Novel ways with willow in flood management

BRITAIN has suffered from a succession of autumn/winter storms, increasingly etched into the collective memory since 2015 when the Met Office began to name these extreme weather events in alphabetical sequence and order. But are there lessons to be learned from the damage and misery caused which could inform the design, development and application of novel materials and methods to mitigate future flooding risks?

WILLOWS BY THE WATERSIDE

This is exactly what happened in the wake of Storm Desmond, which blew over Britain during 4–5 December 2015. With its eye firmly focused on north-west England, Desmond delivered widespread flooding in Cumbria with the Sandylands area of Kendal as badly affected as any. However, what emerged from the floodwaters were some novel ways with native willow to create living flood barriers.

Native British willow species of the genus Salix and in the plant family Saliaceae have the same core characteristics though differing widely in exact form and structure. They range from tall timber-yielding trees like crack willow (Salix fragilis) and white willow (Salix alba) to small shrubs like common osier, and small to medium-sized trees like grey sallow (Salix cinerea) and great sallow (Salix caprea) in between.

Be that as it may, all grow readily from cuttings and are invariably propagated commercially in this way. Regeneration of willow may also occur from natural vegetative propagation processes including rooting of broken and detached branches and the layering of collapsed stems.

Natural regeneration from seed is common but generally restricted to situations and conditions where the inherently tiny Salix seed fall on damp soil or mud within just days of ripening during the early summer months. As such, willows regenerating in this way are usually found along watercourses and in other moist situations like wet meadows.

Willow trees can tolerate temporary flooding but even willows will not grow where the soil has a permanently high water table. That said, a clear association with water and the waterside means native willows, along with native common alder, have always played an important part in the prevention of soil erosion and river bank collapse and, as such, have assumed an important role in the evolution of water management. Willow is the living water diviner with a solitary willow tree growing in a seemingly dry landscape being a sure sign of an underground water source.

Great sallow regenerating from seed on a wet fallow field.

WILLOW’S WAYS WITH WATER

However, what’s now happening in the Kendal area of Cumbria is an altogether more specific and proactive application of living willow trees and shrubs for flood mitigation. Both classic willows (white and crack) and the sallows are planted to create a series of so-called hydro-hedges to slow down and hold back water and hopefully protect hundreds of homes in the area at risk from flooding.

Planting has been taking place at Birds Park, a disused reservoir, with the aim of holding back the thousands of cubic metres of water which could build up under future storm conditions. These living structures are being created by Cumbria Wildlife Trust, with work carried out by volunteers including Kendal Conservation Volunteers and led by Peter Bullard of Cumbria Wildlife Trust.

The Environment Agency is financing the work from a £2.5 million fund for natural flood management pilot projects in Cumbria. Academic and technical input is provided by Dr Nick Chappell, reader in hydrological processes at Lancaster University, who will assess the effectiveness of the project. Birds Park is owned by United Utilities but the facility has not been used for water supply since the 1970s, evolving into a valuable wildlife area.

Despite the stump being peppered with plugs containing systemic herbicide, this unnecessarily felled great sallow tree on a railway embankment defied the odds to produce new spring (shoots) and survive.

According to Peter Bullard, the structures will vary in length from 50 to 100 metres and will be no more than one metre high, with logs harvested from nearby trees used to create the foundations of the structure. Willow and sallow branches are inserted into the ground along both sides of the basal logs to root rapidly in the damp soil conditions, thus creating living posts and providing anchorage and stability to the whole structure.

Flexible Salix stems are subsequently woven in between these living posts in a manner similar to willow basket making which traditionally used native common osier (Salix viminalis). There is a long history of wooden dams being used in Britain to hold back water, although this project is thought to be a first for the application of living trees rather than timber barricades.

Sallow, with its inherently high capacity to coppice, will prove especially useful in this respect. Sallow coppice stools grow quickly in the first years after cutting, and even more rapidly than hazel, to produce coppice poles of log dimensions which are ideal as foundation components of such structures. In addition, the super-fast regrowth shown by sallow in the years after cutting provides ample substantial rods for rooting along the foundations; and especially the highly flexible first and second year spring (regrowth) for weaving between the rooted uprights.

In commercial forestry sallow was generally regarded as an invasive weed of forests and woodland despite unrivalled credentials in regrowth rates, flexibility of coppice rods and durability in wet and damp conditions exceeding those of hazel and sweet chestnut – traditionally crowned as coppice kings.

Cumbrian sheep farmers were also hit by Storm Desmond, sustaining drowned stock, bridge collapse, fencing destroyed and tonnes of gravel from waterways strewn across pastures. However, Steven Clark’s farm in the village of Braithwaite largely escaped, which the owner puts down to an experimental project he had been persuaded to take part in with a paper mill, which involved planting 28 acres of willow crop on his land. And which appears to have prevented the anticipated flooding from the River Derwent.

Brown lesions of anthracnose disease on willow leaves exacerbated by willow trees’ closeness to water and high humidity.

