Structures for Water Storage, Investigation, Planning and Layout (Part -1) - Civil Engineering (CE) PDF Download

Introduction 

The two primary types of valley structures, storage and diversion, were discussed. These are again displayed in Figure 1 for recapitulation.  

FIGURE 1. Structures for harnessing water resources potential of a river

Structures that are created as obstructions across rivers with an intention to store some of the water for future use are called storage dams. They are functionally slightly different from the structures used for flow diversion, called Barrages or weirs discussed in the Lessons 4.1 to 4.3. A diversion structure is primarily meant to create an elevation rise of the river water such that it may flow into a canal, perhaps through out the year or at least during the lean flow period of the river. During high flood in a river, the canal is kept closed in cases like irrigation canals for fear of high sediment entering and depositing on the canal bed. Even if some water is drawn into a canal during river flood season, the main bays of a diversion structures are usually kept wide open to let the flood water pass down the river with the minimum obstruction. 

This is not so in case of storage dams, for which the storage of water especially a portion of the flood flow is primary concern. The storage of water is done with an intention to either reduce the impact of a flood down stream or to use the water beneficially in future. This is achieved by creating an obstruction of sufficient height which creates a reservation on the up stream of the structure. Naturally, since the reservoir would have a finite capacity which would vary with height of the dam and the shape of the river valley on the up stream, any excess flood water has to be discharged through a spillway. Hence, the principal components of a storage dam would be a storage a structure to obstruct river flow, a spillway for discharging excess flood water and outlets for allowing the storage water to be with drawn for the rest of the year for some useful purpose or even let it flow downstream at a regulated quantity.

Some times sluice are provided in the body of the dam to lower the water level in the dam at the time of an emergency it is not necessary that all the principal components of a storage dam be located in the same structure. In fact, all three may be located separately. Of course, the spillway is usually made of reinforced concrete and sometimes combined with a concrete dam. But it may be economical or practically more feasible to construct an earthen or rock fill dam, in which case there has to be separate spill way made of concrete. In fact, it is also not essential for the spill way to be adjacent to the main dam, and can be located any convenient position at the periphery of the reservoir, if that helps in some way. Similarly, out let works may be located at any suitable place in the reservoir and possibly connected to a canal or a tunnel. 

Sometimes dams are constructed to create a head difference for generating power but without using the storage. This may be done due to the requirements of the riparian rights, as in the case of the Salal Dam on Sutlej, which uses the head available to generate power put does not have an outlet or river sluice. Other types of dams include detention dams which are primarily created to retard flood runoff and control flood peaks.  

In this lesson, we shall discuss about the types of dams in vogue, geotechnical and other site investigations that is required for planning the most suitable type of dam and the concept of general layout of an entire dam project including its various appurtenant structures. 

Types of dams 

Almost each dam that has been constructed all over the world is unique. This is so because a particular type is chosen because of the considerations of many factors, as discussed in subsequent sections. In fact, dam engineering brings together a range of disciplines, like structural, hydraulics and hydrology, geotechnical, environmental etc. 

Never the less, primary purpose of a dam is to provide for the safe retention and storage of water. Structurally, a dam must be stable against overturning and sliding, either or within the foundations. The rock or soil on which it stands must be competent to withstand the superimposed loads without crushing or undue yielding. The reservoir basin is created must be watertight and seepage through the foundation of the dam should be minimal.  

Though each situation demands a unique proposal for the type of dam, a broad classification based on the construction material can be made in dividing the types of dams that have been commonly constructed as: 

1. Embankment dams, which are constructed of earth fill and/or rock fill, and
2. Concrete dams, which are constructed of mass concrete. 

Of course, there are some dams that were constructed using rubble masonry, like the Nagarjuna Sagar dam on the river Krishna. But mostly embankment dams are more common for technical and economical reasons all over the world, they account for nearly 80 percent of all the large dams that have been built in modern times. The two main types of dams are further explained in the following paragraphs.

Embankment Dams  

These can be defined as dams constructed of natural materials excavated or obtained from the vicinity of the dam site. The materials available are utilized to the best advantage in relation to their characteristics as an engineered bulk fill defined zones within the dam section. The natural fill materials are placed and compacted without the addition of any binding agent. Two main types of embankment dams that are commonly constructed include the following: 

1. Earth-fill or earthen embankments –These may be classified as dams use compacted soil for constructing the bulk of the dam volume. An earth fill dam is constructed primarily of selected engineering soils compacted uniformly and intensively in the relatively thin layers and at a controlled moisture content. Some of the common sections designed for earth fill embankment dams are shown in Figure 2.

