Slow the Flow Calderdale

A scientific approach to natural flood management

Are We Planning To Flood? – National Flood Forum Conference 2017

 

‘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!

This all resonates with our ‘You Can Slow The Flow’ project, which you can find out more about, and join in with, here.

 

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

Water Table Monitoring – Autumn 2016

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.

Citizen Science

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.

What role can Natural Flood Management play as part of a solution to the flooding problems in the Calder Valley?

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:

http://www.sepa.org.uk/media/163560/sepa-natural-flood-management-handbook1.pdf

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

Where:

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:

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/525893/Mytholmroyd_Action_Plan_FINAL.pdf

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.

 

Case Q (m3/s)
Vegetation Removed (Green) 101
Winter Vegetation (Red) 86

Table 1: Increase in existing channel capacity by vegetation management

Case Q (m3/s)
Vegetation Removed 271
Winter Vegetation 232

Table 2: Increase in channel capacity by raising walls to 1.8 metres above street level and increasing the channel gradient

Case Q (m3/s)
Vegetation Removed 279
Winter Vegetation 242

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.

New swale on Tipside in Todmorden

One of our ‘Source’ partners, Penny Bennett, has written about swale being used on Tipside in Todmorden.  For more information on our partners at the ‘SOURCE’, please see this LINK.

TRIG ( Tomorden Riverside Improvement Group) have been working on Tipside in Todmorden to protect and manage a biodiverse area of open space in the town centre for people and wildlife since 1998.

Tipside is already a flood zone, lying immediately alongside the Calder, it has bunds at either end, installed by the Environment Agency as part of the first stage of the Todmorden flood alleviation works. This allows the river to escape its channel and flood over the open space in periods of extreme rainfall.

Our latest project has been to construct a shallow swale, a saucer shaped scrape about a foot deep, on a particularly wet bit of the site. We wanted to do this to create a new wetland habitat, reinforcing what we already had, and to prevent water ponding on the existing footpath.  Access is often difficult for the many people that use the site after periods of heavy rain, and the path although tarmac has become very muddy since the Boxing Day floods.  The swale will also function as a temporary water feature, helping to slow the flow by taking some surplus water during wet periods and then drying out during the drier summer months.  Once the water reaches a certain level in the swale, it will make its way to the river via a new land drain under the footpath.

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The site after the Boxing Day floods in January 2015, the footpath is in the foreground

We were successful in getting just under £1800 funding from the People’s Postcode Lottery earlier this year and carried out the works over a couple of days in the last week.

Local contractors Mitchell Excavation hollowed out a shallow scrape, ensuring our existing wetland plants weren’t damaged. We have some notable areas of Yellow loosestrife which is one of the only places in the Upper Valley where it is known to occur, and also Purple loosestrife ( no relation!) which is not common in Calderdale either.

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Excavation works start to create the swale

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The earthworks are completed.

Once the hollow had been created we were able to plant our selection of wetland species. We have used plug and small pot plants from Naturescape, a firm that specialise in native perennial plants.  Plugs are tiny seedlings with a bit of leaf and a lot of root and should grow rapidly in Spring.  Seventeen different species have been used, which are all plants which could be expected to be native in this area, and which will tolerate a range of damp to boggy situations.  Plants like soft rush and reed sweet grass provide the background planting with highlights from other plants such as marsh marigold, yellow flag iris and purple loosestrife.  We wanted to create something which looks good for a long period, for example late flowering meadow sweet is included with May flowering cuckoo flower, and will be good for pollinators and other wildlife.

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The planting starts to take shape with the rushes and other plants

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Finishing off the planting

At the moment, the swale is quite dry, because we haven’t had that much rain recently, we are hoping that it will start to fill a bit over the winter, as this will deter dogs and bikes from exploring the area, and allow the tiny plug plants to establish. We will keep you up to date with how it starts to establish!

Penny Bennett 6th December 2016

How ‘mulching’ can help soil absorb rain water?

Beate Kubitz, one of our new colleagues at Slow The Flow Calderdale has written below about the process of mulching which helps the run off of rain water in a heavy rainfall event.  We thought this might prove useful to us all so please have a read.

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‘We agreed to help our friend Scot mulch the ground around his house. We were staying in Truckee, near Lake Tahoe, in California. The area is dry in the summer, and has deep snow in winter. It’s pretty high and mountainous and the slopes are steep. There’s quite a lot of erosion – and interest in natural management to slow winter snow melts, prevent flooding and preserve the soil.

