Greek case studies: Sediment dynamics in the nearshore zone of Gouves (Heraklio, Crete) in relation to erosion (unpublished data 2006)
The area under examination is located between the torrential rivers of Gouvianos and Gournianos, on the North shores of the Heraklion Prefecture of Crete. The area is under an intense wave and wind regime (wave heights >5m during a storm) with the dominant winds being from N and NW and with wave runup (R) reaching elevations of up to of 2.9m on the beach face, resulting in erosion and shoreline retreat. These phenomena have been aggravated by human interventions such as the blockage of sediment fluxes of the aforementioned torrenrs, the construction of small ports, marinas and seawalls. The seabed is covered by sandy material, with the eastward longshore sediment transport to be in the order of 28x103m3 (Poulos et al., 1998).
The present contribution describes the effect of a ‘Meltemi’ event, on the resuspension and transportation of the nearshore sediments. The ‘Meltemi’ or etesians (in Greek means northern wins of yearly occurrence) appear at the beginning of May with low but also fluctuating frequency and with short duration; they preserve their character until the end of June. From the beginning of July, the frequency of Etesians increases reaching its greatest values towards the end of the month, which it then preserves until about mid-September, and then reduces at the end of October. According to the mean daily velocity the Etesians have been distinguished in 3 categories: weak (mean velocity 0-3.3m/s) moderate (3.4-7.9 m/s) and strong (≥ 8m/s) (Carapiperis, 1981). The outcome of this investigation is based upon the deployment of Autonomous Benthic Recorders (ABRs) equipped with an electromagnetic current meter (EMCM), a pressure sensor (wave height and period), and an optical backscatter sensor (OBS) for measuring turbidity (sediment resuspension). The deployment took place in July 2003 and covered a ‘Etesian’event with winds >6 B for a period of approximately 5 days. Figure 6 illustrates the current directions at each station during the study period together with the associated current speeds. It can be noted from Figure 6 that the current speeds increase and decrease relative to the development of the Meltemi event. The highest current speeds are experienced at G1 reaching a maximum of 16 cm/s in peak conditions. It can be seen that at station G1 and G2 there is a strong northerly component, this represents the reverse undertow currents that are produced as waves hit the coastline. The strength of this current is less at G2 due to the frictional effects on the seabed between the two stations. G3 on the other hand shows a more random distribution of current directions. There is still a northerly component but, once again, the strength of the flow has been reduced. Wave-generated currents can also be noted at G3 in terms of the south-easterly direction, this being the predominant wave direction in the area at this time of year.
The explanation for the occurrence of undertow involves the elevation of the main water level within the surf zone. This produces a seaward-directed pressure gradient of water, which on average is balanced by the momentum of the waves directed toward the shore (Komar, 1998). Wave height was seen to increase during the storm period reaching a maximum late on the 27th July (1.4-1.8m) after which it decreased to its previous level. The graphs of wave height (Figure 7) show that there is some degree of correlation between this significant wave height (Hs) and the amount of suspended sediment in concentration (SSC); this is most apparent at Stations G1 and G3 where the peaks of Hs and SSC coincide. At G2, there appears to be a time lag between the peak Hs and the peak in SSC; this time difference at Station G2 shows a lag of approximately 11 hours. It could be possible that the time lag related to the peak seen in G2 represents the movement of the breaker zone. Similarly to Hs, wave period (T) was seen to increase over the duration of the storm, with the SSC distribution appearing also to be related to T, as both parameters rise and fall with the progression of the Meltemi event.
The present study has investigated the effect of an ‘Etesian’ event, on the resuspension of sediments in Northern Crete. The results indicate that wave action is the dominant process in the resuspension of sediments as the nearbed unidirectional flow (current speeds ≤ 0.18 m/s) throughout the ‘Etesian’ event, never exceeding the critical velocity (Ucr) for the threshold of sediment movement. The nearshore wave heights also increased over the event from 0.2 to 1.7m with the larger of the deep water waves were breaking, prior to their arrival in the nearshore region. The wave heights demonstrated a strong linear or exponential relationship to SSC, before peak storm conditions were attained. On the basis of the above, it may be concluded that in addition to the reduction in supply of sediment to the area (terrestrial and longshore), the ‘Etesian’ events during the summer months result in a further offshore transport of nearshore sediment. Although, in the absence of such strong events during the summer period months some of the sediment lost during winter storm events would perhaps allow for to be replenished. However, the observed continuous erosion along this section of the coastline over the last decade suggests that it is unlikely that calm periods, between the summer ‘Etesian’ events and the winter storms to be sufficient in order to replenish lost sediment.
Carapiperis L.N. (1968). The Etesian Winds: On the daily variation of the velocity of the Etesian winds in Athens.20pp
Komar P.D. (1998). Beach Processes and Sedimentation, Second edition. Prentice Hall, Upper Saddles River New Jersey, 544pp
Poulos S.E., Dounas K., Petihakes G. (1998) Study of the sedimentological and hydrological conditions of the Gournes shorezone (province of Heraklio, Crete). Proceedings of the 6th Panhellenic Geographical Congress, Thessaloniki, Oct. 2002, p. 304-311 (in Greek).
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