Wednesday, 10 September 2014

Siphon Solution for Periyar Dam

Siphon Solution for Periyar Dam
History

Mullaperiyar Dam is a masonry gravity dam on the Periyar River constructed between 1887 and 1895 by the British Government. It is simply a wall-like structure, without any sluices, meant to block the west-flowing river and make the water stagnant so that it can be diverted to the eastern side through the tunnel. The troops from Madras and the Portuguese carpenters from Cochin were employed in the construction of the coffer-dams and other structures. A lot of manual labour was involved and worker mortality from malaria was high. It was claimed that had it not been for "the medicinal effects of the native spirit called arrack, the dam might never have been finished". 483 people died of diseases during the construction of this dam and were buried on-site in a cemetery just north of the dam.

There had been days and nights when workers stood as human walls inside the gushing water, preventing the structures from being washed away. The plan would have been abandoned, particularly after a near built dam was washed away. Though the British wanted to wash their hands of the project, Pennycuick sold his wife’s jewellery to mobilise funds and continued the work. And the dam became operational in 1895.

The common people of Madurai and surrounding areas still have great admiration for Major John Pennycuick, whose portrait can be spotted on the walls of homes and small shops amid pictures of gods.

However, the unique idea of harnessing the water flowing in the western side of the Western Ghats and bringing it to the eastern side was first explored in 1789 by Pradani Muthirulappa Pillai, a minister of the Ramnad king Muthuramalinga Sethupathy, who gave it up as he found it to be expensive.

Diversion of Water

The storage can be drawn only through the tunnel to the Tamil Nadu side. The rate of discharge is dependent on the head pressure created by the level of water. There is an extremely huge quantity of Dead Storage provided merely to raise the water level and the pressure at the tunnel. The level of the sluice through which the water drawn by the Tamil Nadu is 15.75 m (48 ft) below the crest of the Dam which leaves a Dead Storage water depth of 41.01 m (125 ft). The existing tunnel is of height 12ft and a sectional area of 150 sqft. The length of the tunnel is 5887 ft, capable of discharging 2100 cusecs. But only 1600 cusecs are drawn through this tunnel.
The excess water could be let out into the Periyar or Idukki reservoir through the spillways located at a height of 136 ft

Dispute between Kerala and TN

The existing dispute between the two states is the dams safety as it is as old as 110 years. Kerala did not object giving water to Tamil Nadu. Increasing the level would add more pressure to the dam's structure.

New dam at Kerala side
The current dead storage level in the dam is 104 ft. If a new dam is built 50 feet below at Kerala side, then the dead storage level will be 154 feet.

A Siphon Solution

The construction of a small dam, at a lower point on TN side, and extension of the tunnel to the shape of a Siphon is a better aternative. The new dam at a lower level increases the suctional force at the entry point of the tunnel. If the tunnel is extended at the entry point downwards by 30 ft, then the dead storage level can be reduced to 75 ft. The costs may be less when compared to the costs for replacing the existing dam with a new one.

Siphon

A siphon allows transfer of fluid from an upper location to a lower one. The fluid is moved upwards from its entry point before it turns down to its exit point. To all appearances it seems as if the fluid is being magically raised upwards without the use of a pump

The frictional energy loss at the walls of the pipe can be compensated by gravitational energy or potential energy of the fluid that move from a higher location to lower point. The more the height difference between both ends of the pipe, the suction force also will be more.

A siphon for water cannot be higher than about 1000 centimeters (33 feet) above the water surfaces.

Advantages of Siphon Solution for the Mullaperiyar Dam

  1. This can reduce the dead storage level of the dam to around 75 ft
  2. Reducing the dead storage will increase the dam capacity. So the catchment areas also can be extended to divert many small waterflows towards the dam.
  3. During the monsoon season Kerala gets excess rain resulting in flooding. When more water is redirected eastwards, the river basins of Kerala will become more safe.

Salient features of the dam and allied structures

The Periyar River is a perennial river which rises from the Sivagiri group of hills in Kerala State and flows through this State and joins the Arabian Sea. Out of its total catchment of 5398 sq. km, 114 sq. km is located in Tamil Nadu far down the river from the Mullaperiyar Dam site and 5284 sq. km lies in Kerala.

The rainfall occurs in 4 out of 5 days during June to November when the rain is practically continuous. Sudden floods from 283.17 to 3398 cumecs (or 10000 to 120000 cusecs) are constantly experienced during most of the year. Sometimes, 76.2 mm (3 inches) of rain is recorded in a period of 4 hours resulting in heavy floods. The project sanctioned in 1884 and started in 1887 was completed by 1895.

Location

Latitude9°31'30" N
Longitude77°8' 45" E

Reservoir

Catchment area 624 sq. km
Full Reservoir level 46.33 m (152 ft)
Maximum Water level 47.24 m (155 ft)
Gross Capacity of Reservoir 443.23 MCM (15.662 TMC)
Gross storage at 104 ft 144.19 MCM (5.092 TMC)
Gross. storage at136 ft 317.43 MCM (11.210TMC)
Gross storage at 155 ft 470.54 MCM (16.617 TMC)
Probable Maximum Flood 8676 cumecs (306399 cusecs)
Recorded Maximum Flood 8453 cumecs (298519 cusecs)

Main Dam

Length 366 m (1200 ft)
Top of dam including parapet 48.16 m (158 ft)
Height of dam from deepest foundation 53.64 m (176 ft)
Top width of dam with parapet 6.4 m (21 ft)
Downstream slope 0.9276 to 1.0
Upstream Slope 1 to 20

Spillway

Location Right Saddle
Crest level 41.45 m (136 ft)
Number of vents (old) 10 in Nos. with size 10.98 m x 4.88m (36 ft x 16 ft)
Number of vents (additional) 3 in Nos. 12.16 m x 4.88 m (40 ft x 16 ft)

Baby Dam

Location Left Bank saddle
Type Composite gravity structure
Top of dam 48.17 m (158 ft)
Length of dam 73.15 m (240 ft)
Height of dam 16.15m (53 ft)

The gross and live storage levels of the reservoir
at various levels are given below

Full Reservoir
Level in feet
Gross Storage
in TMC
Live Storage
in TMC
104 5.1 0.0
136 11.21 6.118
142 12.758 7.666
144 13.299 8.207
152 15.662 10.57
155 - -

Measurements

cusec is a measure of flow rate and is informal shorthand for "cubic feet per second"
commonly used in reference to volume of water in a reservoir or river flow
One cusec = (28.3 litres per second)

TMC stands for Tousand Million Cubic Feet and MCM stands for Million Cubic Meter
One TMC = 28316 Million litres
1 TMC= 28.31684 MCM

Flow Rate calculation

The following data shows the amount of flow in cubic feet per second when there is a height difference between source of the flow and the end point of the pipe

  • The pipe having a radius of one feet is considered here
  • The frictional losses are ignored

Height ftspeed ftpscusecs
5057178
10080252
200113356
500179563
750220690
1000254797
1500311976
20003591126

An easy quick fix

The overhaul of the existing tunnel is costly and involves more complications. Instead a pipe of two feet radius can be erected through the tunnel, in the shape of a siphon. The source of the pipe starts 30 feet below at the mouth of the tunnel. The other end of the tunnel ends at a dam located below. The more the height difference between both ends of the pipe, the more will be the flow rate.

References

Mullaperiyar History

Tracing the Path of Currency
Food Security in India