6.3. Engineering Structures

6.3.1. Catchments

A catchment is an area of land, usually surrounded by mountains or hills, where all water that falls, flows to a common point. When rain falls on the ground in a catchment, it flows by gravity to the ocean or a lake. If the water flows on top of the ground it is called run-off. If it soaks into the ground it is called groundwater. Runoff and groundwater flow into waterways like creeks, rivers, lakes, lagoons and wetlands. This water eventually flows into the ocean. Within catchments, dams are built on rivers to store water and give us a more permanent water supply. Ten millimeters of rain falling on 100 square meters of catchment surface equals 1,000 liters of water; and ten millimeters of rain falling on 1 hectare of catchment surface equals 100,000 liters of water. A water harvesting scheme will only be sustainable if it fits into the socio-economic context of the area and fulfills a number of basic technical criteria.

Slope:The ground slope is a key limiting factor to water harvesting. Water harvesting is not recommended for areas where slopes are greater than 5% due to uneven distribution of run-off and large quantities of earthwork required, which is not economical.

Soils:Should have the main attributes of soils which are suitable for irrigation: they should be deep,notbe saline or sodic and ideally possess inherent fertility. A serious limitation for the application of water harvesting are soils with a sandy texture. If the infiltration rate is higher than the rainfall intensity, no runoff will occur.

Costs:The quantities of earth/stonework involved in construction directly affects the cost of a scheme or, if it is implemented on a self-help basis, indicates how labor intensive its construction will be.

What Is a Drinking Water Catchment? A drinking water catchment is an area of land where rainfall collects in rivers and streams that flow into reservoirs, or seeps into the soil to become groundwater where it is stored in underground aquifers. The captured water later becomes drinking water for the community.

Rivers as a Drinking Water Source The land use around a river and its catchment affects the quality and amount of water in the river. It is important that human land use is managed carefully to make sure rivers remainhealthy. Managing the impacts of all the activities around a river and its catchment is called total catchment management. Rivers are the main source of water that is captured in dams. Once water is collected in dams it can be stored, treated and then be evenly distributed between areas where and when it is needed.

Why Is It Important to Protect Drinking Water Catchments? Protected drinking water catchments provide a significant ‘natural’ barrier to contamination and yield high quality water. By protecting drinking water at the source, the risk is minimized of contamination and reduce the level of treatment required before it is supplied to the community. Source water protection is a crucial step to ensuring safe, good quality drinking water. Land uses and activities within drinking water catchments may adversely impact water quality. There are three main types of contamination:

• Microbiological (protozoa, bacteria, viruses) — often associated with fecal material from humans (from septic tanks or direct water body contact) or domestic animals (such as cows).
• Chemical — often associated with fuel spills, rubbish dumping, pesticides or fertilizers.
• Physical — such as turbidity (cloudiness). This may be caused by erosion and runoff associated with fires, pigs wallowing, and vehicles or horses on unsealed roads or reservoir banks.

How Can We Protect Drinking Water Catchments? When a new drinking water source is developed, existing approved land uses on private land within the catchment may continue. The expansion or development of high risk land uses may not be supported, and some activities may be restricted to protect the source water from contamination and minimize the risk to public health.

6.3.2.Reservoirs

A reservoir (etymology: from French reservoir a "storehouse") is an enlarged natural or artificial lake, storage pond or impoundment created using a dam or lock to store water. Reservoirs can be created by controlling a stream that drains an existing body of water. They can also be constructed in river valleys using a dam. Alternately, a reservoir can be built by excavating flat ground and/or constructing retaining walls and levees. Tank reservoirs store liquids or gases in storage tanks that may be elevated, at grade level, or buried. Tank reservoirs for water are also called cisterns. Underground reservoirs store almost exclusively water and petroleum below ground.

Types of Reservoirs

Reservoirs Dammed in Valleys A dam constructed in a valley relies on the natural topography to provide most of the basin of the reservoir. Dams are typically located at a narrow part of a valley downstream of a naturalbasin. The valley sides act as natural walls, with the dam located at the narrowest practical point to provide strength and the lowest cost of construction.

Photograph 6.2. Reservoirs Dammed in Valleys

In many reservoir construction projects, people have to be moved and re-housed, historical artifacts moved or rare environments relocated. Construction of a reservoir in a valley will usually need a river to be diverted during part of the build, often through a temporarytunnelor by-pass channel. In hilly regions, reservoirs are often constructed by enlarging existing lakes. Sometimes in such reservoirs the new top water level exceeds the watershedheight on one or more of the feeder streams.In such cases, additional side dams are required to contain the reservoir. Where the topography is poorly suited to a single large reservoir, a number of smaller reservoirs may be constructed in a chain.

