This document discusses a methodology for delineating flood prone areas using digital elevation models. It examines using geomorphological characteristics derived from DEMs, like topographic wetness index, slope, and curvature, to identify areas likely to flood. It tests this approach on river basins in Italy. The results show it can accurately delineate flood inundation areas with some overestimation. The methodology works best with higher resolution DEMs and the optimal scale and parameters may vary between sites.

1. 1
Universidad Politecnica de Valencia,
6-10 June 2010
Salvatore Manfreda
Dipartimento di Ingegneria e Fisica dell’Ambiente,
Università degli Studi della Basilicata
Detection of Flood Prone Areas using Digital
Elevation Models
Universidad Politecnica de Valencia, 6-10 June 2010

2. 2
Universidad Politecnica de Valencia,
6-10 June 2010
Flooding
21%

3. 3
Universidad Politecnica de Valencia,
6-10 June 2010
Firenze, November 1966

4. 4
Universidad Politecnica de Valencia,
6-10 June 2010
Flood and Landslides occurences in Italy
- AVI project
(probability of a flood and
a land slide over 50 years)
(http://sici.irpi.cnr.it/ ) (e.g., Reichenbach et al., 1998; Guzzetti et al., 2002)

5. 5
Universidad Politecnica de Valencia,
6-10 June 2010
Motivation
There are essentially two scientific questions posed here:
i) Are there geomorphological signatures useful for the
delineation of flood prone areas?
ii) Is possible to define a simplified approach for the delineation
of flood prone areas?
iii) What is the optimal scale to describe such characteristics?

6. 6
Universidad Politecnica de Valencia,
6-10 June 2010
Digital Elevation Models
The increasing availability of digital terrain models has given a
strong impulse to the development of so called distributed and
DEM-based models.
Digital terrain model obtained through interferometric data
gathered by the space shuttle campaign by NASA with a cell-size
of 90m. (CGIAR-CSI: http://srtm.csi.cgiar.org/)
ASTER GDEM 30m available from June 2009
(http://asterweb.jpl.nasa.gov/gdem.asp )

7. 7
Universidad Politecnica de Valencia,
6-10 June 2010
Description of the study area and
morphological characteristics of the basin
A)flood inundation exposure map of
the Arno river basin where different
codes correspond to different flood
exposure levels (1=P1; 2=P2; 3=P3;
4=P4; 5=stagnant water, and 6=water
bodies);
B)Digital Elevation Model (DEM);
C)log(ad) where ad is drained area per
unit contour length;
D)Surface local slope;
E)Surface curvature;
F)TOPMODEL Topographic index
proposed by Kirkby (1975).
(A) (B)
(C) (D)
(E) (F)

8. 8
Universidad Politecnica de Valencia,
6-10 June 2010
The Geomorphological Characteristics of the
Areas Exposed and Non-Exposed to Flood
Inundation
The conditioned
probability
distributions of the
different
geomorphological
measurements at the 4
exposure levels (P1,
P2, P3, P4) show very
similar shapes and are
very different from the
one conditioned on P0
(areas non exposed to
flood inundation).

9. 9
Universidad Politecnica de Valencia,
6-10 June 2010
The Topographic Index
Stream line
Contour line
The concept of contributing area per
unit contour length
The topographic index was
introduced in the TOPMODEL with
the aim to mimic soil moisture
patterns based on:
• local slope
• basin area
•Topographic convergence
 )tan(/ln a
(Beven and Kirkby, 1979)
D Algorithm

10. 10
Universidad Politecnica de Valencia,
6-10 June 2010
Relationships among topographic index and
flood prone areas Po River and Arno River
Probability distribution of the
Topographic index conditional on the
flood exposure
Po River
Arno River
Flood Exposure
Topographic index
Topographic index

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Universidad Politecnica de Valencia,
6-10 June 2010
Arno River basin
Slope
Flow Accumulation MFD
Resize of DTM:
20m – 40m
60m – 80m
100m – 120m
140m – 160m
180m – 260m
360m – 720m
Risk map PAI
Cell size 20 m 









Tan β
A
TIm
n
d
ln

12. 12
Universidad Politecnica de Valencia,
6-10 June 2010
Error functions
,
from PAIareasFlooding
ITwith ITareasFlooding
ER msm
1001


The objective was to define a threshold
value which minimizes both errors in the
delineation of the flood inundation areas.
Underestimation
.1002
from PAIareasFloodingNon
ITITareas withFloodingNon
ER msm 

Overestimation
(n=0,2; ITms=3.9)
The error ER1 defines the percentage of error in
relation to the correct identification of flooding
areas while ER2 represents the error due to the
overestimation of the method. It should be remarked
that a reduction in the value ITms generally produces
an overestimation of the flood inundation area and
an increasing reduction in ER1.

