Technical Appendix 8
A geomorphic re-appraisal of the River
Severn Roundabout GCR site
Final Report
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Contents
Page
List of Figures
2
List of Tables
4
Executive Summary
5
Acknowledgements
6
1. Project background
7
2. Project aims and objectives
9
3. Methods
9
4. River channel change at the GCR site
10
5. Floodplain morphology at the GCR site
10
6. Subsurface alluvial features and sedimentary sequences at the GCR site 11
6.1 GPR investigations
11
6.2 Coring investigations
26
7. Development of the Roundabout meander loop
26
8. Holocene valley-floor development at the GCR site
32
9. Justification of the proposed GCR site boundary
39
9.1 General criteria for the positioning of the GCR site boundary
39
9.2 The proposed GCR site boundary
39
10. Future research
40
11. A suggested revised GCR Statement of Interest for the
Severn Roundabout site
40
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Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
List of Figures
Page
Figure 1:
Location of the River Severn Roundabout GCR site
7
Figure 2:
The River Severn Roundabout GCR site
8
Figure 3:
River channel change at the Roundabout 1845-2002
12
Figure 4:
River bank erosion and channel deposition at the
Roundabout 1845-2002
13
Figure 5:
Digital elevation model of the Roundabout reach
14
Figure 6:
Valley floor transect showing super-elevation of the floodplain
between the flood embankment and valley side, and the natural
levée
15
Figure 7:
Geomorphological map of the Roundabout GCR site
16
Figure 8:
Geomorphological map of the Roundabout meander loop
showing the digital elevation model with a resolution of 25 cm
17
Geomorphological map of the Roundabout meander loop
showing locations of GPR transects and cores
18
Figure 9:
Figure 10: GPR survey of transect 1 showing three buried palaeochannels 19
Figure 11: GPR survey of transect 2 showing two sets of lateral accretion
units interspersed with channel-fill sediments
20
Figure 12: GPR survey of transect 3 showing buried surface and
palaeochannels
22
Figure 13: GPR survey of transect 4 showing lateral accretion units and
palaeochannel
23
Figure 14: GPR survey of transect 5 showing lateral accretion units and
natural levée
24
Figure 15: GPR survey of transect 6 showing palaeochannels and natural
levées
25
Figure 16: Sedimentary sequence recovered in core 1
27
Figure 17: Sedimentary sequence recovered in core 2
28
Figure 18: Development of the Roundabout meander loop.
29
Figure 19: Erosion and deposition at the Roundabout meander loop
between 1845 and 2002
30
Figure 20: Geomorphological map of the Severn valley at the northern
end of the GCR site
33
Figure 21: Cores from multithread palaeochannels to the west of the
current River Severn at the Roundabout
34
Figure 22: Location of valley-floor cross-sections
36
Figure 23: Valley-floor cross-sections from the Roundabout reach
37
Figure 24: Schematic valley-floor cross-section showing burial of preMediaeval floodplain and development of natural levées
38
2
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 25: The current GCR boundary
41
Figure 26: The proposed GCR boundary
42
3
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
List of Tables
Page
Table 1:
Data sources used to reconstruct historical channel change
Table 2:
Radiocarbon-dated samples from the Buttington reach, River
Severn
31
Radiocarbon-dated samples from the Roundabout reach
35
Table 3:
4
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Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
EXECUTIVE SUMMARY
This project was commissioned by the Countryside Council for Wales (CCW) to
evaluate the Holocene fluvial geomorphology and river channel dynamics of the River
Severn Roundabout GCR site, near Welshpool, with a view to re-appraising its
boundary on a scientific basis. Holocene river channel dynamics at the site were
reconstructed using interpretative geomorphological maps, ground-penetrating
radargrams and 5 floodplain cores. Analysis of maps and aerial photographs dating
from 1845 to 2002 showed that the river channel has been remarkably stable since the
mid-nineteenth century with channel change restricted to small amounts of bank erosion
and in-channel deposition on the outside and inside of meander bends, respectively.
Natural levées border the current river channel and the construction of artificial flood
embankments before the end of the nineteenth century has concentrated recent
sedimentation within a narrow (<500 m) corridor along the present channel. Subsurface
investigations focussed on the Roundabout meander loop suggest that the sinuosity of
the channel increased significantly during a phase of rapid fine-grained sedimentation
which is thought to have begun during the Mediaeval period. Investigations outside the
present GCR boundary indicate that a slowly aggrading anastomosing river system
existed in the reach from 4800 to 3200 BC.
The conservation value of the Roundabout meander is very high because it is probably
the only British example of a highly sinuous, multi-loop, meander which has persisted
with little change for more than 150 years, in spite of having a very narrow neck. Based
on the geomorphological evidence gained through this project it is recommended that
the GCR site be reduced in size (shortened) and focussed on the Roundabout meander
loop which is the main feature of geomorphological interest. The revised GCR boundary
includes the entire Roundabout meander loop and also encompasses an area of valley
floor which lies between the valley side to the east and an artificial flood embankment to
the west. Rapid fine-grained sedimentation in this valley floor area, both before and
after the construction of the flood embankments, led to the development of natural
levees and also restricted lateral river channel migration; these processes were critical
to the development of the present morphology of the Roundabout meander loop.