Our mulching experience was more scientific than I expected. Scot’s friend Michael arrived with a truck load of well rotted mulch and his soil absorption measuring kit. This was a length of pipe with tiny holes drilled in it, like a short sprinkler called a runoff simulator and was equipped with a highly accurate flow meter. We attached it to an outdoor tap. The simulator delivered 4 gallons of water per minute – and the device spread the water along the soil surface in a straight line so you could measure its progress.

The soil in Scot’s garden was super compacted and on a slope. We measured about 6’ from the sprinkler, and set down a marker. It took just 14 seconds from starting the water flow for it to reach the marker. At 4 gallons per minute this means that the ground only took a gallon.

We then spent several hours spreading mulch over the garden to a depth of between 2” and 4”, and tilling it into the ground so that the hard surface was broken up and the mulch was turned into a soil amendment.

Setting up the runoff simulator device for a second time in the same spot, we turned on the water source. The difference was immediately apparent. Instead of flowing straight across the ground, the water soaked into it. We could see its progress, seeping into the air pockets and then trickling over the surface millimetre by millimetre, but after 10 minutes it still hadn’t reached the far marker. By this simple intervention, the soil could now hold over 40 times as much water. Where before a gallon had barely been absorbed by it, it was now holding over 40 gallons securely.

Scot’s friend Michael is a soil scientist who works as a consultant on water sheds in the Tahoe area of California. Amongst his work is ‘helping to keep Lake Tahoe clear’. He explained that he’d managed several soil management interventions to improve its capacity to hold water and stimulate the growth of appropriate vegetation, locking in a virtuous cycle that slowed down water flows and prevented soil and debris washing into the lake.

Although quite different from the Calder Valley, the experiments suggested that we should look at the different water retaining properties of land in the valley and encourage those that hold water above those that do not absorb it.’

More info can be found here – http://ierstahoe.com

Beate Kubitz

Now over 1,500 photos on the river survey map!

Our dedicated volunteers have been working hard to collect the data that informs our work, researching the best places to implement Natural Flood Management (NFM) solutions in the upper Calder Valley.  Over the last 9 months, they have spent an estimated 1000 hours collecting data at a staggering 1,500 locations along the Calder catchment’s watercourses, taking photographs, and carefully measuring data about channel width and depth to inform our flood modelling work.

We have collected the photographic survey into a Google Earth map, available at: http://slowtheflow.net/river-surveys

The Google Earth photo survey  offers a useful reference, and an insight into just how much of the catchment they have covered so far – it is impressive!

There is still plenty to do, though, and soon we will start building 'log jam' leaky dams – if you would like to join our team, please see our ‘volunteers’ page: http://slowtheflow.net/volunteers/

Can you help ‘Slowtheflow’?

Slow The Flow: Calderdale is working to promote the idea of Natural Flood Management (NFM) in the Calder Valley and we are very proud to say that we have had a terrific amount of interest in our website, our Facebook page and Twitter.

Our objectives are simple: to reduce the likelihood of significant flooding events in our towns and villages along the Calder Valley.

After only a few months, we now have over a dozen volunteer river surveyors who are carrying out the first phase of our work by measuring the river network from Walsden and Cornholme all the way down the valley. The data from these surveys will identify where the best locations are to put interventions such as leaky dams and attenuation ponds.

This is just the first step to understand why and how the valley floods. Of course, flooding is exacerbated by and strongly linked to climate change, but is also affected by the significant man-made changes to how we manage land from the top of the catchment right through to the estuaries which take the water into the sea.

The next phase will be to determine where interventions need to go to ‘Slow The Flow’ using the flood modelling which we are contributing to. If you have land or know where these interventions might ‘Slow The Flow’, please let us know. We are also looking for land owners who are interested in NFM and who may want to work with us to help manage the water flow off the hills.

img_0635This is no small task and one which will take many years to achieve but if we want to avoid repeats of over 400 years of flooding in the valley, we simply have to start somewhere.

If you would like to help, please get in touch with us via Facebook on our contacts page.

