Selection of type of dam

 Selection of type of dam

It is very difficult to make decision as to what type of dam will suit best to the particular site condition .Topographical, geological and foundation conditions, availability of materials , suitable site for the location of spillways etc influences the selection of the type of dam. The most common physical factors have been discussed as follows: 

1)  Topography

This is most important consideration for selection of type of dam. In narrow deep valley gravity dam is appropriate while in wide valley, embankment dams are preferable. If however the width of the valley at top is less than about one fourth the height and separate side for spill way is available, Arch dam may be suitable for the best choice. Canyon shape factor (L/H) is calculated if :

(L/H)= 1.5 -5.0; suitable for arch dam

(L/H) >5 ; suitable for concrete gravity dam

(L/H)>10-15 ; suitable for rock fill/embankment dam

Embankment dams are not suitable in narrow deep valley because of the significant different settlement across the slope leading to the formation of cracks. 

2.   Geology and foundation condition

If available foundation is solid sound rock, having no faults or fissures, any type of dam can be constructed on it . If rocks are available but they are fault or fissures, they will have to grouted first before founding gravity dam over them. Poor rock for gravel foundation are suitable for earth or rock fill dam but not for gravity dam. Fine sand or silt foundations are suitable only for either earth dam or for low gravity dam but not for rock fill dam. Gravity dam and rock fill dam, but only after giving proper treatment.

3.  Materials of construction

Dam requires very large quantities of construction materials. Only that dam may prove economical whose materials are available locally or in vicinity of the site. If gravel, sand and crushed stone are available locally, concrete gravity dam may be most suitable. If only fine and crushed grained soils are available earth dam may prove suitable. 

4.   Spillway size and location

For the safety of dam, safe and efficient disposal of flood discharges is very essential. This requires a suitable site for spillway. If the large spill way discharge is required, an overflow concrete gravity dam may be adopted. If separate site for spillway is available and spillway capacity is also small earth dam is preferred. If no separate sites is available for spillway and it has to be accommodated in the dam proper, then concrete gravity dam may be most preferred choice. Sometimes even earth dam may be constructed but some part of the dam which is to be used as a spill way may be constructed of concrete.

 5.   Road way over the dam

If the road way is to pass over the top of the dam, an earth dam or gravity dam would be preferred.

 6.    Length and Height of dam

If the length of dam is very long and height is low, earth dam would be better choice.

If the length is small but height is more gravity dam is preferred.

7.   Life of dam

Concrete or masonry gravity dam have very long life. Earth and Rockfill dam have intermediate life . However, Timber dams are adopted only for temporary storage. 

Selection of site for dam 

a) Dam site should be selected in a narrow valley to reduce the length of dam.

b) For safe and cheap construction, good foundation at moderate depth should be available at the dam site.

c)  It is better if the material of construction is locally available.

d) Bed and side of river should be fairly water tight and basin should cup shape as far as possible.

e) Major portion of the dam should be preferably on a high ground to reduce cost and drainage facility.

f) For large capacity of storage, dam site is the best location at the confluence of the two rivers.

g) Location of suitable spillway should be available.

h) The submergence area of the reservoir should be minimum.

i) A good transport facility to dam site is essential.

j) Catchment should be non-erodible upstream of the dam which is essential to reduce silting the reservoir.

k) A healthy climate at dam site, spacious accommodation for labor, staff quarters are necessary.

l) The sites should have less percolation losses.

m) The dam site should be selected where catchment is maximum.

n)  The dam site should provide deep reservoir with small water surface to reduce evaporation loss and submergence area and to control weed growth.

o) The base site of diversion dam is the location where water way length is minimum.