# A GIS Tool for Mapping Dam-Break Flood Hazards in Italy

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## Abstract

**:**

## 1. Introduction

## 2. The methodological Framework

#### 2.1. Dam-Break Outflow Hydrograph Calculator

#### 2.2. Flood Propagation Calculator

#### 2.3. DEM-Based Dam-break Hazard Mapping Calculator

#### 2.4. The Implementation of the Tool: Libraries and Components

#### 2.5. Validation

## 3. Case Studies

#### 3.1. The San Giuliano Dam test case

#### 3.1.1. 1D Hydrodynamic Study

^{3}/s. The hydrograph that is shown in Figure 7 was used as the upstream boundary condition. A constant hydrometric level in the river section that is closest to the sea was assumed as the downstream boundary condition. The cross-section was extracted using the available topographic map at either 1:5000 or 1:10000. In a few cases, elevation information, specifically 1:25000 topographic maps, from the Italian Military Geographical Institute (IGM), was used. The Manning roughness coefficient for the river channel and floodplain areas was assigned a value of 0.028 m

^{-1/3}and it was assumed to be constant across all areas. This number corresponds to a non-maintained area with dense vegetation. Table 1 summarizes the setup of the modelling experiments.

#### 3.1.2. 2D Hydrodynamic Modelling

#### 3.2. The Gleno Dam-Break Case Study

^{2}in Valle di Scalve (174 km

^{2}), the main tributary valley of Valle Camonica (1460 km

^{2}) in Lombardy, Italy (as showed in Figure 9). In the early morning of December 1, 1923, about 40 days after the first complete reservoir filling, an 80 m long breach opened in the central portion of the Gleno Dam. The collapse was triggered by water seepage at the interface between the masonry base and the overlying structure. The effects of the flood propagation along the downstream river were catastrophic, resulting in 356 deaths and the destruction of three villages and five power stations, as well as a large number of isolated buildings and factories [36]. As a consequence of this catastrophic event, the multiple-arch design, which was in itself not responsible for the accident, was almost completely abandoned in Italy. Pilotti et al. [36] collected and analyzed detailed historical documentation compiled from several archives, in order to reconstruct the event. Furthermore, the authors simulated the event while using a numerical model. Pilotti et al. [36] described the dynamics of the reservoir emptying through a two-dimensional (2D) shallow water simulation and computed a one-dimensional (1D) simulation of the dam-break wave propagation along the downstream valley. All of the information about the model's input data, initial and boundary conditions, and the results, together with the part of the flood extent map that they reconstructed while using the collected historical information, have been published on the following website: http://www.ing.unibs.it/~idraulica/gleno_testcase.htm. Therefore, this information was used for validation purposes for the proposed GIS tool.

#### 3.3. Example Application of Structural Failure for the 250 Existing Italian Masonry Arch and Gravity Dams

## 4. Results

#### 4.1. The Application on the San Giuliano Dam Case Study

^{-1/3}s for this application confirmed this idea. This value of the Manning coefficient, with respect to smaller values (such as the one used by Sole et al. [37]), results in a longer propagation time and a higher water depth, therefore resulting in a larger flooded area.

^{-1/3}s is considerable for dam-break flood events. However, the extremely low value of the false positive rate, or false alarm rate, shows that the GIS tool’s overestimation of the rest of the valley, excluding the steep canyon, is quite low.

#### 4.2. Validation on the 1923 Gleno Dam-Break

#### 4.3. Computational Performance on Large Number of Dam-Break Cases

## 5. Discussion and Conclusion

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**Example of the cross-sectional footprint delineation for an area of the valley downstream from the San Giuliano dam.

**Figure 3.**Example of the flood extent maps for an area of the valley downstream from the San Giuliano dam.

**Figure 4.**Example of the flood water depth maps for an area of the valley downstream from the San Giuliano dam.

**Figure 5.**Schematic representation of the contingency table adopted to test the performance of the Geographic Information System (GIS)-tool in terms of dam-break flood extent map.

