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Title: The Impact Of Particulate Outputs Associated With Timber Harvesting
Author: D Marks S
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_195, Representation ID: 25, Object ID: 1544
Abstract:
Increased river sediment yields associated with forestry are likely to be-particularly significant during harvesting. -Extensive tree planting took place in the UK aduring the 20 years after the Second World War. With,many forests now reaching maturity,. timber harvesting in Great Britain is expected to increase by 70% above 1996 levels in the following two decades. It is therefore important to understand the dynamics .of ,particulate outputs arising -from modern timber harvesting practices in order.to evaluate. the-potential adverse impacts and identify possible prevention and amelioration strategies. Literature was reviewed in order to identify- the forestry practices responsible for enhanced particulate outputs,.and the,potential impacts upon water resources and aquatic life. The long-term record. of. sediment fluxes within the Institute of: Hydrology Experimental catchments at Plynlimon- provided the foundation for a detailed :field study. The existing monitoring network was intensified to enable the investigation of particulate outputs associated with the harvesting of a 13 ha plot in the Nant Tanllwyth catchment. This representedtypical Forest Enterprise (FE) practice in England.and Wales, which normally involves the felling of lo15 ha plots using a range of methods depending on specific site conditions. Detailed studies of suspended sediment concentration, bed load transport, river gravel composition and channel bank erosion were undertaken. In parallel, a detailed record was built up of the harvesting operation (January-June 1996) which involved three separate techniques. Suspended sediment yields for 1995 and 1996 increased by 83% (from 24 to 44 t kma yr-a) in the Tanllwyth catchment subjected to timber harvesting, but by only 44%a(from-16 to 23 t kma yr-a) in the adjacent Hafren catchment, which was only affected by a small-amount of harvesting in the headwaters of its catchment: During 1996, the mean suspended sediment concentration in the Yant Tanllwyth above and below the harvesting site was 2 and 7 mg l-a respectively, while at control sites on the upper. and lower Hafren the mean suspended- sediment concentrations were both 4 mg I-a. Although these initial resultsindicate enhanced suspended sediment outputs associated with the harvesting operation, this increase is much lower than those recorded in previous studies of suspended sediment -outputs associated with larger felling sites before publication of the First-Edition of Forests and Water Guidelines. In the light of critical thresholds for fishery damage, the limited duration of exposure to potentially harmful suspended sediment concentrations is unlikely to significantly harm aquatic life. Bed load yields for 1995 and 1996 increased by-,13% (from 8.6 to 9.7 t kma. yia) in the.Nant . : Tanllwyth catchment, and 15% (from 1.3 to 1.5 t km2 yr-a) in the moorland Afon Cyff catchment. Due to possible lag effects, a longer post-felling time series of bed load data will be required to assess the full impact of the harvesting operation upon bed load yields in the Nant TanIlwyth catchment. Following timber harvesting in the Tanllwyth catchment, river gravel samples-collected by freeze coring revealed an increase in fine material content in the Afon Hafren, below its confluence with the Nant Tanllwyth; However, in the light of published critical thresholds of gravel fines content, none of the samples collected from-watercourses affected by the harvesting operation contained sufficient material to adversely affect salmonid spawning. R and D Technical Report P140 1 Timber harvesting in the riparian zone of the Nant Tanllwyth resulted in a statistically significant increase in the erosion rates of adjacent channel banks. This was associated with a reduction in winter temperatures due to removal of the insulating forest canopy. An increase in the period of sub-zero channel bank temperatures is likely to increase erosion through frost action. The intensive research programme at Plynlimon is supported by studies at other harvesting sites. Higher suspended sediment concentrations were recorded in a catchment (the Afon Biga) affected by a smaller felling operation (6.5 ha) where the steeper gradient and large number of trees in the riparian zone are likely to have contributed to increased particulate outputs. Details of previous sediment pollution incidents associated with timber harvesting investigated by the Environment Agency, and field visits to existing harvesting sites throughout England and Wales, has enabled the identification of the most significant sources of particulate outputs associated with timber harvesting. Combination of the intensive Plynlimon research programmes, monitoring and field observations at other harvesting sites and the collation of information from the Environment Agency pollution archive has given a broad view of particulate outputs associated with timber harvesting upon which recommendations have been based for prevention, amelioration and assessment of these impacts. The most effective and practical measures which can be employed to prevent and / or ameliorate particulate outputs associated with timber harvesting are already supported by existing guidance in the Forests and Water Guidelines (Forestry Authority, 1993) and, more specifically, the FE Wales Harvesting Manual (Killer, 1994), and Forestry Commission Soft Ground harvesting Report (Forestry Commission, 1991b). Results from the detailed monitoring programme at Plynlimon indicate that modern FE harvesting practice is likely to result in less severe particulate outputs than recorded in previous studies, which involved the felling of a larger proportion of a catchment. In the future, the combination of both afforestation and felling in strict accordance with current guidance should further reduce the impacts of particulate outputs associated with timber harvesting. Examples of timber harvesting operations responsible for pronounced particulate outputs that were identified did not conform to current guidance (Forestry Commission, 1991b; Forestry Authority, 1993; Killer, 1994). It is therefore recommended that every effort is made to ensure that such good practice is strictly adhered to by all forest operators, including both FE and private companies / contractors.
Publisher: Environment Agency
Subject Keywords: Sediment; Forestry; Partuclates; Wood
Extent: 130
Permalink: http://www.environmentdata.org/archive/ealit:4384
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