Specification for Grip blocking using Peat Dams

Transcription

1 Technical Guidance Note 1 Specification for Grip blocking using Peat Dams 1. Introduction Moorland drains (grips) have been dug across much of the Yorkshire upland peatlands. Many of these grips have become badly eroded and the impact of drainage combined with other factors has lead to the development of large eroding gullies in many areas. The Yorkshire Peat Partnership is working with Natural England s Environmental Stewardship Scheme to block these grips and to restore the water table to the peatlands and prevent further peat erosion. This document sets out the techniques that the Yorkshire Peat Partnership will use to block drainage grips. These techniques are based on current available evidence and may be subject to further refinement as further research is carried out. It is also likely that, in some circumstances, individual site conditions will require modifications or refinements to these standard techniques. 2. Grip classification Grip blocking techniques will vary depending on the condition of the grip. It is essential, therefore that grips are surveyed prior to the production of a restoration plan. This survey can then be used to produce a detailed grip blocking plan. Natural England have devised a survey protocol which provides all the information needed to develop a grip restoration plan. Parts A and B of this survey will be carried out by the Yorkshire Peat Partnership on behalf of Natural England. Part C can be carried out by any contractor (including Yorkshire Peat Partnership) chosen by the client. It is important to distinguish between grips and gullies in this survey. Grips are man-made channels that run in straight lines and occur in regular patterns which can readily be seen on aerial photographs. Gullies are natural, sinuous channels which may look like streams on the aerial photographs but are actually caused by man-made factors such as drainage, fire, over-grazing or erosion. The survey enables the grips to be classified into 7 categories. These categories are then used to determine the restoration techniques needed. This document concerns grips in categories 1-3.(see Figure 1.1). Grips which are classified in category 1 are further subdivided into sub categories 1b and 1f, those categorised as 1b have been found to be blocked by vegetation or deposition of silt or peat and require no work on them. Grips categorised as 1f are still flowing or capable of flowing after rainfall if dry when surveyed, and thus need damming to restore natural hydrology and prevent continued erosion. Peat dams are not effective for blocking grip categories 5, 6 & 7 and require other techniques such as wood or plastic piling dams. Guidance on these methods is provided

2 in the Yorkshire Peat Partnership Technical Guidance Note 2 - Specification for large grip/gully blocking. 3. Blocking Techniques 3.1. General dam construction All grips should be blocked starting from the top of the grip working downslope Excess water from the grip should be dissipated across the moor. The water should be directed laterally across the moor away from the grip by digging a small crescent shaped overspill channel (see Figure 1.2.) on the down slope side behind the dam The block should be wider ( m) either side than the grip to prevent erosion at the grip edges (see Figure 1.2) Variation in the positioning of the dams is required in order to take advantage of the natural topography The top of the dams should be slightly higher than the surrounding ground level so that impounded water overflows laterally away from the dam and soaks into the bog surface (see Figure 1.2) Placement of the dams must be such that when the water backs up from the dam the grip is filled with water to a level above the base of the next dam up slope (see Figure 1.3) The locations of all dams must be recorded using sub-metre accuracy GPS Category 1 grips: Grip up to 600mm wide; 650mm deep Where these grips are already blocked by vegetation no further work is needed. Where Category 1 grips are still flowing they should be blocked using peat dams as follows: Average spacing of the dams should be 7.5m but adjusted to take account of the gradient and vegetation conditions of the individual grip. On level ground the dams should be no more than 12m apart. On steeper slopes the dams should be no more than 5m apart All peat dams must be constructed using a very low ground pressure 360 o excavator with wide ( bog ) tracks The peat to be used must be well-humified so that it is sufficiently impermeable. A suggested technique is as follows: With the machine straddling the grip strip out the vegetation (to a depth sufficient to ensure the root zone stays intact) to a width of approximately 0.6m either side of

