Dikes and Their Necessity
Dikes in the Netherlands keep the sea out from areas inland for a length of over 350 miles. Windmill pumps are also used to remove water that has seeped through either the dike or from the ground adjacent to the dike. With this system the Netherlands have successfully created large areas of land for habitation and agriculture and have been actually doing this for centuries.
Similar dikes or levees protected vast areas of New Orleans that were affected after the lowering of the water table caused vast land areas to subside by as much as eight feet. These dikes or levees and their maintenance were very much into the news after a number of them were damaged by Hurricane Katrina in 2005.
While these structures are more or less permanent ones, dikes are also quite often built to prevent or reduce flood damage caused by overflowing rivers that may enter inhabited areas. Raising sandbag dikes is a temporary measure to stem flood waters, and it is an exercise that is very easy to undertake as it requires very little material other than bags, sand, or earth, and a sufficient number of people to fill the sand bags and then position them so that they obstruct the path of the water.
In the Netherlands the placing of very large concrete blocks in the sea through mechanized arrangements using cranes and ropeways is a method used to create permanent dikes for the reclaiming of land.
Image Source : Wikimedia: Dike
Site selection for a dike
The alignment of the dike must be such that it can create a barrier between the water source and the areas needing protection. Running the dikes on slightly elevated land would allow the height of the dike to be lower and could also help to decrease its length.
If buildings are being protected, some space must be left between the dike and the building so that foundations are not affected and to also leave sufficient working space for pumps and other maintenance activities. You also need to create a trench at the bottom of the dike to act as some sort of key as dikes can slide due to the pressure of the water against them. The width of the dike should be two to three times the height, with the height itself being determined by the expected flood level and a free board of at least two feet added to this. It would also be necessary to remove all vegetation and grass before starting the base layer for the dike.
Materials for a Dike
Bags made of polypropylene can be used and are most common. You can also use bags made of jute or other material. The normal size of bags is 26 inches by 14 inches. Sand is the easiest material to fill in the bags, though any available earth or clay in the vicinity can also be used. Besides these basic materials you would also need polyethylene sheets to help to make the dike impermeable. Having a few dewatering pumps to keep downstream areas dry would also come in handy.
Building the Dike
Estimate the number of bags required to build the dike. A four foot high dike would require about 7800 bags per 100 foot of dike if width is taken as three times the height. This is based on a volume of the bag being 5 inches height, a width of 10 inches, and a length of 14 inches. You will notice that this is much smaller than the actual dimensions of the bag, but is due to the fact that bags are normally only filled to half the capacity as it is easier to tie them up as well as easier to lift the lower weight of about 35 pounds.
Arrange separate teams to fill and tie up the bags, and other teams to carry the bags to the required location. This is easily done by forming a chain of volunteers from the filling site to the placing site, with people standing just a few feet apart. Let the bags overlap each other, with the filled portion of one bag over the unfilled portion of the previous one. Form one line of sandbags parallel to the direction of the dike. The next adjacent layer should again be staggered so that the filled portion of the bags comes at the junction of the two bags in the adjacent layer. This is similar to the way bricks are placed. Once the base layer is completed, the second layer may be placed in similar fashion with the alignment of these bags being perpendicular to the lower layer. All the layers can be completed leaving a width of at least two or three feet for the top layer.
Now lay out the polythene sheets over the water side of the dike, anchoring them at the base with a layer of sand bags. Let the polythene sheet run to the top of the dike where it can be anchored with sand bags or boards. Your dike is now ready for the expected flood waters.
Pumps can be positioned at intervals on the downstream side of the dike, and sumps may be created to help the pumping and also to observe the water level. Running these pumps may be an intermittent affair and would therefore require constant manning if the dike is to be protected. A flood situation would also require constant manning of the dike itself so that any breaches or leakages can be immediately attended to.
There are times when dikes can get breached due to storm action or weakness in the dikes. This may then require repairs or rescue of the dike in very hostile conditions with water all around. Dramatic ways have been used for such dike rescue, the most famous of them being when a ship was asked to sail into a hole in the dike created by storms, and did so successfully to close the breach. Helicopters or cranes may be used to drop sandbags into the breach in a dike till the situation is under control. If this has to be done manually, it is necessary to attack the breach from both sides and reinforce dike portions on both sides of the breach by widening them sufficiently so that they are less affected by the flood waters. The breach has then to be filled up gradually until both sides meet.
Dikes are man’s valiant effort to stem flood waters created by weather conditions over which he has no control. They are also quite often used to reclaim large areas of land which can help further expansion of towns or to create additional land for agriculture.
Image Source: Wikimedia : Floods
North Dakota State University – Sandbagging for Flood Protection
University of Florida – Building Dikes to Prevent Minor surface flooding (PDF)
How to build a dyke– An interesting video clip