Inclusion of this area of valley floor within the GCR boundary, therefore, provides an
important and necessary geomorphological context for interpreting the Roundabout
meander GCR site.
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Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Acknowledgements
We would like to thank all those who have assisted in any way with this project,
particularly Stewart Campbell for acting as CCW’s nominated project officer. We are
also grateful to Jonathan Morgan, Peter Evans and Tony Gittins (EA Flood Defence,
Shrewsbury) for the provision of cartographic data, and to Dr Eric Johnstone, Dr Gez
Foster, Matt Rowberry and Catherine Swain for their assistance with geomorphological
mapping, coring and GPR survey. Finally, this work would not have been completed
without the kindness and co-operation of the many landowners in the Severn Valley; we
are most grateful to them for allowing us access to their land and for offering us their
generous support.
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Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
1. Project background
The River Severn Roundabout Geological Conservation Review (GCR) site, Powys,
Wales, extends 2.7 km from near Trewern Farm (SJ 26901247) downstream to Rhydesgyn (SJ 28071500) (Figures 1 and 2). It is an exceptionally good example of a lowgradient multi-loop meandering channel that is currently laterally stable. Channel crosssections have low width-depth ratios and the bankfull channel capacity is lower than
farther upstream so that that this part of the river is prone to flooding and is dominated
by overbank sedimentation (vertical accretion) (Higgs, 1997). The site has been
proposed by the GCR for notification as a Site of Special Scientific Interest (SSSI), but
in the interim may be registered as a Regionally Important Geological Site (RIGS) (S.
Campbell, pers. comm.).
This project has been commissioned by the Countryside Council for Wales (CCW) to
evaluate the Holocene fluvial geomorphology and recent river channel dynamics of the
River Severn Roundabout GCR site with a view to re-appraising its boundary (see
Mathews, 1997).
Figure 1: Location of the River Severn Roundabout GCR site
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Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 2: The River Severn Roundabout GCR site
8
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
2. Project aims and objectives
The main aim of this project was to produce a full and up-to-date assessment of all
active and relict geomorphological features of the River Severn Roundabout GCR site,
enabling boundaries to be fixed on a scientific basis.
Four objectives of the study were defined by CCW as follows:
1. To produce a rigorous justification of the proposed site boundary.
2. Systematically to classify all Holocene river terraces and palaeochannels at the
site, and areas of the floodplain and channel.
3. To produce a single colour-coded geomorphological map of the site showing the
location of features of interest (e.g. river terraces, palaeochannels, alluvial fans,
mid-channel and point bars etc.).
4. To elucidate alluvial architectures and Holocene river development at selected
locations within the site. From these evaluate the relationship between Holocene
river development, climate and land-use change in the reach.
3. Methods
To establish the Holocene alluvial history of this reach, including its historical
development, a range of geomorphological and geophysical techniques was employed.
Large-scale maps and aerial photographs of the River Severn (dating from 1845 to
2002) were used to determine recent patterns of channel change at the site (Table 1).
For each map or aerial photograph, channel margins and bars were digitised using
ARC/INFO™ and transformed to National Grid co-ordinates.
Table 1: Data sources used to reconstruct historical river channel change.
Date
1845
Source
Tithe maps for the townships of Tirymynech and Varchoel,
Burgedin and Rhetescyn in the parish of Guilsfield
Scale
1:4,752
1884
1948
Ordnance Survey 1st series. Montgomeryshire sheets 16 SW
and NW
Ordnance Survey 2nd series. Montgomeryshire sheets 16 SW
and NW
B&W RAF aerial photography. Sortie No: 106G/UK/1698
1972
Ordnance Survey sheets SJ 21 SE and SJ 21 NE
1992
Colour GEONEX aerial photography.
2002
Colour CUCAP aerial Photography. Sortie No: ZknMU
1:10,56
0
1:10,56
0
1:10,00
0
1:10,00
0
1:10,00
0
1:10,00
0
1905
In order to reconstruct Holocene river dynamics at the site, detailed surface and sub–
subsurface investigations were conducted. Initially, high-resolution topographic maps of
the valley floor (supplied by the Environment Agency, originally compiled by the Severn
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Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Trent Water Authority in 1977) with contours at half-metre intervals and spot heights at
10 cm intervals, were digitised and used to construct a digital elevation model (DEM) of
the site. Using the DEM, the 2002 CUCAP colour aerial photographs (Table 1) and a
walk-over-survey, an interpretative geomorphological map of the site was then
produced showing terraces, palaeochannels, valley margins and alluvial fans.
