Closure to “ Detection of Flood-Prone Areas Using Digital Elevation Models ” by Salvatore Manfreda, Margherita Di Leo, and Aurelia Sole

The discussers address a number of interesting questions on the original paper. The writers strongly believe that this kind of discussion represents an extraordinary opportunity to exchange ideas and comments on current research, helping both writers and discussers to better address their research tasks and to explain their thoughts. For these reasons, the writers would like to thank the discussers for their critical reading of the paper, which has raised interesting points that deserve to be explained in detail. Focusing on the comments, there are four questions that have been posed by the discussers. These will be addressed in the same order presented in the discussion.

How to cite: Manfreda, S. and Sole, A. Closure to “Detection of Flood-Prone Areas Using Digital Elevation Models” by Salvatore Manfreda, Margherita Di Leo, and Aurelia SoleJournal of Hydrologic Engineering, 18(3), 362-365, (doi: 10.1061/(ASCE)HE.1943-5584.0000693), 2013.  [pdf]

Detection of Flood-Prone Areas Using Digital Elevation Models

The availability of new technologies for the measurement of surface elevation has addressed the lack of high-resolution elevation data, which has led to an increase in the attraction of automated procedures based on digital elevation models (DEMs) for hydrological applications, including the delineation of floodplains. In particular, the exposure to flooding may be delineated quite well by adopting a modified topographic index (TIm) computed from a DEM. The comparison of TIm and flood inundation maps (obtained from hydraulic simulations) shows that the portion of a basin exposed to flood inundation is generally characterized by a TIm higher than a given threshold, τ (e.g., equal to 2.89 for DEMs with cell size of 20 m). This allows the development of a simple procedure for the identification of flood-prone areas that requires only two parameters for the calibration: the threshold τ and the exponent of TIm. Because the modified topographic index is sensitive to the spatial resolution of the DEM, the optimal scale of representation for the performance of the method is investigated. The procedure is tested on the Arno River Basin by using the existing documentation of flood inundations produced by the Arno River Basin Authority for calibration and validation. This approach is applied on 11 subcatchments with areas ranging from 489–6;929 km2 utilizing DEMs of different resolutions with cell sizes ranging from 20–720 m. Results show that the proposed procedure may help in the delineation of flood-prone areas, especially in basins with marked topography. The method is sensitive to the DEM resolution, but a cell size of ∼ 100 m is sufficient for good performance for the catchments investigated here. The procedure is also tested by adopting DEMs from different sources, such as the shuttle radar topography mission (SRTM) DEM, ASTER global DEM (GDEM), and national elevation data. This experiment highlights the reliability of the SRTM DEM for the delineation of flood-prone areas. A useful relationship between model parameters and the reference scale of the DEM was also obtained, providing a strategy for the application of this method in different contexts. The use of the modified topographic index should not be considered as an alternative to standard hydrological-hydraulic simulations for flood mapping, but it may represent a useful and rapid tool for a preliminary delineation of flooding areas in ungauged basins and in areas where expensive and time-consuming hydrological-hydraulic simulations are not affordable or economically convenient.

How to cite: Manfreda, S., M. Di Leo, A. Sole, Detection of Flood Prone Areas using Digital Elevation ModelsJournal of Hydrologic Engineering, 16(10), 781-790  (doi: 10.1061/(ASCE)HE.1943-5584.0000367), 2011.  [pdf]

Utilizzo di tecniche GIS per la delineazione di aree di inondazione nei tronchi incisi, alluvionati e incassati di un corso d’acqua: sviluppo di un metodo geomorfologico applicato al bacino del fiume Basento

L’espressione “rischio idraulico” indica il pericolo di inondazione da parte di corsi d’acqua naturali o artificiali e definisce il manifestarsi di eventi di inondazione che producono danni misurabili a persone o cose (Frescura, 2007). Il progressivo aumento di vittime e danni, causato tanto dal manifestarsi di eventi di inondazione quanto da un’antropizzazione spesso incompatibile con le dinamiche naturali del territorio, ha fatto sì che negli ultimi anni si registrasse un aumento dell’interesse, in ambito scientifico, tecnico e legislativo, attorno al problema della previsione e gestione del rischio idraulico (Plate, 2002). Le cause naturali che possono generare un’inondazione sono molteplici: un evento di pioggia con intensità tale da determinare portate in eccesso rispetto alle normali capacità di convogliamento di un fiume o di un canale di drenaggio; un accumulo di acqua su zone che hanno scarsa capacità di drenaggio e che normalmente non sono sommerse; un deflusso proveniente da aree urbane e suburbane poste a monte; le maree ed il moto ondoso. A queste cause naturali c’è da aggiungere l’effetto legato all’interferenza umana. La costruzione di strutture di controllo quali argini, traverse, dighe e sbarramenti ha cambiato le dinamiche proprie dei corsi d’acqua provocando modifiche sostanziali all’esposizione al rischio di inondazione del territorio. Eventuali collassi delle citate strutture antropiche possono provocare fenomeni di inondazione da un lato ed un ritorno dei corsi d’acqua nei propri percorsi naturali dall’altro.

How to cite: Manfreda, S., L. Giuzio, L. Giosa, B. Onorati, A. Sole, V.A. Copertino, Utilizzo di tecniche GIS per la delineazione di aree di inondazione nei tronchi incisi, alluvionati e incassati di un corso d’acqua: sviluppo di un metodo geomorfologico applicato al bacino del fiume Basento, in Catalogo di morfologie fluviali ed instabilità idrodinamiche nei corsi d’acqua di V.A. Copertino, G. Scavone, V. Telesca, Editoriale Bios, pp. 679-694, (ISBN: 978-88-6093-061-3), 2009. 

THE FLOOD EXPOSURE OF THE ITALIAN TERRITORY THROUGH A GEOMORPHOLOGIC APPROACH

The objetive of the present research was the evaluation of the flood exposure of the Italian territory. For this reason, the national territory has been subdivided in national, interregional and regional catchments according to the actual legislation regarding the soil defence (Law 183 of 1989). Such approach has been used in order to organize, within a Geographical Information System, the existing cartographic and technical documentation on flood exposure and risk produced by the local Agencies recently instituted. Using this documentation, we obtained a partical coverage of the Italian territory that has been extremely useful to explore new methods to define areas where the flood exposure is relatively high. A new method is presented for the definition of the areas exposed to floods that is based on the geomorphological characteristics of the territory. In particular, it was found that the flooding areas can be delineated fairly well trough a modified topographic index ITm = ln(an / tan(b)), where a is the drainage area per unit contour length, tan (b) is the local slope in the steepest descent direction and n is an exponent that was found to be close to 0.15-0.20. Such analyses have been carried out using a digital elevation model at 230 m and one at 90 m of resolution produced by NASA.

How to cite: Manfreda, S., A. Sole, e M. Fiorentino, Valutazione del pericolo di allagamento sul territorio nazionale mediante un approccio di tipo geomorfologicoL’Acqua, N.4, 43-54, 2007. [pdf]