Engineered Leaky Woody Dams (WeLD’s) on Rock Stream Beds

Leaky Woody Dams have been used successfully at Pickering and elsewhere to successfully attenuate the flow of water down the stream and river network.  These dams were constructed by spanning logs above and across the stream bed and securing them in the banks with stakes or by using existing trees.  To install successful LWD’s here in the Upper Calder catchment several technical challenges have to be overcome if they are to remain in place during a flood event.

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featured-news-06-1-862x575-862x575Our early work on the best places to install LWD’s concludes that the upper reaches of our river systems are most favourable for slowing the flow if we are to avoid delaying flows to the point where the flood peaks then later coincide with those from other tributaries in the valley bottoms.  However, many of the stream beds in the upper reaches have rock in the stream beds and it is not possible to knock stakes into either the stream bed or the banks.  Conventional ways of drilling rock to install retention systems for LWD’s cannot be carried out practically using normal plant, as even small drilling rigs will struggle to reach many of the places we have in mind for locating LWD’s.

In Scotland in 2012/13 a new overhead transmission line was constructed from Beauly near Inverness to Denny north of Glasgow, 600 pylon towers were erected many of them in inaccessible places on mountain tops and in remote valleys.  A substantial portion of the final contract value was spent constructing temporary access roads for plant which at additional cost were later reinstated back to their natural state.  We can avoid these pitfalls by engineering LWD’s so that they can be constructed with handheld equipment that can be transported by quad bike and trailer and even wheelbarrow.

A LWD is effectively a tier of logs (the sketch shows three, this could vary).

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The LWD is secured in position on a rock stream bed by using steel dowels  installed by coring (rather than drilling) the rock with a small coring rig which typically are used for coring masonry or concrete and are easily transportable.

 

 

The dowels are sized and spaced to carry the bending and shear forces from the stream flow and the forces transferred into the rock in shear and tension.  Cement grout is ideal for securing the dowels, made from Ordinary Portland Cement and water mixed at site in a barrel using a simple drill powered paddle mixer.  In river the holes are cored by working from wooden baulks placed in the river bed and the coring rig fixed to them, or hand held coring rigs can be used.

The grouting operation uses a short length of plastic tubing or casing that fits snugly into the cored hole, the bar is placed with centraliser spacers in the centre of the cored hole.  Grout is poured into the casing through a short plastic pipe introduced to the base of the cored hole (a tremie), this displaces the water out of the rock socket and up through the top of the casing.  Grouting ceases just before reaching the casing brim.  Once the grout is cured the casing can be removed exposing the dowel or alternatively the plastic casing could be left bonded to the dowel thereby disguising it.  Either way this construction process avoids contaminating the river with grout and is a well proven approach used in bored pile construction below the water table.

These dams are intended to allow the normal river flows to pass underneath along with any fish and attenuate water only in higher flows, with the intention that they also create slower over bank flow, so ideally they will be placed in areas where there are wide, low, flat river banks.  Such interventions are cost effective and can play a significant role in managing downstream flood risk.

By Stuart Bradshaw C.Eng.

Terrain Geotechnical Consultants

 

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PHOTO SHOWS HYDRAULIC-HAND-HELD-DIAMOND-CORE-DRILL. PHOTO CREDIT BRANDON HIR
HOTO SHOWS DIAMOND-DRILLING-RIGS. PHOTO CREDIT BRANDON HIRE
HOTO SHOWS DIAMOND-DRILLING-RIGS. PHOTO CREDIT BRANDON HIRE

Part 2. Engineered Leaky Woody Dams (WeLD’s) on Soft Soil Stream Beds

Leaky Woody Dams have been used successfully at Pickering and elsewhere to successfully attenuate the flow of water down the stream and river network.  These dams were constructed by spanning logs above and across the stream bed and securing them in the banks with stakes or by using existing trees.  To install successful LWD’s here in the Upper Calder catchment several technical challenges have to be overcome if they are to remain in place during a flood event.

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Photos of Leaky Dams

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In the Calder catchment streams are fast flowing in flood events and stream flow forces are consequently greater than perhaps at Pickering, the first blog in this series introduced a method of constructing LWD’s on rock stream beds.  This second blog considers how one might go about securing LWD’s on weak or soft soil banks.

