Process interactions, temporal scales and the development of hillslope gully systems: Howgill Fells, northwest England

Geomorphology, 5 (1992) 323-344 Elsevier Science Publishers B.V., Amsterdam

323

Process interactions, temporal scales and the development of hillslope gully systems: Howgill Fells, northwest England A.M. Harvey Department of Geography, University of Liverpool, PO Box 147, Liverpool, L69 3BX, UK (Received October 29, 1991; revised April 3, 1992; accepted April 6, 1992)

ABSTRACT Harvey, A.M., 1992. Process interactions, temporal scales and the development of hillslope gully systems: Howgili Fells, northwest England. In: J.D. Phillips and W.H. Renwick (Editors), Geomorphic Systems. Geomorphology, 5: 323-344. Hillslope gully development is often controlled by interactions between multiple processes, rather than by individual processes. Such interactions may have both spatial and temporal characteristics. A twenty-year monitoring program of hillslope gullying in the Howgill Fells, northwest England, has identified the roles of both on-slope interactions and slope/ stream coupling interactions, which can be related to several temporal scales. On-slope interactions are primarily seasonal, whereas those related to coupling produce a cyclicity over timescales of up to several years. Over the longer term (decades or more) progressive changes in gully morphology modify the nature of both sets of interactions, and through negative feedback mechanisms may ultimately lead to gully stabilisation. Gully development is controlled by the rates of headwall recession and the basal incision/stabilisationbehaviour. After basal stabilisation, vegetation encroachment onto the eroding slopes takes place faster than headwall retreat, resulting in a finite age and limiting size for gully development. A simple model based on the current rates of development adequately describes the morphology of modern gullies but is inappropriate when applied to an older generation of stabilised gullies. This suggests that there have been changes in sediment generation and removal rates during the late Holocene.

Introduction

Hillslope gullies are important features of Holocene landscapes in many regions, especially where m o d e m hillslope processes are out of equilibrium with hiUslope morphology. This is particularly the case where the Pleistocene to Holocene climatic change saw a shift from glacial or periglacial to fluvially dominated geomorphic regimes, or where major environmental change occurred as the result of climatically or human-induced vegetation change during the Holocene. In extreme cases of hillslope gullying, espeCorrespondence to: A.M. Harvey, Department of Geography, University of Liverpool, PO Box 147, Liverpool, L69 3BX, UK.

cially in dry regions, badland landscapes result. There, the whole slope develops by gully erosion. In less extreme cases, more characteristic of humid regions, individual linear gullies or discrete gully systems partially dissect the hillslopes. Such hillslope gullies are created where the erosional force generated by overland flow exceeds the resistance of the hillslope materials. These conditions commonly occur in two situations; in mid-slope, through a combination of on-slope conditions; and at the slope base, through a combination of onslope and basal conditions. Gully development normally proceeds not by one process but by a variety of processes. Interactions between the processes control the rates and styles of gully development, through

0169-555X/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.

324

their influence on the relationship between erosional and stabilisation tendencies. Process interaction

Process interactions tend to have both spatial and temporal dimensions. Spatially they may involve interactions between on-slope processes, or coupling between on-slope and basal stream processes. On-slope processes include weathering, mass movement, surface and sub-surface erosion processes, on-slope deposition and stabilisation by vegetation colonisation. Within the large literature on dry-region badland geomorphology some papers touch on on-slope process interactions (e.g. Schumm, 1956a; Bryan and Yair, 1982; Harvey, 1982; Bryan, 1987; Finlayson et al., 1987; Imeson and Verstraten, 1988; La Roca Cervignon and Calvo-Cases, 1988; Alexander and Calvo, 1990; Calvo-Cases et al., 1991 ). There are fewer studies of on-slope process interactions on gully systems outside dry regions (e.g. Harvey, 1990, 1992; Harvey and Calvo, 1991 ). In both types region, process interactions influence erosion rates and control the style of morphological development. Although the concept of connectivity within fluvial systems is fairly well established (Brunsden and Thornes, 1979 ), and the functional characteristics of the component zones of the fluvial system well understood (Schumm, 1977), there have been relatively few papers dealing with coupling between these zones. The existing studies tend to deal primarily with the channel itself (e.g. Bull, 1979; Pitlick and Thorne, 1987 ) or with large-scale relationships between sediment source areas and alluvial fan environments (e.g. Harvey, 1989). Studies relating to gully/stream coupling tend to deal with the influence of temporal variations in sediment supply on downchannel sediment movement (e.g. Harvey, 1977; Wells and Gutierrez, 1982; Faulkner, 1988). However, some studies do deal with morphological implications for gully develop-

A.M. HARVEY

ment (Harvey, 1987b, 1988, 1992). The temporal characteristics of process interactions depend on magnitude and frequency behaviour (Wolman and Miller, 1960), particularly on the relative event frequences and lag times of the processes involved (Chorley and Kennedy, 1971 ), and on the recovery times of the landforms (Wolman and Gerson, 1978 ). The simplest style is a random, eventrelated relationship, illustrated by the response to, and recovery from, major sedimentsupplying flood events (e.g. Harvey, 1986, 1987b ). Another common style of interaction is seasonality (e.g. Schumm, 1956a, b, 1964; Harvey, 1974, 1987a), resulting from seasonally variant on-slope processes or seasonally variant coupling effectiveness. Cyclic variations in the style of interaction with a longer than seasonal periodicity, may be produced by weathering-related cyclicity in sediment availability (e.g. Harvey, 1987c; Calvo and Harvey, 1992; Calvo-Cases et al., 1991 ), or by markedly different event frequencies of sediment generation and removal processes (e.g. Harvey, et al., 1979). Over even longer timescales, morphological evolution itself may result in progressive changes in the style of process interaction. On gully systems this trend may lead towards ultimate stabilisation (Beaty, 1959; Harvey, 1988; Wells et al., 1991; Calvo-Cases et al., 1991 ). Such progressive change implies dynamic rather than constant process/form relationships during gully development, and has implications for the interpretation of both currently active and stabilised gully slopes. This paper deals with the morphological implications of changing process interactions during gully development on gullied hillslopes in the Howgill Fells of northwest England. It identifies auto-stabilisation mechanisms that result from the operation of negative feedback, that ultimately may lead to the complete stabilisation of erosional gullies. Hence, hillslope gullies are non-linear, non-equilibrium landform systems, showing progressive change in process/form relationships and potentially fi-

