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The relative effects of a Pinus plantation on the hydrology of an Atlantic dune system. (Newborough Warren case study)
Martin Hollingham (2008)
Site and data Description
Vegetation and Geology
Newborough Forest and Warren form part of a dual spit dune system on the SW corner of Anglesey, Wales , UK . This study concerns the southern dune system lying on the Menai Straits. The current forest area was planted with a mixture of P. nigra and P. sylvestris in 1947. The parts of the forested area adjacent to the warren were clear felled in 1993-1996, the northern clearing was replanted around 1997, the southern clearing replanted in 2001-2003 (figure 2). The remaining forest below the rock ridge has been heavily thinned to between 600 and 250 trees/ ha. Freeman (2008) found that Forest interception was 24% of precipitation and stem flow 1% in a plot near to piezometer 2B (figure 2). The warren consists of a mixture of mobile and fixed dune vegetation, grading to grassland inland. The area is grazed by ponies and rabbits, but the dune system is becoming less mobile inland.
Figure 1 shows the LIDAR 25m topography of the study area. The underlying bedrock exposed at the surface in the forest, overlain by Holocene sand and gravel deposits which grade into marine clay and glacial till inland and north eastwards along the Menai Straits. The Precambrian\ Palaeozoic basement consists of three stepped blocks. Separating the blocks are two faults. (Betson et al., 2002). The first fault is not visible on the surface and runs in a line roughly from the southern wetlands of the lake, Llyn Rhos Ddu to the shore along the forest/ warren boundary. The second fault follows the same axis, 750m north, flowing the edge of the rock ridge, and is visible on the surface (figure 1). The middle block is forested, on the block to the south lies the Warren and to the north the afforested rock ridge.
Betson et al. (2002) measured the particle size of the sands at Newborough and found it to be almost uniformly fine sand in the range 0.1 - 0.3 mm. They also measured the saturated hydraulic conductivity and porosity in the horizontal and vertical orientations in dunes, slacks and close to the coast, they found K to be variable, Ky values <10m/d in slacks, Ky >10m/d in the dunes and coast, and Kxz <10m/d. The overall average effective porosity was estimated as 37%, and the average hydraulic conductivity as 11m/d.
The slope vectors based on the topography indicate that runoff from the higher ground of the rock ridge drains into the sands of the warren west of Llyn Rhos Ddu, and then flows radially out to the sea. The area can be divided into two topological catchments, one largely affected by Llyn Rhos Ddu which is occupied by the major part of the warren, and one affected by the forest. The catchment boundaries are marked on figure 1.
Figure 1 : Map of topography and slope vectors for Newborough Forest and Warren
The majority of runoff and groundwater flow for the Llyn Rhos Ddu affected catchment comes from the agricultural land on the southern slopes of the rock ridge east of the forest. However it is captured by the drains serving Llyn Rhos Ddu which drains out south east along the eastern edge of the warren, past residential developments at Pen Lon into the river, Afon Braint. Llyn Rhos Ddu is in hydrological contact with the sands of the warren, and the level is controlled by a sluice.
The forested affected catchment receives runoff and groundwater flow largely from the forested western part of the rock ridge, the topography directs surface and subsurface flows across the downthrown block towards the western edge of the warren and to the sea.
Copyright © Martin Hollingham