The Hindu 28.07.2009
Rainfall declines at Siruvani Dam
K.V. Prasad
| Water overflows, but heavy silt reduces full storage capacity |
COIMBATORE: After raising the storage to the full reservoir level in the Siruvani Dam in a record 17 days, the South West Monsoon seems to have been done with it. There has been a steady decline in rainfall from third week of this month, almost in line with the forecast that the monsoon is moving northward.
But, if the monsoon is indeed moving away, it is doing so only after providing the second highest rainfall in July in nine years (including this year).
After providing very scanty rain in May (43 mm) and improving in June (349 mm), the monsoon turned vigorous and provided 1371 mm in July (till 24th). The previous maximum was 1472 mm in July 2005. This was the highest in May-August from 2001 to 2008 (see table).
The highest rainfall on a day this year - 229 mm - was recorded on July 16. On other days in the crucial 17-day period, the rainfall ranged from 60 mm to 190 mm.
But, once water began to overflow from the dam on July 18, the intensity of the rain declined. It was 102 mm that day. The decline began the next day: July 19 (34 mm), July 20 (32 mm), July 21 (11 mm), July 22 (25), July 23 (10 mm) and July 24 (8 mm).
Tamil Nadu Water Supply and Drainage Board officials said on Monday that there was no rain at Siruvani catchment over the last couple of days. While the overflow is an eagerly – sometimes desperately – awaited annual event, it does not really indicate that the dam is filled to its 650 million cubic feet capacity.
Of the total dam spread area of 22.7 sq.km., there is silt in many portions. Coimbatore Corporation sources say there is more than 10 ft of silt and this reduces the storage capacity. Though the actual level is 67 ft, water can be drawn only up to 49 ft. The rest is dead storage. The sources say that silt removal is a massive task and it needs the concurrence of the Kerala Government, since the dam is located on its territory.
