J. Tomé M~ rtins, Ó. Ferreira, P. Ciavola & J. Alveirinho Dias

Transcription

1 Partnership in Coastal Zone Management. 1. Taussik & J. Mitchell (eds) 1996, Sam ara Publishing Limited, Cardigan, ISBN ,. Monitoring of Profile Changes at Praia de Faro, Algarve: A Tool to Predict and Solve Problems J. Tomé M~ rtins, Ó. Ferreira, P. Ciavola & J. Alveirinho Dias ;-'t Unidade de Ci~ncias e Tecnologias dos Recursos Aquáticos, Universidade do Algarve, Campus de Gambelas, 8000 Faro, Portugal Abstract: Praia de Faro is an open sandy beach in the Algarve (Southern Portugal), with a high degree of on-off shore sediment exchange in winter associated with a constant change between high and low-to-moderate energy wave conditions. A monitoring study of Praia de Faro was carried out between May 1995 and April1996 in order to obtain an idea of seasonal beach changes, cross-shore profile and alongshore variability. The western part of the monitored area is most impacted by human activity, the dune ridge has been destroyed and beach profiles have smaller sand stocks, leading to a high probability of coastal erosion problems. Overwashes are f requent in this sector, being sometimes responsible for the damage of houses and infrastructure. On the eastern part of Praia de Faro the dune ridge is preserved and beach profiles show a better ability to react to storm events. Beach recovery is generally good and no major sediment losses were found in the short and medium termo Th is can help future artificial beach nourishment, in order to avoid extreme overwashes and damages to properties, since the life span of such kinds of interventions cou ld be enough to make them cost effective. Key words: monitoring, beach profiles, Portugal, predictions Introduction and methodology Praia de Faro is an open, medium energy, mesotidal, sandy beach and part of a sand spit 10.5 km long and m wide (Bettencourt, 1994), that constitutes the western limit of the Ria Formosa barrier island system (Figure 1). Mean significant wave height offshore the Faro coast is about 0.9 m with a mean period less than 5 s (pires & Pessanha, 1986; IH/LNEC, 1994). Predominant wave directions are from the West and the Southwest (68% of occurrences) whilst waves coming from the Southeast (Levante) only account for 25% of the observations (IH/LNEC, 1994). Significant waves higher than 3 m are common in winter, the estimated height for a significant wave with a 5 years return period being 4.8 m, whilst a wave with a 50 years return period is estimated to be about 6.1 m (Pires ' & Pessanha, 1986). The occurrence ofhigh energy wave episodes in winter leads to the existence of severe erosion problems with frequent overwashes ofthe barrier system, especially in places where the dune ridge has been destroyed by human occupation such as at Praia de Faro. Beach behaviour at Praia de Faro is commonly reflective during high tide and intermediate during low tide. Ridge and runnel features are not frequent but a change between berm and bar profiles is common, depending on wave conditions, showing the importance of on-offshore sediment exchange. At low spring tides a 615

2 Partnership in (oastal Zone Management Figure 1 Location of study area ti de terrace can be exposed. To improve knowledge of on-offshore sediment exchanges and related morphological changes at Praia de Faro it was decided to conduct a beach monitoring projecto Beach monitoring by repeated surveys at fixed locations can give the long term systematic change in the profile as weli as short term tluctuations, showing the differences between the summer and winter profiles (Komar, 1976). The survey started on May 1995 at five sites (Figure 1), with three of them (A, B and C) located on the most developed part ofpraia de Faro, and two (D and E), located in places where human occupation is scarce and the dune ridge is still present (E), or was recovered by man (D). Theodolite surveys werecarried out once a month (except August) at low spring tides and each time that an important erosion or accretion event occurred. Sometimes it was impossible to survey ali profiles, especialiy at si te E, due to its distance from the access road. The total amount of profiles surveyed from May 1995 to April 1996 ranges from 11 at site E to 14 at sites A and C. Changes in volume between successive surveys of the same si te were computed to gain an understanding of the magnitude of sediment exchanges. The position of the berm crest in relation to the contact between the dune and the beach, or to the limit between beach and man-made structures was measured in ali profl1es to obtain an idea of horizontal beach displacement and consequently of the possibility of overwashing at each site. 616

