Flash flood risk assessment following the European Water Framework Directive. The case of Marina Alta and Marina Baja (Alicante, Spain). Enrique Ortiz (1)(3), Félix Francés (2), Gianbattista Bussi (1)(2)
ABSTRACT
2D HYDRAULIC MODELING
RESULT: Water depth map of the study zone
Return Period T=500 years 1600 1.- Bco. de Portelles
-NUMBER OF 2D HYDRAULIC MODELS: 22 -NUMBER OF SIMULATED FLOOD EVENTS: 6 for each model -RETURN PERIOD OF MAXIMUM DISCHARGE = 10, 25, 50, 500 years and 2x100 years (in order to check the effect of different flood volumes) -SOFTWARE USED: InfoWorks (Wallingford) -DIGITAL ELEVATION MODEL: Lidar 1x1 m -ROUGHNESS: CORINE Land Use
2.- Río Girona
1400
3.- Barranc de l'Alberca
1200
4.- Barranc de la Losa
Discharge (m 3/s)
5.- Barranc de la Fusta
1000 800 600 400 200 0 1 - 00:00
1 - 06:00
1 - 12:00
1 - 18:00
2 - 00:00
4 – BREAK LINES Return period T = 100 years 1000
EXAMPLE: Girona river (25 km2)
1.- Bco. de Portelles
900
2.- Río Girona
800
3.- Barranc de l'Alberca 4.- Barranc de la Losa
700 Discharge (m 3/s)
The European Water Framework Directive (2007/60), The River Júcar Water Board (Spain) commissioned a study in order to analyse the effect of flooding on different urban centres and hydraulic structures of two counties, Marina Alta and Marina Baja (Alicante, Spain), aiming to redact a basin management plan against risk and following the European Water Framework Directive (2007/60). The first stage, identification of potential flood risk zones, was carried out by coupling a stochastic extreme rainfall generation model, a conceptual distributed hydrological model, and a hydraulic model. The rainfall-runoff model provided the boundary conditions for the hydraulic modelling. 22 hydraulic models were set up with a total of 70 km2 of detailed bidimensional modelling based on digital elevation models 1x1m obtained by Lidar. The results are five maximum water level maps, associated with the 10, 25, 50, 100 and 500 years of return period, for each one of the flood zones. In order to analyse the vulnerability and the economic impact of flooding, the European Water Framework directive was followed: the flood hazard was evaluated considering hydraulic variables (such as the maximum water level) and other variables such as the sediment production and the vulnerability was computed taking into account economical, social and environmental variables (number of affected inhabitants, possible sources of pollution, etc) In order to evaluate the vulnerability as previously explained, it was necessary to elaborate water level – economical damage curves, which were calibrated following the economical damages subsequent to historical floods. Integrating these curves with the results of the hydraulics studies, the risks maps are obtained. Once determined the risk of the studied zones the next step is the elaboration of a management plan in order to reduce the flood risk.
6 – HYDROGRAPHS (BOUNDARY CONDITIONS)
3 – VOIDS (Buildings)
2 – SEA LEVEL INITIAL CONDITIOS
1 – MODEL TOPOLOGY
5.- Barranc de la Fusta
600 500 400 300 200 100 0 1 - 00:00
5 – ROUGHNESS
1 - 06:00
1 - 12:00
1 - 18:00
2 - 00:00
Return period T = 50 years 800 1.- Bco. de Portelles (Ev. 3-1-596)
700
2.- Río Girona (Ev. 3-1-596) 3.- Barranc de l'Alberca (Ev. 2-2-164)
600
4.- Barranc de l'Alberca 2 (Ev. 3-2-014)
Discharge (m 3/s)
CASE-STUDY 22 study areas
500 400 300 200 100 0 1 - 00:00
1 - 06:00
1 - 12:00
1 - 18:00
2 - 00:00
VULNERABILITY ASSESSMENT Methodology:
FREQUENCY COMUNITAT VALENCIANA
1 – ELEMENTAL DAMAGE CURVES 120%
MARINA ALTA and MARINA BAJA (in grey the 22 2D hydraulic models)
The study area is located in the Spanish Mediterranean coast. The zone is affected by severe flash floods due to the “cold drop” effect, especially during the fall season.
