Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for
in Small Island Developing States>
PART D - ANNEXES
Design of Infiltration Galleries
Some of the main aspects to be considered in the design of infiltration
galleries are outlined in this Annex.
Figure 46. Freshwater lens and galleries on Bonriki
Isaland, Tarawa Atoll, Republic of Kiribati.
(A) shows details of
galleries and monitoring boreholes, (b) shows a cross section of the lens
based on salinity measurements in 1980.
Some additional details have also been added based on recent experiences
with galleries, particularly in the Cocos (Keeling) Islands, Indian Ocean.
Figure 47. Freshwater lens and galleries on Home Island,
South Keeling atoll in the Cocos (Keeling) Island: (A) Overall plan of
island, (B) Details in area of housing an dinfiltration galleries.
The exact layout of infiltration galleries is dependent on the boundary
of the freshwater lens and existing land uses, especially the location of
permanent structures. Hence, layouts should tend to be parallel to edges
of lenses and not perpendicular to them, except near the centre. Linear
systems are probably the best approach on islands with limited lateral
extent, and where the freshwater zone is relatively small. On wider
islands, radial or cruciform patterns may be appropriate. Examples of
layouts of galleries are shown in Figures 43 and 44, respectively, for
Bonriki Island on Tarawa Atoll, Kiribati, and Home Island on South Keeling
Atoll, Cocos (Keeling) Islands, Australia. Where possible, areas with
certain land uses should be avoided. These include heavily vegetated
areas; agricultural areas, especially where agricultural chemicals are
used or where pits are dug to the water table (e.g., taro pits); and,
areas of residential and industrial development. In some cases,
infiltration galleries must be fitted to existing land use and other
measures taken. On Home Island (Figure 47), the galleries had to be laid
between existing houses which had earlier been built over the only
freshwater lens on the island. The galleries on Home Island replaced
earlier dug wells and simple extraction systems consisting of pumped wells
with very short lateral pipes extending from their bases. A chlorination
system was necessary to ensure the water was safe for potable purposes. In
contrast, open areas are ideal for construction of galleries and include
airfields, and sports fields and recreational reserves.
Galleries should consist of some form of horizontal, permeable conduit
to allow water to infiltrate from the surrounding saturated zone. Suitable
conduits have been constructed using slotted PVC pipe; porous concrete
pipes made from no-fines aggregate; and, non-jointed hollow concrete
blocks. Conduit systems considered to be generally unsuitable are steel
pipe systems, because of possible long-term corrosion, and open trenches,
because they are susceptible to surface pollution.
Typically, infiltration galleries consist of slotted PVC pipe conduit
systems laid below the water table at or close to mean sea level. These
have the advantage of being easily cleaned if required. If PVC pipes are
chosen, pipes should be of 100 mm or larger nominal diameter to ensure
that they can be cleaned out if necessary in the future; joints should be
either of the rubber-ring or the solvent-cement type (however, as
permanent jointing is not required, pipes could also just be pushed firmly
together to prevent the entry of sediments between sockets and spigots);
pipes should be at least Class 6 water-supply grade for structural
reasons, slotted, and laid horizontally, in straight line sections to
assist with cleaning; synthetic fabric around the slotted pipes should not
be necessary and may in fact cause clogging of slots; and, caps should be
fitted at the ends of gallery pipe sections to prevent the entry of
sediments. If available, selected "no-fines" gravel should be
backfilled around the pipes to prevent the entry of fine sediments into
the infiltration pipes. However, as such gravel is often unavailable, it
may be necessary to backfill with the excavated material and "develop"
the gallery after construction. The "development" procedure
consists of pumping the gallery at a higher rate than under normal
operating conditions with the aim of drawing fine material into the pipe,
from which it can then be cleaned out. This should leave a filter zone
around the pipe consisting of particles which are too large to be drawn
into the pipe. Also, an impermeable barrier such as thick polythene
sheeting should be laid in the trench at a height of about 100 mm to 200
mm above the top of the pipe to assist in minimizing further downward
movement of fine particles into the zone around the pipe (Figures 46 and
Figure 48. Detailed cross section through an infiltration
gallery constructed from horizontal slotted PVC pipes. Vertical
concrete cylinders have been used for the pump well and access manholes.
