Debris ﬂood hazard documentation and mitigation on the ...€¦ · hazards, mainly related to mass transport and erosion phe-nomena. In particular, debris ﬂoods are hydrological

Nat. Hazards Earth Syst. Sci., 12, 1873–1882, 2012www.nat-hazards-earth-syst-sci.net/12/1873/2012/doi:10.5194/nhess-12-1873-2012© Author(s) 2012. CC Attribution 3.0 License.

Natural Hazardsand Earth

System Sciences

Debris flood hazard documentation and mitigation on the Tilcaraalluvial fan (Quebrada de Humahuaca, Jujuy province,North-West Argentina)

G. Marcato1, G. Bossi1, F. Rivelli2, and L. Borgatti3

1CNR-IRPI – National Research Council of Italy, Research Institute for Hydrological and Geological Hazard Prevention,C.so Stati Uniti 4, 35127 Padova, Italy2Departamento de Ciencias Naturales, Universidad Nacional de Salta, Casilla De Correo 529, 4400 Salta, Argentina3Department of Civil, Environmental and Materials Engineering DICAM, Alma Mater Studiorium Universita di Bologna,Viale Risorgimento 2, 41136 Bologna, Italy

Correspondence to:G. Marcato ([email protected])

Received: 22 December 2011 – Revised: 26 March 2012 – Accepted: 28 March 2012 – Published: 13 June 2012

Abstract. For some decades, mass wasting processes such aslandslides and debris floods have been threatening villagesand transportation routes in the Rio Grande Valley, namedQuebrada de Humauhuaca. One of the most significant ex-amples is the urban area of Tilcara, built on a large alluvialfan. In recent years, debris flood phenomena have been trig-gered in the tributary valley of the Huasamayo Stream andreached the alluvial fan on a decadal basis.

In view of proper development of the area, hazard andrisk assessment together with risk mitigation strategies areof paramount importance. The need is urgent also becausethe Quebrada de Humahuaca was recently included in theUNESCO World Cultural Heritage. Therefore, the growingtourism industry may lead to uncontrolled exploitation andurbanization of the valley, with a consequent increase of thevulnerability of the elements exposed to risk. In this context,structural and non structural mitigation measures not onlyhave to be based on the understanding of natural processes,but also have to consider environmental and sociological fac-tors that could hinder the effectiveness of the countermeasureworks.

The hydrogeological processes are described with refer-ence to present-day hazard and risk conditions. Consider-ing the socio-economic context, some possible interventionsare outlined, which encompass budget constraints and localpractices. One viable solution would be to build a protect-ing dam upstream of the fan apex and an artificial channel, inorder to divert the floodwaters in a gully that would then con-

vey water and sediments into the Rio Grande, some kilome-ters downstream of Tilcara. The proposed remedial measuresshould employ easily available and relatively cheap technolo-gies and local workers, incorporating low environmental andvisual impacts issues, in order to ensure both the future con-servation of the site and its safe exploitation for inhabitantsand tourists.

1 Introduction

In the area of Tilcara village (23◦34′55′′ S, 65◦23′50′′ W)there is an urgent need of debris flood hazard assessmentand mitigation. The Tilcara alluvial fan has been built bythe Huasamayo Stream, under arid environmental conditions,in a compressive tectonic context (Sancho et al., 2008). Atpresent, the active depositional lobe is constrained by artifi-cial levees in a narrow sector of the fan. Its ephemeral ac-tivity often results in floods that affect housing and infras-tructures built in the vicinity of the preferential path of thestream.

The Huasamayo Stream is a tributary of the Rio Grande,which flows in a narrow mountain valley located in theProvince of Jujuy in northwest Argentina, 1500 km northof the Capital Buenos Aires. This valley, named Quebradade Humahuaca, was followed by a major cultural route, theCamino Inca, which starts from the Rio Grande springs lo-cated in the desert plateau of the High Andean lands towards

Published by Copernicus Publications on behalf of the European Geosciences Union.

