ORI GIN AL PA PER

Application of geomorphologic knowledge for erosionhazard mapping

Jose Alexander Chavez Hernandez • Jiri Sebesta •

Lubomir Kopecky • Reynaldo Lopez Landaverde

Received: 30 January 2013 / Accepted: 4 November 2013 / Published online: 12 November 2013� Springer Science+Business Media Dordrecht 2013

Abstract An erosion hazard map was elaborated using geomorphologic and lithological

information; this was the base to characterize the erodibility of the territory. The aim of the

proposed methodology is to define the areas where more detailed studies are necessary

(e.g., to estimate rates of soil erosion, mitigation measurements, land use) to prevent future

problems. Field work and remote sensing data (study of historical aerial photographs and

satellite images) were used to understand the geomorphologic evolution and the current

processes taking place in an area; this information was used to group the units according to

its lithology, dynamic and slope inclination. The map was processed using the geo-

graphical information system and categorized in zones of very high, high, moderate, low

and null fluvial erosion hazards. The map covers the Metropolitan Area of San Salvador,

which is experiencing serious problems of mass wasting processes, collapse and settle-

ments of foundations. Most affected areas belong to the Tierra Blanca Joven tephras which

are unsaturated and cover most of the surface; nowadays, the urban projects and infra-

structure resting in this material are suffering from extensive damage. The geotechnical

J. A. Chavez HernandezCzech Technical University in Prague, Faculty of Civil Engineering, Department of Geotechnics,Thakurova 7, 166 29 Prague 6, Prague, Czech Republic

J. A. Chavez Hernandez (&)Oficina de Planificacion del Area Metropolitana de San Salvador (OPAMSS), Diagonal San Carlos 15Avenida Norte y 25 Calle Poniente Col. Layco, San Salvador, El Salvadore-mail: jose.alexander.chavez.hernandez@fsv.cvut.cz

J. SebestaCzech Geological Survey, Klarov 3, 118 21 Prague 1, Czech Republic

L. KopeckyDepartment of Mechanics, Faculty of Civil Engineering, Czech Technical University in Prague,Thakurova 7, 166 29 Prague 6, Czech Republic

R. L. LandaverdeFacultad de Ciencias Agronomicas, Universidad de El Salvador, Ciudad Universitaria,Final Av. Heroes y Martires del 30 de Julio, San Salvador, El Salvador

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Nat Hazards (2014) 71:1323–1354DOI 10.1007/s11069-013-0948-8

information on the tephras shows a decrease in strength and collapsible behavior when

saturated. Due to this, the use of Quickdraw tensiometers (suction) and TMS3 (soil

moisture content) is proposed for monitoring. The methodology of erosion hazard mapping

correlates well with mass wasting reported in the studied area, and for this reason, it could

be a good way to protect the natural resources and improve the land use.

Keywords Geomorphology � Erosion � Denudation � Mass wasting �Tierra Blanca Joven � Unsaturated

1 Introduction

Central America is located along the Ring of Fire where the subduction process causes

high volcanic activity, as well as intense earthquake activity. In addition, it is an area that is

located between the Pacific and Atlantic Oceans, where hurricanes constantly affect El

Salvador and other neighboring countries. The Metropolitan Area of San Salvador (MASS)

(Fig. 1) grew without clear urban planning, and problematic areas are occupied nowadays.

Most of the people with fragile economic resources live in risky areas (on the edge of or

inside the ravines, close to scarps, etc.) increasing their vulnerability to natural hazards.

The problematic areas subject to mass wasting, are located where thick deposits of

volcanic tephra called the Tierra Blanca Joven (TBJ) exist. these are unconsolidated

products of the last plinian eruption of the Ilopango Caldera (536 A.C., Chavez et al.

2012a). TBJ products are an intercalation of falls, surges, ash flows and pyroclastic flows.

Among the two main types of erosion (wind and fluvial) the last one has the greatest impact

and importance in the MASS, for this reason the present effort is focused on fluvial

erosion.

Since 2004 a geological and geomorphological mapping cooperation between the Czech

Geological Survey, the Planning Office of the Metropolitan Area of San Salvador (OP-

AMSS) and The Ministry of Environmental and Natural Resources took place. During this

period extensive field work and understanding of the geological hazards has been shared

and applied. Knowledge and experience concerning lithology and geomorphology evolu-

tion and processes acting in the territory serve as the base to build the proposed zoning

map.

According to Yang and Zhu (2003), nowadays, the methods for researching erosion

apply remote sensing (RS) and geographical information system (GIS), and they are

divided into visual interpretation, spectral analysis, parameter determination and man–

computer interactive interpretation. Some of the authors using these methodologies are Bet

and DeRose (1999); Renscheler and Harbor (2002); Le Bissonnais et al. (2001); Yang and

Zhu (2003); Zhou et al. (2004); Lee (2004); Kim et al. (2005); Behera et al. (2005);

Vrieling (2006); Manyatsi and Ntshangase (2008); Malik (2008); Vrieling et al. (2008);

Perroy et al. (2010); Nigel and Rughoopth (2010); Conforti et al. (2011); Leh et al. (2011);

Bouziz et al. (2011); Martinez–Fernandez and Martinez-Nunez (2011) and James et al.

(2012). Other methods to study gully erosion (Malik 2008) are the continuous monitoring

of headcuts, analysis of isotopes at the boundaries between sediments deposited as a result

of erosion and the dendrogeomorphologic method for estimating the rate of gully erosion

(using in addition to root exposures, corrosion scars on exposed roots or on aboveground

parts of fallen trees and the age of trees within a gully).

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Yang and Zhu (2003) explain that the visual interpretation method is an early method of

using remote sensing images, where the discrimination of objects is made by comparing

the image features such as shape, size, color, shade or texture. This method relies on the

expertise of the interpreter and the quality (e.g., scale, historical data) of the reviewed

information.

