Canacona flash floods (2009): Reports submitted by the National Institute of Oceanography to the Government of Goa

The report is based on a study carried out with the help of National Institute of Oceanography (NIO) and attempts to (1) assemble and analyse available information to describe and identify causes behind the flash floods, and (2) suggest measures to be adopted in Goa to minimise damage arising from similar episodes in future. 

The study comprised of field surveys and analysis of available data, particularly data on precipitation. This work permitted the Committee to infer that the flash floods in Canacona were directly related to about 271 mm of rain that fell on 2 October 2009. The committee noted that though there were no records to suggest that flooding of this magnitude had occurred in Canacona taluka in recorded history, the elements that contributed to the event were not uncommon in Goa. This intense-rainfall event could be looked upon as a warning of the potential of such events to cause damage. Though such events cannot be prevented, the committee suggested that the state should focus on awareness and preparedness for minimising the impact of similar intense-rainfall events.

The committee made one general and four specific recommendations. The general recommendation was that well-known practices in forest management for preventing mudslides (afforestation of mountain slopes, for example) and in river management (such as de-silting of river beds) should receive emphasis and increased investment. These measures would be able to minimise damage from rainfall events of lesser intensity, but higher frequency that occur in Goa. The specific recommendations were as follows -

• The areas vulnerable to mudslides should be mapped and site-specific disaster management plan to face them should be in place at each location with high vulnerability.
• Areas with high vulnerability to flooding due to an intense precipitation event should be identified and a disaster management plan should be evolved at locations that are particularly vulnerable.
• A mechanism for keeping a careful watch should be in place whenever a situation arises with high potential for an intense precipitation event in a vulnerable area. The Meteorological Centre of the India Meteorological Department (IMD), Panaji should form the nerve centre of such a watch.
• The State of Goa should make IMD's "Cyclone Warning Dissemination System" operational in the state.

A brief introduction to these reports is prresented below -

• The first report, by Kessarkar, Srinivas, Suprit, and Chaubey, proposes a method for mapping landslides. It describes the geological reasons behind the landslides that occurred in the Canacona taluka. As noted by the Canacona Flash Floods Study Committee, these landslides occurred on steep slopes; the cascading water triggered the landslides. The method described in this report makes considerable use of satellite data, but relies critically on the field observations at Kuskem. The report list the factors identified as important for landslides in this area and prescribes a method to estimate a Landslide Potential Index and to prepare a landslide-susceptibility map. Though this method has been developed for a specific area, the methodology, with suitable modification, is applicable to other regions as well.
• The second report, by Suprit, Kalla, and Vijith, presents a methodology for assessing flood risk and issuing a warning during an intense-rainfall event. The method analyses topography for river flow and for converting rainfall to river flow. It uses an open-source Geographical Information System (GIS) called GRASS and a simple hydrological model called the rational method. Based on this method and using rainfall data from online Automatic Weather Stations (AWSs), an algorithm is developed for a flood-watch. The potential discharge computed from the data using the rational method is used to develop a simple, graphical method for issuing a warning. Its viability is tested for the event of 2 October 2009 for the Talpona River, but it can be applied to other rivers too.
• The third report, by Sundar, presents the outline of a watch-keeping mechanism with the Panaji observatory of the IMD as its nerve centre. Sundar uses elevation data from the Shuttle Radar Topography Mission (SRTM) and publicly available geographical information such as Google Earth and Google Maps to ascertain which regions of Goa are likely to be prone to floods. An evaluation of the existing network of AWSs and Automatic Rain Gauges (ARGs) then leads to identification of sites where AWSs need to be located in order to augment the existing network in the state.

These three reports are expected to assist in increasing awareness about the potential for similar floods in Goa and help improve our preparedness to face them.