Slowing The Flow Together

Slow The Flow have been working with The National Trust at Hardcastle Crags near Hebden Bridge since 2017 to develop a pilot scheme to reduce flood risk into Hebden Bridge using Natural Flood Management.   

We have been working with our volunteers to build 100s of leaky dams and to create large storage areas. 

Here is our story…

If you attended the virtual premier of our film, Slowing The Flow Together,  (funded by The Environment Agency and in Partnership with the National Trust at Hardcastle Crags), here are the answers to the questions we were asked. 

If you need any more information not outlined below,  please see the relevant pages on the rest of our website or email us at


We would be happy to help. Send us an email to for more help and advice.

Our website contains a wealth of information which community groups can use to set up their own Slow The Flow group.

See our YouTube channel for a full set of videos from our mini conference on the subject – Youtube

It would also be worth contacting the Environment Agency (EA) too – try your catchment co-ordinator in the first place.

Initially it was members of the local community getting together after the major flood event of Boxing Day 2015. We recognised we needed to do something to reduce the impact of flooding and not simply wait for the authorities to develop and implement their plans. This has proved to be sensible as the flood alleviation plan for Hebden Bridge is still at the design stage, almost 5 years after the Boxing day flood and we have suffered another major flood event (February 2020) plus a number of very near misses.

Identifying what we could do as a community group we carried out a number of river surveys looking for potential sites for NFM measures. We quickly identified Hardcastle Crags, owned by the National Trust as an ideal location. The Trust were more than supportive of the idea and gave us all the help and encouragement needed.

As soon as we can get back into Hardcastle Crags we will be extensively marketing and getting people back into the woodland volunteering. We can do this with special considerations to social distancing and will welcome anyone who wants to get involved.  The only limitations we have are physical ones in Hardcastle Crags working in a woodland environment so you will need to be able to walk a couple of miles and be able to climb inclines. 

If you are physically or mentally challenged in some way and you do want to help,  please still get in touch and we will try and find you a role which suits your specific limitations.

Keep an eye on our social channels and all the information will also be here on our website.

Our work has been featured on both national (BBC Countryfile) and local TV, (ITVs Calendar and BBC Look North) as well as local and national radio. We commissioned the video to be able to showcase our work. Getting TV commissioning editors to produce a programme dedicated to NFM is quite a challenge, but we are working on it and our social media presence is such that anyone searching for NFM will come across our name.

Later in the year there is a Panorama programme (BBC) on climate change which we have contributed to. Not sure if we will make the final cut or the date for the broadcast.

I do know that Emma at Mott McDonald is preparing CIRIA guidance on this. The Environment Agency have a number of consultancies on their framework for NFM in a number of capacities. We have worked with the private sector on a pro-bono basis.

Yes, volunteering is currently suspended. The best way to find info is check the National Trust Hardcastle Crags and Slow the flow Facebook and websites. When it starts back up again, it’ll all be on there.

This is part of the long-term plan. We have delivered a pilot programme of engagement with local school children in conjunction with a local educational charity.  This focused on introducing children to basic hydrology, NFM and ultra-sonic measuring devices used in flood monitoring.

STF is run entirely voluntarily and we all hold down day jobs.  This is one area we would like to develop so if you have any specific skills, time or ideas as to how this might work,  please do contact at  We always welcome new initiatives but our time is limited so any help is always appreciated.


Seed funding to enable community groups to establish their own ‘Slow The Flow’ Group.

Help identify large landowners with sites suitable for NFM.

Help community groups monitor and evaluate the effectiveness of their NFM interventions.  Supplying evidence that can secure funding from Local Economic Partnerships would be a great start.

Both funding and technical. They really have been very supportive. They see us as a good example of community resilience. We are very grateful to them for the support they have given us.

Securing funding from public bodies is always a rigorous procedure and quite time consuming. Many iterations of our proposals were required. It requires technical skills and resilience to secure the funding and a very specific skill set.  You also need credibility when applying so make sure you have all of your policies and processes in place first.


Have a look at the recent CIWEM seminar for an excellent presentation on beavers into Cornwall. Calderdale is not natural habitat for beavers. They prefer more undulating topography.

We think they will last for 10 years plus. The hard wood logs will last longer than soft wood.  There is no specific funding for their maintenance but like any feature in the landscape they are the responsibility of the landowner to maintain. It is expected that the up-coming Environmental Land Management Scheme, being developed by DEFRA to replace Basic Farm Subsidy, will contain provision for public funding for public benefit.

Good question. It is necessarily long term and the new Environmental Land Management Scheme should address long term management. We have looked at millponds but the costs can be large with the removal of sediment being the main problem.  Locally two have been converted successfully as both had available land where silt could be buried. The area is not particularly suited to Pumped Storage systems.

Another good point. Slopes are quite critical. Not too steep. Not too shallow. Through the Working with Natural Processes mapping tool, this suggests areas where NFM might be best applied. This from Defra is a very good working document

The point at which they become less effective also changes from catchment to catchment depending upon how each tributary meets the main river.