The spectral analysis method interprets digital remote sensing images and object

spectra. The results depend upon the spatial and temporal resolution of the used remote

sensing information, image processing and the appropriate identification of the spectra that

represent the erosion. If is executed by a GIS expert, then assistance of soil erosion experts

is needed to standardize the knowledge to be applied.

The parameter determination method (Yang and Zhu 2003) is obtained from remote

sensing images for soil erosion models to estimate soil loss. This method needs a solid data

base (preferably of the studied area), in addition to remote sensing images with a good

spatial and temporal resolution.

Man–computer interactive interpretation is a combination of data stored in a computer

and the experience of experts using GIS, and for this reason, it should be more reliable

because it is a combination of the above methods. Enhanced results are expected if the

conditions for the above methods are satisfied.

Visual interpretation like aerial photogrammetry (Bet and DeRose 1999; Vrieling 2006

and James et al. 2012) and satellite images assists to examine and quantify gully and

erosion remotely, but aerial photos allow a better differentiation of ravine types than

Fig. 1 Metropolitan Area of San Salvador (MASS) and location of Las Canas basin (OPAMSS 2010)

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satellite imagery. Also, the digital elevation models (DEMs) help in the study of landscape

change by identifying locations of geomorphic stability or change.

Vrieling (2006) recommended that for any erosion study using satellite data, it is

important to evaluate the scale required and the information that can be extracted from the

different types of satellite imagery, but usually empirical relations and field data will be

necessary. The timing of satellite images (Vrieling et al. 2008) are critical and are linked to

the erosive season and climate, vegetation types and land management.

The most widely used soil erosion model (Vrieling 2006) is the Universal Soil Loss

Equation (USLE) which is an empirical model of long-term averages of sheet and rill

erosion on short slopes (based on plot data collected in eastern USA). For this reason, the

empirical relationships of the USLE may not be valid in different environments; also the

erosion and deposition processes are not included in the USLE. To have a good model,

Renscheler and Harbor (2002) recommended looking for the user needs, abilities, data

availability and the procurement of good results.

The described methods have been successful in defining qualitatively or quantitatively

the areas affected by erosion, but they do not describe explicitly the reasons, evolution and

dynamic of the phenomena. Most of the methods are mainly GIS based, and field work is

not regularly done; instead, available information is used frequently.

The understanding of the dynamic processes that change the morphology and the

identification of problematic deposits are only some of the key issues to build a geological

hazard zone. The intention of the project is to introduce an erosion hazard zoning meth-

odology for improving the land use and urban development using visual interpretation, GIS

tools and field work, emphasizing in the zones where denudation processes could mostly

affect. The methodology aims to define the areas where more detailed studies are necessary

(e.g., to estimate rates of soil erosion, mitigation measurements, land use) to prevent future

problems.

Because it is based on geomorphology and lithological information, the zoning map

represents and explains the evolution of the territory. This information helps not only

outlining the erosion hazard but also in understanding the processes that could control it,

and the necessary considerations for the mitigation works (structural and non-structural)

that could better suit each situation.

2 Lithological information

Lexa et al. (2011) summarized the current knowledge of the geologic setting and litho-

stratigraphic units of the MASS. The principal ones there are: the Late Miocene–Pliocene

Balsamo Formation, the Late Pliocene–Early Pleistocene Cuscatlan Formation and the

Late Pleistocene–Holocene San Salvador Formation (Fig. 2). A general lithological map

for the MASS, combining geomorphological information is shown in Fig. 3.

The Balsamo Formation represents a basement of the MASS and is formed by the

remnants of andesite stratovolcanoes Panchimalco, Ilopango and Jayaque (Fig. 1). The

Balsamo Formation (Lexa et al. 2011) is composed of lava flows, coarse to fine epiclastic

volcanic breccias and breccias/conglomerates. It is is covered by younger volcanic rocks

of the Cuscatlan and San Salvador Formations. In areas not affected by retrograde erosion,

a layer of laterites with a thickness of a few meters is found on the top of the formation.

Rocks of the Balsamo Formation show different states of weathering and fracturing, but

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mostly have low erodibility. The laterites on top of the Balsamo Formation could be prone

to the effects of erosion, especially close to the scarps and upstream areas.

According to Lexa et al. (2011) the Cuscatlan Formation, covered by the tephras of the

San Salvador Formation, are comprised of silicic domes, tuffs, ignimbrites and volcanic

sediments related to Jayaque and Ilopango calderas, interstratified locally with basaltic and/or

andesitic lavas. The erodibility of these materials is low. Tuffs of this formation are con-

solidated (Fig. 4). Ignimbrites of the Cuscatlan Formation show different degrees of welding

(Lexa et al. 2011). Moderately welded facies dominate over unwelded and strongly welded

ones. The ignimbrites show a blocky or columnar jointing and form table-mountains and

Fig. 2 Lithostratigraphic units of the MASS (After Chavez et al. 2012a; Lexa et al. 2011). Ka*: Correlationwith marine tephras horizons. PL Plan de la Laguna, FB Boqueron Flow, TBJ Tierra Blanca Joven, TBTierra Blanca, VSS San Salvador Volcano

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cuestas. The lavas of the San Jacinto domes showing a different level of fracturing asociate

with marginal blocky breccias.

The San Salvador Formation (which is the youngest) includes products of basalt–

andesite stratovolcanoes (Lexa et al. 2011) associated with the evolution of the Central

Graben (San Salvador volcano) as well as interstratify silicic tephra/ignimbrites of the

Coatepeque and Ilopango calderas. The location of the tephras was controlled by the

direction of the winds, the erosion processes and explosive force during the eruption.

According to Chavez et al. (2012a) the more important strata there are: Tierra Blanca 4

(TB4), Tierra Blanca 3 (TB3), Tierra Blanca 2 (TB2) (between them exist paleosoils) and

Tierra Blanca Joven (TBJ) which are the youngest plinian to phreatoplinian products of the

Ilopango Caldera; also strata from the San Salvador stratovolcano like Apopa, G1, G2

including different lava flows, debris flows and tephras are included (Fig. 2).

