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Wave revetments for inland dams, ponds and lakes

Water is an intrical part of our lives. From an aesthetic point of view it is amazing to have a pond of some sort in our living vicinity, it creates a park atmosphere and facilitates relaxation and stress release. It is also nice to notice the variety of wildlife that congregates around and makes use of such ponds / dams. An entire ecological community in our presence to value and stand in awe of. From a building and civil aspect, dams and ponds form an intricate part of storm water attenuation. These structures are essential for the survival of our infrastructure and instead of just having the structure grey and drab it is endeavoured to blend it into the surroundings and amalgamate it aesthetically into the overall scenery. Agriculture relies heavily on dams and lakes for the supply of water to game and livestock.

In all the various aspects where dams, ponds and lakes are used, these structures have their own set of design requirements and coefficients that need to be considered. However, to some degree, the problem of wave action on the banks of such structures is common to all applications.

Revetment structures of various types are employed to address this problem. There are two essential requirements that need to be considered when designing wave revetments on earth slopes for inland dams (Watermeyer, 2003):

  The revetment must be designed with a stable toe to prevent sliding or base unravelling.
  Slope stability beneath the revetment, especially under rapid draw-down conditions.

The appropriate design of gabion revetments can address both issues as mentioned above.

Depending on the nature and size of the dam, pond or lake, an appropriate wave theory needs to be decided upon. A choice exists between calculations related to (Shore Protection Manual, 1977): a) Shallow water gravity waves, or b) Deep water gravity waves. Generally it is the Deepwater gravity wave calculations that are involved in the design of wave revetments (Watermeyer, 2003).

Factors that need to be considered when designing wave revetments include (this is not an absolute list) aspects such as Significant wave heights, Wind set-up, Wave energy, Wave run-up, Wave run-down, Wind waves along rivers and canals as well as Effective fetch, Fe, for calculation of Hs.

Where:

Fe = Effective fetch in KM, allowing for shore retardation of development of wave heights.
Hs = Significant wave height in m = average height of the highest one-third of the waves in a wave spectrum and is only equalled or exceeded by 13% of the waves generated by a particular wind speed (SANCOLD Report No 3, 1990).

Rock Rip-Rap is often used for wave revetments. The Hudson Formula is used to determine the size of rock used in the rip-rap. The Hudson Formula predicts realistic behaviour of rip-rap compatible with field observations (Watermeyer, 2003). Rip-rap layers should be placed over a filter layer. River mattresses can be used to cage the rip-rap rock and this structure can then be used in

conjunction with the filter layers to form a structurally functional wave revetment. It is recommended that the long axis of rocks used for rip-rap not be more than 2,5 times the short axis (Watermeyer, 2003). This compares favourably with the standard of rock that is recommended for River mattresses (rock not being more than 2/3 the height of the basket).

The under and filter layers below the rip-rap are of importance when designing wave revetments. The permeable layers should comprise of a gravel (which can be crushed stone) over sand (which should preferably be silica) filter. They filter under layers are important for:

  For pressure relief purposes and for dissipation of water hammer type pressures caused by wave impacts.
  For prevention of loss of fine material from the earth embankment which can otherwise be pumped through the filter fabric.

It is recommended that the permeable filter layer be compacted to 98% Proctor maximum density. All layers should be compacted by vibration (Watermeyer, 2003).

When considering geofabrics as filters, it should be remembered that the greater the flow conveyance of the combined first underlayer and filter layers, the greater the stability of the rip-rap. Geofabrics do not provide a good substitute for such flow conveyances as a stabilising factor. The size of the underlayer that is placed on the filter fabric should be such that it does not damage the fabric during movement of the rip-rap under wave action (Watermeyer, 2003).

The use of river mattresses to encase the rip-rap rock will contribute to preventing the movement of the rock under wave action and thus contribute to the protection of the filter fabric below the mattress.

As with all gabion and river mattress applications the filter fabric must be capable of preventing the loss of fine materials from the embankment earth fill without becoming blocked and thus impermeable. It must be remembered that in during wave action there is a two-way flow. If the geofabric becomes blocked it will be subject to lifting during wave draw-down. As with all fabrics, if the embankment is comprised of disersive clayey materials, then a suitable sand filter should preferably be used (Watermeyer, 2003).

Geofabrics (of the non-woven needle-punched polyester type) can have very low coefficients of sliding friction and may introduce potential planes of sliding failure on steep revetment slopes (Watermeyer, 2003). The use of a river mattress could alleviate this problem. With the rock packed tightly into the mattress compartments, the entire mattress can be anchored into the embankment at the top end with "Y" fencing standards. This will prevent the river mattress from sliding down the embankment.

Ultimately the filter requirements associated with wave revetments are much more burdensome than the filter requirements associated with many one-way flow drainage conditions (Watermeyer, 2003).

Unlike with concrete-filled and cement-stabilised soil, wave revetments where the permeable filter layer must drain into the reservoir at the toe if the revetment (Watermeyer, 2003), when river mattresses are used in wave revetments, the entire face of the revetment is permeable, enhancing the flow conveyance of the wave action in both directions.

When the River Mattresses are filled appropriately (over filled slightly as per the Land Rehabilitation Systems installation guidelines), the rock is held tightly in position. This will prevent movement of the rock during water hammer type pressures caused by wave impacts. Should the rock not be packed properly, the resultant movement of the rock could cause the rock itself to be eroded slightly and this will affect the functioning of the revetment. Further as the rock moves within the wire basket it rubs the galvanising off the wire exposing areas where accelerated corrosion of the wire can take place thus affecting the life span of the material and the entire structure.

Watermeyer, C.F. 2003. Wave Revetments for Inland Lakes. Journal of the South African Institution of Civil Engineering, 45(4), Pages 10-17, paper 559.



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