2.14.1 Design Procedures – Batter Drains
A four step process is provided for the design of batter drains:
Step 1 – Dimensions
Batter drains should have a minimum depth of 300 mm.
Hydraulic capacity of a batter drain is normally defined by the allowable head water level upstream of the drain’s inlet.
Step 2 – Foundations
The lining of the batter drain should be adequately anchored to the foundations to avoid slippage or separation, with a maximum distance of 3 m between anchorage points.
In cases where prefabricated units need to be bolted together, it is important that all bolt holes are sealed with a flexible sealant to allow for flexural movement.
Step 3 – Inlet design
The inlet area should be protected against possible scour resulting from accelerating flow velocities (usually more important on temporary batter drains). This protection is necessary to prevent water from either undermining the top of the batter drain, or being diverted along the edge of the lining (the most common cause of failure).
For temporary batter drains (i.e. drainage chutes) and during the early revegetation stage of permanent batter drains, sand or gravel bags can be used to direct inflow towards the centre of the chute.
Step 4 – Outlet design
Typically, an energy dissipator will be required.
The outlet may consist of a bed of nominal 150 mm rock (minimum) placed with a minimum bed thickness of 250 mm or at least 1.5 times the maximum rock size. Typical dimensions of the rock bed are:
|L||=||6De metres long (minimum)|
|W||=||T + 0.6 metres wide at the batter drain outlet, expanding to T + 0.5L + 0.3 metres at the end of the dissipator.|
|De||=||equivalent pipe diameter (m) of the batter drain flow area|
|T||=||top width (m) of flow in the batter drain|
Batter drains should be lined each side with a minimum 300 mm wide (turf) grassed filter strip or rock to control side erosion caused by splash. In areas where the total disturbance is to be minimised, or where introducing turf is undesirable, other forms of erosion control such as geotextiles or concrete may be preferred.