geomorphic dynamics of gullies developed in sandy slopesof...

Landform Analysis, Vol. 17: 91–97 (2011)

Geomorphic dynamics of gullies developedin sandy slopes of Central Spain

Ana Lucía1, José Francisco Martín-Duque1, Jonathan Benjamin Laronne2,Miguel Ángel Sanz-Santos1

1Dept.of Geodynamics and Institute of Geosciences (CSIC-UCM), Complutense University of Madrid, Madrid,Spain²Dept. of Geography & Environmental Development, Ben Gurion University of the Negev, Beer Sheva 84105,Israele-mail: [email protected]

Abstract: Gullies developed on sandy lithologies are scarce. Such landforms have developed in the sandy deposits of theCretaceous ‘Utrillas’ facies within the Segovia province of Central Spain. They appear at the slopes of a group of mesas andcuestas located at the edge of the northern piedmont of the Guadarrama Mountains of the Spanish Central System. The ac-tivity of the different geomorphic processes acting in these gullies, as well as their connectivity, are being characterized andquantified. This study was preceded by reconnaissance methods, whereas presently technologically advanced and more ac-curate techniques are being utilized. The new methods are applied in an experimental catchment (1.26 ha). They include: i)an automatic Reid-type (formerly termed Birkbeck) slot bedload sampler for continuous monitoring of bedload flux and forcontinuous sampling of bedload; siphons for sampling the suspended load; and a Parshall flume to monitor water discharge.Jointly, these instruments allow to study the fluvial dynamics at the catchment mouth; ii) topographic surveys undertaken bya Terrestrial Laser Scanner (TLS) to quantify the activity of gravitational, overland flow and fluvial processes in differentsediment source areas within the gully basin; iii) micro runoff and erosion plots to monitor the infiltration and erosive re-sponse of different Hydrologic Response Units (HRU) comprising the interior of the catchment. This ensemble of novelmethods has started providing patterns of sediment movement within the gully system.

Keywords: sand, gullies, Terrestrial Laser Scanner (TLS), Reid bedload sampler, erosion-runoff plots, Spain

Introduction

Gullies and badlands are usually associated withclayey lithologies (García Ruiz & López Bermudez2009). Gullies developed in sands or sandstones areexceptional, in part explaining why few studies (e.g.:Boardman et al. 2003, Crouch 1990, Lindquist 1980,Okagbue & Ezechi 1988, Osterkamp & Toy 1997)have been undertaken on such gullies. In most ofthese studies only reconnaissance methods havebeen used. Therefore, obtaining accurate data aboutthe geomorphic activity and connectivity of sandygullies is expected to cover a knowledge gap.

The studied sandy gullies of Central Spain are lo-cated at the edge of the northern piedmont of theCentral System (Fig. 1), appearing at the slopes of agroup of mesas and cuestas. These landforms are not

only interesting because they form in sands, but par-ticularly as it has become obvious that theirformation was triggered by quarrying activities start-ing at least 800 years ago (Moreno 1989, Vicenteet al. 2009). The sediment yield from these gulliesfrequently affects human land use, burying roads,crops and pasture fields, as well as damaging villagestructures. Therefore, human activities and gully dy-namics are intricately interconnected in this region,having interacted since the beginning of gully forma-tion.

For both academic and applied purposes, ourbroad objective is to understand the geomorphicprocesses which operate within these gullies. De-tailed description of the specific aims are describedelsewhere (Lucía et al. 2007). They focus on evalua-tion of the rates of different processes acting within

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these landforms, their frequency of occurrence, theirtriggering factors and the interconnection and cou-pling between them (Harvey 2001, Thomas 2001).

Based on a detailed landform analysis of 75 gul-lies in the area, the geomorphic activity was charac-terized (Lucía et al. 2008). The more active processesin the catchment are mass movements at theheadwalls and sheet erosion on the sandy innerslopes and catchment ridges. Both processes providesediments to the channels. The sediments trans-ported to the dry washes are evacuated by pulses offluvial activity, depositing sediments onto small allu-vial cones at the toe of the slopes. Most of the sedi-ment stored in the channels is sand. Therefore, themajority of the sediment is transported as bedloadinstead of in suspension, the latter predominating inmost gullies at the catchment scale (Mathys et al.2003, Gallart et al. 2005, García Ruiz et al. 2008).