325

DEVELOPMENT OF HILLSLOPE GULLY SYSTEMS

nite temporal and spatial limits to their growth. An empirical model is developed, based on the results of a 20 year record of field monitoring of gully evolution. The model relates rates of gully extension and stabilisation, identifying the finite temporal and spatial scales for gully development in this environment.

Hillslope gullying in the Howgill Fells

Historical and functional aspects Hillslope gullies are common in many upland areas in northern and western Britain (e.g. Harvey, 1985; Harvey and Renwick, 1987; Ballentyne, 1991 ), dissecting steep, glacially or periglacially modified hillslopes. Some may relate to incision following late Pleistocene/ early Holocene climatic change, but many appear to have resulted largely from human-induced vegetation change during the Holocene. In some areas gullies are active today. The Howgill Fells, northwest England (Fig. 1 ) is a humid, cool temperate area with mean annual precipitation of ca. 1400 ram, cool, wet summers and wet winters with moderate frost and snow. In the Howgills there are two sets of hillslope gullies. Large, now stabilised older gully systems, and smaller areas of active gullying (Figs. 1 and 2) dissect Pleistocene soliflucted glacial deposits, over Silurian mudstone bedrock. Both sets show interesting aspects of slope/stream coupling. Isolated, mid-slope gullying is rare. The two sets of gullies appear to be primarily related to basal stream activity, either lateral erosion by the main streams or incision of steep tributary channels. The older gullies may include several phases of gullying but the majority appear to relate to human-induced vegetation change on the hillslopes in the 10th century AD (Harvey et al., 1981; Harvey, 1985; Miller, 1991 ). In many cases there are large debris cones or alluvial fans where the gully systems meet the main valleys, indicating that as the

result of sediment overload, coupling of sediment supply zones to main channels was imperfect. During gully erosion and fan deposition, sediment supply exceeded the transporting power of the streams. In other words, actual stream power was below the threshold of critical stream power (Bull, 1979). The active gullies are much smaller and less extensive than the older gullies, but also demonstrate an important coupling behaviour with the basal streams. Not only is basal scour by the stream fundamental to maintaining active gully erosion, as will be discussed below, but the gullies play a vital role in supplying sediment to the modern stream system. Most stream channels in the Howgills are relatively stable, single-thread, and often meandering. Over the last forty years these have shown only slow rates of channel change. Their morphology is closely related to drainage area and sediment size characteristics (Harvey, 1987b, 1991 ). Downstream from active gullies, channels are braided, unstable, have steeper slopes and widths almost an order of magnitude greater than the stable single-thread channels. The coupling of hillslope gullies to the streams is clearly fundamental for the dynamics of the modern fluvial system.

Howgill gullies: process interactions Gully development has been monitored at the Grains Gill and Carlingill sites (Fig. 1 ) at a variety of temporal and spatial scales since 1969. Over the whole period, six-monthly observations have been maintained at Grains Gill. These include observations of headwall retreat rates, and through sequential photography using methods similar to those used by Suwa and Okuda (1988), of morphological change, erosion and deposition on the gully slopes, basal zones and within the stream channel. Over shorter timescales, sediment yield at Grains Gill has been assessed from sediment trap data for the periods 1971-1972

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DEVELOPMENT OF HILLSLOPE GULLY SYSTEMS

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and 1975-1977. Since 1987 more detailed observations of erosion and deposition have been maintained at the Carlingill sites, specifically related to the influence of coupling on process interactions. From these data it is now possible to consider the temporal and spatial variations in on-slope and coupling interactions and how these relate to the longer term sequence of gully development.

On-slope processes A clear seasonality of on-slope process interactions has been identified (Harvey, 1974, 1987a, b), with winter processes dominated by mass movement, and summer by overland flow erosion. Winter sediment yields, in part influenced by freeze-thaw and snowmelt activity, are much higher than those for the summer half-year (Table 1 ). Particle size also differs,

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DEVELOPMENTOF HILLSLOPEGULLYSYSTEMS

329

TABLE 1 Mean sediment yields (kg/m2), by six-month season, from sediment trap data 197 l-1972 and 1975-1977, on filled and unfilled slopes at Grains Gill gully site (from Harvey, 1987b ) Rilled sites (2 sites)

