Lower rainfall has been cited as the cause of falling dam levels. The worse affected dam is the new Sungai Selangor dam located near Kuala Kubu Baru. It was reported that in mid-September the water had receded to 40% of its capacity.

To find out the cause, evaluation was carried out using all relevant information and data from SPLASH website and rainfall records at Kuala Kubu Baru station obtained from the Meteorological Services Department.

The Sungai Selangor dam was completed in April 2003. The impounding of the dam started on 25/04/03 and it reached its full supply level on 13/04/04. Its total storage capacity is 230 million cubic meters (mcm).

The dam is designed as a regulating dam, storing excess water during wet season and releasing water to the river for treatment during dry season.

It supplies water to two treatment plants under Sungai Selangor Phase 3 project. One is at Rasa and the other is at Bukit Badong with a combined capacity of 1.05mcm per day.

The current output from the two plants since early this year is only about 0.4mcm per day.

The rainfall, extrapolated from the rainfall records, shows that 1,446mm of rain was recorded from 25/04/03 to 13/04/04 when the dam was been filled and 1,414mm from 01/01/05 to 15/09/05 when the two treatment plants were in operation.

A simple calculation shows that 259mcm of the dam's impounded water was unaccounted for from 01/01/05 to 15/09/05 while only 103mcm of water was drawn off for treatment during this period.

The low water level in the dam was definitely not due to drought as there was more rainfall during the first nine months of this year than the corresponding periods of the last two years. Therefore the contention that lower rainfall is the cause of falling dam levels cannot hold water.

Something seems to be amiss as far as this dam is concerned. Urgent investigation and action by the relevant authorities are needed.

Note:
1. An edited version of the above was published in The Star on 12/10/05.
2. Calculations of unaccounted for water in the dam is given below:

Sungai Selangor dam
Unaccounted for water (UFW) in the dam
from 01/01/05 to 15/09/05 (258 days)

UFW = amount of water in storage on 01/01/05
+ amount of water from rainfall between 01/01/05 and 30/09/05 - amount of water left in the dam on 30/09/05
- amount lost through evaporation
- amount drawn off to supply
all in million cubic meters (mcm)

Amount in storage on 01/01/05:
The dam of 230 mcm storage capacity was full = + 230 mcm

Amount from rainfall: extrapolated from rainfall records
1,446mm of rain was recorded
from 25/04/03 to 13/04/04 to fill the dam
1,414mm of rain was recorded
from 01/01/05 to 15/09/05
:. Amount from rainfall = 230 x 1,414 = +224 mcm
1,446

Amount left in the dam:

At 40% full, the amount of water in the dam
On 15/09/05 = 0.4 x 230 = - 92 mcm

Amount lost through evaporation
= (4.6mm/day) x (6 sq km) x 258 days = not significant

Amount drawn off to supply at 400 million litres per day
= 0.4 mcm/day x 258 days = - 103 mcm
UFW is therefore = + 259 mcm
or as % of stored water = 259/(230+224) x 100 = 57 %

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Data obtained on Sungai Selangor dam

(A) Data from SPLASH Website - www.splash.com.my

A.1 Dam storage capacity = 230 million cubic meters (mcm)
A.2 Impoundment area = 6 sq km
A.3 Impounding of dam started on 25 April 2003
A.4 Date when dam reached full supply level 13 April 2004

(B) Rainfall data from Perkhidmatan Kajicuca Malaysia (PKM)

B.1 Station Hospital Kuala Kubu Baru
B.2 Evaporation Average 4.6mm from 1/1/05 to 15/9/05

(C) Operation of dam from 01/01/05 to 15/09/05

C.1 Average daily treatment plants production = 0.4 mcm