Rend. Online Soc. Geol. It., Vol. 39 (2016), pp. 27-30, 3 figs. (doi: 10.3301/ROL.2016.39)

© Società Geologica Italiana, Roma 2016

ABSTRACT

In this study, two sediment source inventories produced in 1994 and

2006 have been compared and an analysis of sediment connectivity has

been carried out in a 5 km2 headwater catchment (Rio Cordon catchment,

Eastern Italian Alps). The 2006 sediment sources inventory was produced

through an integrated approach encompassing field survey and

interpretation of geomorphometric parameters (i.e., openness, surface

roughness and wetness index) computed on a LiDAR-derived 1-m

resolution DTM. The 2006 inventory aimed at updating a sediment source

dataset dating back to 1994, which had been implemented by means of

traditional techniques, i.e., field survey and interpretation of aerial

photographs. Moreover, a topography-based index of connectivity (Cavalli

et al., 2013) has been applied in order to evaluate the potential connection

of 2006 sediment source areas with regard to the channel network. The

analysis indicates that using a geomorphometric approach based on high-

resolution LiDAR DTM in combination with field survey helps obtaining

reliable and detailed sediment sources inventory and improving sediment

delivery assessment.

KEY WORDS: sediment source areas, sediment connectivity, LiDAR,

DTM, multitemporal analysis.

INTRODUCTION

In mountain catchments, the characterization and

quantification of sediment supply to the channel network is

essential for undertaking sediment management strategies. The

identification of type, extent and location of sediment sources

in a catchment and the analysis of the degree of linkage

between those sources and the channel network are key steps

for assessing sediment delivery.

The compilation of landslide inventories is a consolidated

approach to document location, extent and typology of

landslide phenomena at different scales and environment (e.g.,

Brardinoni et al., 2003; Mondini et al., 2014). Several methods

and techniques can be used to prepare landslide inventory

maps. A comprehensive review of existing methodologies can

be found in Guzzetti et al. (2012).

Recent advances in surveying technologies and better

availability of high resolution Digital Terrain Models (DTMs)

(Tarolli, 2014) offer the opportunity to perform accurate

studies aimed at the assessment of shallow landsliding and

erosion processes in mountain basins (Ardizzone et al., 2007;

McKean & Roering, 2004; Tarolli et al., 2012). The increasing

availability of high resolution DTMs, especially if derived via

LiDAR technology (Cavalli & Tarolli, 2011), extends the

applicability and potentialities of topography-based approaches

for the characterization of sediment dynamics and connectivity

(Cavalli et al., 2013; Trevisani et al., 2012).

In this work, two inventories of sediment source areas,

including not only landslides but also areas featuring active

surficial erosion, dating back to 1994 and 2006 and produced

using different techniques in a 5 km2 headwater catchment,

have been compared. A topography-based index of

connectivity (Cavalli et al., 2013) has been then applied in

order to evaluate the potential connection of 2006 sediment

source areas with the main channel network. The study is

aimed at showing the capability of high-resolution DTMs and

geomorphometry in the detailed and precise sediment sources

assessment and sediment dynamics characterization in

mountain catchments.

Multi-temporal analysis of sediment source areas and sediment

connectivity in the Rio Cordon catchment (Dolomites)

Marco Cavalli (a), Paolo Tarolli (

b), Giancarlo Dalla Fontana (

b) & Lorenzo Marchi (

a)

_____________________________________________________________________________________________________________________________________________________

(a) Research Institute for Geo-Hydrological Protection, National Research Council, Corso Stati Uniti 4, 35127, Padova, Italy. E-mail: marco.cavalli@irpi.cnr.it

(b) Department of Land, Environment, Agriculture and Forestry, University of Padova, Via dell'Università 16, 35020, Legnaro, Italy.

Document type: Short note.

Manuscript history: received 22 October 2015; accepted 07 January 2016; editorial responsibility and handling by Sebastiano Trevisani.

_____________________________________________________________________________________________________________________________________________________

Fig. 1 – The four information layers used for mapping sediment source

areas: a) orthophoto; b) shaded relief map; c) roughness index; d) slope/area

index. Higher values of roughness index can enhance deposits related to slope processes (white dashed circles in c). Lower values of slope/area

index depict areas prone to saturation as debris-flow channels (white arrows

in d).

M. CAVALLI ET AL. 28

STUDY AREA

The Rio Cordon catchment is located in the Dolomites

(Eastern Italian Alps). Catchment area is 5 km2, elevation

ranges between 1763 and 2748 m a.s.l. and the average slope is

27°. The geological setting of the basin is characterized by

dolomite rocks cropping out in the upper part and volcanic

conglomerates, sandstones and calcareous-marly rocks

dominating the middle and lower parts. Quaternary deposits

(moraines, scree deposits and landslide accumulations) are also

common. The area presents a mean annual rainfall of about

1100 mm; precipitation occurs mainly as snowfall from

November to April. Vegetation cover consists mainly of

mountain grassland (60%) and widespread shrubs (15%), while

forest stands (spruce and larch) occupy 6% of the total area in

the lower part of the catchment (Trevisani et al., 2010). In the

Rio Cordon catchment, a high-resolution Digital Terrain Model

(DTM) with 1-m resolution was generated using LiDAR data

acquired from a helicopter during snow free conditions in

October 2006 (Cavalli et al., 2008).

METHODS

SEDIMENT SOURCE INVENTORIES

The 2006 inventory

Field survey aimed at a preliminary sediment sources

identification and mapping were carried out in the late summer

2006, just few weeks before the LiDAR survey, with hand-held

GPS receiver, laser rangefinder, measuring tape and digital

photo camera. Active sediment sources identified are active

talus, surficial erosion areas, mainly due to intensive cattle

grazing, shallow landslides, eroded stream banks, and minor

debris flow channels and deposits.

