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Flooding rekindles discussions about dykes as one of the possible ways of controlling the surging waters that may cause death, destroy infrastructure and sweep crops, leading to untold suffering and hunger.

In this issue of Irrigation News, we look at these structures and their use.

Dykes are flood mitigation structures created in a naturally occurring ridge/river that prevents water from flowing beyond a certain height. These flood control structures are used to protect coastlines, riverbanks, agricultural fields, homes, urban areas and other properties. Dykes act as the first line of defence against overflowing rivers, storm surges, rising seas and oceans. They are constructed by excavated earth materials and/or sand bags.

Application of dykes

In rivers and streams, dykes are constructed in river banks to prevent the flood waters that damage properties. Examples are the Budalang’i dykes constructed on river Nzoia and the Daua dykes in Mandera.

Other types of dykes are constructed on the coastlines to secure them from water tides that might spill over and destroy properties.

The recommended standard dimensions of a dyke are as follows:

Height/dept h Top Width Bottom Width Dyke slope Remarks
Dyke jointly used as community road 1.0 (minimum) 2 to 3 6 to 7 1:2 Dyke surface is protected by sodding
Ditch for drain 0.5 1.5 0.5 1:1 Apart from 0.5 to 1.0m from foot to dyke

What to consider during construction:

  • Dykes are constructed on both banks along river/canal/stream.
  • Alignment (toe of slope) of the dyke is set apart 0.5m to 3.0m from the existing canal banks considering stability.
  • Extreme bending portion is improved with smooth alignment.
  • Embankment materials are basically obtained in the adjacent borrow pit clay soil.
  • Extra embankment (10 to 30cm height) is needed on the crown (top surface for surface ground sinkage (subside).
  • Top surface of dyke crown is inclined towards side ditches for surface drainage.
  • Side drain at the land side is provided for drainage in the land area.
  • Slopes and top width (crown) are covered using sod (turf) to protect it from flow attacking.

Lower Nzoia Irrigation Project Phase 1 currently at 65.8% completion and 85.5% of its objectives is to control the perennial flooding of River Nzoia through the construction of a dyke as well as link the Bunyala Irrigation Scheme to the Lower Nzoia gravity intake through a canal.