1874 G. Marcato et al.: Debris flood hazard documentation and mitigation on the Tilcara alluvial fan

its confluence with the Rio Leone, 150 km southwards. Sincethe time of hunters and gatherers, the valley has been a nat-ural route for people, goods and ideas. For these reasons,since 2003, it has been included in the UNESCO Word Her-itage list. As several mass wasting processes affect the Que-brada de Humahuaca, limiting its development and posingrisk to hamlets, infrastructures and to the peculiar landscapeof the area, UNESCO itself has encouraged Argentina “tocarry out an environmental assessment of the proposed flooddefence project in order to gauge its impact on the outstand-ing universal value of the Valley”. In fact, besides Tilcara,almost 20 villages are located along the valley, but the pop-ulation is significantly decreasing, also as a secondary con-sequence of the high frequency of mass wasting phenomenathat cause relevant damage to structures and infrastructures,halting traffic on transportation routes. Moreover, these massmovements threaten the farming activities, exploiting the ter-races along the valley floor, with the effect of reducing thearea of the land available for cultivation and therefore thesustenance of the population.

In particular, landslides and debris floods are the main haz-ardous phenomena affecting the slopes and the valley floors,inducing also serious consequences on the erosion and sed-imentation activity of the Rio Grande (Chayle and Wayne,1995; Cencetti et al., 2001; Cencetti, 2008). Most of theseevents occur on the Quaternary alluvial fans that are signif-icant landforms within the Quebrada de Humahuaca land-scape. Among these, the Tilcara alluvial fan represent aninteresting geomorphological asset, as it incorporates spec-tacular morphostratigraphic features; at the same time, itsevolution is a major natural hazard threatening human set-tlements and activities.

Alluvial fans are fan-shaped geomorphological featuresprogressively generated by deposition of eroded sediment(NRC, 1996). Since ancient times and all over the world, al-luvial fans have been occupied by human settlements and re-lated economic activities, because of mild slope and smooth-ness of topography, land fertility and relative closeness torivers (Schick et al., 1999). Especially in high mountain en-vironments, alluvial fans are exposed to a number of naturalhazards, mainly related to mass transport and erosion phe-nomena. In particular, debris floods are hydrological mass-transport phenomena characterized by gently sloping deposi-tion areas and massive bedload transport, with limited boul-der transport and deposit thickness (Aulitzky, 1980). The in-herent velocity and energy of these phenomena and, to someextent, their unpredictability can cause loss of lives and sig-nificant economic damage if the material cannot be containedin the main stream, eventually inundating parcels, buildingsand roads. As the triggering and subsequent evolution is veryrapid, it is generally quite difficult to implement effectivemonitoring devices and related thresholds and early warn-ing systems. As a consequence, structural risk mitigation hasbecome a priority and the classic approach is to implementcontrol works to constrain the stream in a sector of the fan or

to limit and filter the sediment that can be transported duringan event.

However, in areas where there is a significant anthropicpressure and particularly in developing countries, the needfor exploitable land may lead to underrate the risk, to buildinsufficient countermeasures and not to respect buffer zones(Burton and Kates, 1964; Blaikie et al., 1994). Moreover, de-veloping countries necessarily place higher priority on foodproduction, leaving fewer resources for sustainable risk man-agement procedures (Clarke and Munasinghe, 1995). Thesefactors increase the vulnerability and, in order to plan ef-fective mitigation works, it is important to consider thesocio-economic context of the region prone to natural haz-ard (Alcantara-Ayala, 2002).

The control structures must be dimensioned for high in-tensity events to guarantee a high degree of safety, especiallyif the land use limitations, are removed in case of ongoingurbanization; actually, these kind of countermeasures are ex-pensive and the political pressure may sometimes lead to un-derestimate the risk and install buffer solutions. Sometimes,in order to finance control works, it may be possible to relyon private investments, especially if the frequency of disasterevents is sufficiently high (Keefer, 2009).

Moreover, the ongoing climate changes can result in morefrequent events. In particular, the El Nino Southern Oscilla-tion (ENSO) climatic phenomenon has been widely referredto because of its impact on economies and human activities indeveloping countries, also as a consequence of more intensemass wasting phenomena. The relationship between interan-nual atmospheric and oceanic circulation oscillations, con-trolling precipitation regime and landslide activity phases,has been observed in these areas. In Latin America, the tem-poral distribution and frequency of landslides triggered byprecipitation seem to be strictly conditioned by the ENSOclimatic cycle (Moreiras, 2005), and also at longer timescales (Trauth et al., 2003). It can induce enhanced precipi-tation, leading to an increase in pore-water pressure in soils,lateral scouring of rivers and over steepening of hillslopes,eventually resulting in shallow failures and large mass move-ments.

