Attenuation Research

This pilot study, presented initially to government and industry at CIWEM’s   Catchment-2011   UK conference,  explores the use of dense vegetation at field boundaries to restrict flow and thereby create incremental natural porous  ‘green reservoirs’ within the floodplain which attenuate overbank discharge.  Flood-risk reduction is primarily flood-peak reduction. The work indicates that simple and sustainable flood water management techniques can offer significantly increased floodplain storage with low impact, which in turn can provide a real reduction in flood risk. Ongoing,  focus is extended to the  Multiple Benefits  of High Friction floodplains in the Camlad sub-catchment approached through a SCIMAP framework.

  1. Whilst flood resilience is expedient to manage the legacy built environment, it is essentially reactive. Attenuation is a pro-active tool of human endeavour.
  2. A report on the research carried out to date (Experimental modelling of exceedance flows)    is here . . .
  3. Evaluation of the technique in terms of funding and the pragmatic demand from community agencies for Multiple Benefits is here. In particular the function of porous structures (robust thick hedgerows) as agricultural buffer strips and as corridors of biodiversity. Current pilot schemes are investigated   . . . .  more
  4. The challenge of modelling of these green reservoirs in real-time during real flood events using high-speed processors to provide evidence for effective communication to stakeholders    . . . .   more 

Successful analysis of flood waters requires an understanding not only of the physical and practical elements of catchment management – but also of the human contribution of upstream landowners for the benefit of vulnerable downstream communities.

Beyond this there is potential for pilot studies on the ground to quantify multiple benefit from enhanced hydraulic agricultural and amenity buffer strips ( more . . .)

Graphic by  from an original sketch
Keywords: Flooding, Exceedance, Low-cost, Wellbeing, Green Reservoirs, Multiple Benefit, Mitigation, Attenuation, 2D modelling, Riparian, Sustainable, Biomass, Communication, Transparency, Local, Hydraulic-Friction, WFD_water_quality_DWPA, Pollution, Biodiversity
Link keywords: BBC state of water – Woods & McAll
(Header images are geneated at random – click  here  for  header captions)
  Support;  Ecosystem‘;   Action;   Scale;   Ponds;   Reservoirs;   P.O.S.T.;   Notes
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Hybrid grass ‘could reduce flooding impact’

Mark Kinver,  BBC News Environment reporter, writes:-

“A hybrid farmland grass, developed by a team of UK researchers, could help reduce flooding, a study has shown. A team of plant and soil scientists said tests showed the new cultivar reduced run-off by 51%, compared with a variety widely used to feed livestock. They added that rapid growth and well developed root systems meant that more moisture was retained within the soil rather than running into river systems.

Slowing the flow to reduce peak flows

High Friction Grass – photo courtesy Dr Kit McCleod, James Hutton Institute

The novel grass is a hybrid of perennial ryegrass (Lollium perenne) – which is widely planted by farmers for grazing livestock – and meadow fescue (Festuca pratensis), which has environmental stress-resistant characteristics.

Co-author Kit Macleod, senior research scientist at the James Hutton Institute based in Aberdeen, said a long-term project had been developing novel forage grasses but their environmental benefits had not really been tested.”

More on greener grass (courtesy BBC online)

The findings appear in the journal Scientific Reports.


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Defence, Resilience and ATTENUATION . . .

” . . .   abide these three, but the greatest of these (for the wellbeing of communities) is ATTENUATION . . .”  (with apologies to 1 Corinthians 13:13)
Stakeholders in flood prevention weave diffuse threads of support and wellbeing for communities

Policy and governance become more robust as they draw together such diverse threads,  ensuring as they do so that rivers are valued and managed as conveyors of a clean critical natural asset (capable of multiple benefit in both drought and flood), not despised as drainage channels of the commercially developed  environment.