Figure 2. Principal types of earthen embarkment dams
(a)    Homogeneous with toe drain,
(b)    Homogeneous with chimney drain & horizontal blanket,
(c)    Zoned with clay core chimney drain & horizontal blanket,
(d)    Zoned with earth & rockfill

2. Rock-fill e mbankments – In these types of dams, there is an impervious core of compacted earth fill or a slender concrete or bituminous membrane but the bulk of the dam volume is made of coarse grained gravels, crushed rocks or boulders. Typical sections of rock fill dams are shown in Figure 3. The stability of the outer shell of a rock fill dam relies on the frictional forces acting in between each rock gravel piece which ensures its safety against sliding kind of failure during earth quakes. 

Figure 3. Principal types of rockfill embankment dams, with the following features: (a) Vertical day core & drains, (b) Inclined day core & drains.(c)    Upstream decked with asphaltic or concrete membrane & drains ;(d)    Central asphaltic membrane with drams.

It may be observed that shell of the rock fill dam is more permeable than that of an earth fill dam. Mostly, the water is prevented from flowing down by the impervious core of the rock fill dam.

Embankment dams are advantageous in the following major aspects: 

1. These are suitable for river valleys of any type: steep gorges or wide valleys
2. Can adapt to a broad range of foundation conditions, ranging from good rock to even permeable soil type of foundation
3. Uses naturally available materials
4. Relatively less costly 

Amongst the disadvantages, it may be raid that they have greater susceptibility to damage than concrete dams due to the possibility of getting washed away during an over tapping of the spill way which may occur if there is a flash flood in the river and the gates of the spill way are not operated in time or the spill way itself is of inadequate capacity. 

A further disadvantage of the embankment dam when compared to a concrete dam is that the former requires to have a separate spill way in contrast to the latter, where the spill way may be integrated within the dam body itself. Also earthen embankments are prone to concealed leakage, perhaps due to faulty construction or internal erosion in the dam body or in a pervious foundation.  

Concrete Dams

The use of mass concrete in dam construction started from about 1900 for reasons of ease of construction and to suit complex designs, like having spill way within the dam body. From about 1950 onwards, mass concrete came to be strengthened by the use of additives like slag or pulverized fuel ash, in order to reduce temperature induced problems or avoid undesirable cracking or to reduce the total cost of the project.

Amongst concrete dams, too, there are many varieties, the principal types of which described below.

1. Gravity dams (Figure 4a): A gravity dam is one which depends entirely on it’s own mass for stability. The basic gravity profile is triangular in shape, but for practical purposes, is modified at the top. Some gravity dams are slightly curved in plan, with the curvature being towards the river upstream. It is mostly due to aesthetic and other reasons, rather than having an arch action for providing greater stability.  

Figure 4(a) Concrete Gravity dam

2. Buttress dams (Figure 4b): This type of dams consist of a continuous upstream face supported at regular intervals by buttress walls on the down stream side. These dams are generally lighter than the solid type of dam but likely to induce slightly higher stresses at the foundation since most of the load now passes through the buttress walls and not spread uniformly over the foundation as in a solid gravity dam. 

FIGURE 4b Buttress Dam

3. Arch dams (Figures 4c and d): These types of dams have considerable upstream curvature in plan and rely on an arching action on the abutments through which it passes most of the water load on to the walls of the river valley. This type of dam is structurally more efficient gravity dams and greatly reduces the volume of concrete required. Of course, a prime necessity in constructing an arch dam is to have sound foundation and abutments.   

FIGURE 4. Principal types of concrete dams(continued); (c) Arch gravity dam : (d) Cupola or double curvature - arch dam.

These are a few other types of concerted dams that have been constructed, a couple of which have been illustrated in Figures 5a and b.  

Figure 5a. Hollow gravity dam

Some of these, it may be noted, are made of reinforced concrete, and not mass concrete for gravity dam. Some of major advantages of having a concrete dam are listed below: 

1. Concrete gravity, hollow gravity or buttress dams are suitable to all kinds of river valleys- narrow or wide, provided that good rocky foundation is available at moderate depths below the river bed.
2. Concrete dams are not sensitive to over topping, unlike the embankment dams. However, the water over topping the concrete dam may destroy the foundation down stream due to the impact of the falling water.  
3. Concrete dams may accommodate a crest spill way, or sluice ways through the body of the dam, which is not possible for embankment dams. The cost of having a separate spill way channel is thus avoided.
4. Concrete dams are more resistant to withstand seismic disturbance. 
5. Cupola or Double curvature arch dam is an extremely strong and efficient structure, provided a narrow valley exists with good rock in abutments and foundation.