At present, a specific hydrological study of the Tilcara areais part of the integrated and multidisciplinary investigationplan set up in order to document, assess and mitigate hydro-geological hazards. The research activity includes geologicaland geomorphological investigations together with the anal-ysis of social and economic aspects in order to assess theimpact of mitigation measures on the urban environment.

2 Study area

2.1 Alluvial fan processes and dynamics

Alluvial fans are depositional landforms produced wherechannels enter a zone of reduced stream power (Harvey,

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G. Marcato et al.: Debris flood hazard documentation and mitigation on the Tilcara alluvial fan 1875

2004). This situation is found at mountain fronts and attributary junctions, where the sediment load is normally de-posited. The evolution of these landforms are driven by thelong-term evolution of the landscape, with reference to thetectonic regime and the rate of uplift, and occur in all climaticsetting. In particular, fan morphology is directly related towater and sediment supply, which in turn is controlled byclimate. Alluvial fans can be classified in debris-flow andfluvially-dominated, on the basis of the primary process thatdelivers sediments to the fan itself. The predominance of oneprocess on the other is witnessed by either sediment type andtexture, or surface morphology; it depends on the catchmentcharacteristics and on the water and sediment mixture rheol-ogy. It is clear that debris flow are more common where sed-iment availability is high and in steep catchments, whereasfluvial processes prevail in large, less steep basins (Harvey,2004).

Debate occurs over the relative roles of tectonics, baselevel changes and climate in governing the evolution of al-luvial fans, even if climate is meant to play a major role infan dynamics, either in dry or in humid regions (Bull, 1991).

In arid environments alluvial fans are composed of looseclastic materials deposited during extreme magnitude-lowfrequency flood events, originating in the upper sector ofthe catchment (Graf, 1988). Frequently, the widening ofthe channel and its eventual avulsion can convey the water-sediment mixture to large areas of the fan; as a result, the re-turn period in each area may vary, exceeding many decades.

2.2 The Tilcara alluvial fan

The study area is located in northwest Argentina near theborder with Bolivia, in the Quebrada de Humahuaca val-ley, where the Rio Grande de Jujuy river flows. The ele-vation of the valley floor spans from 3440 m a.s.l. near thevillage of Iturbe to 1350 m where the Rio Grande flows inthe Rio Reyes; the valley width ranges from 100 m up to3 km (Fig. 1). The valley, which has an asymmetrical pro-file, is surrounded by Sierra Agular and Chani on the westside, which exceeds 6200 m allowing for the presence of per-manent glaciers, and the Sierra Zenta-Tilcara on the east side,which has lower peaks about 5000 m high, with no glaciers.

The Rio Grande is characterized by scattered dischargevalues, mainly due to climatic variability. At the station ofSan Juan, 180 km downstream from the springs, the max-imum measured discharge is 358 m3 s−1 and the minimum3 m3 s−1.

From the geological point of view the Quebrada deHumahuaca is one of several intramontane reverse-faultbounded basins of the Cordillera Oriental thick-skinned foldand thrust belt. In the study area, folded, faulted and jointedPrecambrian and Cambrian geological units prevail. Pun-coviscana Formation is a Precambrian low grade metamor-phic unit with slates and phyllites. The Cambrian rocks, be-longing to the Meson group, constitute the principal core of

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Fig. 1. Geographical setting. The Quebrada de Humahuaca region in the Province of Jujuy.

Fig. 2. Structural sketch map of the area (after Sancho et al., 2008). Image source: Google Earth

Fig. 1. Geographical setting. The Quebrada de Humahuaca regionin the Province of Jujuy.

the ridge (the sierra). These are multicolored orthoquarzitesand quarz-rich arenites (Ramos et al., 1967; Turner, 1970;Amegual and Zanettini, 1974). The general structure is re-lated to a homoclinal fold, with N–S direction, dipping east-ward. Regional N–S faults have produced the uplift of largeareas and determined the incision of N–S valleys, such asthe Quebrada de Humahuaca (Turner, 1970; Mon and Sal-fity, 1995).

