Simulation of 2008 Pellice River flood

Transcription

1 Simulation of 2008 Pellice River flood 1 L. Natale 1, G.Petaccia 1 Department of Hydraulic and Environmental Engineering, University of Pavia, Via Ferrata 1, Pavia, Italy Abstract petaccia@unipv.it This work is focused on the simulation of the most recent flood event on Pellice River, in Italy, which caused inundations and landslides. Rainfall measurements of the event were available, together with a Digital Elevation Model and orto photographs of the whole reach. A 25km long reach was considered, from Fenile to Po confluence. The one-dimensional topography was generated using ORSADEM, an interactive tool entirely developed by University of Pavia researchers. The software allows to automatically extract river-bed cross sections from DEM and to perform one-dimensional hydraulic calculations, integrating De Saint Venant equations with three different numerical schemes. Roughness parameters can be assigned in a very simple way and the flooded area is automatically mapped. The flood waves flowing from upstream watershed into the river reach of interest were reconstructed by conceptual hydrologic models. The results were compared and confirmed by the limited data recorded during the simulated flood event. Introduction The increased socioeconomic relevance of flood waves studies and their shift towards integrated flood risk management concepts have brought to the development of complex methodologies for river flood simulation. The most popular approaches to fluvial hydraulics modeling are based on the full De Saint- Venant [1] equations which remain the most powerful and accurate model largely used by many engineering packages [2], [3], [4], [5]. Nowadays, the airborne survey techniques (LiDar) allow us to produce a detailed digital elevation model (DEM); the Geographic Information Systems (GIS) tools make easy the application of DEM to 1D hydraulic models and the mapping of inundated areas [6], [7], [8]. Some codes for professional applications, use ArcGis extensions to process the geospatial data required for the hydraulic calculations [9], [10]; instead, ORSADEM, used in the study presented here, include the GIS tool. ORSADEM is an interactive computer code developed by researchers of Pavia University, which extracts river-bed cross sections from DEM, performs unsteady flow hydraulic calculations and automatically maps flood prone areas. The modeling core of ORSADEM integrates De Saint Venant equations written in conservative form by means of the following up-to-date numerical solvers: 1. SANA: a Lax Friedrichs type solver written on a staggered grid [11], 2. ROE: an upwind solver with centered and upwind treatment of source terms [12], 3. HLL: a Riemann solver with centered and lateralized source term treatment [13].

2 These numerical schemes were tested by simulating laboratory experiences [14] as well as real world situations [15]. These schemes handle with difficult transcritical conditions, typical of mountainous river. ORSADEM maps automatically and with high precision, flood prone areas. Moreover it displays water depth and discharge hydrographs as well as the position of assigned water level during the whole simulation. In this paper HLL scheme was used; HLL is first order accurate in space and time, written for a Finite Volume formulation, with a centered treatment of source terms [16], [17]. Case Study During the last decades many important floods caused major threat to human life and infrastructures in North Western Italy. In Pellice River floods occurred in November 1994, June 2000, October 2000 and May The Pellice River, that flows in Piedmont Region, is Po River tributary (Figure 1); its watershed, at the Po confluence, has area of 947 km 2, average elevation of 1498 m a.s.l., average slope of 44%, and concentration time of 7.19 hours. Chisone River is its major tributary; Chisone basin, closed at the confluence with Pellice River, has extension of 604 km 2, average elevation of 1666 m a.s.l. and concentration time of 6.5 hours. Figure 1: Location of Pellice and Chisone river basins and its hydrological stations Three hydrologic stations are located in Pellice basin, see Figure 1, whose elevations are shown in table 1. Table 1: Elevation of hydrological stations Station Elevation ( m a.s.l.) San Martino Chisone 410 Villafranca Pellice 260 Luserna San Giovanni 475 The reach simulated in this paper is 25 km long, from Fenile to the confluence with Po River, after one half of the reach the confluence with Chisone river is located. The riverbed is described by 205 cross sections, spaced approximately 120 m, extracted from the DEM by means of ORSADEM tool. The Manning roughness coefficients were defined performing preliminary simulations in the

3 different areas of the reach according to land-use maps and orto photographs. A particular attention was paid to estimate the hydraulic resistance of vegetated areas [18]. The roughness was given separately for the main channel and the flood plains, and was evaluated using Cowen method to take into account the reach resistance [19]. The flood of May, 28 th to 30 th, 2008 was studied. The flood was generated by two different storms occurred in a very short time interval. These storms, coming from South, gave its contribution mainly in the Pellice basin, and moved towards Chisone basin with lower intensity. Hourly rainfalls, recorded in the stations of table 1, were used to evaluate the average rainfall hyetograph on the Pellice and Chisone Basins. From these data, together with some non continuous recordings of discharges at Fenile station, a conceptual hydrological model was used to evaluate the entire discharge hydrograph of Pellice river at Fenile Station. Same procedure was performed for Chisone basin, at San Martino Station. The discharge hydrograph of Chisone river at the confluence with Pellice river was then evaluated, for hydrologic similitude, extending the rainfall runoff model of Chisone basin, calibrated at S. Martino station, to represent the flood wave at the confluence. The flood waves evaluated in this way are shown in Figure 2. Figure 2: Discharge hydrographs for Pellice and Chisone Since the flood in the final part of Pellice River is due to the superimposition of the two waves, of the main watercourse and of Chisone river, the sequence of the two flood wave was studied. The simulation assumed initial constant flow of 100 m 3 /s at Fenile and 100 m 3 /s coming from Chisone confluence. As downstream boundary condition the water elevation in Po river was imposed at the confluence between Pellice River and Po River. Results and comments

4 In Figure 3, Pellice River maximum water elevation profile is shown. Figure 3: Maximum water elevations for Pellice River As an example of ORSADEM results, the automatic mapping of a part of the flooded area downstream the Chisone River confluence is represented in Figure 4. Most of the flood is confined in the main channel, which is well defined. ORSADEM interface highlights areas pertaining to abandoned channels that were inundated during the flood. Figure 4: Example of flooded area mapping Figure 5 compares measured and simulated water levels at Villafranca Piemonte station

5 located 7 km upstream the Po confluence, where the Po backwater effect is negligible. The comparison between simulated and observed data shows a good agreement in the maximum predictions of water levels and discharges, as well as in the flood wave shape. To evaluate the peak routing of the studied reach both Pellice flood wave and Chisone flood wave were propagated separately. As a result the Pellice maximum discharge is reduced of 10%, while the Chisone maximum discharge is reduced only of 2%. Figure 5: Comparison between simulated and observed water levels Conclusions The 2008 flood wave on Pellice River was simulated from hydrological data. The river channel geometry was generated using ORSADEM and the unsteady flow simulations were performed using HLL numerical solver of De Saint Venant equations written in conservative form. The results of the simulation show that the numerical model predicts satisfactorily the water levels. ORSADEM results to be a useful tool to simulate flood waves propagation in natural rivers. References [1] Cunge J.A., Holly F.M, Vervey A. (1980). Practical aspects of Computational River Hydraulics, Pitman Publ. Inc. [2] Bates, P.D., De Roo, A.P.J. (2000). A simple raster-based model for flood inundation simulation. Journal of Hydrology Vol. 236, pp [3] HEC-RAS (2002). HEC-RAS, River analysis system, user manual. US Army Corps of Engineers, Hydrological Engineering Center, Davis, CA, Report No. CPD-68.

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