The Calder catchment sits almost entirely on what is commonly known as Millstone Grit, which is a type of sandstone deposited in ancient river deltas some 3.2 million years ago.  Due to the deposition process certain reaches of our rivers flow through weaker sandstones which are susceptible to weathering which decomposes the sandstone rock to a soil, where present in river banks this soil is often water softened and has low in-situ strength.

Our early work on the best places to install LWD’s concludes that the upper reaches of our river systems are most favourable for slowing the flow if we are to avoid delaying flows to the point where the flood peaks then later coincide with those from other tributaries in the valley bottoms.  However, some of the banks adjacent to the stream beds in the higher reaches have a covering of soft soils sitting above the sandstone bedrock and simply knocking in timber stakes may not prove successful or indeed may not achieve the required capacity  A conventional way of installing stakes deeper is to use piling plant and install driven piles in place of timber stakes, however, construction cannot be carried out practically using normal plant as even small mini piling rigs will struggle to reach many of the places we have in mind for locating LWD’s.

In Scotland in 2012/13 a new overhead transmission line was constructed from Beauly near Inverness to Denny north of Glasgow, 600 pylon towers were erected many of them in inaccessible places on mountain tops and in remote valleys.  A substantial portion of the final contract value was spent constructing temporary access roads for plant which at additional cost were later reinstated back to their natural state.  We can avoid these pitfalls by engineering the LWD’s so that they can be constructed with handheld equipment that can be transported by quad bike and trailer and even wheelbarrow.

A LWD is effectively a tier of logs (the sketch shows three, this could vary).

eng-leaky-woody-dams-on-soil-724x1024

LWD is secured in position on soft soil stream banks by using driven tubes installed using a Grundomat.

The tubes are sized and spaced to carry the bending and shear forces from the stream flow and the forces transferred into the soil in shear and tension are then reinforced accordingly with steel dowels.  Cement grout is ideal for filling the tubes, made from Ordinary Portland Cement and water mixed at site in a barrel using a simple drill powered paddle mixer.  Where soft soil banks are present with a rock bed then a combination of Grundomat piles and cored in dowels from the previous blog  is the way to go.

A Grundomat  is a small pneumatic soil displacement hammer driven by compressed air, so the equipment requires a small compressor and a petrol driven generator.  They are typically used for installing underground services without having to excavate trenches but can also be used for light piling.

These dams are intended to allow the normal river flows to pass underneath along with any fish and attenuate water only in higher flows, with the intention that they also create slower over bank flow, so ideally they will be placed in areas where there are wide, low, flat river banks.  Such interventions are cost effective and can play a significant role in managing downstream flood risk.

By Stuart Bradshaw C.Eng.

Terrain Geotechnical Consultants

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grundomat-1-300x225

Flooding – we can’t build ever-higher walls!

The householder stoically pointed to the level, on her living room wall, of this year’s floods – just above head height. Then she indicated the then record flooding level in 2005 – around waist height.

Things are clearly getting worse, and the effects of climate change are living up to the predictions that scientists have been making for at least a decade – principally of more extreme weather events driven by increased warmth in the atmosphere.

If you do what you’ve always done, you’ll get what you’ve always had

Collectively, we – and particularly the highest levels of government – have known for years that this was coming, and have chosen to ignore its potential to tear communities and local economies apart. In much the same way as WW1, ‘The War to end all Wars’, patently failed to end all wars, we appear to have learnt little from the devastating 2007 and 2012 floods.

The 2008 Pitt Review was set up to seek answers, to prevent such events happening again. To some extent it was successful in doing so, resulting in the 2010 Flood and Water Management Act (FWMA), which made specific provision for Sustainable Drainage Systems (SuDS). In particular, Schedule 3 set out to overcome the long-standing barrier to SuDS implementation, that of adoption and maintenance of SuDS systems. It also aimed to make SuDS a mandatory part of the planning process for all new developments.

During the course of the coalition government (2010-2014) Schedule 3 was consulted, delayed and dissipated, until finally, at the start of 2015, it was completely neutered, and any pretence at making meaningful provision for SuDs adoption was dropped. In an effort to abolish any ‘hindrances’ to house building, SuDS would not be required for developments of less than 10 houses, and in any case, only where “reasonably practical”. This is a great get-out clause for unwilling developers, but is inconsistent with the best industry knowledge – that there is no reason why SuDS should not be the norm for developments – see below.

