Proposed flood defences - Rivelin allotments at risk?

[dutch]

I often walk by rivelin and was here during the 2007 floods. It doesn't get too bad usually and wonder why they don't use the upstream structures to buffer occasional extreme rainfall.

Guess whole thing is a bit like driving all your life with a spare until you get a flat one day.

[BHRemovals]

I do belive there are some open meetings about these proposels

[Eater Sundae]

Looks like some allotment owners might start growing rice.

 

Looking at the size of the embankment, it surprises me how high it would be, ie that the local land slopes as much as it does between the Hagg Hill area and the position of the embankment.

 

I no longer walk in the area, so it doesn't affect me directly, and I might think differently if I did, but generally it looks like a good idea.

 

It's not clear to me what they will do with the area which can be flooded? Do the allotments stay there in the knowledge that they may/will be flooded occasionally? Is the area retained as a park, with free access and Riverside paths for most of the year, except when flooded, or will it be fenced off?

 

In the past I've driven, walked and cycled many times along that section of rivelin valley road, and walked along the riverside path, but never really noticed the slopes of the land. it always seemed pretty flat, except of course for the natural slope associated with a river. Only since reading this thread have I looked at the land as I drove along there yesterday.

 

If this goes ahead, it would result in a large and fairly deep lake if and when it operates.

[six45ive]

This is a response to the proposals on the RVCG Facebook page. Pop over and give it a like to keep updated.

 

"In response to your front page article on Wednesday and the comment included on page 11 by Cllr Bryan Lodge I feel it necessary to state that it was never the intention of The Rivelin Valley Conservation Group to be alarmist but simply to ensure that because the consultation was started in August when most people are on holiday that Sheffielders were fully acquainted with the facts.

If the councillor had attended one of the consultation meetings and asked the pertinent questions our representatives did he would know that the computer model used by the Council's consultants indicated that in Rivelin's case the 2 impounding reservoirs would need to hold back a volume of water that would mean a depth of some 10 metres at the embankments, 80 metres wide and running back up the river and Nature trail for several hundred yards Holding back this pressure of water will involve the engineering construction of a full reservoir dam and with a safety element to ensure no overtopping this would mean an 11 metre embankment.

This water would be held until the flood risk had subsided when it would be released gradually to allow for re-use. Should the Rivelin scheme proceed we are faced with a massive obstruction to people being able to walk freely up the most accessible easy-going countryside walk in the City, but with the safety hazard of a 10 metre deep unfenced reservoir. The impact on the landscape of these structures and the drowned debris strewn areas where the impounded water had been will be enormous.

Although the scheme may be funded through government grant I would question whether the Council has any contingency plans for the subsequent maintenance of reservoir dams, the complicated sluices and their gates and the reinstatement of footpaths and retaining walls after each flood.

The wildlife corridor so vital to the movement and interaction of the

valley's wildlife would be severed by the dams and sluices and significant areas temporarily inundated by the impounded waters. The wildlife of a pennine river and its banksides is very special and unique to that particular habitat.

To suggest days, if not weeks, of interrupting the flow and drowning of areas irrespective of nesting seasons for birds and small mammals, the sequence of insect development which is specific to different water conditions and the flowering times of our native flora is nothing short of criminal. There has to be an alternative to a complete obstruction of the river and Nature trail.

Alternatives include better maintenance of the river systems, particularly at confluences and bridges, the repair of some of the existing mill dams, construction of new impounding dams which do not interrupt the river and footpath networks and a conversation with Yorkshire Water regarding the use of Rivelin reservoirs.

The drawing down of the Lower Dam by 1 metre would more than compensate for the volume of the scheme being suggested and would have minimal impact on the supply to the treatment Works.

The facts put together by our professional experts cannot be disputed by councillors and we hope Sheffielders will use the consultation process to make their views on the value of each affected valley known. The viability of alternative schemes does need to take full account of the Value, even if unquantifiable, of our ability to access and enjoy the wild and industrial heritage that makes Sheffield the unique City it is.

Roger Kite M.B.E.

Ex Chairman and currently Conservation Adviser to Rivelin Valley Conservation Group"

Edited October 9, 2016 by six45ive

[Eater Sundae]

I asked, above, what would be done to the area which would be subject to flooding. I presume it would need to be cleared of trees, walls, hedges, allotment stuff such as huts, greenhouses etc, otherwise this would become entangled with river borne debris each time it floods. I expect they would need to leave it with a smoothish finish, such as grass or bare earth or some other compacted surface.

 

Once flooded and then drained, the area would become wet and muddy/slippery. Is seems likely that they would have to detour all the paths to put them above the top flood level. From a H&S viewpoint, I wonder if they would feel it necessary to remove all lower, riverside paths to avoid having situations where paths just disappear under floodwater and where people might slip.

 

There is a gate at Rother Valley which can operate to hold back water (to prevent flooding downstream towards Rotherham), but that just increases the area and depth of existing lakes (wild life and fishing lakes, I think). I doubt that the depth increases by much, and certainly not by 10m. The sluice gate is under an access bridge, IIRC, and would probably only add around 1 to 2 metres upstream depth, at most. Does anyone know how they manage lakeside/riverside paths in that case?

