Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/579379
october 2015 WET NEWS 15 A £3.1M project to build a long-awaited scheme to help reduce the risk of flooding in Northallerton has been completed. The scheme works by storing fluvial flows upstream of the town that will create space in culverts for the surface water to drain away. The North Yorkshire town and surrounding villages have been plagued with flooding since the early 20th century, with major floods most recently in 2000, 2008 and 2012 that caused widespread disruption and damage including to the town's Friarage Hospital. When funding was finally secured a†er a long wait, the Environment Agency (EA) wanted to develop a scheme that was sustainable with low- maintenance needs and conforming to SuDS principles of holding back flood waters on the outskirts of the town |and discharging flows safely into the town's culverted watercourses at a controlled rate. The solution designed is based on two large Hydro-Brake Flood Alleviation vortex flow controls, installed in newly- refurbished culverts on the outskirts of the town. They enable excess water to be held back in specially built flood storage basins on the approach to the culverts. "The Sun and Turker Becks have repeatedly overtopped and threatened the town," reports the EA area project manager Ian Cooke. "That's why, a†er consulta- tion and looking at similar suc- cessful schemes in other areas, we decided to use upstream storage to reduce the flooding risk. The resulting scheme needs very low maintenance, and is resistant to blockage. Our chosen solution meant that we used vortex control technology. "In addition, the ability to carefully-size the flood allevia- tion vortex flow controls means we could minimise the back-up storage requirements, even under high rainfall conditions. As a result, the risk of the farm- land on the outskirts of North- allerton flooding and threaten- ing nearby developments and the town centre will be minimal in future." Peak flow The Northallerton catchment, with the villages of Brompton and Romanby is in a low lying river valley, surrounded by steep, well-drained agricultural land which makes it susceptible to flash flooding. Largely culverted, the Turker and Sun Beck watercourses run through suburbs and the centre of the town, causing flood threats to amenities including Grade II listed buildings, conservation areas and listed monuments as well as the Friarage Hospital. At times of peak flow, excess floodwaters can overtop the watercourse in • Northallerton has been plagued by flooding issues. the eA decided a sustainable solution with low- maintenance needs was the answer. Minimising flooding through spin ONSITE Flood deFeNce the agricultural land on its approach to the culverts, sending flows to cause flooding in the East of the town. The rivers in the catchment are constricted by their channel capacity, bridges and culverts and recent CCTV surveys have revealed some were in a poor state of repair, or could be threatened with collapse should flows continue at high volumes. As part of the wider flood alleviation scheme, the EA undertook a programme of repairs to rectify these defects in advance of works to create the storage areas. Lower flows would also reduce back-ups in the remaining combined sewerage entering the culverts. On top of this, frequent floods have already necessitated the ground floor of Friarage Hospital to be reconfigured so as to reduce major disruption from flooding, although it remains vulnerable. Hidden Protection To reduce the risk of flooding to about 170 properties, the EA has adopted a scheme designed with advanced self-activating vortex flow controls to protect against a minimum 1 in 75 year fluvial flood event. Detailed hydraulic modelling was conducted for the Northallerton catchment before designing the new scheme. Cooke continues: "The scheme is designed to also improve protection to the area ProjEcT SPEcS • reduce the risk of flooding in Northallerton • Install two large Hydro-brake Flood Alleviation vortex flow controls • develop a scheme that was sustainable with low-maintenance needs • conduct detailed hydraulic modelling for the Northallerton catchment before designing scheme "We will be able to make comparisons over 2014/2015, particularly between beckton and long reach, where travelling bridges have been retained, as the plant is not being encroached to the same degree. With the minimal headroom requirements of the Zickert top and bottom scraper technology, we were able to create an initial low volume cover design" Ian Cooke, EA the scheme is aimed at reducing the risk of flooding to about 170 properties the Sun and turker becks have repeatedly overtopped and threatened the town two Hydro-brake Flood Alleviation vortex flow controls have been installed in newly-refurbished culverts on the outskirts of Northallerton • the Hydro-brake Flood Alleviation vortex flow control's internal geometry allows water to flow unrestricted • the cone-shaped vortex flow control devices installed in the culverts are based on an industry- standard method of flow attenuation • A series of new stepped trash screens have been installed • the advanced vortex flow controls are self- activating NEEd To kNoW 1 Major floods in 2000, 2008 and 2012 caused widespread disruption in Northallerton 2 Frequent floods have necessitated the ground floor of Friarage Hospital to be reconfigured 3 the turker and Sun beck watercourses run through suburbs and the centre of the town 4 More water is able to flow through the culvert unimpeded, meaning less flood storage is needed