Environmental Engineering Reference
In-Depth Information
of another valley or lower down, close to another surface river. This flow pattern might be
quite different during the dry season.
A further complication at some sites is blanketing of the valley floor by either residual
soils, or sediments deposited by rivers or lakes. Obvious sinkholes or subtle depressions
on these valley floors indicates that material is or has been eroding or collapsing into cav-
ities in the underlying karst. The river and storage area at Lar are underlain by up to
600 m of lake sediments, and before impounding the water table was about 200 m below
the river bed. Such rivers and storages are referred to as “supported” or “hanging”.
As a result of this degree of complexity, it is not possible to present a set of models for
storages in karst areas, equivalent to those on
Figure 2.45
. Also, it can rarely be safely
assumed that a karstic rock mass will become less permeable at depth. This means that
assumption i) on which the Figure 2.45 models are based, does not usually apply. It might
appear that the right abutment ridge at Khao Laem Dam fits Model (b) (see Section 3.7.2).
The ridge was formed by cavernous limestone with a dry season water table above the val-
ley floor but below the proposed FSL. However, if the dry season water level in Khao Laem
ridge had been above the proposed FSL, it would not have been safe to assume a Model
(a) (i.e. watertight) situation, unless it could be proved that the limestone:
- Became free of interconnecting cavities, and effectively impermeable, at depth, or
-Was not within a bed which emerged at a lower level, either downstream, or in another
valley.
A classification of karst river valleys has been compiled by Ruquing (1981) using expe-
rience gained during engineering works in the Yunnan and Guizhow Provinces of
Southwest China. The classification includes notes on the potential of each valley type for
the storage of water. Ruquing (1981) recognizes 5 categories of 10 types, but notes that
the classification is far from complete.
2.12.3
Features which may form local zones of high leakage, from any storage area
Some proposed storage areas underlain mainly by non-soluble rocks may contain localised
individual features of very high permeability, which could potentially act as drains. Such fea-
tures may include:
-
Buried channels, alluvial or glacial (see Sections 3.9.1 and 3.12.1);
-
Lava flows with clinker (Section 3.2.2.1);
-
Lava tubes (see Section 3.2.2.1);
-
Some open-jointed fault zones (see
Section 2.3.2
)
;
-
Abandoned underground mine workings.
It is probable that such features would rarely cause significant leakage from storages fit-
ting Models (a) or (b), because leakage paths would be too long and gradients too low.
However they could form effective drains, if located within or through the outer ridges in
Models (c), (d) and (e).
Any of these features could occur also within a storage area underlain by soluble rocks
and might influence its watertightness. It is also possible that an isolated bed of cav-
ernous, soluble rock might occur within a storage area formed almost entirely by non-
soluble rock and influence its watertightness.
2.12.4
Watertightness of storages underlain by soils
Many dams and weirs are built in the valleys of large rivers and the reservoir watertight-
ness controlled by the alluvial or glacial soils. Often the water tables are relatively high
