Winter Sandy Protections of the Northern Adriatic Coast against Flooding: Preliminary Results

Transcription

1 Journal of Coastal Research SI ICS2009 (Proceedings) Portugal ISSN Winter Sandy Protections of the Northern Adriatic Coast against Flooding: Preliminary Results C. Corbau, U. Simeoni, R. Archetti, A. Peretti and M. Farina Dept. of Earth Sciences University of Ferrara, Ferrara 44100, Italy DISTART University of Bologna, Bologna 44136, Italy Servizio Tecnico Bacino Po di Volano Regione Emilia Romagna, Ferrara 44100, Italy ABSTRACT CORBAU, C., SIMEONI, U., ARCHETTI, R., PERETTI, A. and FARINA, M., Winter sandy protections of the northern Adriatic coast against flooding: preliminary results. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), Lisbon, Portugal, ISSN Coastal erosion induces a constant and often permanent loss of territory. This phenomenon is more alarming when associated with the lowering of the hinterland territory because of the subsidence. This trend sets the conditions for more frequent and vast floodings due to sea storm and high water events. Such floodings not only bring remarkable uneasiness to people and local economies but, sometimes, they can provoke a serious catastrophe, such as during the inundation of Venice in Fortunately, because of the weather-marine and geomorphological conditions of the coast of Emilia-Romagna, in the northern Adriatic sea (Italy) floodings provoke only damages principally to infrastructures and human activities. In order to reduce these risks, temporary sandy levees are constructed during the winter period by the owners of the infrastructures: the material used is usually taken from the lower part of the beach and deposited without following a suitable procedure. If these morphological variations are not accurately evaluated, they may induce changes on the beach and increase the actual erosion. In order to reduce such phenomena and improve the performance of these temporary interventions, a study has been conducted, which aims to determine the sea storm impact on the sand levee and to evaluate the most appropriate constructive modalities and the possibility to use sand coming from the cleaning of the beach during the summer period, which is today accumulated in dumps. A methodology based on the use of the SBEACH (Storm-induced BEAch CHange) software is presented. First results are encouraging. More simulation will be performed in order to validate the methodology ADDITIONAL INDEX WORDS: Sea storm impacts, protection, sand levee, SBEACH, Emilia Romagna, northern Adriatic INTRODUCTION Coastal zones are important for the economy, the environment and the Mediterranean culture. They are naturally highly dynamic areas, containing a large variety of the world s most productive habitats, and represent high ecological values. They are also attractive areas, especially in the Mediterranean, and constitute important economic resources. In fact the coast allows industry, urban development and residential activities, tourism and recreation, transport, fisheries, aqua- and agriculture, etc. However 95% of the world beaches are eroding (PRANZINI & ROSSI 1995) due to human activities and moving dynamics. Beside wave and wind actions, the main reasons for erosion are related to human activities. Tourism for instance has favored the construction of hotels, houses, roads and harbours but they have often been built too close to the shoreline and are now threatened by coastal erosion as underlined by the abundant literature dealing with coastal management problems (CARTER 1988; FABBRI 1990, SIMEONI and BONDESAN, 1997; VALPREDA and SIMEONI, 2003, ARCHETTI, in press). Such problems are observed along the coast of Emilia-Romagna Region in Italy, northern Adriatic, as this area provides large revenue for coastal communities. Furthermore this coastal zone is exposed to risk from coastal flooding and erosion during storms (GAMBOLATI and TEATINI, 1999; CIAVOLA et al., 2007; BEACHMED, 2008). During storm events beach and dune erosion occur quickly causing shoreline retreat and damage to the seaside infrastructures (bars, restaurants, hotels), which may be partially destroyed. For this reason the owners of the infrastructures commonly build temporary soft protections at the end of the tourism period. These protections consist of sand transported from the lower part of the emerged beach to the upper part where it is accumulated as a levee. The sandy levees are sometimes associated with sand fences. Such constructions, common along the coast of Ferrara, are often realised without permission from the local authorities, which suggest the use of external sand or advice the realisation of sand fences. As the owners of the seaside infrastructures will continue to resort to such practices, this study aims to furnish them indications for the correct realisation of the protections. The main objectives are: to record and describe all the temporary protections realised along the Lido delle Nazioni, to analyse the short term evolution of this littoral in order to assess the performance of the different protection s systems, to perform some numerical modelling, by using a two dimensional numerical model SBEACH (LARSON and KRAUS, 1989), for calculating beach erosion produced by storms waves and water level and finally to find the geomorphological and sedimentological characteristics of the more efficient protection s system against storm impacts. The present paper 1194