It’s not a one-off…

As we think about Appleby, Carlisle, York and Calderdale – this year’s principal targets – we should remind ourselves of other major flooding events: in Somerset and Southern England during Jan-Feb 2014; Cumbria in 2009; Gloucestershire and South and East Yorkshire in 2007; Cumbria again in 2005; and Boscastle in 2004. In addition there have been many other serious events across the UK, with riverside towns, such as Morpeth and York, often being worst-hit.

Image: Rescue team boat on the street. 2015 Floods in York, corner of Huntington Road and Park Grove (with thanks to resident, Bea Thackeray)
Image: Rescue team boat on the street. 2015 Floods in York, corner of Huntington Road and Park Grove (with thanks to resident, Bea Thackeray)

The government’s standard response is to build ever higher walls and defences. Whilst these are partially effective, they can be, and are being, over-topped. We have to ask, can we keep building ever-higher walls to separate us from our rivers? What if it rains on the ‘wrong side’ of the wall? Are we willing to accept the potentially ugly and divisive effect of flood barriers on our river banks?

What can we do about it?

So what else might we be able to do? Three main things:

SuDS on new developments
Retrofit SuDS
Catchment scale land management

SuDS

One wonders how bad things have to get before the government realises that this year’s economic growth targets cannot outweigh the need to develop sustainably for future generations. SuDS should simply be standard for all new developments.

As the 2015 CIRIA SuDS Manual states,

“Provided that drainage is considered early enough in the outline design of a new development then there is no reason why SuDS should not become the norm everywhere…. Where SuDs are designed to make efficient use of the space available, they can often cost less to implement than underground piped systems “.

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Image: SuDS Basin at Manor Park, Sheffield (with thanks to Roger Nowell at Sheffield City Council)

 

Retrofit SuDS

Whilst it is vital to embed SuDS in all new developments, 80% of the buildings (and therefore their surroundings) which will be in use in 2050, are already built.

If we only fit SuDS to new developments, we can only deal with, at best, 20% of the building stock likely to be in use in 2050 – by which time climate change effects will be far more serious. These effects are already overwhelming existing settlements and their communities. We have to build resilience into our existing building stock, by re-purposing largely redundant open space and verges for short-term, but large-scale, water attenuation during storm events.

Done well, this can also have multiple benefits of creating more beautiful, less mundane, places to live, as well as improving biodiversity. In turn, this provides health benefits, through increased outdoor activity, and improved air and water quality. The list of potential benefits is big enough to fill an article all by itself. (Susdrain ‘why retrofit?’)

Catchment Scale Land Management

When rain falls on the Lake District or the Pennines or the Peak District, and towns downstream flood, there is a limit to the contribution that SuDS can make. The water causing the rivers to overtop has been shed too rapidly from drained moorland and heavily-grazed and trampled fields. Drains and rivers are cleared to encourage rapid runoff, with almost inevitable consequences as the water collects and is channelled through downstream towns.

Water can be held back: the natural drainage principles of SuDS can apply as well to large rural areas as they do to small urban areas. ‘Making Space for Water’ is a long-established phrase, one that was often heard but little acted upon in the UK during the 2000’s; whilst Dutch cities like Nijmegen on the Waal (Rhine) undertook major planning, landscape and engineering works in their ‘Room for the River’  project to deal with what they saw as inevitable – the need to live with greater variation in water flows – both flood and drought.

The ability of trees, and particularly woodland, to hold back storm water and improve infiltration of rain into ground water and aquifers, is well-known. But as a society we have tended to question and play with the theory, rather than getting on and looking at land holistically – as something other than just a food factory.

In the Calder Valley, Treesponsibility  and The Source  have been doing great work and setting a good example for many years. We need to help them and scale up their efforts, and implement Natural Flood Management (NFM) at a catchment scale as standard.

We hope to contribute by using its members’ expertise in the science of flooding combined with their local knowledge, to advise on the most effective locations for NFM interventions in the area.

A Suite of Solutions

There is no question that we need ‘hard’ flood defences to help contain rising waters from rivers and surges from the sea. But the compelling evidence of the importance of ‘soft’ defences, like SuDS, water-sensitive land management and Making Space for Water, has been sidelined for too long.

Soft defences – green infrastructure rather than grey infrastructure – are often highly cost-effective and multi-functional. Preoccupation with easing the way for economic growth is understandable, but is a very selective and dangerously blinkered view of a much larger picture – one in which the effects of climate change will become increasingly hard to ignore.

