Aquaculture Engineering

Unit 11 - Water distribution and control structures

Water control structures or Gates

Definition
Gates are part of water distribution system in an aquacultural farm and are installed to control the water flow. Following are the basic requirements of a gate.

• It should be of sufficient capacity to handle the required flow of water during pond draining and filling.
• It should be properly located and carefully constructed taking into the account the pond bottom level and channel bed level for an efficient flow of the water and complete drainage as and when required.
• It should be made of a material which can withstand the saline action and where fouling action is minimum.
• Self weight of the gate as far as possible should be lighter but not sacrificing the structural requirements. It should require minimum additional foundation depending upon the soil characteristics and should be economical and easy to construct.
• It should be firmly situated at the bottom of pond or channel and linked with the dikes to prevent seepage and possible collapse.
• It should be easy to operate and maintain, and should have proper provision for filtering predatory fishes, their eggs and other organism.

Main gate
The main gate connects the main channel of the pond system to the source of water. In a tide fed farm, it regulates the exchange of water. It is the biggest gate in the pond system and is usually made of reinforced cement concrete (R.C.C.). Depending upon the local site conditions and on the size of farm number of main gate may be more than one. Main gates may have one or more openings, size of each opening being usually 1 to 1.2 m in width for easy handling. A gate with wider opening will require a lifting mechanism for operation of the gate. The height of the main gate depends on the combine height of highest tide and the river flood. Generally the height of main gate is taken as that of the main or peripheral dike. It is located at a place where fresh sea water can be obtained most of the time. A typical design of main supply gate is shown in fig. The various components of the main gate are floor, apron, cut off walls, side walls, wing walls, walking plat form, flash boards and screens.

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.

Secondary gates

Secondary gates regulate the flow of water from the main feeder channel to the secondary feeder channels. In many farms, they are also used to convey water from the secondary feeder channel into the ponds. Generally they are made of concrete, R.C.C., brick masonry or wood. A typical design of secondary gate is show in fig. The secondary feeder channel bed should be wider than its supply gate by at least 0.5 m on both sides to minimize scouring.

Pond inlet gates
Pond inlet gates regulate the flow of water from feeder channel into the various ponds. The two most commonly used pond gates are P.V.C. pipe made and open type. They are usually designed for allowing free fall of water from feeder canal into the ponds. A typical design of P.V.C. pipe made pond inlet gate is shown in Fig. It consists of P.V.C. elbow or bend and two pieces of P.V.C. pipes. The top edge of the elbow or bend is usually flushed with the bed of the feeder channel and at the bottom; a P.V.C. pipe is horizontally fixed into it. The vertical P.V.C. pipe piece fits into the top edge of the elbow or bend fitting and is detachable. The length of this vertical stand pipe should be higher than the maximum designed depth of water above the particular inlet gate in the feeder channel. The flow of water into the pond is accomplished by taking out the vertical pipe and is checked by pressing it to its original position. The outer diameter of both P.V.C. pipe pieces and the inner diameter of the bend or elbow should be same. The discharge capacity of this type of pond inlet gate can be computed by using the equation.

D = Internal diameter of the pipe (m)
g= Acceleration due to gravity 9.81 m/sec²
H = The vertical height of the water at its free surface in the feeder canal above the central axis of the horizontal pipe (m)
C_d = Co-efficient of discharge, value of which may be taken equal to 0.81 for length of pipe equal to 3 times the diameter i.e., for L = 3 D
C_d= 0.71 for L = 25 D, C_d= 0.64 for L = 50 D

A typical design of open type pond inlet gate is shown in Fig. It is generally made of R.C.C. or wood. Usually two rows of grooves are made vertically in the side walls, one for mounting wooden planks or cast iron plate shutter and the other facing towards feeder canal for mounting screen shutter. Flow of water into the pond is regulated by sliding the wooden planks or cast iron plate shutter vertically up or down. The free fall discharge through open type of pond inlet gate can be computed by using the following equation.

Where,
Q = Discharge through the open type inlet gate m³/sec
b= breadth of the inlet gate or width of flow (m)
H₂=Height of pond water above the bottom or floor of the gate (m)
H₁ =Height of pond water above the top of the shutter or vent opening (m)
C_d = Discharge co-efficient value of which may be taken equal to 0.62

A typical design of open type pond inlet structure
Unless the head over the upper sill is less than the depth of the shutter opening value of Q can be obtained by using the expression Q = C_d A (2gh) ^½ . The head is being measured to the centre of the opening. Sometimes pipes are used instead of rectangular vents in the pond inlet gate. The size for one ha pond inlet gate should not be less than 300mm of the pipe diameter.