Environmental Engineering Reference
In-Depth Information
coastal environments such as estuaries and tidal marshes. In upland areas they are com-
mon to coastal plains, peneplains, and glacial terrain, which all have characteristically
irregular surfaces and numerous depressions.
Formation
The growth of a swamp from the gradual destruction of a lake is illustrated in
Figure 7.59.
Vegetation growing from the shores toward the center is gradually filling in the lake.
Various types of plants are responsible for the accumulation of the organic material. As
each type is adapted to a certain water depth, one succeeds another from the shore out-
ward. Some plants float on the surface, while others are rooted to the bottom. Floating
plants form mats that live above and die below. These catch sediments and in the course
of time may form a thick cover. Beneath the mat is water or a thick black sludge. In time
the mat may support large plants and bushes, and eventually the basin becomes filled
completely with semidecayed organic matter.
The lush growth, by accelerated transpiration, can dry what remains of the lake water,
and swamp trees move onto the firmer ground. As the vegetable matter decomposes, peat
deposits are formed and continue to grow in thickness with time. Some that have been
forming since the end of the Wisconsinin glaciation have reached thicknesses of 20 ft.
Diatomaceous Earth or Diatomite
Diatomite, also referred to as diatomaceous earth, is an accumulation of diatoms, micro-
scopic plants that secrete siliceous material in lakes or swamps. It has a very low unit
weight, high porosity, and an absence of plasticity accounted for by the round shape of its
hollow silica shells.
Engineering Characteristics
Granular Deposits
Granular materials are deposited around the lake perimeter in the form of beaches, deltas,
and dunes. The coarser materials are borrow sources. Beach deposits may be moderately
compact, but most deposits are in a loose state.
Lake Body Soils
Silts, clays, and organic materials generally compose the lake body soils. Freshwater clays
evidence stratification; saltwater clays do not. In an existing lake the materials are soft,
weak, and compressible.
Former lake beds are large flat areas which can be found covered with swamp or marsh
vegetation, or tilled as farmland because of rich modern soils, or existing in remnants as
FIGURE 7.59
Gradual destruction of a lake by filling with marsh
and plant growth. (From Longwell, C. R. et al.,
Physical Geology
, 3rd ed., Wiley, New York, 1948.
Reprinted with permission of John Wiley & Sons Inc.)
