Coupling between hillslope gully systems and stream channels in the Howgill Fells, northwest England: temporal implications/Le couplage des systèmes de ravins et des lits fluviaux dans les Howgill Fells, nord-ouest de l
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Coupling between hillslope gully systems and stream channels in the Howgill Fells, northwest England: temporal implications/Le couplage des systèmes de ravins et des lits fluviaux dans les Howgill Fells, nord-ouest de l'Angleterre : signification temporelle - article ; n°1 ; vol.3, pg 3-19

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Géomorphologie : relief, processus, environnement - Année 1997 - Volume 3 - Numéro 1 - Pages 3-19
Abstract Hillslope gullies, cut in glacial till, supply sediment to the channel of Carlingill, a small upland stream in the Howgill Fells, northwest England. Coupling between the two zones is fundamental to the dynamics of the whole system. Gully development is initiated by stream erosion at the slope base, and maintained by the periodic removal of the eroded sediment by stream floods. As gully development proceeds the strength of coupling between gully and stream channel decreases, and is ultimately broken, leading to the stabilisation of the gullied slopes by revegetation. The coarse sediment supplied by the gullies becomes the stream bedload controlling the stream dynamics. The channels in zones away from gully sediment sources tend to be narrow, single-thread, relatively stable channels. Those downstream of gully sites tend to be wide, often braiding and unstable. Monitoring of sediment movement between gullies and stream channels, at two spatial scales, over temporal scales of 6 and 25 years, has identified a decrease in coupling with progressive gully development, causing a decrease in the frequency of sediment supply events and an increase in the complexity of sediment storage within the system. Over the longer term, two phases of gully development can be identified. Large gullies developed in the late Holocene, probably ca 1 000 yrs BP, which overloaded the system, causing deposition of large debris cones and alluvial fans. More recently, over the last ca 200 years, smaller gullies developed and locally stabilised. Their transient influence on channel morphology has been identified through the application of lichenometry to the valley floor morphology. Coupling relationships are fundamental to the geomorphology of slope and channel systems in this environment, over a range of timescales.
Résumé Des ravins entaillent des versants, façonnés dans une couverture morainique et déversent des sédiments dans le lit du Carlingill, petit torrent des Howgill Fells, au Nord Ouest de l'Angleterre. La définition des liens de couplage qui existent entre ces deux éléments du système est fondamentale pour comprendre la dynamique du système dans son ensemble. La formation des ravins est liée à l'érosion de la base du versant par le torrent; ils se maintiennent grâce au travail de curage des crues qui emportent périodiquement les sédiments qu'ils y ont apportés. Cependant, au cours de l'évolution des ravins, l'importance des liens qui les relient au torrent se relâche pour finalement disparaître, ce qui conduit à leur stabilisation par revégétalisation de leurs pentes. Les sédiments grossiers fournis par les ravins deviennent la charge de fond du torrent et un élément fondamental de sa dynamique. Les lits fluviaux hors des zones ravinées ont tendance à être étroits, à chenal unique et relativement stables. Ceux situés en aval des versants ravinés sont larges, instables, et souvent disposés en tresses. L'étude des transports de sédiments entre les ravins et les lits des torrents, à deux échelles spatiales, et plusieurs échelles de temps (périodes de 6 et 25 ans) met en évidence l'affaiblissement du rapport entre ces transports et la progression du ravinement; ceci provoque une décroissance de la fréquence des événements producteurs de sédiments et une augmentation de la complexité des relais des stocks sédimentaires dans le système. Sur le long terme, nous avons pu mettre en évidence deux phases de ravinement. De grands ravins se sont développés pendant l'Holocène récent, probablement autour de 1 000 BP. Ils ont fourni au système d'énormes surcharges sédimentaires, déposées sous forme de grands cônes de débris et cônes torrentiels. Plus récemment, depuis 200 ans environ, de plus petites ravins se sont développés puis localement stabilisés. Leur influence temporaire sur la morphologie des lits fluviaux a été mise en évidence par des études de lichénométrie appliquées à l'analyse de la morphologie du fond des vallées. Le couplage ravins-lits fluviaux est fondamental pour expliquer la géomorphologie des pentes et des lits fluviaux dans ces milieux, à plusieurs échelles de temps.
