Carolyne Locher meets up with Jon Davis, construction director at PHB Contractors, to discuss the work its Lakes & Ponds subdivision carries out, as well as the difference a pond can make to woodland settings and how they can benefit woodland owners.

2020 has seen the UK’s wettest February, followed by the UK’s driest spring since records began in 1884. The last line written on an information board next to a dew pond in Kingley Vale National Nature Reserve is a wake-up call. Written by the reserve’s manager Steve Walker, it states: “In periods of drought, a pond is literally an oasis in a desert.”

“The dew pond referred to is in the SSSI (Site of Special Scientific Interest) yew woodland 300 metres from the Reserve entrance,” says Jon Davis, construction director at PHB Contractors, responsible for its Lakes & Ponds subdivision. “This pond, 12 metres in diameter and approximately 1.3 metres deep, is surrounded by ancient yew grove and scattered mixed deciduous trees on three sides, with pasture chalk grassland on the fourth.”

Jon is speaking from PHB’s two-acre yard in Albourne, West Sussex, during a hot week towards the end of June. He and his team are continuing to adapt to a new normal following eight weeks of pandemic lockdown and he welcomes the chance to sit still for an hour.

Forestry Journal: Jon Davis.Jon Davis.

PHB’s Lakes & Ponds division has created (and refreshed) dew ponds throughout the South Downs National Park. “We have built three now [at Kingley Vale]. In this instance, it was a case of re-excavating a small hollow dug out 15 to 20 years ago.”

The whole process sounds relatively straightforward. Before works began, PHB was advised on what access they were allowed. “In this particular case, we had to set up base in the car park 1.5 miles away and we could not store anything on site. It took time to get the excavator, travelling at 1.5 mph, down to site. The clay was re-handled by a dumper to get it down there. Once in the reserve, access to site was through a substantial gap between the yews.”

The surrounding trees received a crown lift from PHB’s two full-time arb subcontractors. Then an 8-tonne excavator re-dug a larger hole, setting aside the spoil (soil and indigenous flora) for later reinstatement. To fix the water level, a timber-edged shelf (FSC-approved treated softwood) was built around the hole. All were lined with a sandwich of geotextile, EPDM liner and a layer of clay.

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Jon explains: “The clay protects against puncturing by animal hooves and provides a natural substrate from which the pond can re-wild itself. Feathering spoil out from the pond edge, whatever flora was dormant should self-set and germinate. A client may encourage pond life by introducing a water sample taken from the original pond. I think Steve Walker’s idea was to partially fill the pond to start it off and then to let nature take its course. With two men full-time, and me there for a day overseeing the installation of the liner, it took a week to complete the operation.”

Jon, now 47, has been with PHB for 16 years. Following an HND in rural resource management (from Seal Hayne Agricultural College) Jon worked in horticultural engineering, as a sales agent for John Deere, in golf course construction and maintenance, joining PHB as 360-degree excavator machine operator. Progressing to contracts manager two years later, he became construction director (and shareholder) in 2018.

Forestry Journal: Clearing historic parkland pond.Clearing historic parkland pond.

The company covers three areas of business: ‘Self-Drive Plant Hire’ – hiring out 45 excavators and 35 dump trucks; ‘Engineering & Groundworks’– building foundations and substructures for buildings, bridges, railways and electric vehicle charging points across the country; and ‘Lakes & Ponds’ – creating and desilting static bodies of water, or bodies of water fed by rivers and streams running in one end and out of the other.

PHB’s Lakes & Ponds division works with public bodies, charities and private clients, and their work is evenly split between creation and desilting.

“We construct bodies of water of all sizes, from ponds 10–15 metres across to lakes 2–3 acres in size. Some years we have more desilts, in others two or three really nice construction jobs. Most construction projects are for private clients and we do more when the economy is strong, our work being dependent on the financial climate and who is spending money. Between 2012 and 2016, we had some nice works, many funded through Natural England’s Parkland Restoration Scheme. As the funding was spent and the financial climate changed, these types of contracts became fewer.”

Forestry Journal: Restored woodland pond before refilling.Restored woodland pond before refilling.

With 14 full-time staff and up to 10 sub-contractors, PHB offers a complete turnkey service. “From an initial [paid-for] survey, we give clients the CAD drawings, the calculated volumes of silt and options of where it could go. That is the starting point.

