Rain, rain, rain, can result in pluvial flooding!
Don’t do this at home but if you turn the taps on full flow, without the plug in, the waste pipe under the basin will fill and water will start to rise into the bathroom basin. Ultimately, the basin might fill up and you could flood the bathroom. This would happen if the flow out of the plughole, down the waste pipe, was less than the flow in from the taps. The basin overflow might help but the “pressure head” pushing water through the overflow is often low, so its effectiveness can be limited. Flooding is more likely if the pipes are not clean.
Why do parts of Hastings flood?
The flooding of the Priory shopping centre in Hastings is the same phenomenon but instead of the basin waste pipe, there is an underground drainage system (including storage capacity). If more water flows in, than flows out, eventually water floods out above ground.
The central cricket ground (where the shopping centre was built) often flooded in the past. A report on Wikipedia states: “The ground had previously flooded in 1866, when one resident noted that it had been underwater many times within their memory.” Online reports from Hasting’s historians describe severe flooding in 1913.
This part of Hastings is low-lying and water runs down from the surrounding hills. The situation has got worse over the years because of impervious developments on the hills. Rainwater collects and flows quickly down to the centre.
Drainage is by gravity and overflows to the sea. The “pressure head” is low. On high spring tides the “pressure head” is lower as the overflow outlet is submerged.
So, if the inflow from rainstorms around Hastings coincides with a high spring tide, the “basin” overflows. Where does the water go? Unlike your overflowing basin the water cannot go through the bathroom floor, down through the ceiling and into the lounge! Hastings centre is low-lying land surrounded by hills and defences against tidal flooding along the seafront. (Tidal flooding and pluvial flooding are different). There is nowhere for the excess water to go! It sits in the central area until the tide drops.
It is a man-made arrangement. Left to nature this part of Hastings would be subject to tidal flooding on high tides. Our predecessors kept the sea away by building flood defences. They left Priory meadow undeveloped as marsh land and then a cricket ground. At various times “engineered” drainage systems were provided to hold or remove surplus rainwater through high tide periods. The Old Steine in the centre of Brighton is similar. The huge Victorian stormwater sewers, under the Old Steine in Brighton, are worth a visit. Sometimes engineered solutions need to be updated!
I don’t know the details of the drainage in Hastings, so won’t attempt to identify solutions, which are likely to be complex and expensive. Simplistically, reduce the flow in, increase the flow out or create additional storage capacity. None would be easy!
How does Rye avoid flooding?
How does this all relate to Rye? Well, parts of Rye are low-lying and the area is protected by defences against tidal flooding. In that respect there is some similarity. The good news is that Rye doesn’t sit in a localised hollow and is not surrounded by hills, which have been developed. Also, there are very large areas of marshland close by.
Inland of the sluice at Strand Quay and beyond Scots Float, the Rother and the Tillingham are non-tidal. Thanks to the tidal flood defences, these rivers do not experience the peaks of high tides. The marsh land is protected, drained by miles of ditches, which feed water to the rivers or to the sea. The water level in rivers and ditches can be lowered twice a day, every day, at low tide. It is a man-made arrangement developed and proven over many years.
The developed low-lying areas of Rye are above the water level in the ditches and non-tidal rivers, so rainwater can drain from roads and pavements to the ditches and rivers. In some areas the below ground drainage systems are shallow, with shallow gradients. The “pressure head” in these systems is low and, in some places, there are pumps to help keep water below ground during periods of high rainfall. Such engineered arrangements need good maintenance. Rye is fortunate to have Anthony Kimber and REACT, planning ahead, keeping watch and nudging action.
The large areas of low-lying marsh close to Rye are a big asset. Water will find its way to the lowest point, so surplus water finds its way to the low-lying land, filling the drainage ditches and flooding the fields. A very large volume of surplus water can be accommodated by the ditches and a relatively shallow depth of water on the huge area of marsh. Being close to the sea and tidal rivers, the drainage ditches can be emptied at low tide. It is an advantage that the water level in the river Tillingham can be lowered, via the sluice at Strand Quay, on a low tide in advance of predicted rainfall. The sluice is an important asset.
