If you couldn’t make our public meeting about our ‘You Can Slow The Flow‘ urban SuDS project, there is now a chance to catch up!
Please help us to share this information widely, the more people get involved and make small changes (whether your property floods or not!) the greater difference we can make to flood peak levels in Calderdale.
Hope to see you at our next public meeting on 27th June, where Calderdale Council’s new NFM officer, Rob Twiggs, will speak about his role and how he is supporting Natural Flood Management projects.
Amanda McDermott CMLI – Partnerships Officer at Slow The Flow Calderdale
A simple and cheap idea to reduce water getting into our rivers.
Since we set up Slow The Flow Calderdale last year and being involved in Natural Flood Management (NFM) in the Calder Valley, and as someone who is new to the issue of flood prevention, I was keen to put into practice some of what I have learnt over the last 18 months at my own house.
We live in Luddendenfoot in a terraced house with 2 gardens at the front and one at the rear. As well as the usual flower beds and lawn, our larger garden at the front of our house accommodated space for 2/3 cars so I was keen to adopt some simple principles around Sustainable Drainage (SuDS) to reduce the amount of rain water entering the drains around my house and ultimately into the river Calder Calder.
As you will see from the pictures below, the concrete flags were fairly unsightly and did nothing for the aesthetics of our front garden. Also when it rained, the rain water simply ended up pooling and making its way into the drains.
So I decided to pull up the flags, put down some compost, topsoil and grass seed and create a small lawn. The rain water now goes down into the soil ensuring that this little bit of garden does not contribute to the rising flood waters in the event of heavy rain.
It cost around £60 to do all this and alongside the planters I made from pallets, I have made a small and simple contribution to Sustainable Drainage in the valley. This tiny scheme is not going to stop the Valley flooding, but if every household made a few changes to our own gardens, then we can make a difference to our environment and help prevent flooding in the areas in which we all live and work.
To find out how you can make a small difference in your own backyard, visit our ‘You Can Slow The Flow’ pages
Adrian Horton – Communications Officer at Slow The Flow Calderdale
As Slow The Flow Calderdale prepare for various interventions to Slow The Flow at Hardcastle Crags, the question of which type of Leaky Woody Dam (LWD) is coming under consideration. The subject is complicated because we must ensure that the right schemes are put into place in the correct locations. We are learning from the experiences from our colleagues at Stroud and Pickering about the best methods to use and this knowledge and experience is critical to the success of the scheme at Hardcastle Crags.
Horse jumps or beaver dams?
The Leaky Woody Dams used at Pickering are what I term the “horse jump” type in that they resemble the tiers of logs used in horse jumping. They rely on what civil and geotechnical engineers term “passive earth pressure” to keep them insitu. In the case of a retaining wall there are two types of earth pressure operating and keeping the wall in equilibrium. The retained soil which is tending to push the wall over which is termed “active earth pressure” and the soil in front of the wall at the lower level resisting this action which is called “passive earth pressure”. In the horse jump type of LWD the timbers are pushed against the downstream soil by the stream forces and the passive earth pressure resists this action in the same way that the soil in front resists the active earth pressure in a retaining wall.
One can argue that these types of LWD’s are not leaky enough and are subject to large forces and several have failed in practice. The Pickering Beck is not as fast flowing as some of our upland rivers in the Calder Valley. This means that our LWD’s if built this way in the larger streams could have much greater hydrodynamic forces on them than those at Pickering and Stroud.
There are two forces from the stream flow at work, hydrostatic and hydrodynamic, the former is the force from water at rest (like in a swimming pool) and the second is a function of the velocity of the water which in turn is a function of the gradient and the roughness of the stream bed.
Since our visit to Stroud, we tend to favour the more random placement of logs, rather like a beavers dam than a horse jump.
These are leakier and will have lower forces on them. In the Stroud scheme, they pin the logs together with steel pins. They first drill them then drive the pins through into the ground. An improvement on this would be to secure some logs together with threaded dowels, whereby a bar is driven through at least two logs and a plate and washer is installed on each end of the bar clamping them together. Some designs use steel wire to bind the logs however this will rot through before the logs do. These random types of dam will gather brash over time, this will reduce their leakiness but the brash has a tendency to fail and wash through under high forces so they are self regulating. The horse jump types do not allow brash to “pop” and open up to let water through, particularly if the logs are straight and quite uniform.
