North Street Keighley, West Yorkshire
North Street in Keighley is one of the few streets remaining in this historic town nestled in the South Pennines in Yorkshire. Built at the confluence of two rivers, the town has long been related to the mills, powered by these watercourses, and the manufacture of textiles and textile-related machinery continued right up until ten years ago.
Over the past two hundred years the population of Keighley has risen tenfold, and with the rising population, traffic increases. Much of the town was remodelled in the 1960s, but the older parts of the town suffered from too many vehicles competing for too little road space.
Following several consultations, the local council decided to make some alterations to the road network in and around the town centre, to alleviate the traffic gridlock that was becoming a feature of the town centre.
The old Keighley College building that fronted onto North Street was demolished, making space for an additional traffic lane, and provided a public open green space. Existing street trees had to be removed to allow for the road works, so new trees were specified in large soil volume tree pits, supported with GreenBlue Urban RootSpace soil cell systems.
To maximise the value of the new tree planting, it was decided to use these tree pits as storm water attenuation and treatment pits, taking water from the carriageway, passing it through a stone layer on the top of the tree pit and then filtering it down through the Arborsoil Hydro soil mix provided for the root growth. The water then transferred through a clean stone layer to an underdrain to a stormwater drain. This method of conveyance gives an acceptable level of pollutant removal and slows the water down so that the existing drainage network can cope with the increasingly violent storm events.
Calculations: The SuDs tree pits were designed to accept highway water runoff resulting from the creation of an additional 356sq.m of new carriageway coping with a rainfall intensity assumed to be 50mm/hr; peak runoff rate was determined to be 4.95 l/s.
Using the 6 hours, 2-year return for a storm surge it was calculated that 7128 litres (7.128cu.m) of storage would be required.
It was further determined that there would be 7.77cu.m of water storage from the free board-level above the gravel surface along with the available void space of the gravel/slate ‘mulch’ before any infiltration to the tree pit soil actually took place.
It was calculated that the soil element of the tree pits could accommodate 20.16cu.m of water storage along with a further 2.88cu.m of water storage in the void space of the gravel to the base of the tree pits. Total storage capacity was estimated to be 30.81cu.m prior to any secondary infiltration to the surrounding strata or absorption from the trees, obviously, some assumptions were taken such as void ratio for the gravel set at 40% and the use of sandy loam soil with soil water saturation being 150mm water per 300mm soil depth.
From these calculations, the joint tree pit should be able to sustain peak rainfall for up to three and a half hours for a 1 in 30-year storm surge event.
Working alongside City of Bradford Metropolitan District Council and SIG Geotechnical Ltd, the project bought together irrigation and aeration, Root Management for guidance along with utilities, Mesh, Geonet and ArborSoil Hydro, specifically blended as a result of research and testing perfect for SuDS tree pits.
The trees have shown amazing growth over the past two years, which is typical for trees planted in these systems; uncompacted soil with good porous surfacing allows maximum gaseous exchange, and every storm event washes fresh nutrient into the tree pit. In summer 2020, noticing that the trees were still experiencing astonishing new shoot extension growth, GreenBlue Urban commissioned an independent arborist to undertake sap flow analysis; measuring the ability of the tree’s vascular system to transport water to the canopy. The highly technical test equipment uses Heat Pulse Method probes to assess tree vigour and gives a real-time reading of the moisture uptake.
Using the algorithm below, tree vigour assessment can be calculated:
Sf (cm^3/hr) / Sc (cm^2) = TVA VPD (kPa)
Where Sf is Sapflow and Sc is Stem circumference.
On the day of testing, the Vapour Pressure Deficit was low at 0.5kPa, meaning that the humidity level was quite high. (VPD is the measured difference between 100% water potential in the atmosphere and actual water potential) The higher the humidity level, the less water can evapo-transpire from the leaf surface, so the test data showed a water draw of only 1.34 litres per hour. In a warmer and drier environment (average VPD is 1.5kPa) this would rise to as much as 4.0 litres per hour! The TVA measured 10,53, which is very high, showing exceedingly healthy trees. Bear in mind that these trees have only been planted for two years, these trees are demonstrating healthy steady growth and will increase in size and water draw with every year. Using trees as part of a Sustainable Drainage System is one way of creating additional capacity over time – as the trees grow they intercept more water on the tree canopy and they draw more water from the ground, therefore becoming increasingly efficient.
This project clearly displays how imaginative tree planting can provide numerous benefits without high levels of cost – and continue to provide these benefits for decades.
Watch our video on Keighley, North Street here.