Some tectonic lineaments, mainly strike slip faults trend-ing SE–NW, conjugate to principal thrusts (Salfity et al.,1984), drive the drainage pattern of the tributaries of RioGrande, like Huasamayo and Huichara streams.

Over the bedrock, an extensive Quaternary cover is found,which was generated by gravity processes acting in the up-per part of the slopes, together with alluvial phenomena inthe valley bottom. The morphology of this area is strictlycontrolled by lithological and structural features of the rockmasses and by ancient and active tectonics (Marcato et al.,2009). Mass wasting phenomena are in fact conditioned bythe notable tectonic uplift of the area (Chayle and Aguero,1987). The present-day geomorphological evolution of theQuebrada de Humahuaca is largely controlled by the mo-bilization of the thick deposits generated by the physicalweathering of the rock masses. In particular, the action of se-vere frost shattering in periglacial conditions has also a majorrole in the production of an extensive debris cover.

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The marked asymmetry of the valley flanks conditions thetype and activity of processes and landforms. On the east-ern flank two terraces formed by Rio Grande can be found,together with huge incised alluvial fans generated by the dif-ferent tributaries originating from the eastern cordillera. Thedeposits are represented by poorly consolidated coarse sedi-ments, easily involved in mass transport phenomena. Thesedeposits are mainly organized in terraces, pediment coverunits and alluvial fans (Robinson et al., 2005) (Fig. 2).

The Rio Grande basin has a mountain arid climate type,with high intensity and irregular summer storms and coldwinters, especially in the central part of the valley, where thevillage of Tilcara is located. Here, the annual average rain-fall is 136 mm yr−1 as a consequence of the orographic bar-rier formed by the oriental peaks, which prevents the cross-ing of the cloud systems generated from the Atlantic anticy-clone. Moreover, nearly all precipitations occur during thesummer season, with meteorological events that can be veryintense. Unfortunately, no continuous data are available onprecipitation regime. At present, the only available climaticdata in Tilcara is the average precipitation on a monthly ba-sis (the maximum is in January, with 42 mm on a 1934–1990dataset).

The vegetation cover of the basin, consisting mostly of xe-rophytes, is scarce; therefore, the soil can undergo intensesuperficial erosion during high intensity rainfall events.

The Tilcara alluvial fan has an area of 5 km2, with adrainage catchment in the order of 120 km2. The HuasamayoRiver is the main feeding channel, on a length of 12 km anda difference in altitude of more than 2000 m. Therefore, thelongitudinal profile is short and the slope is relatively high,being in the order of 10–11◦ (Fig. 3).

The geomorphological evolution of the Tilcara alluvial fanis characterized by a complex segmented development pat-tern (Sancho et al., 2008). Remnants of old evolutionarystages are recognizable; nowadays, they are in a higher po-sition with respect to the actual thalweg of the Huasamayostream, due to compressional tectonic movements. TheQuaternary deposits are characterized by conglomeratic andsandy facies (Sancho et al., 2008). The conglomerates aremainly clast-supported, with a sand matrix and a weak ce-mentation. The sandy facies are arranged in lenticular ge-ometries. The morphostratigraphic features suggest that eachfining-upward sequence begins with a debris flood, erodingthe base of the channel and depositing sediments as the en-ergy of the flow decreases. The entrenchment of the channelallows the sand lenses to fill it. Normal sedimentation phasesoccur between two surge events in the distal part of the fan(Sancho et al., 2008). The terraced arrangements of these de-posits, constituted by poorly consolidated materials, form anearly inexhaustible sediment reserve that can be transportedby flooding events.

Present-day fan activity is mainly due to debris floods,which develop as very rapid surging flow of water that gains

energy in the steep channel therefore transporting a greatamount of sediments.

