A multi-temporal inventory for constraining earthflow source-to-sink pathways in the Sillaro

River basin, Northern Apennines

Sharon Pittau1, Matteo Berti1, Giovanna Daniele2, Marco Pizziolo2, and Francesco Brardinoni1

1BiGeA, University of Bologna, Bologna, Italy (sharon.pittau2@unibo.it)2Servizio Geologico, Sismico e dei Suoli, Regione Emilia-Romagna, Bologna, Italy

EGU2020-10391EGU General Assembly 2020© Pittau et al. All rights reserved 1

Evaluation of landslide sediment supply to river systems is fundamental for modelling landscape evolution and for tackling applied issues on river sediment management.

While the study and monitoring of individual earthflows are widespread, limited quantitative information are available about the contribution of these processes to the sediment budget at the basin-scale.

Introduction

In the Tuscan-Emilian Apennines the predominance of clay-rich lithologies originates earthflows, that in these settings are the dominant agents of hillslope sediment transfer.

The compilation of earthflow inventories at the basin-scale is important to understand how they affect thelandscape evolution.

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Objectives

Through the compilation of a multi-temporal earthflow inventory (1954-2018), we aim to:

i. characterize earthflow source-to-sink sedimentary pathways, with special reference to sedimentdelivery to ephemeral and perennial streams;

ii. explore possible litho-topographic controls on earthflow size, frequency and recurrence;

iii. test how relevant information could complement the existing inventory of the Emilia-RomagnaRegion, for evaluating earthflow hazard and risk potential.

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The Sillaro Basin

- Study basin: located in the Northern Apennines, Italy

- Study basin area: ~ 138 km2

- Channel type: wandering (single & multi- thread channel reaches)

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- Study basin: located in the Northern Apennines, Italy

- Study basin area: ~ 138 km2

- Channel type: wandering (single & multi- thread channel reaches)

The Sillaro Basin

- Dominant geology: argillites & siltstones of the Ligurian domain

- Dominant landslide type: earthflows

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• Multi-temporal mapping of earthflows across 12 historical aerial photo sets from 1954 to 2018:

195419691976198819962000200620082011201420162018

MethodsInitiation zone

Transport zone

Total disturbed area

Deposition zone

• Mapping of the ‘Total disturbed area’:initiation + transport + deposition

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• We mapped all earthflows visible on sequential orthophoto mosaics. In particular, we mapped all movementsabove a minimum width of 4 meters. Below this threshold, it was not possible to consistently identifyearthflows and delineate the relevant perimeter, especially on the 1954 orthophotos, which are the coarserset.

Methods

• Implementation of a database with the following earthflow attributes:

i. Year of occurrenceii. Recurrenceiii. Areaiv. Geology

v. Land usevi. Sourcevii. Sink

1954 2006

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EARTHFLOW SOURCE-TO-SINK PATHWAYS

SOURCE SITES

Morphology at initiation:

- Gully headwall- Hillslope (open slope)

SINK SITES

Morphology at deposition:

- Gully channel (1st and 2nd order)- Connected tributary (> 2nd order)- Main channel- Hillslope- Anthropogenic barrier (eg, road)

9 combinations identified

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MULTI-TEMPORAL INVENTORY:

Number of earthflows = 1085

Total disturbed area = ~ 6 km2

Average area = 5690 m2

Median area = 2800 m2

Range area: min 200 m2

max 97610 m2

Results

→319 single movements

→766 recurring movements

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Multi-temporal aerial photo interpretation and mapping allow depicting:(i) the pace and extent of recurring earthflows at a given site(ii) the degree to which the flow lobe travels all the way down to the colluvial fan, or involves only part of the

tributary channel.

1988 2000 2014 2018

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TEMPORAL DISTRIBUTION

0

10

20

30

40

50

60

70

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

1954 1969 1976 1988 1996 2000 2006 2008 2011 2014 2016 2018

Eart

hfl

ow

nu

mb

er/y

ear

Eart

hfl

ow

area

(km

2)/

year

Year

Total disturbed area Number

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Open slopes

SOURCE TO SINK PATHWAYS

Number of mapped earthflows

62.9 % 37.1 %

Gully headwalls5

47

(5

0.4

%)

11

7 (

10

.8%

)

8 (

0.7

%)

11

(1

.0%

)

18

9 (

17

.4%

)

27

(2

.5%

)

3 (

0.3

%)

4 (

0.4

%)

17

9 (

16

.5 %

)

0 - 10

10 - 100

100 - 500

> 500

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7

Open slopes

13

14

Gully headwalls

17

9

70

50 14 47 20 10 3

14

6

0 - 10

10 - 50

50 - 100

> 100

RECURRENCE

Number of earthflow sites

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Comparison between the Sillaro and the Emilia-Romagna Region (RER) inventories

THE SILLARO INVENTORY

• Spatial scale: Sillaro river basin

• Temporal scale: from 1954 to 2018

• Contents: only active earthflows

• Year of occurrence: specified

• Update: mainly for landslides that caused damage, performed overwriting the old landslide mapping

• Update: for each of the 12 orthophotos available, performed mapping a new polygon

THE RER INVENTORY

• Spatial scale: Emilia-Romagna region

• Temporal scale: from the mid 90’s until today

• Contents: active and dormant landslides

• Year of occurrence: unspecified (indicated only for the last update of some landslides)

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RER inventory

Sillaro inventory

Active landslide

Dormant landslide

We aim to contrast and integrate the Sillaro and the Emilia-Romagna Region inventories.

Some advantages:• create a more complete inventory• integrate data on events that caused damage• check if there is a correlation between dormant and

recurrent earthflows

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Active landslide

Dormant landslide