Aquaculture Engineering

Water control structures or Gates - Main gate 2

Unit 11 - Water distribution and control structures

Water control structures or Gates

Main gate

The gate rests on a floor which serves as its foundation. The elevation of the floor should be lower than the lowest pond bottom elevation inside the pond system of the farm. Preferably it should be lower than lowest tide to ensure complete drainage of the pond system. The gate foundation must be rigid and stable. Often bamboo poles or coconut timbers are used as piles for providing foundation for the structures. Sometimes layers of boulders and gravel are also used along with the piles. The extended and clear part of the floor upstream and downstream of the gate is known as apron. It provides protection against scouring action of flow.

Cut off walls are necessary at both ends of the gate floor if it lies over alluvial soils, for its safety against seepage and consequent piping action. Side or breast walls are used as a retaining wall for the dike fill. In the side walls, grooves are made for fixing screens and flash boards. Most of the main supply gates are provided with 4 grooves two at each sides. Usually the top of the walls are made the same height as that of the dike. Sometimes buttress supports are also built for strengthening the side walls and to reduce seepage flow along the side walls.

Wing walls in the main gate provides transition of the flow from water source such as tidal creek to the gate and then from gate to the channel. They also help in retaining the earth on both sides of the gate. Cat walks or walking plat forms are provided between the two side walls. Usually they are made of RCC slabs or wooden planks. Screen made of high density polyethylene or nylon meshes are attached to a wooden rectangular frame which fits into the two outer grooves of the gate–one at channel side and the other towards the water source. They prevent entry of predators along with incoming water and exist of cultured fish. Amount of water flowing through the gate is controlled by the flash boards. They are generally made of wooden planks of size 5 cm thick and 30 cm wide inserted into the remaining two grooves of the main gate. A one meter wide opening of the main supply gate needs a channel bed of at least 3 m wide.