River, Coastal and Estuarine Morphodynamics: RCEM 2007 – Dohmen-Janssen & Hulscher (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-45363-9

Autogenic cycles of sheet and channelised flow on fluvial fan-deltas

M. van Dijk & G. PostmaSedimentology Group, Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht,The Netherlands

M.G. KleinhansFluvial Research Group, Department of Physical Geography, Faculty of Geosciences, Utrecht University,Utrecht, The Netherlands

ABSTRACT: In the development of fluvial fan-deltas incised channels are usually regarded as signatures ofchanges in climate, tectonics or base level of the receiving basin. The role of autogenic mechanisms influencingthe evolution of these deltas is poorly known. Experiments were performed to study the intrinsic evolution offluvial fan deltas without external variations of base level, climate and/or tectonics. Water and sediment were ledonto a submerged shelf, where flow deceleration caused the formation of fluvial deltas. The deltas evolved bysheet flow, incidentally alternating with channelised flow. The transition from sheet flow to channelised flow wasinitiated by an increased surface gradient caused by aggradation of the delta apex. This led to incision, headwarderosion and ultimately to channelisation. Renewal of the sheet flow occurred after the incised channel was backfilled. The back filling was initiated by bar deposition, caused by flow deceleration. The flow bifurcated aroundthis bar, causing bank erosion and additional flow deceleration. The process of autogenic incision and back fillingcontrasts with commonly recognised autogenic processes; lateral shift of channels and active parts of fan lobes.The autogenic incisions can be easily confused with climate, tectonic and sea-level driven erosional events. Ourflume models suggest that autogenic erosion might just as well be part of the intrinsic evolution of the systemand need to be distinguished from allogenic signatures in modern and ancient fluvial fan-delta systems.

1 INTRODUCTION

Alluvial fans and fan deltas are common sedimentaryenvironments along basin margins. They often consti-tute thick sediment wedges that form in geologicallyshort periods of time and contain valuable, high resolu-tion records of tectonic, climate and sea-level change.Alluvial fans are formed of coarse sediments, createdwhere high-bed-load streams enter zones of reducedstream power, and deposit their coarser fraction oftheir loads (Bull 1977). Fan deltas are considered theircoastal prism when alluvial fans prograde in a standingbody of water (Nemec and Steel 1988). Moscariello(2005) distinguished two broad fan categories, whichare referred to as 1) alluvial fans, deposited mainly bydebris flows, and 2) fluvial fans, deposited mainly bysediment fluid flows. Research has been carried outto unravel the effects of catchments type (e.g. Blair1999), climate changes (e.g. Postma 2001, Nemec andKacanzi 1999), tectonic movements (e.g. Blair 2000,Gawthorpe and Collela 1990, Garcia-Garcia et al.2006) and base-level variations (e.g. Nichols 2005,Gutsell et al. 2004) on the morphology, sedimentologyand architecture of both modern and ancient fan deltasand alluvial fans.

Recently, the interest for the intrinsic aspectsof fluvial systems has grown, since Miall (1996)named them autogenic processes to distinguish themfrom allogenic processes such as climate change asdescribed above. During the past decade, Muto andco-workers have developed an entirely new concept ondelta-scale systems from experimental and numericalmodelling studies; autostratigrapy (Muto 2001, Mutoand Steel 1992, 2001, 2004, Muto and Swenson 2006,Muto et al. 2007). It investigates the behaviour of deltasduring constant sea-level fall and rise, while variableslike discharge etc. are kept constant. They showed thatthe experimental deltas showed predictable and quan-tifyable responses (‘autoretreat’ and ‘autobreak’) overa large range of different allogenic conditions (Muto &Steel 2001, 2004). As the experiments were performedwith constant sea-level rise and fall, the investigateddeltas were still influenced and modified by the chang-ing base level. To investigate the behaviour of deltaswith the absence of all external forcings, the exper-iments reported herein were carried out without anyvariations in base level.

A disadvantage in the study of autogenic behaviourof fluvial systems is the lack of proper field studyobjects. During the Holocene the allogenic conditions

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Figure 1. Schematic drawing of the experimental setup.

have not been constant, and in fact they are still chang-ing. Therefore, additional knowledge must come fromexperimental research where allogenic conditions canbe kept constant. We aim to quantify the vertical andlongitudinal changes in fluvial fan-delta depositionrelated the different flow mechanisms through timewith the following objectives: 1) determine autogeni-cally controlled morphodynamics of fluvial fan deltas;2) relate observed behaviour to observed flow mech-anisms; and 3) discuss implications for real worldsystems.