                       Polemic     Technical    Archive   ATREPO
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Innovation, Flood Risk and PR14

were on the menu of a stimulating and well attended Environment Agency seminar  on Wednesday 12th December 2012, in London.  Disarmingly,  this wide-ranging UK public service is inviting innovation in five particular fields,  namely:-

      1. The joining-up of  catchment risk  to avoid splitting the care of WATER_ASSET from WATER_LIABILITY.  However
            • Water-the-asset is in private hands , whilst
            • Water-the-liability remains a public burden perhaps less concerned with innovation
      2. The scarcity of water (ie ‘private’ solution of drought and quality risk)
      3. Regulation (with talk of obligations on ‘ sustainable’ management)
      4. The worsening exceedance stats of natural water (relating to FRM and insurance thresholds)
      5. The joining up of catchment resilience

Innovative attenuation numerical modelling suggests that rural ‘Glastir-style’ (ie funded) buffer strips with added hydraulic function can join up  ii.   and  1v.   to achieve the goals set out in   i.  (above)

  High Friction Flood Plains,  which increase green reservoir capacity, offer  multiple benefits without lateral impact, and at low cost !!

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Evidence or Action ?

Water conferences – there was an excellent one recently on ‘Ecosystem’ service – promote both evidence from stakeholders and action from operating authorities. Sometimes, often for reasons of cost, these two worthy goals conflict.  For the supply and control of water within catchment ecosystems, challenging questions arise.  So, what would be best for community resilience:-

  1. More guidance ? . . .  Y:
    • To facilitate automated online assessment of ‘Present-Value’ and ‘Cost-Benefit’ to aid submission to funding authorities – for example, methodical computation of the ‘Eco-Value’ of high-friction floodplains (hedgerows and woodlands) in respect of biodiversity, pollution, amenity, infestation, morphology and flooding;  ie to quantify the multiple benefit potential of rural ‘sponges/filters’ which surround urban vulnerability.
  2. More evidence ? . . .  Y:
    • But primarily to join-up existing evidence, plans (RBMP, CMP)  and directives (WFD, soil, floods)
  3. More actors ? . . . Y:
    • River Trusts active in all catchments funded by PES beneficiaries to broker buyers and sellers in the Ecosystem market, and to facilitate delivery of Ecosystem good practice
    • Water supply industry connection in all catchments
    • Insurance industry representation of urban value-at-risk
  4. More sectors ? . . .  N:
    • Just joined up thinking covering both water supply/quality (ie asset issues) and conveyance/control (ie liability issues) . . .  for example, the role of the Vyrnwy water supply reservoir in Flood Risk Management.

Answers given above reflect a public-interest flood-prevention perspective.  Diffuse solutions favour adaptive action over lugubrious evidence gathering.

(For the 2008-2011 numerical research, data, reports, etc  click  here .)

Keywords: Ecosystem, PES, Trusts,

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Ecosystem Landuse and Nature

  “We are not creatures of circumstance; we are creators of circumstance” 

  • In developed countries,  landuse is managed – has been usually for centuries – to yield productive agriculture from wilderness scrub.
  • Managed  features can be soft, benign and sustainable, but can they remain natural ?
  • For the added benefit of, say, an Ecosystem Service, a natural feature will demand proactive design input.
  • Even more so to achieve multiple benefits. Model & design to create wellbeing.
  • Flood attenuation and biodiversity ‘corridors’ are two such benefits/services; when linked, they have particular potential.
  • Thus two support themes compete for flood vulnerable communities
        • Proactive managed ecosystem features to deliver attenuation
        • Reactive FRM ‘fire’-fighting by resilience.

 Above is illustrative of a tributary floodplain in an existing natural state . . .

Here ‘marked-up’ with  porous attenuation ecosystem features (aka dense wide hedgerows) .  Addition of such ‘Eco/NFM’ could be expected to yield increased flood storage in the region of 20% with minimal lateral impact.

The graphic below shows the CFD mesh of a 2D variable friction model of the Upper Severn in Mid Wales – the ‘worms’ being dense wide high friction hedgerows.

Ecosystem Service,  Sustainable, Flood, Attenuation,  Multiple Benefit, NEA_2011
Original quotation attributed to Benjamin Disraeli (1804-1881)
The images above are illustrative only and do not depict precisely a real location.
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Scale and Diversity

of Diffuse Fluvial Attenuation in Natural Flood Management
Reviewing comment at and correspondence relating to the recent PostNote seminar,  the range and  diversity of opinion is surprising.   In fact Flood Risk could come a close second to sovereign debt-management – where economists can present polar opposite risk assessments, never mind defensive strategies – for the attenuation of  boom/bust extreme events !!
Key words/issues arising :-
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Attenuation Super Ponds

This attenuation feature has been in the news.  The following pdf’s relate:-

In the second pdf, the final report, there are 31 instances of the word ‘natural’, 18 of ‘natural process’, 42 of ‘SUDS’, zero of ‘NFM’, zero of ‘Natural Flood Management’, 7 of ‘filter-strips’ as hydrologic features, but with no reference to  filter strips as hydrodynamic tools.