After the flood : The National Flood Resilience Review

Extreme flood events have been etched into the public consciousness since the Book of Genesis and stories of Noah. These dramatic events  have impacted communities and their legacy across the generations. The River Calder has a very long history of notable floods: 1615, 1673, 1722, 1775, 1866, 1891, 1901, 1920, 1935, 1938, 1944, 1945, 1946, 1947, 1962, 1965, 1967, 1975, 1978, 1982 (June, Aug, Dec), 1986, 1989, 1990, 1991, 1992, 1995, 2000, 2006, 2008, 2012 (June, July), and 2015 (Nov, Dec). These historical events can help us to predict future events. Each event helps us calibrate, validate and verify statistical and numerical models to help predict future events.

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St Elizabeth Flood, Netherlands, 1421 caused between 2,000 and 10,000 casualties.

Boundary markers such as this one outside Moyles can help calibrate or verify models.

Boundary markers such as this one outside Moyles can help calibrate or verify models.

But what of the future? We cannot predict exactly where and when the next extreme event will happen. Will it be across the whole catchment, or confined to a sub catchment?  The Government has just published its National Flood Resilience Review in response to the events of December, and which was ordered in part to ‘provide a ‘stress test’ of our nation’s resilience to flooding, so improving our understanding of the possible implications of extreme events. In doing this we will also review whether the assumptions in current modelling are still sound’.

 

The results of this stress test for the Calder Valley can be found here.

They make for sober reading:

‘The Met Office based rainfall predictions on recently recorded extreme events, and added substantial but plausible additional uplifts, of between 20% and 30% for each of the six standard climatological regions of England and Wales, determined from modelling and analysis of monthly rainfall records for these regions. The Met Office has a 90% confidence that monthly rainfall in any of the six regions will not exceed these modelled levels at any time over the next ten years’.

‘The difference in flood extent between the December 2015 extent and the stress test extreme rainfall scenario is an increase of 31% (0.8ha) for Hebden Bridge and 43% (1.7ha) for Mytholmroyd, reflecting the shape of the flood plain in this location. Under this scenario up to 400 more properties would be flooded in the Mytholmroyd area and a similar number in Hebden Bridge’.

BUT …and it is an important but, it should be noted: ‘Even with this increase in flood depth, the modelled stress test flood levels remain 0.15 to 0.95 m below those in the published Environment Agency Extreme Flood Outlines. Consequently, the areas that would be affected by this plausible extreme rainfall scenario in Hebden Bridge and Mytholmroyd are likely to be within existing areas known to be at flood risk’.

Bearing in mind the predictions of the Intergovernmental Panel on Climate Change, the coefficients used in modelling which were revised post-2007 will be further refined and increased with more accurate weather forecasting based on each catchment allowing early warning of these events into the future.

Sphagnum on Walshaw Moor Study

Summary Short Study:

“A modelling study and investigation into how annual burning on the Walshaw Moor estate may affect high river flows in Hebden Bridge.”

Date: first delivered 24th June, 2016; this version, amended and partly extended, delivered 21st July, 2016.

Author: Dr Nicholas A. Odoni, Honorary Fellow, Department of Geography, Durham University

For (research client): Treesponsibility, 10 Broughton St, HEBDEN BRIDGE, W. Yorkshire, HX7 8JY

Introductory notes/cover points:

(i) in this summary, “WME” means ‘Walshaw Moor Estate’, and likewise “HB”, Hebden Bridge;

ii) further details of assumptions, model set up, results, e are available on request.

AIM: simulate the effects of annual patch burning on the WME; use OVERFLOW1 to generate flow hydrographs at HB; compare peak flows under different burn cases with the base case (control) peak flow; from the results assess whether burns are likely to raise or lower peak flows, and by how much.

MODEL SET UP: use a 50 m source DEM of the whole HB catchment, with the channel network and geometry inferred therefrom; devise a base case ‘Manning map’ – the ‘grass- heather’ case – for the whole of the catchment, comprising a mix of cotton and moor grass species and heather; trees and woodland ignored (omitted) from the modelling; reservoirs assumed to be storage-neutral and to allow unimpeded through passage of water; channels all assumed to be unimpeded and allow free flowing, open passage of water; grips, ditches and drains ignored; bankside areas all assumed to be unimpeded and free flowing; use Natural England images to derive and map the approximate outline and extent of the WME for implementation in the model and simulations.

