Detection of Flood Prone Areas using Digital Elevation Models

1Universidad 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


Flooding21%


Firenze, November 1966


Flood and Landslides occurences in Italy - AVI project

(probability of a flood anda land slide over 50 years)

(http://sici.irpi.cnr.it/ ) (e.g., Reichenbach et al., 1998; Guzzetti et al., 2002)


MotivationThere 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?


Digital Elevation ModelsThe 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 )


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)


The Geomorphological Characteristics of the Areas Exposed and Non-Exposed to Flood InundationThe 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).


The Topographic Index

Stream line

Contour line

The concept of contributing area per unit contour length

The topographic index wasintroduced in the TOPMODEL withthe aim to mimic soil moisturepatterns based on:• local slope• basin area•Topographic convergence

)tan(/ln a

(Beven and Kirkby, 1979)D�Algorithm


Relationships among topographic index and flood prone areas Po River and Arno River

Probability distribution of theTopographic index conditional on theflood exposure

Po River

Arno RiverFlood Exposure

Topographic index

Topographic index


Arno River basin

Slope

Flow Accumulation MFD

Resize of DTM:20m – 40m60m – 80m

100m – 120m140m – 160m180m – 260m360m – 720m

Risk map PAICell size 20 m

Tan βATIm

ndln


Error functions

, from PAI areas Flooding

ITwith IT areas FloodingER msm 1001

The objective was to define a thresholdvalue which minimizes both errors in thedelineation of the flood inundation areas.Underestimation

.1002from PAI areas Flooding Non

IT ITareas with Flooding Non 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.


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)


Arno River sub-catchments

11 10

2 3

4

5

6

7

8

9

1

Sub -catchments


Results 1/2


Results 2/2


DEM Resolution affects morphological indexes

(Wood, 1996)


Scale Dependence of theModified Topographic Index

The spatial distribution of the topographicindex is inevitably linked to the cell-size ofthe adopted DEM (Zhang and Montgomery,1994). This dependence is investigatedcomparing the errors ER1 and ER2 obtainedusing the topographic index by Kirkby andits modified version (ITm) computed fromDEMs with different cell-size.The scale dependence analyses were carriedstarting from a digital elevation model withcell-size of 20m. From this model, DEMswith different resolution were obtainedthrough aggregations of the first andintermediate hydrological elaborations forthe construction of the topographic indexand modified topographic index


Scale Dependence and Associated Errors


Patameter n vs spatial scale

n


Patameter t vs n


VALIDATION


CONCLUSIONSThe 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.


Thanks for your attention…

Publication related to this research lineManfreda, 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 Hydrological Signature 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 allagamentosul 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.

Detection of Flood Prone Areas using Digital Elevation Models

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