Lexa et al. (2011) describe the most important tephras of San Salvador Formation. For

these layers, there is very poor geotechnical information available, as they are not usually

identified during the geotechnical studies:

The Plan de la Laguna pyroclastic ring surrounding a maar consists of pyroclastic

breccias, agglomerates, and scoria that belong to a phreatomagmatic event. Erodibility for

this material is low.

The Arce and Congo tephra units belonging to the Coatepeque caldera represent distal

facies of plinian fall-type deposits (Lexa et al. 2011). Their erodibility is high.

Fig. 3 Simplified lithological map for the MASS, combining geomorphologic information. TBJ TierraBlanca Joven, TB Tierra Blanca, SS San Salvador

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Products of the San Salvador volcano are mostly pumice/scoria horizons with inter-

mediate erodibility. Basaltic-andesitic lava flows of the San Salvador volcano (with null

erodibility) are interbedded with Ilopango and San Salvador tephras. Areas of badlands

with intense erosion have formed in the SE area (Fig. 3) of San Salvador volcano, where

thick brown color tephra deposits are present.

TB4 consists of moderately sorted coarse pumice fall deposits with thin layers of fine

ash at the bottom and top. The erodibility is intermediate for this layer.

TB3 (which erodibility is intermediate/high) consists of fine ash deposits with variable

quantities of accretionary lapilli and fine-to-coarse pyroclastic surge deposits.

The TB2 consists of poorly sorted dacite pumice fall deposits and intercalations of fine

pyroclastic surge/fall deposits. The erodibility is intermediate/high.

The description of the units of TBJ is on Fig. 5. A graphical representation of grain size

curves of TBJ fall ‘‘G’’ samples is displayed in Fig. 6, ranging from silty sands to sandy

silts. Similar results were obtained by Hernandez (2004) for most of the TBJ units.

Close to Ilopango caldera the thickness of TBJ reaches 60 m (Chavez et al. 2012a);

also in TBJ there are present areas of badlands (zones of intense erosion with high

density of drainage), where mass movements, underground erosion, sheet run off, rill and

gully erosion are intensive (Fig. 7). A current problem in the MASS is the urbanization of

the badlands, where landfills (usually for housing projects) is a normal procedure (Figs. 8

and 9). During past earthquakes, pronounced settlements of foundations were observed

in landfill areas of TBJ, leading to severe structural damages (Bommer et al. 1998). In

some cases TBJ was inadequately compacted, leading to additional problems of erosion

during heavy precipitation.

Chavez et al. (2012a, b, 2013) have dealt with the hazards and geotechnical knowledge

in the MASS, focusing mainly on the Tierra Blanca Joven tephras (TBJ) (unsaturated

material), indicating the decrease in strength and collapsibility of TBJ when the increase in

moisture reduces the suction and cementation.

Fig. 4 Consolidated Tierra Blanca tuffs of the Cuscatlan Formation in the northern part of the MASS. Theerosion is low in this area

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The normalized major element composition (weight) obtained by XRF spectrometry

(Communication of Spera 2012) of a G unit fall deposit of TBJ is 72 % SiO2, 15.3 %

Al2O3, 3.50 % Na2O, 2.58 % CaO, 2.3 % K2O, 1.99 % FeO, 0.95 % Fe2O3, 0.75 % Mg,

0.33 % TiO2 and 0.1 % MnO.

Samples of the units D, F and G of TBJ were studied using the environmental scanning

electron microscope (ESEM) as shown in Figs. 10 and 11. The results are consistent with

the comments by Rolo et al. (2004) since there is an important presence of voids (where

capillary water can be present) and among the grains there are no strong links, suggesting a

metastable structure. According to the results of the quantitative data of the ESEM, most of

the material is composed of silica-rich volcanic glass, feldspars and a small portion of

clays. Of the samples assessed with the ESEM (Units D, G and F), RTG analysis were

made in the clay fraction. The composition of the clay mineral fraction (Communication of

Fig. 5 Units of Tierra BlancaJoven (TBJ) tephras (AfterHernandez 2004)

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Lexa 2011) is identical for all samples, and the main alteration mineral is Al–Mg smectite

without significant participation of Fe. Minor alteration minerals represent mordenite

(zeolite) and poorly crystalline mineral of the kaolinite group; both of these minor minerals

Fig. 6 Area of grain size curves of TBJ ‘‘G’’ unit samples (build from data of Molina et al. (2009); Avalosand Castro (2010); and own results)

Fig. 7 Problems of erosion, collapse and mass movements connected with the presence of ‘‘Tierra BlancaJoven’’ (TBJ) tephras. a Planar erosion on the shores of Ilopango caldera; b recent gully near Las CanasRiver; c lateral erosion in Las Canas River; d failure of rainwater pipelines on filled stream

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are in the fraction bellow 2 micrometers. Beside alteration minerals, there is presence of

plagioclase and quartz and minor amount of illite that may represent admixture from

altered rocks in the volcanic explosion center.

The ESEM was used also to study (Romero and Simms 2008) the contribution of the

smectite in the behavior of TBJ in a sample of TBJ ‘‘G’’ unit; in order to do that, the

pressure inside the ESEM chamber was changed to provoke low and high moisture. As a

result, it was observed that swelling between grains took place only in some small sectors

of the sample (Fig. 12); in the other parts of the sample, the change was minimal meaning

that the swelling process is poor, which is consistent with the collapse behavior observed in

TBJ (Chavez et al. 2012a).

The small presence of clay minerals in TBJ (1–8 %, Communication of Henriquez

2013) agrees with the behavior of collapsible soils (Houston and Houston 1997; Ng and

Menzies 2007) which are soils composed by silts and fine sand-sized particles with small

amounts of clays and have low density, but are relatively stiff and strong in their natural

state. The cementation of the collapsible soils could consists of dried clay, salts, oxides,

interparticle forces in clayey soils and chemical precipitates, which may have been added

after deposition. The compacted soils can be collapsible also.

