Geology Reference
In-Depth Information
than replace it with a purpose-designed buffer, par-
ticularly if the buffer is broadly in an appropriate
place in the landscape. Although field observations
and measurements could be used to evaluate an
individual buffer, a more generic approach is
required for developing farm management plans.
This can best be achieved by using erosion models.
This chapter describes the application of an
erosion model for the assessment of buffer per-
formance at two field sites, where measurements
were made of sedimentation within the buffer
and the characteristics of the buffer architecture
over a 14-month period. The first step is to decide
which of the many erosion models found in the
literature is the most appropriate to use. A selec-
tion procedure is developed based on establishing
criteria that the models should satisfy. The cho-
sen model is then applied to the field conditions.
Attention is paid to the way the model is set up
to give the best representation possible of the
pattern of runoff movement over the land surface
and the resultant spatial distribution of erosion
and sedimentation. The model output is compared
with the measured data and a judgment is made
on whether the model is suitable for evaluating
existing buffers and for making recommendations
on how their performance might be improved.
at the downslope end established in 2001/2002
under the Countryside Stewardship scheme. The
grass varies between 50 and 200 mm in height in
the winter and 300 and 500 mm in the summer
with a ground and canopy cover of some 80-100%.
The buffer seems to be well-maintained. During
the study period, the field above the buffer was
under maize during the summer months and bare
over the winter.
The second field of 97,000 m
2
on a sandy clay
loam soil of the Hodnet Series has a 6.9 m long
grass strip at its downslope end, below which is a
fence, followed by an area of shrubs, small trees
and an irrigation pond. The field is subject to
erosion when bare of crop, and in 2004 a gully
with an average width of 1.07 m and an average
depth of 0.45 m had developed perpendicular to
the buffer. At the base of the gully, covering the
buffer strip from front to back, was a 33 m wide
area of sedimentation (Plate 11). The field slopes
down to the buffer at 7°, but the slopes also
converge across the field towards the gully form-
ing contributing side slopes at an angle of 3-4°.
Sediment also covered the buffer at the corner of
the field below a set of farm tracks where the land
was used for turning vehicles. Along the rest of
the buffer, the grass covered 60-100% of the
ground surface, although the grass within the
back 3 m was rougher and more tussocky than
that at the front, and there was a 50 mm step from
the field surface up to the buffer edge, most
probably brought about by ploughing. The field
was subsequently planted to wheat, after which
the gully became less deep and the grass started
to re-emerge through the sediment in the buffer.
In the spring of 2006, a grass waterway was
planted along the line of the gully. This appeared
to prevent any further gully development and
reduce the transport of sediment (Plate 12).
At both field sites the buffer areas were instru-
mented to assess their ability to trap and retain
sediment eroded from the slopes above (Owens
et al
. 2006). Sediment was collected on astroturf
mats installed within the buffer at the front
(upslope boundary), mid-buffer and back (down-
slope edge) (Plate 13). The mats were installed in
December 2004 and were inspected for sediment
13.2
The Study Area
The study area comprises two arable fields within
the basin of the River Tone, Somerset, England,
where the hilly land is known for its high rate of
erosion and high delivery of sediment to
watercourses (McHugh
et al
., 2002; Murdoch &
Culling, 2003). The mean annual rainfall of the
basin ranges from 800 to 1000 mm, with highest
monthly totals in December and January and the
driest months from April to July. July and August
are the warmest months when the mean daily
maximum temperature is 21°C; January is the
coldest month with mean daily minimum tem-
peratures of 1-2°C (Meteorological Office, 2005).
The first field, on a sandy clay loam soil of the
Crediton Series, has an area of 39,000 m
2
, a slope
length of 195 m at an angle of 2°, and a grass buffer
