Environmental Engineering Reference
In-Depth Information
The main types of deposits shown on
Figure 3.37
are as follows:
- Lag deposits, usually gravel or boulders, occur along the base of the river channel and
are moved only during peak flood times. They are usually uniform sized and in the active
river channel may have very high voids ratio and permeability. Where preserved at the
bases of abandoned or buried channels their voids may be “choked” by sand or by fines.
- Point bar deposits, usually sands and gravels, are deposited on the insides of bends in
the stream. During normal flows these materials occur above the lag deposits along the
whole of the channel with their upper surfaces in the form of migrating dunes. The
cross-bedding seen in the bar deposits results from preservation of some of these dunes.
The point bar deposits are usually coarser at depth, becoming finer towards the top.
- Levees of fine sands and silts are formed along the top of the river banks where these
coarser materials are deposited more quickly than the fine silts and clays when flood-
waters overtop the banks.
- Flood-plain deposits are usually fine silts and clays, deposited in thin horizontal layers
during floods. In situations where the soils dry out between floods, desiccation cracks
are formed and these may be preserved as sand infilled joints if wind-blown or water-
borne sand is deposited in them. Small tubular holes left by burrowing animals or
decomposing vegetation are also common in floodplain deposits. These may be pre-
served open or else infilled by sand or fines.
- Oxbow lake deposits occur in lakes formed during floods when parts of the meander-
ing channel are cut off from the stream. The nature of the deposits depends on whether
the channel is abandoned slowly (chute cutoff) or abruptly (neck cutoff) as shown. In
each case the fine-grained soils are usually near normally consolidated and are at least
partly organic due to the presence of rotted vegetation.
Not shown on Figure 3.37 are bar deposits which can be seen at low flows along
straight parts of meandering streams and also in the channels of “braided” streams. Such
bars may migrate quite rapidly with aerial photographs taken 10 years apart showing sig-
nificant changes in the stream bed geometry.
Cary (1950) reports that gravel bars in several fast-flowing rivers in U.S.A. contain elon-
gated lenticular deposits of essentially uniform-sized “open-work” gravel. He reports many
open-work gravel lenses in glaciofluvial deposits in the north-western U.S.A., and comments
on the large voids and extremely high permeabilities of these materials. The authors have seen
open-work gravels and cobbles in which the large voids have become “choked” with sand,
which clearly must have migrated into the voids after the original deposition of the gravels.
Cary considers that open work gravels are probably formed in fast-flowing rivers at the
downstream ends of rapidly aggrading bars. He suggests that in these situations eddies
sometimes occur, which remove finer gravels and sand, leaving only the coarse materials
which form the open-work deposits.
In arid or semi-arid climates where stream flows are intermittent, it is common to find
cemented layers in the lag and lowest bar deposits of meandering steams and of other e.g.
braided streams. This cementation occurs as the waters dry up and the most common
cements include gypsum, calcite and limonite. Cementation is relatively common also in
the floodplain deposits.
It is not uncommon to find timber, in some cases the remains of large trees, buried in
channel or floodplain deposits. The timber is often well preserved, particularly where
groundwaters are highly saline. In some situations the timber is partly or wholly rotten
and may have left gaping voids in the alluvial deposit.
It is difficult to generalise about the properties of alluvial soils, because of the extremely
wide range of soil types. The following are some observations which may be taken as a
general guide.
