Orshi Dam – Basic Catchment Information
Prepared by Rob Delaney, 12th June 2015
The location of embankment breach is;
Google Earth (GE) provides an image of the reservoir at full water level taken on 31st Dec 2005. The
breach is said to have occurred in 2012 and the dam hasn’t been functional since. The GE image also
shows what appears to be slumping on the downstream slope of the embankment and a pool of
water which make indicate seepage through the embankment.
The catchment area has been calculated to be 334 km2. GE was used to calculate the catchment by
tracing the surface water flow paths and then marking on the watersheds. Finally a polygon was
drawn for the entire catchment. GE Pro has a feature which provides the area of a polygon.
The centre or mid-point of the catchment was found using a website (www.earthpoint.us) and
pasting in the catchment polygon from GE. The location of the catchment mid-point is;
The average annual rainfall for the catchment is 216mm. This is based on taking the coordinates of
the catchment mid-point (14.93, 23.94) and using The World Bank Group – Climate Change Portal
(http://goo.gl/OCHbnI ). The data series for a period 1900 to 2009 was used. On an average year
there is only significant rainfall over 4 months, from June to September, with some 86% of the total
annual rainfall occurring in just 2 months (July and August).