Taking advantage of the enhanced representation of the

topographic surface given by the 1-m resolution DTM, some

geomorphometric indices were computed and used as an

integration of the field survey to produce a detailed sediment

source map.

Orthophotos and shaded relief map derived from LiDAR

DTM are well-known information layers used to produce

landslide inventory maps. In this study, three different

geomorphometric indices were used to further improve the

interpretative capability of LiDAR data helping in the

identification and mapping of larger and difficult access

sediment source areas, roughly surveyed in the field: openness,

roughness and slope-area indices (Fig. 1).

The positive openness index (Yokoyama et al., 2002),

expressing the degree of dominance or enclosure of a location

on an irregular surface, is an angular measure of surface relief

and horizontal distance and was used to enhance visualization

of LiDAR DTM. A roughness index (RI) (Cavalli et al., 2008;

Cavalli & Marchi, 2008) contributed to the recognition of

morphological features by emphasizing local elevation

variability that may not be apparent on the shaded relief map

(Fig. 1c). A third hydrological based index, the slope-area

index (SA), calculated as the ratio between local slope and the

specific catchment area, was applied in order to better mapping

debris-flow channels (fig. 1d). SA describes the spatial pattern

of saturated areas since it is algebraically related to the

topographic wetness index.

The 1994 inventory

An inventory map of sediment sources of the Rio Cordon

catchment, carried out in 1994, was also available. Sediment

sources were identified through field survey with measuring

tape and altimeter and aerial photographs (1:25,000 scale)

interpretation and then mapped at 1:10,000 scale.

The 1994 map was scanned, georeferenced, and finally

digitized in order to be compared in GIS environment with the

2006 inventory of sediment sources. The georeferenced map

was also loaded on the handheld GPS facilitating considerably

the field survey conducted in 2006, where each 1994 sediment

source area was surveyed and reclassified on the basis of the

typologies previously mentioned.

SEDIMENT CONNECTIVITY

The assessment of connectivity was carried out by means of

a topography-based index of sediment connectivity (IC)

(Cavalli et al., 2013). The index expresses the potential

connection between the different parts of the catchment and in

this work is aimed at evaluating the potential linkage between

sediment source areas of 2006 inventory and the Rio Cordon

channel network. IC was computed through SedInConnect 2.0

(Crema et al., 2015) using RI as the weighting factor and, thus,

the 1m DTM as the only input. Further details on the

methodology can be found in Cavalli et al. (2013).

RESULTS AND DISCUSSION

INVENTORIES COMPARISON

The two inventories of sediment source areas are shown in

Fig. 2. Sediment sources identified and mapped in 2006 in the

Rio Cordon catchment were 419, ranging in area from 7 to

47,000 m2, with a total area of about 650,000 m

2 (12.9 % of the

total catchment area). Landslides (165 areas) and surficial

erosion (98 areas) represent about the 62% of total sediment

source areas, whereas active talus covers 32.4% and debris-

flow related areas are quite limited (11.6%).

This map was compared with the inventory of 1994 in order

to evaluate changes occurred during the 12 years period. The

2006 inventory shows 127 more sediment sources (150,000

m2) than the 1994 inventory, mostly located in the upper and

middle parts of the catchment. Nineteen old sediment sources

were no more active and several other sediment sources

showed revegetation in the 2006 field survey. In general,

results suggest that discrepancies between 2006 and 1994

inventories are not due to a general increase of erosion

processes in the catchment. Besides the still intensive cattle

grazing which may have emphasized erosion in some areas and

the occurrence of a large landslide (2,000 m2) in 2001, the

MULTI-TEMPORAL ANALYSIS OF SEDIMENT SOURCES AND SEDIMENT CONNECTIVITY ANALYSIS

29

increase in 2006 inventory can be mainly ascribed to the

different methods used to produce the inventories.

CONNECTIVITY ANALYSIS

The standardized value (z-score) of IC for each individual

sediment source against the downstream distance from the

catchment outlet is presented in Fig. 3. This analysis highlights

a different pattern between the lower sector of the catchment

(downstream distance to the outlet from 0 to approximately

2500 m), where sediment sources on hillslopes are effectively

connected to the channels, and the middle and upper parts of

the study area. IC values negative or close to 0 at distances

longer than 2500 m suggest that the low gradient and the

presence of several depressions characterizing the middle part

of the catchment strongly affect sediment dynamic favoring

deposition and retaining of the sediment coming from the

active talus and other large sediment sources from the Rio

Cordon channel.

CONCLUSIONS

The main outcomes of this study can be summarized as

follows:

(i) Morphometric indices derived from LiDAR DTM

have proven useful in identifying and accurately

mapping larger and unreachable sediment source

areas;

(ii) Using morphometric indices in combination with field

survey lead to more reliable and detailed sediment

sources assessment than using traditional techniques,

especially in difficult terrain condition like in the

upper areas of headwater alpine catchments;

(iii) Connectivity index proved to be a valuable tool to

characterize sediment sources in terms of sediment

dynamics, highlighting the role of the morphology of

the middle part of the catchment in decoupling

upstream sediment sources from the Rio Cordon main

channel.

Fig. 2 – The two sediment source areas inventories compiled in the Rio Cordon catchment.

Fig. 3 – Standardized average value of IC for each individual sediment source plotted against the downstream distance from the outlet. Symbol size

is proportional to the area of the sediment source.

M. CAVALLI ET AL. 30

ACKNOWLEDGMENTS

Part of the research for this paper has been carried out in the frame

of a PhD at the University of Padova (PhD course Land, Environment,

Resources and Health), funded by CNR IRPI.

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