Assessment of the Longshore Sediment Transport at Buarcos Beach (West Coast of Portugal) through Different Formulations

Sérgio H. C. D. Larangeiro (1) , Filipa S. B. F. Oliveira (2)

(1) Portuguese Hydrographic Institute, Lisboa (PT)
(2) National Laboratory of Civil Engineering Lisboa (PT)

The aim of this paper is to apply some of the most practical longshore sand transport formulations and to evaluate their adequacy to estimate the total (suspended and bed load) longshore sediment transport rate (LSTR) at Buarcos Beach, in the West Coast of Portugal. The straightforward methodology here present is simple to apply and therefore useful to quickly obtain a first estimative of the littoral transport in coastal engineering studies.
Buarcos Beach is an important seaside resort located in the Atlantic West coast of Portugal. It has a NW-SE general orientation and an extension of approximately 2.8km (Figure 1). Despite its general orientation, this stretch of coast has an irregular shape due to the existence of several inflections in the shoreline orientation. Almost the total extension of the beach is covered by hard rock outcrops, which have an onshore-offshore orientation that extends, in average, from 2m depth above chart datum (CD) up to a depth of about 1m below CD. The beach sediments are mainly medium and coarse sands. The mean water level (MWL) is 2m above CD.
The hydrodynamic conditions in the nearshore region of Buarcos Beach were derived based on the transformation of the annual wave climate known at 10.0m depth CD in front of the beach. Such wave climate, based on wave records from January 1984 to December 1996, resulted from transference, by mathematical modelling, of the wave regime registered at a waverider station located at 89.0m depth CD (Capitão et al., 1997).
Buarcos Beach dynamics has been studied by several authors. Estimations presented indicate that the LSTR falls within the range 200 - 1 500×103 m3year-1. More recently, Larangeiro et al. (2003) concluded that the littoral sediment transport at Buarcos Beach is about 1 000×103 m3/year (directed southwards).This result, obtained through the application of more complex mathematical modelling, was based on the same wave data used in the present study and is in agreement with field measurements (Vicente and Clímaco, 1986). It was used as reference to validate the accuracy of the formulations here applied.
Seven well-known formulations for calculating the LSTR rate were applied in the present study. The commonly used CERC predictive formula (US Army Corps of Engineering, 1984), based on the principle that the LSTR is proportional to the wave energy flux factor, was used considering two different values for the dimensionless empirical coefficient of proportionality, K: an early design value of 0.77 given by Komar and Inman (1970) and a more recent value of 0.25 derived from an empirically based relationship for the K parameter proposed by Valle, Medina and Losada (1993). It was also applied a formulation obtained from Komar and Inman (1970) based on considerations of the mechanics of sand transport that relate the LSTR to the wave and current parameters. Based on fluorescent sand tracer and streamer traps experiments, Kraus et al. (1982, 1988) proposed two predictive empirical models relating the LSTR to the wave and current conditions. Those models were both applied here. The LSTR method proposed by Walton (1980), which includes the Longuet-Higgins cross-shore current distribution model was also included in this analysis. Finally, the two LSTR formulae developed by Kamphuis et al. (1986) and Kamphuis (1991) from dimensional analysis, which include the influence of the wave period, beach slope and sediment grain size were also evaluated.
From the results obtained (Table 1), one can conclude that the predictions given by the CERC equation with empirical coefficient of 0.77 and the formulations developed by Kamphuis et al. (1986) and Walton (1980) are unrealistically high: about 5.9 and 6.0 and 4.5 times larger than the value assumed as reference (1 000×103 m3year-1). The CERC equation with empirical coefficient of 0.25 and the formulations proposed by Kraus et al. (1982, 1988) also revealed an overpredictive behaviour but of a lower order of magnitude (1.9, 2.5 and 1.8 times higher than the reference value). Kamphuis (1991) and Komar and Inman (1970) were found to give accurate predictions.

Figure 1 - Location of Buarcos Beach

Table 1 - Results from the LSTR equations

Reference Equation LSTR x103 (m3/year)
Komar and Inman (1970) C1 5 933
Valle et al. (1993) C2 1 922
Komar and Inman (1970) K&I 1 212
Kraus et al. (1982) KR82 2 472
Kraus et al. (1988) KR88 1 819
Walton (1980) WT 4 451
Kamphuis et al. (1986) KP86 6 063
Kamphuis (1991) KP91 1 062

References