Ground penetrating radar (GPR) transects (6 in total, covering 920 m) were carried out
using PulsEkko equipment with 100 MHz antennae to establish sub-surface alluvial
sedimentary structures and sequences. Differential GPS was used to provide the highresolution survey data necessary to correct the GPR traces for the effects of surface
topography. Furthermore, two sediment cores were collected using a percussion corer
to ‘ground truth’ the GPR survey, and to collect organic material for radiocarbon dating.
Although surface and sub-surface investigations focussed on the existing Severn
Roundabout GCR site, they were not solely restricted to this area. Geomorphological
mapping and coring were also undertaken across the width of the valley floor; this
provided additional data to augment interpretation of the Roundabout site.
4. River channel change at the Roundabout GCR site
There has been very little river channel change in the study reach since 1845, and there
have been no changes in overall channel planform (Figure 3). The channel pattern has
been remarkably stable with only small changes occurring principally at the major
meander bends. Figure 4 plots sediment erosion and deposition in the reach between
1845 and 2002 and illustrates the very minor changes which have taken place
throughout the reach since 1845.
The pattern of channel change in this reach provides a marked contrast with the River
Severn GCR site at Caersws (Maas et al., 2001). The rapid changes in the Caersws
reach have included a number of meander loop cut-offs, in contrast to the Roundabout
site where the Roundabout meander loop itself has remained stable for over 150 years
despite having a very narrow neck (10 m). There is also a great contrast in the
geomorphological and sedimentological character of the two reaches. The river channel
in the Roundabout reach has a low gradient (0.0004 m m-1) and is confined between
high banks (4.8 m above water level) formed predominantly of silts and clays. In
contrast, the Caersws reach has a higher gradient (0.0021 to 0.0029 m m-1), the river
channel is largely unconfined and bounded by erodible composite gravel banks, which
supply sediment to the numerous bars in the reach.
5. Floodplain morphology at the GCR site
Figure 5 shows a DEM of the Roundabout GCR site and surrounding Severn Valley;
two topographic features are clearly evident. First, the elevation of the floodplain
decreases away from the present river channel indicating the development of natural
levées on the east and west banks of the modern river channel (Figure 5). Natural
levées are created by the vertical accretion process and form when suspended
sediment from overbank flows preferentially settles out close to the river bank. Second,
artificial flood embankments, built before 1884, run sub-parallel to the present river on
both banks, but are discontinuous on the right (east) bank due to the proximity of the
valley side. Between the artificial flood embankments there has been accelerated
sedimentation, resulting in much higher floodplain elevations between the flood
embankments than outside them (Figure 6).
10
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 7 presents an interpretative geomorphological map of the Roundabout reach
produced from combining data from the walk-over survey data, DEM and colour aerial
photographs. The surface topography of the floodplain within the GCR site is very
subdued except in the areas immediately adjacent to the present channel.
Nevertheless, a large number of palaeochannels has been identified within the reach,
many of which appear to show a similar sinuosity to the current channel. To the east of
the river, four coalescing alluvial fans have been identified, which in places have been
truncated by river erosion. In particular, the lateral extent of the most northerly alluvial
fan (SJ 2800013700) suggests that it once occupied a much larger area of the valley
floor. To the north-east of the Roundabout meander loop a single fragment of a much
higher terrace has been identified; it lies approximately 3 m above the valley floor and is
higher than some of the alluvial fan sediments.
Figure 8 shows an enlarged section of the geomorphological map in the immediate
vicinity of the Roundabout meander loop superimposed on the digital elevation model
(DEM). This clearly shows the subdued nature of floodplain topography in the GCR site
in that many of the features mapped in the walk-over survey cannot be identified even
on a 25 cm vertical resolution DEM.
6. Subsurface alluvial features and sedimentary sequences at the GCR site
6.1 Results of GPR investigations
Figure 9 shows the location of the six GPR transects which were surveyed in the
Roundabout GCR site. The transects were located within, and adjacent to, the
Roundabout meander loop with the aim of providing sub-surface data to aid
interpretation of the meander’s development. A velocity of 0.065 m ns -1 was used for
each transect on the basis of prior information about the likely nature of the sedimentary
sequence which was thought to be dominated by silt and clay. The depths of individual
units within the sedimentary sequence, and elevations above Ordnance Datum, are
based upon this assumed velocity. Cores taken from this area (see Figure 9) confirmed
that the sediments are dominated by silt and clay and the elevations given in the
radargrams, therefore, provide a reasonable estimate of the true elevation.
Transect 1
Figure 10 shows the radargram for GPR transect 1. This transect runs in an east-northeast direction from the neck of the Roundabout meander loop across the core of the
meander. It shows three palaeochannels (Figure 10; 1-3), one at approximately 50 m
from the beginning of the transect, the second at 120 m and the third at 200 m. All three
palaeochannels are about 60 m wide. Palaeochannel 1 is approximately 1.25 m deep
and asymmetric in form, with a bar developed at the western side of the channel.