3 Monitoring Profile Changes at Praia de Faro Results., Using ali profiles from ali surveys it was possible to determine accretion and erosion envelopes for each site for the analysed year, aliowing the computation of the maximum observed variability. ln Table 1 the difference in volume per beach length unit (m 3 /m) between the.two referred envelopes is presented, for each site, together with maximum and minimum sand stock volumes above the Chart Datul,P. (level of the lowest astronomical tide, 2 m below mean sea levei). At the seaward limit the point where the envelopes cross the zero was used whilst the landward limit was the interface between beachl dunes or beach/man-made structures. Figure 2 compares the envelopes for sites E (absolute maximum sand stock measured) and A (absolute minimum sand stock measured). Volume changes between successive surveys were computed to understand the rate of beach change during summer and winter, at each site. ln Figure 3 time is plotted against cumulative volume change in module (3A) and against volume change between successive surveys (3B). Figure 3A outlines the parts of the year when greater sediment exchange (steepest parts of the plot) and smalier sediment exchange (smoothest parts of the plot) occurred. Figure 3B shows the volume change between successive surveys and identifies the biggest erosion and accretion events. Changes in distance between the berm crest and the beachl dune limit or the beach/man-made structures interface are plotted against time for profiles A and E in Figure 4. Whenever more than one berm was present, the distance to the seaward berm crest was used. Whenever berms did not exist (e.g. after storms) the levei of maximum wave run-up was considered. Therefore, if the distance to the reference limit is zero, it means that overwashes occurred or that the dune was eroded or touched by the sea. The graph aliows a comparison between the beach behaviour at different sites, namely about overwashes during high energy wave conditions or berm recovery during low energy periods. Table 1 Envelope volumes and sand stock SiteA Site B Site C Site D Site E Volume difference between upper (accretion) and lower (erosion) profile envelopes (m 3 /m) Maximum sand stock above the Hydrographic Zero (m 3 /m) Minimum sand stock above the Hydrographic Zero (m 3 /m) 617

4 Partnership in (oastal Zone Management g c 6 o.. to.. > 4 iii 2 O S ite A t O S ite E g c 6 o i i: 4 iii. -#,-. 8 '~--- 2 O O O Horizontal distance {m) Figure 2 Erosion and accretion envelopes at sites A and E Discussion From the analysis of the different surveys realised during one year of monitoring it was possible to see that one berm was generally present at Praia de Faro. However, in winter, the berm can be absent or badly defined during several weeks at sites A and B. A second herm is frequent after low energy periods at sites D and E. Changes in profile morphology were not direct1y related to a transition from winter to summer but to changes from high to low energy conditions, especially at sites A, B and C. During low energy conditions, with swell waves approaching from the West Southwest, the beach shows an accretionary trend with predominant onshore sediment transport, leading to the creation of a new berm ar to an increase in volume of the existent one. With such wave conditions a beach cusp system is normally generated, causing important longshore variations in beach profile. Profiles spaced m can have a difference in sand volumes ofup to 10 m 3 /m, depending on their position relative to a cusp trough or cresto During high energy conditions, with swell waves higher than m approaching from the West-Southwest, the beach shows a typical erosion or storm profile, either without a berm or with erosion ofthe existent ones (Figure 5). With such wave conditions the beach face is less steep and beach cusps are destroyed. 618

