The local geology, the intended use of the water collected, and where/when the water is meant to be released are all defining parts of a reservoir. Some reservoirs are single-purpose, and are only meant to fulfill one goal: irrigation, flood control, water supply, etc. In recent years however, more and more multi-purpose reservoirs are being designed to fulfill at least a few of these. Different sources will organize the ‘types’ of reservoirs based on a variety of aspects, and you can find a lot of definitive “main kinds of reservoirs,” that leave out a few classifications. Below is a fairly comprehensive list and tool for identification with all these different sources in mind.
Retention vs. Detention
This distinction is based on how long water remains in the reservoir before it’s released for treatment, distribution, or wherever it’s next destination may be. Certain purposes are benefitted more directly by a particular reservoir function, and therefore are almost always found in those specific design-cases.
Retention Basins
Permanent bodies of water that always maintain a certain level of water are considered retention, or wet ponds. Retention ponds can be a major beneficial force when it comes to sedimentation, reducing turbidity, and removing toxins and pollutants through biological forces.
Detention Basins
These are temporary reservoirs that store water in cases of flooding, or as a source of additional water or pressure during periods of peak-demand when used as a part of water-distribution systems.
Location
The water-source and natural geography is integral to a reservoir’s design and function. A reservoir can be formed by filling an excavated basin or constructing concrete embankments, depending on where and how you intend to trap and use your water.
Dammed Valleys
These reservoirs rely on the natural landscape to capture water, usually placed in a narrow valley downstream of a watershed. The river is diverted via a channel while the dam is constructed. The river is then slowly released back into the reservoir, trapping water within the dam and the valley itself.
Some reservoirs are built by making an already existing lake even bigger and entrapping the water via excavated hills or additional dams.
Coastal
These storage reservoirs hold freshwater along the coast. These typically are placed at the mouth of a river in order to contain flooding.
Bankside
Bankside reservoirs are filled with pumps or siphons alongside a river. This can be done by excavating a new basin, and/or by constructing waterproof embankments. These must be lined with an impermeable lining; even poured concrete will require an underlayment in order to be completely waterproof.
These are often used in retention ponds, as the long-term water storage allows natural forces to settle sediment and digest toxins.
On-Stream vs Off-Stream
On-stream reservoirs are located directly in the path of a watercourse, blocking the natural flow of water. This can be done by impounding the water via embankments, dams, or other forms of capture.
These will have more serious impacts on the local waterway, the animals living within the water, and the ecosystems along the lake or river’s edge. Changing the flow of water and the ambient temperature can drastically affect how fish or other aquatic animals eat, mate, and where they lay their eggs.
Off-stream reservoirs are built a distance from a nearby river or stream. Water is diverted to storage by building waterproof banks, aqueducts, or canals.
Storage vs Distribution
This distinction is made depending on what kind of water is being contained, how long it stays within the structure, and where it is distributed.
Storage reservoirs are used to maintain water supply for many uses: agriculture, domestic, energy, etc. These are typically filled via the natural flow of a river or other waterway.
- Water within most storage reservoirs is untreated and needs to be pumped through a water treatment system before being delivered to its destination if being distributed for potable uses.
- Long-term storage can occur in these reservoirs in order to maintain a consistent supply year-round. These include most retention ponds, as the long-term stay of the water inside the reservoir means that biological forces can begin to work on absorbing toxins and pollutants. This also gives time for sediment to settle or be caught in traps, reducing the water’s turbidity as well as trapping the hard metals and pollutants that can be found in floating sediment.
- Distribution or Service reservoirs collect and drain a specific amount of water, typically as part of a city’s specific water supply. This water is already treated and serves as an additional potable-water source during emergencies. These reservoirs can also be used to maintain constant pressure in distribution mains and to fill in gaps during variations in demand. Service reservoirs are often placed near the center of its distribution zone in order to reduce pressure losses caused by friction.
Surface Reservoirs
Surface reservoirs are constructed at or below ground level, therefore also known as ground reservoirs. The treated water stored in these reservoirs is pumped to elevated reservoirs from which it is supplied to the consumers via gravity and pipes.
Surface reservoirs usually contain two compartments, so that one can be used while the other is being cleaned or repaired. These are connected via control valves. Overflow pipes are positioned in order to drain at full supply level to maintain a constant level of water.
Elevated Reservoirs
Elevated reservoirs are, understandably, elevated from the ground. These reservoirs are also known as overhead tanks and can come in a variety of shapes and sizes to match the local architecture. Although, most are found in your typical city water-tower ball on legs fashion.
These are used when a surface reservoir is unable to produce sufficient head (water pressure and the resulting energy due to higher elevation). While water must be pumped up to these reservoirs, gravity assists in head during distribution.
Standpipes
A standpipe is a vertical cylindrical tank resting directly upon the ground without any supporting towers. A standpipe’s diameter ranges from 10 to 15 m and its height varies from 15 to 30 m. These are typically made from steel, reinforced concrete cement, HDPE, or other impermeable materials. Steel standpipes tend to be more common as it is very difficult to ensure concrete standpipes are watertight.