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Source : Persée ; Ministère de la jeunesse, de l’éducation nationale et de la recherche, Direction de l’enseignement supérieur, Sous-direction des bibliothèques et de la documentation.

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Adrian M. Harvey
Coupling between hillslope gully systems and stream channels
in the Howgill Fells, northwest England: temporal implications/Le
couplage des systèmes de ravins et des lits fluviaux dans les
Howgill Fells, nord-ouest de l'Angleterre : signification
temporelle
In: Géomorphologie : relief, processus, environnement. Mars 1997, vol. 3, n°1. pp. 3-19.
Citer ce document / Cite this document :
Harvey Adrian M. Coupling between hillslope gully systems and stream channels in the Howgill Fells, northwest England:
temporal implications/Le couplage des systèmes de ravins et des lits fluviaux dans les Howgill Fells, nord-ouest de l'Angleterre :
signification temporelle. In: Géomorphologie : relief, processus, environnement. Mars 1997, vol. 3, n°1. pp. 3-19.
doi : 10.3406/morfo.1997.897
http://www.persee.fr/web/revues/home/prescript/article/morfo_1266-5304_1997_num_3_1_897Abstract
Abstract Hillslope gullies, cut in glacial till, supply sediment to the channel of Carlingill, a small upland
stream in the Howgill Fells, northwest England. Coupling between the two zones is fundamental to the
dynamics of the whole system. Gully development is initiated by stream erosion at the slope base, and
maintained by the periodic removal of the eroded sediment by floods. As gully development
proceeds the strength of coupling between gully and stream channel decreases, and is ultimately
broken, leading to the stabilisation of the gullied slopes by revegetation. The coarse sediment supplied
by the gullies becomes the stream bedload controlling the stream dynamics. The channels in zones
away from gully sediment sources tend to be narrow, single-thread, relatively stable channels. Those
downstream of gully sites tend to be wide, often braiding and unstable. Monitoring of sediment
movement between gullies and stream channels, at two spatial scales, over temporal scales of 6 and 25
years, has identified a decrease in coupling with progressive gully development, causing a decrease in
the frequency of sediment supply events and an increase in the complexity of sediment storage within
the system. Over the longer term, two phases of gully development can be identified. Large gullies
developed in the late Holocene, probably ca 1 000 yrs BP, which overloaded the system, causing
deposition of large debris cones and alluvial fans. More recently, over the last ca 200 years, smaller
gullies developed and locally stabilised. Their transient influence on channel morphology has been
identified through the application of lichenometry to the valley floor morphology. Coupling relationships
are fundamental to the geomorphology of slope and channel systems in this environment, over a range
of timescales.
Résumé Des ravins entaillent des versants, façonnés dans une couverture morainique et déversent des
sédiments dans le lit du Carlingill, petit torrent des Howgill Fells, au Nord Ouest de l'Angleterre. La
définition des liens de couplage qui existent entre ces deux éléments du système est fondamentale
pour comprendre la dynamique du système dans son ensemble. La formation des ravins est liée à
l'érosion de la base du versant par le torrent; ils se maintiennent grâce au travail de curage des crues
qui emportent périodiquement les sédiments qu'ils y ont apportés. Cependant, au cours de l'évolution
des ravins, l'importance des liens qui les relient au torrent se relâche pour finalement disparaître, ce qui
conduit à leur stabilisation par revégétalisation de leurs pentes. Les sédiments grossiers fournis par les
ravins deviennent la charge de fond du torrent et un élément fondamental de sa dynamique. Les lits
fluviaux hors des zones ravinées ont tendance à être étroits, à chenal unique et relativement stables.
