Need for water harvesting at Awang Sekmai, Manipur and Artificial Recharge Techniques
- Part 2 -

L Sherjit Singh *

New Year 2013 revellers at Sekmai, Manipur on 1st January 2013 :: Pix - Deepak Oinam

I) Direct Methods

A) Surface Spreading Techniques

These methods are suitable where large area of basin is available and aquifers are unconfined without impervious layer above it. The rate of infiltration depend on nature of top soil if soil is sandy the infiltration will be higher than those of silty soil. The presence of solid suspension in water used for recharge clogs the soil pores leading to reduction in infiltration rate i.e. recharge rate. Water quality also affects the rate of infiltration. The various spreading methods are as below:-

1. Flooding

This method is suitable for relatively flat topography. The water is spread as a thin sheet. It requires a system of distribution channel for the supply of water for flooding. Higher rate of vertical infiltration is obtained on areas with undisturbed vegetation and sandy soil covering.

2. Ditch and furrows

In areas with irregular topography ditches or furrow provide maximum water contact area for recharge. This technique consists of a system of shallow flat bottomed and closely spaced ditches / furrow which are used to carry water from source like stream /canals and provide more percolation opportunity. This technique required less soil preparation and is less sensitive to silting. Generally three pattern of Ditch & furrow system are adopted (i) lateral (ii) dendritic & (iii) contour. In area of low-transmissibility the density of ditch & furrow will be high.

3. Recharge Basin

This is the most common method constructed parallel to stream channels for artificial recharge they are either excavated or enclosed by dykes and levees.. In this method, water is impounded in series of basins,. The size of basin may depend upon the topography of area, in flatter area will have large basin. The most effective depth of water in basin is 1.25 m (Baumani New York) because lesser or greater depths resulted in reduced rate of infiltration. This method is applicable in alluvial area as well as hard rock formation. The efficiency and feasibility of this method is more in hard rock formation where the rocks are highly fractured and weathered.

4 Bench terracing

Bench terracing involves leveling of sloping lands with surface gradients up to 8 percent and having adequate soil cover for bringing them under irrigation. It helps in soil conservation and holding runoff water on the terraced area for longer durations, leading to increased infiltration and ground water recharge. For implementing terracing, a map of the watershed should be prepared by level surveying and suitable benchmarks fixed. A contour map of 0.3 m contour interval is then prepared. (To be contd)

Depending on the land slope, the width of individual terrace should be determined, which, in no case, should be less than 12 m. The upland slope between two terraces should not be more than 1:10 and the terraces should be leveled. The vertical elevation difference and width of terraces are controlled by the land slope. The soil and weathered rock thickness required, vertical elevation difference and the distance between the bunds of two terraces for different slope categories are furnished

5 Contour bunds and contour trenches

Contour bunding, which is a watershed management practice aimed at building up soil moisture storage involve construction of small embankments or bunds across the slope of the land. They derive their names from the construction of bunds along contours of equal land elevation. This technique is generally adopted in low rainfall areas (normally less than 800 mm) where gently sloping agricultural lands with very long slope lengths are available and the soils are permeable. They are not recommended for soils with poor internal drainage e.g. clayey soils.

Contour trenches are rainwater harvesting structures, which can be constructed on hill slopes as well as on degraded and barren waste lands in both high- and low- rainfall areas. The trenches break the slope at intervals and reduce the velocity of surface runoff. The water retained in the trench will help in conserving the soil moisture and groundwater recharge.

6 Nalah bunds and check dams

A check dam is a small dam, which can be either temporary or permanent, built across a minor channel, or drainage ditch. Similar to drop structures in purpose, they reduce erosion and gullying in the channel and allow sediments and pollutants to settle. They also lower the speed of water flow during storm events. Check dams can be built with logs, stone, or sandbags. Of these, the former two are usually permanent or semi-permanent; and the sandbag check dam is usually for temporary purposes.

Also, there are check dams that are constructed with rockfill or wooden boards. These dams are usually used only in small, open channels that drain 10 acres (0.040 km2) or less; and usually do not exceed 2 feet (0.61 m) high.They are also called Jack Dams. Many check dams tend to form stream pools.

Under low-flow circumstances, water either infiltrate into the ground, evaporates, or seeps through or under the dam. Under high flow (flood) conditions, water flows over or through the structure. Coarse and medium-grained sediment from runoff tends to be deposited behind check dams, while finer grains are usually allowed through.

7 Gully plugging

Torrential flow of water cuts into the soil, forming rills which are gradually deepened into gullies. Gully plugs can be mechanical or vegetative barriers across the gullies to slow down the water flow and check further extension of the gully. Vegetative gully plugs are made by planting of hardy species such as chrysopogon, Vetiver or Munj grass.

8 Percolation ponds

Infiltration ponds (also called infiltration basins or percolation ponds) are large open water ponds that are either excavated or in an area of land surrounded by a bank, and normally will not exceed 15,000 m3. They store rainwater but with the main aim of infiltrating the water to aquifers where it can be extracted using boreholes, hand-dug wells, or nearby springs. They are constructed in areas where the base of the pond is permeable and where the aquifer to be recharged is at or near the surface.

9 Stream modifications

Seepage from natural streams or rivers is one of the most important source of recharge of the ground water reservoir. When total water supply available in a stream / river exceeds the rate of infiltration, the excess is lost as run off. This run off can be arrested through check bunds or widening the steam beds thus larger area is available to spread the river water increasing the infiltration.

The site selected for check dam should have sufficient thickness of permeable bed or weathered formation to facilitate recharge of stored water within short span of time. The water stored in these structures is mostly confined to stream course and height is normally less than 2 m. To harness maximum run off, a series of such check dam may be constructed

10 Irrigation methods

In irrigated areas water is sometimes deliberately spread by irrigating cropland with excess water during dominants winter or non irrigating seasons. The method requires no additional cost for land preparation because the distribution system is already installed. Even keeping irrigation canals full contribute to recharge by seepage from the canals. Where a large portion of the water supply is pumped the method has the advantage of raising the water table and consequently reducing power costs.

To be continued...