Summer (Apr-Oct) Winter (Oct-Apr) Annual

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Total sed. yield

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with summer winnowing by overland flow producing less stony sediment yields (Table 1 ). Surface morphology also shows a marked seasonality, with an annual "Perth Amboy" type of rill network development (Schumm, 1956b). The network develops to a maximum in late summer before it is destroyed by frost action the following winter (Harvey, 1987a, 1988), a pattern repeated from year to year (Fig. 3). There are also spatial variations in the style of on-slope process interactions that relate to the position of the slope in the context of slope/ stream coupling (see below). The gully slopes can be classified as follows: basal slopes, feeding directly to the main streams; gully-head slopes, with convergent rill systems feeding a main gully channel; gully-side slopes feeding laterally into a gully channel; and stabilising slopes, disconnected from the gully or stream network and feeding sediment onto a re-vegetating debris slope. Rill network development and evidence for processes in inter-rill areas, on 15 study plots on Carlingill gullies, have been monitored since 1987 (Harvey, 1990; Harvey and Calvo, 1991 ). The results are reported in detail elsewhere (Harvey, 1992) but are summarised here. Rill network development (Fig. 4a), expressed by a numerical index based on clarity/continuity, depth, density and complexity of the rill network, shows a strong seasonality, as expected, but also siterelated variations. Maximum development occurs on gully-head and basal sites, and mini-

mum on stabilising sites. Processes observed in inter-rill areas also show a clear seasonality (Fig. 4bi) and site-related differences (Fig. 4bii). In general, erosion rates are highest and the morphology most dynamic, where strong coupling to the gully or stream network, i.e. in basal and gully-head sites, fosters rapid removal of eroded sediment. This is confirmed by the contrasts evident in the sediment yield data (Table 1 ) from Grains Gill, between traps on filled (equivalent to gully-head) and nonfilled (equivalent to gully-side and stabilising ) sites.

Coupling Sediment produced by erosion of the gully slopes is fed downslope to one of several storage areas. Suspended sediment in runoff is fed directly to the stream system and transported away. The stones accumulate in the storage areas together with both fines and stones derived by mass movement processes. The main storage areas are within the gully channels and in the basal zones. In the latter case they occur either as debris cones at the foot of the main gully channels, or as debris aprons below open slopes. Periodically, stream floods remove this accumulated sediment and incorporate it in the stream bedload, maintaining the high flux of coarse sediment through the fluvial system and maintaining continued erosional activity on the gully slopes (Harvey, 1991 ). Previous work on coupling identified threshold rainfall events for sediment produc-

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DEVELOPMENTOF HILLSLOPEGULLYSYSTEMS

FREQUENCY OF SCOUR, 1 9 6 9 - 8 9

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Fig. 5. Grains Gill: frequency of basal scour 1969-1989, in relation to site type, site type illustrated by sketch.

tion and removal, and estimated their frequencies from long term rainfall records for neighbouring rainfall stations. An average of ca. 3035 sediment producing events (including snowfalls) per year was indicated, and an average return period of ca. 2 years for basal removal (Harvey, 1977, 1987b; Harvey et al., 1979 ). Given the random variability between sites, this leads to a 2-5 year cyclicity of debris accumulation and removal. Now that 20 years of observational data are available it is possible to examine event frequency of basal scour in real, rather than indirect terms, and to explore more fully its spatial variations. All gully bases along Grains Gill have been monitored over the period 1969-1989 and have been classified into linear gully channels, debris cones and debris aprons (Fig. 5 ). There are 6 linear gully channels, 6 debris cones and 8 debris aprons, of which 2 are below scars created by erosion during the study period (in

331

1975 and 1976) and one below a scar which became vegetated during the study period. From six-monthly sequential photography of the gully bases from fixed points, basal scour has been classified into minor scour, where some removal can be identified from the photographs but major morphological changes are absent, and major scour where partial or total removal of debris has taken place (Table 2). The six-monthly observations can be grouped into three according to the magnitude and extent of basal scour. For 17 of the 40 periods (Class 0, Table 2 ) little or no basal scour was observed. On these occasions almost no sites show major scour, with the exception of site AN. At that site degree of scour was difficult to determine between its initiation by erosion in 1975 and ca. 1982, during which time the nearvertical scar fed sediment directly into the stream. For those 17 periods no more than six sites showed even minor scour. Thirteen periods showed moderate scour (Class 1, Table 2) with up to three sites recording major scour and 5 to 13 sites with at least minor scour. Ten periods (Class 2, Table 2 ) showed major scour at four or more sites and at least minor scour at 9 to 20 sites. Six out of the ten are s u m m e r periods, suggesting a s u m m e r dominance of high stream floods, but not as marked as was suggested in an earlier analysis of scour at the upper gully site alone (Harvey, 1987b). The major scouring events are not uniformally distributed over the 20-year period. None occurred between 1971-1975, 1977-1981 and 1985-1989. These were primarily periods of low storm rains, and no long term trend is indicated. Indeed, another major flood occurred in 1990 after the end of the data period, breaking the "dry sequence" of the late 1980's (Harvey, 1992). Ten major scouring floods in twenty years accords with the suggested ca. 2 year return period for basal removal (Harvey, 1977, 1987b; Harvey et al., 1979 ), however there are marked differences between gully channel, debris cone and debris apron sites (Table 2, Fig. 5 ). Only

332

A.M. HARVEY

TABLE 2 Grains Gill, frequency of basal scour, 1969-1989 Linear

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333

DEVELOPMENT OF HILLSLOPE GULLY SYSTEMS

within the gully channels is major scour as frequent as once every 2 years; on the debris aprons it averages once every ca. 3 years, and on the debris cones once every ca. 6 years (Fig. 5 ). This may be in part due to random spatial variability in flood effectiveness beyond the confines of a gully channel and in part to increased erosion thresholds for larger debris cones at the mouths of the larger (lower angle) gullies. For the debris cones and debris aprons (excluding site AN; see above) there is a relationship between headwall distance from the mainstream (L) and the return period of major scouring events (I) which may be expressed as follows: I = 1.4L °'°5

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This implies a decrease in the strength of coupling and an increase in the complexity of the sediment storage and removal system, with gully growth. On debris aprons there is a simple cyclicity of accumulation and removal with a mean periodicity of ca. 3 years. On linear gullies sediment is fed to the gully channels by each sediment-producing event (ca. 30-35 p.a.), stored for up to ca. 2 years within the channel, then transported downchannel to the debris cones by events with a return period of ca. 2 years. On some occasions it may be transported directly to Grains Gill channel, but alternatively it may be stored in the debris cones, only to be removed by scour on average once every ca. 6 years.

