IV. Retreating Shorelines
Chapter IV : Evaluation Of Impacts And
Interactions
1
Introduction
301
In chapter II, environmental impact, viz. consequences, of man-made structures
upon coastline evolution has been repeatedly stressed. We will therefore limit us
here to some additional or concomitant considerations.
Coastal engineering problems may be classified into four general categories :
shoreline stabilization, backshore protection (from waves and surge), inlet
stabilization, and harbor protection (Fig. 35). A coastal problem may fall into
more than one category. Once classified, various solutions are available to the
coastal engineer. Some of the solutions are structural ; however, other techniques
may be employed such as zoning and land-use management. We deal primarily
with structural solutions, but these considerations may also apply to other types of
solutions.
Fig. 36 shows the structures or protective works in the four general coastal
engineering problem classifications and lists the factors that must be considered
in analyzing each problem area. Hydraulic considerations include wind, waves,
currents, tides, storm surge or wind set-up, and the basic bathymetry o,f the area.
Sedimentation considerations include the littoral material and processes (i.e.,
direction of movement; rate of transport, net and gross ; and sediment
classification and characteristics), and changes in shore alignment. Navigation
considerations include the design craft or vessel data, traffic lanes, channel depth,
width, length and alignment. Control structure considerations include the
selection of appropriate protective works (shape, size, site) keeping in mind the
potential effects on adjacent areas. Economics must consider maintenance costs,
and interest and amortization of initial investment.
As a short term effect scour at structure's toe will form a trough. Scouring can
undermine the toe leading to settlement of stone in a rubble mound breakwater.
This may be countered by overbuilding the cross section, or by providing excess
stone at the toe to fill the anticipated scour trough. The toe of a vertical structure
may be protected similarly against scour by the use of stone. Impermeable cutoff
walls at the base must be used to protect a gravity wall from undermining by
scour. As a general guide, the maximum depth of a scour trough below the
natural bed is about equal to the height of the maximum unbroken wave that can
be supported by the original depth of water at the toe of the structure.
Chapter IV : Evaluation Of Impacts And
Interactions
1
Introduction
301
In chapter II, environmental impact, viz. consequences, of man-made structures
upon coastline evolution has been repeatedly stressed. We will therefore limit us
here to some additional or concomitant considerations.
Coastal engineering problems may be classified into four general categories :
shoreline stabilization, backshore protection (from waves and surge), inlet
stabilization, and harbor protection (Fig. 35). A coastal problem may fall into
more than one category. Once classified, various solutions are available to the
coastal engineer. Some of the solutions are structural ; however, other techniques
may be employed such as zoning and land-use management. We deal primarily
with structural solutions, but these considerations may also apply to other types of
solutions.
Fig. 36 shows the structures or protective works in the four general coastal
engineering problem classifications and lists the factors that must be considered
in analyzing each problem area. Hydraulic considerations include wind, waves,
currents, tides, storm surge or wind set-up, and the basic bathymetry o,f the area.
Sedimentation considerations include the littoral material and processes (i.e.,
direction of movement; rate of transport, net and gross ; and sediment
classification and characteristics), and changes in shore alignment. Navigation
considerations include the design craft or vessel data, traffic lanes, channel depth,
width, length and alignment. Control structure considerations include the
selection of appropriate protective works (shape, size, site) keeping in mind the
potential effects on adjacent areas. Economics must consider maintenance costs,
and interest and amortization of initial investment.
As a short term effect scour at structure's toe will form a trough. Scouring can
undermine the toe leading to settlement of stone in a rubble mound breakwater.
This may be countered by overbuilding the cross section, or by providing excess
stone at the toe to fill the anticipated scour trough. The toe of a vertical structure
may be protected similarly against scour by the use of stone. Impermeable cutoff
walls at the base must be used to protect a gravity wall from undermining by
scour. As a general guide, the maximum depth of a scour trough below the
natural bed is about equal to the height of the maximum unbroken wave that can
be supported by the original depth of water at the toe of the structure.