Modelling the impact of the extension of the north jetty of the Mondego river inlet on the adjacent southern beaches

Modelling the impact of the extension of the north jetty of the Mondego river inlet on the adjacent southern beaches Oliveira, J. N. C. 1 Abstract: The need to protect coastal populations and mitigate the shortage of sediments and the consequent erosive effect observed in the Portuguese west coast, led to the massive construction of groynes and revetments along the study area, a coastline extension of 30 km south of Figueira da Foz. This study characterizes the wave climate and the induced potential sediment transport south of the Mondego river s inlet, from 1952 to 2010. Based on the statistical analysis of the wave parameters time series (hindcast) validated for the study period, a wave climate with high interannual and seasonal variations and highly energetic was identified, with a mean wave direction of 299.5ºN and extreme obliquities 290/311ºN in winter/summer (January/August), and 40% of significant height occurrence greater than 2 m. The calculated potential longshore sediment transport, based on the numerical model LITDRIFT, revealed highly variable annual figures, ranging between 580-1500 x10 3 m 3.year -1, with south directed drift averaged 385 x10 3 m 3.year -1 during 78% of the study period. The cross-shore distribution of the longshore transport was identified in the active zone, with variable length (~2000 m). Between 2008 and 2010 the north jetty of the Mondego river s inlet was extended about 400 m, motivating the main goal of this study: to evaluate the impact of this extension on the southern beaches, using the coastline evolution numerical model LITLINE. The model was calibrated for the period 1996-2001 and validated for the period 2001-2008, based on measured coastlines and synoptic wave climate data. The prediction simulations for the 12 subsequent years to the extension point out a widespread retreat of the coastline, enhanced downdrift of the existing groynes. Keywords: Numerical modelling, Wave Climate, Longshore sediment transport, Coastline evolution, Erosion, Figueira da Foz. 1. INTRODUCTION The Portuguese coast supports a variety of essential activities that take advantage of its natural, economic and cultural value. However, the growing coastal occupation is often incompatible with the natural dynamics, and the degradation of the coastal system due to coastal erosion results from non-integrated intervention strategies. The regeneration of the coast is a complex and slow process, which is not possible without the correct understanding of the coastal dynamics in order to support sustainable land use planning models and validate risk and intervention management policies in the coastal area (Santos et al., 2014). The results and analysis presented in this extended abstract are a part of the work developed and presented in the masters dissertation Modelling the impact of the extension of the north jetty of the Mondego river inlet on the adjacent southern beaches (in Portuguese), by the same author, Oliveira (2016), and further results and analysis can be found in the said dissertation. This study focuses in a coastal stretch located in central-west part of Portugal (Figure 1), featuring natural sandy beach coast in all its extension, interrupted by a rocky headland in Pedrógão. This physiographic unit is bounded at north by the Mondego river s inlet, with its two jetties (40 8'45" N and 8 52'42" W), and at south by the river Lis inlet, with two small jetties (39 52'50" N and 8 58'18" W). The coastline evolution in this stretch was heavily influenced by human interventions, dating back from the middle of last century. Amongst those interventions, the following can be highlighted: the construction of Mondego river inlet s jetties (1961-1965); the sand extraction on the Figueira da Foz beach (1973-1996); dredging to enable passage through the inlet; river and port bank and bottom maintenance interventions; the beach nourishment 50 m south of the south jetty (1973-1975); the construction of the revetments in Gala-Cova, Lavos, Leirosa and Pedrógão (1975-1979); the construction of a groyne in Leirosa (1978), a groyne in Costa de Lavos (1979) and a field of five groynes in Gala-Cova (1979); and lastly, the extension of the inlet's north jetty (2008-2010). The goal of this study is to assess the impact of the last mentioned intervention in the coastal stretch. Since the morphological variations of the shoreline are caused by the longshore sediment transport gradient and the interaction with the various structures along the coastline, the knowledge of the sea waves characteristics, sea 1 Instituto Superior Técnico, ULisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal, joao.c.oliveira@tecnico.ulisboa.pt 1