Owned by the Swedish firm Iggesund Paperboard, which makes premium cardboard packaging for a number of high-profile clients, the manufacturing facility at Workington had recently invested £108 million in a new biofuel boiler to reduce energy costs. They were anxious to divert from fossil fuels and into a sustainable source of biomass for fuel, and with a mind on keeping their carbon footprint as low as possible the company focussed on sourcing locally.

A previous study had shown that willow trees would be the most appropriate energy crop since they grow rapidly (up to six inches extension growth in seven days) and do not have especially deep-seated root systems which could be a problem for farmers trying to grow arable crops nearby.

Steven Clark had planted the willow the previous year and, although not yet ready to harvest, it paid dividends for flood mitigation, slowing down the flow by intercepting rain and impeding the flow of water over ground and in the soil.

Protecting farmers, farmland and livestock against flooding had not figured in Iggesund’s reason for signing up farmers to its ‘grow your income’ scheme when it started in 2013. At that time, the focus was on feeding the biomass plant with a forecast requirement of 600,000 tonnes of biomass every year. The discovery of willow as an effective flood barrier was a collateral benefit and a happy by-product of the project, said Ulf Löfgren, the managing director of the Swedish-owned mill.

Uncut white and crack willow trees and their hybrids will grow into massive and immaculate trees.

PLANTING AND ESTABLISHING

Planting and establishing willow is little different than planting any other crop where planting material is vegetative propagated cuttings. For willow the fields are first cleared of weed growth by spraying with glyphosate herbicide, then ploughed, followed by power harrowing in the autumn with a cover crop added prior to planting the willow cuttings in the following spring or early summer period. Soil should be slightly acidic, which means a pH value of between 6.5 and 7.0.

Willow cuttings of 20 cm are planted in a double-row format so that harvesting machinery can pass over the same tram lines, thus reducing any compaction of the soil. The overall aim is to establish 15,000 willow cuttings per hectare of land. Once established the willow is harvested once every three years, with the working life of a coppiced willow stool expected to be a minimum of 25 years. If the land does flood then harvesting can be rolled over to the following year.

Early-spring flowering sallow provide rich nectar sources for bees and butterflies like the peacock emerging from winter hibernation.

HOW DOES WILLOW MITIGATE FLOODING?

So, how does willow help to manage floodwaters? Willow species have a high capability to increase hydronic roughness, which basically means a willow plantation is exceptionally adept at slowing down the speed with which water flows across the floodplain while trapping sediment, which translates into a significant reduction in the amount of debris carried by the floodwater. Conversely, floodplains which are entirely of closely grazed grassland do not provide much hydronic roughness and therefore exercise little brake on the flow of water.

Such situations clearly require tree species with resilience, which is seen first hand in sallow. The last few years have seen nothing short of a massacre of trees and woodland lining embankments along the country’s railway network, including the widespread removal of very old trees nowhere near the railway track and in no way impacting on the safe running of trains.

A number of very old sallows – Salix caprea, approaching 100 years old – were felled along the East Coast Mainline in my Hertfordshire home town. The stumps of felled trees were peppered with plastic plugs of glyphosate herbicide. This herbicide-delivery system is currently touted as the eco-friendly way of killing tree stumps, although littering the landscape with plastic plugs doesn’t appear to be very eco friendly to me. Be that as it may, these stubborn old sallows failed to die and promptly sent up masses of spring which two years on is approaching two metres tall.

Over-stood grey sallow coppice on a railway embankment.

GREATER BIODIVERSITY BUT SOME PESTS AND DISEASE

However, there are other good reasons for growing willow besides earning from burning. A number of Cumbrian farmers engaged in the Iggesund project have commented on a boom in biodiversity seen in stands of willow as opposed to grassland. Increased frequency of deer, hare, heron and a variety of songbirds is frequently mentioned, and, according to Robert Woods from the Teesside Environmental Trust, willow is capable of attracting no less than 266 species of insects including 173 different moths and 51 different kinds of beetle. Insects such as bees and butterflies coming out of over-winter hibernation may also utilise willow catkins (flowers) as an early-season source of nectar at a time when little else may be available.

On the downside, willow has its own specific insect pest and fungal disease problems. These include the giant willow aphid (Tuberolachnus salignus), one of the largest aphids in the world with a body length coming in at between 5 and 5.8 mm; and a fungal disease called willow anthracnose causing black necrotic spots on the leaves, made worse because the branches of many willow species typically hang down near the soil which, more often than not, is persistently wet or damp.

Over-stood willow coppice at the edge of the Brewery pond at Monken Hadley in North London, cut decades ago and now supporting more than six sturdy stems.

BEAVERS EAGER FOR WILLOWS

The past master of timber-built dams is the native Eurasian beaver (Castor fiber) which has been coppicing willow trees by the waterside for far longer than human beings. This native mammal was made extinct across England and Wales by the 12th/13th century but hung on in Scotland until the 16th century. The Eurasian beaver is now making a significant comeback in the UK.

Willows feature prominently in the tree species felled by beavers to construct dams and create deep and slow-moving pools of water, while young willow regrowth features prominently in the beaver diet. Willow benefits from the beavers’ gnawing, cutting and dam-construction activities because willow stems placed by beavers within their dams continue to grow, thus creating a natural, self-reinforcing construction material.

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