This design was used for six galleries on Home Island, Cocos (Keeling)
Both manual and mechanical excavation methods can be utilised for
gallery construction. The choice of method, which may include a
combination of the two, is dictated by local factors including the degree
of difficulty of the excavation and the cost and availability of manpower
and equipment. Some important factors to be considered during construction
include, if possible, excavation in short stages to minimise the amount of
open trench at any given time, which reduces the amount of dewatering
required, and limits the amount of erosion of disturbed soils and possible
contamination of the water source. Care must be exercised in the use of
dewatering pumps as over-pumping can lead to local intrusion of saline
water; pumping rates should be just sufficient to enable excavation below
the water table, and subsequent laying of conduits and backfilling
operations. Also, salinity tests should be performed regularly as
excavation advances to ensure that significant salinity increases are not
detected either due to encroachment into higher salinity groundwater zones
or to dewatering at inappropriately high rates. Tests should be conducted
at about 20 m intervals during excavation. If high-salinity areas are
detected, the gallery alignment or overall length may require adjustment.
Conduits should be laid at such a level both to prevent dewatering
during low tides and saltwater intrusion during high tides or periods of
peak freshwater use. These problems may result if the conduits are too
high or too low. It is suggested that the inverts of conduits should be
set at between 100 mm and 300 mm below mean sea level (MSL), depending on
the head of water above mean sea level (Figure 48). No allowance for
possible long-term sea level changes is suggested because of uncertainty
of the magnitude of changes. If sea level does rise, as is most likely
within the next 50 years, infiltration gallery conduits would be slightly
lower within the freshwater lenses than at present. This should not
necessarily be a problem, particularly if there is an accompanying
increase in precipitation which could increase the thickness of the
freshwater lens. A more substantial effect on some small low-lying islands
is the possible long-term inundation of certain areas. However, if the
inundated areas are away from the freshwater lenses, the amount of
disturbance could be minimal.
Based on Darcy's Law for groundwater flow, an approximate
solution method was developed for estimating the horizontal length of
gallery systems for the Cocos (Keeling) Islands (Falkland, 1988). The
length of a gallery, Lg (in metres), can be expressed as:
Lg = W.Qg / 8.k.D.h (1)
W = width of extraction area (m);
flow from gallery (m3/day);
k = permeability (m/day);
= approximate mean thickness of freshwater zone (m); and,
allowable drawdown (m).
The extraction area for a particular gallery is assumed to be that area
of the lens influenced by the gallery. Based on the areal extent, A (with
length, L, and width, W), of an assumed extraction area, the length of a
gallery, Lg, can be equated to L - W. This implies that gallery is shorter
than the length of the extraction area, L, by half the width (W/2) at each
end. Within this area, the extraction or pump flow from the gallery, as
defined above, should be proportional to the sustainable yield of the lens
divided by the lens area. This can be expressed as:
Qg = A.QS / AL = (Lg + W).W.
Qs / AL (2)
A = extraction area (m2) = W.L
sustainable yield of entire freshwater lens (m3/day or
AL = area of lens (ha).
Methods to assess the sustainable yield of freshwater lenses are
outlined in Annex 4. The length of gallery, Lg, can thus be
obtained by simultaneous solution of equations (a) and (b) for a given set
Pipe slots should allow the passage of water but should minimise the
passage of sediment particles. It is recommended that the slot width not
exceed 1.5 mm. Entrance velocities at the slots should be below a certain
maximum value to prevent clogging. Huisman (1972) mentions that, for the
design of radial collector wells, which are similar in design to
horizontal gallery pipes, the maximum entrance velocity (v) may be
calculated using the square root of the permeability (k) of the
surrounding aquifer material divided by 60 (both v and k are expressed in
m/sec). This design criteria is considered too conservative in most cases.