Palaeochannel 2 is around 1.75 m deep and appears to have a more symmetric crosssection. The bases of palaeochannels 1 and 2 are at approximately 58 m A.O.D.
Palaeochannel 3 is shallower and flatter in form (approximately 0.5 m deep) with its
base at approximately 57.5 m O.D. The different cross-sectional shape of this
palaeochannel may be due to the transect crossing it obliquely. The similarity in the
base elevations of these palaeochannels and the pattern of overlying sedimentary units
suggest that they all formed on an old surface which has subsequently been buried by
more than 4 m of fine-grained sediment which has smoothed over and hidden the
subsurface topography. The water level in the current channels is around 59 m,
according to the detailed topographic maps produced from aerial photographs.
11
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
1845
1884
1900
1948
1972
1992
12
N
0
Metres
500
1000
River channel
Figure 3: River channel change in the Roundabout GCR site 1845-2002.
2002
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
N
Channel
Deposition
Erosion
Erosion/deposition
Metres
0
500
1000
Figure 4: River bank erosion (red) and channel deposition (blue) in the Roundabout GCR
site 1845-2002.
13
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 5: Digital elevation model of the Roundabout GCR site and surrounding Severn
Valley showing the location of the cross-section in Figure 6.
14
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 6: Valley-floor transect showing super-elevation of the floodplain between the flood
embankment and valley side (a), and the natural levée (b).
The base of the channel is likely to be 1-2 m below normal water level, which is
comparable with the elevations of these palaeochannels. At the eastern end of the transect
a series of steeply dipping reflectors indicates that the western bank of the current channel
has experienced lateral accretion during this period of fine-grained sedimentation. This
suggests that the river channel may have moved eastwards by approximately 40 m since
the onset of rapid fine-grained sedimentation.
Transect 2
Transect 2 runs from north to south within the core of the Roundabout meander loop. At
the northern end there are sequences of steeply dipping reflectors, indicative of lateral
accretion, interspersed with the flat-lying reflectors of a channel fill (Figure 11). This
suggests that the channel may have moved northwards by as much as 95 m during the
period of fine-grained sedimentation. To the south of this sequence are two channels, with
their bases at about 57 m and 56 m O.D., which have no surface expression. The
sedimentary pattern of the channel fills suggests that they have been inactive during the
period of fine-gained sedimentation. At the south end of the transect is an alluvial unit
boundary, with the older and higher surface to the south, which has only minimal surface
expression despite being a pronounced form near the base of the visible sedimentary
sequence.
15
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 7: Geomorphological map of the Roundabout GCR site.
16
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
17
Figure 8: Geomorphological map of the Roundabout meander loop superimposed on the 25 cm vertical resolution DEM.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
18
Figure 9: Geomorphological map of the Roundabout meander loop, River Severn, showing locations of GPR transects and
cores.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
19
Figure 10: GPR survey of transect 1 showing the three buried palaeochannels.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
20
Figure 11: GPR survey of transect 2 showing two sets of lateral accretion units interspersed with channel-fill sediments.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Transect 3
In transect 3 (Figure 12), which runs from south to north, there is a series of steeply dipping
reflectors, interpreted as a lateral accretion sequence, at the southern end. To the north of
this is a sequence of flat-lying reflectors, which is thought to correspond to the older alluvial
unit apparent at the southern end of transect 2. The surface topography of this buried
alluvial unit suggests that there may be at least two palaeochannels on it, which have no
topographic expression at the current ground surface. At the northern end of transect 3
there is a channel which may correspond to the southernmost channel in transect 2.
Transect 4
The fourth GPR transect (Figure 13), which runs from south to north, shows a steeply
dipping sequence of reflectors at the southern end, suggesting lateral accretion and
channel migration during the period of fine-grained sedimentation. Between 40 and 90 m
from the southern end of the transect there appears to be a broad palaeochannel. The
width of this channel (70 m), in comparison to other buried channels at a similar depth (60
m), and its flat cross-sectional profile suggests that the GPR transect may have crossed
the channel obliquely. Alternatively, the ‘channel’ might have been a flat surface between
two prominent natural levées. These features have no topographic expression at the
current ground surface.
Transect 5
Transect 5 runs from east-south-east to west-north-west on the west bank of the Severn,
and lies to the north-west of the Roundabout meander loop (Figure 14). At the eastern end
of transect 5 is a series of steeply dipping reflectors interpreted as a lateral accretion
sequence. To the west of this is a natural levée and then a relatively flat surface. Near the
western end of the transect is a pronounced palaeochannel which has its base between 56
and 57 m O.D. This palaeochannel is a prominent feature on the current ground surface.