As an alternative to inundation,  flood defence  ‘ponds  engineered dry will be agriculturally productive – at least until the overspill of a 30-40RP uninsurable loss-event – with downstream low-lying areas defended in the meantime by diffuse hydrodynamic green reservoirs (harnessing the NFM capacity of the upstream floodplain).  Attenuation therefore,  ‘point’ or ‘diffuse’, presents options and  useful benefits to operating authorities, but, in general terms, the more diffuse, the less the impact.

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Green Reservoirs

 The concept of ‘green’ flood storage featured in both technical presentations at the POST Seminar  (House of Commons 17th January 2012).

 It would seem to be a key message to the Select Committee.  Green solutions are also a theme of CIWEM 2012 (20Mar12).  Natural Flood Management (NFM) by ‘green’ porous reservoirs demands multiple intermittent engineered features at strategic locations to achieve significant cumulative effect without lateral impact

Select Committee chair Anne McIntosh (MP Thirsk/Malton) seemed  a  tad uncertain of the science of NFM.  Yet natural processes do deliver (more . . .).  For sure, uncertainty is a problem, but so is hesitation for vulnerable communities. 


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The Parliamentary Reply . .

. . . from Select Committee Chair, Anne McIntosh MP, Member for Thirsk  . . .

. . .  (to the question posed below at this week’s POST UK seminar) could perhaps best be described as a cautious,  maternal,  albeit slightly negative “We’ll see” .   The Chair sought moral support from colleagues, but few other parliamentarians were present. The negative thinking was that end-users of  ‘water-the-asset’ should not necessarily have to fund directly, even in small part,  natural networks (ie rivers) conveying such communal good .  This NFM commentary can not quite see the logic in this, and would reflect:

  • Natural attenuation yields effective multiple benefit, not least in diffuse incremental filtration of DWPA in the delivery of good water
  • Water is the same medium whether displaying characteristics of an asset or those associated with liability
  • Time is ‘on-side’ in this equation.  A fractional annual financial contribution would accumulate significantly when multiplied by the RP years between loss-carrying flood events.  Even 0.25% pa is defendable (more . . . ).
  • Regulation: For such a ‘fee’, private (water) industry would stimulate, even regulate, public (environmental) provision to the benefit of all.   FRM taxpayer grant would be routed competitively via both private (Water Industry) and public (Environment Agency) institutional bodies. A nice win-win-win.

A supplementary challenge (effectively to NFM scientists) was laid before the group attending Committee room 12.  From the Chair, the question was asked  “Is the evidence in place yet that NFM methods are effective . . . if not why not”.  In broad terms, the scientific reply was that more monitoring was needed.   The Chair and the floor both commented also that statistical uncertainty, connected so closely to scientific approach,  should not paralyze outcomes and compromise delivery (more . . . ).                      Top

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A Parliamentary Question . .

re: Equitable Funding of  Natural Flood Defence within Water Supply 

Does Parliament see a role for the Private Water industry in the funding of sustainable flood defence along rivers which provide essential conveyance of water managed, primarily, for urban users ?   This question is posed (to the Select Committee on Flooding) against the background of similar National Energy  Supply obligations to sustainability through the funding of renewables.

  • This key framework aspect was not highlighted by Pitt; neither is there provision in the 2010 Act.
  • By way of example in Mid Wales, there is little evidence of balanced management of Lake Vyrnwy or of any flood attenuation provision alongside new ‘blue’ pipelines in the floodplain.
  • There is widespread acceptance that capital grant frameworks favour the provision of water industry hard assets rather than the deployment of equivalent natural methods.
  • In broad terms, 5-10% risk is catered for by legacy flood defence structures; 1.5% risk is covered by Insurance; between these two there is gap which undermines the wellbeing of comunities. Happily,  there are some 30 years to fill this gap in FRM – ideal for ‘sustainable’ systems.  (Top)
  1.                           Water  –   an Asset and a Liability 
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Select Committee Seminar