“We use trusted partners to design and draw up the more intricate ponds or lakes. From there, all works, from removing vegetation and initial groundworks, exponential earthworks and earthworks handling, to post-works landscaping, we do in-house. For desilts, some clients, like Kew Gardens Wakehurst Place, perform their own pre-works wildlife surveys, tree removal and post-works landscaping. Ecological legislation, for example the importance of the great crested newt, has intensified in the last 15 years and if there are mitigations for wildlife or ecology, we bring in trusted subcontractors to write mitigation plans and method statements for us to follow.”

Forestry Journal: Desilting a woodland pond.Desilting a woodland pond.

The desilting of Westwood Lake (2011/12) for (DEFRA-sponsored) Royal Botanic Gardens Kew, Wakehurst, was not so straightforward and took months. Westwood Lake is a designated historic landscape feature, one in a series of ‘furnace’ (or ‘hammer’) ponds used centuries ago in the High Weald’s iron industry. Ardingly Brook flows into Westwood Lake and out again via a series of regulated sluice gates, through a wetland conservation area and into Ardingly Reservoir. The lake supported fish (perch, roach, and trout) and supplied 20,000 litres of water a day to Wakehurst’s garden irrigation systems.

Decades of silt accumulation had reduced the overall capacity and landscape value of the lake and was about to obstruct the water draw-off point to pumps pushing water into the irrigation system. Kew invited PHB (among others) to assess how much silt needed removing.

Jon surveyed using the ‘boat and prod’ method. “There is no substitute for getting in a boat and using a measuring staff to prod about, feeling the silt’s consistency and where it starts and stops. I have tried using floating sonar technology a couple of times but found it too inaccurate to determine the level of detail we like for a client’s survey. We presented Kew with CAD plans and sectional drawings of where 10,000 m3 (wet volume) lay. We were invited to bid for the job.”

Forestry Journal: Pumping decades of silt from the lakebed.Pumping decades of silt from the lakebed.

Desilting has three phases: preliminary work, finishing work and the desilt itself. Preliminaries for Westwood Lake included wildlife surveys, which revealed that no mitigations were needed. PHB’s electrofishing stock survey revealed that 20,000 fish needed re-homing in Ardingly Reservoir. As well as supporting juvenile mayflies and damselflies, the silt was found to be a spawning ground for a protected species, the brook lamprey. Under licence, specialist subcontractors removed them by hand for re-homing upstream.

Site access was explored and what to do with the silt agreed on. “We try to keep silt on site, heaped in specially prepared areas or left in lagoons to dry before being spread on land or used to fill in hollows. Removing silt to a donor site or landfill is a last resort. In Waste Acceptance Criteria tests (landfill), silt scores highly in organic carbon, meaning that the elements will rot and continue to degrade. As a filling material, dried silts can subside over time rather than compacting.” Westwood’s silt was to be dewatered on site for future use as an agricultural fertiliser.

Forestry Journal: Installing silt trap.Installing silt trap.

Surrounded by SSSI ancient woodlands and formal gardens, there was no room to create dewatering lagoons close by. It was decided to pump the silt through a 6-inch steel pipeline laid 65 metres vertically and 500 metres linearly through the woodland into temporary spoil pits (100 m x 40 m and 30 m x 30 m) built (by PHB) in an old Christmas tree plantation. The Environment Agency granted an Environmental Permit for PHB to create and operate the silt lagoons. Natural England gave special permission to widen and upgrade the SSSi woodland track before laying the pipeline.

The lake was drained over time and the lakebed was found to be an unstable mix of sandy clay soils. “Vibration is the enemy. A tracked machine can move across it once, but if it stops with the engine left running it will begin to sink; the base is like quicksand. At Westwood, we used 30 timber bog mats
(5 m x 1 m x 150 mm depth) for the machines to track out on. We had two 14-tonne excavators and an 8-tonne digger ‘haymaking’: two machines cut and transferred silt to a central point, a third machine re-handled it into a skip. The skip contained a macerating pump which chopped up and mixed the silt with water before it was pumped back to the lake edge.” A booster pump fed the watery mix uphill and into the lagoons.

“We found silt with high organic content (leaves, twigs, fish detritus and wildfowl faeces) towards the outlet. A hard wall of silt (up to 6 ft in places) at the lake head had such a high mineral content (clay soil or sandy soil), you could see the layers of various inputs that had washed down over the years.”