Figure 1 Study area and detail of the main structures and adjacent urban centers. (Source: Google Maps, December 2014). level, topo-hydrography and sedimentology is critical in determining the mobilized sediment volume that induces the evolutionary process of the shoreline. The local wave climate in this coastal stretch was analyzed for the period 1952-2010. Using synoptic sea level data, the potential longshore sediment transport south of the Mondego river s inlet was calculated, for the same period, through numerical modelling (LITDRIFT). Finally, the effect of the north jetty s extension in the coastline evolution of this 30 km stretch was assessed for a prediction period of 15 years, using numerical modelling (LITLINE). 2. DATA AND METHODS 2.1. Wave climate and sea level The time series of the wave parameters significant height (Hs), peak period (Tp) and mean direction (Dir) in the position of geographic coordinates 9 00'W and 40 00'N, in front of the study area, resulted from the application of a sea wave generation and propagation spectral model to the north-east Atlantic Ocean (Dodet et al., 2010), with a spatial resolution of 0.5 and a time step of 6 hours. The model was validated by the authors against multiple buoys data, including the Figueira da Foz buoy for the period of 1993-1995. The wave climate characterization in the study area is based on the statistical analysis of the wave regime and its seasonal and interannual variability. For the parameters Hs, Tp and Dir, the frequencies of occurrence were estimate in left included class intervals with amplitudes of 0.50 m, 2 s and 10º, respectively. The statistical parameters average, standard deviation, minimum and maximum, and 1st, 2nd and 3rd quartiles of the time series corresponding to the total and seasonal regimes were also estimated. The considered seasonality is based on: the maritime seasons winter (October to March) and summer (April to September); and the monthly variations. The considered sea level time series takes into account both the astronomical tide and the storm surge component, obtained using hindcast data. 2.2. Longshore sediment transport modelling Topo-hydrography and sedimentology The selection of the representative cross-shore beach profile (with spatial resolution 5 m) was based on a morphological characterization of the 2

study area, the coastal stretch between the inlets of the Mondego and Lis rivers, by Oliveira and Brito (2015). The representative sediment median diameter, d 50=0.30 mm, considered along the cross shore profile, was obtained by calculating the equilibrium profiles associated to different d50 and adjusting them to the representative profile, and the sediment geometrical spreading, d 84 /d 16 =1.30, was based on information from previous local studies. Potential longshore sediment transport The potential longshore sediment transport (LT), was obtained with the use of LITDRIFT, a twodimensional model in the vertical plane (2D-V) that calculates the sediment transport in the longshore component, a part of the deterministic numerical models system LITPACK that describes the major coastal physical processes (DHI, 2014). This model is based on the intra wave Quasi-3D numerical model LITSTP, which describes the time-varying distribution of both suspended and bed loads within the wave period in combined wave and current motion. The propagation of the wave climate from deep-water to the bathymetric -14 m Chart Datum (CD) was accomplished with the use of the LITDRIFT module Transfer Wave Climate. The potential LT, referred to as Qs hereafter, was analyzed in its components: Q s, the potential LT directed to the left of an observer in land facing the sea; Q s, directed to the right of the same observer; and the sum of the absolute and relative values of the two previous components, respectively, the total Qs, Q s, and the resulting Qs, Q s Res. The statistical analysis of the annual Qs was complemented with the analysis of relevant years in terms of LT: the year of average Q s, and the years of average and extreme Q s and Q s. 2.3. Coastline evolution modelling The correct characterization of the behavior and phenomena inherent to the coastal dynamics of the study area is crucial in building a reliably model to reproduce the reality. The conceptual model adopted (Figure 2) was set according to the requirements of the numerical coastline evolution model, LITLINE, based on