Transect 6
Transect 6 runs from north-north-east to south-south-west, almost perpendicular to
transect 5. At the northern end of the transect there is evidence for a natural levée
associated with the nearby palaeochannel (see Figures 9 and 15). The prominent
palaeochannel visible in transect 5 crosses transect 6 at about 70 m from the northern end
(Figure 15; 1). The asymmetrical profile of this feature in the radargram is consistent with
its position on the outside of a meander bend. On the southern side of this palaeochannel
there are two narrow lateral accretion sequences. These appear to have been formed
during infilling of the channel and do not appear to have been associated with lateral
channel migration. A second palaeochannel (Figure 15; 2), which has its base below 57 m
A.O.D., lies between 40 and 50 m from the northern end of the transect. This
palaeochannel, which has no topographic expression at the current ground surface,
appears to have been partly buried by a levée associated with palaeochannel 1.
21
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 12: GPR survey of transect 3 showing buried surface and palaeochannels.
22
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 13: GPR survey of transect 4 showing lateral accretion units and palaeochannel.
23
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 14: GPR survey of transect 5 showing lateral accretion units and the natural levée.
24
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 15: GPR survey of transect 6 showing palaeochannels and natural levées.
25
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
6.2 Results of coring investigations
In addition to the six GPR transects, two sediment cores were collected from
palaeochannels within the Roundabout GCR site (Figure 9). These cores were collected to
obtain organic material for 14C dating and to provide information on the sedimentology of
the alluvial fill.
Core 1
Core 1 was taken from a buried palaeochannel which was identified from GPR transect 1
(Figure 10); 5.5 m of sediment were recovered. The dominant size fractions in the top 5 m
of the sequence were clay and silt (see Figure 16). However, below 2 m the sediment also
contained sand and below 4 m gravels were present. In general, the sediments become
coarser with depth, with sand and gravel dominating the sequence between 5.5 and 6 m
below the surface. This corresponds well with the GPR data which show the base of the
palaeochannel between 5.5 and 6 m. A negligible amount of organic material was
recovered from this core and was deemed insufficient for radiocarbon dating.
Core2
Core 2 was taken from a prominent palaeochannel to the west of the flood embankment
(see Figure 9). It reached a depth of almost 3 m (see Figure 17). In the upper half metre of
the core silt was the dominant size fraction. Below this was a layer of silty clay. The
remainder of the sediments consisted of clayey silts interspersed with layers of sandyclayey silt. Sediments from the lowest metre appeared to be similar to the sandy-clayey
silts recovered above although the majority of the sediment from this unit was lost during
recovery. This core also yielded insufficient organic material for radiocarbon dating.
7. Development of the Roundabout meander loop
In order to elucidate meander development mechanisms at the Roundabout reach, subsurface features (palaeochannels and lateral accretion deposits) visible in the radargrams
have been plotted onto the surface geomorphology map (Figure 18). Buried
palaeochannels are shown in green and the dark purple arrows show the direction of river
channel migration based on the buried lateral accretion deposits. The distribution of buried
palaeochannels and lateral accretion sequences suggests that the sinuosity of the channel
increased significantly during the period of rapid fine-grained sedimentation, with foci of
channel migration on the inside of the individual loops of the meander. A sequence of
possible former channel margins, inferred from the locations of lateral accretion deposits, is
shown as a series of black dotted lines in Figure 18. The observed patterns of
sedimentation strongly suggest that lateral accretion sequences will also be present on
three further meander loops associated with the Roundabout feature (pink dashed arrows).
Figure 19 shows that the pattern of erosion and deposition inferred from sedimentary
sequences is consistent with that shown in cartographic analysis of channel changes
between 1845 and 2002. An approximate estimate of the onset of rapid fine-grained
sedimentation within the reach can be obtained using the rate of deposition on the inside of
meander loops during this period. The pattern of reflectors in the radargrams, particularly
those in Figures 10 and 11, suggests that the appearance of lateral accretion units was
simultaneous with the onset of vertical accretion; individual reflectors show both lateral
26
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 16: Sedimentary sequence recovered in core 1. See Figure 9 for core location.
27
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 17: Sedimentary sequence recovered in core 2. See Figure 9 for core location.
28
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
29
Figure 18: Development of the Roundabout meander loop.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
N
Channel
Deposition
Erosion
Erosion/deposition
30
0
Metres
50
100
Figure 19: Erosion and deposition at the Roundabout meander loop between 1845 and 2002.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
accretion on channel banks and vertical accretion burying palaeochannels and other
features along their length. This pattern is indicative of a significant change in
sedimentation style at that time. During the period 1845 to 2002 there was 17.25 m of
lateral accretion at the apex of the northern loop of the Roundabout meander core; a rate
of 0.11 m per year (see Figure 19). Extrapolating this figure based on the length of the
lateral accretion sequence recorded in GPR transect 2 suggests that the onset of rapid
sedimentation occurred around AD 1020. However, rates of lateral channel migration are
likely to have decreased significantly as the depth of fine-grained sediment increased. This
suggests that this phase of sedimentation began somewhat later than AD 1020.