The monetary cost of flooding (£1.3 billion in insured losses on Boxing Day 2015 – not to mention uninsured losses… ) should be sufficient evidence that we need a new approach, one which contains a suite of solutions including comprehensive catchment-scale SuDS, land management and flood defences. Otherwise, the cost to communities will be far more than just economic.

The stoic three-times flood victim, on the evening news, can only hold out for so long. The implementation of a broader, more effective set of solutions is urgently required and long overdue.

By Bill Blackledge CMLI
Vice Chair of The Landscape Institute’s Technical Committee

Adapted from a post originally published on 6th January 2016 for 2B Landscape Consultancy Ltd

2B ‘do’ Dutch water management…

2B are lucky enough to know an eminent Dutch sustainable water management expert, who also arranges professional tours of projects. Bill had met Marnix de Vriend a couple of years ago at a conference on water management in Hull, so when we went over for the Floriade we emailed him to ask for ideas of interesting places to visit in Holland.  Marnix, with great generousity, arranged for us to go on a whistlestop tour of Holland and some of his projects.

SuDS proven to work at all scales

Our journey focused on sustainable drainage (SuDS) and working with natural processes (WWNP) to manage water effectively. We took in projects of differing scales – from disconnected downpipes in the small village of Beek, to a huge dyke relocation in the city of Nijmegen that will allow space for the River Waal (the Rhine in Germany) to overflow safely in future years. It was a real treat that at each of our locations Marnix arranged for us to meet local people involved in the project.

Beek was a wonderful example of community ownership and pride in the environment. The project mainly comprised the disconnection of downpipes and other surface water sources from the main sewers.  This allows the village to cope with storm events, by providing additional water storage capacity, and by preventing overflow of foul sewers – two of many benefits of sustainable drainage systems.

The residents of Beek have taken action, their homes don’t flood any longer, and they have a more beautiful town, with rills and gravity-fed water features. (Beek is also proud to be in the mountainous area of Holland – some areas are 19m above sea level!)

RILLS ANIMATE THE STREETS OF BEEK AND MANAGE WATER ON THE SURFACE RATHER THAN IN DRAINS
RILLS ANIMATE THE STREETS OF BEEK AND MANAGE WATER ON THE SURFACE RATHER THAN IN DRAINS

Disconnected downpipes outside each house feed into gullies along the side of the road. Every house in the chain had to agree to be involved, or the system as a whole would fail

2B AND FRIENDS WITH A GRAVITY FED SUDS FOUNTAIN IN BEEK
2B AND FRIENDS WITH A GRAVITY FED SUDS FOUNTAIN IN BEEK

We also visited the massive Dyke relocation project on the River Waal (the Rhine in Germany) at Nijmegen, site of the “Bridge Too Far” drama from WW2.  This immensely ambitious project has engaged the entire city to adopt a new strategy.  The plan recognises the need to make space for water, due to an increasing likelihood of future flooding events along the River.  Whilst some homes are being displaced, the plan involves new ways of bringing communities together. It invests in creating better places, which respect the power of the water and enable communities to live successfully alongside the River.

Marnix and his contacts were extremely welcome guides. People who are involved in the projects can explain the problems they have overcome, and the reasons for the decisions they have taken.

The Dutch think we are strange

It was interesting to see how a different country’s politics and priorities mean that their society’s attitude to SuDS is quite different to ours. Water is very much at the front of their minds, and they think we are rather backward when they hear that we are still cleaning up after floods every time, rather than preventing them.  We can’t help but agree!

By Amanda McDermott CMLI 

Adapted from a post originally published on 19th October 2012 for 2B Landscape Consultancy Ltd

2B AND FRIENDS ON THE BOAT MOORED ON THE WAAL IN NIJMEGEN, WHERE VISITORS CAN LEARN ABOUT THEIR HUGE AND AMBITIOUS PROJECTS.
2B AND FRIENDS ON THE BOAT MOORED ON THE WAAL IN NIJMEGEN, WHERE VISITORS CAN LEARN ABOUT THEIR HUGE AND AMBITIOUS PROJECTS.
NIMEGAN WAAL ‘ROOM FOR THE RIVER’ (HTTP://BIT.LY/1YZROAR) PROJECT
NIMEGAN WAAL ‘ROOM FOR THE RIVER’ (HTTP://BIT.LY/1YZROAR) PROJECT