Report on the Doley Brook & Wetland Works
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Report on the Doley Brook & Wetland works - The Acres, Gnosall. February to March 2015. Michael Deegan Gnosall Parish Councillor 1 Background The drainage work on the Doley Brook started on February 13th 2015 and took three days to complete. The work came about as a result of a meeting between Gnosall Parish Council, the Environment Agency (EA) and Staffordshire County Council Farms in May 2014. The meeting followed an unconsented encroachment of drainage work in April 2014 to the brook beside 150m of The Acres (owned and managed by Gnosall Parish Council). The drainage works were being carried out by the new tenant farmer upstream at Plardiwick Farm (a Staffordshire County Council property). It emerged the tenant farmer believed that recent Parish Council management of the Doley Brook through The Acres was causing flooding upstream on the floodplain at Plardiwick Farm. The drainage project of the brook through the Parish Council land was subsequently agreed to test this assertion. The work had been delayed since autumn 2014 until dry conditions in late January/February enabled the contractor (Foxley Earthworks) adequate access. The main focus of the works was to cut the vegetation on the brook to maintain a central open channel within the watercourse. This stretch of the brook runs through the Acres from the Hollies Brook confluence to the electricity sub-station by the A518 road culvert. Above: Doley Brook looking south from Boardwalk. (1) February 13th and (2) February 16th. 1. The Acres The Doley Brook The Doley Brook is classified by EA as a Main River and as a headwater is a tributary watercourse of the River Trent Catchment. Extensive drainage was carried out by the National Rivers Authority in 1980 to canalise the brook, which heavily modified the channel. Despite regular de-silting and drainage of the brook through The Acres until 2007, the main field to the east of the brook became increasingly boggier in the late 1990’s and much of the allotments area (created in 1981) had to be abandoned as marshland. The Acres Restoration project in 2012 aimed to create a more sustainable future for the site and make the most of the brook and wetland. Working with EA and Farming Floodplains for the Future, the brook was cleared and re-engineered with a two-stage channel. A new silt- lagoon was installed to intercept sediment being washed down the Hollies Brook and an overflow channel (and pools) was created to take excess water through six acres of floodplain. This area was further reconnected to the brook by removal of the drainage spoil- banks, allowing the floodplain in this contained wetland to provide a huge area for flood- storage to further protect land downstream, as well as restoring a valuable wildlife habitat. 2 The Works The contractor was authorised to take a more rigorous cut than planned because of the surprising quantity of silt in the channel (only three-years since it was last cleared in 2012). Over a period of three days, the contractor carried out maintenance on the Doley Brook all the way through The Acres downstream to the A518. As well as clearing more sediment than envisaged, the contractor also removed the two log- deflectors installed to create meanders. Unfortunately the squeezed-channel had become completely clogged-up and the deflectors starting to act as dams. They were an innovative element of the 2012 project, but with hindsight there was not enough velocity or flow in the brook to scour out a meander in the watercourse. Left: Doley Brook looking South from the boardwalk. (3) February 13th before the works and (4) February 15th immediately after the works. Silt Lagoon Trap Another key objective was to clear the newly created silt lagoon. This was installed at the confluence with the Hollies Brook (in 2012) to intercept the excessive load of sediment washed down the Hollies Brook system (as a result of intensive farming practices upstream). The lagoon was completely full of sediment and has now been fully cleared. Although this feature has proved successful, it clearly will require clearing at least once a year. Above: The Silt Lagoon from two different angles. (5) February 13th and (6) February 16th. 3 Overflow Channel The overflow channel created in 2012 through the wetland was not cleared on this occasion. There was not a significant issue with silt or weeds in the channel and this will now be addressed next autumn/winter as part of the Parish Council’s ongoing Acres and Doley Brook maintenance programme. Left/below: Foxley Earthworks on site. (7) Feb 13th and (8) Feb 15th. 2. Water level Monitoring Photographs were taken of the gauge-board located in the Doley Brook at the most northern end of the Acres allotments. The waterlevel of the brook (and in the wetland) did appear to drop slightly during the drainage works, but has since risen closer to its initial level (despite the dry weather). Above: Doley Brook gauge board. (9) February 13th before the works, (10) February 16th immediately after the works and (11) March 15th one month afterwards. 