Attachments
Summary
1. Introduction
The 2006 Annual Flood Report aims to fulfil two primary roles: it provides a summary of the flood year 2006 and it collates important data on the flood regime. These data will, in time, accumulate to provide a primary regional resource for flood research and the collation of historical reference material.
2. About floods in the Mekong Basin
2.1 The size of Mekong floods in their global context
Based on the data in the World Catalogue of Large Floods (IAHS. 2003), extreme floods on the Mekong are compared to those upon other global river systems with catchments in excess of 500,000 km2, revealing that the river is amongst the world maxima classified upon the basis of peak discharge per unit area and very close to the global limit for rainfall generated flood runoff.
2.2 The historical geography of floods on the Mekong mainstream
An analysis is undertaken of the temporal and spatial nature of floods along the Mekong mainstream, revealing that the river basin is far from geographically homogenous with regard to the nature and severity of the flood season in any given year.
2.3 The nature and analysis of floods on large rivers
The quantitative definition of the magnitude of a flood exclusively in terms of its peak discharge is a useful and sufficient statistic in the case of small river basins where the duration of flood events is usually only a matter of several days. On large rivers the flood hydrograph has a much longer duration, which in the case of the Mekong is a matter of several months. The multivariate aspects of the hydrograph therefore need to be simultaneously taken into account in the assessment of flood risk and magnitude. In this report modern multivariate statistical technology is used; this brings together the peak flood discharge, the volume of the flood hydrograph and the duration of flows above critical thresholds.
2.4 Temporal aspects of the Mekong flood regime
The onset and duration of the flood season in the Lower Mekong Basin is clearly an important variable from year to year. In keeping with broader definitions of hydrological seasonality on the mainstream adopted by the Environment and Fisheries Programmes within the Mekong River Commission (MRC), four flow seasons are identified, namely the flood season, the dry season and two transition seasons. The hydrological indices adopted to define the onset and closure of each one are presented along with a historical assessment of how these dates have varied from year to year. It is demonstrated that these temporal variables have been remarkably consistent over the last 80 to 90 years and have a surprisingly small inter-annual variability. Studies of the palaeoclimate during the Holocene are quoted that suggest these temporal aspects of the Mekong flood regime have been unchanged over the last 5,000 to 6,000 years.
Throughout Part 1 the events of 2006 are set within this wider geographical, historical and temporal context and a number of graphical techniques are presented that could be adopted as standards for the comparative assessment of the Mekong flood for any year that is under consideration.