3 Hydrogeological hazard and risk in the Tilcaraalluvial fan

3.1 Hydrogeomorphic processes in the Huasamayostream catchment

The high average slope of the basin generates a system withgreat potential energy, that can lead to intense alluvial phe-nomena. At the same time, the occurrence of slope failuresin the upper part of the Huasamayo catchment can causethe formation of small landslide dams and frequently hyper-concentrated flows are generated as a consequence of dambreaching. More often, it is the shear stress induced by thehigh velocities of the water itself that causes debris floods.In fact, from the basin the water is collected in a narrow,steep rock canyon called Garganta del Diablo; then the chan-nel cuts into old deposits, with sufficient energy to mobilizea great amount of sediment from the lateral scarps and fromthe channel bottom, entraining the debris and transporting ituntil the channel opens in the active fan. Here the flux startsto decelerate, depositing its load.

At the beginning of the eighties, in order to limit the areasubject to flooding to the most active depositional lobe and toprotect the urbanized portion of the Tilcara alluvial fan, de-flection walls have been constructed. Unfortunately, in 1984an high magnitude event broke the artificial levees, damag-ing more than 30 houses (Azarevich et al., 1999). More-over the flux-constraining approach, limiting the spreading ofthe flood, has caused massive aggradation during the years,therefore requiring the construction of higher levees, with in-creasing maintenance costs. These processes have generateda hanging channel, that in some places could be up to 5 mabove the houses nearby (Fig. 4). It is clear now that the risk,instead of being reduced, has been increased by the instal-lation of the deflection levees and that a different approachhas to be considered, to reduce the hazard and the risk for theTilcara inhabitants.

The documentation of past events is inadequate to performa sound hazard assessment. In particular, preferential pathsand inundated areas haven’t been mapped. Therefore, the de-posit map and an evaluation of the deposited volume are notavailable, together with pre- and post-event channel geome-tries. As a consequence, at this stage of the research, bothsemi-empirical scaling relationships (Griswold and Iverson,2008) and dynamic modeling (Hungr, 1995) cannot be ap-plied in order to obtain hazard maps on the alluvial fan.

Concerning the main valley, pushed westwards by the tele-scopic evolution of the alluvial fan, the Rio Grande wasforced to bend and therefore to erode his alluvial terraceson the right bank, where the National Route 9 stretches. Theroad suffered from damage due to a number of slope failures,



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Fig. 1. Geographical setting. The Quebrada de Humahuaca region in the Province of Jujuy.

Fig. 2. Structural sketch map of the area (after Sancho et al., 2008). Image source: Google Earth

Fig. 2. Structural sketch map of the area (after Sancho et al., 2008). Image source: Google Earth.

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Fig. 3. The Tilcara alluvial fan. The Huasamayo Stream catchment is visible in the background. The

location of Tilcara village with reference to the active depositional lobe is represented. Note the

location of the National Road, which can be involved in flooding events. Image source: Google

Earth

Fig. 3. The Tilcara alluvial fan. The Huasamayo Stream catchment is visible in the background. The location of Tilcara village withreference to the active depositional lobe is represented. Note the location of the National Road, which can be involved in flooding events.Image source: Google Earth.



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Fig. 4. At present, the thalweg level (A) is up to 5 meters higher than the ground level (B) in the

outskirts of Tilcara Village

Fig. 5. Land use in the Tilcara alluvial fan. Image source: Google Earth

Fig. 4. At present, the thalweg level(A) is up to 5 m higher than theground level(B) in the outskirts of Tilcara Village.

mainly caused by toe erosion; at that time engineering workswere designed and gabion walls were installed. Furthermore,due to the Huasamayo periodical floods, the Rio Grande waspartially dammed in 1998 and 1999, Routa 9, farming areasand a camping site were flooded during these events.

The damming of the Rio Grande has become a major haz-ard since the floodplain is narrowing due to the alluvial fandevelopment; downstream the confluence, the hamlet and itsancient and important farming activities of Maimara lay justa few meters above the thalweg therefore in a risky positionin the event of a major dam breach.

3.2 Socio-economical setting and vulnerability issues

In the Quebrada de Humahuaca, the hydro-geological pro-cesses are strictly related with the socio-economical set-ting. The population of the Department of Tilcara is of10 000 inhabitants. At present, the demography is chang-ing, ascampesinos are leaving the valley, also as a conse-quence of the high frequency of mass movements that dam-age parcels, structures and infrastructures. For example, theRuta Panamericana National n. 9, the main road connectingArgentina with Chile and Bolivia, is affected by landslidephenomena, and the connection and the transport of goodscan be frequently interrupted. Even the strategic railway Fer-rocaril General Belgrano was also abandoned in 1992, dueto increasing maintenance costs. At the same time, masstransport phenomena jeopardize cultivated fields, reducingthe land available for farming, but they can also partially ortotally dam the Rio Grande causing flooding. In this scenariothe farming activity are struggling to survive and sustenancefor the local population is becoming more expensive.