The other problem with the horse jump type of LWD is digging into the banks which in rock stream beds is not very practical in parts of the Calder Valley. Our upland streams also contain a lot of boulders, these are ideal for securing randomly placed logs because they are themselves fairly randomly placed. Dowels can be used to do this and there are ways of doing this explained in our previous blog HERE. You can use either rotary percussive drills or core barrels all of which can be powered by a small petrol generator. You can use cement grout to secure them or products like Hilti Hit HY 200 ( https://www.hilti.co.uk/anchor-systems/injectable-adhesive-anchors/r4803) which are ready mixed grouts and come in injectable tubes. Boulders can also just be used to act as passive resistance without the need for dowels if circumstances allow.
Horses for courses and horse jumps for stream courses too!
Having said all this, we do think there is a place for the horse jump type of LWD in parts of the Crags. It is more like a plate weir than an LWD in many ways. We have found with plate weirs that if you leave a gap at the bottom as shown here, you get a lot of scour in the stream bed.
It is better to put a slot in the weir plate or a series of portholes above the stream bed which you can post drill with an auger or hole saw. If you place stone downstream of the slot it helps protect the bed from scour like this:
As such, we could use logs in tiers in small incised channels probably a metre or less wide, in this situation you are more likely to find soil banks through which the channel has incised itself. There are many like this in Hardcastle Crags and it is these we will be working on for the early part of this project.
The idea is to create small attenuation ponds behind each one and put several in a cascade so that the pond extends as far back as it can without intercepting the next plate weir upstream. If there is a bit of over bank flow well that’s a bonus wetting up the woodland floor in the process, but you are also creating a lot of little ponds and slowing the flow.
We have a thesis by an MSc student at University of Leeds who carried out some research on the effectiveness of plate weirs in the Hebden Water catchment available here at this blog: http://slowtheflow.net/results-of-slow-the-flow-research-conducted-on-plate-weirs-installed-in-brooks-on-the-hebden-water-catchment/
Silting could be an issue with the traditional horse jump type of dam but with a slot silt only goes as high as the bottom of the slot due to fast flow of water through it which keeps it clear, silting of course reduces the attenuated volume a bit but it doesn’t seem to stop them functioning which is evident from the experimental plate weirs detailed in the blog above and shown in the photographs.
In many ways it’s horses for courses and one type of obstruction is fine in one place but unsuitable in another.
Lastly, another idea is to use “Pecafil” as LWD’s or plate weirs. These are polypropylene fluted boards (used for “For Sale” signs) reinforced with steel mesh used for casting concrete foundations and have effectively replaced timber formwork. It is very light and might be ideal for remote places for forming LWD’s in small watercourses. The only problem is its colour its yellow! http://www.maxfrank.co.uk/uk/products/formwork-technologies/pecafil-permanent-formwork.php
So we have plenty of options at Hardcastle Crags and around our significant catchment. Some work is already underway by contractors in the Crags, our work in the Crags will start in April with volunteer days on dates to be announced and we will be using a variety of different LWD’s to Slow The Flow.
If you would like to be involved, please get in touch with us HERE.
© Stuart Bradshaw BSc(Hons) MSc DIC CEng MIStructE M.ASCE FGS
Chris Uttley, Rural SuDS Project Officer at Stroud District Council has published a series of six case studies that illustrate the natural flood management work being undertaking in the catchment of the Stroud River Frome in Gloucestershire. They have worked with a wide range of farmers, private land owners, woodland owners and partners to install over 280 different structures on 18km of headwater stream.
Approximately 21% of the catchment area now discharges through natural flood management features located primarily in the headwaters.
The case studies are a selection of the work sites in the Stroud Frome catchment were chosen to be representative of private & NGO ownership. They are a mixture of woodland & farmland and different techniques and construction processes. They describe the Natural flood Management works, the reasons for them, how they were carried out and by whom and the consents required and the indicative costs.
All make very interesting reading. Please click on the links below.
