Recommended for

Areas having slope up to 5%.

Presence of productive shallow aquifer/s.

Methodology

Shallow trenches break the slope at intervals and reduce the velocity of the surface runoff generated in an area. It also helps in arresting the soil erosion, increasing soil moisture and supports in enhancing the artificial groundwater recharge too.

Before construction of shallow trench, it is necessary to know the presence of the aquifer in the area.

Shallow trench is usually constructed along the contour (line of equal altitude) and perpendicular to the water flowing over the surface area. Surface runoff/ stormwater generated over the barren/ empty areas etc is captured in the shallow trenches and allowed to seep down into the soil and murum (weathered material) present below.

A pit of 1 m x 1 m x 2 m (L x W x D) is excavated using a JCB machine. Keep a berm or cushion of 30 to 50 cm (depending upon soil) after it and then excavate the next one. If shallow aquifer thickness is more than 2 meters, then the length can be increased up to 5 meters.

The pit is lined from two sides by construction of an unplastered double brick wall.

Fill the shallow trench with proper scientific filter media. Lay down boulders bed of 1 m thickness, overlain by gravel bed of 400 mm thickness. On the top add 300 mm of sand layer. Above the sand bed provide an outlet pipe in the trench to drain out excess water collected. This is helpful in cleaning the filter after the monsoon.

Depending upon the areal extent of shallow aquifer and presence of recharge area, construct such pits continuously along the contour line. Please refer to the drawing for further detail.

The excavated material from these pits is removed and disposed off properly.

Process

The shallow trenches are purposefully constructed across the slope. Hence, the runoff (stormwater) generated within the project area is captured & collected into the shallow trenches.

The stormwater consists of a heavy load of silt, clay, suspended material, etc. As soon as the stormwater starts collecting over the filter bed, the particulate matter will be filtered within sand, gravel, and pebble layers. This will also reduce the velocity of the stormwater and allow it to seep into the soil/murum/aquifer present down below.

Around 5 lac litres of rainwater will get stored and recharged if a recharge trench of 100 meters length is constructed. This is equivalent to arresting rainfall up to 100 to 120 mm.

The rate of absorption decreases after saturation of the soil moisture and aquifer and then water remains stagnant in the recharge trench.

Precautions

During heavy rainfall spells or flood prone situations, maximum stormwater should be diverted to stormwater drainage. Only a limited quantity should be allowed to flow into the recharge trenches.

Cover the entire shallow trenches with MS grill to protect from any mishaps.

Strictly follow all the prescribed technical norms while constructing the recharge trenches. The faulty construction may lead to damage to the aquifer.

Maintain the thickness of the berm between two pits. The berm is helpful in maintaining the water pressure within the trench.

Maintenance

Depending upon the presence of silt load the recharge trench will get silted during every monsoon. Hence, immediately after monsoon remove the sand and gravel layers along with the silt. Wash them properly and again lay down in the trench. After every two years the pebbles should be removed and cleaned and add again in the recharge trench.