River Management Techniques. Upper Wharfedale Best Practice Project (Information Series No 6)

1.0. Applying a more holistic approach to river management


1.1. Moorland grip blocking

BACKGROUND INFORMATION

During the 1960s to 1980s drainage channels or ‘grips’ were constructed over extensive areas of Pennine moorland in the belief they would benefit both grouse and livestock. It has now been shown that moor-gripping has done little to increase the productivity of vegetation. Instead, it has resulted in detrimental disturbance of the natural hydrology, damaged the Sphagnum (bog moss) plant communities, caused erosion of the moorlands through over-deepening of grips, increased silt deposition in downstream rivers and increased the likelihood of flash flooding.

1.1

LOCATION:
Oughtershaw Moss
SD 836820-865813

AREA: c.318ha

DATE UNDERTAKEN:
Started Summer 1999 and completed Autumn 2000

COST:
A rate of £13/hour was charged by a local contractor to block an estimated 42km of grips. Additional costs included £325 for the supply and delivery on site of 100 straw bales and 15 heather bales. The overall cost of grip blocking was £4,200.

The Upper Wharfedale catchment includes approximately 17km˛ of intensively gripped moorland. This is an area of degraded blanket bog, dominated by cottongrass with limited amounts of Sphagnum, which is being managed under Countryside Stewardship to enhance the vegetation communities. The principal method used to restore the natural hydrology is to block existing grips, thereby encouraging water retention on the moor and helping to restore ‘peatland’ plant communities. By doing this, it is hoped to increase the amount of Sphagnum and also to restore the cover of dwarf shrub species, such as ling, bilberry and cross-leaved heath.

DESCRIPTION

The area of land under consideration rises from 380m to 500m above sea level. The highest part of the land consisted of deep blanket peat which is hagged in places. The sloping ground consisted of shallower peat over mineral soil exposed in some grips. The soil map of the area shows the soils as belonging to the Winter Hill Soil Association, consisting of thick very acid raw peat soils, perennially wet. A plan was developed from aerial surveys of the area providing the location and density of existing grips. A study of the plan, combined with a foot survey of the site, indicated the following:

  • Many grips were gradually re-vegetating naturally, particularly where individual grips were short and on level/gently sloping ground;
  • Bare peat in grips indicated ongoing erosion present throughout the area, but particularly on the deeper peat, on long sections of grips (with greater flow), and on steeper slopes, including where grips entered the natural gills;
  • Hagging, possibly caused or accentuated by rapid run-off from grips, although some areas were starting to re-vegetate.

DESIGN

A small pilot study area was selected where two different techniques could be used to block the grips and, to help increase the understanding of the issues, a programme of water level monitoring was also initiated. Two trial grips were selected for blocking with either straw or heather bales to determine the efficacy of the method, against comparison with the use of peat scooped from the ground adjacent to the grip. The majority of the remaining gripped area was blocked using peat – a method successfully employed by English Nature in earlier work undertaken in Arkengarthdale, North Yorkshire.

Blocking of grips was undertaken using a tracked digger vehicle to construct peat dams and/or to compress the straw/heather bales into the grip where these were used instead. Due to the variability of the ground, guidelines could not be followed rigidly and contractors were encouraged to spend time assessing the situation in order to site each dam in the most beneficial location. It was found to be advantageous to use local farming contractors, who had knowledge of the terrain and the skills to operate the machinery required in these conditions.

SPECIFICATION

Location

Grip blocking was initiated at the uphill end of a grip system in order to reduce or slow water flow into the middle and lower sections. Some dams to block grips should always be located at the uphill end of a grip system, and not just in the middle or lower sections. Grips chosen for blocking included those which are actively eroding or which feed into hagged areas. Grips across level, or raised/basin mire areas were also chosen, allowing other grips to in-fill naturally.

Spacing

Dam spacing was such that the water level at the top of the dam reached the base of the next uphill dam. This was not practical for steeply cut and eroding grips, and here the objective was to dam the higher feeder grips and hold back water flow. In shallowly sloping to level situations, dams were constructed at 10-15m intervals to slow water flow and speed up vegetation growth.

Construction

Dams were made of peat taken from adjacent land to a depth of 30cm and compacted into the grip. The length of dam along the grip was one to two times the width, and more for deeper grips. The dam height was aligned to the surrounding vegetation surface, ideally with overflow seepage to the sides, and not back into the original grip, so as to allow water to  overflow and escape laterally into the adjacent land away from the main grip channel. The exception was where this created ponding of water to a depth of more than 60cm, and thus a significant hazard to livestock. In such cases the dam height was kept to 60cm. The value of using peat dams is that the vegetation readily re-colonises.

SUBSEQUENT PERFORMANCE

To date, the majority of the gripped areas have held through severe winter conditions and water level monitoring gave an early indication that grip blocking reduces the downstream run-off volume in the grip channel by about 24%.

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