5 Monitoring Profile Changes at Praia de Faro r=======, ~.ê 3 A l.- " - " 8ite A Sand _ - -! ~ ite B replacemen,l iteC. ~ ite D/ I! ;" E _ --- // 1: ~100 't> r.!!,.... _ ":..- -" - - ~ediiiie'; - - I :::I 50 ' - E l~~~~~~~;;~~~-~"-~"~~~:~~=-~s~e~d~im~e~n~t--~ex=c~h=a~ng:e~ ~1 <3 O ln -..:.=-::..."'-.=-:---- exchange I O> O> ~ ~ ~ ~ ~ ~ ~ <t: Z LL <t: 0) 80 -,---..., :::-:------:-----, , 'iii > 3 B I Sand replacement at Site B o 60 C 8 I O ::J ~ o.-.40 ~ 8 5i..ê O <t: ~ g 20 ; ; - - ~ 1\}- I,g ~ O I ;o...,~-:..-..-~IJ/--I "\...--c!_' ~--=-=-=-:...:-""--'=-'+_'õ~r, lh 0)0) \ \11, I ~ ~-20 \ /, ~I ~ " II.c 0 /~... lij II fj).º~ g~ \C~ O) -40 til til '-...J º til 0-0_ - E ~ ~ e UJ O~..g ~_UJ_cn w._=cn"' l > ~ ~ ~ ~ ~ iv' g> ~ ~ 9- :E <t: Z LL... 1 Figure 3 Cumulative volume change (3A) and volume change between successive surveys (38) for the period studied 60 E -; 50 u r::: CI) 40 ~ os:! CI) ~ o 30 -CI) 20 u r::: C'II -1/1 i5 10 O Itl Ol iv' :E 1-=-="8 ite" A I " " 8 ite E I Figure 4 Berm crest distance to reference points for sites A and E Zero values mean overwashes or dune erosion 619

6 Partnership in Coastal Zone Management 10 Site A -E /01/ c o /01/ &'IS > CD 4 iii 2 M.S.L O Figure 5 O Horizontal distance (m) Storm and swell profiles for site A Notice that changes were observed in only one day When southeasterly (Levante) waves prevail the breaker angle is normally large (more than 10 0 ) and, even if the waves are small, the beach tends to develop a storm profile, with erosion of the berm and of the cusp crests, sometimes inducing the existence of a notorious scarp. ln a previous work (Andrade, 1990) the generation ofbeach scarps at Praia de Faro was also associated with southeasterly waves. Annual on-offshore sedimentary exchanges were high, reaching maximum values of about 100 m 3 /m (Table 1)over the entire surveyed year (accretion beach envelope versus erosion beach envelope). A large difference was observed between available sand stocks from site to site, with eastward sites having the highest sand volume available. The difference in sand volume between the eastern and western proflles was about 100 m 3 /m considering the accretion envelopes and about 90 m 3 /m considering the erosion envelopes, thus showing the eastern sites greater resistance to storm erosion. Short term on-offshore exchanges were also important, with maximum storm erosion reaching about 42 m 3 /m between successive surveys at site B Oanuary 1996) and maximum beach recovery of 22 m 3 /m/day at site A Oanuary 1996, Figure 5). The highest short term exchanges occurred at sites A, B and C, while sites D and E showed a less extreme behaviour. Nevertheless, the general response to erosion or accretion events was similar at all sites (Figure 3B). On-offshore exchanges were most intense in October-November 1995 and January 1996, while between May and September 1995 and after February 1996 sand exchange was not very high. This trend is quite evident at sites A, B and C. while site D has an intermediate behaviour and si te E shows almost a constant exchange rate between May 1995 and April1996 (Figure 3A). The trend at site E can be related to the higher robustness of the profile (higher sand stock available) along the year allowing a more appropriated response to the prevailing wave conditions, while proflles at sites A, B and C seem to be far from that behavio due to their lack in sand stock. 620