Rather a velocity of double this value, which is often used for permanent
vertical well screens, is suggested. An example of a slot pattern for 225
mm PVC pipes used for the Vaipeka gallery extension on Aitutaki is shown
in Figure 49, and Figures 46 and 47 provide an outline of design options
that have been found useful in galleries on Tarawa and Home Islands.
It is also good practice to install manholes (or access holes) at
spacings not exceeding 100 m to enable conduits to be desilted if
necessary. Manholes should be about 1 m in diameter and be constructed
using suitable, corrosion-resistant materials such as prefabricated
cylinders made from fibreglass, concrete, ferrocement, or, where
available, local materials such as coral slabs. Joints between pipes and
manholes, and between manholes and manhole bases, should be well sealed,
preferably with concrete, to prevent water from entering the system; and,
manholes should be fitted with sealable lids. The top of the manholes can
be covered if required. Further, in addition to the manholes which are
primarily installed for inspection and maintenance purposes, it is useful
to install testing points at selected locations by inserting PVC tees into
the gallery pipes, and extending the tee to the surface. To allow the
entry of a salinity probe, the pipes should be about 50 mm in diameter.
Generally, one pump per gallery is sufficient. Pump wells or sumps can
be constructed using the same materials as for manholes. Joints between
the pump wells and incoming infiltration pipes and the pump well base
should be properly sealed, as for manholes. These measures are important
to ensure that the only water entering the system is through the slots in
the pipes, and, hence, to prevent local overpumping and consequent
excessive drawdown, and contamination from surface sources. Pump wells
should be properly covered to exclude animals, birds and sunlight, and the
rims of pump wells should be set about 0.5 m to 1.0 m above ground level
to prevent local inflow during heavy rainfall to minimise the chances of
contamination. The base (inlet) of pump suction well should be set so that
it is at least slightly above mean sea level, although, in some cases, it
may be necessary to set the inlets slightly below mean sea level to
prevent the pump running dry at very low tides. Float switches should be
set to turn the pump off when the water level approach the base of the
well to prevent the pumps from sucking air
The types and sizes of pumps are dependent on the required yield, the
head, and the length of the gallery. There is a trade off between pump
rate per gallery and the number of galleries. The product of these two
quantities must be no greater than the sustainable yield. Hence, it is
good practice to set the pump rates at a rate somewhat lower than the
calculated rate until the effects of long-term pumping have been
monitored. Also, to minimise disturbances to the freshwater lens, pumping
should be continuous rather than intermittent, although this may not be
possible if solar pumps (with no battery back-up) are utilised. In this
case, pumping should be continuous throughout the pumping period. The
prime consideration is to minimise the drawdown in the well due to the
pump to prevent seawater intrusion. Mather (1975) suggests a maximum
drawdown due to a pump of 30 mm (0.03 m) except for very thick freshwater
lenses. Drawdowns of between 30 mm and 50 mm may be considered reasonable
for most lenses and were used in the design of galleries in the Cocos
(Keeling) Islands (Falkland, 1988). Similarly, there have been a number of
approaches to the selection of pump rates for galleries.
Extent of Use
For freshwater lenses in the Trust Territory of the Pacific Islands (now
Federated States of Micronesia), Mink (1986) suggested a pump rate of 0.10
gal/min/ft (or 1.8 m3/day/m) of gallery. Examples of pumping
rates from operational galleries on coral atolls include:
- A total pumping rate from seven Kwajalein galleries with a combined
length of 2 120 m of about 340 m3/day in 1980 (Hunt and
Peterson, 1980; Peterson, 1984) -- This pumping rate is equivalent to 48
m3/day per gallery or 0.16 m3/day/m of gallery.
Using the sustainable yield estimate of about 520 m3/day
(Peterson and Hunt, 1981), the upper pumping rate for the galleries
would be approximately 0.25 m3/day/m of gallery.