a one-line theory and part of the LITPACK system mentioned above, and is described in the following section. Conceptual model LITLINE calculates the coastline position based on the sediment transport determined by the transport rate tables previously generated with the LINTABL tool, through successive calls to LITDRIFT. From an initial coastline position, LITLINE determines the coastline evolution in time by solving the continuity equation for sediment volumes (Eq.1): y c (x) t = 1 h act (x) Q(x) t + Q sou(x) h act (x) x Eq. (1) in which, y c is the coastline position, t is time, Q is the LT expressed in volume; x is the longshore position; h act (x) is the active height of the cross-shore profile; x is longshore discretization step and Q sou (x) is the source/sink term expressed in volume. The control of the littoral drift in the boundaries is achieved through the control of the bed roughness, parameter k, to adjust the potential transport determined by the LITDRIFT model to the sediment drift defined in the conceptual model, and is considered variable in time (non stationary). In the north boundary, the Mondego river inlet, the sediment input is conditioned by the sand retention effect of the north jetty, the dredging operations in the inlet and sand extraction and nourishment at the adjacent northern beaches. In the south boundary, the Lis river inlet, sediment availability and dynamic equilibrium conditions are admitted (Santos et al., 2014). The depth of the offshore limit of the active zone of the representative cross-shore beach profile considered was -11.60 m CD (vertical chart datum), corresponding to an active length of approximately 1500 m. The top of the dune was set at 12 m CD, and the beach berm at 5 m CD, resulting in an active height of 16.6 m. Figure 2 Conceptual model adopted for the coastline evolution modelling with LITLINE. 3

The mean orientation of the coastline was considered equal to the mean orientation of the isoline of MSL (mean sea level) in the coastal stretch in 2011, 19.6 N, likewise for the direction of the baseline adopted as the longshore axis, which spatial step considered was 10 m. Coastline data The coastline position depends essentially on factors such as the oceanographic and sediment characteristics, the geomorphological context and the human intervention. The coastlines used in the calibration and validation processes were extracted from digital terrain models based on topographic surveys from INAG (so called at the time), for the years of 1996 and 2001 (July), and topo-hydrographic surveys from DGT, for the years of 2008 and 2011 (Oliveira and Brito, 2015). Calibration, validation and prediction The model calibration consists in determining the sediment transport (implicitly the value of the calibration parameter, k) in the study area that yields the most correct simulation of the coastline evolutionary trends, that is, the best fit between measured and numerical coastlines. The topohydrographical and sedimentological features considered are the ones mentioned in the longshore sediment transport modelling section. The implementation of this process in the period 1996-2001, resulted in k = 0.0050 m, value which was validated for the period 2001-2008, holding constant the other conceptual model parameters. The prediction period considered in this study, 2008-2022, was defined based on the study of Teixeira (2006), in which the sediment accumulation process in Figueira da Foz beach due to the extension of the north jetty was considered to stabilize in approximately 12 years time after the completion of the construction works, similarly to what had happened at the time of its construction in 1965. After the calibration and validation of the model, two scenarios were simulated: the real scenario, in which the mentioned extension was built, and a hypothetical scenario without its construction. The assessment of the jetty s extension impact was based on the comparison of these two scenarios. In both simulations, it was considered the wave climate in the 15 years prior to the completion of the extension along with astronomical tide predictions. To analyze the results efficiently, the coastal stretch was divided into five smaller stretches, conditioned by the existing transverse structures, both built and natural: 1) from the south jetty of the Mondego river inlet to the last groyne of the Gala-Cova groyne field (EGC); 2) from the last groyne of the EGC field to the groyne in Costa de Lavos (ECL); 3) from the ECL to the groyne in Leirosa (EL); 4) from the EL to the headland in Pedrogão; and 5) from the headland in Pedrogão to the north groyne of the river Lis inlet. The effect of the variability of the wave climate on the coastline evolution was assessed through the modelling of an alternative time series in the prediction period. 