NHA TRANG PROJECT2013-2014

Aims of the projectTwo main issues:

• Understanding and quantifying the impact of typhoons and winter storms on the beach of Nha Trang,

• Assessing the recovery processes of the beach after an extreme events.

Strategy:

• Describing the geomorphodynamic functioning of the beach for regular forcing,• Monitoring the temporal coastal evolution and assessing the associated forcing.

How to do that?

Multi-scales approach:• Assessing the wave transformation from the

depth of closure point to the swash zone,• Addressing the associated sediment transport.

the depth of closure point correspond to the location beyond which the beach is no longer sedimentologically active. Its depth corresponds approximately to 4.5m.

by courtesy of Prof. Son, National Institute of Oceanography, Nha Trang

Morphodynamics of Nha Trang beach

What to measure?

What to measure?

The swash zone is the most energetic part of the inshore domain and is responsible of the shaping of the upper beach profile apart of storm episode. Because the turbulence becomes predominant, the sediment transport equations based on rolling, saltation or sheet flow model are no longer valid in the surf and swash zones (Aagaard & Hughes 2006, Elfrink & Baldock 2002, Masselink & Puleo 2006).

Current microprofiler Vectrino

Directional velocity measured at high frequency (64Hz) every 1mmover a range of 30mm

High frequency (20Hz) variation of bottom elevation

Swash zone measurements

30mm

t0 …t4

Measurements in the swash zone are difficult to conduct and also, the processes are difficult to model. However, some equations are available to estimate the rush up and back swash.

The trajectory of the cell of width and height δ, locating near the front of water rushing up the beach after collapsing of a bore can be written in a similar manner than a ballistic motion equation (Hughes 1995):

The term in τ takes into account the effect of friction on the displacement. The shear stress for hydraulically rough turbulent flows is:

This equation can be solved for uprush but also backwash (Hughes & Baldock 2004). The asymmetry between uprush and backwash is obtained by using different values for f and δ, for uprush and backwash, and also amending the depth of swash. The change in these parameters is related to the water saturation of porosities (Puleo & Holland 2001).

where f is the empirical Darcy-Weisbach friction coefficient

Swash Zone