According to Toy et al. (2002), the soils with a medium texture are more erodible,

especially those with large amounts of silt (like TBJ, see Fig. 6), these soils tend to create

surface runoff, the soil particles are easily detached and the sediments are easily trans-

ported (TBJ has a low density, connected with pumice presence and the saturated specific

Fig. 8 1949 aerial photograph of upstream of Las Canas River and also is marked the urban developmentnowadays

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Fig. 9 Problems of settlements in badlands occupied by urban project (Cumbres de San Bartolo, Figs. 8, 13and 15). a Area affected by underground erosion, the sideway clearly proves problematic; b settlements thatalso damaged the street. Photos are from 2009, nowadays still there are settlements and damages in the areaafter repairs were made

Fig. 10 ‘‘D’’ unit a and ‘‘F’’ unit b of TBJ in environmental scanning electron microscope (ESEM)

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gravity of ‘‘G’’ unit is 2.41). Other factors affecting the erodibility include the soil

structure, which is the arrangement of soil aggregates and the presence of agents that

temporary bind these components.

3 Geomorphology information

Sebesta (2006) divided the Metropolitan Area of San Salvador (MASS) in structural–

tectonic, denudation as well as accumulation units and forms.

In the MASS, there is an important presence of tectonic scarps, which are combined

with the structural hillsides. These areas, due to its high tilt and high fracture (due to

tectonic movements), are susceptible to mass wasting processes. Among the main struc-

tural surfaces (Fig. 1) are the San Salvador stratovolcano, Ilopango caldera, Guazapa

Volcano, Planes de Renderos caldera, Loma Larga stratovolcano, Nejapa volcano, Jayaque

lavas and ignimbrites; also cinder cones, maars (an important presence in the northern

part), domes, shield volcanos in the north, lava flows and the young structural surfaces of

Ilopango (preserved plains, some affected by the erosion and exposing older Ilopango

deposits). There are scarps in the majority of the volcanoes along the MASS. Also tectonic

remnants (volcanic bodies affected by tectonic) and remnants of old volcanoes are present

(old stratovolcanoes of Panchimalco, Jayaque, Old Ilopango and Old San Salvador) (Lexa

et al. 2011). As the years have passed, the urban growth has increased in the foothills of the

volcanoes; including the more recently active (San Salvador stratovolcano and Ilopango

caldera).

Fig. 11 ‘‘G’’ unit samples of TBJ in environmental scanning electron microscope (ESEM)

Fig. 12 Change of moisture inside the environmental scanning electron microscope (ESEM) produceswelling in a small sector of TBJ, ‘‘G’’ unit; a low moisture and b high moisture content

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Most of the morphostructures are covered by Ilopango caldera and San Salvador volcano

tephras and some parts by tephras of Coatepeque caldera too, and for this reason, are affected

by intensive denudation processes, predominantly where Tierra Blanca Joven (TBJ) and other

pumices are present because of its young age and properties. The combination of mass

wasting processes increase the quantity of transported sediments in ravines and rivers.

The areas of underground erosion are located mostly adjacent to scarps, canyons,

streams and craters. Erosion (connected with anthropic activities) affects also inside the

urban area as the majority of the pipelines are in bad condition, resulting in the constant

failure and damages of roads and sectors of housing projects. Also the ravines and valleys

are now filled by urbanized areas, and sometimes, these old drainages serve as groundwater

flows (Fig. 8), where underground erosion is acting. All of these processes lead to the

creation of caves, piping and gullies, ranging in size from small to large, that in a given

period produce a collapse of the surface (Figs. 7, 9).

The combination of change in the base level, anthropological changes, climate and the

tectonic setting are the main external controls (Chavez et al. 2012a) of the denudation rates

in the proximal area of the Ilopango caldera. The upstream of Las Canas basin (Figs. 1,

13), a group of braided and meandering channels, is among the more problematic places in

the MASS, being a good example of the external controls of the erosion. The base level of

Las Canas basin is the artificial lake for generating electricity Cerron Grande (North of the

MASS), and during 2001, a period of drought (Gonzalez et al. 2004) changed the level

dramatically, perhaps this event started a sequence of adjustments and feedback over the

entire drainage system (Fig. 10 of Chavez et al. 2012a). The upstream of Cerron Grande

(Las Canas River) is an area covered by unconsolidated TBJ deposits with a thickness of

more than 60 m, and the middle basin and downstream are covered by consolidated tuffs of

Tierra Blanca (older deposits of Ilopango caldera of the Cuscatlan Formation). Floods,

change of level in the stream bed and change of course (meander migration) were observed

in the past years (Figs. 1, 13, 14, 15). In the period of 2008–2010, the level of the riverbed

decreased approximately 8 meters (point 2 in Fig. 14, 16).

The valleys upstream of Las Canas River are affected by localized faulting showing a

structural control in the drainage. The faults are young because they cut through TBJ. At

the moment, the activity and movement of these faults are unknown, but the movement

could cause channel migration and affect sedimentation patterns in the river.

The human activities (Figs. 8, 13, 14, 15, 16) in the river system such as deforestation,

urbanization (modifying the evapotranspiration), increased irrigation, broken waterlines

and agriculture practices can affect the water flow and sediment quantity, and also channel

engineering works like dams, modification of the river channel and flood controls can

disturb the equilibrium of the system. Some alluvial plains of the drainage system are

occupied nowadays, and the hydraulic area reduced by housing projects increasing the

denudation problem. Sand extraction in the bed stream of Las Canas River is an important

activity in the area (Fig. 17). Approximately 1,300,000 kg of sand is extracted daily from

Las Canas River (El Diario de Hoy, p. 59, 01/07/2011), and due to this activity, the course

of the river is diverted and lateral walls are carved.

3.1 Geomorphologic units

The structural–tectonic units and forms are as follows:

Volcanic scarp steep hillsides of volcanoes (crater, maar and caldera scarps); the rocks

are generally fractured and hydrothermally altered, and for this reason, they are affected by

fluvial erosion. Erosion is often accompanied by landslides, debris flows or creep.