3. RESULTS AND DISCUSSION 3.1. Wave climate Wave regime The wave regime in the study area is characterized by an average Hs of 2.15 m and the following predominant classes: 1.00-1.50, 1.50-2.00 and 2.00-2.50 m, with 26, 23 and 14% of occurrence, respectively (Figure 3). The average Tp is 11.6 s, and the classes with higher frequency are 10-12 s (27%), 12-14 s (27%) and 10-8 s (21%) (Figure 4). About 90% of the wave events direction range from 270 to 330ºN with an average Dir of 299.5ºN and the most frequent classes 310-300ºN (23%), 290-300ºN (21%) and 310-320ºN (16%) (Figure 5). The analysis of the aforementioned parameters reveals that the median values are 1.78 m, 11.4 s and 301.2ºN, respectively, and that 50% of the events have Hs in the class1.31-2.64 m, Tp in 9.5-13.4 s and Dir in 289.4-312.9ºN. Figure 3 Hs histogram (for classes of amplitude 0.5 m) south of Figueira da Foz. Figure 4 Tp histogram (for classes of amplitude 2 s) south of Figueira da Foz. 4

Figure 5 Dir polar histogram (for classes of amplitude 5º) south of Figueira da Foz. The combined analysis of the relations Dir-Hs, Dir-Tp and Tp-Hs revealed that: i) 50% of the events range between: Dir 280-320ºN and Hs 1.00-2.50 m; Dir 280-310ºN and Tp 8-16 s; Tp 8-12 s and Hs 1.00-2.50 m; ii) 90% range between: Dir 260-340ºN and Hs 0.50-4.50 m; Dir 270-340ºN and Tp 6-18 s; Tp 6-16 s and Hs 0.50-4.50 m; iii) the most frequent Dir class is 310-300ºN (23%), associated with Hs 1.00-1.50 m (6%) and Tp 10-12 s (7%); iv) the most frequent Hs class is 1.00-1.50 m (26%), associated with Tp 8-10 s (10%); v) the most frequent Tp class is 10-12 s (27%), associated with Hs 1.50-2.00 m (9%); vi) Hs 9.5 m are exclusively associated with Dir 270-290ºN and Tp 16-22 s. Seasonality In the maritime winter/summer (Figure 6): i) the average Hs is 2.66/1.63 m and the prevailing class is 1.5-2.00/1.00-1.5 m (19/37%); the average Tp is 12.9/10.2 s and the prevailing class is 12-14/8-10 s (39/35%); the average Dir is 293.4/305.6ºN and the prevailing class is 290-300/300-310ºN (24/25%); ii) the quartiles analysis for Hs, Tp and Dir, respectively, revealed that 50% of the events fall into the classes 1.65-3.36/1.15-1.89 m, 11.4-14.5/8.5-11.4 s and 284.5-306.4/ 296.0-317.1ºN, and that the median values are 2.33/1.48 m, 12.8/10.0 s and 295.6/306.8ºN; iii) 75% of the Hs events range between 1.00-3.50/0.50-2.00m, 80% of the Tp events range between 10-16/6-12s and 73/79% of the Dir events range between 280-310/290-320ºN, resulting in west/northwest incoming directions; iv) 16/1% of the events have Hs 4.00 m. The monthly analysis revealed that the average values of Hs and Tp, respectively, are maximum in January, 3.02 m and 14 s, and minimum in July, 1.39 m and 9 s. The Dir monthly averages are restricted to the range 289-311ºN, and registered extreme west/northwest (290/311ºN) obliquity in January/August. 3.2. Longshore sediment transport modelling Potential LT Interanual variability The four components of the LT were obtained for each of the 59 years in study. The high interannual variability of the potential LT (Qs) is notorious and there is no pattern in its temporal variation. The annual average of Q s is 597 x10 3 m 3 (54% of the years in analysis registered a higher value) and the maximum and minimum values are 915 and 283 x10 3 m 3, respectively. The annual average of Q s is 321 x10 3 m 3 (44% of the years in analysis registered a higher value) and the maximum and minimum values are 829 and 95 x10 3 m 3, respectively. The annual average of Q s is 918 x10 3 m 3 (39% of the years in analysis registered a higher value) and the maximum and minimum values are 1 498 e 578 x10 3 m 3, respectively. The annual average of Figure 6 Dir double and polar histograms for the maritime summer and winter (coastline direction, 19.5ºN, marked with dashed line). 5

