a backup may not be so fortunate for those living below. You can get back flow flaps in waste water pipes and I suspect these may be a requirement by insurance companies in places like Fishlake and the Calder valley which have suffered floods in recent times.

Richard

[N_Cook]

Thanks for that, that term had passed me by. So for my purposes the term might justifiably be "domestic rainfall surcharging of sewers" assuming there is not a tighter specific term for it.

[N_Cook]

Yes clay soil is an allowable derrogation , to use a sewer. But I still fail to understand why, the wider general case, with all the rain falling on roads, pavements, car standing areas going into the road storm drain system , and run-off from clay soil gardens , why the proportionally quite small roof run-off is directed into the sewer system.

Like you I'm , but only a couple of metres,sufficiently higher than the local flood-prone area. My contribution to relieving the local sewer system from extreme rainfall , because a local choke point in the sewer system means some low lying houses get flooded with sewage+rain every 10 years or so.

with a bonus of gaining a rain-gauge of sorts. I'm only aware of one other person , elsewhere in a flood prone area of Hampshire, who has similarly diverted all roof run off into hardy shrubbery in his garden, as there is yearly repeated local sewage contaminated flooding in his area, and nothing done about it by council/EA/water company . My slightly more complex mod is directed to robust shrubbery and concrete path and also doubles as a rain-rate rainfall gauge, measuring the throw of the water , cross-calibrating with the local tipping-bucket professional rain gauge

Tapping my downpipe during heavy rain and overflowing gutter recently, it did not sound hollow and making a small hole in it,water jetting out. So decided to make a mod. To unblock that downpipe would be a workup anyway as continuous 18 foot long run to the gutter and I'd have probably sawn across it and founnd or made a breakable union in the join for next time.

There are problems with sewer back-flow preventers, they quite easily block themselves or jam apparently, with just the regular 3 Ps.

[alan_m]

Can you suggest a cheap work-around?

[N_Cook]

Its not like all this electricity "smart" meter guff, no access required to indoors of any old-stock house. Once the reason is explained to a house owner and permission granted and agreement on where the outflow would be directed. Somone comes along , cuts the downpipe, fits an elbow and a horizontal run of matching pipe. I've no idea how many people would refuse to this diversion, knowing how selfish people are,as it is not protecting them personally in the vast majority, so perhaps a very low percentage of people volunteering foe such works.

[N_Cook]

Any waterbutt filler plus overflow diverter back into the downpipe, can be retained, as long as the sewer bypass elbow is fitted below the waterbutt connection.

[nightjar]

That is going to happen even where the systems are separated. Sewers are not watertight, so surface flooding will get in through manholes and get contaminated with sewage that way.

[alan_m]

So most of the water is diverted directly from the drain to the surface where it ends in the same place in many areas - the road rainwater drains.

I have flower beds in my front garden (either side of concrete slab for a car) and divert any rainwater directly to these beds. I live in an area of the UK that usually has the lowest level of rainfall. In a heavy prolonged rainfall there can be a substantial amount of water flowing from my front garden to the road. Most of the people along my road have have completely concreted or paved over their whole front garden area for car hard stands and any diversion of rainwater would flow directly to the road

I also have a large back garden and again divert rainwater first to a water butt and then the overflow from this down a pipe to further down my garden.

While I have made these modifications the same for all properties along my roads would only result in flooded roads and probably little difference to the amount of water entering the aged sewerage system. My property was built with a shared foul and rainwater "sewer" and I suspect due to the age of the properties all the houses down my road are similar.

According to the local water/sewage company one of the biggest problems in my area appears to be fatbergs. A lot of take-aways have sprung up over the last decade on the main road near to where I live although the water authorities are also blaming domestic customers who pour used oil and fat down the drains rather collecting it in a container and taking it to the local recycling centre for disposal.

I would suggest that one of the biggest problems with overflowing sewers etc. is that in many areas there is no opportunity for rainwater to soak into the ground with everyone wanting off road parking - and with new builds having pocket handkerchief sized gardens that have a paved over patio area.

[Fredxx]

<Snip>

In theory new builds must have sufficient soakaways to cope with surface water from concreted areas.

The issue is perhaps with older builds, made worse with a sewer system that was never designed to cope with new builds non-surface water.

[Bob Eager]

Quite. It can be a bit complicated.