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Badlands are formed if Tierra Blanca Joven (TBJ) or young San Salvador volcano tephras

are covering the volcanic scarps (Fig. 18a). According to the inclination of the unit, the

mass wasting processes can increase its relevance (e.g., \20� when the hazard is

moderate).

Fig. 13 View of the floodplain of Las Canas River; on the right the urban project Cumbres de San Bartolo(Figs. 8, 9, 15)

Fig. 14 Upstream of Las Canas River (lower-left corner of Fig. 8) where it is possible to observe changeswith the help of aerial photographs (1949 and 1984) and Google Earth (2007 and 2011)

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Fig. 15 Downstream of Fig. 14 (Fig. 13, and upper-right corner of Fig. 8) where is possible to observechanges with the help of aerial photographs (1949 and 1984) and Google Earth (2007 and 2011)

Fig. 16 Points 1 (16a) and 2 (16b) of Fig. 14, upstream of Las Canas River (2010), where vertical erosionis intense (approximately 8 m in 2 years). In August of 2012, the bridge in the photo 16a collapsed as aresult of erosion processes

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Tectonic scarp the materials of the tectonic scarps are fractured and with some degree of

weathering and frequently are covered by tephras, which means that it is a good envi-

ronment for fluvial erosion (Fig. 18b). Erosion increases if the inclination of the slope is

higher or if the weathering is more intense and also if covered by accumulations of

unconsolidated tephras. Erosion is often accompanied by landslides, debris flows or creep.

Structural surface of volcanic edifice hillsides of old stratovolcanoes, calderas, lava and

shield volcanoes; these surfaces have different inclinations and different permeability; if

the inclination is elevated, then it is more impacted by erosion but it depends upon the

covering tephras, being TBJ and young San Salvador volcano ashes more prone to be

affected. In the surface of the weathered shield volcanoes, generally, there is the presence

of a paved surface of lava blocks, which give protection against erosion.

Dome accumulations of viscous blocky lava with steep slopes nowadays covered by

Ilopango unconsolidated tephras. The tectonic modified its surface. Usually, the perme-

ability is high and lava blocks are very resistant. The circulation of groundwater is deep

due to high tectonic disintegration.

Maar low and wide crater produced by a phreatomagmatic eruption, it usually has a

steep crater slope and can be affected moderately by mass wasting problems. Most of the

maars are opened by erosion, and there are drainage outlets (except Plan de la Laguna

maar). This unit can have important groundwater recharge.

Cinder and/or scoria cones usually monogenetic volcanoes and usually associated with

stratovolcanoes or calderas. Cinder cones have low altitude and steep hillsides. These

cones show a moderate fluvial erosion.

Stratovolcano composite volcano with intercalated layers of lava, ashes, scoria, bombs,

as well as mud flow and debris flow deposits. The erosion (moderate) corresponds with

local geological conditions and inclination, but the upper part is always more affected by

mass wasting processes.

Diastrophic blocks relief of blocks chaotically organized, sagging slowly within the Central

Graben, even inside calderas (Fig. 19a). The majority is covered by recent volcanic products

and reworked materials. The blocks are formed by epiclastic volcanic breccias, ignimbrites or

lava rocks. If covered by TBJ or other tephra units the erosion could be intensive depending

upon inclination but it is mostly moderate. The permeability could be high.

Fig. 17 Extraction of sand in Las Canas River. The river is diverted from its course, and lateral walls arealso excavated

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Tectonic remnant of volcanic form volcanic edifices that still preserve some parts of its

original form. These volcanic edifices are impacted by tectonic, specially the domes, lava

volcanoes or epiclastic accumulations of the ancient volcanoes (Fig. 19b). Weathering or

hydrothermal altering is common, and mass wasting processes can be expected in the

drainage system and eventually debris flows can happen. Since the relief is ancient and

equilibrated, the erosion and mass movements are not intensive; but when the hillsides are

covered by San Salvador Formation tephras, then these processes can occur.

Younger structural surface usually plane surfaces with low inclination built by recent

polycyclic volcanic and exodynamic deposits, where mass wasting processes are very low.

This surface which is not yet eroded is distant from the headcut. The edges close to erosion

hillsides (especially badlands) are impacted by mass movement processes, underground

erosion and surface erosion.

Undistinguished lava flows areas with low relief forming the remnants of ancient vol-

canoes or the slopes of local volcanoes, commonly from shield volcanoes. The lavas are

weathered, and usually there is a presence of a paved surface by lava blocks (when

weathered material is washed away by erosion) that protect against the erosion (Fig. 20).

Fig. 18 Large elevation representative scarps in the MASS. a Volcanic–tectonic scarp of Ilopango calderathat is constantly affected by mass wasting processes; b tectonic scarp of Planes de Renderos caldera (LomaLarga volcano) affected from time to time by debris flows

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Structural surface smooth surface or plane surfaces, built by recent lavas, more resistant

ignimbrites (Fig. 4) or block earthflows lahars. This unit is not affected by erosion as the

low inclination or low erodibility.

The denudation forms and units are composed by exhumed surfaces, erosion hillsides,

badlands, V-shape streams, ravines, wide valleys, paleo-drainage, landslide scarps and

debris flows paths. The denudation processes occurring upstream of the basins produce

large quantity of sediments that are transported and deposited along the drainage system; if

enough sediments is produced, then aggradation and degradation in sectors of the bed

stream can be expected, causing excavation of the channel or horizontal movement

(meanders for example). Tectonics also controls the pattern and behavior of some drainage

system.

The denudation units and forms are:

Badlands surface with a high density of drainage system (Figs. 13, 21) and with

intensive mass wasting processes. There is a presence of badlands mostly in the fall and

Fig. 19 a Diastrophic blocks west from the MASS, it is possible to see the morphology with differentlevels; b The Guaycume Volcano, a tectonic remnant of volcanic form, is in front of the picture; in thebackground Guazapa Volcano (Fig. 1)

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pyroclastic flow deposits of TBJ, accumulation of reworked material and weathered sur-

face of old lavas, which are an unsaturated, unconsolidated and erodible materials. Also

there is a presence of badlands in areas close to San Salvador volcano, where important

layers of unconsolidated ashes (brown ashes) are present. In TBJ, liquefaction can be

expected also because it is capable to absorb 40–30 % of water (Lomnitz and Schulz 1966;

Bommer et al. 1998; Rolo et al. 2004; Chavez et al. 2012b). The slopes are affected by

collapse, flowage and mud flows during the rainy season and earthquakes.