As trustees we have a range of skills that have turned out to be complimentary: engineers, project managers, landscape architects and communication skills. We mostly use a ‘walk over’ procedure to identify where NFM interventions are best placed. A combination of topography, hydrology, basic science and availability of materials combine to produce an NFM area plan. Access issues and the availability of volunteers also feature in our planning. These skills have been developed over the years and are almost second nature to the seasoned trustees and volunteers who turn out for Slow The Flow.


We acknowledge that monitoring is critical to the evidence base for NFM.

The project was set up by local volunteers who were keen to do something in a catchment that frequently floods, it took time and effort to get the funding to carry out the works, there was no additional funding made available for baseline data collection. Nevertheless, we have sought to capture time lapse footage and identify flow through pressure transducers. This has built a partial picture. Since we have started on this project in 2016 funding has been made available to universities through the Natural Environment Research Council to look at the efficacy of NFM

We have two streams adjacent to each other, one with and the other without interventions, so with the stream flow data, it is possible to do that.

We think they will last for 10 years plus. The hard wood logs will last longer than soft wood. After that period the hydrology around these gullies might have radically changed anyway with more flow shed over the woodland floor.

The project is spilt between small leaky dams in feeder streams and large leaky dams across the main river, Hebden Water.  The small leaky dam construction is undertaken by volunteers and because the stream flow forces are small there are no special requirements other than blocking up the streams with tiers of logs.  Leaky dams are placed in succession up each stream course and spaced so that stored water upslope determines where the next one will be placed.  The large leaky dams which have high stream forces have been engineered and are tethered with steel rope fixed to dowels grouted into the bedrock or boulders on the stream bed, these are constructed by forestry professionals.

One of the universities has been doing a survey using lasers which provide a 3D image of the ground behind a leaky dam.  This data provides an indication of the sediment storage each dam provides.

Measuring flows over plate weirs in small feeder streams is being undertaken, measuring flows in the main rivers is being attempted using ultrasonic which provide height of stream flow hydrographs, these could be converted to discharge hydrographs with a rating curve.  As volunteers, a rating curve is a lot of work to produce and measuring stream velocities in high flows is potentially very dangerous.  One can see delays in the stream flow data downstream compared to upstream at peak by plotting stream height against time. Reduction in discharge on the feeder streams is also discernible.

Indeed, it is part of a wider programme of working with natural processes that will make our landscape more resilient. But no, we have not made any assessment of this.

Not sure this has formed part of any research this might be worth picking up with universities.

Hardcastle Crags occupies an area of the Hebden Water catchment. Hebden Water joins the River Calder at a confluence in Hebden Bridge town Centre. The site is approximately 197 hectares and we have worked with Slow the Flow to install leaky dams and natural flood management interventions across most of the suitable areas of the woodland.

The Hebden Catchment is 59 km2 above Hebden Bridge most of our work has been in the  Crimsworth sub catchment (11.82 km2) but our works only touch a small proportion of this catchment to date.

You’re right, Hardcastle Crags covers a very small percentage of the Upper Calder catchment. The National Trust work with various colleagues and fellow land owners including Yorkshire Water to develop a much larger natural flood management project working on more of a landscape scale to undertake moorland restoration across our land holdings from Marsden Moor in the South to Hardcastle Crags in the North. The National Trust are also currently delivering a natural flood management project in partnership with Yorkshire Water on moorland sites above Todmorden.

The Hebden Catchment is 59 km2, although much of our work has been in the Crimsworth sub catchment (11.82 km2). Our works only touch a small proportion of this catchment to date.

We did not have the resources to carry out the detailed modelling. The project was set up by local volunteers who were keen to do something in a catchment that frequently floods, Hardcastle Crags is owned by the National Trust so obtaining permission from one landowner was relatively easy so this was the main driver behind project location.  In that location there are multiple feeder streams which were obvious choices for placing small leaky dams, there were also three floodplains containing gullies which were blocked up with leaky dams.  The woodland has been thinned to provide the materials for the leaky dams which has increased the under storey and will in time increase the available roughness on the woodland floor allowing water to infiltrate. Since we started the project detailed opportunity mapping has been produced by the Environment Agency and be can be found here

Land Issues

See the EA document Working With Natural Processes where there are a number of case studies relating to lowland catchments and coastal areas. This is a very useful working document which should be of great help on this and very many other questions.

We are already working over in Todmorden on Yorkshire Water land at a site at Gorpley. The Calderdale Council NFM Grant Fund is designed specifically to fund schemes like this and others.  If you have land which you think might be useful for NFM, please complete the form here – and someone will be in touch with more information.

Volunteers are required everywhere and when the Covid-19 crisis is over, we will be re-establishing working groups at Hardcastle Crags. 

