The flood hazard in Nam Chun Sub watershed, Phetchabun Province was assessed by Saowanee Prachansri in March, 2007 (Prachansri, 2007). The following paper is extract from her research.
Background
Floods are probably the most recurring, widespread, disastrous and natural hazard. Adverse impacts include loss of life, property amage, contamination of water supplies, loss of crops, and social dislocation and temporary homelessness. One of the causes of floods is heavy ainfall that exceeds the absorptive capacity of soil and the flow capacity of rivers. Flood disasters are often increased in intensity and everity by deforestation, poor drainage condition, and land use changes that increase agricultural activities and urbanization. All of these activities contribute to increased rates of runoff and higher flood peaks.
The frequency of flash floods seems to have increased during the past half century in many parts of the world (Wiskow and van der Ploeg, 2003). Flash floods are a major threat to human life and infrastructures and the damage is much more severe than normal floods (Foody et al., 2004). This is because flash flooding happens at much faster rates than regular floods. They are distinguished from other types of flooding by the short time scales over which flood-producing rainfall occurs within a few hour and over small spatial scales (ODonnell, 2002). Flash floods are very short-lived floods lasting from several hours to a few days (Choudhury et al., 2004).
Flooding, both riverine floods and flash floods have damaged life and property since the earliest inhabitants settled in Thailand. Flash flooding is common during the Monsoon season (June to October). Flash flood events are usually generated by heavy convective precipitation over a relatively small area. Additional important factors that play a major role in their occurrence include catchment morphometry, initial soil moisture content and infiltration capacity of the soil. To better understand these issues, a more pragmatic look at the actual factors contributing to the flooding process is needed for instance the alteration of flow paths, sealing of soil surfaces and other human activities that alter land cover which affect the physical properties of soils. Such activities are considered to be a possible cause for changes in runoff peak flow behavior and this subsequently affects the magnitude of flooding in lowland areas. To better understand how these changes affect flood characteristics downstream, it is necessary to analyse and quantify how soil physical properties influence the volume of runoff from a catchment area and changes in land use influence the characteristics of flash flooding.
Flood hazard study was carried out in Namchun watershed and the lowlands of Lomsak plain. For any flood hazard assessment runoff modeling is very crucial. For runoff modeling in Namchun watershed various approaches were applied (Prachansri, 2007; Solomon, 2005) such as index method, the SCS curve number method and a semi physical approach.
Objectives
The main objective of this research is to assess the causes of flash flood hazard at downstream areas of the Nam Chun watershed, taking into account the influence of soil and land cover parameters upstream. The specific objectives of the research are the following
To assess the effects of different land cover types on soil properties To assess the effect of soil parameters in influencing runoff To assess the downstream consequences in terms of flood hazard by linking the upstream catchment runoff model with a downstream 1D2D-flood model
References
Choudhury, N.Y., Paul, A. and Paul, B.K., 2004. Impact of costal embankment on the flash flood in Ban gladesh: a case study. Applied Geography. http://www.sciencedirect.com
Prachansri, S., 2007. Analysis of soil and land cover parameters for flood hazard assessment: a case study of the Nam Chun Watershed, Petchabun, Thailand. M.Sc. Thesis, ITC, Enschede, 92 pp.
To assess the effects of different land cover types on soil properties