Results of research conducted on Plate Weirs installed in brooks on the Hebden Water catchment – By Stuart Bradshaw from Slow The Flow Management Team.
In January 2016 I wrote a blog and published it on LinkedIn entitled “Pound for pound could this simple weir save our uplands from future flooding”, https://www.linkedin.com/pulse/pound-could-simple-weir-save-our-uplands-from-future-stuart-bradshaw, this pre-dates my involvement with Slow the Flow Calderdale which came later in the year. I wrote this blog in the aftermath of Boxing Day 2015 and the devastating flooding which deluged the Upper Calder Valley and other parts of Northern England thanks to Storm Eva. In fact it is largely because of this blog that I then met Amanda McDermott and Robin Gray who are both now valued and active committee members of StFC and it was only following these introductions that I first considered the possibility of forming a group of like minded individuals who were keen to try and do something positive to reduce flood risk here in the Calder Valley.
The “Pound for pound” blog also opened up connections with University of Leeds Department of Geography from where a young post-graduate student named Alex Clark approached me to see if he could do some research on the efficiency of plate weirs, and in particular on the weirs I had installed here on the Hebden Water catchment. I was more than happy to help and so Alex and I went about setting up some pressure gauges upstream and downstream of two weir cascades and the results from this research are now available here for anyone who is interested: HB_Dissertation_A Clark Final
I will let Alex’s report speak for itself but it is apparent certainly to me that there is some definite positive benefits to be gained from installing small obstructions in watercourses in appropriate places, whether its plate weirs or woody debris dams. These type of interventions are just some of the ideas we will be implementing beginning soon in Hardcastle Crags with the National Trust, just one of our projects underway with Slow the Flow Calderdale.
If you want to get involved then please get in touch. In fact when I think of it, the “Pound for pound” blog has brought us quite some way since the dark days of January 2016.
Stuart Bradshaw BSc(Hons) MSc DIC CEng MIStructE M.ASCE FGS
Since March 2016, our river surveyors have been surveying various rivers in the catchment in the Calder Valley. As a result of all this very hard work, we have now completely surveyed the Hebden Water Catchment including at Nutclough, Crimsworth Dene and Hebden Water itself.
Measurements have been taken every 20 metres on these stretches, recording the channel width, depth, bed type and unusual features along the way. Never before has so much information on our river network in the catchment been recorded in such detail and this will now feed into the river models currently being undertaken in the area. We are all very fortunate to have such beautiful scenery on our doorstep and it has been a great way of spending weekends exploring parts of the river network normally unseen and unexplored.
One of our newest teams is now embarking on a new survey in Walsden Water taking in information from all the tributaries feeding into this important stretch of water in the upper catchment. Beate Kubitz, who is heading up our new team has written a personal account about their first foray into Walsden Water to survey this stretch of river.
Beate Kubitz writes………………….
‘We decided to survey the upper reaches of Ramsden Clough between two foot bridges. It was easier to survey from the top bridge to the lower one, although we encountered two impassable waterfalls, which meant we had to detour around them, each time accessing them from different directions.
This river section is around 250m long and varies between 2 and 5m wide. Its banks are steep and wooded, with few flood plains although there are opportunities for creating deeper, larger pools at some points.
Figure 2: Typical river view – point 3, 60m below bridge
At about 80m below the upper bridge there is a waterfall (impassable today). There is definitely some drainage entering the stream from the left hand bank. Above this point is a depression which could be a holding area, although the drainage doesn’t currently appear to feed it.
Figure 3: Above point 5, 80m below bridge
Figure 4: View of point 5 (80m below bridge) from above
There is one rocky, steep to almost vertical, tributary at about 185m below the upper bridge. A large pool has formed where it joins the main stream.
Figure 5: View of waterfall into pool, points 10 and 11 85m below bridge
Figure 6: Steep tributary entering pool at point 10/11 85m below bridge
Apart from these there are multiple drains and smaller inlets which gather water into the stream (an example photographed at point 3, 60m below the upper bridge). The riverbed is either stony or slabbed with large sections of bedrock. This probably contributes to its fast flow. There are a few waterfalls over natural stone weirs in the upper section.