This is consistent with the findings of a geomorphological survey 5 km upstream at
Buttington undertaken to provide contextual information for an archaeological investigation
(see Macklin et al., 2002). Radiocarbon dates from samples obtained at the Buttington site
are shown in Table 2; conventional radiocarbon ages were calibrated using Oxcal version
3.5. The sample from Buttington core 1 was underlain by at least 2 m of clay indicating that
the onset of fine-grained sedimentation occurred before deposition of the sample. The
sample from Buttington core 3 was recovered from a channel fill of gravelly clay underlying
more than 3.5 m of silt. The onset of fine-grained sedimentation appears to have occurred
after sample deposition at this site. The sample from the river bank was recovered from
below a structure associated with a much lower level of the floodplain and therefore
probably also pre-dates the onset of rapid fine-grained sedimentation. These three dates
suggest that rapid vertical accretion began at the Buttington reach sometime after the
beginning of the tenth century and before the end of the thirteenth century.
Table 2: Radiocarbon dated samples from the Buttington reach, River Severn
Sample location
and depth
Laboratory
number
Conventional 14C
Age (yr BP)
Buttington core 1
Palaeochannel B
Depth 2.75 m
Buttington core 3
Buried
palaeochannel
Depth 3.75 m
Buttington river bank
Depth 3.31 m
Beta 164537
820±40
Beta 166217
1040±40
Beta 164538
1010±50
Calibrated 14C Age (2
sigma calibration;
calendar years)
AD 1070 – 1090
AD 1120 – 1140
AD 1140 – 1160
AD 890 – 1040
AD 1100 – 1120
AD 1140 – 1160
AD 890 – 930
AD 950 – 1170
The twelfth century landscape at Buttington, including the remnants of the waterside
structure, has been buried by alluviation to a depth of more than 3 m. Sedimentary
sequences at the Roundabout show similar evidence of a buried landscape to those at
Buttington. The subdued nature of the surface topography contrasts with the prominent
features visible at depth in the GPR transects at both sites. It has been suggested that the
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Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
River Severn may have had more than one active channel in the Buttington reach until the
twelfth century but increased rates of sedimentation after this date rationalised the river into
a single channel (see Macklin et al., 2002). The GPR data suggest that this is also a
possibility for the Roundabout reach.
The integration of geomorphological mapping, coring and GPR data at the Roundabout
meander loop provides a model of valley-floor development in the reach during the last
1,000 years; recent and probably relatively rapid sedimentation has progressively buried
the former valley floor, smoothing variations in former topography, and has partially ‘frozen’
the Roundabout meander loop in its present form through the restriction of lateral migration
by increasingly high cohesive banks.
8. Holocene valley-floor development at the GCR site
Geomorphological and sedimentological investigations of the valley floor outside the flood
embankments have provided evidence of a significantly different fluvial landscape which
existed in this reach before the Mediaeval period. Figure 20 presents a geomorphological
map of the Severn valley floor at the northern (downstream) end of the Roundabout GCR
site and shows that there is a significant difference in the morphology of the valley floor
between those areas close to the current river channel, and within the present flood
embankment, and those that are farther away from it. Those areas farther away from the
modern river are characterised by a complex pattern of palaeochannels indicative of a
multithread channel network. Sediment cores recovered from three palaeochannels
(Figures 20 and 21) revealed a sequence of blue-grey clay-dominated sediments
suggesting that an organo-clastic anastomosing river (Nanson and Knighton, 1996)
previously occupied this portion of the Severn valley. Coarser sediments from the lower
part of core 2 are thought to be a channel-fill sequence. Four organic samples from the
three cores were submitted for radiocarbon dating and returned dates ranging from BC
3520 – 3120 to BC 4910 – 4680 (see Table 3 and Figure 21). These dates suggest that
these anastomosing conditions were present for at least 1,180 years during the midHolocene and, since the youngest of these dated deposits lies more than two metres below
the current ground surface, may have persisted until much more recent times.
The radiocarbon-dated samples confirm that sediments of Holocene age exist to a depth of
almost five metres in this reach. This contradicts earlier suggestions that glacio-lacustrine
sediments are overlain by a thin veneer of alluvium in this area. For example, Thompson
(1982), in a schematic valley cross-section, suggested that the thickness of recent alluvium
in this area is little more than a metre and that glacio-lacustrine alluvium of unknown
thickness underlies the entire valley floor and the neighbouring outwash plain. Although
glacio-lacustrine alluvium has been found on both sides of the Severn valley in this reach, if
it underlies the entire valley floor this must be at depths greater than 5 m.