A reconnaissance study was undertaken during2007–2008 to monitor the main processes which movesediments within the gullies (Lucía et al. 2008). Re-connaissance methods are suitable when no previousdata are available and when erosion and sedimenta-tion are located in specific areas (Hudson 1997). Be-ing the case, erosion pins were used to determine thevariation in pedestal heights in inner divides, therebymeasuring sheet erosion. For the quantification of thesediment yield in different catchments, two types ofsediment traps were installed – a small gabion dambuilt in a scour channel and a metal pit box located atthe outlet of a gully. The quantification of the volumeof cone deposits on roads after storm events com-pleted the reconnaissance of the geomorphic activity.The above-mentioned methods allowed to measurehigh erosion rates: on average 11.7 mm sheet erosionin some inner gully divides and 115 Mg ha–1 yr–1 of sed-iment yield in a gully for a single rainfall event (69mm; I30 = 72 mm hr–1), occurring on Sept. 9, 2008. Theobtained rates indicate the high mobility of the sandsin which the gullies form.

A second phase of the research on thegeomorphic activity of the sandy gullies includes theuse of more accurate, technologically advancedmethods to measure processes in one experimentalcatchment.

A set of instruments has been deployed at theoutlet of this catchment to measure the suspendedload, the bedload and the water discharge, the lattertwo by automatic devices. Aiming to studygully-channel coupling, detailed topographical sur-veys with Terrestrial Laser Scanning (TLS) are beingcarried out at selected locations. These areas werelocated according to an accurate survey of sites inwhich slope and channel activities are connected.Also, to better understand the hydrological responseof hydro-geomorphic units forming the interior ofthe gullies, a set of erosion and runoff microplotshave also been installed.

The use of these methods allows studying thegeomorphic dynamics of this selected gully, consid-ered to be representative in this area. Specifically,the use of TLS to monitor slope and channelinterconnectivity with respect to sediment move-ment and the installation of the automatic bedloadsampler at the gully outlet are novel and of interestwithin the framework of gully erosion studies.

Study area. The Barranca de los Pinosexperimental catchment

The Barranca de los Pinos (Fig. 2) is one of themany well developed sandy gullies scattered through-out the slopes of a set of mesas and cuestas of thenorthern piedmont of the Guadarrama Mountains ofCentral Spain. The gullied catchments occupy an area

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Ana Lucía, José Francisco Martín-Duque, Jonathan Benjamin Laronne, Miguel Ángel Sanz-Santos

Fig. 1. Location of the sand slope gullies in the SegoviaProvince, UTM coordinates, 30 zone (a) and view of theinterior of one of the studied sandy gullies (b)

exceeding 18 km2, underlain by Cretaceous sedimentsof marine (limestone and dolostone) and fluvial(sands, gravel and clay) origin. The catchment iscapped by limestone and dolostone. The rest of thecatchment substrata are mainly sands of two differentgeologic formations – Arenas de Segovia and Arenasde Carabias (Alonso 1981). The main difference be-tween the two sandy formations is the dinimished co-hesion within the lower slope. The sand is exposed in85% of the gullied area, and only 15% of the gulliedsurface is covered by limestone-dolostone colluviumderived from the ungullied slopes and the caprock.

The area of the catchment (1.26 ha) was deter-mined by a detailed topographical survey using a dif-ferential GPS. Altogether 90% of this area is gullied.The derived Digital Elevation Model (DEM) of 2×2m shows steep slopes (20° and 60°) in the gullied sur-face. The main channel heads westwards, hence thedominant aspects of slopes are south and north.These slopes are dissected by secondary gully chan-nels, which are more abundant in northern slopes.The drainage density of the channels visible in anorthophoto of 0.5 m pixel size is 40.9 km km–2. Themain channel has a longitudinal slope of 6.6% and itssediment is medium sand (D50 = 0.555 mm; Fig. 3).

The soils developed on the limestone-dolostonecaprocks and colluviums are rendzic and calcicleptosols, and sandy cambisols where the substratumis sand. Most of the gullied surfaces lack soils; thesandy rocks and their regolith are exposed. Within thegullies the vegetation is very scarce, with some pinestands on sandy substrata. These were introduced 60years ago by the Regional Forestry Agency of the re-gion. The colluvium is covered by scattered junipersand holm oaks. The experimental catchment is lo-cated within a broad region, the climate of which isContinental Mediterranean. The mean annual rain-fall (registered nearby during 79 years) is 677 mm.The mean temperature is 11.9°C (INM 2001).