Progressive changes during gully development The development of a gully from a steep t streamslde scar takes place by a combination of headwall recession and incision of the main gully channel. As incision proceeds, the slope of the channel is reduced (Fig. 6 ). The rate of incision is ultimately limited by a concurrent reduction in stream power, or by the channel cutting through the relatively erodible Pleistocene sediments into the more resistance underlying bedrock (as in gully G see Fig. 6).

During this sequence the strength of slope/ stream coupling decreases and the sediment transport system becomes more complex (see above). Reduction in the frequency of sediment removal from the slope bases causes a progressive change in the style of the on-slope process interactions. Reduction in slope angle from an initial slope of ca. 70 o for basal slopes, 50-60 ° for gully-head slopes and ca. 40 ° for gully-side and stabilising slopes, suggests also a reduction in erosion rates. The eroding gully surfaces, surveyed in 1969 and again in 1989, have been classified according to their coupling relationships (see above ), into basal, gully-head, gully-side and stabilising slopes. Debris cover, partially vegetated slopes (i.e. with sporadic plants, < 50% cover) and vegetated slopes ( > 50% cover) were also recorded (Fig. 7 ). Over the period 1969-1989, gully-head slopes have become reduced in area and many gully-side or stabilizing slopes have become partially or more fully vegetated. This is a long-term trend, independent of basalscouring flood frequency. It has implications for on-slope processes, indicating an overall reduction in sediment production and a change in the balance of surface processes away from wash-related and rilling processes. Changes in rill network composition during progressive gully stabilisation can be illustrated by the gully D rill systems especially over the period since 1976 (Fig. 8). Prior to ca. 1976, the annual cycle of rill development masks any obvious signs of longer term trends (Fig. 3 ). However, over the whole period, but particularly since 1976, stabilisation trends are present, with vegetation colonisation proceeding up-slope, debris burying the slope-base and a progressive decrease in the size of the erosional area (Fig. 8). Morphometric data derived annually from the autumn maximum rill network illustrate these trends. Figure 9 shows the progressive decrease in erosional area (Ae) over the monitoring period, especially since 1976. Over the whole period erosional area is almost halved. Until 1982

334

A.M.HARVEY -60

- 50

G 40

-1ta :::r

30 X "'" " ' F

J

~o" o <

20

E~.

""-".'~"" -. CN

~ 3

10

i

i

go

,o

30

2o

fo

D i s t a n c e ( m ) from s t r e a m

Fig. 6. Grains Gill: gully channel profiles. Xindicates erosion into bedrock.

the steady reduction in rill length (Fig. 9, EL) maintained fairly consistent drainage densities, but since then the rapid decrease in rill length has resulted in a dramatic fall in annual maximum drainage density (Fig. 9, Dd(e) ). This may reflect the existence of a threshold drainage area, below which rilling processes become less sustainable. These changes though do not appear to have had an impact on the composition of the drainage net. Maximum order for the two rill networks fluctuated between 3rd and 4th order over most of the period. Both 1st and 3rd order numbers decline over the period (Fig. 9, nl, n3), but no trend in 1st to 3rd order bifurcation ratio can be identified (Fig. 9, nl/n3). Similarly mean length of I st order segments is unchanged (Fig. 9, E1 ). These trends are clearly related to progressive stabilisation: very little of the variability can be ascribed to rainfall variation (Harvey, 1988, 1990). The overall evolution of gully morphology

depends on the relationship between gully extension by headwall erosion and stabilisation of the lower slopes by vegetation colonisation. Headwall recession rates have been assessed from measurements from fixed points at sixmonthly intervals. Headwalls have been classified into those above gully-heads, gully-sides (riUed and unrilled), and direct supply slopes (Fig. 10). From the first 12 years of data, recession rates for gully-heads have been calculated from the field observations. These can be directly related to the steepness of the hillslope into which the gully is cut, by the regression equation: E=0.016+0.57U

N = 10, R = 0 . 7 6

(2)

where U ( m / m ) is the slope through which the gully is cut and E ( m / y r ) is the recession rate (at the time of measurement, not the mean rate throughout the period of gully development). The approximate age of each of the 10 gully heads at Grains Gill has been estimated from

335

DEVELOPMENTOF HILLSLOPEGULLYSYSTEMS + l -- ~ - " ~'~. ~" ~

1969

=

A, %~~,~

------

\

Headwall recession 1969-89 Headwall Ridges

--Y- -Y-- Breaks of slope

g

(

1989

~

~,

(~

\~

ACTIVE SLOPES

STABILISING SLOPES

~

Basal slopes

I'~

Stabilising

I~

Gully heads

~

Debris

~

Gully sides

~

Partial vegetation

~

Vegetation

\'c,-

\

~ ",,',,

0 I

Metres

30 I

Fig. 7. Grains Gill: changes in gully surface conditions and status of gully slope units 1969-1989.

this regression equation (Harvey, 1987a). With the exception of gully G, a complex multiple-aged gully, these range from ca.30-40 years for the smaller gullies to ca. 125-175 years

for gullies F, H, and J (for locations see Fig.