Ceux situés en aval des versants ravinés sont larges, instables, et souvent disposés en tresses. L'étude
des transports de sédiments entre les ravins et les lits des torrents, à deux échelles spatiales, et
plusieurs échelles de temps (périodes de 6 et 25 ans) met en évidence l'affaiblissement du rapport
entre ces transports et la progression du ravinement; ceci provoque une décroissance de la fréquence
des événements producteurs de sédiments et une augmentation de la complexité des relais des stocks
sédimentaires dans le système. Sur le long terme, nous avons pu mettre en évidence deux phases de
ravinement. De grands ravins se sont développés pendant l'Holocène récent, probablement autour de 1
000 BP. Ils ont fourni au système d'énormes surcharges sédimentaires, déposées sous forme de
grands cônes de débris et cônes torrentiels. Plus récemment, depuis 200 ans environ, de plus petites
ravins se sont développés puis localement stabilisés. Leur influence temporaire sur la morphologie des
lits fluviaux a été mise en évidence par des études de lichénométrie appliquées à l'analyse de la
morphologie du fond des vallées. Le couplage " ravins-lits fluviaux" est fondamental pour expliquer la
géomorphologie des pentes et des lits fluviaux dans ces milieux, à plusieurs échelles de temps.between hillslope gully systems and stream channels Coupling
in the Howgill Fells, northwest England : temporal implications
Le couplage des systèmes de ravins et des lits fluviaux dans les Howgill Fells,
nord-ouest de l'Angleterre : signification temporelle
A. M. Harvey *
Abstract
Hillslope gullies, cut in glacial till, supply sediment to the channel of Carlingill, a small upland stream
in the Howgill Fells, northwest England. Coupling between the two zones is fundamental to the dynamics
of the whole system. Gully development is initiated by stream erosion at the slope base, and maintained
by the periodic removal of the eroded sediment by stream floods. As gully development proceeds the
strength of coupling between gully and stream channel decreases, and is ultimately broken, leading to the
stabilisation of the gullied slopes by revegetation. The coarse sediment supplied by the gullies becomes
the stream bedload controlling the stream dynamics. The channels in zones away from gully sediment
sources tend to be narrow, single-thread, relatively stable channels. Those downstream of sites tend
to be wide, often braiding and unstable. Monitoring of sediment movement between gullies and stream
channels, at two spatial scales, over temporal scales of 6 and 25 years, has identified a decrease in cou
pling with progressive gully development, causing a decrease in the frequency of sediment supply events
and an increase in the complexity of sediment storage within the system. Over the longer term, two phases
of gully development can be identified. Large gullies developed in the late Holocene, probably ca 1 000
yrs BP, which overloaded the system, causing deposition of large debris cones and alluvial fans. More
recently, over the last ca 200 years, smaller gullies and locally stabilised. Their transient
influence on channel morphology has been identified through the application of lichenometry to the val
ley floor morphology. Coupling relationships are fundamental to the geomorphology of slope and channel
systems in this environment, over a range of timescales.
Key words : gully development, sediment supply, channel geomorphology, lichenometry.
Résumé
Des ravins entaillent des versants, façonnés dans une couverture morainique et déversent des sédi
ments dans le lit du Carlingill, petit torrent des Howgill Fells, au Nord Ouest de l'Angleterre. La définition
des liens de couplage qui existent entre ces deux éléments du système est fondamentale pour comprendre
la dynamique du système dans son ensemble. La formation des ravins est liée à l'érosion de la base du
versant par le torrent; ils se maintiennent grâce au travail de curage des crues qui emportent périodi
quement les sédiments qu'ils y ont apportés. Cependant, au cours de l'évolution des ravins, l'importance
des liens qui les relient au torrent se relâche pour finalement disparaître, ce qui conduit à leur stabilisa
tion par revégétalisation de leurs pentes. Les sédiments grossiers fournis par les ravins deviennent la
charge de fond du torrent et un élément fondamental de sa dynamique. Les lits fluviaux hors des zones
ravinées ont tendance à être étroits, à chenal unique et relativement stables. Ceux situés en aval des ver
sants ravinés sont larges, instables, et souvent disposés en tresses. L'étude des transports de sédiments
entre les ravins et les lits des torrents, à deux échelles spatiales, et plusieurs échelles de temps (périodes
de 6 et 25 ans) met en évidence l'affaiblissement du rapport entre ces transports et la progression du rav
inement; ceci provoque une décroissance de la fréquence des événements producteurs de sédiments et une
augmentation de la complexité des relais des stocks sédimentaires dans le système. Sur le long terme, nous
avons pu mettre en évidence deux phases de ravinement. De grands ravins se sont développés pendant
l'Holocène récent, probablement autour de 1 000 BP. Ils ont fourni au système d'énormes surcharges sédi
mentaires, déposées sous forme de grands cônes de débris et cônes torrentiels. Plus récemment, depuis
* Department of Geography, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, England
Géomorphologie : Relief, processus, environnement. 1997, n° 1 pp. 3-20 4 A. M. Harvey
200 ans environ, de plus petites ravins se sont développés puis localement stabilisés. Leur influence tem
poraire sur la morphologie des lits fluviaux a été mise en évidence par des études de lichénométrie
appliquées à l'analyse de la morphologie du fond des vallées. Le couplage " ravins-lits fluviaux" est fon
damental pour expliquer la géomorphologie des pentes et des lits fluviaux dans ces milieux, à plusieurs
échelles de temps.