Q s Res is 276 x10 3 m 3 (61% of the years in analysis registered a higher value) and the maximum and minimum values are 718 e 300 x10 3 m 3, respectively. The littoral drift is predominantly south, in 46 of 59 years (78%). The following years can be highlighted for their relevant characteristics and extreme behavior: i) 1994, the year which registered the Q s closest to the average for the study period, 917 x10 3 m 3. In this year, the values of Q s and Q s were 679 and 238 x10 3 m 3, respectively; ii) 1956, the year which registered the Q s closest to the average for the study period, 600 x10 3 m 3, and 1986, the year which registered the maximum Q s, 915 x10 3 m 3 ; iii) 1974, the year which registered the Q s closest to the average for the study period, 326 x10 3 m 3, and 1978, the year which registered the maximum Q s, 829 x10 3 m 3. Q s : year of average value The combined analysis of the parameters H rms, Dir e Qs reveals that northern events are mostly associated to wave heights lower than 2 m and are more frequent during the maritime summer. The most energetic events, with the highest H rms, are associated to Dir values lower than 300ºN and occur during the maritime winter, inducing LT towards north. Nevertheless, Q s Res in this maritime season is directed south. Annual sediment budget and cross-shore distribution of Qs The analysis of the annual sediment budget, the diagram that relates Q s to classes of H rms and Dir, at the starting of the cross-shore profile (-14 m CD), along with the analysis of the cross-shore distribution of Q s and Q s in 1994 (Figure 7), reveals that: i) events with H rms 2.00-5.00 m and Dir 295-305ºN, have the most significant sediment mobilization capacity; ii) the cross-shore profile active zone starts approximately at depth -11.30 m CD and has an extension of 1500 m; iii) the LT increases shoreward: there is a first peak at depth -5 m CD, at approximately 650 m from the shore (MSL isoline), caused by the existence of a submerged bar; and two more peaks, at depths of -2 and 1.5 m CD (at 230 and 20 meters from the shore, respectively), due to the sudden slope change. Seasonality For a detailed assessment of the Qs seasonality, the period from April 1994 to March 1995 was analyzed in order to cover two consecutive and complete maritime seasons: i) the maritime winter registered the largest Q s value, 524 x10 3 m 3, and in summer only 60% of this volume was mobilized, 313 x10 3 m 3 ; ii) in the maritime summer/winter, the sediment mobilization potential was divided into 15/22% north and 85/78% south, resulting in south drift of 43/57% of the total volume mobilized in this summer-winter cycle. In summary, despite the seasonality of the littoral drift, more intense in the winter, the Portuguese west coast south resulting drift trend is evident in both seasons. Q s : years of average and maximum values The year 1956/1986 registered the average/maximum Q s. In this year, the average H rms was 1.36/1.65 m and the mode was 1.45/2.23 m, with wave heights rising up to 5.50/7.90 m. The average Dir was 302/300ºN and the mode was 292/290ºN, more rotated north than the normal to the coastline average direction (289.5ºN). Figure 7 Annual sediment budget (Q s for classes of incident Hrms and Dir), and cross-shore distribution of Q s and Q s, in 1994. 6

The events with H rms and Dir in the classes 3.00-6.00 m and 290-300ºN are responsible for the extreme Q s registered in 1986, in which the active zone starts at depth -12.90 m CD and has 2270 m of extension, greater values than those of the year with average Q s. Q s : years of average and maximum values The year 1974/1978 registered the average/maximum Q s. In this year, the average H rms was 1.57/1.61 m and the mode was 2.43/1.17 m. The average Dir was 300/299ºN and the mode was 285/291ºN, more rotated south/north than the normal to the coastline average direction (289.5ºN). The events with H rms and Dir in the classes 3.00-6.00 m and 290-300ºN, and the events with H rms greater than 3.00 m and Dir 270-285ºN, are responsible for the extreme Q s registered in 1978, in which the latter southern events are responsible for the north directed drift. The active zone starts at depth -12.30 m CD and has 1970 m of extension, lower values than those of the year with maximum Q s but still higher than those of the year with average Q s. 3.3. Coastline evolution modelling Coastline evolution post-extension Figure 8 presents the predicted numerical coastlines for the 1 st of July of the years 2010, 2014, 2018 and 2022, resulting from the simulations of the coastline evolution in the scenario with extension of the north jetty, along with the coastline measured in 2008. The results obtained in the calibration and validation stages suggest the model doesn t properly simulate the coastline evolution in the stretches 1 and 2 due to the deviation between the local coastline orientation and the mean orientation of the coastal stretch in study, a consequence of the requirements of this type of models (morphologic longshore uniformity). Additionally, the model considers the direction of the groynes perpendicular to the baseline (requiring only their length and the