Evolution towards dynamic equilibrium

According to Chapter 6, there are 4 evolutionary stages (Figure 6.4) in the evolution of relocated inlets with similar characteristics to Anc˜ao Inlet.

Anc˜ao Inlet

The first two years of Anc˜ao Inlet evolution were presented in Chapter 6. According to that Chapter, Anc˜ao Inlet was in Stage 1 in June 1997. Transition to Stage 2 included inlet channel enlargement, reaching equilibrium values in April 1998. During Stage 2, the inlet was acting as a sediment trap while developing both the flood- and the ebb-tidal deltas. Anc˜ao Inlet reached the end of Stage 2 one year after the opening (July 1998). Therefore entering Stage 3 by October 1998, the transition through Stage 3 occurred during the second year, showing full characteristics of a mature migrating inlet by May-July 1999. Sediment bypassing was occurring at Anc˜ao Inlet during the second year of its evolution. Only on one occasion, after high-energy SW storm conditions, did Anc˜ao Inlet show Stage PS characteristics (January 1999). The main characteristics and the time that Anc˜ao Inlet spent in each stage are summarised in Table 8.2.

As can be seen from the results, from July 1999 to September 2000, Anc˜ao Inlet was still a mature migrating inlet, therefore, in Stage 3. The period from July 1999 to September 2000 is considered a low-energy period. According to several authors (i.e., Morris et al., 2001; Williams et al., 2003, see Chapter 6), most of the migration of Anc˜ao Inlet occurs during high-energy conditions. Therefore, due to the general low-energy conditions (Figure 8.2), inlet migration during this period was small (Figure 8.5) and Anc˜ao Inlet width (Figure 8.3a) and Ω (Figure 8.3b) had maximum values in March 2000. The highest-energy ‘Levante’ event occurred in April 2000 and probably induced the decrease of inlet width and Ω that was observed in the following months (June and September 2000). The volumetric evolution of the main morphologic areas also reflects the low-energy conditions.

From September 2000 to April 2001 the study area was under high-energy SW conditions that included some major SW events (Figure 8.2a), while the fre- quency of occurrence of ‘Levante’ conditions was low, with just one ‘Levante’ high- energy event in February 2001 (Figure 8.2b). Therefore, in January and April

2001, Anc˜ao Inlet was showing Stage PS characteristics. The high-energy condi- tions increased the longshore sediment transport and induced sediment transport towards the inner parts of the inlet and rapid eastward migration of the inlet channel. The inlet width increased by January 2001 as a consequence of the SW storms and strongly reduced by April 2001, due to the growth of the easternmost spit of Anc˜ao Peninsula (see Appendix A). The rapid growth of the peninsula and also of the ebb delta (see Figure 8.4b) can be related to the high rates of eastward sediment transport associated with the SW storms. Simultaneously, the infilling of the channel area produced a decrease of the Ω that reached values similar to those at the opening (Figure 8.3b).

July 2001 (Figure A.16) represents a transition from Stage PS back to Stage 3 under calm conditions (Figure 8.2). There is a slight decrease in the volume of the flood delta and some accumulation in the channel and ebb delta (Figure 8.3c and 8.4). Eastward migration occurred but with much lower rates than in the previous months (Figure 8.5); inlet width (Figure 8.3a) and Ω (Figure 8.3b) showed a slight return to its previous values.

Fuzeta Inlet

Fuzeta Inlet was in Stage 1 in July 1999. The first year of its evolution cor- responds to the low-energy period mentioned above. The volumetric evolution corresponding to the first year of Fuzeta Inlet was characterised by accumulation on the flood delta and erosion or no significant changes in the rest of the mor- phological units (see Table 8.1 and Figure 8.3c and 8.4). According to Chapter 6 the transition from Stage 1 to Stage 2 includes inlet channel enlargement until reaching equilibrium values and flood and ebb delta development. Inlet channel width and Ω underwent large increments during the first year. According to the results, Fuzeta Inlet channel reached dynamic equilibrium by July 2000. How- ever, the deltas were not fully developed, the flood delta being more developed than the ebb delta. Therefore, by July 2000 Fuzeta Inlet had not reached the end of Stage 2 but eastward migration had already started.

The second year of evolution of Fuzeta Inlet (July 2000 to August 2001) was characterised by a low-energy summer that had some ‘Levante’ events and a very energetic winter with major SW storms (Figure 8.2). The volumetric evo- lution showed accumulation in all of the main morphological units (Table 8.1) and therefore the entire inlet area was acting as a sediment trap. Both of the

deltas showed accumulation (Figure 8.4) and the inlet channel underwent infilling processes (Figure 8.3c). The major SW storms that occurred during this winter arrived very refracted at Fuzeta Inlet. It seems possible that the incident angle was too large for the waves to pass towards the internal parts of the inlet and change the inlet to Stage PS. However, the energy of the winter events was suffi- cient to cause some erosion on the ebb delta. The incident SW waves also caused eastward migration and inlet enlargement (Figure 8.3). However, Ω did not show significant variations (Figure 8.3c). After April 2001, the flood delta underwent strong accumulation processes until reaching its maximum volume by August 2001 (Figure 8.4a). The inlet channel underwent small variations until June 2001 and it then underwent infilling processes until August 2001 (Figure 8.3c). The accumulation on the flood delta and the infilling of the channel could be related to the effect of some small ‘Levante’ events that pushed the sediments to the inner areas of the inlet. The analysis of the second year of Fuzeta Inlet evolution shows that the ebb delta was already in dynamic equilibrium by October 2000 since only small variations occurred afterwards. The flood delta showed some strong variations after October but since they were related to the high-energy events, the dynamic equilibrium can be accepted for October 2000. Therefore, by Octo- ber 2000 (15 months after the opening) Fuzeta Inlet reached the end of Stage 2. After that time, Fuzeta Inlet entered Stage 3, thus being considered as a mature

migrating inlet by April 2001. However, the strong variations in the channel (Fig-

ure 8.3) and flood-tidal delta volume (Figure 8.4a) that Fuzeta Inlet underwent after that date, imply that the inlet did not reach a full development as a mature

migrating inlet. Stage PS was defined as the stage after direct exposure to high-

energy conditions. Therefore, Stage PS for Fuzeta Inlet, directly exposed to SE conditions, would be reached after a high-energy SE event. However, Stage PS was never reached by Fuzeta Inlet, probably due to the lack of persistency of the high-energy ‘Levante’ conditions. The main characteristics as well as the time that Fuzeta Inlet spent in each evolutionary stage are summarised in Table 8.2.