Beach Morphodynamics and Profile Sequence for a Headland Bay Coast
Keywords:Embayed beaches, Brazilian sandy beaches, sheltered beaches.
AbstractThis paper presents a sequence of beach profile for a headlands and bay coast. Shape analyses of the embayed beach, identification of the predominant wave direction, beach and nearshore profiles, sedimentology characteristics, hydrodynamic conditions at the beaches, morphodynamics and morphometric data for 17 beaches on central-north coast of Santa Catarina, Brazil, were obtained. Beaches are classified in three main groups: (1) exposed; (2) semi-exposed; and (3) sheltered. The exposed beaches had an indentation ratio smaller than 0.39 and the dominant south waves are approximately parallel to the coast (angle smaller than 40°). The beaches can be divided into three mainly groups. (a) Reflective beaches have coarse sand (0.59mm-0.94mm) and steeper nearshore slope (1:40) associated with a very narrow coastal plain <1Km). (b) Intermediate beaches with one nearshore bar have medium sand (0.30mm-0.45mm) and gentle nearshore slope (1:100-1:200) and a developed coastal plain-island bars systems. (c) Dissipative beaches have fine sand (0.20 mm) and a gentle nearshore (1:200) morphology. When two or more nearshore bars are present the coastal plain contains fore dune ridges. The semi-exposed beaches have a large indentation ratio (0.37-0.49) and the wave has an approximate angle greater than 40°. They are partially exposed to southerly waves. There is a alongshore beach morphodynamic change that is function of distance between headlands, shape of bay, wave breakers, grain size and relative tidal range. When Hb<Ho in the diffraction zone, reflective conditions (coarse grain) or dissipative/ low tide terrace to mud flat conditions (fine grains) are possible. Generally, in the central position (Hb ≥ Ho) the beach is a dissipative non-barred system or low tide terrace (fine sand). In the case of medium sand, the beach is reflective. Sheltered beaches are influenced only by diffracted waves or local wind waves. They are totally sheltered from the more energetic ocean waves that come from the south. Normally, the wave approximates with an angle greater than 50° and RTR is large. They can be divided into: (a) reflective (coarse and medium sand) and (b) dissipative non-barred or low tide terrace (fine sediment). However, further research is necessary for sheltered beaches, because it is very difficult to include them in the classification proposed in the literature. These types of beaches are in the low limit between wave and tidal dominated environments and a small change in the wave height results in a modification from tidal to wave domain and vice-versa and for this type of beach the source of sediment and consequently grains size define the beach shape and slope (concave or convex). In direction to a more universal classification will be necessary intrudes the shape of beach in the parameters.