3 the grip and to a depth of 0.2m into the base of the grip for a length of approximately 1.5m upstream of the dam site and place the stripped vegetation to the side of the grip for later use Using peat gathered from the grip to the upstream side of the dam site, turn peat over in area stripped of vegetation to create a wedge-shaped dam 1.2m thick, making sure that the dam is keyed into both sides of the grip by 0.6m and the base by 0.2m (see Figure 1.2). The peat dam should be finished 0.8m higher than the surrounding ground to allow for settlement Create a runoff via a crescent-shaped shallow overflow channel to the lower side of the grip upstream of the new dam, ensuring excess water can be dispersed onto moorland without causing subsequent surface erosion (see Figure1.2) To the upstream side of the dam re-profile the sides to approximately 35 o and for a distance of approximately 1m to ensure there is no deep water hazard and to slow down flow (see Figure 1.2) Revegetate all bare peat (including the top of the dam) using the vegetation previously set aside to prevent oxidation of the peat (See Figure 1.2) In many situations category 1 grips should be completely re-profiled between dams. A suggested technique is as follows but it is recognised that individual contractors may have other reprofiling methods available: The vegetation either side of the grip immediately upstream of the dam is removed and used to cover part of the dam. The newly exposed peat is then turned over to lift the base of the grip and the steep side slope is re-profiled to approximately 45 o. The peat from the sides and base are worked over and compacted to create a gentle undulating curve (see Figure 1.4). Ensure that the downhill edge of the grip is slightly lower than the uphill edge to facilitate even overflow of collected water allowing even re-wetting and avoiding creation of secondary erosion channels. Moving up the grip the next section of vegetation is removed and used to cover the previously exposed and now re-profiled peat. Repeat this process until along the length of the grip. At the top end of the grip section unbroken turves from adjacent to the grip may be needed. Make good the area where the turves are taken from by teasing in the surrounding vegetation Category 2 & 3 Grips Early erosion; >600mm-1000mm wide; >650mm 1000mm deep) and deeply scoured: >1000mm-1500mm wide; >1000mm- 1200mm deep) grips: These categories of grip should be blocked using peat dams and a suggested technique is as follows: Average spacing of the dams should be 7.5m but adjusted to take account of the gradient and vegetation conditions of the individual grip. On level ground the dams should be no more than 12m apart. On steeper slopes the dams should be no more than 5m apart. Placement of the dams must be such that when the water backs up from the dam the grip is filled with water to a level above the base of the next dam up slope. Placement of the dams must be such that when the water backs

4 up from the dam the grip is filled with water to a level above the base of the next dam up slope (see Figure 1.3) All peat dams must be constructed using a very low ground pressure 360 o excavator with wide ( bog ) tracks The peat to be used must be well-humified so that it is sufficiently impermeable With the machine sitting on the low side of the grip strip out the vegetation (to a depth sufficient to ensure the root zone stays intact) to a width of approximately 1m (Cat 2)-1.5m (Cat 3) either side of the grip and 0.3m into the base for a length of 2.5m (Cat 2) 3.5m (Cat 3) upstream of the dam site and place the stripped vegetation to the side of the grip for later use Initially using peat gathered from the grip to the upstream side of the dam site, turn peat over in the grip to create a wedge-shaped dam in the area stripped of vegetation 1.2-2m (Cat 2) or 2-3m (Cat 3) thick (see Figure 1.2). It is likely that additional un-oxidised peat will be needed to complete the dam. This should be gained from a borrow pit adjacent to the grip. The borrow pit must be located within 90 o or 180 o of the machine but within easy reach without moving the excavator. Strip the vegetation from the borrow pit in as large an unbroken turf as possible and store to one side for later use. Excavate un-oxidised peat from the borrow pit and complete the dam making sure that it is keyed into both sides of the grip by 1m (Cat 2) -1.5m (Cat 3) and the base by 0.3m. The peat dam should be finished 1-1.5m (Cat 2) or 1.5-2m (Cat 3) higher than the surrounding ground to allow for settlement (see Figure 1.2) Make good the borrow pit by stretching the previously stored vegetation across it thus leaving a shallow depression. Use all other previously excavated vegetation to cover any bare peat Create a runoff via a shallow crescent-shaped overflow channel to the lower side of the grip upstream of the new dam, ensuring excess water can be dispersed onto moorland without causing subsequent surface erosion (see Figure 1.2) Category 2 & 3 grips should be completely re-profiled between dams. A suggested technique is as follows but it is recognised that individual contractors may have other reprofiling methods available: The vegetation either side of the grip immediately upstream of the dam is removed and used to cover part of the dam. The newly exposed peat is then turned over to lift the base of the grip and the steep side slope is re-profiled to approximately 45 o. The peat from the sides and base are worked over and compacted to create a gentle undulating curve (see Figure 1.4). Ensure that the downhill edge of the grip is slightly lower than the uphill edge to facilitate even overflow of collected water allowing even re-wetting and avoiding creation of secondary erosion channels (see Figure 1.1). Moving up the grip the next section of vegetation is removed and used to cover the previously exposed and now re-profiled peat. Repeat this process until along the length of the grip. At the top end of the grip section unbroken turves from adjacent to the grip may be needed. Make good the area where the turves are taken from by teasing in the surrounding vegetation.