Forestry Journal: Ducks awaiting their lake back.Ducks awaiting their lake back.

The silt dewatered quickly. “We excavated during the week and the lagoons settled over the weekend. On Sunday nights, I made turbidity checks (ensuring the silt had settled out) before letting the water flow back into the lake for remixing with the silt or to flow out through the penstock (underwater sluice gate).”

To finish, PHB created a 30-metre-diameter silt trap, slowing down volumes entering the lake by having the water percolate over a reed bed before reaching the main body of water. “The reed bed is capable of holding up to 1,000 m3 of silt. The reeds are chopped and harvested every year and the construction should last for 20–25 years. We would expect it to be 8–12 years before Kew needs to consider desilting the reed bed.”

Preliminaries took a year to organise. Pre-works (paths and dewatering pits) took five weeks to construct. “The desilt itself started late in November 2011 and we came out at Easter 2012. Within that, the lake was flooded for two weeks, frozen two weeks, and we had our Christmas shutdown.” Westwood Lake’s capacity was increased by 40 per cent.

Post-works, for the first year the lakeside was quite ‘raw’. “A lot of trees were cleared around the banks before we started.” Wakehurst’s teams began to re-landscape the banks and within 18 months PHB were back refreshing paths around and down to the lake. “It was good to be back almost immediately. Now, eight years on, it looks incredible, a major enhancement of the landscape. I am really proud of that one.”

Forestry Journal: Recent dewpond – two years on.Recent dewpond – two years on.

Working with natural substrates, such as clay soils and silts, it is not practical for PHB to give specific guarantees, but Jon will happily investigate an issue. It takes approximately 25 years before a lake needs desilting again, depending on the soil strata (sand/topsoil) and land use within the outlying catchment area. If the body of water is ‘on stream’, flooding events can reduce this cycle to 15 or 20 years (PHB do perform attenuation works). “We are not going back to lakes and ponds already desilted within my lifespan at the company just yet!”

PHB’s teams have just returned to a work site begun before lockdown, renovating a lake for the Wildlife & Wetland Trust (Arundel). “We had been there for approximately six weeks, held up by rain and flooding, when a two-week window of dry weather at the beginning of March allowed us to get started. We then had to shut down and leave site for eight weeks, furloughing direct employees and laying off subcontractors.”

PHB’s Lakes & Ponds division launched formally in 2006 when Jon became contracts manager. “We never really marketed hard as our reputation spoke for us. During the last five years our turnover has doubled, and we are trying to catch up with our marketing, upgrading our website and proactively marketing the business as a whole. We want to do more of the same kind of work, with another couple of crews doing it.”

“What angle are you taking with this article?” Jon asks.

“Climate change and drought,” I respond, asking in return: “What would you say is the benefit of a pond to a woodland setting and woodland owner?”

“It is about bringing biodiversity back into an otherwise waterless landscape. Birds, insects, invertebrates and mammals will naturally be attracted to any body of water, whether it is in a field or a forest,” Jon responds.

For this article, Steve Walker wrote: “Jon has installed three ponds at Kingley, the last one in February this year. We need some rain to fill it up now! When I became reserve manager in 2016, we were down to one functioning pond. Although great in itself, a network of ponds is more valuable, especially in the face of climate change and drier summers, ensuring amphibians (and other aquatic species) can move across the landscape (essential for genetic diversity). We now have four ponds on the reserve and have shifted our focus to the surrounding landscape. We (with volunteers) recently restored a farm pond to the east and are working with other farmers to reinstate ponds within their agri schemes. Hopefully, in future, these ponds will provide stepping stones, connecting up the nature reserve with other freshwater habitats further afield.”

Information board beside Dew Pond in SSSI yew woodland, Kingley Vale National Nature Reserve:

‘The shallow, saucer like bowls of dew ponds are a characteristic feature throughout the South Downs. Traditionally used for watering sheep, shepherds would line them with puddled clay to prevent water from seeping down into the porous chalk. Today most dew ponds are better valued for their wildlife. Not only do they provide a breeding place for frogs, toads, and dragonflies, but also food and habitat for a host of other species from grass snakes to bats. In periods of drought, a pond is literally an oasis in a desert.’ Steve Walker, reserve manager, Natural England

All images: PHB’s Lakes & Ponds division.

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