How to calculate water losses through seepage?

Water storage reservoirs are created either by constructing a dam across a river or by just making a tank to store the water supplied from various sources. The intention behind constructing water storage reservoirs is based on the particular requirement of the person, organization or the government. The size of the reservoir varies- based on the need and can be as small as a pond to large reservoirs of capacity around millions of cubic meters.  As the industrial sector had an enormous growth during the last century, the demand for water has increased. The need to construct larger dams is the outcome of this demand. At present, agriculture sector is the largest consumer of water for irrigation followed by domestic and industrial use.

In this increasing demand for water, industries having their own water reservoirs can’t afford to have leakages especially when a large cost is involved in pumping it from the source. Water reservoirs are prone to losses through evaporation and seepage. In this article, we will discuss about seepage and its mitigation methods.

Seepage is the slow escape of a liquid through porous material or small holes.  In water reservoirs, seepage is the main crucial issue behind water losses.  Seepage generally occurs when the water escapes vertically through the bottom of the pond and horizontal filtration of water through the dykes. The water releases through such conditions is called seepage water. For effective water management of reservoirs/dams/or small ponds, seepage calculation has become crucial. The stability of any water reservoir is inversely proportional to the seepage i.e. if the seepage is high; stability is low and vice versa.

There are many factors that affects amount of seepage water from the reservoir. The reasons behind seepage are – wall of the reservoir, slope, soil type, bedrock type, dykes and volume and pressure of water.

New reservoirs have greater seepage problem

New reservoirs release more seepage water than old reservoirs. It is mainly due to the good soil structure of the new reservoir. Soil particles bind together and form the soil structure. The arrangement of soil pores between these particles influence the water movement and hence extremely important in seepage assessment study. On the other hand, a reservoir that has been filled with water for some time breaks down the soil structure. It occurs due the deposition of organic materials at the bottom of the reservoir that seals the soil pores at the bottom of the reservoir and reduces permeability.

Seepage calculation

There are two major types of seepage occurring in the artificial reservoirs. First is the horizontal seepage and the second one is vertical seepage. The amount of vertical seepage is dependent on the soil texture and soil structure at the pond bottom. For example, if the soil structure allows water to penetrate it, there will be more seepage. As discussed in the previous section, composition of soil particles forms the soil structure. If the soil is coarse or sandy, it is more permeable.  The same case occurs with the horizontal seepage. The water escapes through the walls of the reservoirs due to higher permeability induced by the materials used to build the walls, water pressure and slope.

As discussed in previous section of this article, there are various factors affects the rate of seepage. However, the material used in the construction and type of soil in the bed of the water reservoir, mainly determines the rate of the seepage. The table below gives the rate of seepage losses in millimeters per day from various soil types;

Sr. No	Natural soil type	Seepage water losses (in mm/day
1	Sand	25.00 – 250
2	Sandy loam	13.00 – 76
3	Loam	8.00 – 20
4	Clayey loam	2.50 – 15
5	Loamy clay	0.25 – 5
6	Clay	1.25 – 10

Source: ftp://ftp.fao.org/fi/CDrom/FAO_Training/FAO_Training/General/x6705e/x6705e02.htm

To calculate quantity of water escaping as seepage, following formula can be used;

Seepage water (in m³/Day) = Seepage losses (in m/Day) X Surface area of the pond (in m²)

Example;

Surface area of the pond = let’s take 1,00,000 m² (Width in meter X Length in meter)

Seepage losses = 14 mm (Soil type: Loam, by taking the average of 8 and 20)

Seepage water (in m³/Day) = 1400

 

Preventive measures to restrict seepage

There are few important methods to restrict seepage in the reservoirs.

For new reservoirs: During construction phase, use of impermeable material for the walls of the reservoir helps in restricting seepage.  Construction materials with high clay content i.e. minimum 55% restricts crack formation. This material includes organic material, top soil, decomposing material and material with high mica content.

For existing and new reservoirs-

• The upstream and downstream reservoirembankment needs to be protected by covering with topsoil and planting grasses or covering with rocks. It helps to reduce erosion.
• The base of the reservoir can be covered and compacted with clay soil. Lime can be added to make it more robust.
• Synthetic liners can be applied to reservoirs to restrict seepage. There are various Reinforced and nor reinforced liners available in the market. Liner stops the penetration of water and avoids seepage. It  is not the cost effective method but if compared with production losses of the organization due to unavailability of water (resulted by water seepage); this cost is minimal.

Note: This is only one of the methods to estimate the rate of seepage. There are many other methods that can be employed to estimate the rate of seepage correctly.

Image credit:  Mohylek, on Wikipedia