2 Winter Sandy Protections of the Northern Adriatic Coast Against Flooding presents the preliminary results and mainly the numerical simulations. STUDY AREA Physical Environment The examined coast stretch, Lido delle Nazioni, is a small seaside resort in the Northern Adriatic Sea that is about 3 km long, at about 7 km from Porto Garibaldi (Figure 1). It is a microtidal environment and is defined as low gradient sandy coast. Beaches are about 50 m width and 2 m height and are often characterised by a well-developed berm. The coastal dunes have been largely destroyed for the construction of seaside infrastructures and presently only some small dunes may be observed at the two extremities of this stretch. P29N P28N P26N 2002) During the last two decades, the regional authorities avoid the construction of rigid structures and favor soft solutions such as nourishment (SIMEONI et al., 2003). Meteorological and Hydrodynamics Characteristics The prevailing winds are blowing from NNE-E, from ESE-SSE and from NW (IDROSER, 1982). The analysis of the wind data (December 2003-december 2004) indicates that the mean wind velocity is about 2.4 m/s (ARPA, 2004). The more frequent winds characterised by a velocity greater than 10 m/s are those blowing from ENE and then from NW but their velocity reaches only 8 m/s. The maximum tidal range is about 0.9m at spring tides, while it decreases down to 0.3m at neap tides. The wave climate is usually mild, with significant wave heights lower than 1 m. The prevailing waves are principally from ENE and secondly from ESE. The maximum wave heights are related to the waves from ENE and E. The more frequent sea storms are from N060 to N120, but the strongest ones are from N-NE (IDROSER, 1981). Storms with oneyear return period, with direction NE-E-SE, have wave heights around 3m and periods of 7.5 s (IDROSER, 1996). According to the wind, waves and morphological data, to the effects induced by the man-made structures on the beach profile, and to the orientation of the river mouths, the direction of the longshore transport of sediment is towards the north. DESCRIPTION OF THE INTERVENTIONS At the end of September each year the owners commonly build sandy levees in front of their properties in order to protect them against winter storms. Figure 1. Location of Lido delle Nazioni. The lines represent the profile used for the present study. The evolution of the area points out to a regressive phase occurring after the 50s, due to fluvial discharge reduction and subsidence. The recent evolution of the coast may be resumed in a period of shoreline progression or stability during the 19 th century followed by a decrease of the positive trend at the beginning of the 20 th century. Between 1950 and 1970 regression occurred along the entire coast due to fluvial discharge reduction and subsidence (BONDESAN et al., 1978, SIMEONI et al., 2003). This negative trend has also been accentuated by the construction of harbour jetties of Porto Garibaldi that have modified the hydro-sedimentary dynamic. In order to reduce the intensity and energy of waves on the coast, to limit the shoreline retreat and to favour sand deposition on the beach, different rigid structures were built between 1960 and 1980 (IDROSER 1996). For instance in breakwaters were constructed in front of Lido di Pomposa and Lido delle Nazioni (FIERRO, 1998). The breakwaters are about 100 m in length and the distance between two adjacent structures is about m. These structures are 200 m distant from the shoreline. However the erosion has not been contained by these engineering structures. In fact, during the last decade erosion occurred along the costal stretch of Lido delle Nazioni (R.E.R, Figure 2. Photos of the winter protections realised along the Lido delle Nazioni: a) operation, b) sand levee, c) sand levee associated with sand fences and d) nourishment with external sand. Most of the time the sand is taken from the lower part of the beach and deposited on the upper part (Figure 2a, b). Sometimes, sand fences, associated or not with levee, are installed on the upper part of the beach in order to retain the sand transported by the wind (Figure 2c). When the emerged beach is too narrow, a nourishment is performed using sand coming from the cleaning of the beach during the summer period and deposited in a local dump (Figure 2d). After the winter period the remaining sand is redistributed along the beach profile. 1195