The older of the two radiocarbon-dated samples from core 2 (see Figure 21) was
recovered from a unit of sandy-clayey silt overlain by a gravelly-silty sand. The sample was
obtained from immediately below the boundary between the two units. The change in
sediment size probably reflects a change in climatic conditions affecting the reach. The age
of the coarser unit corresponds well to a period of increased flood frequency and
magnitude identified in investigations on British rivers (see Macklin et al., in press).
32
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
33
Figure 20: Geomorphological map of the Severn valley at the northern end of the GCR site.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
34
Figure 21: Cores from multithread palaeochannels to the west of the current River Severn at the Roundabout.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Table 3: Radiocarbon-dated samples from the Roundabout reach.
Roundabout
Grid
Sample Lab. ID Conventional Calibrated 14C Age
14
Reference Type Number
C Age (yr
(2 sigma)
BP)
SJ 263 148 Plant
Wk 15770
4611±36
BP 5470 – 5350
material
or BP 5340 – 5280
or BP 5170 – 5130
or BP 5110 – 5070
Roundabout
SJ 267 140 Wood
Wk 15771
4481±36
BP 5300 – 5030
or BP 5020 – 4970
BC 3350 – 3080
or BC 3070 – 3020
Roundabout
SJ 267 140 Wood
Wk 15772
4708±43
BP 5590 – 5500
or BP 5490 – 5320
BC 3640 – 3550
or BC 3540 – 3370
Roundabout
SJ 267 142 Wood
Wk 15773
5906±46
BP 6860 – 6820
or BP 6810 – 6630
BC 4910 – 4870
or BC 4860 – 4680
Sample
Location
Calibrated 14C Age
(Calendar Years)
BC 3520 – 3400
or BC 3390 – 3330
or BC 3220 – 3180
or BC 3160 – 3120
Figures 22 and 23 show the location and topography of three valley-floor cross-sections,
and illustrate the height relationship between those areas formed by an anastomosing and
aggrading river channel during the mid-Holocene and those adjacent to the current river
channel.
All three cross-sections show that the highest elevations on the valley floor are found
between the flood embankments. Elevations decrease outside the flood embankments.
The lowest floodplain elevations are found in areas between 0.7 km and 1.5 km from the
current river channel. Evidence for a slowly aggrading anastomosing river system is found
in areas of the floodplain with low elevations (see Figure 20). The presence of natural
levées along the current river channel has been shown at a larger scale in Figure 6. This
provides a model of Holocene valley-floor development at the site which is illustrated
schematically in Figure 24: a slowly aggrading anastomosing river system deposited the
blue-grey sediments (pre-Mediaeval alluvium) and produced a relatively flat floodplain. In
the Mediaeval period there was a significant increase in sediment supply to the reach. The
anastomosing river system was rationalised into a single channel. Natural levée formation
began adjacent to the river channel. The building of flood embankments near to the river
channel concentrated the deposition of overbank sediment in areas close to the river
producing the marked difference in elevation between areas within and outside of the flood
embankments.
Dating of the change in rate of sediment supply is currently based on extrapolation of
recent rates of channel change and correlation with the Buttington reach, 5 km upstream.
Further field investigations at the site may yield datable organic material from the area
adjacent to the current river channel which would enable this approximate date to be
confirmed or modified. Additional field investigations may also enable this model of
Holocene valley-floor development to be refined. At present two issues remain outstanding.
Firstly, the current model suggests that blue-grey sediment deposited by the anastomosing
river system should be present in the banks of the current river channel, most probably on
the outside of meander bends. Exposure of channel-bank sediments within the reach are
35
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 22: Location of valley-floor cross-sections.
relatively poor but it is still surprising that an outcrop of this material has not yet been
found. Secondly, comparison of the former floodplain surface apparent in GPR radargrams
with the floodplain in areas where sediments deposited by the anastomosing river are
found close to the ground surface shows a discrepancy in elevation of up to 2 m. For
example, the floodplain surface associated with buried palaeochannels in GPR transect 1
(see Figure 10) lies at a depth of approximately 5 m below the current floodplain surface.
The floodplain surface in areas of multithread palaeochannels associated with
anastomosing river conditions lies up to 3 m lower than the floodplain surface adjacent to
the current river channel. This difference is not yet adequately accounted for in the model
of Holocene valley development.
36
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
37
Figure 23: Valley-floor cross-sections from the Roundabout reach.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
38
Figure 24: Schematic valley-floor cross-section showing burial of pre-Mediaeval floodplain and development of natural
levées.
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
In summary, geomorphological and sedimentological evidence from the River Severn
valley floor at the Roundabout reach contain a detailed record of Holocene valley
development dating back to at least BC 4910 – 4680. The development of the valley floor
in this reach has been complex, involving a phase of slow aggradation under anastomosing
river conditions as well as a more recent phase of rapid sedimentation along the margins of
the current single-thread channel. The large width of the valley floor in this reach coupled
with the lack of datable organic material from more recent sediments mean that the pattern
and timing of valley-floor formation are not yet fully understood. Further research at this site
will aim to address these problems in order to refine the current model of valley
development and precisely date the various stages.