position at the shore), resulting in a computed direction of the structure different from the real. This effect is visible in Figure 8, where these stretches align parallel to the baseline over the years, a reorientation with no physical relation to the constructed structures. On the downdrift side of the ECL, the model predicts alternating erosion and accretion, with maximum retreats and advances of 52 and 78 min 2010-2011 and 2012-2013, respectively. In the following years a decrease of these values is expected, except for the year 2021-2022, in which an advance of 64 m is expected. Updrift of the EL the reverse behavior is expected for the same years, with maximum advances and retreats of 77 and 85 m in 2010-2011 and 2012-2013, respectively, and retreats up to 65 m in 2021-2022,120 m south of the EL. The same analysis in the vicinity of the southern structures reveals a repetition of this behavior in the same periods, with a decrease in the intensity of the advances and retreats with the increasing distance from the jetty, especially north of the revetment located updrift the Pedrógão headland. This LT gradient along the study area is not specifically attributed to the extension of the north jetty but to the physiographic unit s morphological response as a whole to the action of the wave regime. Updrift the river Lis inlet north jetty, the model predicts high sediment accumulation that seems unreasonable according to the local characteristics. In fact, the model does not Figure 8 Measured coastline of the study area in 2008 and numerical coastlines for the years 2010, 2014, 2018 and 2022, in the scenario with extension of the north jetty, obtained on July 1 of the respective years. 7

simulate the dynamics resulting from the dispersion caused by the river flow, invalidating the predictions for the area adjacent to the inlet. Extension impact The impact of the jetty s extension was assessed by comparing the evolution scenarios with and without the extension of the jetty. The analysis of its impact on the annual coastline variation (from January to December), revealed that the extension reduces the coastline variation amplitude in the stretches 1, 2, 4 and 5 (numbered from north to south), of an average of 12, 1, 2 and 4 m, respectively. For the abovementioned stretches, the reduction occurs in 69, 38, 62 and 62% of the prediction years, while in the stretch 3 reduction of the coastline variation amplitude only occurs 23% of the time. In the latter, an average amplitude increase of 1 m is expected. Figure 9 presents the predicted numerical coastlines for the years 2016 and 2022 (July 1), obtained for each scenario, along with the coastline measured in 2008. The numerical coastlines display similar trends in the central parts of stretches 3 and 4, and in the updrift vicinity of the existing structures throughout the study area, with the exception of stretch 1 in which, as mentioned, it s considered that the model doesn t reliably replicate the real evolutionary trends. In the year of 2022, downdrift of the EGC field and the Pedrógão headland, the coastline retreat effect is especially notorious. By comparing the 2022 numerical coastlines, it is evident that a general retreat of the coastline is expected as a consequence of the north jetty s extension. Table 1 shows the average variation induced by the jetty extension on the coastline position, in each year (July 1) of the analysis period, for the study area and the individual stretches. Negative values correspond to a retreat induced by the extension and the intensity of the color represents the intensity of the retreat. These results confirm the general erosion effect induced by the extension, especially in stretches 3 and 4, where the model is more reliable. Table 1 Average annual variation of the coastline position due to the jetty extension, for the study area and the individual stretches. Average variation of the coastline position [m] Study Stretch Period area 1 2 3 4 5 2010-2011 -0.2 4.1-3.1 0.0 0.0 0.0 2011-2012 -0.8-11.6 1.3 0.4 0.1-3.7 2012-2013 -0.6 1.8-1.9-0.8-0.2-1.2 2013-2014 0.1 1.9-0.4 0.0 0.1 0.0 2014-2015 -0.1 2.1-1.0-0.3 0.1-0.4 2015-2016 -0.7-4.3-0.3 0.0-0.2-2.4 2016-2017 -0.9-10.0 0.4-0.3-0.2-2.4 2017-2018 -1.0-7.2 0.0-0.3-0.1-4.1 2018-2019 -0.8-5.1-0.5-0.3-0.3-1.9 2019-2020 0.5 18.9-3.2-0.3 0.2 0.4 2020-2021 -0.1 9.1-4.8-0.4 0.1 1.3 2021-2022 -0.3 4.8-4.1-0.3 0.0 0.6 2010-2022 -4.9 4.5-17.6-2.6-0.5-13.7 Figure 9 Prediction of coastline evolution in the study area for the period 2008-2022 with and without the extension of the north jetty. 8