 

The more I think about this idea for Rivelin, and now I understand how deep it would be, the impact would be large. As noted above, this would be a full blown dam. It's a pity they couldn't use lots of small impounding dams such as all the little dams which were/are already there from its old industrial days, by keeping them all just a few metres lower, each, but allowing for extra depth and volume during heavier river flow.

 

---------- Post added 10-10-2016 at 06:43 ----------

 

http://www.rivelinvalley.org.uk/downloads/Riv_flood_storage.pdf

 

The link above leads to another link which shows more detail for the two Rivelin dams. I don't want to derail this thread about Rivelin, but I am interested in also looking at other rivers (in particular the Porter, which is part of the Sheaf proposals). Can anyone point me towards more details on the other schemes ie something similar to the Rivelin document posted by WalkeyIan.

Edited October 10, 2016 by Eater Sundae

[six45ive]

Yes Eater Sundae. All of those things are incredibly serious concerns and they aren't costed in the £80 million fund as I understand it. That money is purely for building the structures, the clean up after a flood would have to come out of council coffers.

The fundamental point here is that this is all unnecessary because we already have the engineering solutions in place further up the city's valleys. The important point to note in the letter I posted is that the bottom Rivelin reservoir only needs to be lowered 1 metre to take up the capacity of the two dams being proposed!

This seems to be no more than an opportunistic and ill thought out approach to the fact that the government are throwing some money at the council and they have to find a way to spend it so.........hey, sod the consequences.

This is entirely unnecessary across the city's valleys and will do way more damage than good when you bear in mind that the structures may never need to be used.

[bluecanary]

This seems to be no more than an opportunistic and ill thought out approach to the fact that the government are throwing some money at the council and they have to find a way to spend it so.........hey, sod the consequences.

 

This was my thought too......

[Eater Sundae]

Yes Eater Sundae. All of those things are incredibly serious concerns and they aren't costed in the £80 million fund as I understand it. That money is purely for building the structures, the clean up after a flood would have to come out of council coffers.

The fundamental point here is that this is all unnecessary because we already have the engineering solutions in place further up the city's valleys. The important point to note in the letter I posted is that the bottom Rivelin reservoir only needs to be lowered 1 metre to take up the capacity of the two dams being proposed!

This seems to be no more than an opportunistic and ill thought out approach to the fact that the government are throwing some money at the council and they have to find a way to spend it so.........hey, sod the consequences.

This is entirely unnecessary across the city's valleys and will do way more damage than good when you bear in mind that the structures may never need to be used.

 

Just a thought...

 

Prior to the creation of the National Rivers Authority, (the forerunner of the Environment Agency), the responsibility for prevention of river flooding lay with the local water authority. So, if the systems hadn't been split up and sold off, Yorkshire Water would still have had a vested interest in keeping the reservoir level lower to provide emergency storage.

 

As things stand, it might prove difficult to arrange the joined up thinking necessary to arrange for such a drop in reservoir level.

 

It does seem to be an obvious solution, though.

 

The other rivers in Sheffield don't all have the same opportunities as provided by a reservoir on the doorstep, but where there is one it does seem to be a good solution.

 

However, just a further thought...

 

I don't know how water is abstracted from the river, normally, and what would happen if the rate of abstraction had to suddenly increase to provide flood storage. In these circumstances, the water entering the reservoir would be much dirtier than normal, and carrying much more debris. This might overwhelm the inlet screens, both in terms of water flow rate and the quantity of debris. Also, the extra fine solids in the water will settle out in the reservoir, which in time will compromise the storage capacity.

 

Edit. I've just checked. The storage volume associated with the two temporary dams on the Rivelin valley corresponds to just over 4 days capacity of the water treatment works. (ie 330,000 m3 is > 74,000 m3/day). If those were designed to fill in 1day, then for the reservoir to provide an alternative storage, it would need to accept water from the river at 4 times the normal rate. If flooding prevention assumes filling in, say, 12 hours, then the rate goes up to 8 times. This would require a much greater capacity draw off from the river, and much bigger screens. This may prove to be a very costly modification. It all depends on the rate of temporary storage required , which will be part of the calculation of storage volume required. Only if the temporary storage corresponds to a flooding problem associated with 4 or 5 days of heavy rain, and not a much shorter period of flash flooding, would the existing draw off from the river be adequate.

 

Another possible issue may be the need for the water treatment works to maintain a minimum quantity of water, to cope with droughts, equipment failure, etc. The water companies will be required by Ofwat to meet certain requirements, no more (as the funding is not there) and no less (they must meet the minimum standard). On that basis, it might not be possible to just drop the level of the reservoir. It might actually be necessary to increase the height of the reservoir, but then not fill it. But that probably would not be possible due to relative river levels.

Edited October 10, 2016 by Eater Sundae

[six45ive]

A lot of what you say deals with legalities regarding a private company such as Yorkshire Water but this should be the easiest of issues to overcome. All that's required is negotiation and joined up thinking by all parties involved and, if need be, a change in regulation to enable this simplest of solutions to come about.

Also, your point about this not being a solution for all of Sheffield's main rivers is wrong. All of the rivers except the Sheaf are supplied by reservoir water upstream; a quick look on a map will highlight this quite starkly.

Edited October 10, 2016 by six45ive

[Guest makapaka]

The drawings not clear to me. Where on rivelin valley road would this start/finish?