2 EXPERIMENTAL DESIGN

2.1 Setup

The experimental setup consisted of a shelf and a rect-angular duct acting as a feeder channel (Fig. 1). Thehorizontal shelf measured 4.5 by 3.5 m. The bed ofboth the shelf and the channel consisted of sand. Theshelf was covered by 1 cm of water.An adjustable drainlocated in the basin ensured a constant water level. Thechannel was 5 cm wide and 1 meter long and consistedof smooth impermeable walls. The outlet of the ductwas located in the centre of one of the edges of theshelf, consisting of the same impermeable walls toensure that no water or sediment was lost. The ductwas positioned horizontally, and its width was chosencarefully to ensure that no alternating bars would occurin the duct at the range of discharges used to avoid vari-ations in the outflow direction at the outlet of the duct.The sediment was mixed with the water flow by meansof a sediment feeder. The sediment feeder delivered a

constant sediment volume by means of a worm gear.Deviations of the delivery by the feeder were measuredto be less than 1% over 15 minutes. The grain size ofthe sediment used in the experiments was chosen suchthat sediment transport would occur as bed load, withFroude numbers around unity.

2.2 Measurements

Every four hours the experiment was halted and thesurface topography was measured in detail by meansof photogrammetry (a way of measuring topogra-phy based on stereoscopy). An automated movableplatform contained a set of cameras, which took pho-tographs of the surface from different angles simul-taneously. With help of specially designed software(SANDPHOX™), the images are processed into a Dig-ital Elevation Model (DEM) of the sand surface. TheDEMs are here presented as shaded relief maps (Figs3–4), processed in the commercially available programSurfer. The vertical accuracy of the photogramme-try is 250 µm and the horizontal accuracy 100 µm atthe very least. To prevent reflection by the runningwater, we stopped the experiment and drained the watercarefully before each photogrammetry session.

3 RESULTS

3.1 Autogenic behaviour

3.1.1 Stage 1: sheet flow and fractionationFrom the start of the experiment the delta was beingcongregated by sheet flow, i.e. a continuous shallowsheet of water flowing over the delta. As a result,the delta was smooth, and almost circular in shape(see Fig. 2). After 12¼ hours the sheet flow startedto fractionate. As a result of enlargement of the sur-face area of the delta plain, it could no longer sustaina single continuous sheet flow, and the flow started tobreak up in smaller, but still shallow and wide flows.This has been named ‘fractionated flow’ by Whippleet al. (1998). After fractionation occurred, the deltacontinued to grow in diameter, but the smoothness ofthe shoreline of the delta was lost, as shown by Fig-ure 3A. An important feature of the delta at this pointis the zonation of flow mechanisms (Fig. 3A). On theapex sheet flow continues as a single sheet, but aroundthat a zone of smaller and shallow flows exists. These‘fractionated sheet flows’ then developed into distinctsmall-scale channels, which left a rill-like mark on thedelta plain. These small-scale channels migrated later-ally across the delta plain together with the upstreamfractionated sheet flows, leading the more irregularlyshaped shoreline (Fig. 3A). Flow migration meant thatat a given location, water flow was active only part ofthe time, leading to lower sediment transport capacity

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Figure 2. Shaded relief map of the delta after 12 hours ofevolving. Note the smooth, circular shoreline of the delta,which is basically a slip face with a height of one cm (theimposed water depth) and the angle of repose as gradient.Thesmall square indicates the area of which the average elevationwas measured, shown in Figure 4.

averaged over longer periods compared to the sheetflows on the apex. These lower transport capacity justdownstream of the apex formed an impediment for thesediment on the apex, and as result the sediment wasdeposited on the apex, leading to aggradation and over-steepening of the gradient from the apex towards theshoreline.

3.1.2 Stage 2: transition to channelised flowAfter the delta grew and aggraded for almost 40 hours(see Figs 3A and 4), a scour occurred on two third ofthe distance from apex to the delta shoreline, in frontof the entering flow (Fig. 3B). This scour was initiatedby the increasing gradient from apex to delta shorelineas the apex aggraded. The increasing gradient causedflows to accelerate, and led to incision. Once devel-oped, the scour attracted increasingly more water as itdeepened, and the water jetted from the scour towardsthe shoreline. This concentration of flow caused depo-sition of a lobe of sediment in front of the delta.As evenmore water was converging into the scour, headwarderosion caused upstream migration. Within 30 minutesit reached the feeder channel, and a fully confinedchannel occurred (Fig. 3C), producing an erosionalsurface extending to the delta apex.

3.1.3 Stage 3: Channelisation and channel fill-upAfter the flow became fully confined within the erodedchannel walls, part of the delta plain was deprived fromits supply of water and sediment, and was abandoned(Fig. 3C). As a result, part of the shoreline of the delta

was starved of its supply of sand, and all the mate-rial was deposited at the channel mouth. This area ofthe delta therefore showed high progradation rates asa delta lobe was formed (see also Fig. 4, change inlength). As progradation continued, the lengthening ofthe channel caused the overall gradient to decrease,leading to loss of momentum. The decreasing capac-ity of the flow initiated deposition of the sediment itcarried, and a bar was formed in the middle of the chan-nel (Fig. 3D). The remaining flow bifurcated aroundthe bar while at the same time two scours occurredupstream and to the sides of the newly formed bar(Fig. 3D). These scours started to erode the walls ofthe channel, and caused it to widen (compare the widthof the channel in Figs 3C and D).This led to shallowingof the flow, more flow deceleration and more sedimentwas deposited adjacent to the mid-channel bar. Eventu-ally, the bar migrated upstream, and the entire channelwas backfilled. The zones of sheet flow, fractionatedsheet flow and small channels were reestablished, andcontinued until a new cycle of incision, channelisedflow and channel filling was reinitiated.