Mots clés : ravinement, charge de sédiments, morphologie des chenaux, lichénométrie.
Introduction
The dynamics of fluvial systems are controlled by coupling between zones of sed
iment supply, storage and transport (Schumm, 1977). Of fundamental importance
within upland fluvial systems is the coupling between hillslope sediment sources and
the channel. This controls the volume of sediment delivered to the channel, hence the
potential sediment flux of the channel system and, especially in the case of coarse sedi
ment, the style of channel adjustment. There is also evidence of coupling influencing
the rate of hillslope erosion, where erosion by hillslope gully systems is influenced by
local base-level conditions. Coupling involves the connectivity and the sensitivity of
the system (Brunsden and Thornes, 1979), depending on how variations in sediment
supply relate to the threshold of critical power (Bull, 1979). Sediment overload may
cause aggradation ; sediment deficit may cause incision.
Within the literature, attention has been given to coupling within the channel ne
twork (Knighton, 1989; Phillips, 1992), with some work on sediment provenance
(Woodward et ai, 1992). Of work on hillslope-channel coupling, some is concerned
with downstream response (Brizga and Finlayson, 1994; Harvey, 1987a), and some
with the mechanics of sediment generation, storage and delivery (Harvey, 1991).
Relatively little has dealt with temporal variations in coupling relationships (Harvey,
1977; Harvey et ai, 1979), and even less with their spatial implications.
Three sets of questions may be asked. (1) To what extent is coupling synchronous
and related to individual events, or lagged involving seasonality or other cyclicity, or
progressively changing over longer timescales ? (2) How do these timescales interact,
and do the interactions show spatial variations between hillslope and channel loca
tions ? (3) What are the implications for erosion and deposition within fluvial systems,
and how do these relate to landform development?
The study area
In this paper these questions are addressed in relation to hillslope/channel coupling
in the Howgill Fells, northwest England, over timescales ranging from the individual
event to Holocene landform evolution. The study area experiences a humid temperate
climate, with a mean annual precipitation of ca 1 400 mm, cool wet summers and wet
winters with moderate frost and snow (Harvey, 1992). The Carlingill valley in the
Howgills (fig. 1) is a small (4.6 km2) steep (135-650 m) catchment, dissecting Silurian
mudstone geology. The area was glaciated during the late Pleistocene, with déglacia
tion occurring ca 15 000 BP (Harvey, 1985). Although some of the upper catchment is
deeply trenched into bedrock, elsewhere the hillslopes are mantled with late
Pleistocene soliflucted till. A late Pleistocene stream terrace is cut by the Holocene val
ley (Harvey, 1985; Harvey et ai, 1984).
Géomorphologie : Relief, processus, environnement, 1997, n° 1 pp. 3-20 Coupling between hillslope gully systems and stream channels
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: Relief, processus, environnement, 1997, n° I pp. 3-20 6 A. M. Harvey
Two sets of Holocene gully systems dissect the till-covered hillslopes, an older, now
stable "fossil" set and modern gullies. The older gullies are large features which fed
fans and debris cones. Numerous radiocarbon dates from various sites within the
Howgills indicate that the main wave of hillslope gullying and fan or cone deposition
was initiated in the 10th century AD, probably in response to human-induced vegetation
changes following the Viking settlement of this part of northwest England (Cundill,
1976;Harvey, 1985; 1996; Harvey et ai, 1981; 1984;Miller, 1991).
The modern gullies are smaller and closely related to the modern streams. Some are
active, currently supplying sediment to the streams ; others are stabilised and inactive.
