Pond dyke - Main or peripheral dike

Unit 8 - Brackish and marine water fish farms

Marine water fish farms

Pond dyke
Main or peripheral dike
Main or peripheral dike form the boundary of the entire pond system. The purpose of this dike is to retain water for culture operation as well as to provide overall protection to the farm from floods, spring and storm tides etc,. Main dike encloses the entire farm. Its design and strength depend upon the prevailing conditions at the site. The vertical distance from the ground surface to the top of dike is the designed height for construction. This height must consider allowances for waves and floods; shrinkage and settlement and free board. The total height of the main dike above the ground level of a brackish water pond can be computed by the following formula.
Fig1
Where,
Hm = Height of main or peripheral dike (m)
Ht = Highest astronomical tide above the zero datum or mean lower low water.
Hg = Elevation of ground surface with respect to zero datum or MLLW (m)
Fa = Flood allowances
Hf = Free board
%S = Percentage of soil settlement allowance


The amount of soil settlement depends on the type of soil material used for construction, soil water content during construction, amount of compaction done, speed of construction and characteristics of foundation material. When mechanized earth moving equipments are used, 5 to 10 percent of allowances may be provided for settlement. In many coastal aquaculture ponds, however the settlement allowances used is between 20 to 25 percent. Depending upon the organic matter content in the soil, following values of soil settlement may be used.

Fig2

For soil with exceptionally high organic matter content, settlement allowance requirement may be 40 percent or even more.

Free board is provided as a safety factor to prevent over topping of dyke. It is the vertical distance from the surface of water level in the pond at its design depth to the top of the dyke after settlement. A free board of 0.6 meter to 1 meter is provided in the main dyke. A minimum of 0.6 meter free board is necessary in the case of well compacted peripheral dyke.

Side slope is necessary for stability of dyke, the flatter the slope, the more stable it is. The outer slope and the inner slope of the dyke depend upon soil texture and prevailing site conditions. The minimum required slope for different soil textures is given.

Fig3

The minimum slope of dyke at outer edge should be 1:5:1 and preferably 3:1 if it is exposed to strong floods and storm surges. Unstable soils may require 4:1 or flatter slopes. The top width or the crown of dyke depends upon the height of dyke and its purpose. Engineering Standard requires a minimum top width of 2.4 m for all dykes between 3 to 4.5 m high. Dykes between 4.5 to 6 m require a minimum top width of 3.7 m. However in actual practice most of the main dykes of a coastal aquaculture farms are built with a top width between 1.2 to 2.5 m. If the dyke is to be used as a road way a minimum of 3.7 m top width should be provided. The base width or bottom width of dyke depends upon the depth of water; top width of the dyke and type of soil. The base should be sufficiently wide so that the seepage line should not appear above the toe on the downstream side of dyke. Berms are constructed at dyke base to provide additional stability to dyke or when there is excess soil.
Last modified: Wednesday, 27 April 2011, 9:42 AM