A detailed analysis of the abovementioned values within each stretch revealed that on the downdrift side of the ECL, the extension will induce maximum additional coastline retreats of 2.2 m and advances of 1.6 m, with consecutive retreats in the last four years of the prediction period, where the maximum value is expected. Updrift of the EL the opposite trend is expected, except in 2022 where the model predicts again an increase of the coastline retreat due to the extension. Until 2018, on the downdrift side of this groyne, the amplitude increase in the coastline variation is notorious. After 2018 the coastline variation s amplitude decreases for the rest of the prediction period. Updrift of the Pedrógão headland, the coastline evolution trend is similar to the predicted updrift the EL. Downdrift of this natural structure, the coastline variations are greatly enhanced by the extension: maximum additional retreats of 14.6, 21.2 and 7.7 m are expected in the years 2011, 2015 and 2019 and maximum additional advances of 12.2, 14.1 and 11.5 m in the years 2012, 2021 and 2022. The comparison of the coastline variations induced by the extension of the jetty with the average retreat expected in the study area for the scenario without the extension reveals that the erosion caused by the jetty extension has less significance in the coastline evolution than the dredging operations in the Mondego river inlet. The analysis of the wave climate variability effect on the coastline evolution revealed that the incidence of a wave climate with constant characteristics for a long period of time increases the coastline variation amplitudes and, consequently, the coastline retreats in the case of an erosive trend. 4. CONCLUSIONS AND FUTURE WORK The fact that this analysis is based on a very long wave climate time series strengthens the relevance of the displayed parameters. The results show that the coastal stretch under consideration is subjected to a high energy wave climate with a strong seasonal variability. The analysis of the potential longshore sediment transport for this 59-year period reveals a high interanual and seasonal variability, with Q s ranging from 578 to 1 498 x10 3 m 3. This study also made possible the characterization of average and extreme years of Qs, including the assessment of the active extension variability, ranging from 1500 m in a year of average Q s up to 2270 m in a year of extreme Q s. The erosive effect of the extension of the north jetty of the Mondego river s inlet in the adjacent southern beaches has been identified with varying intensity throughout the study area, while maintaining the overall trend retreat of the shoreline. The areas most affected by the lack of sediment caused by this extension are located downdrift of the built structures, especially in Leirosa, and this effect is still visible in Pedrógão, about 27 km from the jetty. The lack of sediment in the study area is enhanced by the extension of the jetty but it s mostly due to the dredging operations in the Mondego river inlet. The weakness of coastline evolution models for coastal stretches with deviant alignments relatively to the baseline is also confirmed. 5. REFERENCES DHI (2014). LITPACK - An integrated modelling system for littoral processes and coastline kinetics, Short introduction and tutorial, Copenhagen, Denmark, 64 p. Dodet, G., Bertin, X. e Taborda, R. (2010). Wave climate variability in the -East Atlantic Ocean over the last six decades, Ocean Modelling, Vol. 31, pp 120-131. Oliveira, F.S.B.F. (2014). Morphological characterization of the littoral stretch between the inlets of the Mondego and Lis rivers, Laboratório Nacional de Engenharia Civil, DHA/NEC, Lisbon. (in Portuguese) Oliveira, F.S.B.F. e Brito, F.A. (2015). Morphological coastline evolution south of the Mondego river inlet from 1975 to 2011, Proceedings of VIII Congresso sobre Planeamento e Gestão das Zonas Costeiras dos Países de Expressão Portuguesa, Universidade de Aveiro, Aveiro, CD-ROM, 15 p. (in Portuguese) Oliveira, J.N.C. (2016). Modelling the impact of the extension of the north jetty of the Mondego river inlet on the adjacent southern beaches, Dissertação de Mestrado em Engenharia Civil, Instituto Superior Técnico, Universidade Técnica de Lisboa. (in Portuguese) Santos, F.D., Lopes, A.M., Moniz, G., Ramos, L. e Taborda, R. (2014). Coastal areas management The challenge of change, Grupo de Trabalho do Litoral, 242 p. (in Portuguese) Teixeira, A.A.T. (2006). Coastline evolution south of the port of Figueira da Foz mathematical model studies, Instituto Superior Técnico, CEHIDRO - Grupo de Costas e Portos, Lisbon, 25 p. (in Portuguese) 9