The active gullies play an important role in the modern system as the major sediment
supply points for the streams. Within the Howgills as a whole, two alluvial channel
styles are evident, separated by a threshold, reflecting adjustment to low and high rates
of sediment supply. Channels in catchments without active gullies are single-thread,
relatively stable channels. Those downstream of active gullies are wide, often braided,
unstable channels (Harvey, 1977; 1987a; 1991).
The gullies themselves are closely related to coupling relationships with the stream
channels. Most gullies were initiated by stream erosion at the slope base and have
developed by headwards erosion upslope. Throughout their development, coupling is
important for both gully and stream processes. Erosion events on the gullied slopes
occur ca 30-35 times per year (Harvey, 1977 ; Harvey et al, 1979) in response to sea
sonally variable process interactions (Harvey, 1974; 1987b; Harvey and Calvo, 1991).
Sediment accumulates in debris cones at the base of the gullies until removed by a
stream flood and incorporated into the stream bedload. Such events occur with a return
period of ca 2-5 years (Harvey, 1977 ; Harvey et ai, 1979), resulting in a cycle of cone
development and removal.
As gully development takes place, the strength of gully/stream coupling decreases.
Basal scour becomes weaker (Harvey, 1992), process interactions on the gullied slopes
change and erosion rates decrease (Harvey, 1994), leading to total severance of the
coupling relationship and the stabilisation of the gully slopes by re vegetation. A model,
based on interactions between gully extension and revegetation rates, suggests under
present environmental conditions, a maximum length of gully of ca 40-50 m, and a
maximum period of gully development prior to stabilisation of ca 160 years (Harvey,
1992). So far, no attempt has been made to apply this model beyond the context of
modern gullies. This paper attempts to do that, within the context of landform deve
lopment on the valley sides and valley floor of the Carlingill valley.
Coupling within the active system and changes over the short term : (1 968-94)
The emphasis of previous work (Harvey, 1992; 1994) was on temporal aspects of
gully processes. Field monitoring of the study sites (fig. 1) has been continued and the
data now allow consideration to be given to the coupling relationships, the complexity
of sediment storage and to the periodicity of sediment supply to the stream channel. In
addition, field survey of the valley floor throughout Carlingill allows temporal and spat
ial variations in channel stability to be considered in the context of changes in
gully-stream coupling.
Géomorphologie : Relief, processus, environnement, 1997, n° 1 pp. 3-20 Coupling between hillslope gully systems and stream channels 7
Gullies
Six-monthly data for Grains Gill gullies (fig. 1), based on the analysis of sequential
photography, are now available for a period of 25 years. These data, relating to the fr
equency of basal scour (tab. 1), confirm the trends identified earlier (Harvey, 1992).
Scour is more frequent on debris aprons at the foot of small scars and on linear gully
channels, than on large debris cones issuing from larger gullies. This suggests that
sediment storage becomes longer in duration and more complex as gullies develop.
Using the same criteria to classify events as previously (Harvey, 1992), the extended
data confirm what had been suggested earlier : an overall reduction in event frequency
in recent years, expressed by differences between the return periods calculated for the
25 years and those for the 20-year period quoted previously (tab. 1). It is not clear how
far this trend is due to progressive gully stabilisation or to climatic fluctuation. Over
the last decade there has been a decrease in the incidence of major summer events,
effectively eliminating the summer dominance of high stream floods suggested pre
viously (Harvey, 1987a; 1992).
Mean Recurrence Intervals (in years)
Major Events(a) Minor Events(b)
Linear channels (n=6) 1.1(1 • 1) 3.6(2.1)
(gullies J, H, G, F, D, C)
Debris Cones (n=6) 1.7(1 6.5(6.1) •5)
(gullies J, H, G, F, E, C)
Debris Aprons (n=7) 4.7 (3.0) 1.7(1 .3)
(gullies J, E, С, В 1 , B2, AB, A)
Data for 25 year period 1969-94 ; figures in brackets refer to 20-year results 1 969-89 (see Harvey, 1 992).
Data based on analyses of 6-monthly sequential photography. Major events (a) are defined as those where
either partial or total erosion of debris has occurred ; minor events (b) as those where some erosion can be
recognised, but where no major morphological changes have taken place (see Harvey, 1 992).