1). The headwall recession rates based on the 20year data are slightly higher than those based

336

A.M.HARVEY

Fig. 8. Grains Gill, Gully D rill systems: vegetation colonisation and morphological change over the period 1969-1988.

on the 12-year data, previously published (Fig. 10), but are of the same order. As well as confirming the regression relationship above, these data suggest a relationship between recession rates (again, short-term actual rates) of the gully heads and gully length (L, m ), where: E = 0 . 9 4 L -°51

N = 10, R = - 0 . 6 1

(3)

With only ten data points this is significant at only the 10% level, but does give a crude indication of how recession rates may decrease with gully size/age. This itself may be partly due to a "decoupling" effect, but also to the general convexity of the upper parts of the hillslopes into which the gullies are cut. The rate of vegetation colonisation upwards from the base of the slope is as important for gully development as the rate of headwall recession. Little quantitative data exists, but this is a theme under current investigation. Some indication can be gained from measurements made on gully D of up-slope limits of semi-continuous vegetation (i.e. > 50% cover) (Fig. 11 ). Mean rates of vegetation encroachment up slope are approximately 0.35-0.4 m /

yr. By linking together estimates of headwall recession rates, and vegetation colonisation rates it possible to consider the overall rate of gully development. This will be dealt with in the next section.

Implications: towards a model for gully development The majority of Howgill gullies were clearly initiated by stream-induced basal erosion or slope failure creating a small streamside scar. The sequence of gully development (Table 3 ) from initiation to ultimate stabilisation can be seen as the result of interactions between erosional and stabilisation tendencies. The most likely position for scar development (stage 1, Table 3 ) would be where steep valley-side slopes are undercut by the stream. Many of the active gullies are located in interfluve positions between older stabilised gullies, which is where slopes are steepest (e.g. Gully F on Grains Gill and several small active gullies on Great Ulgill and Small Gill; locations, Fig. 1 ). Not all scars will develop into gullies, some may

DEVELOPMENT

OF HILLSLOPE 140

337

GULLY SYSTEMS

Ae

120 E 100

80

•200 • 150

M r - 100

3

50 1.4-

(*)

Dd

1.2. E 1.0v 0.80.60

.

4

-

~

6O

4O

10

2O

0

1o5

iJ 1

.

0

1

1970

1

1

1

1

1

1

1

1

1975

i

,

1980

i

i

i

,

~

i t

19 5

Fig. 9. Grains Gill, Gully D rill systems: changes in the morphometric properties of rill networks over the period 19691987 (relates to the annual maximum, autumn rill network development): Ae, erosional area; EL, total rill length; Y~LI, total length of first-order rills (Strahler system ); Dd(e), drainage density of the erosional area; n ,, n3, numbers of I st, 3rd order rill segments; LI, mean length of I st order rill segments.

338

A.M.HARVEY ~~1

Debrts

~'~

Main rill network

....

Divides

~-.

Gully edges

( "

"-\\ ',,

~

0

\'~.

~\ "\)

l

Metres

30 I

\ x\ % . ~

•. -,...../-~

"\"

_f

~

," ~

Mean rate of headwall recession, m/yr 1969-89 Gully Heads (n =10)

0.18 (0.12)

(Range 0.08-'~ 0.38)

Upper sides (rilled) (n : 1 5 )

0.11 (0,10)

(Range 0.05-~ 0.28)

Direct Supply Slopes (rilled) (n=3)

0.09 (0.07)

(Range 0.06--,-0.11)

........ Lower Sides (unrilled - stabilising ?) (n--9)

0.05 (0.04)

(Range 0.03 ~ 0 . 0 7 )

....

Fig. 10. Grains Gill gullies: mean annual headwall recession rates, derived over the 20 year period 1969-1989• Figures in parentheses relate to the 12 year period 1969-1981 (see Harvey, 1987a).

revegetate and stabilise almost immediately (stage a, Table 3). Development into a gully (i.e. to stage 2, Table 3 ) would seem most likely if the erosional area is sufficient to allow rill development, and if the debris produced continues to be removed by stream erosion. If not, vegetation colonisation may take place before the scar has had a chance to develop. Over the 20-year observation period, 5 small scars alongside Grains Gill (in less than 1 km of valley) have stabilised by vegetation growth and

3 have been initiated. Of these, one is developing into a gully by incision and headwall retreat, but the fate of the other two is uncertain at present. If at any time during gully development, the basal stream migrates away from the slope base, as apparently happened below Gully U (Harvey, 1992 ), early stabilization by vegetation colonisation will be initiated (stage b, Table 3 ). Otherwise, the gully will progressively develop along the lines described in this paper.

DEVELOPMENTOF HILLSLOPEGULLYSYSTEMS

339

30

25 @

Q. 0 20 @

el 7_"

15

~

j

S

//

/ , ~'~-,,.///"\ '~"1___J

E lO q o o

A

5 0 1969-70

1974-75

I 1979-80

i 1984-85

I 1987

Fig. 11. Grains Gill, Gully D: vegetation limits 1969-1987, at three sample sites, mean rates are approximately0.350.4m/yr. Concurrent reductions in the strength of coupling, and in slope angle of the lower slopes, allow debris accumulation and vegetation colonisation to take place from the base up. This constitutes an important negative feedback mechanism, resulting from gully development itself, and leading to a decrease in erosion rates. This is the stage that has been reached by most of the larger gullies. Eventually, vegetation colonisation will out-pace headwards recession. The gully system will stabilise (stage c, Table 3 ) as erosional area is reduced below that able to sustain a rill network. Several of the larger gullies appear to be reaching this stage (e.g. the upper parts of gullies D and G on Grains Gill and the upstream rill system on gully M in Carlingill). Along Cadinglll valley are several midslope scars, located above now-stabilised gully slopes which appear to be the end product of this process. This overall sequence would suggest that under any one set of local environmental conditions, unless rejuvenation occurs, there is a finite timescale and a limiting size for gully development that can occur before auto-stabilisation takes place. Figure 12 represents an attempt to quantify

TABLE3 Stages in gully e v o l u t i o n

STABILISATION

EROSION 1 I

Scar initiation by basal scour

1 2 I

3

Development into a gully

Progressive development of the gully by headwards erosion. Slope/stream coupling weakens as the gully develops