Figure 7: Typical mix of bedrock and stones found along this stretch
The last waterfall in this section is of manmade construction and (at 3-4m) higher than the natural falls upstream. It is crossed by a wooden footbridge, which clears the riverbed by 0.7m at the central point.’
Slow The Flow Calderdale would like to say a huge thank you to Beate and her team for their efforts at Walsden. Not only have they managed to record a huge amount of information, but they have been to stretches of water not normally visited by members of the public to gain this data and information.
The information recorded in the upper catchment will be hugely important in analysing the amount of water which is then discharged into the river network which in turn has a significant impact all the way through to Brighouse and beyond.
More information on our River Surveys and the results collected can be viewed on our River Survey pages.
If you would like to help Beate and her team, please contact us for information.
‘Are We Planning To Flood?’
Recently Slow The Flow: Calderdale attended The National Flood Forum Conference 2017.
The conference was a very useful day, with a variety of delegates, including The Environment Agency, The Water Authorities, Community Flood Groups, Academics, Politicians, Councils, Lead Local Flood Authorities (LLFA), Insurers, Construction Industry Professionals, and Reporters.
Throughout the sessions, there was a helpful dialogue around flooding in relation to the planning system from all of these different perspectives. As ever, it was reassuring to hear from other community flood groups dealing with some of the same issues that we face.
Key Themes and Learnings
I will stick largely to reporting on Slow The Flow: Calderdale’s key areas of interest (i.e. Inland NFM, and how community groups can work effectively with other organizations) so as to try and keep it to a blog post, rather than an essay! There were also several other points of interest, such as FloodRe insurance, Mental Health following flooding, tidal surges, climate change evidence, vulnerable communities, and household resilience measures.
Sustainable Drainage Systems (SuDS) in urban areas
Bob Haddon from Shifnal Flood Partnership Group (SFPG) discussed the need for developers to be ‘more than just builders’. Shifnal has a great deal of planned new build development, and the community has been forced to consider the issue of what significant amounts of rainwater run-off from new hard surfaces would do to flooding. SFPG are strong advocates of urban Sustainable Drainage Systems (SuDS) and are working hard to ensure that “when developers are granted permission for building, they are responsible for the provision of adequate attenuation systems, which also provide betterment to the existing flash flood potential”.
Phiala Mehring from the Loddon Valley Residents Association (LVRA) echoed this, with her description of the ‘Hatch Farm’ case study – an intended building site on a flood plain, in an area that already floods. LVRA are working hard to get consideration of SuDS written in to outline planning conditions (rather than as reserved matters, which is where they currently tend to sit, if anywhere).
Other pearls of wisdom for other community flood groups, from Shifnal, Loddon Valley, and others, included:
- Studying historical maps is important.
- Knowing your catchment area (particularly walking it in the rain!) is enormously helpful.
- Think long term (longer than that… 30 yrs is not long term, try 150+)
- What the local fishermen don’t know about their river is not worth knowing – talk to them.
- Endeavour to become consultees on planning applications – it is much easier to have an influence before applications are approved.
- 75% of Local Plans have no mention of managing carbon. Neighbourhood Plans can make a huge difference.
- ‘1 in 100’ flood statistics are not helpful. Plan for the worst case scenario.
The importance of retrofitting SuDS
Sue Illman (Landscape Architect, and CIC Champion for Flood Mitigation and Resilience) was also eloquent on the problems caused by development without a SuDS strategy. She emphasised the need to retrofit SuDS to existing developments where possible.
There will be, of course, ongoing development, partly driven by the need to build significant numbers of houses… so yes, we need to integrate SuDS into all new development, to stop things getting worse. However, the situation is already worsening due to climate change.
Recent revisions from the EA now estimate that peak flood levels are likely to be around 25% higher, and could be up to 105% higher, in 100 years’ time.
Therefore, we must retrofit solutions to existing developments, as well as implementing SuDS on all new developments, even in order to maintain existing flooding levels – and certainly if we wish to improve the situation.
As Phillip Harker from the Homes and Communities Agency (HCA) put it:
“We must recognise the consequence of getting this wrong, in order to understand the importance of getting it right”
You Can Slow The Flow!
