News letter Dam edition

Ingeokring Newsletter
8.2 Effects of dissolution below the canals and the irrigation
area
During the helicopter survey made in June 1986 it was observed
that at many places where gypsum is present near
the earth surface (for example near the camp site), sinkholes
of varying size (0.1-3 m diameter) are present (Photos 5 and
6).
It may be expected that dissolution along the main canal
and primary and secondary canals also could take place
giving rise to leakage and surface subsidence.
The situation below a canal can be analysed as follows
(Figure 14, from James & Lupton (1978)):
dM
- = K dA(Cs
- C)
dt
where M is the amount of gypsum dissolved at time t, A is
the surface area of gypsum in contact with fresh water and C
is the concentration of dissolved gypsum in the water (since
this is constantly refreshed this may be taken as zero).
The linear retreat of the gypsum surface below a canal may
also be written as -dZ/dt (see Figure 14) and the amount of
gypsum dissolved per unit of time:
dM dZ
- = .A.ρ
dt dt
Combining (6) and (7), taking C=0:
dZ K d C s
- =
dt ρ
Again K d and C s have to be estimated and depend on flow
rate, temperature and concentration of sodium chloride in
the groundwater.
Example 1. In the case of a silt layer between massive gypsum
and the canal (Figure 14):
Take K d =10 -4 m/s and a low hydraulic gradient, then v~10 -5
m/s. If the silt contains salts, then the groundwater may
have dissolved sodium chloride (NaCl) in it and estimates of
K d =5x10 -5 m/s and C s =10 kg/m 3 may apply. This gives
(Equation 8):
dZ -4
5 × 10
-
- = = 2.16 × 10
7
dt 2320
(6)
(7)
(8)
= 6.8m / year
Example 2. If no sodium chloride is present, then K d =10 -5 m/s
and C s =2.5 kg/m 3 and
dZ 2.5 × 10
-
- =
= 1.08 × 10
8
dt 2320
Example 3. If free flowing canal water is flowing directly
over a gypsum rock surface, with a velocity of 0.5 m/s, then
at 23 °C: K d =4x10 -5 m/s (James & Lupton, 1978). Since hardly
any sodium chloride is present in Wabi Shebelle water,
C s =2.5 kg/m 3 .
-5
-5
dZ (4 × 10 ).(2.5)
-
- =
= 4.3 × 10
8
dt 2320
These values are high and show that protection against contact
with fresh water is necessary. They also explain why
sinkholes may develop rapidly in the wadis of the irrigation
area during the rainy season, the only time when water is
flowing over these areas. Consider a locality where silt containing
some sodium chloride salts is present above gypsiferous
rock (the conditions of example 1). Say a localised
flow of water during the rainy period occurred for a period
of two weeks, then a retreat of the underlying gypsum surface
of 6.8 m/year x (2/52) = 0.26 m could occur. This is sufficient
for a small sinkhole to develop (Photo 6).
These calculations can be considered as examples. Comparable
calculations for different rock types are shown in Table
1. The calculated solution rates are regularly confirmed by
observations and reported in literature (James et.al., 1981).
Using actual site measurements of flow velocities and electrical
conductivity measurements of the water and groundwater,
these could be tailored to the real situation.
8.3 Design of monitoring program
= 0.34 m / year
= 1.36m / year
Before the actual construction starts, many data can be
gathered and monitored that would aid in the refining of
the prediction of dissolution behaviour of gypsiferous rocks
and soils under hydraulic structures in the irrigation area.
Monitoring of piezometric levels in the piezometric stations
at the dam site, together with temperature and electrical
conductivity measurements will give information on hydraulic
conductivity and also whether there is any fluctuation of
the salinity of the groundwater. Also the variation in C
(solubility) over the site and in the irrigation area can be
obtained this way (by similarly measuring the electrical conductivity
of the groundwater in other boreholes).
During and after construction it is highly advisable to monitor
the groundwater, to be aware of any changes in conditions.
After construction, the groundwater should be con-
Dam edition | Double Issue 2007/2008 | 36