At the same time, the number of tourists visiting the Que-brada is noticeably increasing, thanks to the declaration ofUNESCO, that included the site in the World Cultural Her-

itage list. During summer season, due to tourism, the popula-tion can reach 20 000 units and the demand for new buildingplots to assign for hotels and resorts is very high. Moreover,people are coming from other parts of Argentina to work forthe touristic industry. As no urban planning is available in themain villages,barrios de emergencia, have been constructedin the floodplain or on alluvial fans to accommodate the newworkers. The hydrogeological risk in these areas is very highand, due to the chronic lack of funds, the countermeasureworks that have been constructed are generally underesti-mated and are also affecting the sediment transport rate ofthe basin. The engineering works tend to solve the prob-lems locally, most of the times losing the big picture: often,their long-term impact turned out to be damaging (Chayleand Wayne, 1995). For example, some deflection walls thathave been built are narrowing the tributary channels thus ex-posing risk for other boroughs, by increasing aggradation lo-cally. On the other side fields previously devoted to agricul-ture have been purchased by investors for building touristicfacilities, therefore impacting the same “cultural landscape”,as UNESCO calls it, the tourists are coming to visit (Repisoet al., 2007).

In Tilcara area, the urbanized portion of the alluvial-fan is1.3 km2 (Fig. 5) and for the local population, the chance tobuild on the active sector of the fan, which has an area of0.2 km2, is particularly appealing.

It is possible to roughly estimate that the new allotmentscould accommodate almost 1500 people if the local buildingtypology and density is maintained. The economic outcomefor the village could be extremely positive and this is an im-portant element to consider when evaluating different possi-ble mitigation strategies. The income generated by tourismis virtually the only source of money for financing the reme-diation works. A building compensation plan could be anexcellent resource for fund a radical risk mitigation for thewhole Tilcara village, since relocating is almost impossible.It is important to underline that the urbanization of the area isan almost inexorable process and therefore it is necessary todesign a compromise solution to meet the safety, economicand environmental requirements.

4 Risk mitigation

4.1 Risk mitigation issues in natural and culturalheritage sites

In general, in the management of natural and cultural her-itage sites, experts and planners should cooperate and sup-port public administrations in order to ensure long-term con-servation goals and the safety for both population and visi-tors, carrying out:

– the documentation of past and present hazard and riskconditions;



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Fig. 4. At present, the thalweg level (A) is up to 5 meters higher than the ground level (B) in the

outskirts of Tilcara Village

Fig. 5. Land use in the Tilcara alluvial fan. Image source: Google Earth

Fig. 5. Land use in the Tilcara alluvial fan. Image source:Google Earth.

– the evaluation of conservation issues in the context ofthe geomorphological processes acting in the area, in-cluding risk and impact implications;

– eventual design of countermeasures, both structural andnon-structural, minimising environmental and visualimpact;

– correct information about the natural hazards for popu-lation and visitors.

All the interventions should be designed respecting lowenvironmental impact issues. Suitable remedial measuresshould be chosen paying attention to visual intrusion inscenic areas or to visual impact on the natural conditions ofthe site (i.e. shapes, colours, dimensions, materials etc.).

In this specific case, some possible low-impact solutionscould employ gabions built using local materials, coveredby rock blocks simulating local structures and building prac-tices.

4.2 Possible risk mitigation actions in Tilcara

The assessment of debris flood hazard and hazard mappingcan be performed by means of a number of different meth-ods able to simulate the flow evolution and to predict theflow path and inundated areas on alluvial fans. In these pro-cedures, the documentation of past events is of paramountimportance, both for the application of empirical and semi-empirical equations and for the calibration of mathematicalmodels.