Methods

The experimental methods are concentrated in arepresentative experimental catchment (Barranca delos Pinos, Orejana Municipality, Segovia Province).They are grouped in three categories: fluvial activitymeasurements of sediment transport and water dis-charge, slope activity measurements by means ofTLS and erosion and runoff measurements at theplot scale.

Fluvial activity measurements – sedimenttransport

The fluvial activity is being monitored at the outletof the catchment. Bedload is monitored continuouslyand automatically by two independent, side-by-sideReid-type slot samplers (Reid et al. 1980). This appa-ratus samples and weighs a slot width portion (theslots can vary from 0 to 160 mm; with the maximumaperture, both samplers represent 26% of the channelwidth) of the total bedload transported in the chan-

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Geomorphic dynamics of gullies developed in sandy slopes of Central Spain

Fig. 3. Grains size distribution of sediment in the mainchannel of the experimental catchment

Fig. 2. Barranca de los Pinos experimental catchment and location of Hydrologic Response Units (HRU): 1 – exposedhigher cohesive sands, 2 – limestone-dolostone colluvium deposits on higher cohesive sand substrata, 3 – exposed lowercohesive sands, 4 – limestone-dolostone colluvium deposits on lower cohesive sand substrata, 5 – exposed sands underpine canopy, 6 – ungullied slope

nel. The cumulative mass of the sediment enteringand filling each sampler is monitored by means of apressure transducer connected to a pressure pillow(having a pressure-mass calibration) upon which aninternal box is located. A separate, calibrated pres-sure transducer is located inside the sampler but out-side the inner box to monitor the pressure of the watercolumn. These vented pressure transducers are con-nected to a data logger registering each 30 s the aver-age of three 10 s readings. For every 30 s time intervalthe difference in added pressure between the pillowand the water column is the added pressure of sam-pled bedload. Details of the functioning and use ofthe Reid bedload sampler are provided elsewhere(Laronne et al. 2003).

The specific characteristics of our two samplersare:– slot width is adjustable (0–160 mm);– the volume of each of the metal boxes is 0.225 m3.

This dimension was calculated from the sedimentyield assessments made in the two sediment traps,gabion and metal box (Lucía et al. 2008). Becauseof the size of the inner box, the sensitivity of thebedload sampler is estimated to be 0.3 kg(Laronne et al. 2003). The effectiveness of thesampler is considerably reduced when it is 80%full (Habersack et al. 2001), hence bedload datawill be used up to this stage of sampler filling;

– the length of the slot is 65 cm, so designed given that90% of the GSD is sand and is mainly transportedby saltation and the saltation length of the sandparticles was estimated using the following expres-sion:

λb/D = 3D* 0.6 T0.9 (1)

where:λb/D – is the nondimensional transport rate with anaccuracy of 50%,D* – the nondimensional sediment diameter,T – the transport stage parameter; the calculated sal-tation length is 8.8 cm, hence the saltations lenght isexpected not to exceed 18 cm; to ensure that sandwill saltate beyond the slot, a broad safety factor wasapplied.

A Parshall flume was installed downstream of thebedload sampler to monitor water discharge. Theelevation of the water surface is measured in a still-ing well by a vented pressure transducer. This trans-ducer is also connected to the data logger, so themeasurements of bedload and water discharge arelogged simultaneously.

Altogether six siphon samplers were installed inone of the sides of the Parshall flume. They wereplaced at different heights to sample the suspendedsediment during hydrograph rise. Figure 4 shows thesetting of the equipment at the outlet of the experi-mental catchment.

Slope activity measurements by means ofTerrestial Laser Scanner (TLS)

Detailed topographical surveys are being carriedout with a TLS to quantify the activity of gravita-tional and runoff (e.g., rilling and cone deposition)processes in sediment source areas within the experi-mental gully catchment. The TLS model is a LeicaScan Station 2, which can reach a precision of 2 mmat a scanning distance <120 m. In total, 8 gully loca-tions have been identified and scanned with a spatialresolution which ranges from 0.5×0.5 cm to 1×1 cm.The point cloud (Fig. 6) is ‘cleaned’ using the Cy-clone software. In this process, the scattered vegeta-tion is removed from the data to obtain a representa-tion of the gully surface. The cleaned cloud isexported in a .ptx format to the Polyworks software,where a Triangulated Irregular Network (TIN) digi-tal model is constructed.