3.2 Cyclic behaviour

Four complete cycles of channel incision and fill-uphave been observed within the experiment, and a fifthphase of incision was just initiated when the experi-ment was stopped. The delta behaviour throughout theexperiment is shown in Figure 4. The periods of inci-sion and subsequent channelised flow are indicatedby the light-grey bars, whereas the white parts depictphases of sheet flow. Furthermore, the height of theapex of the delta is shown by the thick black line (seethe axis on the left), while the other two thinner linesexpress the change in length (measured from apex tothe delta edge, parallel to the inflow) and width (mea-sured perpendicular to the inflow) of the delta (see axison the right).

The pattern occurring shows erosional events atquite regular intervals, and have comparable dura-tions. Therefore this type of autogenic mechanism canbe named ‘autocyclic’. The erosion occurring duringchannelised flow is clearly shown by the elevation ofthe apex of the delta, which decreases by at least 10 mmduring every incisional event. The height of the apexcontinues to grow, despite the incisions. The rate ofgrowth, however, is declining. This can be explainedby the continuously increasing lateral size of the deltathrough time. Furthermore, the start of a period ofchannelisation is accompanied by an increase in thechange in length (see Fig. 4, at 40 and 120 hoursinto the experiment), which represents the effect ofconcentration of the water flow due to channelisationand corresponding delta progradation. In contrast, thewidth of the delta increases just before periods of chan-nel initiation. This relates to the delta aggrading itsapex to be able to initiate incision.

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Figure 3. Shaded relief maps of the delta through a cycle of built up, incision, channelised flow, and channel fill. (A) Thedelta after 70 hours, showing zonation of the flow and small channels towards the delta shoreline. Note that although no ‘areacovered with water’ is indicated, in Figures 4B and B the entire delta plain is covered with water. (B) The delta after 38 hours,showing a scour hole which caused the flow to converge and eventually led to channelisation. (C) The delta after 81 hours,with fully channelised flow. Note the abandoned areas on the delta plain and starving of the adjacent delta shoreline. (D) Thedelta after 85 hours. The channel widened compared to Figure 4C, and bar formation commenced.

4 IMPLICATIONS

These experiments show that the behaviour of a fluvialfan delta under constant conditions is more complexthan previously thought. The alternation of periods ofincision and channelised flow with periods of sheetflow dominance is very similar to the results reportedby Kim et al. (2007). They propose a variation in sloperelated to storage and release of sediment to accountfor shoreline migrations. Essentially, the periods ofsheet flow dominance in our model represent storageof sediment, the delta building up its slope for incisionto be initiated, whereas during channelised flow sedi-ment is released, leading to increased progradation. AsFigure 4 shows, the change of the length and width ofthe delta is far from constant. They both show periodsof increased and decreased growth, representing thestorage and release of sediment.

The fact that subsequent periods of incision do notcut down to the level of the previous periods of chan-nelised flow, has important implications. It means that

the erosional surfaces are preserved within the archi-tecture of the deltaic body. These erosional surfaceswill look very similar to erosional surfaces formedby allogenic causes, such as climate changes or tec-tonic movements. In fact, it might well be impossibleto distinguish between allogenic and autogenic formederosional surfaces. This means that incisions previ-ously interpreted as caused by allogenic processes,might also have an entirely autogenic origin.

5 CONCLUSIONS

Autocyclic aspects of fluvial fan delta evolution havebeen investigated by means of laboratory experiments.An experiment was performed, in which all external(allogenic) variables (water and sediment input, andsea level) were kept constant. As a result the pro-cesses observed had to be of an autogenic nature.Measurements done during the experiments consisted

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Figure 4. Behaviour of the delta throughout the experiment. The light grey blocks represent periods of channelised flow,whereas the white blocks represent sheet flow dominance. In total 5 incision events have been observed. However, the finalevent does not show up as a wide bar, as the experiment was stopped just after incision commenced. The fat black line indicatesthe average height of the delta apex (measurement location shown in Figure 2), while the other two lines show the change inlength and width of the delta. Note that the delta elevation is measured in mm (left axis), and the changes in length and widthin % (right axis).

of detailed topographic scanning (DEM’s) and videoimages. Our main conclusions are:

– Autocyclic behaviour on fluvial fan deltas consistsnot only of sheet flow and flow fractionation andlateral migration of channels as commonly sug-gested, but also by cycles of fan incision that leadto channelisation.

– The transition from sheet to channelised flow wasobserved to be initiated by aggradation controlledoversteepening of the apex of the delta, caused byzonation of sheet flow, fractionated sheet flow andsmall-scale channelised flow. The transition backto sheet flow was initiated by flow decelerationand bar formation and subsequent backfilling ofthe channel.

– Incisions on alluvial fans or fan deltas which havebeen attributed to allogenic forces (climate, tecton-ics and base level), might actually have been causedby the intrinsic behaviour of the system itself.

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