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14 Tides and Waves on Vegetated Coasts
Fluid muds are often described as thixotropic substances that possess the
rheological properties of pseudoplastics and Bingham plastics. Bulk density
ranges from 1.03 g/cm 3 , slightly greater than that of sea water, to a yogurt
like consistency with density greater than 1.30 g/cm 3 . Mechanisms that lead
to the accumulation of fluid muds are poorly understood. It is believed that
the most important mechanisms are: trapping by waves and coastal currents
and estuarine-type inner-shelf circulation.
Many observations indicate that the attenuation rates of surface waves propagated over muddy bottoms are much greater than normally attributed to
bottom friction dissipation. An experiment at the front of the Surinam River
showed that the wave energy loss between two stations at 22 km (water depth
7.1 m) and 4 km offshore (water depth 1.3 m) was about 96% (Wells, 1983).
Although water depth at the inner station was only 1.3 m, waves did not break,
nor did they undergo the typical shoaling and transformation that leads to a
height increase just prior to the time of breaking. The spectra at the outer station show a combination of swell (T = 9-10 s) and wind-induced waves (T =
3-5 s). The ratio wave height/water depth remains nearly constant at 0.19,
which is a much lower value than the 0.55 associated with the limiting wave
height (see Chap. 4). Various theories have been proposed to describe the
interaction between water waves and a soft bed. The interested reader should
consult Massel (1996a) for a more detailed discussion.
14 Tides and Waves on Vegetated Coasts
Fluid muds are often described as thixotropic substances that possess the
rheological properties of pseudoplastics and Bingham plastics. Bulk density
ranges from 1.03 g/cm 3 , slightly greater than that of sea water, to a yogurt
like consistency with density greater than 1.30 g/cm 3 . Mechanisms that lead
to the accumulation of fluid muds are poorly understood. It is believed that
the most important mechanisms are: trapping by waves and coastal currents
and estuarine-type inner-shelf circulation.
Many observations indicate that the attenuation rates of surface waves propagated over muddy bottoms are much greater than normally attributed to
bottom friction dissipation. An experiment at the front of the Surinam River
showed that the wave energy loss between two stations at 22 km (water depth
7.1 m) and 4 km offshore (water depth 1.3 m) was about 96% (Wells, 1983).
Although water depth at the inner station was only 1.3 m, waves did not break,
nor did they undergo the typical shoaling and transformation that leads to a
height increase just prior to the time of breaking. The spectra at the outer station show a combination of swell (T = 9-10 s) and wind-induced waves (T =
3-5 s). The ratio wave height/water depth remains nearly constant at 0.19,
which is a much lower value than the 0.55 associated with the limiting wave
height (see Chap. 4). Various theories have been proposed to describe the
interaction between water waves and a soft bed. The interested reader should
consult Massel (1996a) for a more detailed discussion.