Surf Zone Sediment Size Variation, Morphodynamics, and Hydrodynamics during Sea/Land Breeze and El-Norte Storm in Sisal, Yucatan, Mexico

This study compares the effects of a mesoscale pressure system (i.e., cold front: referred to as El-Norte) and local pressure systems (land and sea breezes) on the hydrodynamics, beach profile, and particle size distribution of sediments within the surf zone. The effect of onshore and offshore wind has been investigated using measurements of hydrodynamics (water elevation, current velocities and direction), geomorphology (surface sediment samples and beach profiles), and meteorological observations (wind, temperature and atmospheric pressure). The experiment was conducted on Sisal beach located in the Gulf of Mexico along the northern coast of the Yucatan Peninsula from March 27th 2014 to April 11th 2014. For a 50 m cross section of the beach (i.e., the surf zone) hydrodynamics were analyzed by estimating turbulent kinetic energy, kinetic energy dissipation rate and bed shear stress using the measurements obtained from Acoustic Doppler Velocimeters (ADV). Significant wave height and wave period in the surf zone were computed using pressure sensors from ADVs. A pressure transducer located in the swash zone provided additional water free-surface elevation data to help assess wave energy dissipation. During sea breeze cycles with wind velocities reaching up to 15 ms-1, rapid changes in wave height over a short period of time were observed. One El-Norte storm event affected the study site during the field campaign with maximum wind speeds of 15 ms-1, and significant wave heights of 1.5 m and 0.3 m in water depths of 10 m and 0.4 m, respectively. This study will test five hypotheses: (1) El-Norte is more effective in skewing sediment size distributions toward coarser grain sizes compared to regular sea-breeze events, (2) sea-breeze events result in deposition of sediment within the surf zone and erosion in the swash zone, in addition to onshore sandbar migration, (3) an individual El-Norte event can cause significant alterations to the beach profile resulting in a steeper beach profile and significant erosion in the swash zone and surf zone, (4) the undertow currents are stronger during El-Norte than during sea/land-breezes and play an important role in the offshore sandbars migration

Wind-Driven Nearshore Dynamics in the Gulf of Mexico

Coastlines around the Gulf of Mexico are dynamic, due to prevailing energetic wind systems such as frequent cold fronts and diurnal wind systems. In the last two decades, more research has focused on the surf zone’s complex coupling effects between winddriven waves and currents. However, there is still a need for further field based experiment to elucidate, (1) how offshore cold fronts impact nearshore morphodynamics and sediment dynamics, when compared to onshore fronts, (2) what is the main physical forcing that controls the surf zone and the inner-shelf region current circulation during relatively intense onshore and offshore wind events, and (3) does the cumulative effect of sea breeze cycles result in more morphodynamic variation than cold fronts? Therefore, two field based studies were conducted in the Gulf of Mexico to advance our understanding of the complex coupling effects between wind-driven waves and currents, including turbulence quantities, sediment transport parameters, and morphodynamic processes. Both field experiments included time-series of highly resolved hydrodynamics and suspended sediment concentration obtained at different cross-shore locations across the surf zone. The first field experiment was conducted at a sea breeze dominated beach on Sisal, Yucatán Peninsula, México. Time-series observations suggest that the impact of sea breeze cycles on the nearshore hydrodynamics and morphodynamics is comparable to the effect of onshore-directed cold front, and cumulatively the sea breeze cycles will result in higher sediment loss. Regardless, it is also suggested that the cumulative accretional of the land breeze cycles can be sufficient to compensate for the loss of sediment by either the sea breeze cycles or the cold fronts. The second field experiment evaluated surf zone hydrodynamics and sediment dynamic processes within the upper Texas coast during three offshore cold front events and three onshore Gulf breeze events. Observations show enhanced eastward suspended sediment transport following the passage of cold fronts, while westward transport was experienced during the Gulf breeze events. The study suggests that during late fall and early spring seasons, Galveston Island will experience higher erosion rates in the eastside, while accretion of sediment on the west side will occur