7 Monitoring Profile Changes at Praia de Faro The frequency of profile changes from October 1995 to January 1996, together with the small variability during the rest of the year, indicates seasonal variations, pointing out that beach profiles are more unstable in winter. However, it is not possible to taik about "summer" and "winter" profiles for sites A, B and C since in winter there is a constant and rapid change between both kinds (Figure 5). The terms "storm" profile and "swell" or "berm" profile should be used instead. Site E has a more constant cross-shore sedimentary exchange along the year (Figure 3A) ;t that the terms "summer" and "winter" profile could be correctly applied to this si te. During the study year three major erosion events occurred (October 1995, November 1995, January 1996), which were followed by periods of accretion (Figure 3B). All the erosion events were responsible for a landward shift of the berm crest or even for complete erosion of the berm (Figure 4). During the second and third erosion episodes overwashes occurred at sites A, B and C, while at site D the dune limit was touched by the swash only during the January 1996 evento At site E, at least one berm was always present, with its crest located at a distance from the dune never smaller than 20 m Oanuary 1996). Sites B and Chave the Praia de Faro coastal road as a landward limit, which was overwashed and damaged by the sea in January A considerable amount of sand was spread over the road at several points, not only by wave action but also by strong winds that occurred during the storms. At the beginning of February 1996 local authorities started to remove the sand from the road to inject at site B (Figure 3) and near sites C and D. From May 1995 to April 1996 the net volume change at all sites was positive or near to zero, ranging from -6 m 3 /m (site C, the only negative value) to +43 m 3 /m (site E). Sites A and C showed a small volume change (less than 10 m 3 /m), while sites D and E showed respectively a net accretion of 32 and 43 m 3 /m. Site B presented a net accretion of 41 m 3 /m, due to artificial sand injection. These values show that beach recovery at Praia de Faro was generally good, even after a winter with three erosion events, except for profile B, where natural beach recovery was helped by human intervention. Conclusions Praia de Faro is a dynamic open beach with very important and rapid sediment exchanges which are well reflected by morphological variability. The five sites studied showed similar trends in response to erosion or accretion events during the monitoring period (May 1995 to ApriI1996). However, significant differences were recorded between sites regarding available sand volumes and erosion consequences. The eastern sites (D and E) presented higher sand stock volumes and a better dynamic equilibrium in relation to prevailing wave conditions whilst the western sites had smaller sand levels, allowing erosion events to cause overwash. Despite the sarne annual beach variability, it was possible to see that sites located in developed areas (A, B and C) presented more erosion problems caused by sporadic retreat associated to high energy events. At sites B and C natural changes in profile morphology are now controlled by the existence of a coastal road and parking spaces located in the area of profile variability. This situation is responsible 621

8 Partnership in Coastal Zone Management for a constant need for sand injection at these sites, to decrease the effects of overwashes and to help beach recovery after storms. Beach recovery at Praia de Faro was generally good after high energy events with rapid creation of new berms or increase in size of the existent ones. Sedimentary exchange between bar and berm was almost always observed, with a few exceptions at the western sites where such interaction was sometimes not so obvious. This capability of beach recovery opens good possibilities for future projects of artificial nourishment at Praia de Faro, since it seems that there is no major sediment loss on the short and medium term, allowing the replenishment to have a long life span and cost effectiveness. Acknowledgements The authors are grateful to ali field work participants for their important help. Paolo Ciavola acknowledges the support ofeu-hcm Project n. ERB/CHRX/ CT94/0541. A special acknowledgement to Mr A. Klein for his useful comments on data analysis. References Andrade, C O ambiente de barreira da Ria Formosa, Algarve-Portugal. PhD Thesis, Universidade de Lisboa. 645 pp. (unpublished). Bettencourt, P Les environments sedimentaires de la côte sotavento (Algarve, Sud Portugal) et leurs évolution holocene et actuelle. 2 volumes. PhD Thesis, Université de Bordeaux I. (unpublished). IH/LNEC Final report of sub-project A: wind wave climatology ofthe Portuguese coast. Report PO-W A VES 6/94-A, Instituto Hidrográfico, Lisbon. 80 pp. Komar, P Beach processes and sedimentation. Prentice-Hall, Englewood Cliffs, New Jersey, USA. 429 pp. Pires, O. & Pessanha, L Wave power climate of Portugal. ln: Hydrodynamics of ocean wave-energy utilisation. lutam Symposium Lisbon Eds: Evans, D. & Falcão, A.F. Springer, Berlin, Germany. pp ) 622