UK Parliamentary Seminar, 17th January 2012: P.O.S.T. Briefing

A meeting to review Natural Flood Management (defined by P.O.S.T. as the alteration, restoration or use of landscape features) systems,  and to inform parliamentarians of this novel way to reduce flood risk. The policy drivers of this approach,  the scientific basis, and the implementation of such inland natural flood management strategies are reviewed here (POSTnote-396).  (Top)

Download the full report

Natural Flood Management (PDF PDF, 4 pages, 321.5 KB) Opens in a new window

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‘Natural’ Ecosystem Attenuation in Action

Upper Severn Overbank Flow shows promise. 

A routine overbank flood (c. 2-3yr RP) occurred on Wednesday 4th January in the Upper Severn reaches. Whilst unremarkable in itself and some 500mm below the critical stage of local embankments, the EA newly streamed hydrographs show clearly the ‘natural’/current attenuation afforded, in particular, by the Munlyn-Buttington reach.

It is notable that the rising ‘limbs’ of the Munlyn and Buttington hydrographs lagged Abermule by approximately 1 hour and 4 hours respectively (0430, 0515, and 0815). However the peaks at Munlyn and Buttington lagged Abermule by approximately 3 hours and 11 hours respectively (1115, 1415, and 2200). Furthermore the durations of the three hydrograph peaks (considered from  ‘rising-inflection-point’ to ‘falling-inflection-point’) were 13 hours, 16 hours and 24 hours respectively.  In general terms, the landscape between Munlyn and Buttington is relatively natural;  it has evolved ad-hoc. Whilst there are infrastructure crossings and some legacy flood structures , it is as yet unengineered in respect of flood-risk.

It is suggested that, if the duration at Buttington had also been 16-17 hours  (ie if the floodplain had been dysfunctional, say canalized), then the peak-stage at Buttington would have been higher ? This needs little argument; it is not uncertain.

By the same token – indeed the reverse of the same coin – if the floodplain were to be ‘soft-engineered’ (ie more functional  and thereby the peak at Buttington of longer, say 30 hour duration)  evidence indicates that Buttington would  be then of lower stage.

The proposal therefore is that Incremental ‘Natural’ Attenuation is capable of reducing downstream risk by extending upstream duration,  and does not cause impact by ‘pushing’/increasing  flood-water-stage elsewhere.  (Top)

1. Two significant tributaries join the Severn between Abermule and Munlyn, but no significant tributaries enter the equation on the Munlyn-Buttington (c. 10km) reach.
2.Cross-sections at Munlyn and Buttington are broadly similar.
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 Ongoing matters of interest and comment
  1. CIWEM & FRM  (20Mar12)
    • Water supply and flood risk – a marriage for 2012 ??
    • Both conveyed to the altar by UK Rivers
    • Environment Agency brief on “The Greening of Flood Risk Management”
  2. NFM Roll-out: Moorland; Farmed-Upland; Floodplains; Infrastructure; SUDS
    • max features for minimum impact
  3. Ecosystem Services
    • Paid ecosystem resource
    • Section 106 offset; PR09; PR14
    • CSF; Catchment Sensitive Farming
  4. Multiple Benefits from rural buffer-strips: a Welsh perspective.
  5. Outcome realization: Uncertainty v. Delivery
  6. 2D v 1D models; convergence v. divergence; anuga
  7. Run-off Attenuation:
    • 1D theory
    • Remote sensing of potential RAF locations; Perfect coursework for geographers
    • Networked research groups
  8. NFF Conference (7th March 2012, SOAS, London)
    • No mention of NFM, RAF’s, SUDS, Green Reservoirs, etc
    • Nor the EA’s “Greening of Flood Risk Management” (above )
    • More to Flood risk than insurance and resilience ??
  9. Point v. Diffuse sources
    • Point-source pollution relatively easy; DWPA by culture and best practice, both more difficultLikewise attenuation:
    • DWAA still on the drawing board
  10. Open source v. proprietary modelling code
    • ownership; flexibility; economy
    • anuga; R2D
  11. Main river as listed conveyance structure as canals – regulated by local trusts briefed to deliver WFD good water by best practice



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