This ‘time zero’ DEM surface will be comparedwith later DEMs after gravitational movements orintense channelized (rill) runoff erosion occur insediment source areas. This volume subtraction ofDEMs will allow quantification of erosion and its lo-cation.

Erosion and runoff measurements at a plotscale

Six different Hydrologic Response Units (HRU)represent the experimental catchment. These havebeen identified according to their homogeneity in li-thology, slope, surficial deposits and vegetation can-opy. To understand their different hydrological anderosive response, micro-plots (0.25 m2) have beendeployed to compare their infiltration capacity, run-off and sediment production.

In order to not to disturb the studied catchmentsurface where fluvial activity is being monitored, rep-resentative locations of the six HRUs have been cho-sen in two adjacent gullies separated by the divide ofthe main gully. Three micro plots (0.25 m2), which

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Fig. 4. Setting of equipment installed at the outlet of theBarranca de los Pinos experimental catchment

act as replicas of the experiment, have been installedin each HRU. Therefore, a total of 18 micro plots arecurrently under study. The HRUs have slight varia-tions in aspect and gradient. To exclude the influ-ence of these factors, the 18 micro plots were in-stalled at constant gradient and aspect.

Rainfall

An automatic rain gauge (HOBO type) of 0.2 mmaccuracy was installed. This rain gauge is located atmid-catchment elevation.

Results

The monitoring of the described new techniquesstarted at the beginning of the hydrological year2009–2010. Considering that the autumn of 2009 wasexceptionally dry in the Segovia region, the accumu-lated precipitation represents less than the 50% ofthe mean annual till the 20th of November of 2009,(data obtained by the National Agency of Meteorol-ogy, AEMET). Only two small flow events were reg-istered at the experimental catchment of theBarranca de los Pinos.

Figure 5 shows the record of the first event, whichoccurred on October 1, 2009. The bedload transportstarted after a cumulative rainfall depth of 7.1 mmand occurred with a mere 1.8 cm water depth in thechannel. The cumulative rainfall depth during theevent was 10.8 mm and the maximum intensity for a

duration of 30 min was 3.2 mm h–1 or 0.36 mm min–1

= 26.6 mm h–1. Interestingly, bedload has occurredat the inception of the flow during 6 min and was ini-tiated from a flow depth of 0.35 cm to cessation at a0.30 cm. Although the flux of bedload is small withreference to the shallow (low shear stress) flow, val-ues in the range 0.2–0.5 kg sm–1 are considered high.Hence, considerably higher fluxes will occur in some-what larger flow events.

Figure 6 shows an image of the point cloud ob-tained with the TLS at one of the inner locations ofthe experimental catchment. It corresponds to thecaprock of the gully’s headwall; the pine canopy andthe sandy slopes are also seen. Because no move-ment has occurred as yet, a comparison of Digital El-evation Models (DEM) has not yet been made.

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Fig. 5. Record of the first monitored bedload event

Fig. 6. Output of one of the eight scanned areas in the inte-rior of the monitored gully

Discussion

The methods initiated at the Barranca de losPinos experimental catchment are deployed to re-cord the geomorphic activity within this sandy gully.The interest of such monitoring derives not onlyfrom the landscapes being rare, but also from thenovelty of the two techniques (Reid-type bedloadsampler and TLS) for this environment (sand slopegullies), by the joint use of different methods to tracksediment movement, and due to the interrelationsbetween human activity (quarrying), gullying and itseffects. The scarce literature on the geomorphic ac-tivity on sand gullies is expected to secure consider-able interest in the forthcoming results from this ex-perimental catchment.

Acknowledgements

The Barranca de los Pinos experimental catch-ment is a part of the research project CGL-2006-07207 and CGL2010-21754-C02-01, financially sup-ported by the Spanish Ministry of Science and Inno-vation. We thank Jorge Martin Manzanas, CristinaMartín-Moreno, Antonio and Victor MuńozGonzález and Javier Lucía for logistic and overall as-sistance.

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