I

a

_I

b

=[ Early stabilisation initiated I

c

--

-I

Immediate stabilisation

I

by channel migration

i -

Eventual stabitisation by vegetation colonisafionof the lower slopes and encroachment upslope

this general model by utilising the data on headwall recession and vegetation encroachment from the previous section (Figs. 10 and 1 1 ). There, gully length is plotted against gully age, assuming gully initiation as a streamside scar of a given size (represented by area a in Fig. 12), followed by gully development at headwall recession rates within the range presented in Fig. 10. The extreme rates plotted, 0.3 and 0.1 m/yr, both seem unlikely for sus-

340

A.M. HARVEY OLDER GULLIES

5

0

50

0

"5

100

150

200

250

300

350

MODERN GULLIES

60

Active Lower slopes vegetated I.I Recently stabilised i I Reactivated 'older gullies'

•

E 10

5

5O

o

I J v

. n

,

/

, ~q 50

r-np 1oo Gully length (m)

///

Z

I 30

c

J

i

i I ! I

z

2o f

I 10 . - l - - , c ~

^~,/J

P

/ "//

t

0

/

// /"

0

f

50

I

I

I

1

100

150

200

250

Gully age (years)

Fig. 12. Generalised model of gully development based on rates of headwall recession and rates of up-slope vegetation colonisation. Insets show frequency distribution of gully lengths for modem and older gullies in the Cadingill drainage area. Area a relates to gully initiation by basal erosion and the first 5-10 years of gully development to lengths 5-10 m. Following gully initiation, E is headwall recession rate, variously estimated from the data in Fig. 10. In the regression equation it relates to short-term recession rates, not long-term means. The regression line is assumed to have an origin, after gully initiation, of 10 m at 10 years, after which short-term recession rates progressively decline as the gully develops. The pecked lines on either side of the regression line, together with the lines relating to E=0.1 and 0.3, assume development from 5 m × 10 yr to 10 m X 5 yr, and can be regarded as limiting cases. Vis vegetation encroachment rate, estimated from the data in Fig. 11. Vegetation enroachment is assumed to begin once the gully has reached 20 m in length (see text). Shaded area represents most probable combinations of gully age and length for gully stabilisation.

tained gully growth given the range of data in Fig. 10. More likely to be representative is one of the central curves (0.2 m/yr, or one of the curves from the regression equation). Most of the existing gullies longer than ca. 20 m show the beginnings of vegetation colonisation on their lower slopes. Let us assume that vegetation encroachment starts when gullies reach this size; on the best estimates of gully growth rate this would be ca. 55 yr after gully initiation. Vegetation encroachment then pro-

ceeds at 0.35-0.4 m/yr upslope. Curves plotted for these rates intercept the growth curves after a further 70-125 yr, i.e. 125-180 yr total, and at gully lengths between 25 and 45 m. Under current growth and stabilisation rates these figures would represent the approximate temporal and spatial limits to gully development. The morphological sequence that would result from the mean rates of gully growth and stabilisation presented in Fig. 12 is illustrated in Fig. 13.

341

DEVELOPMENT OF HILLSLOPE GULLY SYSTEMS

How realistic are the figures suggested above? The model is only a first approximation and may be subject to several sources of error. Firstly, the field results may be non-representative. This is unlikely for the headwall erosion rates; it has already been demonstrated (Harvey, 1987) that current erosion rates (with a mean ~ 0 . 2 m/yr) accord with estimates based on air photographs for gross gully development since 1948. However, the estimates for vegetation encroachment rates are crude and with more data could obviously be refined. Observations on Gully U since 1970

( H a r v e y , 1 9 9 2 ) suggest that t h e rate m a y be

faster and/or nonlinear, resulting in an earlier stabilisation and a smaller limiting size. Also, vegetation growth may be initiated on gullies smaller than 20 m in length, and younger than 70 years old. Again this would foreshorten the lifespan and reduce the size of the ultimate gully.

Despite these shortcomings the results do seem to accord fairly well with the actual size range of the modem gullies (Fig. 12, inset), albeit slightly underestimating the maximum size. The only active gullies markedly longer

GULLY DEVELOPS

GULLY STABILISATION

AFTER

30

...... iii!iiiii !iiiiiiiiiiii....

20

10

0

1~0 0

10

[~

Erosional slopes

• ....

Debris Stabilising slopes

0

10

0

10

0

al0oft

3O

. a12~///I =1~// J

E o~

are ~/

a

20

/~"

/

leo

3'0

ao

Initial scar

a ,n "~

A s s u m e d errosional d e v e l o p m e n t after 10 y e a r s

/

/ //blsO

a __ 1__ P r e d i c t e d errosional d e v e l o p m e n t z~- eo after 2 5 - 160 years

~./.f / / ~ /

a 2~'//////b'/////"

b o so -

• I

2=0

111

40

o~

10

Active erosion at gully base ,..uoe0

0,e.r.,

10

aol//////j~mo.7///~

brs ,so -

0 bO-50

°_

,

,

,

10

20

30

'40

510

Basal stabilisation a fter,

-

years

610

Distance from stream (m)

Fig. 13. Schematic representation of the development of gully long profiles, from initiation to stabilisation, on the basis of mean values for erosion and revegetation rates shown in Fig. 12.