We also want to encourage other groups to start to look at their own localities and using our methodology here and throughout our website to set up your own NFM schemes in your street, village, town and city.  There is too much to do by just one organisation so we must encourage other groups to take this on for themselves. 

Please do contact us at if you need specific advice and we will help when we can. 

The Calder Valley as surrounded by upland farms where sheep and beef are the main products. The increased and temporary inundation by flood water is pretty minimal in the scheme of things.  There is some resistance from some but we, and others, work hard to promote the merits of NFM and flood alleviation to landowners and farmers and any dis-benefit could be funded through the new Environmental Land Management Scheme.

We have been involved with quite a few attenuation ponds now around Calderdale and the message is getting across that they do indeed empty after the flood has passed. More though still needs to be done to reassure farmers and landowners.

Through the proposed new Government Environmental Land Management Scheme public goods will be funded by public funding

More locally there is a Calderdale Council NFM grant Scheme is designed for this purpose.  The scheme is currently closed but we do expect it to open again later in the year.  If you have land which you think might be suitable for NFM, please tell us about it here –

See the EA document Working With Natural Processes where there are a number of case studies relating to lowland catchments and coastal areas. This is a very useful working document which should be of great help on this and very many other questions.

Landowners are ultimately responsible for features in the landscape.  Just like dry-stone walls, hedges, fences and tree plantations it falls to the land owner to maintain them in good order. The up-coming Environmental Land Management Scheme is likely to incentivise landowners to carry out public good, such as NFM measures, for public funding. In reality the leaky debris dams we have constructed will require little maintenance during their lifetime (10 to 15 years).

The hope is that ELMs is longer term than 20 years but we agree there probably needs to be a change in the responsibilities associated with land management.

First, a catchment wide approach to flood alleviation is required, highlighting easy win NFM measures. Landowners should be paid a fixed rate per m3 water stored/hedgerows and trees planted etc. The system needs to be as straight forward and easy to engage as possible. Hurdles such as Planning consent and Ordinary Water Course consent should not be obstacles to NFM implementation. NFM interventions need to be maintained over time and monitored for their efficiency and contribution to flood alleviation.   

We are at an early stage in how NFM will be managed over the long term – it is about making attenuation part of mainstream land management rather than a simple one-off. It is hoped that the new Environmental Land Management Scheme will have answers with funding for farmers and landowners for public benefits.

Great question, thank you. See answers above. There have also been several presentations on ELMs and NFM on the CIWEM website –

Sustainable Urban Drainage Systems (SuDS)

Green roofs do well with the right species, and there is plenty of evidence out there (The Green Roof Centre is a good starting point) but for ourselves we only have anecdotal evidence of this. The green roof on our trustee Amanda’s shed has fared very well, which she puts down to species selection combined with a solar powered, water butt fed, irrigation system. The first test of the Flood Warden Store green roof will have been this unusually dry May of 2020 – which, during this unusual period of COVID-19 lockdown, we haven’t managed to visit, so we don’t know!  

The short answer is yes! To some extent – we need a significant increase in uptake in order to make a real difference to flood alleviation. We hope that the more precedents pop up, the more likely local developers are to see Sustainable Drainage Systems (SuDS) as the norm, and include it automatically.

By engaging with the Local Authority, we’ve been able to influence Calderdale’s emerging Local Plan. There is, crucially, a new Flooding SPG to go with it, into which we have been able to have some input, that sets requirements for SuDS. The latest draft we’ve seen still doesn’t go quite as far as we would like. We’d prefer SuDS to be required on ALL developments – all sizes, all flood zones, and retrofit as well as new build (through new build, we can only really stop the problem worsening. To tackle the flooding we already have, we must retrofit!)

Our opportunity mapping pilot project (OMPP) has been fairly successful so far, even without any significant marketing (we’re waiting to advertise in earnest because we are currently producing/peer reviewing a report that attempts to quantify the implications of actually implementing all of the interventions in the OMPP, and rolling out something similar over the wider catchment).

Various schemes in that pilot area, influenced by the mapping, are popping up – we hope to roll it out more widely, this will need significant funding though. 

A couple of interventions we have been directly involved in are retrofit examples – Hebden Bridge Town Hall courtyard and the Flood Store container green roof. There are quite a number of case studies on our website now, the majority of which are retrofit.

The success, particularly of retrofit projects, can be hard to quantify! We set up a bespoke monitoring system for the rain garden planters at Hebden Bridge Town Hall, the results of which do show that they are successful:

They are bespoke – details of their construction are in the case study. We had hoped that the local fabricator could develop them as an ‘off the shelf’ product with local suppliers, but the end cost was going to be prohibitive, particularly for a domestic setting, so they have had to abandon that for now.

A rain garden is basically just any planting that you allow to flood from time to time. It can be a raised planter e.g. at Hebden Bridge Town Hall courtyard, but can also be a planted dry pond / swale etc, or simply a low area in the garden, that naturally intercepts water, or that a disconnected down pipe or water butt overflow leads to.