**Figure 7.**San Giuliano dam-break outflow hydrograph according to the methods of Sole et al. (1997). Source: [37].

**Figure 8.**San Giuliano dam-break outflow hydrograph according to the approach that used the 2D propagation model of [40].

**Figure 11.**(

**a**) Comparison of the flooded area estimated by the proposed GIS tool for the San Giuliano potential dam-break with the flooded area calculated by Sole et al., 1997 [33]; (

**b**) Comparison of the flooded area estimated by the proposed GIS tool for the San Giuliano potential dam-break with the flooded area calculated by the 2D hydrodynamic model proposed by Cantisani et al., 2013 [40].

**Figure 12.**Comparison of the flooded area estimated by the proposed GIS tool for the first 2.3 km of the Gleno Dam-break with the flooded area (i) simulated by the hydrodynamic model and (ii) obtained from historical reconstruction both by Pilotti at al. [36].

**Figure 13.**GIS tool example application of instantaneous dam-break due to structural failure for the 250 existing Italian mansory arch and gravity dams: (

**a**) an example of the flooded areas calculated for the potential failure of 250 dams; (

**b**) an example of the water depth map for a dam area in Northern Italy; (

**c**) an example of the wave arrival time map of a dam in Northern Italy; and, (

**d**) an example of a loss of life map for an area of a dam in Southern Italy.

Method | Spatial Resolution | Initial Conditions | Upstream Boundary Condition | Downstream Boundary Condition | Manning's Coefficient |
---|---|---|---|---|---|

The proposed GIS tool | 0.5 cross section per km | Dry | Dam-break outflow hydrograph of fig. 1 | None | 0.06 m^{-1/3}s |

Sole et al. [37] | 1 cross section per km | Discharge of 1 m^{3}/s | Dam-break outflow hydrograph of fig. 7 | Sea level | 0.028 m^{-1/3}s |

2D hydrodynamic modelling | 10 m DEM | Dry | Dam-break outflow hydrograph of fig. 8 | Critical water depth | Spatial variation |

**Table 2.**Results of the GIS tool performance compared, respectively, with the 2D propagation model approach and with respect to Sole et al., 1997 in terms of a flood extent map for the San Giuliano case study: false positive rate r

_{fp}, true positive rate r

_{tp}, false negative rate r

_{fn}, and accuracy, sensitivity, and specificity.

r_{fp} | r_{tp} | Accuracy | Sensitivity | Specificity | |
---|---|---|---|---|---|

2D propagation model [40] | 10.1 | 78.5 | 86.5 | 75.5 | 89.9 |

Sole et al. [37] | 19.9 | 59.7 | 74.2 | 57.7 | 80.1 |

r_{fp} | r_{tp} | Accuracy | Sensitivity | Specificity | |
---|---|---|---|---|---|

Historical reconstruction of flooded area | 2.2 | 78.7 | 94.8 | 78.7 | 97.9 |

Flooded area simulated by hydrodynamic model | 1.5 | 86.9 | 96.8 | 86.9 | 98.5 |

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Albano, R.; Mancusi, L.; Adamowski, J.; Cantisani, A.; Sole, A.
A GIS Tool for Mapping Dam-Break Flood Hazards in Italy. *ISPRS Int. J. Geo-Inf.* **2019**, *8*, 250.
https://doi.org/10.3390/ijgi8060250

**AMA Style**

Albano R, Mancusi L, Adamowski J, Cantisani A, Sole A.
A GIS Tool for Mapping Dam-Break Flood Hazards in Italy. *ISPRS International Journal of Geo-Information*. 2019; 8(6):250.
https://doi.org/10.3390/ijgi8060250

**Chicago/Turabian Style**

Albano, Raffaele, Leonardo Mancusi, Jan Adamowski, Andrea Cantisani, and Aurelia Sole.
2019. "A GIS Tool for Mapping Dam-Break Flood Hazards in Italy" *ISPRS International Journal of Geo-Information* 8, no. 6: 250.
https://doi.org/10.3390/ijgi8060250