342

than 50m in length are re-activated older gullies or complex gullies such as gullies D and G on Grains Gill. When however, the size frequency distribution of the older gullies is compared, it is clear that this model cannot be applied to their development. They have a much greater size range. Gullies, even of the modal size for the older gullies of 80-100 m length (Fig. 12 ), would require 350-400 yr for their development, at maximum current rates. Gullies in excess of 280 m in length would have required much longer. This seems unlikely given the relatively narrow age-range for 14C dates from alluvial fan sediments derived from Howgill gullies. Evidence from 11 14C dates relating to three major sets of gullies indicates that all three seem to have been eroding over a similar period around the 10th century AD (Miller, 1991 ). The alternative possibilities are either that different processes were operating then, or that process rates were different from today's. The very long gullies may have been initiated in midslope as discontinuous forms, and subsequently linked together. This, however, seems unlikely for the rest of the gullies, as the majority appear to be basally connected to the main stream systems. For these gullies, higher rates of headwall recession and/or slower rates of vegetation colonisation would have been necessary to allow for much greater and more rapid gully development prior to stabilisation. The causes of gully development for the two sets of gullies appear to be different. The modern gullies could be regarded as intrinsic to the modern system, locally triggered where the streams cut scars at the base of steep till slopes. The older gullies appear to be extrinsically caused by widespread, apparently synchronous vegetation changes on the valley sides, triggered by human activity (Harvey et al., 1981; Harvey, 1985; Miller, 1991). Under these circumstances the rate of operation of geomorphic/re-vegetative processes may well have been different. Unlike the modern gullying, to which the stream system appears to ad-

A.M. HARVEY

just simply by a change in channel style, the older gullies fed massive amounts of sediment into the system causing widespread alluvial fan and valley aggradation (Harvey, 1985 ). Conclusions

Rates of erosion and stabilisation on the modern Howgill gullies, monitored over a 20yr period, appear to provide a satisfactory basis for understanding the changing process/ form relationships during gully development. The interaction between erosional and stabilisation trends is critical. The stabilisation mechanisms, here dominantly vegetation colonisation, create a negative feedback which ultimately limits gully growth, both spatially and temporally. In other environments other mechanisms may have the same effect. In dryregion badlands, slope reduction, the development of a stone cover and lichen as well as higher plant colonisation may have similar effects, but over longer timescales (Calvo-Cases et al., 1991 ). Under present conditions in the Howgill Fells, which may be representative of other upland areas of northern Britain, the limits to the growth of basally-induced gullies appear to be approximately 50 m of headwards erosion in up to ca150 yr of gully development prior to intrinsically-induced stabilisation. In about the 10th century AD in the Howgills, under different environmental conditions, gully growth continued much further, prior to stabilisation. During gully development there are progressive changes in on-slope process interactions, sediment production rates, and the strength of slope/stream coupling, which can be related to the sequence of gully development. It is important that future studies of gullying, whether primarily concerned with erosion rates or with process/form relationships, take into account the possibility of such progressive changes, and that short-term studies are put in their longerterm contexts.

DEVELOPMENT OF HILLSLOPE GULLY SYSTEMS

Acknowledgements I am grateful to the staff of the Drawing Office, and Photographic sections of the Department of Geography, University of Liverpool for producing the illustrations. References Alexander, R.W. and Calvo-Cases, A., 1990. The influence of lichens on slope processes in some Spanish badlands. In: J.B. Thornes (Editor), Vegetation and Erosion. Wiley, Chichester, pp. 385-398. Ballantyne, C.K., 1991. Holocene geomorphic activity in the Scottish Highlands. Scott. Geogr. Mag., 107: 8498. Beaty, C.R., 1959. Slope retreat by gullying. Geol. ~oc. Am. Bull., 70: 1479-1482. Brunsden, D. and Thornes, J.B., 1979. Landscape sensitivity and change. Trans. Inst. Br. Geogr. New Ser., 4: 463-484. Bryan, R., 1987. Processes and significance of rill development. In: R. Bryan (Editor), Rill Erosion: Processes and Significance. Catena Suppl., 8: 1-15. Bryan, R. and Yair, A., 1982. Perspectives on the studies of badland geomorphology. In: R. Bryan and A. Yair (Editors), Badland Geomorphology and Piping. Geobooks, Norwich, pp. 1-12. Bull, W.B., 1979. Threshold of critical power in streams. Bull. Geol. Soc. Am., 90: 453-464. Calvo-Cases, A. and Harvey, A.M., 1992. Morphology and development of selected badlands in southeast Spain. Earth Surf. Proc. Landforms, in press. Calvo-Cases, A., Harvey, A.M. and Paya Serrano, J., 1991. Process interactions and badland development in SE Spain. In: M. Sala, J.L. Rubio and J.M. Garcia-Ruiz (Editors), Soil Erosion Studies in Spain. Geoforma Ediciones, Logrono, pp. 75-90. Chorley, R.J. and Kennedy, B.A., 1971. Physical Geography: a Systems Approach. Prentice Hall, London, 370 PP. Faulkner, P.H., 1988. Gully evolution in response to both snowmelt and flash flood erosion, western Colorado. In: V. Gardiner (Editor), International Geomorphology, 1986, Vol. I. Wiley, Chichester, pp. 947-969. Finlayson, B.L., Gerits, J.J.P. and Van Wesermael, B., 1987. Crusted microtopography on badland slopes in southeast Spain. Catena, 14:13 l - 144. Harvey, A.M., 1974. Gully erosion and sediment yield in the Howgill Fells, Westmorland. In: K.J. Gregory and D.E. Walling (Editors), Fluvial Processes in Instrumented Watersheds. Inst. Br. Geogr. Spec. Publ., 6: 4558. Harvey, A.M., 1977. Event frequency in sediment production and channel change. In: K.J. Gregory (Edi-