Erosion hillside hills of erosion valleys (ravines, streams, canyons, rivers). The erosion

increases if the inclination is high (Fig. 22). The erosion hillsides covered by soft,

unconsolidated materials like weathered layers, TBJ or San Salvador volcano ashes are

more affected by erosion. Flowage, mud flows and debris flows can be expected in these

areas.

Weathered area these areas of deep weathering are usually remnants of the relief of

structural surfaces located near the hydrological division (Balsamo Formation), where

retrograde erosion is not acting yet. Remnants of weathering areas in the scarps are

especially affected by erosion.

Structural exhumed surface Originated where the denudation processes stopped; this

unit corresponds to planar to inclined surfaces but with good lithological conditions against

erosion (old ignimbrites, reworked block material or lavas). There are plains with different

heights that belong to ignimbrites and lavas located in deep drainage systems (Fig. 23).

This unit has moderate erosion hazard if the inclination is [15�.

In the MASS, a lot of the urban growth is now occupying areas of alluvial cones,

floodplains and mass movement, being affected constantly during the rainy season each

year. The accumulation units and forms are as follows:

Polygenetic fill without drainage these are closed places that suffer floods and depo-

sition, but are not affected by erosion.

Polygenetic polycyclic volcanic plain plane surfaces at the foothills of volcanoes,

formed with reworked and recent volcanic products that cover the diastrophic blocks

(Fig. 24). This unit has low erosion hazard. The edges with the erosion hillsides can be

impacted by mass movement processes, underground erosion and surface erosion.

Fig. 20 Paved surface by lava blocks in the northern part of the MASS. This surface protects againsterosion (after weathered material is washed away by erosion)

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Fig. 21 a Area of Badlands close to Ilopango caldera where mass wasting processes act constantly;b badlands close to Las Canas River. In these areas, a high density of drainage systems are developed

Fig. 22 Erosion hillside in the southwest part of the MASS. The erosion has wash away the soil and thebedrock can be seen

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Sanitary landfill anthropic area of poor compaction and surface changes with time. Low

erosion, biological processes and gas emanation can be expected.

Alluvial cone (fossil) plane surface and currently not affected by floods. The mass

movements and erosion are not important.

Fluvial terrace this unit has a restricted extension near the rivers (on the floodplain or

river bed), usually not affected by floods and has low erosion hazard.

Polygenetic fill with drainage these are closed places that suffer floods and deposition.

These are surfaces with variable lithological conditions (alluvial or fluvial gravel, but also

older deposits of Tierra Blanca) and with few flood problems but moderate erosion hazard.

Alluvial cone (active) this area of high erosion hazard is impacted by the constant

accumulation of deposits, and the riverbed can constantly change its position, starting an

erosion and deposition process in another place.

Floodplain the river channel in this area is constantly affected by erosion and deposition

processes (Fig. 25). Also lateral erosion is expected.

Mass movements accumulations composed by the deposits of important polygenetic

mass movements, landslides, debris flows, debris avalanche and rock fall. This unit has

high erosion hazard. If located close to a drainage system then it can be mobilized.

4 Methodology

4.1 Zoning map of fluvial erosion hazard

The zoning map of fluvial erosion in the MASS is the first attempt to locate the more

problematic areas and represent, in a general way, the level of denudation of the volcanic

morphology of the MASS. This information will help in the control and proper develop-

ment of the territory.

The methodology was composed by the study of existing literature such as the geo-

logical maps made by Bosse et al. (1978), Hradecky et al. (2004) and Lexa et al. (2011);

the geomorphologic map (made by the authors of this paper, Sebesta and Chavez) aerial

photographs (1949, 1984 and 2000); 1:25,000 topographic maps and the use of satellite

Fig. 23 Structural exhumed surface (south of the MASS) that belongs to ignimbrites in deep drainagesystems

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imagery (Quickbird of the year 2008 and Google Earth); giving prominence to the areas

where intense erosion was observed, this information helped during field work to identify

areas that needed more attention.

The base of the map is a proper interpretation of aerial photographs (using stereoscopy)

which served to define the geomorphologic surface units and forms, described previously.

It was also helpful to the 10-m elevation contours and the digital terrain model to display

the surface morphology (shadow map, obtained with the Hillshade command of ArcGIS

9.3) (Fig. 1). The historical aerial photographs allowed studying the territory before the

urban development and also to recognize the anthropic interventions, for example, the

filling of ravines or streams (Fig. 8). In addition to visit areas of interest, the field work

helped to verify and corroborate information obtained after the interpretation of aerial

photographs.

The map units were grouped according to resistance to erosion, dynamic, lithological

composition of the rock and soil, inclination of the slope, geomorphology, maturity of the

profile, equilibrium of the longitudinal profile of the river and distance to the local base

level of erosion (position in the basin upstream, middle basin or downstream). Another

aspect taken into account was the density and the depth of recent rills and gullies, which

represent the current intensity of active erosion processes.

Fluvial erosion is present on the entire region of the MASS, but with a different

intensity and therefore as a first approach, three levels of hazard were chosen for general

fluvial erosion (Fig. 26) from low, moderate to high. The more dangerous areas correspond

to the surface covered by the unconsolidated Tierra Blanca Joven (TBJ) tephras, where

areas of badlands are developed. The badlands are associated with gullies, being areas of

strong relief intensely dissected by ravines (a high density of them). Included in the

badlands are some regions covered by natural vegetation that currently are in equilibrium,

but if disturbed inadequately, then it is possible to expect denudation problems as men-

tioned before. Also in the category of high hazard, the scarps of the morphostructures were

included. The erosion is concentrated at the head of the ravines, and the walls are disrupted

by mass movements, as well as underground or subsurface erosion (Charlton 2007).

