DAMS DEMYSTIFIED
What is a dam?
A dam is a structure or barrier constructed across a river or a stream to store water on its upstream side, forming a reservoir or a lake. The reservoir or lake created by dams not only suppress floods but provide water for various needs, including irrigation, water supply, hydropower generation, recreation and navigation. The side where water is stored is called upstream side while the other is the downstream. When the structure is 15 metres or more in height, it is referred to as a large dam, while small ones are less than 15 metres in height.
Types of Dams
Dams are majorly classified by the materials they are made of as described below:
Fill or Embankment Dams
The construction materials are basically soil (sand, loam, clay, and so on) and rocks, with a trapezoidal or nearly trapezoidal cross section. The dam is built by compacting successive layers of earth/rock using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides. Fill dams are further categorised as shown below depending on the materials used for construction:
| Category | Type | Description |
|---|---|---|
| Earth fill Dam | Homogenous | The embankment is constructed using homogenous earth material and is usually economical and rapidly constructed but usually of low strength |
| Zoned Type | Earth material is used for the centre of the embankment, sandy filter materials and rock fill materials are used for the upstream and downstream sides | |
| Rock fill Dam | Earth Core | Earth material is used for the centre of the embankment, sandy filter materials and rock fill materials are used for the upstream and downstream sides |
| Concrete | Embankment consisting of only rock materials is covered with concrete face slab on the upstream slope | |
| Asphalt Faced | The upstream slope of the rock fill dam is covered with an asphaltic membrane |
Concrete Dams
The construction materials are mainly concrete made by the conventional method of placing concrete in blocks or by the recent developed methods where slumpless concrete is compacted with vibratory compactor, and covered with conventional concrete. Concrete dams are further categorised with reference to the behaviour of the structure in resisting the forces imposed on it and the main categories are summarised below:
| Type | Description |
|---|---|
| Gravity Dam | These are solid concrete structures that maintain stability against design loads from and the mass/strength of the concrete and its geometric shape |
| Buttress dam | This is usually an upstream structure or wall constructed with a sloping concrete slab supported on a number of buttresses on the downstream side. |
| Arch Dam | An arch dam is a curved with the convex portion facing upstream. It resists major portion of water pressure by arch action which transmits most of horizontal water thrust behind them to the abutments by the arch action. |
Other structures necessary for dams
River Diversion This structure is constructed to enable satisfactory handling of the stream flow during construction and is vital to the success of the work. The different types of diversion systems involve the construction of tunnels, concrete culverts, steel pipeline and open channels.
Spillway This structure is provided to dispose of surplus floodwater safely which cannot be stored in the reservoir. Spillways are invariably provided in all the dams and often called safety valve for the dam. It is necessary to provide a spillway of sufficient capacity so as to avoid water from overtopping the dam. Overtopping may lead to failure of dam resulting in serious damage to the property.
Outlet Works This structure is necessary to make provision for the discharge of water as and when it is required for irrigation, water supply, hydropower etc. It is normally constructed by providing a pipe passing under or through the dam, with discharge controlled by valves.
Typical figure showing an embankment dam and its associated structures
Studies necessary for dams
Hydrological studies to evaluate the long- and short-term stream flows, rainfall and storage reservoir characteristics of the proposed dam and its catchment;
Geological studies to determine the effect of the location and construction of the foundations, abutments, dam itself, and spillways. Investigations are carried out to determine the degree of permeability of the rock formation on which the dam is to be placed which may affect the hydrostatic uplift pressures as well as to determine the degree of grouting necessary to control possible foundations seepages;
Geotechnical studies to evaluate the availability of earth and rock materials and their suitability for dam construction or as concrete aggregates;
Environmental studies to evaluate the short- and long-term effects of the proposed reservoir and the construction operations on the region aesthetics, plant and animal, ecology, fish preservation and public amenity of the reservoir and downstream reaches; and
Studies to identify ideal sites for the auxiliary structures for the dam such as the diversion system, spillway and outlet works.
By Eng. Andrew K. Mutua (Koei Africa Ltd)