343 tor), River Channel Changes. Wiley, Chichester, pp. 301-315. Harvey, A.M., 1982. The role of piping in the development of badlands and gully systems in southeast Spain. In: R. Bryan and A. Yair (Editors), Badland Geomorphology and Piping. Goabooks, Norwich, pp. 317-335. Harvey, A.M., 1985. The river systems of north-west England. In: R.H. Johnson (Editor), The Geomorphology of North-west England, Manchester U.P., pp. 122-142. Harvey, A.M., 1986. Geomorphic effects of a 100 year storm in the Howgill Fells, northwest England. Z. Geomorph., 30: 71-91. Harvey, A.M., 1987a. Seasonality of processes on eroding gullies, a twelve year record of erosion rates. In: A. Goddard and A. Rapp (Editors), Processes et Mesure de l'Erosion. CNRS, Paris, pp. 439-454. Harvey, A.M., 1987b. Sediment supply to upland streams, influence on channel adjustment. In: C.R. Thorne, J.C. Bathurst and R.W. Hey (Editors), Sediment Transport in Gravel Bed Rivers. Wiley, Chichester, pp. 121150. Harvey, A.M., 1987c. Patterns of Quaternary aggradational and dissectional landform development in the Almeria Region, southeast Spain: a dry-region tectonically active landscape. Die Erde, 118:193-215. Harvey, A.M., 1988. Seasonal and longer term development of rill systems on eroding slopes (Abstract). In: Int. Geogr. Union COMTAG Syrup., Amsterdam/ Leuven 1988. Abstracts of Papers. Harvey, A.M., 1989. The occurrence and role of arid-region alluvial fans. In: D. Thomas (Editor), Arid-region geomorphology, Bellhaven/Frances Pinter, London, pp. 136-158. Harvey, A.M., 1990. Gully development in the Howgill Fells: the role of process interactions (Abstract). IBG Ann. Conf., January 1990. Glasgow, Abstracts of Papers, 13. Harvey, A.M., 1991. The influence of sediment supply on the channel morphology of upland streams: the Howgill Fells, northwest England. Earth Surf. Proc. Landforms, 16: 675-684. Harvey, A.M., 1992. Influence of slope/stream coupling on process interactions on eroding gully slopes: Howgill Fells, northwest England, in review. Harvey, A.M. and Calvo, A., 1991. Process interactions and rill development on badland and gully slopes. Z. Geomorph. Suppl., 83:175-194. Harvey, A.M., Hitchcock, D.H. and Hughes, D.J., 1979. Event frequency and morphological adjustment of fluvial systems in upland Britain. In: D.D. Rhodes and G.P. Williams (Editors), Adjustments of the Fluvial System. Kendall/Hunt, Dubuque, IA, pp. 139-167. Harvey, A.M., Oldfield, F., Baron, A.F. and Pearson, G.W., 1981. Dating of post-glacial landforms in the central Howgills. Earth Surf. Proc. Landforms, 6: 401412.

344 Harvey, A.M. and Renwick, W.H., 1987. Holocene alluvial fan and terrace formation in the Bowland Fells, northwest England. Earth Surf. Proc. Landforms, 12: 249-257. Imeson, A.C. and Verstraten, J.M., 1988. Rills on badland slopes: a physico-chemically controlled phenomenon. In: A.C. Imeson and M. Sala (Editors), Geomorphic Processes in Environments with Strong Seasonal Contrasts. I. Hillslope Process. Catena Suppl., 12: 139-150. La Roca Cervignon, N. and Calvo-Cases, A., 1988. Slope evolution by mass movements and surface wash. In: A.C. Imeson and M. Sala (Editors), Geomorphic Processes in Environments with Strong Seasonal Contrasts. I. Hillsiope Processes. Catena Suppl., 12: 95102. Miller, S.Y., 1991. Soil chronosequences and fluvial landform development: studies in SE Spain and NW England, Ph.D. Thesis, Univ. of Liverpool. Pitlick, J.C. and Thorne, C.R., 1987. Sediment supply, movement and storage in an unstable gravel-bed river. In: C.R. Thorne, J.C. Bathurst and R.D. Hey (Editors), Sediment Transport in Gravel-Bed Rivers. Wiley, New York, pp. 151-183. Schumm, S.A., 1956a. The role of creep and rainwash on the retreat of badland slopes. Am. J. Sci., 254: 693700. Schumm, S.A., 1956b. Evolution of drainage systems and

A.M.HARVEY slopes in badlands at Perth Amboy, New Jersey. Geol. Soc. Am. Bull., 67: 597-6646. Schumm, S.A., 1964. Seasonal variations in erosion rates on hillslopes in Western Colorado. Z. Geomorph., Suppl., 5:215-238. Schumm, S.A., 1977. The Fluvial System. Wiley, New York, 338 pp. Suwa, H. and Okuda, S., 1988. Seasonal variations of erosional processes in the Kamikamihori Valley of Mt. Yakedale, northern Japan. In: A.M. Harvey and M. Sala (Editors), Geomorphic Processes in Environments with Strong Seasonal Contrast. II. Geomorphic Systems. Catena Suppl., 13:61-77. Wells, N.A., Andriamihaja, B. and Solo Rakotovololona, H.F., 1991. Patterns of development of Lavaka, Madagascar's unusual gullies. Earth Surf. Proc. Landforms, 16: 189-206. Wells, S.G. and Gutierrez, A.A., 1982. Quaternary evolution of badlands in the southeastern Colorado Plateau, USA. In: R.B. Bryan and A. Yair (Editors), Badland Geomorphology and Piping. Geobooks, Norwich, UK, pp. 239-258. Wolman, M.G. and Gerson, R., 1978. Relative scales of time and effectiveness of climate in watershed geomorphology. Earth Surf. Proc. Landforms, 3:189-208. Wolman, M.G. and Miller, J.P., 1960. Magnitude and frequency of forces in geomorphic processes. J. Geol., 68: 54-74.