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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)

Calderdale Catchment Plan and its development. Workshop held on Saturday 6th August

Technical workshop actions

Maintenance draft actions

Helping Ourselves (Part 2); The importance of river surveys and a pilot project for Hardcastle Crags

Slow The Flow have applied via Source Partnership for a grant from the Environment Agency to install small structures at up to sixty identified sites in watercourses in Hardcastle Crags. Our engineers have been out looking at the rivers and the surrounding banks to determine the best places to install interventions in the Crags, National Trust owned land. The structures will either be small wooden plate weirs or small leaky woody dams made from logs, their purpose to “Slow the Flow”, our maxim and one that is hoped will reduce the flood peak and help prevent catastrophic flooding to our downstream towns and villages.

Phase 1 of the project will be for small interventions on brooks and ditches with some study work in preparation for Phase 2. Phase 2 programmed for 2017 is for larger leaky woody dams on Hebden Water itself, along with attenuation ponds.
Slow The Flow have also applied for additional grant aid again from the EA for further studies into similar interventions in Crimsworth Dean on Crimsworth Dean Beck, this is rather more complicated and expected to take longer to come to fruition due to land ownership issues and the fact that the river bed is designated a Site of Special Scientific Interest by Natural England.
It is hoped this project will be a pilot that will shine a spotlight on the Calder Valley and the steps we are taking to help ourselves, at least some of the work will be carried out by volunteers, so if you want to help yourself or your neighbours here is your opportunity, it is hoped work will commence in the Autumn.

In order to secure funding one of the stumbling blocks we have to get over relates to the way the EA is funded and a requirement to demonstrate the benefits of the proposed interventions. This is no easy task, however it is not insurmountable as our colleagues at Pickering can vouch for. We believe we can make a case for these works by a combination of river level monitoring and river (computer) modelling. Currently the EA have only one river gauge on Hebden Water, below Valley Road bridge, a much better picture of the river in flood would be attained by introducing further gauges on structures upstream of Valley Road. This can be done relatively cheaply using these types of gauges which cost around £300 each.

River modelling was one of the first objectives of Slow The Flow and for good reason. The Pickering project came about by effective river modelling to substantiate the benefits of introducing interventions into stream flows, they were lucky in some respects because the Pickering Beck was the subject of a Defra funded project so their modelling (or at least the initial phases) was funded and it was relatively straightforward to use these models to make an adequate case . At the present juncture we are being asked to substantiate benefits without any funding in place, a classic chicken and egg situation.

We will have to apply some pressure if we are to make any headway, this is ongoing but in the meantime, the data that we will eventually need for this exercise still needs to be collated. We have a small team of river surveyors but we need more hands on deck if we are to obtain the geometric data in timely fashion that is needed to compile the river models. So here again we return to the “Helping ourselves” principle that I wrote about in Part 1 of this blog.
Why have the EA not surveyed the rivers sufficiently, surely they must have done this for their current river models?

The EA’s remit is for the main river, the Calder, the tributaries fall under the responsibility of the Lead Local Flood Authority, Calderdale MBC. The EA have geometric survey data only for the Calder and for a short stretch of each tributary. The modelling we are proposing introduces out of channel flow caused by introducing interventions in the stream flow, so the channel depths, widths and the channel roughness play a significant role in the modelling process. A rougher channel with say cobbles and boulders in the stream bed slows the water as opposed for example to a concrete lined channel. It is this data which can only be obtained by fieldwork that is central to our case.

If you can help the cause then please get in touch via our website.

Stuart Bradshaw C.Eng.

New technology has helped flood modelling ?

Traditionally models have used gauging information from gauging stations managed by the Environment Agency. New technology is increasingly being applied. LiDAR ( Light Detection and Ranging) derived from satellites is particularly useful to derive Digital Elevation Models [ DEM] coupled with GIS ( Geographical Information System) they can be used for to test flood models. Mobile terrestrial LiDAR  mounted on a vehicle can capture georeferenced data of features in a floodplain including vegetation, barriers, structures and even kerb heights.

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Yes but are these models accurate ?

Models can be tested against real events such as the Boxing Day Floods of 2015.

The growth of digital media in the public realm ( CCTV) or through social media allows the calibration of models with real events, particularly useful in urban areas where the combination of infrastructure below ground adds to the complexity of flood events.

Anecdotal and historical data for example, using historical flood data from County Records Offices is really useful .

Citizen-generated data such as the survey work by the Calder Catchment Flood Studies Network helps to calibrate these models.

For example, in Oxford, the Oxford Flood Network has affixed their own sensors to structures on inundated flood plains. This is not just about generating more data, this project helps wider initiatives to communicate flood risk.

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http://oxfloodnet.co.uk/

Robin Gray CMLI 

Development Manager 

South Pennines Local Nature Partnership 

Pennine Prospects 

 www.watershedlandscape.co.uk

 

Calderdale Catchment Plan and its development. The latest workshop held on Saturday 2nd July

We’d like to keep you up to date with the Calderdale Catchment Plan and its development. The latest workshop held on Saturday 2nd July in Halifax focusing on Governance arrangements. Please find attached the notes written up from the work on the day that will directly inform how the Catchment Plan process is run and provide an agenda for the first meetings of the different Catchment Plan working strands. They are Resilience taking place on the 9th July at Hebden Bridge town hall, Natural Flood Management, Strengthening defences, Using existing infrastructure taking place on the 16th July at Hebden Bridge Town hall and maintenance taking place on the 23rd July at Hebden Bridge Town Hall..  

Following the initial work strand meetings, these working groups will continue to invite people to get involved. If you are interested in getting involved please email avril.south@environment-agency.gov.uk to register. 

We look forward to working with you:

Notes from governance group

Governance group decisions

CCP revised principles

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