4 3. Rainfall Specific precipitation records for the site are not monitored. However, according to Met Office figures, the total rainfall recorded in nearby Stafford during February was 42mm and for early March (to the 12th) was 27mm. The Environment Agency confirm that February was drier than average and likely to be slightly cooler than average, with March continuing in the same pattern. Nationally, Met Office statistics show that the UK has had its sunniest winter since 1929 while figures for temperature and rainfall are fairly average for the West Midlands. Right: Rainfall Summary for England and Wales for January – March 2015 (www.gov.uk/government/statistics/weekly- rainfall-and-river-flow-summary-4-to-10- march-2015) 5 4. Long-term Affect The Acres One month after the works were completed, a further set of comparison photographs of the Acres were taken, in addition to shots of the brook and pools upstream to Doley Common SSSI. Although the weather has remained relatively dry over the past month, the level of the brook through the Acres has remained stable and virtually unchanged since before the drainage works. This comparison is particularly marked from than from the boardwalk bridge over the brook – looking both north and south. Above: Doley Brook looking North from the boardwalk. (12) February 13th before the works, (13) February 16th immediately after the works and and (14) March 15th one month afterwards. Below: Doley Brook looking South from the boardwalk. (15) February 16th immediately after the works and (16) March 15th one month afterwards. 6 Upstream – Plardiwick Farm to Doley Common SSSI As part of the post-works appraisal, a visit was made to the floodplain upstream to assess the effect of the clearance through The Acres. Regardless of the drainage and low rainfall, the level of the brook flowing past Plardiwick Farm and through Doley Common SSSI is close to bank-top and the adjacent pools are still holding significant water. Above: (17) Pools adjacent to the Doley Brook. March 15th The Plardiwick Farm field appears visibly lower in height than the immediately adjacent Doley Common SSSI land to the north. It is possible that the land was always lower. However, it is more likely the underlying peat soils have shrunk as a result of drainage during the drier decades of the 1980’s to 1990’s, when the land was able to be drained and utilised for more intensive farming-use. Weather conditions since the late 1990’s have been much wetter and climate change predictions indicate that periods of high or extreme rainfall will be more common in the near future. Above: (18) Plardiwick Farm (left) next to Doley Common SSSI (right). Doley Brook in foreground. 7 It should be noted that the field also in the floodplain on the opposite side of Plardiwick Farm (southwest of Hollies Farm) is dominated by reeds (Phragmites australis) with scattered willows. This wetland habitat is quite extensive and will have taken many years to develop, indicating an already established high groundwater table on this side of the brook. Above: (19) Reedbed habitat on eastern side opposite Plardiwick Farm. March 15th It was also evident that the cultivated fields above this opposite side of the brook side are ploughed down the contours of the slopes. Although an unploughed strip of reedbed is left at the bottom, there were a number of eroded gulleys into the brook that add to the diffuse pollution downstream – showing that excessive silt being deposited into the system is clearly not just a problem from the Hollies Brook catchment. Above: (20) Eroded gulley down arable slope into the Doley Brook. March 15th (21) Vehicle tracks into Doley Brook creating gulley channels at the bottom of an arable slope. 8 5. Biodiversity Value along the Doley Brook The land on either side of the Doley Brook (immediately south of Doley Common SSSI) is currently of considerable wildlife habitat importance. In particular, the lower field of Plardiwick Farm along the Doley Brook contains notable areas of open water and wet grassland. Lowland Wet Grassland and such healthy Rivers & Streams are designated as habitats on the Staffordshire Biodiversity Action Plan (SBAP). Of particular interest was the important assemblage of wetland birds (predominantly wintering wildfowl and waders) using the Plardiwick Farm pools and surrounding wet grassland. During the March visit a flock of approximately 40 Lapwing, an SBAP priority species, was observed. There were also large numbers of Teal, Wigeon and Mallard. Lesser numbers of Snipe and Redshank were also in evidence.