3 Corbau et al. The sand levee may have different forms: triangular, trapezoidal or parallelepiped. They are generally parallel to the shoreline but they may have a U shape in order to encircle the infrastructure. The height is variable between 0.5m to more than 1.5m (Figure 3). At the base, the width ranges from 5 m to 15m. METHODOLOGY The present study aims to simulate the impact of storms on some beach profiles measured along the Lido delle Nazioni. Although different cross-shore sediment transport mathematical models are available in the literature, we decided to use the SBEACH (Storm-induced BEAch CHange) numerical simulation model as it is widely used to calculate storm-induces beach profile changes (THIELER et al., 2000; CAROLL, 2004; MENDOZA, 2008). This model was developed by the USACE (US Army Corps of Engineers) to calculate beach and dune erosion under storm wave action (LARSON and KRAUS 1989, LARSON, KRAUS and BYMES 1990). Figure 3. Topographic profile before and after the realisation of the sand levee (September 2005) The primary application of the SBEACH model is to predict cross-shore beach morphology, on a scale of feet or meters, resulting from storm events on a time scale of hours or days (LARSON et al., 1989; WISE et al., 1996). A fundamental assumption made by the authors of SBEACH is that within the short time frame of a storm event the primary transport mode is normal to the shore, and thus longshore transport is assumed to maintain continuity. The use of the SBEACH model needs the acquisition of profile data as well as wave and water level data and qualitative calibration. After the input data (profile, wave and water level) was obtained, a calibration procedure was performed using a single profile. The procedure included a model run using default calibration values and then the values were adjusted for each calibration parameter in an attempt to improve the results. In general, the SBEACH model is calibrated through the adjustment of two parameters, the transport rate coefficient, K, and the coefficient for slope dependent term, ε. Sediment grain size information (effective grain size= 0.22 mm) was obtained from previous investigations. Furthermore the calculation of the eroded volume was performed by using BMAP (profile comparison) that computes the volume for the two profiles The topographical profiles were surveyed on the 21th of October 2008 with a dgps RTK and the bathymetrical survey was realised one week later with a digital echosounder associated to the dgps. Different sources of data on wave and sea water level (hereinafter swl) close to the study site are available and have been collected. The data sources are: - Operationally forecasted wave data by SWAN forecasting service of the Adriatic Sea (data available at -measured wave data by the Wave bouy in the Northern Adriatic; -tide data collected by a tide gauge installed in the Ravenna harbour ( Moreover, during short time periods; - wave and swl data have been collected during field campaigns carried out in shallow water, with an ADCP (Acoustic Doppler Current Profilers) installed on a 4 m bottom. The instrument is able to measure swl, Hs, Tp and currents. Description of the instruments configuration during a similar field campaign carried out in 2002 and in 2006 is given in ARCHETTI et al. (2003) and in ARCHETTI AND LAMBERTI (2007). The data have been used to set up the methodology, as explained in the following session. RESULTS The simulation was performed on three profiles: P26N, P28N and P29N. Profile P26N is located at about 500 meters south of P28N while the distance between P28N and P29N is about 250m. No sand levee has been realised on the profile P26N because it is situated on a natural area. The measured profiles are shown in Figure 4. P26N is characterised by the presence of a dune of 3.4m height. The slope of the seaward flank of the dune is about 5 and the emerged beach slope ranges from 1 to 2. The beach width, considered as the distance between the foot of the dune and the shoreline is about 50 m. The 6m isobath is situated at 1200 m from the shoreline. Furthermore, at 1 m depth and at about -3.5m, the profile presents a planar zone. Profiles P28N and P29 are quite similar. The sand levee of P28N is situated at about 40 m seaward of the infrastructure and at about 30m from the shoreline. The levee present on P29N is situated at 80m from the bar and at about 40 m from the shoreline. The seaward slope of the levee is about 12 for P28N and 7 for P29N. Furthermore P29N is also characterised by the presence of a bar at 1.5m depth. The 6 m isobath is at about 1300 m from the shoreline Figure 4. Topo-bathymetrical profiles used for the simulation: P26N (grey line), P28N (black line) and P29N (dashed line). In order to run preliminary test with SBEACH, we have chosen as input waves and swl conditions, the condition measured during a typical sea storm in the Northern Adriatic Sea. The preliminary study aims to define a methodology for the study of the beach erosion due to storm and to determine the impact of the sand levee. Wave data were collected during the storm recorded from the 10 th to the 22 th December 2007 in front of Ravenna at 4 m depth. 1196