PELIGROSITY MAGNITUDE It is necessary to assess VULNERABILITY in order to evaluate the risk
METHODOLOGY Examples of flash floods damages affecting the study zone
Stochastic extreme rainfall generator:
Rainfall –runoff model:
RAINGEN
TETIS
(Salsón and García-Bartual, 2003)
(Francés et al., 2007)
VULNERABILITY
100%
% of damage
SPAIN
4 – RISK ASSESSMENT
3 – VULNERABILITY CURVES
2 – OBSERVED DATA
80% 60% 40% 20%
RISK
0% 0
1
2
3
Water depth (m) First floor rooms
Garages
Garden
Roadway cleaning
Roadway damage
Veichles in garages
In this stage of the work, elemental damage curves are elaborated. This curves represent the percentage of total damage versus the water depth. Every land use (taken from CORINE) has a different elemental curve, since the damage is proportional to the value of the affected goods.
Observed data of damages caused by a real flash flood (October 2007) are collected (insurance remboursments, police reports, city hall official reports, …), which will be use to estimate the damages in € depending on the water depth. The same flood is also modelled by means of the hydraulic model, in order to obtain observed water depths.
In this stage, elemental curves are transformed into vulnerability curves (water depth versus damage in €), taking into accout the different land uses and the different residential densities. These curves are calibrated taking into account the damages in € caused by the observed flood used for calibration (October 2007). The result is a curve which relates water depths with the damages quantified in euros that it will generate.
F =1
h =∞
The total damage sin € are calculated for each return period, using vulnerability curves and hydraulic modeling results. The hydraulic model provides water depths, which are turned into monetary damage by means of the vulnerability curves, depending on the land use and the density of population. This damages are computed for every town. Later, the total risk in euros is calculated, taking into account the probability of the flood event and the damages generated by the same flood event. The following formula is used (D is the damage in €, V(h) is the vulnerability curve, F is the cumulative frequency of the flood):
D = ∫V (h)dFH = ∫V (h) f H (h)dh F =0
h =0
Hydraulic model:
INFOWORKS (Wallingford)
VULNERABILIT Y ASSESSMENT
- 368 synthetic rainfall extreme events covering the whole study area - Hydrological simulation of the 368 events x 3 initial moisture states (10%, 40% and 80%) in more than 250 subcatchments - Probability estimation of maximum discharges in every subcatchment - 2D hydraulic modelling of 22 areas of special interest (5 events simulated, with 10, 25, 50, 100 and 500 years return period) -Vulnerability assessment by means of calibrated damage curves (damages vs water depth)
EuropeanSocietà Geosciences Union General Idrologica Assembly 2011 Le Giornate Vienna | Austria Italiana| 03 – 08 April 2011
REFERENCES
CONCLUSIONS -Following the Water Framework Directive, a multidisciplinary study of flood risk has been carried out; - The study has integrated different models (rainfall generation, rainfall-runoff, bidimensional hydraulic) to obtain flood mapping associated with a probability value; -Damages vs water depth curves have been calibrated using real floods damage data, in order to calculate the cost of flood depending on water depth and land use; -The damages in € caused by different floods has been calculated and associated with its cumulative probability, in order to provide a value which may used for cost-benefit analysis in remediation and mitigation structures design
Salsón, S. and Garcia-Bartual, R.: A space-time rainfall generator for highly convective Mediterranean rainstorms, Nat. Hazards Earth Syst. Sci., 3, 103– 114, 2003 Frances, F., Velez, J.I., and Velez, J.J. Split-parameter structure for the automatic calibration of distributed hydrological models, Journal of Hydrology, 332 (1-2), 226-240, 2007.
(1) Hidrogaia s.l., Avda. Juan de la Cierva, 27, Parque Tecnológico 46980 - Paterna – Valencia, Spain
dell’Idrologia 2011, Bologna 1- 2 Dicembre 2011
(2) Research Institute of Water and Environmental Engineering, Universidad Politécnica de Valencia, Spain (3) Idrologia e Ambiente s.r.l., Riviera di Chiaia 72, 80122, Napoli, Italy