This explanatory text is about pluvial flooding and was prompted by the recent flooding at Priory Meadow in Hastings.
Good maintenance and vigilance – important factors to avoid flooding
Pluvial flooding presents by far the lesser of the flood risks in Rye. Flooding from the sea (tidal flooding) is of greater concern.
A crossover between the above and Anthony Kimber’s article about flood risks, published last week, is the flooding at Strand Quay. At any one time this might be tidal, pluvial or a combination of both. In 2016 the area flooded, over high tide periods, during dry weather. Tidal seawater could be seen rising up in road gullies during a rising spring tide. Flap valves on the drainage outlets at the quay wall were checked and, thankfully, this solved the problem (at that time).
Clearly, heavy rainfall can result in pluvial flooding at this location because the drains become less effective or closed, during high spring tides. However, whether the flooding is by rain (pluvial) or from the sea (tidal), the depth of water is limited by the crown of the road. Above this crest the water flows down to the south-west side and then away to the lower farmland beyond Winchelsea Road. In terms of flooding, it isn’t serious but it creates dangerous road conditions, with vehicles forced to the wrong side of the road. It is therefore important to ensure that drains and valves are maintained. Thankfully Anthony Kimber and his team keep watch, nudge those responsible for maintenance and keep people informed via Rye News.
Image Credits: Anthony Kimber .
Thanks Geoffrey, that’s very informative! I have always been struck by how rarely Rye floods outside the small hill on which the oldest part of the town sits, and can see it’s due to good water management. It’s all the more important that Southern Water and any other agencies involved are given the resources they need to maintain and improve the defences.
Mum and Dad’s home in Ashenden Avenue was flooded on a number of occasions when I lived there (1953 – 1965) due to a combination of high rainfall, a high spring tide and a “storm surge”. After the first event, Mum would always check the Tide Table in Rye Fixtures and get very worried if it started to rain before a predicted high tide.
Later, my career in physical oceanography included being in charge of the Group at the Proudman Oceanographic that produced tidal predictions, including those published in the Rye Fixtures.
Whenever I gave talks about the relevance of our work, I always mentioned Mum’s fear as an example of the psychological damage that can be caused by extreme natural events, as well as the obvious physical damage.
Thank you Graham for your first hand memories. It is important that where drains and ditches are shallow, they are maintained. Water flow in shallow gradient drains and ditches is slow, so silt drops out of suspension, rather than being carried away.
If drains and ditches are not maintained, they lose capacity and they cause greater resistance to flow. That’s a double whammy.
We are again in a period of high spring tides. The water level in the Tillingham was lowered this morning on the low tide. This creates capacity to accommodate rainwater, well below general ground levels.
Despite the low level of the water in the Tillingham, the water level in the main ditch just north railway line was fairly high. The water level in the feeder ditch at the end Ashenden Avenue was even higher. That ditch is silted to more than half its intended depth (the evidence is the concrete pipe under the little bridge).
The main ditches running around Gibbets Marsh car park are overgrown and holding water. The risk of flooding in this area would be reduced if the ditches were maintained, re-establishing their capacity and freedom to flow. Also, the outlet from the main ditch on the west side of the River Tillingham needs to be checked.
Who has the responsibility to maintain and clear those ditches round Gibbets Marsh etc?
Thanks Geoffrey. The “little bridge” that you mentioned was built in 1953 when two new houses were built at the end of Ashenden Avenue – number 13 (Mum and Dad’s) and number 15.
The flooding occurred when the feeder ditch to the Tillingham overflowed because the Sluice gates were closed to stop sea water flooding in from Rye Bay, due to a high tide and storm surge. It’s a tricky decision to time the closure to allow as much river water to drain out but not allow too much sea water in.
But, as you argue, it’s a “no brainer” to keep the ditches and streams as free from silt as possible, through regular maintenance. What a pity that you are having to point this out some 60 years after the little bridge was built.
It’s a no brainer really, back in the day dredging was a familiar sight to all in Rye, the Strand and Rother were dredged regularly, all the ditches that feed into the main rivers were cleared, look at the ditches on the marsh, they are bigger and kept clear.
We can blame global warming but a lot of our problems are poor management, spiders, newts and plants are now more important than flooding.