Some months ago the house almost opposite ours was obviously having problems with the drains. Pretty sure this is a combined sewer - ours is, but it comes out in a side road. Most houses were built in the 1920s and

1930s, although ours was 1903.

There have been regular visits to that house by a van with a high pressure hose system. This escalated to a big yellow tanker with high pressure hose and a wide bore suction hose. This continued through many regular visits.

A little while ago we were advised of a pending road closure from the company that does sewerage (we get water from another company). This was for ten days (i.e. two weeks of working days). The road is busy, and a bus route, so it's been weirdly quiet.

They came and they dug using pneumatic drill and a small digger. And dug. And dug. After a few days they dug another hole about 10 metres away. Then they filled that in. By this time they had two days left. On the last day we got a letter through the door telling us that it was a bit more complicated, and they needed up to another two weeks.

The following week, a low loader arrived with a socking great digger, also used as a crane. They enlarged the hole sideways (to about 6 metres square). And they dug. And dug. They also shipped in a Portakabin for the workers to use.

Then they shipped in steel shuttering for the sides of the hole. Long stuff. They told me that they were going down an estimated seven metres.

The lengths of shuttering were about seven or eight metres long. They are supporting all four sides of the hole and are nearly flush with the road surface now. There is a long ladder disappearing into the hole.

This afternoon (10 minutes ago) a truck pulled up with another load of steel shuttering. They are supposed to finish tomorrow...

I'm not bothered, it's lovely and quiet. But they haven't reached the sewer yet. Imagine trying to do another one just for surface water, all down the road...

[Fredxx]

Those dimensions sound like surface water holding tank size.

[N_Cook]

A pic of a close to flooding event

the local chokepoint manhole, photographed through an air balance vent hole in the cover, with normal view to the right. Two 8 inch pipes in and one 12 inch one out, same cross-section area but water does not like changing direction. Two horizontal in and one vertical out to the low level wide-area 18 inch sewer pipe system. The piece of toilet tissue on the wrung evidenced a previous extreme rainfall event but after the last pressure-spray and bowser bulk scouring by the utility . Curiously the swirl is always 3-way , despite 2 inputs and nothing in the benching form to induce a 3 way swirl. For the period of maximum rainfall rate, ignoring any lesser rainfall periods in the same event. Locally t= 2minute duration, r= 530mm/min rainfall rate from the local rain gauge t=3min , r= 340mm/min t=5min,r= 200 t=10 min,r= 95 t=20 min, r=45 t=40 min,r= 20 from monitoring rainfall events for the situation leading to rising level inthat manhole

[N_Cook]

Who goes around replenishing the silted up infil and so non-functional soak-aways or French drains? and how long do they function before silting up?

[Bob Eager]

No, they tell me they are digging to reach the sewer.

[Owain Lastname]

A lot of Hampshire is on clay, which isn't very permeable, so surface water will stay on the surface and flood unless there flood drains.

Owain

[N_Cook]

Perhaps blockage/collapse from dilapidations like this

part of the local sewer system, stills from CCTV survey here

Originally presumably fairing at the pipe junction but hydraulic pressure or ground heave caused failure and mutual transfer of local river water and sewage problems, plus some hollow of unknown size for a future sewer collapse . Builders "fiiting" pipes by boring through and protruding pipe and hence trapping and blocking flow, a nice gallery of horrors.

[pinnerite]

I have applied for planning permission to replace my battered single story garage with an extention as my immediate neighbours did two years ago.

I live in the notorious borough of Harrow. After the plans were submitted, the planning department demanded a heritage asseessment. I did that myself. They apparently liked it.

But they then demanded a flood risk assessment.

The architect found a specialist firm to do it at £585 plus VAT.

However, I live on the side of a hill that runs all the way south to central Harrow where the Civic Centre lives. That is where the planning office is.

The thing is this website will tell you the altitude of your property above sea level.

<POST CODE>

From this I find the planning office is 60 Metres (197 ft) above sea level, whereas my home is 90 Metres (295 ft) above se level.

So if there is a flood, their office is going to under 30 Metres of water before I need to start paddling.

And so (again) why couldn't they save me from having to commission a 45 page report that is clearly unnecessary?

(Rant over),

Alan

[nightjar]

When the road is covered in a couple of feet of water, it doesn't make any difference. It still gets down through the manholes and mixes with the sewage.