Bank-side Reservoir Where water is pumped orsiphonedfrom a river of variable quality or quantity, bank-side reservoirs may be built to store the water.

Photograph 6.3. Bank-side Reservoir Such reservoirs are usually formed partly by excavation and partly by building a complete encircling bund orembankment, which may exceed 6 km (4 miles) in circumference (https://en.wikipedia.org/wiki/Reservoir - cite_note-ICEQueenMary-7).

Both the floor of the reservoir and the bund must have an impermeable lining or core: initially these were often made ofpuddled clay, but this has generally been superseded by the modern useofrolledclay. The water stored in such reservoirs may stay there for several months, during which time normal biological processes may substantially reduce many contaminants and almost eliminate anyturbidity. The use of bank-side reservoirs also allows water abstraction to be stopped for some time, when the river is unacceptably polluted or when flow conditions are very low due to drought.

Service Reservoir Service reservoirs store fully treated potable water close to the point of distribution. Many service reservoirs are constructed aswater towers, often as elevated structures on concrete pillars where the landscape is relatively flat. Other service reservoirs are entirely underground, especially in more hilly or mountainous country. Service reservoirs perform several functions, including ensuring sufficient head of water in thewater distribution systemand providing water capacity to even out peak demand from consumers, enabling the treatment plant to run at optimum efficiency. Large service reservoirs can also be managed to reduce the cost of pumping, by refilling the reservoir at times of day when energy costs are low.

Photograph 6.4. Service Reservoir

6.3.3 Dams

In ancient times, dams were built for the single purpose of water supply or irrigation. As civilizations developed, there was a greater need for water supply, irrigation, flood control, navigation, water quality, sediment control and energy. Therefore, dams are constructed for a specific purpose such as water supply, flood control, irrigation, navigation, sedimentation control, and hydropower. A dam is the cornerstone in the development and management of water resources development of a river basin. The multipurpose dam is a very important project for developing countries, because the population receives domestic and economic benefits from a single investment. Demand for water is steadily increasing throughout the world. There is no life on earth without water, our most important resource apart from air and land. During the past three centuries, the amount of water withdrawn from freshwater resources has increased by a factor of 35, world population by a factor of 8. With the present world population of 5.6 billion still growing at a rate of about 90 million per year, and with their legitimate expectations of higher standards of living, global water demand is expected to rise by a further 2-3 percent annually in the decades ahead.

Photograph 6.5. A view from a dam

But freshwater resources are limited and unevenly distributed. In the high-consumption countries with rich resources and a highly developed technical infrastructure, the many ways of conserving, recycling and re-using water may more or less suffice to curb further growth in supply. In many other regions, however, water availability is critical to any further development above the present unsatisfactorily low level, and even to the mere survival of existing communities or to meet the continuously growing demand originating from the rapid increase of their population. In these regions man cannot forego the contribution to be made by dams and reservoirs to the harnessing of water resources.

Photograph 6.6. Aerial view of Sayamaike dam built in the 7th century and still in use today

Seasonal variations and climatic irregularities in flow impede the efficient use of river runoff, with flooding and drought causing problems of catastrophic proportions. For almost 5,000 years dams have served to ensure an adequate supply of water by storing water in times of surplus and releasing it in times of scarcity, thus also preventing or mitigating floods. With their present aggregate storage capacity of about 6 000 km³, dams clearly make a significant contribution to the efficient management of finite water resources that are unevenly distributed and subject to large seasonal fluctuations. Most of the dams are single-purpose dams, but there is now a growing number of multipurpose dams. Using the most recent publication of the World Register of Dams, irrigation is by far the most common purpose of dams. Among the single purpose dams, 48 % are for irrigation, 17% for hydropower (production of electricity), 13% for water supply, 10% for flood control, 5% for recreation and less than 1% for navigation and fish farming.

Water Supply for Domestic and Industrial Use

It has been stressed how essential water is for our civilization. It is important to remember that of the total rainfall falling on the earth, most falls on the sea and a large portion of that which falls on earth ends up as runoff. Only 2% of the total is infiltrated to replenish the groundwater. Properly planned, designed and constructed and maintained dams to store water contribute significantly toward fulfilling our water supply requirements. To accommodate the variations in the hydrologic cycle, dams and reservoirs are needed to store water and then provide more consistent supplies during shortages.