The Economics of Upper Catchment Small Scale Attenuation Ponds versus A Capital Intensive Flood Alleviation Scheme
Since the Boxing Day flood of 2015, Slow The Flow have promoted Natural Flood Management (NFM) approaches to supplement hard defences in the armoury of flood prevention techniques. These can often be implemented at low cost bringing immediate benefits, rather than taking years in planning, procurement and construction which is so evident with the Flood Alleviation Scheme (FAS) at Mytholmroyd. One technique we are keen to promote is the provision of attenuation ponds designed to normally be dry, but store water during the onset of a storm, draining slowly and finally emptying completely when the storm has passed. The inbye land below the moorland, which is drained enclosed farm land, is ideal for locating these ponds. They can be installed to intercept small stream courses and buried culverts, reducing flooding to properties locally.
Basin at Oldroyd protected 5 houses from flooding
An attenuation pond was constructed at Oldroyd near Todmorden in 2016 and features in the blog, the link for which is given below. The benefits are clear to see, the pond and inlet channels collect water safely storing it away from vulnerable properties, it delivers a lower flow rate and clean filtered water safely into the landscape, and lowers and delays the overall flood peak in the main river system.
http://slowtheflow.net/flood-prevention-at-oldroyd/#more-5477
More could be done
The scoping studies for Calderdale Council’s NFM Grant Scheme undertaken in the Spring of 2018 by The Source Partnership identified as many as 43 attenuation ponds of which around 30 received funding, and of these to date, 8 are completed. Under different funding streams there are a further 3 complete and operational this includes the one at Oldroyd. The approximate cost of the 43 scoped ponds was £600,000 delivering approximately 29,000 m3 of storage. This is a cost per cubic metre of around £21, which includes compensating landowners, 2 years maintenance payments and additional features such as tree planting and swales to deliver each as a complete small scale NFM project. These additional features also have some water storage and slowing capacity, which is ignored in this assessment.
There are plenty more undiscovered opportunities, including many disused mill ponds.
Compare to Mytholmroyd FAS
In comparison, if we consider the main section of the Mytholmroyd Flood Alleviation Scheme (FAS*) one can estimate an area of 22,000 m2 – using the polygon tool in Google Earth. The additional storage above the flood plain is determined by multiplying this figure by an average depth of new flood wall above the flood plain as follows:
River | Reach | Approx. Channel Area (m2) | Approx. Average Height Of Flood Walls Above Flood Plain (m) | Approx. In Channel Volume of Flood Storage Above Flood Plain (m3) |
Calder | Vale Upholstery to New Bridge | 5432 | 1.00 | 5432 |
New Bridge to Caldene Avenue | 1792 | 1.35 | 2419 | |
Caldene Avenue to Whitehouses | 4989 | 1.65 | 8232 | |
Whitehouses to Hawks Clough Bridge | 7530 | 1.00 | 7530 | |
Elphin Brook | Calder to Railway Arches | 1015 | 1.10 | 1117 |
Railway Arches to Cragg Road Bridge | 1077 | 1.00 | 1077 | |
21835 | 25807 |
So by these estimates which relate specifically to Mytholmroyd, the additional storage volume above the flood plain provided by the FAS is of the order 26,000 m3 at a cost of say £33 million which equates to approximately £1270 /m3 of storage, this total storage is approximately the same order as the overall storage provided by the planned 43 attenuation ponds but at 61 times the cost per m3.
Photo: Crags Business Park attenuation pond under construction April 2018
There is no suggestion here that attenuation pond storage can entirely supersede the requirement for in-channel storage, as clearly attenuation ponds will store water when in channel capacity remains still available, reducing their ability to store water towards the peak of a flood. This means that considerably more attenuation volume is required than in channel volume to take account of this issue. However, the economic argument for more attenuation volume is really the central point of this blog and it is clear that a lot more can be provided than currently planned at a sensible economic cost, which may even reduce the cost for example of the planned Hebden Bridge FAS.
Could they be made more efficient still?
Ideally, attenuation ponds should only be filling just as the in channel volume is approaching bank full. So this begs the question, could landowners be responsible for controlling their pond outflows by closing a control gate? If you have volunteered your land for the greater good of the community then it is also likely that you would take the time to control the outflow from your attenuation pond given adequate warning, so perhaps the answer to this question is yes.
A simple timber control gate for use on an attenuation pond outlet is given in the sketch below, with an extension to the flood warnings that are issued to the general public by text message, a text with when to close the gates could be issued by the Environment Agency to those landowners with attenuation ponds. Not all will respond, some may be out, others on holiday, which requires a degree of flexibility, but the technology is there and the benefits could be significant at a fraction of the cost of major public works. A further extension to this idea would be remote operation of the control gates, which although more expensive could be a very powerful way of providing relatively inexpensive flood prevention measures.
* The area of the River Calder channel in its enhanced state (post FAS) between the bridge at Vale Upholstery and the stone arch bridge at Hawksclough plus the Elphin Brook as far as Cragg Road.
