SRA End Of Year Report 2015-16: Pioneer dredging
This is the opening section of Part 2 of the SRA End of Year Report 2015-16, covering Somerset Rivers Authority’s Enhanced Maintenance Programme of works in 2015-16.
Pioneer Dredging (high level carriers)
Pioneer dredging and de-silting tasks were completed for Somerset Rivers Authority by Environment Agency teams, on both high and low level carriers, to improve channel conveyance and water management.
Increased conveyance capacity also allows dissolved oxygen in the channel to be regularised so that fish can breathe comfortably. (On heavily silted channels with reduced flow rates dissolved oxygen levels can fall to critically low levels).
A specification for de-silting tasks was developed with Natural England to minimise disruption to flora and fauna on established channel-side habitats and limit the removal of silt to that accumulated on the channel bed of watercourses.
Background: After the floods of 2013-14, Environment Agency investigations indicated that de- silting certain sections of the Rivers Parrett and Tone would do more to reduce the duration and depth of flooding than any other single action in the Flood Action Plan. Consequently, in 2014, 8km of the Parrett and Tone were dredged. Recent hydraulic modelling has shown that this 8km dredge – combined with a wide range of other improvements, such as the Environment Agency’s £2.5m investment in temporary pumps and pumping facilities – would make a big difference if there were to be a repeat of flooding on the scale of the winter of 2013-14. Effects would include:
- significantly cut the risk of flooding to 129 of the 142 properties reported to the Environment Agency as having flooded in Northmoor and Saltmoor
- reduce closure of the A361 to about 3 weeks instead of the 9-10 weeks experienced
- clear the Moors of water more quickly
Following the 8km pioneer dredge, the Environment Agency, on behalf of Somerset Rivers Authority, investigated the effectiveness of dredging elsewhere, with input from local people and the Internal Drainage Boards.
Ten potential dredging sites were identified and assessed, including a section of the River Parrett immediately downstream of where the 8km dredge had finished, from Northmoor Pumping Station down towards the M5 and Bridgwater.
In October 2015, the SRA’s Board voted to dredge the first 750m of this section of the Parrett, downstream from Northmoor Pumping Station. Of the 10 potential sites scrutinised, action here offers the greatest physical benefits to people and property. Dredging will help to reduce flood risk to homes and buildings, damage to agricultural land and disruption to roads and railways. It could cut peak flood levels by between 50-80mm in Northmoor and reduce the duration of a flood like that of 2014 by 3 – 5 days, in combination with the 2014 8km dredge and additional pumping.
After the Board’s approval, the Environment Agency – on behalf of the SRA – awarded a ‘design and build’ contract to Galliford Try, Black & Veatch and Land & Water. These companies worked on the 8km dredge of the Parrett and Tone in 2014.
Work was scheduled to follow on from an interim-funded maintenance dredge of 2.2km of the Parrett, upstream of Northmoor Pumping Station, that was pioneer-dredged in 2014. (For more information about this dredge, see the section below on ‘Maintaining dredged profiles’).
Problem: Limited conveyance capacity of the Parrett, downstream of Northmoor Pumping Station.
Being achieved: Land & Water began dredging downstream of Northmoor Pumping Station on April 25, their work being overseen and managed by the Environment Agency for the SRA. Due to the width of the river, narrow banks, poor access, and the dangers of high-voltage overhead cables, most of the dredging (600m) will be done using a pontoon-mounted excavator on the river. It is not possible to dispose of dredged material directly to the existing banks. It has to be placed in barges, transported by tug to Westonzoyland Pumping Station, unloaded into tractor and trailer, then incorporated into adjoining farmland as a soil conditioner.
Generally, the dredging is only removing silt from one side of the river to provide the required flow area. As with the 8km dredge and the 2.2km maintenance dredge of the Parrett, the aim is to achieve a cross-sectional area of around 70m2 within the constraints of maintaining existing river banks and hard defences so as not to reduce their stability, and avoiding environmentally sensitive areas and protected species.
A shorter length (150m) of dredging was undertaken from the bank adjacent to the road leading to Northmoor Pumping Station and Moorland. The road was closed for three weeks from 31 May (during weekday working hours) to allow material to be transported safely by tractor and trailer.
By 21 June, more than 9,000m3 of silt had been removed; the final total is expected to be 13,500. This pioneer dredge should be finished by the end of the third week in July 2016.
Background: The Cripps is used by the Environment Agency as a channel for moving potentially huge amounts of water from the Brue down to the Huntspill or from the Huntspill up to the Brue. Operations are controlled via sluices and Gold Corner pumping station, which dominates the junction of the Cripps, South Drain and the Huntspill.
Gold Corner and the Huntspill were built in the early 1940s. Their primary purpose was to supply the 4.5 millions of gallons of water needed daily to help produce explosives at Royal Ordnance Factory Bridgwater. However, for nearly 75 years, the combined system has also helped to prevent flooding in the Brue Valley and to maintain water levels suitable for farming and environmental benefits. Nearby are five valuable wildlife Sites of Special Scientific Interest together with the Huntspill River National Nature Reserve.
Problem: The Cripps had not been dredged for many years. Significant depths of silt were reducing its capacity for diverting flows from the Brue to the Huntspill and vice versa.
Achieved: Approximately 22,500m3 of wet silt was removed from the whole of the Cripps (2.5km) by two long-reach excavators and placed on adjacent land, under the Water Resources Act (Section 167).
The task was completed in November 2015 by Environment Agency Operations Teams (Wessex). Dredging has increased the capacity of the Cripps and helped to maintain local farming and environmental benefits.
Background: Throughout the Brue Valley Catchment, water is collected and distributed via rhynes, drains and channels, so as to obtain desired levels. The main artery of this water management system is the River Brue. Optimising its capacity is vital for ensuring that the frequency, depth and extent of flooding are better controlled.
The aim of dredging part of the Brue is to remove accumulated sediments from the river channel and thereby improve conveyance to the extent recommended by the SRA’s Dredging Strategy (which is in development, and due to be presented to the next SRA Board meeting on July 27). Work will support the implementation of existing Water Level Management Plans in the Brue Valley Catchment and contribute directly to Somerset’s 20 Year Flood Action Plan. It should provide more water of better quality to help achieve and maintain favourable conditions in adjacent protected sites on the Moors (SSSIs, SPA, Ramsar Wetland of International Importance).
Achieved: Preparatory work has been carried out for dredging 4km of the Brue from North Drain Pumping Station to Westhay. Ecological and environmental constraints, and hydrological advantages, have been examined. Plans for works on the Brue are being shaped by the findings and proposed recommendations of the SRA’s nascent Dredging Strategy, which has involved extensive work by consultants from HR Wallingford.