MAIN RAINFALL-RUNOFF SCENARIO: run the numerical experiment (simulations) as an uncalibrated model application, using a hypothetical rainfall-runoff scenario (no observed data available at the time the work was conducted and during first writing of this Summary); rainfall is assumed as a steady, wet day, based on a multiple of the observed rain at Pickering, North Yorkshire, on Christmas and Boxing Day, 2015; total applied rainfall c. 73 mm over the first 24 hours, and a total c. 82 mm over 29 hrs; prior ground condition assumed to be thoroughly wet following weeks of rainy weather.

SIMULATING ‘BURN’ CASES: Manning’s ‘n’ relationships for burnt ground derived using data in Holden et al. (2008)2; segment hillslope3 burns mapped according to the geomorphology of the catchment and structure of the stream network; patch burns mapped as random patches of cells within larger, 250 m x 250 m blocks, these in turn randomly sited in the WME; simulate 1st – burns of each individual segment hillslope; 2nd – burns of 2%, 4%,6% and 8% of the area of the WME; 3rd – long period burns (10-18 yrs) using the same annual percentage burns as before, with burn effect periods of 4, 8 and 12 years, the burn effect declining inverse exponentially as the vegetation recovers; assume vegetation recovers to its previous, unburnt condition at the end of the burn effect period; run replicates of simulations to stabilise variance in results; assume that no over-burning4 is allowed; assume that no burning is allowed in any cells immediately next to streams or water bodies; assume that no burning occurs on any part of the catchment of the Hebden Water outside the boundaries of the WME.

MAIN RESULTS (details available on request):

NOTE: the stage heights are estimates only, being the middle value from a range of stage height calculations at HB based on the supposed hydraulic geometry of the channel there, and ignoring possible backwatering effects during high flows caused by the Hebden Water flowing into the River Calder

  1. Whole segment hillslopes, tested one at a time.

(a) Burns in 63 of 68 individual segment hillslopes increase flow peaks in HB (mean increase is 0.04 cumecs, 0.1 cm; max. increase 0.146 cumecs, 0.4 cm). (b) There is a clear positive correlation between the hillslope area burnt and the increase in the flow peak (R2 of 0.29, p<<0.001). (c)  In the other 5 segment hillslopes, complete burns reduce the peak flow, but the reduction in each case is negligible (<0.003 cumecs, <0.01 cm).

  1. Grouped patch burns, total area of burns ranging from 2%-8% of the area of the WME. (a) All combinations and spatial arrangements of burn patches raise the peak flow in HB (2% burns, mean increase 0.07 cumecs, 0.2 cm; 4% burns, mean increase 0.15 cumecs, 0.4 cm; 6% burns, mean increase 0.22 cumecs, 0.6 cm; and 8% burns, mean increase 0.30 cumecs,0.8 cm).  (b) There is a strong correlation of the total area burned in patches with the increase in the flow peak (R2 of nearly 1.00, p<<0.001).
  1. Grouped patch burns, 2%-8% annual burn areas as above; burn effect periods lasting 4, 8 and 12 years, but with exponential declining effect as the patch vegetation recovers to its prior, unburnt condition; simulations run for 6 years beyond whole burn effect (vegetation recovery) cycle to explore wider long term effects.

(a) The effect of long term burning and burn rotation management at a given annual percentage rate is roughly double that of the same percentage burn area for one year only. This is found for all burn effect and vegetation recovery periods.  For a 2% annual burn area, the mean increase in the flow peak at HB is c. 0.14 cumecs (0.4 cm), 1.95 times the effect of a single 2% burn; for a 4% annual burn area, the equivalent figures are 0.30 cumecs (0.8 cm) and 2.02 times; for 6%, 0.46 cumecs (1.2 cm) and 2.07 times; and for 8%, 0.61 cumecs (1.6 cm) and 2.07 times.  (b) For a given annual percentage burn area, the increase in the flow peak is itself increased by lengthening the burn effect and vegetation recovery time, although the additional effect is c. 1/10th that of applying long term burn rotations.  Together, the annual percentage burn area and burn effect (recovery) account for most of the variance observed in the flow peak increase at HB (adj. R2 of c0.996; for both variables, p<<0.001).