Table 1 - Grains Gill Gullies : Frequency of Tableau 1 - Ravins de Grains Gill : fréquence des
scour (1969-1994). For locations see figure 1 sapements (1969-1994). Localisation cf. fig. I
The complexity of the sediment system is revealed by the 2-month ly data from the
Carlingill gullies (U, M and L, fig. 1), now available for a 7 year period. Major sed
iment generating events are overwhelmingly winter phenomena, but major runoff
events can occur in any season. Return periods for erosional and depositional events,
as defined previously (Harvey, 1994), have been calculated from these 7 year data
(tab. 2). The frequency of major basal scour at the foot of gullies L and M is of the
same order as that found on Grains Gill gullies. The lower frequency on gully L than
on gully M is best explained by the lack of scour at one site on gully L (L5, fig. 1 ). The
two major events occuring within the study period removed all debris at the other sites
on gully L, but although affecting all three cones on gully M, only trimmed their dis
tal parts. Erosional events are much rarer on the decoupled debris slopes of gully U,
Géomorphologie : Relief, processus, environnement, 1997, n° 1 pp. 3-20 A. M. Harvey
than in the storage/transport zones of the other two gullies (tab. 2). The high slope ero
sion rates on gully L are expressed in the high frequency of major depositional events
on the debris aprons below.
Mean Recurrence Intervals (in years)
Basal stream Erosional Depositional
scour events events events
Sites Major Minor Major Minor Major Minor
Gully L
(debris aprons) 4.4 1.2 3.3 0.6 0.7 0.4
Gully M
(gully channels) - - 2.7 0.5 4.3 0.5
Gully M
(debris cones) 3.7 1.6 2.0 0.5 6.1 0.6
Gully U
(debris slopes) - - 13.2 2.6 1.6 0.4
Gully L : Basal slopes/debris aprons : data available Ju 1988-Feb 1995 ; number of sites : 4, L2-L5 ;
number of observations at each site : 37.
Gully M : Linear channels : data available Oct 1987-Feb 1995 ; number of sites : 10, 3 or 4 in each channel
zone MC1-3 ; number of observations at each site : 42.
Gully M : Debris cones : data available Oct 1987-Feb 995 ; number of sites : 9, 3 on eacl i debris cone
MF 1-3 ; number of observations at each site : 42.
Gully U : Decoupled stabilising slopes : data available July 1988-Feb 1995 ; number of sites : 4, U45
3 within UO; number of observations at each site : 40.
Table 2 - Carlingill Gullies : Mean frequencies Tableau 2 - Les ravins de Carlingill : fréquences
of erosional and depositional events on trans moyennes des phases d'érosion et de dépôts dans
les sections de transit et d'accumulation des port and storage zones of gullies U, M, L,
(1987-95) in relation to style of coupling. ravins U, M, L, (1987-1995) en fonction du type
For locations of sites see figure I. de couplage. Localisation cf. fig. 1
These general results mask contrasts between the gullies in terms of complexity and
spatial characteristics of the sediment storage and transport system. On gully L the simple
directly-coupled basal slopes undergo sedimentation synchronously with erosion above.
There is little erosion of the debris aprons except when induced by stream activity.
On gully M the situation is more complex. On few occasions does erosion occur syn
chronously through any one of the three gullies (Ml-3, fig. 1). On some occasions
erosion in a feeder channel is associated with deposition on the cone ; on others deposi
tion in the does not extend to the cone below. On a few occasions, mostly
in winter, deposition may take place throughout one gully/cone system. Major erosion
may occur in the upper gullies in response to local runoff, or distally in response to basal
scour by the stream. This is a much more complex response to runoff and sediment gene
ration than on gully L. Apart from a few major dominantly erosional or depositional
events, sediment movement appears to take place in a series of pulses down each gully,
with a lack of synchroneity within any one gully system or between neighbouring gullies.
Géomorphologie : Relief, processus, environnement, 1997, n° 1 pp. 3-20 between hillslope gully systems and stream channels 9 Coupling
On gully U the situation is relatively simple, with deposition on the debris slopes
(UO and U45, fig. 1) occuring in response to major winter sediment generation, but
with no complexity induced by basal scour on this decoupled system. The upper part
of site UO, in the gully channel, is dominantly erosional, feeding sediment onto the
debris slope below. Occasionally scour extends onto the upper part of the debris slope.