Fig. 24 Polygenetic polycyclic volcanic plain at the foothill of San Salvador volcano (Picacho)

1344 Nat Hazards (2014) 71:1323–1354

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The areas of tectonic scarps, erosion hillsides, the volcanic bodies and lavas which are

covered by tuffs have moderate hazard.

The plain areas of TBJ with less inclination have low hazard; included in this classi-

fication are the consolidated tuffs of Tierra Blanca (old deposits of Ilopango caldera of

Cuscatlan Formation) lavas, ignimbrites and epiclastic rocks.

In order to have an enhanced and better erosion hazard map (Fig. 27), a slope map using

the 10-m contour curves was combined with the geomorphologic units map (using the

same principles explained above). To obtain the slope map, the spatial analyst extension of

ArcGIS 9.3 was used.

To classify the slope units in degrees, the Slope/Degree tool of ArcGIS 9.3 was used in

the digital terrain model raster of 10-m elevation contours. The obtained raster was clas-

sified in different range (0�–15�, [15�, 15�–25�, [25�; 0�–20� and [20�) using the

Reclassify tool of ArcGIS 9.3. The slope classification of each geomorphologic unit and

form was made according to its intrinsic characteristics (not all were classified). The final

slope ranges for the classified units were chosen after making different verifications with

Fig. 25 Problematic floodplains in the MASS. a Acelhuate River where urban projects now are inside theoriginal floodplain and are frequently affected by floods; b 2009 photo, upstream of Las Canas River (Point3 of Fig. 14 and same area of Fig. 16) where it is possible to see the abandoned riverbed as intensivedenudation in the area progresses

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different ranges. The process ended when the results resembled the behavior observed in

the field and aerial photographs. According to Conforti et al. (2011), it has been observed

that the gully erosion propensity increases rapidly for hill slopes above 20� and on slopes

with a concave shape.

The chosen geomorphologic units to be classified by slope were as follows: erosion

hillside and the structural surface of volcano edifice (0�–15�; 15�–25� and [25�); the

structural exhumed surface (0�–15� and [15�); and tectonic and volcanic scarps (0�–20�y [ 20�).

After the slope raster was classified, it was transformed to shapefile, and using the

intersect tool of ArcGIS 9.3, the information of each polygon (shapefile) of the geomor-

phologic units and forms was crossed with the classified slope map (Fig. 28). A polygon

shapefile of geomorphologic units (where some of the units were classified in slope

degrees) was the final result.

According to slope, aerial photography interpretation, geomorphology, lithology,

satellite images and field work, the erosion hazard classification for zoning (Fig. 27) is

presented next:

Very high Badlands, tectonic scarps (with inclination [20�), volcanic scarp (with

inclination [20�; comprising the whole unit when badland is present), erosion hillside

(with inclination [25�), structural surface of volcanic edifice (with inclination [25�),

diastrophic blocks in caldera with badlands, weathered areas.

Fig. 26 General erosion hazard map of the MASS

1346 Nat Hazards (2014) 71:1323–1354

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High Erosion hillside (inclination between 15� and 25�), structural surface of volcanic

edifice (inclination between 15� and 25�), alluvial cone (active), floodplain, mass move-

ments accumulations.

Moderate Erosion hillside (with inclination\15�), structural surface of volcanic edifice

(with inclination \15�; also complete units of cinder and/or scoria cone, maar, strato-

volcano), tectonic remnants of volcanic form, structural exhumed surface (with inclination

[15�), polygenetic fill with drainage, diastrophic blocks in Graben, tectonic scarp (with

inclination \20�), volcanic scarp (with inclination \20�).

Low Alluvial cone (fossil), fluvial terrace, structural exhumed surface (with inclination

\15�), structural surface, structural surface of volcanic edifices (domes and shield vol-

canoes), sanitary landfill and undistinguished lava flows.

Null Polygenetic fill without drainage, polygenetic polycyclic volcanic plain, younger

structural surface.

The proposed methodology proved to have a good correlation between the problems of

mass wasting reported in the studied area (OPAMSS 2007). In spite of the working scale of

the maps used to build the erosion hazard map (1:25,000 topographic maps), more than

80 % of the areas affected by mass wasting were in zones of very high, high and moderate

erosion hazard, and for this reason, this methodology could be a good way to improve

urban development planning and land use.

Using the erosion hazard map can aid choosing the areas where more detailed studies

are needed. It is recommended that a program comprising government and private agencies

Fig. 27 Fluvial erosion hazard map of the MASS

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studies the erodibility, erositivity and erosion modeling in the problematic areas, and

proposes a policy of control and proper land use according to the results, which will be

revised from time to time according to their effectiveness. Knowledge of the main external

controls such as the fluctuations of the base level, anthropological changes, climate and

Fig. 28 Phases of construction of erosion hazard map; a geomorphologic map; b slope on degrees andc resulting erosion map

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tectonic is essential, since they can change the behavior and channel form in a drainage

system. According to Kim et al. (2005), some of the largest water reservoirs for electrical

generation in El Salvador have lost 13.5–48 % of their usable volume because of

Fig. 29 Almost vertical slopes of Tierra Blanca Joven (TBJ) in Las Canas River (linked to suction andcementation). Temperature changes, rains and river flow produce lateral, planar and rill erosion generatingslab failure and mass movements. a and b are photos of Las Canas River which is affected constantly bylateral erosion; c and d shows the problem in a smaller stream, tributary of Las Canas River

Fig. 30 Monitoring of moisture content, temperature and suction in a TBJ slope using TMS1 (two devices)and Quickdraw tensiometer (left side)

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sedimentation and are expected to become unusable in several decades if sedimentation

continues at the current state.

Erosion inventories could include direct measurements of erosion in the field in small

areas, as representative plots, slopes and small watersheds; also the erosion can be mea-

sured with laboratory tests, where it is possible to have more control of some parameters.