At present, no continuous records of rainfall and dischargeare available for the Huasamayo river basin, therefore it isnot possible to perform a general hydrological study of thebasin. Moreover, the aggradation and erosion phenomena

in the channel have not been mapped at a detailed scale af-ter major past events. Even though it is unquestionable thatthese studies are mandatory in order to design countermea-sure works (Marchi et al., 2010), though the analysis of land-forms and the description of the events given by the localpopulation it is possible to outline two possible mitigationscenarios for the village of Tilcara. The first scenario isconservative and it consists in the periodical removal of thematerial accumulating in the alluvial fan, and in the mainte-nance and reinforcement of natural and artificial levees (seeFig. 3). Even though in developed environments this solutioncould be the most cost-effective and less landscape impact-ing, in a socio-economical situation like in the Quebrada deHumahuaca, it would be extremely difficult to implement. Infact, since there is little municipal control over new construc-tions, the anthropic pressure on the fan would probably leadto a progressive narrowing of the active depositional lobe,with increasing hazard and risk for the population. Besides,since the public funds set aside for engineering measures arelimited, the risk is that the magnitude of the event for whichthey are designed could be inadequate to guarantee a suffi-cient level of safety; in this sense, the 1984 episode is sadlyillustrative of this tendency. Moreover, in case of two or moresubsequent events, the channel would not be effective.

The second approach is extremely radical, but it could bean opportunity to solve definitively the hazardous conditionsof Tilcara. An artificial channel could be designed on the leftbank of the Huasamayo River in order to divert the flood-waters laterally, preventing them from entering the sedimentladen deposits of the fan (Fig. 6), (L. Marchi, personal com-munication, 2011). At present this is just a hypothesis, asstudies on the project discharge and on the rheology of thewater and sediment mixture that can be transported fromthe upper part of the basin are lacking. Anyway, given thefact that areal-type measures are not feasible in arid environ-ments, the diversion of stream channel at its outlet from theupper catchment appears to be a viable option.

The channel entrance could be placed at 2745 m a.s.l., atthe exit of the narrowest part of the Garganta del Diablowhere the natural average thalweg slope is 25 %. Then,across the alluvial bank, the channel would enter at 2715 min a preexisting gully, and from there the water would flownaturally towards the Rio Grande. The channel would be550 m long with a slope of 5 % in the first track, which wouldbe 400 m long, and 6.5 % in the rest of its course. If weconsider a trapezoidal section for the channel with a 10 mbase and 30◦ scarps, we can roughly estimate a volume of500 000 m3 to be removed. To assure that the artificial chan-nel would not be affected by avulsion, the debris would beplaced at the outlet of the Garganta forming a protective damimmediately downstream of the artificial channel in order todirect the flow into the artificial channel itself. Given thatthe excavating material would be poorly consolidated sandand gravel, the digging and transport process could be per-formed simply by caterpillars, keeping the construction costs



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Fig. 6. Sketch of the proposed countermeasure. A. Location of the diversion channel. B. Cross

sections of the artificial channel and picture of the material to be excavated. Image source: Google

Earth

Fig. 6. Sketch of the proposed countermeasure.(A) Location of the diversion channel.(B) Cross sections of the artificial channel and pictureof the material to be excavated. Image source: Google Earth.

relatively low. Moreover, the financial costs of the engineer-ing work could be carried by the investors that would exploitthe Tilcara alluvial fan.

In the design of such a structure, some issues must betaken into account, besides the need of a low environmen-tal and visual impact. Firstly, in order to assure an adequatedegree of confidence the deflection dam must be sufficientlyhigh to avoid overtopping even in case of multiple, succes-sive events; in fact, sediment accumulated upslope of the em-bankment could decreases the total freeboard of the structure.For this reason the estimate height of the dam is 25 m abovethe thalweg. The feasibility of such an embankment has tobe analysed and designed in further studies, considering both

hydraulic and geotechnical properties of the soils. Moreover,the margin of safety has to be extremely high because the riskof overflow and/or the breaching of the protective dam couldin the end cause higher damage with respect to the same floodevent in the natural state.

