Enhancing shoreline advance by ploughing the intertidal beach: Physical simulation

Understanding shoreline behaviour and developing tools to deal with erosion has increasing interest nowadays. Coastal erosion and accretion produce changes on the beach width. These changes condition the uses given to dry beach and coastal areas. As the beach becomes narrower, the hazard of coastal areas increases. Additionally, due to the tourism, the demand and interest for wider beaches in early spring have risen.

Natural and human factors determine shoreline evolution. Storms erode beaches during winter, and calm weather conditions produce accretion. Assisted recovery techniques aim to propose new soft engineering methods that enhance accretion during calm periods. These human interventions need to be thoroughly analysed to ensure their effectiveness. In this study, we propose the ploughing of the intertidal beach area to accelerate the natural recovery process of the beach.

The effect of ploughing the intertidal area of a beach has been analysed through real scale physical simulations in the wave-current-tsunami flume (COCoTsu) of IHCantabria. The effect of the ploughing was monitored by measuring the sand transported shoreward with cell pressures beneath sediment trap boxes. The channel was longitudinally split into two equal channels (1 m wide each), one of them with plane sloping sand and the other including five crests and holes emulating a real plough made by a tractor. The comparison of both sides derives the effect of the ploughing.

Simulated geometry includes wave generator, 11 m of flat bottom, 17 m of concrete variable sloping fixed bed, 10 m of sand with D₅₀ = 0.318 mm movable bed, 2 m of trap box for continuous capturing and weighting shoreward transported sand and 10 m of wave dissipators. Concrete and sand slopes were designed to mimic the real geometry of a sandy beach intertidal accreting bar.

Sixteen experiments were conducted with fixed wave dynamics and bottom geometry and varying water level. Wave conditions were irregular waves with Hs = 0.3 m and Tp = 7 s, which produce dimensionless fall velocity Ω ≤ 1.5 ensuring accretion over the sandy bottom. Water level ranged from the level of the top of the sand to 50 cm above it. Additionally, one test was conducted with rising water level from -20 cm to 50 cm (from the top level of the sandy area), emulating a rising tidal cycle.

Hydrodynamics and morphodynamics were measured continuously during each experiment by means of 16 free surface elevation sensors, 4 ADV, 2 OBS, 8 pressure cells and 6 video cameras. Bottom load sediment transport was calculated as the difference of the measured total load (pressure cells beneath the aforementioned sand trap boxes) and suspended load sediment concentration measured by the OBS. Additionally, the laser scanner accurately determined the initial and final 3D geometry of the movable bed area.

All this data allows the analysis of the suitability of ploughing technique for accelerating natural accretion processes. Preliminary results show that ploughing affects the roughness of sandy bottom, increasing the wave dissipation and with a variable effect on sediment transport depending on the water level.