Ideas for new policies to help
Suggestions from the floor included the idea that development sites should not be allocated a ‘provisional number’ of dwellings until a realistic SuDS plan for the site has been created. Another delegate suggested that developers should be required to produce a site flood plan at outline planning stage, and that it should be binding. Too often the drainage strategy for outline permissions is allowed to be ‘watered down’ (forgive the unintentional pun) as the project moves on into detailed design phase. Too much deviation from the original plans can have taken place, by the time that projects are built, for the original sustainable drainage concepts to work effectively.
Collaboration between community groups and public bodies
Phiala opened her talk with “Like many people in the room, flooding is not my day job”.
It was humbling to think of how many in the full lecture theatre had come voluntarily, because they care for their community, and spend countless hours of their free time trying to help.
It was generally encouraging to hear a number of tales of the good work and collaboration that is going on across the country. Public bodies and community volunteers are both groups that are being stretched. Through working together, we can all alleviate the pressure for each other to some extent. We have knowledge, skills and resources that are different, and can support and complement one another.
Hannah Burgess, speaking from a Lead Local Flood Authority (LLFA) perspective was clear that in order to support each other best, it is not a negative thing to remember that it is a ‘Them’ and ‘Us’ situation from both sides. We all have a role in checking that each other are doing things correctly, and trust is two-way.
Slow The Flow: Calderdale are grateful to our partners in the Calder Valley. We are continuously learning how to work together most effectively, and on the whole can consider our collaborations a success. In this instance, without our friends at the Environment Agency, we would not have known about and attended the conference – thank you!
More information can be found at http://www.nationalfloodforum.org.uk/nff-conference-2/
Written by Amanda McDermott CMLI
Written by Adrian West-Samuel from Moors for the Future Partnership.
The team at the Moors for the Future Partnership (MFFP) undertakes a range of conservation works to reverse more than 200 years of damage caused by industrial pollution and wildfires that left large areas of the Peak District and South Pennine uplands bare of vegetation. I had the good fortune to be involved in Moors for the Future’s dipwell monitoring campaign over a period of a few months towards the end of last year.
Why monitor the water table?
To give some background to the monitoring: there are a number of dipwell sites located around the Peak District and South Pennine upland areas. We use them to compare the impact the conservation works are having on water table levels, water storage and channel peak flow.
MFFP is also examining the effect of its work on flooding. Previous projects including Making Space for Water (One and Two) found that practical stabilisation of degraded moorland can add benefit to reducing flood risk at the same time as delivering other benefits. Work undertaken (including gully blocking and re-vegetation) helped to reduce the impact of flooding downstream by holding water back and increasing the time it takes for rainwater to reach a river during a storm.
For the dipwell campaign, a team of 15 staff, dedicated volunteers and a university placement student checked out the water levels on 10 sites over 11 weeks. In total, more than 9000 measurements were taken across the Peak District and South Pennines every Wednesday. We were out in all weathers, walking across rough terrain on open moorland, without the aid of footpaths or tracks, to each well.
What are dipwells?
The dipwells are 1 metre long tubes, with pre-drilled water access holes, that have been pushed down into the peat – just leaving a small section visible above the ground surface. They are randomly located within a 30 metre square to form small clusters. There are usually a number of clusters positioned within areas of restoration treatment so that comparisons can be made between the different treatment types.
The monitoring itself is fairly low-tech with the essential kit amounting to a plastic pipe and metre long measuring stick. On a dark background, however, black dipwells that only protrude a few centimetres can be difficult to spot! But armed with a GPS and site map the method works well and once located the readings can be completed in a matter of minutes.
The measurements are all taken on the same day to give a snapshot of water levels across a range of sites across the Peak District and South Pennines, spanning from Chatsworth to Skipton. This work will provide invaluable information on the effect of our conservation work, which aims to ‘rewet’ the moors and provide the right conditions for plants like sphagnum moss, which holds a large amount of water as well as being vital to peat formation in active blanket bog. Monitoring takes place on intact peat which has not been damaged by industrial pollution or wildfires, and areas of bare peat as well as areas that have undergone conservation work.