- A total pumping rate in 1992 from 23 Tarawa galleries at Bonriki (17
galleries: Figure 46) and Buota (6 galleries) with a combined length of
about 6 900 m of about 1 300 m3/day, which is slightly less
than the estimated sustainable yield (Falkland, 1992) -- This equates to
an average pump rate of 56 m3/day per gallery or 0.19 m3/day/m
of gallery. On an areal basis the pumping rate at Bonriki is equivalent
to about 7.3 m3/day/ha, based on the total area of the lens.
- A total pumping rate from six galleries on Home Island, Cocos
(Keeling) Islands (Figure 47) of approximately 110 m3/day which is
equivalent to the sustainable yield (Falkland, 1992) -- This pumping
rate equates to 19 m3/day per gallery or 0.06 m3/day/m of gallery.
From these data, it can be seen that the average pumping rates for the
galleries on Kwajalein and Tarawa are approximately the same, while that
for Home Island is about one third of that rate. It should be noted that
the freshwater lens on Home Island is much thinner than at the other two
locations. Examples of pump rates from galleries on islands other than
atolls include one from the Pacific and one from the Caribbean:
- A total pumping rate from the Vaipeka gallery on the island of
Aitutaki, Cook Islands (Figure 49), of about 480 m3/day or about 3.4
m3/day/m of gallery -- This is much higher than the rates for atolls as
there is a sizeable catchment area for the gallery. The original gallery
was constructed parallel with the shoreline at the base of a volcanic
rise, 140 m in length, and a total pump rate, shared between two pumps,
of about 5.5 l/s. The extension to the gallery is 130 m and will
initially have one pump rated at about 3 l/s (Falkland, 1994, 1995). A
second pump of the same capacity may be installed on the extension if
monitoring proves this to be feasible.
- A total pumping rate from the open trench galleries on the limestone
islands of New Providence and Andros, Bahamas, of 120 m3/day per 300 m
gallery (Lloyd et al., 1993) -- This is equivalent to 0.4
m3/day/m of gallery. However, recent modelling suggests that this pump
rate is too high and that an upper pumping limit of 58 m3/day per 300 m
of gallery (or 0.19 m3/day/m of gallery) should apply (Lloyd et al.,
1993). This lower value is very similar to the rate adopted for Tarawa.
From the above discussion, gallery-pump rates for atolls and small
limestone islands should probably be less than 0.25 m3/day/m of gallery,
and, in the case of very fragile lenses, should probably be less than
about 0.10 m3/day/m of gallery.
Tony Falkland, ACTEW Corporation, Australia
Falkland, A.C. 1988. Cocos (Keeling) Island Water Resources and
Management Study. ACT Electricity and Water Report No. HWR88/12, ACT
Electricity and Water, Canberra. 211 pp + appendices.
Falkland, A.C. 1992. Review of Tarawa Freshwater Lenses, Republic of
Kiribati. ACT Electricity and Water Report No. HWR92/681, ACT
Electricity and Water, Canberra.
Falkland, A.C. 1994. Review of Vaipeka Gallery Extension and Water
Supply Needs. Australian International Development Assistance Agency,
Falkland, A.C. 1995. Vaipeka Water Gallery Extension Project,
Aitutaki, Cook Islands. Australian Agency for International
Development, Canberra. 62 pp + appendices.
Huisman, L. 1972. Groundwater Recovery. Macmillan, London.
Hunt, C.D. and F.L. Peterson 1980. Groundwater Resources of
Kwajalein Island, Marshall Islands. Water Resources Research Centre
Technical Report No. 126, University of Hawaii. 91 pp.
Lloyd, J.W., Yu Yong Hong, and D.W. Peach 1993. The Management of
Groundwater Abstraction in Islands Using Trenches: A Bahamian Example.
In: Study and Modelling of Saltwater Intrusion into Aquifers,
Proceedings of the 12th Saltwater Intrusion Meeting, Barcelona. pp.
Mather, J.D. 1975. Development of the Groundwater Resources of Small
Limestone Islands. Quarterly Journal of Engineering Geology,
Peterson, F.L. and C.D. Hunt 1981. Groundwater Resources of
Kwajalein Island, Marshall Islands. Water Resources Research Centre
Technical Memorandum Report No. 63, University of Hawaii. 38 pp.