Vulnerabilidade e riscos naturais a eventos de alta energia nas praias semi-urbanas e naturais do litoral de Aquiraz, Ceará

Abstract

This work aimed to identify and map zones that are vulnerable to erosion hazards and impacts to urban structures as well as to beach and dune areas. The study site is located at the town of Aquiraz, distant 25 km from the capital, and includes the beaches of Prainha, Japão, and Porto das Dunas. In chapter 02, the study site was characterized, and the following aspects were analyzed: occupation, geology-geomorphology, physiographic, climatology and oceanography. Chapter 03 corresponds to the coastline evolution analysis for the past 56 years using the DSAS software. It was found that the coastline is going through a progressive erosion phase. In the years between 1958 – 2014, the AEPR of Prainha ranged from -0.02 to -5.15 m/year, in Japão the values ranged from -0.07 to -1.29 m/year and in Porto das Dunas, values for the AEPR ranged from -0.29 to -2.61 m/year. Chapter 04 analyses the oceanographic aspects with special emphasis on the swell waves. The methodology has covered topics such as wave over wash and overtopping, sea level height, and meteorological tides. In the field, wave height, period and direction were measured, during the spring and neap tide periods, from September 2011 to April 2012, every fortnight and the waves observed had a maximum and a minimum height of 2.3 and 1.1 meters, respectively, in the months of December and February. The wave directions were NNE-NE e ESE on the first and second semesters, respectively. Chapter 05 and 06 covers the sediment dynamics and topography. Sediment samples were collected at the dune and at the beach face in 11 spots along the three beaches and at the 11 spots of high wave frequency (Japão). Data from the beach profiles and the sediment samples were analyzed in the laboratory using specific computer software’s. In the seasonal period, classification indicates beaches with medium and coarse granulometry for 2011 (up drift and berm), and medium granulometry sediments for 2012. In high frequency waves, the dune was made up of medium to coarse sands during spring tides and of medium to fine sands during neap tides. From the beach profiles created, 6 of them presented erosion (profiles 1, 2, 5, 7 and 10). In the seasonal profiles, sediment mobilization happened at the dune base and at the foreshore. In the longitudinal profile, there was a low sediment transfer, except for points 9 and 10. In the seasonal sediment balance, erosion was observed in May and sediment deposition was observed in August. The longitudinal sediment transport rate showed Prainha as having the highest values in 2011. In Japão, sediment transport was higher in 2012. In Porto das Dunas, significant sediment transport took place in May. 11 Chapter 07 focuses on the compilation of methods for classifying the coastline vulnerability and hazards to erosion for the next 10, 25 and 50 years. From quantitative and qualitative data analysis, it was found that the beaches studied have a medium to high vulnerability, and regarding the hazards, Prainha seems to be in higher risk (-28.40, -71.10, -142.20 meters) followed by Porto das Dunas (-18.86, -30.39, - 60.67 meters) and Japão (-12.01, -30.00, - 60.0 meters). All this information will be useful to the community and government for decision making, development plans and for land use programmers.