A rewarding team activity
Many of the sites are in remote locations and walking across moorland terrain can be challenging. The weather is also testing and often very changeable. But, as a result, the days out were interesting and always rewarding (that feeling sometimes developed a few hours later when back home, warm and dry!). I got to meet some of the other likeminded people involved and our shared passion meant days in good company. At different stages of their journeys and with different motivation there have been plenty of experiences and career advice to exchange.
I love the change of season and the moors offer plenty of indicators to assist and observe this process throughout the year. Spring brings a selection of birds to the moors as the habitat provides an ideal environment for their nesting sites. Summer sees the chicks fledge and then later the flowering heather that moves the colour palette firmly into the pink/purple spectrum. And then the cooler autumn temperature triggers the coat moult of the iconic moorland species: the mountain hares. It’s a privileged time of year to be out on the moor as it offers an occasional glimpse of hares during their white colour stage.
Bring on next autumn’s dipwell campaign!
Find out more about the Moors for the Future Partnership.
Traditionally flood alleviation has been implemented by civil engineering using “hard” solutions such as higher river walls, levees and tunnelling. These still have a significant role to play in the battle with our elements; however, we may be coming to a point where these traditional methods of controlling the path of flood water are no longer sufficiently adequate. This is particularly so if we as a nation are going to manage the effects of climate change. The following discussion considers as an example a single location in the Calder Valley where such solutions may no longer be quite enough: Caldene Avenue in Mytholmroyd, which experienced serious flooding on Boxing Day 2015. However, by careful implementation of Natural Flood Management interventions our existing infrastructure can be supplemented, even future proofed.
The Scottish Environmental Protection Agency (SEPA) have published an excellent handbook on Natural Flood Management (NFM) which is available here for those wishing to learn more:
A real hydrograph to understand Caldene Avenue variables
Our science page goes a long way to explaining the mechanics of fluvial flooding using a diagrammatical hydrograph. The hydrograph here in Graph 1 illustrates the actual conditions measured at the Caldene Avenue river gauge in Mytholmroyd. The gauge measures the height of the river at 15 minute intervals and the graph shows the 72 hour period beginning 00.00 hours on 25th December 2015.
The second Graph 2 presents river flow rate in cubic metres per second (m3/s or Cumecs) for the same period, the calculations that convert river level to river flow are relatively straightforward and they form the basis of many of the commercially available river modelling software programs.
The approach uses the fixed variables relating to the river channel, width, water depth and channel gradient and a formula first developed by Robert Manning in 1889 (and known as Manning’s equation) as follows:
V = (R2/3So1/2)/n
V = Mean flow velocity (m/s)
R = Area of flow / Wetted perimeter
So = Channel gradient
n = Manning’s roughness coefficient.
Once the flow velocity is calculated the discharge Q, (m3/s) through the channel can be calculated by multiplying the velocity by the area of the flow.
Manning’s roughness coefficient
Manning’s roughness coefficient is a variable that represents the drag on the water from the sides and bottom of the channel. The rougher the surface, the slower the flow, hence a stream containing large cobbles and boulders will have a much higher “n” value than one made from concrete or masonry. Vegetation also plays a role in channel roughness with higher roughness expected in summer than in winter.
As the river channel approaches Caldene Avenue bridge the gradient slackens from around 1 in 155 to around 1 in 890. This slows the velocity, along with the obstruction from the bridge deck which further reduces the channel capacity to convey water.
The channel cross section in this part of the river contains a narrow deeper channel around 5.0 metres wide centrally within a wider channel with an overall width of around 18 metres. In December 2015 the channel shoulders were covered with vegetation, albeit much reduced from the summertime. Nevertheless, these would have been rougher than the central deeper part of the channel which is lined with cobblestones or setts. A photograph taken in April 2009 shows broadly how the channel would have looked in December 2015 below (Plate 1):
Plate 1: April 2009
In the Autumn of 2016 the channel vegetation was removed by the Environment Agency (EA) exposing the setts as can be seen in the following photograph (Plate 2). The EA had identified “the benefits in reducing flood risk through management of channel vegetation” in their Action Plan for reducing flood risk in Mytholmroyd available here:
Don’t we want to SLOW the flow?