To design these control works, an estimate of the maxi-mum flood event is required, assessing peak discharge andtotal flow, sediment yield and composition. The peak dis-charge is needed to design the channel geometry, i.e. thecross-section and the slope. Sediment characterization isnecessary to estimate the amount of deposited material to beexpected along the channel path and especially at the junc-tion of the natural stream with the artificial channel. In order



to design properly the countermeasure works, consideringthe 30◦ bending angle and even more the sudden change ofslope at the junction, a 3-D hydraulic and dynamic modelwill be mandatory to estimate the behavior of the flow andthe probability of obstruction and spill of the flood out of thechannel or the gully.

Characterization of groundwater flow is also needed. Infact, monitoring and modeling of groundwater tables andflows are mandatory during design and construction phasesof check dams, embankments and artificial levees. At thisstage, specific data are lacking. Nonetheless, in this arid re-gion, considering the regime and scarce quantity of precipi-tation, the nature of infiltration and pore water pressure buildup is strongly transient, considering the grain size and sort-ing of deposits, which are relatively permeable. Moreover, intributary valleys, no permanent inflow by rivers and glacier ispresent and only small perched aquifers are expected. This isalso testified by the vegetation cover, which is very sparse inthe whole catchment. Therefore, these issues can be consid-ered of minor importance both for the long- and short-termstability of the embankment. Anyway, during the design, se-vere conditions have to be accounted for, with reference topore pressure build up during storms and rapid drawdownafter a debris flood event.

Another important aspect that has to be considered is theeffect of the flow diversion further downstream, at the con-fluence with the Rio Grande (Fig. 6). In fact, the outletof the natural gully lays in front of the village of Maimaraon the other bank of Rio Grande river and in the furtherphase of studies it will be essential to assess the stability ofthe Maimara river bank, considering that erosion phenomenamay occur (Best, 1986). It might be necessary to build a pro-tection wall, even though the angle of confluence is rathersmall.

5 Conclusions

In developing countries, the effectiveness of mitigationworks has to be evaluated considering the socio-economiccontext, the environmental impact and the effects of climatechanges.

The area of Tilcara, in the Quebrada de Humahuaca, ischaracterized by huge volumes of poorly consolidated de-bris, that might be mobilized by mass wasting phenomena,namely landslides and debris floods. In particular, debrisfloods phenomena cause damage to infrastructures and agri-cultural activities and also to the cultural asset of the CaminoInca, which stretches along this valley and is included in theUNESCO World Cultural Heritage (UNESCO, 2003).

In this framework, a mitigation strategy aimed at avoidingpotential damages should be implemented in order to pre-serve the natural condition of this fragile environment andits population. Given this situation, two opposite needs haveto be guaranteed: the preservation of the peculiar landscape

as it is, without further human interventions, and the protec-tion of populations and their goods, in order to provide themfuture chances of development.

In arid and high mountain environments, low-impact arealmeasures such as terracing and revegetation are not feasi-ble. At the same time, the construction of embankments andditches to protect the built-up area cannot be the ultimate so-lution, especially where anthropic pressure is high and con-tinuous maintenance is needed. Hence, structural watershedmanagement has to be considered.

In Tilcara, a viable option is to divert the stream channelat its outlet from the upper catchment. This solution is im-pacting and relatively expensive, but the resulting environ-mental and landscape drawbacks can be accepted consider-ing that the protection of highly valuable sites is guaranteedand that the means of financing the countermeasure workscould be found in private investors. In order to minimize theexpected impacts, the proposed remedial measures shouldemploy available technologies and local hand, incorporatinglow environmental and visual impacts issues. This approachto the mitigation of mass wasting phenomena affecting theTilcara alluvial fan should bridge the gap between the needof preservation, the request of safety and the yearning fordevelopment of the local population, keeping in mind UN-ESCO recommendation.

Acknowledgements.The Authors wish to thank two anonymousreviewers and the Editor for their suggestions, which have helpedto improve the earlier version of the manuscript. We are thankfulto Lorenzo Marchi for the constructive discussions of debris floodprocesses and possible mitigation actions. The research was fundedby National Research Council of Italy, International short mobilityProgram.

Edited by: F. LuinoReviewed by: two anonymous referees

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http://dx.doi.org/10.5194/nhess-10-547-2010

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http://whc.unesco.org/en/list/1116

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Debris ﬂood hazard documentation and mitigation on the ...€¦ · hazards, mainly related to mass transport and erosion phe-nomena. In particular, debris ﬂoods are hydrological