Environmental Engineering Reference
In-Depth Information
The effects of reduced wave heights on coastal systems will vary from site to
site. It is known that the richness and density of benthic organism are related to
such factors as relative tidal range and sediment grain size (Rodil and Lastra, 2004),
so changes in wave height can be expected to alter benthic sediments and habitat
for benthic organism. Coral reefs reduce wave heights and dissipate wave and tidal
energy, thereby creating valuable ecosystems (Lugo-Fernandez et al., 1998; Roberts
et al., 1992). In other cases, wave height reductions can have long-term adverse
effects. Estuary and lagoon inlets may be particularly sensitive to changes in wave
heights. For example, construction of a storm-surge barrier across an estuary in the
Netherlands permanently reduced both the tidal range and mean high water level by
about 12% from original values, and numerous changes to the affected salt marshes
and wetlands soils were observed (de Jong et al., 1994).
Tidal energy converters can also modify wave heights and structure by extract-
ing energy from the underlying current. It has been suggested that the effects of
structural drag on currents would not be significant (Minerals Management Service,
2007), but few measurements of the effects of tidal/current energy devices on water
velocities have been reported. Some tidal velocity measurements were made near
a single, 150-kilowatt (kW) Stingray demonstrator in Yell Sound in the Shetland
Islands (Engineering Business, Ltd., 2005). Acoustic Doppler current profilers were
installed near the oscillating hydroplane (which travels up and down the water col-
umn in response to lift and drag forces) as well as upstream and downstream of
the device. Too few velocity measurements were taken for firm conclusions to be
made, but the data suggest that 1.5- to 2.0-m/s tidal currents were slowed by about
0.5 m/s downstream from the Stingray. In practice, multiple units will be spaced far
enough apart to prevent a drop in performance (turbine output) caused by extraction
of kinetic energy and localized water velocity reductions.
Modeling of the Wave Hub project in the United Kingdom suggested a local
reduction in marine current velocities of up to 0.8 m/s, with a simultaneous increase
in velocities of 0.6 m/s elsewhere (Michel and Burkhard, 2007). Wave energy con-
verters are expected to affect water velocities less than submerged rotors and other,
similar designs because only cables and anchors will interfere with the movements
of tides and currents.
Tidal energy conversion devices will increase turbulence, which in turn will alter
mixing properties, sediment transport, and, potentially, wave properties. In both the
near field and far field, extraction of kinetic energy from tides will decrease tidal
amplitude, current velocities, and water exchange in proportion to the number of
units installed, potentially altering the hydrologic, sediment transport and ecological
relationships of rivers, estuaries, and oceans. For example, Polagye et al. (2008) used
an idealized estuary to model the effects of kinetic power extraction on estuary-
scale fluid mechanics. The predicted effects of kinetic power extraction included: (1)
reduction of the volume of water exchanged through the estuary over the tidal cycle,
(2) reduction of the tidal range landward of the turbine array, and (3) reduction of the
kinetic power density in the tidal channel. These impacts were strongly dependent
on the magnitude of kinetic power extraction, estuary geometry, tidal regime, and
nonlinear turbine dynamics.