MAIN CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER WORK:

  1. Any arrangement of burn patches on the WME, wherever situated, increases the flow peak at HB.
  1. There is a clear (p<<0.001) positive correlation between the area burnt each year and the increase in the flow peak at HB. Thus, the bigger the annual burn area, the higher the increase in the flow peak is likely to be compared with the base case.  This implies that for the rainfall-runoff scenario modelled here, patch burning on the WME is likely to work in opposition to any measures implemented on the moor to reduce the flood peak at HB.
  1. Long term annual burning at a given percentage rate roughly doubles the increase in the HB flow peak compared with a burn of that percentage area for one year only. This result is to be expected because long term rotation burning will increase the overall area of the WME which is to some extent affected by burning, whether a particular patch has only just been burnt or is in partial recovery of the vegetation.  Depending upon the density of present and previous burning, therefore, the number of patches so affected by burning may range from between about 20% and 100% of the moor’s area.
  1. Longer vegetation recovery times also raise the increase in the peak flow predicted at HB, although the effect is about 1/10th of the burn rotation effect. This implies that provided the vegetation in any patch is able to recover fully from previous burns, the increase in the flow peak at HB caused by burn rotation should broadly stabilise over the longer term.  This raises the question as to whether repeated burns, over a rotation cycle, themselves affect vegetation recovery times.  This is possibly significant if the cycle of burning leads to a change in the species cover of burn patches which have been repeatedly burned over decades or longer, although this aspect of the ecology and hydrology of the moor-peatland system has not been explored here.
  1. Possible further work to consider: repeat the tests using calibrated applications of the model, the calibrations derived from two or more observed rainfall-runoff events; set up the model to apply to the catchment at a finer spatial resolution, preferably 5 m or 10 m, so that grips and drains can also be modelled and their influence included; incorporate a more nearly correct base case land cover and channel geometry, for example including areas of trees or scrub where known, also areas dominated by Sphagnum and bog sympathetic species; also consider incorporating any stream obstructions or local modifications of the flow path or stream geometry. A more detailed study using a model incorporating more complete physics e.g. JFLOW, would also be informative and provide greater physical realism over a wider range of different rainfall-runoff scenarios and prior wetness conditions.  Such a model could also possibly incorporate a treatment of reservoir storage and discharges that is more realistic than the ‘storage-neutral’ treatment used here.

REFERENCES AND GLOSSARY

  1. Odoni NA and Lane SN, 2010. Assessment of the Impact of Upstream Land Management Measures on Flood Flows in Pickering Beck using OVERFLOW.  Report for Forest Research as part of “Slowing the Flow at Pickering and Sinnington Project”.
  2. Holden J, Kirkby MJ, Lane SN, Milledge DG, Brookes CJ, Holden V, and McDonald AT.
  3. Overland flow velocity and roughness properties in peatlands. Water Resources Research, 44, WO6415, doi: 10.1029/2007WR006052, 2008.  11 pages.
  4. “Segment hillslope”: every reach in the network receives water from both the upstream reaches flowing into it and the hillslopes adjacent to it, on either side of the channel. The latter are termed ‘segment hillslopes’.  The area of segment hillslopes varies from one reach to another, owing to the variations in the geomorphology of the landscape and the connectivity of the stream network.  Odoni and Lane (2010), referenced above, provides more explanation.
  5. “Over-burning”: where patches are burnt in the long period scenarios (burn rotations), it is assumed that the vegetation cover on the patch recovers over a period of years after the burn. Burning is generally assumed to occur only on patches that have recovered completely from prior burning, so that the vegetation and ground cover have regained the same density and composition as they had before the burn occurred. Over-burning therefore refers to those occurrences of burning of a patch of ground before completion of the recovery cycle, so that full recovery of the vegetation to its prior (unburnt) condition has not been achieved.

A Natural Flood Management Pilot Project at Hebden Water and Crimsworth Dean Beck, Hardcastle Crags, Hebden Bridge, West Yorkshire

Further to our meeting on Friday 22nd July 2016 concerning the above matter where I undertook to combine the separate reports for Hebden Water and Crimsworth Dean Beck, herewith please find the report attached with links to photographs and the Google Earth files embedded within both of the attached files.

I have included costs for river level monitoring and river modelling and I have removed the SuDS pilot and the proposal for full restoration of the Crimsworh Dean millponds, leaving in the run-off interception study.  There are further costs included at Section 7.0 which have been prepared by Craig Best at the National Trust, this relates to forest floor restoration and is intrinsically linked to NFM, hence its inclusion.  We have had a very considered look at attenuation volumes as I hope you will appreciate in the time and with the tools available to us.

A lot of unpaid effort has gone into this application so we as group all hope this proposal will be given serious consideration. Slow The Flow  StFC Pilot Project Grant Application Report, Rev.B. 12.11.16(2)

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