Infiltration Galleries - Subsurface Sources of Water for Water Supply

An infiltration gallery is a horizontal or nearly horizontal tunnel with permeable boundaries that are constructed near highly permeable aquifers to permit groundwater to infiltrate into the same. 

Infiltration chambers - LID SWM Planning and Design Guide (sustainabletechnologies.ca)

These structures are rectangular in cross-section and are called horizontal wells.

Infiltration Galleries - Subsurface Sources of Water for Water Supply
Image Credits: Prodyogi

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Fig. 2. below shows the cross-section of an infiltration gallery that is constructed near a river stream. It is generally constructed in highly permeable aquifers with a high water table so that an adequate head is available for the gravity flow of groundwater into the gallery. 

Fig.2. Plan of Infiltration Gallery

Based on how infiltration galleries are laid, they can form different types as shown in Figures 2 (a) and (b).

Working and Construction of Infiltration Galleries

1. Infiltration galleries are constructed near a permeable aquifer with a high-water table and a continuous recharge of perennial flow.
2. They are laid parallel to the riverbeds at depths of 3m to 6m.
3. These galleries collect water by intercepting and collecting water through gravity flow.
4. The horizontal pipe may be made of vitrified clay or concrete or brick of 0.5 m to 1.5 m in diameter.
5. The pipe in the infiltration gallery is perforated and is surrounded and packed by means of a permeable boundary which is formed by a 3-layer graded filter. This graded filter is made of:
1. 16 cm ballast (25 to 50 mm size) around the pipe.
2. 16 cm large pebbles (12 mm to 24 mm) form the intermediate layer.
3. 24 cm pebbles (3mm to 10 mm size) forming the outer layer.
6. The water enters the tunnel either through the side or bottom.
7. The purpose of filter arrangement in varying sizes of aggregates is to allow clear water percolation with low entrance velocities.
8. The gallery is laid at a slope leading to a central collecting shaft, from where the water is pumped to the surface for use or distribution.
9. For long-length distribution from infiltration galleries, manholes can be provided at intervals of about 100 m for inspection and maintenance.

Calculation of Flow/Yield Capacity of Infiltration Gallery - 

The analysis of flow towards the infiltration gallery placed parallel to a stream can be calculated as:

q = k [H² – H_o²]/2L

Fig.3. Calculation of Flow into Infiltration Gallery - Pictorial Representation

Where, 

L = distance between the gallery and the source

H = Heigh of the saturated zone at distance L

Ho = Depth of water in the gallery

K= Coefficient of permeability

q = discharge passing through the vertical section of the gallery per unit length of the gallery.

The equation is derived as per Darcy Law and accounts under the following assumptions:

1. Soil is isotropic, homogeneous, and incompressible
2. The tangent of the angle of inclination of the water table i.e., the slope is equal to its sine
3. The flow is uniform and horizontal throughout the depth of the aquifer.

The yield capacity doubles, if there is a water table on the other side of the tunnel.

Features of Infiltration Galleries

1. One or more galleries are constructed that together connect to a central point like a spring box or a hand-dug well. Hence these center-point water collection structures are called collector wells.
2. Infiltration galleries are constructed in combination with other means of water supply systems so as to meet the increasing water demand as the gallery alone cannot meet the demand of a big population.
3. They are constructed in a way that is prevented them from getting contaminated. A safe distance of generally 30 meters is maintained from latrines and any contaminated areas. The safe distance varies with the site and location of the gallery.

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