Environmental Engineering Reference
In-Depth Information
In this situation there is potential for leakage, but leakage paths would be so long and
at such low gradients, that loss from the storage would probably be negligible. The
shaded area indicates where small amounts of the storage water will enter, fill voids and
cause the water table to rise locally.
It is likely that this model represents many storages in non-soluble rock areas with low
to moderate rainfall.
Model (c).
This storage is elevated well above an adjacent valley, and separated from it by
a high, broad, ridge. The water table within the ridge rises above the proposed FSL. There is
no potential for leakage from the storage to the valley, so the storage should be essentially
watertight. A small amount of storage water will fill voids above the water table in the adja-
cent valley side (shaded) and the water table will rise slightly and meet this valley side at FSL.
Model (d).
The topography and geology are the same as Model (c), but the water table does
not rise above the proposed FSL. The reasons for this may include any or all of the following:
-The climate is drier;
-The surface runoff is greater;
-
The rock is more permeable and drains more rapidly towards the valleys.
There is potential for leakage. However, because of the high rock cover and long poten-
tial leakage path, significant leakage would be unlikely. A small amount of storage water will
fill voids in the rock above the water table in the adjacent valley side (shaded) and should
cause some rise in the water table.
Model (e)
. The storage is elevated, as for Models (c) and (d), but the ridge forming its rim
is low and narrow. As a result of this, the rock beneath the ridge and below FSL is likely to
be affected by de-stressing, and hence more permeable than that at Models (c) and (d). If the
rainfall is high, as in the case shown, the water table may rise slightly beneath the ridge, but
would be unlikely to rise above the proposed FSL, as at Model (c). When the storage is filled,
seepage from it will cause the water table to rise, to at least the position of the dotted line.
There is potential for leakage from the storage across to the lower valley (see arrow).
2.12.2
Watertightness of storage areas formed by soluble rocks
As discussed in Chapter 3, Section 3.7 Carbonates and Section 3.8 Evaporites, areas
underlain by these soluble rocks often exhibit karst topography and may be cavernous to
great depths. In carbonate rocks, depths of more than 300 m has been proven at Attaturk
Dam (Riemer et al., 1997) and about 600 m at Lar Dam (Salambier et al., 1998). Ruquing
(1981) records cavities developed to about 1200 m below the Yilihe River in China.
The evaporites are more soluble than the carbonates and pose much more difficult prob-
lems in dam engineering. The factors affecting watertightness of storages are essentially the
same for both, but there have been many more dams built in carbonate areas. Therefore, the
following discussion will deal with principles and experiences from carbonate rock areas.
The carbonate rocks can occur in beds ranging in thickness from a metre or so up to
several hundred metres, e.g. see Figure 3.29, a cross section at Lar Dam. They commonly
occur together with beds of non-soluble rocks, in uplifted sequences which have been
folded, jointed and displaced and locally crushed by faulting. Such complex rock masses
will have been affected by tectonic movements and dissolution at various times during
their long geological history. In the more recent episodes they have been selectively dis-
solved and eroded, both at the surface, producing deep river valleys and underground
producing interconnected caverns and complex flow paths.
There can be many types of direct connections between surface and underground
streams. For example, during the wet season a surface river may flow into a tunnel or shaft,
flow underground for a kilometer or more and emerge as a spring, either high on the side
