Solai dam on May 9 2018 burst amid heavy rains leading to loss of lives, others nursing serious injuries as well as destroying property worth of millions. In another incidence, a dam used to irrigate flower in Limuru broke down its embankment discharging water downstream to the valley of river Karura stretching 3 kilometres. This leads to the question: what causes failure of earthen dams and what should we know more about these types of dams?
Dams can be classified on the basis of the function and use, structural design, materials of construction, shape of cross section and hydraulic design. Most of the dams constructed are earthen dams since they are made of locally available materials like clay, gravel, sand and silt. They are much cheaper to construct compared to gravity dams which require cement and steel while also they are favorable to a variety of sites unlike gravity dams.
When dam bursting disasters happens, the need for more dams to be constructed can be wished away, which should not be the case. Kenya’s Vision 2030 provides concrete ideas on the infrastructural programmes for construction of earth dams. Water, like other liquids, exerts a lot of lateral pressure wherever it is contained, always pushing outward and thus the requirements to adherence of standards for earth dams construction.
Earthen dam design follows a sequence of steps which includes obtaining geotechnical, topographical and hydrological data. This involves calculation of daily water use requirement, calculating required dam reservoir storage volume, collecting monthly rainfall data, estimating catchment area water outflow, estimating evaporation and seepage losses. Based on the information obtained, the dam’s structural specifications such as dimensions, height of embankment, slopes, crest width spilling capacity and volume of earthworks are determined based on selected dam site topography.
Depending on the size of the dam, an environmental impact assessment (EIA) should be carried out to determine, analyse and present the environmental impact, formulating remedial measures to mitigate the negative impacts.
How Earth Dams Are Constructed
Earth dams, are built by excavating on the path of a river then piling lumps of soil to build the walls around a reservoir, which would arrest rain or run-off water. The blockage, also known as the bank, is strengthened by compacting the soil. It is advisable to carry out a test to determine if the soil in use has the required bearing capacity, and whether it is moist enough to ensure a strong bondage.
Since the walls are simply made by piling additional layers, the piles tend to be wider at the base and thinner as the dam grows taller. The risk of collapse when the water levels are low to medium is lower because the walls are thicker, meaning there is high absorption capacity. But when the dam fills up and water begins overflowing, the thin brim is quickly compromised to give way, spilling over at the lowest point.
The entire weight of water is then exerted at this point to forcefully tear the wall all the way down. Once the water has found a point of weakness, it is not possible to contain the force with which it will find its way out. A properly built dam should have a spillway that is below the highest point of the wall, to evacuate excess water during flooding. Engineers also advise that walls should have grass to increase stability of the dam and prevent erosion. Walls should also be steeper on the outside than inside. A 33-degree slant is recommended for the inner side, against 20 degrees downstream.
Causes of Earthen Dam Failure
The main causes of dam failure include extreme rain volumes, design error, geological instability caused by changes in water levels during filling or poor surveying, poor maintenance especially of outlet.
Some of the modes of failure caused by hydrological conditions includes overturning of a dam section which takes place when the resultant force cuts the base of a dam. Another mode of failure is overtopping which occurs when the water level in the reservoir exceeds the height of the dam and flows over the crest. To avoid overtopping adequate spillway and outlet should be provided.
Hydrological failure can also be as a result of overstressing of structural components which may be as a result of reservoir surcharge; thus spillway of sufficient capacity to pass peak flood should be provided. Also side slopes opinion upstream and downstream should be flat enough so that the shear stress induced in the foundation is within the shear strength of the materials used. Lastly Erosion of earth spillways can also result to dam failure thus upstream slope should be protected against erosion by wave action while the crest and downstream slope should also be protected against erosion.
Depending on the size of the dam, dam failure can lead to destruction of property or even loss of lives. Routine deformation monitoring of seepage from drains in and around larger dams is necessary to anticipate any problems and action taken before structural failure occurs. Most dam designs incorporate mechanisms to permit the reservoir to be lowered or even drained in case of such problems.
This article first appeared in the Hortfresh Journal, July – August 2018 Edition