Reducing the friction coefficient speeds up the flow. That is desirable at this point in the river system, where riverside homes and businesses are under direct threat from the river overtopping its banks. Higher up the catchment the notion of “slowing the flow”, from which our group takes its name, has the effect of flattening the hydrograph as explained on our science page. Clearly this also is a desirable outcome for flood risk because it lessens the problem needing to be dealt with downstream in towns.
Plate 2: December 2016
Effects of changing the variables
We can see an estimate of the existing channel capacity at Caldene Avenue bridge in Table 1. The only variable to change between the two calculations is Manning’s “n” for the channel shoulders. The bankfull capacities are drawn as a red and a green line on the hydrograph.
|Vegetation Removed (Green)||101|
|Winter Vegetation (Red)||86|
Table 1: Increase in existing channel capacity by vegetation management
Table 2: Increase in channel capacity by raising walls to 1.8 metres above street level and increasing the channel gradient
Table 3: Increase in channel capacity by raising walls to 3.0 metres above street level and leaving the channel gradient as existing
One can see the improvements that have already been made by removal of the vegetation in terms of the increased channel capacity, about a 15% increase is demonstrable, nevertheless there is a way to go to prevent flooding given a similar flooding event as the hydrograph does still break considerably through the green bankfull line. Clearly the intentions, as we already know, are to modify the bridge at Caldene Avenue and we may expect a deepening of the channel here as well to steepen the slackening channel gradient. These measures will increase the channel capacity. However, to remain in bank, the river walls would have to be raised to around 1.8 metres above the current level of Caldene Avenue to cater for a flood of the magnitude experienced on Boxing Day 2015. The further calculations reported at Table 2 demonstrate this, and 1.8 metres compares with the value given by the EA in the Mytholmroyd plan noted above. Leaving the channel gradient the same as currently would require the walls to be raised to 3.0 metres as per Table 3. However, the new bridge deck has to be at a similar level to the current one – as a higher bridge deck would mean traffic could not access it either from Burnley Road or Caldene Avenue. This must be the reason the river channel is to be widened at this point as raising the walls would have to be accompanied with raising of the bridge deck and the latter is not possible.
Cost comparison between hard engineering and NFM
Altering existing infrastructure such as Caldene Avenue bridge is expensive and disruptive. Slowing the flow higher up the catchment to “squash” the hydrograph below the bankfull capacity can be a far cheaper and greener partial solution. NFM is not the panacea, some civil engineering is clearly required – mitigation is the key message, but NFM can be implemented now, it is civil engineering by the people for the people. Think of the waste created by the demolition of this bridge and the adjacent retaining walls, the resources used in the construction of the new bridge and of course what of the carbon footprint to all of this! Not so with leaky woody dams for example, or creation of small attenuation ponds or swales in the fields above the valleys. As the Table 1 calculations illustrate, supplementation of our existing in bank capacity in the main river channels can be enhanced by vegetation management (reducing roughness) at relatively low cost. Further supplementation using NFM (increasing roughness higher up the catchment) can also play a role at significantly lower cost to society than traditional hard engineered approaches can offer. Furthermore, NFM interventions implemented widely can futureproof some of our existing and (even newly built) infrastructure against more serious events than that occurring on Boxing Day 2015.
NFM – part of the ‘suit of armour’
This example serves to illustrate the effects on our existing infrastructure of our current and future flood risk and the role Natural Flood Management can play in the flood risk management armoury. Clearly the EA appear to be softening on the implementation of NFM, given that both the Mytholmroyd and the later Calderdale Catchment Plan identify catchment wide measures including potential Natural Flood Management and upstream storage. However, in my view, the process is too protracted and should be higher up the agenda than it currently is.
Slow The Flow pilot project at National Trust Hardcastle Crags – a first step
It is excellent news that we have a pilot NFM project – but that is all it is, “a pilot”, and currently with limited funding. We cannot simply keep raising river walls, but we can quickly and cheaply implement NFM in our higher catchment. £10m is earmarked for the Mytholmroyd scheme in the aforementioned plan – that could buy a lot of leaky woody debris dams!
©Stuart Bradshaw CEng.