9. Justification of the proposed GCR site boundary
The Roundabout meander is the main feature of interest in the GCR site, being a unique
example of a highly sinuous meander loop which has persisted with little change for more
than 150 years, despite having a very narrow neck. Other low-sinuosity meander bends
within the present GCR site are much less noteworthy and are an almost ubiquitous feature
on most British rivers. Since the River Severn Roundabout GCR site was first established
the flood embankments (argae) have been realigned and enlarged and a significant
number of field boundaries, used to define the original limits of the GCR site, have
disappeared (see Figure 26). Based on the available geomorphological evidence
presented above, several criteria were used in the re-evaluation of the GCR boundary.
9.1 General criteria for the positioning of the GCR site boundary
 The proposed GCR site should include geomorphological features of direct
relevance and interest to the Roundabout meander loop. We believe that the
present GCR site is too large and should be shortened and centred upon the
Roundabout meander loop.

Ideally the GCR boundary should follow geomorphic boundaries. However, given
the subdued surface topography in the reach, flood embankments and field
boundaries have been used as well. In practice, the boundary follows hedge lines,
roads, argae and other permanent linear features. Where it has not been possible to
follow such features, the boundary follows a straight line between two permanent
features (e.g. two hedge lines).

Although the interpretation of the development of the Roundabout meander loop has
involved the use of both surface and sub-surface data, the proposed revised GCR
boundary only respects surface features. Given the fact that most sub-surface
features are buried by at least 4 m of alluvium, and are unlikely to be damaged by
normal agricultural activity, we do not see the need to include within the revised
GCR boundary buried features which presently lie outside the boundary.
9.2 The proposed GCR site boundary
The following section describes the location of the proposed GCR boundary, based on the
criteria outlined above. The description commences from the southwestern corner of the
proposed boundary (west side of the river), and is described in a clockwise direction (see
Figure 26).
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Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
The upstream limit of the boundary crosses the River Severn west-north-west of Trewern
Farm; on the east side of the river it follows field boundaries in a east-south-east direction
towards Trewern Farm, where it turns north. After 300 m it turns east until it meets the
minor road to Trewern Farm. It follows this road north and then east to Old Mills. The
boundary then follows field boundaries around Old Mills and continues north for
approximately 300 m. It then follows a field boundary east for 100 m before turning north
across a field to meet another field boundary. It follows field boundaries north for
approximately 300 m and then west for 100 m to meet the river. The boundary then runs
upstream west along the centre of the river channel around a small meander loop,
generally heading west. It then joins the argae at the point where the course of the argae
begins to run parallel to the river. The boundary then follows the argae to the upstream end
of the site.
The GCR boundary proposed in this report is shorter than the current GCR boundary. The
total area of the proposed GCR site is 0.46 km2 compared to 0.83 km2 for the original
boundary; this is a decrease in area of 44 %.
10. Future research
This study has identified the main surface and subsurface geomorphological features in the
Roundabout meander loop and has attempted to identify the controls on meander and
valley-floor development. Nevertheless, a more detailed understanding of river valley
development can be achieved through further geomorphological investigations. Some of
the main avenues for further research are outlined below.
Analysis undertaken by Macklin et al. (1994) and cited in Taylor and Lewin (1996)
upstream at Welshpool has shown that the floodplain sediments contain heavy metals
which can be used to date historic alluvial deposits and determine rates of floodplain
sedimentation. It is likely that similar levels of heavy metals exist at the Roundabout site
and that the technique could be used to complement radiocarbon dating.
Further subsurface investigations could be used at the Roundabout site to examine the
nature of the buried alluvial surface and more accurately determine its depth. Similar
methods could also be used to determine the location and nature of the buried boundary
between the mid-Holocene surface and that which existed prior to the onset of fine-grained
sedimentation.
11. A suggested revised GCR Statement of Interest for the Severn Roundabout site
The Roundabout meander on the River Severn, near Welshpool, mid-Wales, is an
exceptionally good example of a low-gradient multi-loop meandering channel that is
currently laterally stable. A detailed history of the Roundabout meander loop, and of valleyfloor development during the last 7,000 years, has been established from
geomorphological and sedimentological investigations. The Roundabout meander loop
increased in sinuosity and became partially ‘frozen’ in its current form due to rapid finegrained sedimentation which probably began during the Mediaeval period. This accounts
for both its tortuous planform and its persistence and stability. This period of rapid
sedimentation has also buried the previous floodplain surface in parts of the valley floor,
forming the prominent natural levées along the current river channel. Previously, a slowly
aggrading anastomosing river system was present in this portion of the Severn Valley.
40
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 25: The current GCR boundary.
41
Technical Appendix 8: A geomorphic re-appraisal of the River Severn Roundabout GCR site
Figure 26: The proposed GCR boundary.
42