4 Winter Sandy Protections of the Northern Adriatic Coast Against Flooding In Figure 5 the time series of the Hs, direction, Tp and direction are shown. The wave direction was NNE (Bora storm) with the maximum recorded wave height at the site 1.8 m and off shore (in Cesenatico) Hmax = 3 m. Figure 5. Characteristics of the wave data used for the simulation: H (top), Dir (centre) and T (bottom) The data shown in figure has been used to simulate the short term beach erosion due to a storm with SBEACH. Table 1: Principal results obtained from the simulation performed using SBEACH model. Profile P26N P28N P29N Retreat of 1m 0.5 m 6 m 3.6 m Retreat of 0.5m 12 m 11 m 20 m Retreat of 0m 0.5 m 6.8 m 4.4 m The results of the different simulations are presented in Figure 6 and in table 1. These simulations clearly indicate an erosion of the emerged part of the profiles. The shoreline retreat ranges from 0.5 m for profile P26N to 6.8 for P28N. The retreat of 0.5 m is greater for P29N (up to 20m) than for P26N (12m) and P28N (11m), while the retreat of 1m ranges from 6 m (P28N to 2 m (P26N). Figure 6: Simulations realised on profiles P26N, P28N and P29N. The black line represents the initial profile and the dashed one is the result of the simulation The simulations also indicate that the impact of the sea storm event appears different for every profile. In fact the calculations realised with BMAP indicate an erosion of about 8.8 m 3 /m on the upper beach of the profile P26N (from the dune to the shoreline), of about 15.8 m 3 /m and 13.8 m 3 /m respectively for P28N and P29N. As observed on Figure 6 the eroded sediment is deposited seaward at a depth comprising between 0.5 m and 2m for P28N and P29N and up to 3m for P26N. Moreover the simulations also indicate an erosion of the bar observed on profile P29N. Finally, the wave run up, defined as the maximum vertical extent of wave up rush on a beach or structure above the still water level (SWL) and calculated by SBEACH, is 1.63m, 2.17m, 1.93m for P26N, P28N and P29N respectively. DISCUSSION AND CONCLUSION The impact of storms in the coastal zone provokes a series of processes inducing erosion of emerged beach and inundation. When these processes occur in urbanized coasts, the change on the morphology of the beach is usually accompanied by damage to existing infrastructures. In order to protect their estates, the owners build sand levees during the winter period. The realised simulations, even if they have not been calibrated due to the lack of storm-induced beach response data, are in 1197

5 Corbau et al. agreement with the scientific literature since the material eroded on the foreshore and dune regions during the storm is transported and deposited seaward. Different parameters related to storm characteristics such as storm duration and wave period modulate the morphodynamic response due to storm. However, some of these effects are also directly related to the coastal morphology (HEQUÉTTE et al., 2001) and relative shoreline orientation (COOPER et al., 2004), among other factors. In fact, according to the simulations, a variability in profile shapes with the controlling factors is observed. Moreover the storm induced erosion is minor for the profile presenting a natural morphology (without sand levee) compared to the other profiles. It seems that two factors influence the erosion of the emerged beach: the beach slope and the width between the shoreline and the dune or the sand levee. Obviously, minor erosion is associated to wider beach and gentle slope: the shoreline is almost stable for P26N while shoreline retreat is observed for the two other profiles. However, the present results do not allow the evaluation of the importance of these two factors. In addition, the simulations indicate that profile P28N presents the highest value of run-up and therefore the maximum vertical extent of wave uprush on the beach. This value is typically calculated in function of a nondimensional surf similarity parameter that takes into account the beach slope (BATTJES, 1994). Since the role of the winter sandy levees is to protect the tourist infrastructures, it is important to determine the shape of the beach profile and sand levee that will induce the minor run-up. Such evaluation is complex since the levees are composed by sandy material and the beach morphology is continuously changing. The continuous monitoring of the profiles (about 50 profiles along a 3 km stretch) and the shoreline will allow to calibrate the model and subsequently to define the solutions that offer the best protection. Finally, the analysis of the results from the present monitoring (winter ) will be useful for a future experiment on a pilot site. This experiment, foreseen for the winter , aims to test different geometries and diverse modalities in order to establish suitable guidelines for the owners. LITERATURE CITED ARCHETTI, R. AND LAMBERTI, A., Study of hydrodynamic induced by low crested structures through image processing. In: Jane McKee Smith (ed.) Proc. of the 30th Intern. Conf. on Coastal Engineering (San Diego, California, USA), pp ARCHETTI, R., Study of the evolution of a beach protected by low crested structures using video monitoring. Journal of Coastal Research. Issn In press. 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