13. 13
Universidad Politecnica de Valencia,
6-10 June 2010
Testing the New Approach on the Arno River
A) the areas exposed to flood inundation;
B) the map of areas with topographic
index superior to the threshold value of
8.40;
C) error distribution given by the
difference between the map A and B;
D) map of areas with modified
topographic index superior to the
threshold value of 3.90;
E) error distribution given by the
difference between the map A and D.
(Manfreda et al., 2008)

14. 14
Universidad Politecnica de Valencia,
6-10 June 2010
Arno River sub-catchments
11 10
2
3
4
5
6
7
8
9
1
Sub -catchments

15. 15
Universidad Politecnica de Valencia,
6-10 June 2010
Results 1/2

16. 16
Universidad Politecnica de Valencia,
6-10 June 2010
Results 2/2

17. 17
Universidad Politecnica de Valencia,
6-10 June 2010

18. 18
Universidad Politecnica de Valencia,
6-10 June 2010
DEM Resolution affects morphological indexes
(Wood, 1996)

19. 19
Universidad Politecnica de Valencia,
6-10 June 2010
Scale Dependence of the
Modified Topographic Index
The spatial distribution of the topographic
index is inevitably linked to the cell-size of
the adopted DEM (Zhang and Montgomery,
1994). This dependence is investigated
comparing the errors ER1 and ER2 obtained
using the topographic index by Kirkby and
its modified version (ITm) computed from
DEMs with different cell-size.
The scale dependence analyses were carried
starting from a digital elevation model with
cell-size of 20m. From this model, DEMs
with different resolution were obtained
through aggregations of the first and
intermediate hydrological elaborations for
the construction of the topographic index
and modified topographic index

20. 20
Universidad Politecnica de Valencia,
6-10 June 2010
Scale Dependence and Associated Errors

21. 21
Universidad Politecnica de Valencia,
6-10 June 2010
Patameter n vs spatial scale
n

22. 22
Universidad Politecnica de Valencia,
6-10 June 2010
Patameter t vs n

23. 23
Universidad Politecnica de Valencia,
6-10 June 2010
VALIDATION

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Universidad Politecnica de Valencia,
6-10 June 2010
CONCLUSIONS
The methodology proposed for the delineation of areas exposed to
flooding offers an estimation of the flooded areas with a very low
underestimation and a more relevant overestimation. This may be due to
terrain characteristics and to protective measures which cover conspicuous
portions of the territory otherwise subject to flooding.
The geomorphological method described here is a valid preliminary tool
in contexts where there is a lack of data for detailed hydrologic and
hydraulic simulations.
The methodology may benefit from the use of a higher resolution DEM.
The errors associated with different scales are deeply influenced by the
grid cell resolution and tend to decrease when using a more detailed
description of the topography.
The threshold values of ITms were found to vary with the reference scale.

25. 25
Universidad Politecnica de Valencia,
6-10 June 2010
Thanks for your attention…
Publication related to this research line
Manfreda, S., M. Di Leo, A. Sole, Detection of Flood Prone Areas using Digital
Elevation Models Journal of Hydrological Engineeering, Journal of Hydrologic
Engineering, Vol. 16, No. 10, September/October 2011, pp. 781-790
(10.1061/(ASCE)HE.1943-5584.0000367),2011.
Fiorentino, M., S. Manfreda, V. Iacobellis, Peak Runoff Contributing Area as
HydrologicalSignature of the Probability Distribution of Floods, Advances in Water
Resources, 30(10), 2123-2144, 2007.
Manfreda, S., A. Sole, e M. Fiorentino, Valutazione del pericolo di allagamento
sul territorio nazionale mediante un approccio di tipo geomorfologico, L'Acqua, n.
4, 43-54, 2007 (In Italian).
Manfreda, S., A. Sole, M. Fiorentino, Can the basin morphology alone provide
an insight on floodplain delineation?, on Flood Recovery Innovation and Response,
WITpress, 47-56, 2008.