Sedimentation occurs most frequently on the upper parts, with only major flow
events transporting sediment to the base of the slope.
Stream channels
The gullies and other scars along Carlingill valley feed sediment to the stream, cau
sing the channel locally to form wide, unstable braided reaches. This is evident when
the modern channel pattern is considered in relation to the locations of major sediment
supply points (fig. 2). Braiding occurs in several zones of channel instability which
show channel changes since 1968 (fig. 2). Braiding occurs near the scars and tributary
junctions between Small Gill and Haskaw Gill. There is braiding downstream of gully
U, although sediment supply from the gully ceased with decoupling some time ago. In
1 968 the channel was braided at gully M, and still is braided at gully L.
Channel widths reflect these local zones of instability (fig. 3). From the gorge outl
et above Small Gill, channel widths steadily increase downstream to the Grains Gill
confluence, from ca 3 to 5 m in a distance a little under 2 km, broadly in accord with
the regional relationships for stable single-thread channels (Harvey, 1987a; 1991).
Superimposed onto this trend are the much wider reaches, in zones of channel instabil
ity adjacent to sediment supply points (fig. 3).
Under modern conditions sediment supply causes otherwise stable channels to jump
a threshold to a wide unstable habit, but only locally, forming small-scale sedimentat
ion zones (Church and Jones, 1982). There is little downstream propagation of these
unstable zones. As sediment supply from stabilising gullies declines, channel stabilisa
tion follows, but with a time lag between gully and channel stabilisation.
Changes 1968-94
The geomorphic changes that have taken place over the last 26 years can be identi
fied from 1968 air photos. A field survey of Carlingill gullies and channel was carried
out in 1970, with periodic observations made between then and the commencement of
detailed monitoring in 1987. Since 1968-70 there have been major changes on all
monitored gullies, with headwall extension but progressive basal stabilisation (Harvey,
1992; 1994). A few new scars have been created since 1968, but many others show a
reduction in erosional area and progressive stabilisation over the same period. Three
gullies at the downstream end of the Carlingill reach, active in part in 1968, have sta
bilised since. The 1968 channel of (fig. 4) shows more extensive unstable
braided reaches than today, including a greater degree of braiding in the Haskaw Gill
area, more extensive braiding downstream of gully U and a now-stabilised braided
reach at gully M. The overall trend since 1968 has been towards gully stabilisation,
weakening of coupling and reduction in the instability of the stream channel.
Géomorphologie : Relief, processus, environnement, 1997, n° 1 pp. 3-20 |
10 A. M. Harvey
MODERN SYSTEM ACTIVE IN 1994
Acttv» gully «ration
Ri oulty turfacM
Active channel 1094
Act*. „««.МИ»
| Or«v.l MdtoMMi dMortad aine» 1M0
Fig. 2 - The modern channel of Carlingill, sho Fig. 2 — Le lit actuel du Carlingill montrant les
wing braided reaches and zones of channel sections à chenaux tressés et les zones d'instabilité
instability within which channel change has taken où le changement de cours s 'est produit depuis les
place since 1968 air photos. Also shown are areas of photographies aériennes de 1968. On voit égale
active slope erosion 1994 (based on field survey ment, sur les versants, les zones d'érosion active
observées sur le terrain en 1994. 1994)
The medium term : progressive change over the last two centuries
Previous work at the upper end of Carlingill valley (Harvey et al, 1984) used
Rhizocarpon geographicum lichenometry to identify the sequence of deposition on the
valley floor over the last ca 150 years. This paper extends that work throughout the val
ley, to test whether sedimentation zones can be related to previously active gullies. In
addition to mapping the valley floor, lichenometry has been extended to exposed boul
ders and stones on the formerly gullied hillslopes.
Lichenometry of the valley floor used the same procedure to correlate alluvial sur
faces as the previous work, using the maximum Rhizocarpon geographicum thai lus
diameter within mapped sedimentary units, calibrated with the same curve corrected to
take into account the period since the previous survey. The results identify marked
Géomorphologie : Relief, processus, environnement, 1997, n° 1 pp. 3-20