Some possible methods to measure erosion (Toy et al. 2002) are the change of weight, the

variation in the surface, changes in the channel sections and sediment recollection in

erosion plots or watersheds. Choosing the appropriate method will depend upon available

resources and working scale; going from cheaper studies such as the use of small parcels,

use of stakes or erosion pins, erosion bridges or frames, linear elevation measuring

instruments, monitoring of reference points, remote sensors and study of fluvial geomor-

phology; to the more expensive photogrammetric method, laser scanning of topography

and LIDAR. The measurements can be made before disturbing the surface to establish a

baseline of erosion conditions or after preservation, and to determine effectiveness of

Fig. 31 Results of monitoring of G unit fall using TMS device during a short rain the 2 of July of 2013

1350 Nat Hazards (2014) 71:1323–1354

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control practices (Toy et al. 2002). Channelization, building of dams and dredging in rivers

can move the problem to other areas downstream or upstream.

The collapse or failure mechanism of TBJ was first described by Hernandez (2004), but

it was not studied in more detail in spite of the problem that generates in ravines, rivers and

housing projects throughout the year (Figs. 7, 9, 16, 29). In the lateral walls of rivers

(especially in TBJ), the erosion (slab failure) is episodic, varying from year to year;

vegetation may develop in the material deposited during a mass movement (Toy et al.

2002) protecting the material to be removed by the flow of the river or stream. But a large

flow of an unusual event can remove the protection provided by the vegetation.

The device TMS1 of TOMST is actually being used (along with the Quickdraw ten-

siometer) (Fig. 30) for monitoring (Fig. 31) and understanding the moisture and temper-

ature changes (temperature levels: -10, 0, ?12 cm relative to soil surface when installed

vertically) that can affect the behavior of the TBJ and provoke mass wasting and degra-

dation of the hillsides increasing the deposits entering into the drainage systems (Chavez

et al. 2013). Soil moisture is (communication of M. Sanda 2012) based on time domain

transmission (TDT) principle for the full range of soil moisture (50–200-MHz operation

range in water to air, respectively) by the TMS (TOMST measuring system) device.

Currently, a calibration curve is made for TBJ deposits (Fig. 32) and is expected that the

use of a new version (TMS3) for monitoring will take place during 2013. Both probe

readings (TMS and tensiometer) are unaffected by different soil types, soil temperatures or

salinity conditions.

Fig. 32 Preliminary results using TMS1 in different units of TBJ of sectors of the Metropolitan Area of SanSalvador (MASS)

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5 Conclusions

For urban planners, civil engineers and governmental agencies, the recognition of the

lithology, geotechnical properties, hazards and geomorphologic processes on the territory

are essential. This will improve the urban planning, land use, environmental care, building

projects and agriculture, leading to a reduction in mass wasting problems and collapse.

In spite of the current trend in erosion assessments having a tendency to use remote

sensing and parameter determination, the proposed methodology aims for the gathering of

geomorphologic information in the field, understanding the dynamic of each area and using

stereoscopic historical aerial photos to understand past, present and future behavior; also

historical data of remote sensor information are very helpful. When comparing the pro-

posed erosion hazard map with recent mass wasting problems (Fig. 27), a good correlation

was observed, in spite of the working scale. For a more detailed map, it will be necessary to

improve the scale of the topographic, lithological and geomorphologic information. It is

recommended that an updated land-use map is also included in the analysis in order to

improve the results and understand future evolution in the studied area.

After identifying the areas affected by erosion (with the help of the erosion hazard map),

then detail studies of the erodibility, erositivity and erosion modeling can be proposed

using simple or more advanced methods according to importance and resources. The

results can help in building a policy of control and proper land use, which will be revised

from time to time according to their effectiveness.

The methodology presented in this paper is based mostly on visual interpretation

combining geomorphology, lithology and slope conditions, but also field work and

experience are important in proposing a qualitative classification (very high, high, mod-

erate, low and null). The areas covered by unsaturated Tierra Blanca Joven (TBJ) tephras

are intensively affected by erosion and drainage changes taking place during a short period

of time, disturbing the urban projects and causing environmental damages (loss of soil,

increasing erosion in all the drainage systems, disturbing the natural fluvial environmental

and decreasing the lifetime of the artificial lakes used for generating electrical power, as

large quantities of sediments are transported). For TBJ, some important aspects for

research are the sensitivity to moisture content changes, cementation, suction, weathering,

evapotranspiration and vibrations. The identification of the problematic layers prior to a

land-use change or building projects will aid in deciding the risk management strategy and

to avoid and be prepared for future problems. This has to include engineering geological

mapping (Chavez et al. 2012b) containing geomorphology, geology, seismicity, hydro-

geology, geological hazards and geotechnical information; also modeling of unsaturated

soils (Chavez et al. 2013) and rock mechanics is necessary.

The monitoring of problematic layers using low-cost and practical devices like TMS or

Quickdraw tensiometer will help in understanding the evolution and changes, of suction

and water content, experimented in different places of the MASS. Significant information

such as the relation between rain, permeability, evapotranspiration, weathering processes

and critical soil moisture content of TBJ is expected.

Acknowledgments The work had been funded in the framework of cooperation between the CzechEmbassy in Costa Rica, Oficina de Planificacion del Area Metropolitana de San Salvador (OPAMSS), CzechGeological Survey, Czech Technical University in Prague (CVUT), Ministerio de Medio Ambiente yRecursos Naturales de El Salvador (MARN) and Universidad de El Salvador (UES). Authors acknowledgesupport of the Czech Geological Survey, Oficina de Planificacion del Area Metropolitana de San Salvador(OPAMSS), Agronomy Faculty and Civil Engineer School of Universidad de El Salvador and the Geo-logical Survey of the Ministerio de Medio Ambiente y Recursos Naturales de El Salvador (MARN). Special

1352 Nat Hazards (2014) 71:1323–1354

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thanks go to Cecy, Andres and Daniel Chavez for assistance during this project. We are grateful to thereviewers and editors whose remarks improved the quality of the paper.

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