5 3.4. Category 4 grips (Grip which is wide in relation to depth: >1500mm-2000mm wide, >1200mm-2000mm deep). These categories of grip are at the limit of what can be blocked using peat dams and in some circumstances other methods may be more appropriate Average spacing of the dams should be 7.5m but adjusted to take account of the gradient and vegetation conditions of the individual grip. On level ground the dams should be no more than 12m apart. On steeper slopes the dams should be no more than 5m apart. Placement of the dams must be such that when the water backs up from the dam the grip is filled with water to a level above the base of the next dam up slope. Placement of the dams must be such that when the water backs up from the dam the grip is filled with water to a level above the base of the next dam up slope (see Figure 1.2) All peat dams must be constructed using a very low ground pressure 360 o excavator with wide ( bog ) tracks The peat to be used must be well-humified so that it is sufficiently impermeable With the machine sitting on the low side of the grip strip out the vegetation (to a depth sufficient to ensure the root zone stays intact) to a width of approximately 1.5-2m either side of the grip and 0.3m in the base for a length of between 3-4m upstream of the dam site and place the stripped vegetation to the side of the grip for later use Initially using peat gathered from the grip to the upstream side of the dam site, turn peat over in the grip to create a wedge-shaped dam in the area stripped of vegetation 3-4m thick (see Figure 1.2). It is likely that additional un-oxidised peat will be needed to complete the dam. This should be gained from a borrow pit adjacent to the grip. The borrow pit must be located within 90 o or 180 o of the machine but within easy reach without moving the excavator. Strip the vegetation from the borrow pit in as large an unbroken turf as possible and store to one side for later use. Excavate un-oxidised peat from the borrow pit and complete the dam making sure that it is keyed into both sides of the grip by 1.5-2m and the base by 0.3m. The peat dam should be finished 1.5m higher than the surrounding ground to allow for settlement (see Figure 1.2) Make good the borrow pit by stretching the previously stored vegetation across it thus leaving a shallow depression. Use all other previously excavated vegetation to cover any bare peat Create a runoff via a shallow overflow channel to the lower side of the grip upstream of the new dam, ensuring excess water can be dispersed onto moorland without causing subsequent surface erosion (see Figure 1.2) Category 4 grips should be completely re-profiled between dams. A suggested technique is as follows but it is recognised that individual contractors may have other reprofiling methods available:

6 The vegetation either side of the grip immediately upstream of the dam is removed and used to cover part of the dam. The newly exposed peat is then turned over to lift the base of the grip and the steep side slope is re-profiled to approximately 45 o. The peat from the sides and base are worked over and compacted to create a gentle undulating curve (see Figure 1.4). Ensure that the downhill edge of the grip is slightly lower than the uphill edge to facilitate even overflow of collected water allowing even re-wetting and avoiding creation of secondary erosion channels. Moving up the grip the next section of vegetation is removed and used to cover the previously exposed and now re-profiled peat. Repeat this process until along the length of the grip. At the top end of the grip section unbroken turves from adjacent to the grip may be needed. Make good the area where the turves are taken from by teasing in the surrounding vegetation Other Methods The Yorkshire Peat Partnership recognises that with previous projects specified by several organisations and with the restoration work having been carried out by numerous experienced contractors, a number of differing techniques have evolved, thus the Yorkshire Peat Partnership will consider recommending the acceptance of tenders based on methods which differ from those specified, provided that the tenderer supplies a compelling case for their preferred method, supported by a method statement and diagrams if necessary, along with a price for the specified method and a price for their preferred method. TT/July 2010 (Mod TT Sep 2011)

7 Figure 1.1: Grip classes Category 1b - Blocked grip Category 3 - Eroding and deeply scoured; >1000mm- 1500mm wide; >1000mm-1200mm deep Category 1f - Flowing grip up to 600mm wide; 650mm deep Category 4+

8 Figure 1.2: Peat dams

9 Figure 1.3: Spacing of grip/gully dams

10 Figure 1.4: Reprofiling