Floodplain sediment from a 100-year-recurrence flood in 2005 of the Ping River in northern Thailand

S H Wood,A D Ziegler

doi:10.5194/hess-12-959-2008

Abstract

Abstract. The tropical storm, floodwater, and the floodplain-sediment layer of a 100-year recurrence flood are examined to better understand characteristics of large monsoon floods on medium-sized rivers in northern Thailand. Storms producing large floods in northern Thailand occur early or late in the summer rainy season (May–October). These storms are associated with tropical depressions evolving from typhoons in the South China Sea that travel westward across the Indochina Peninsula. In late September, 2005, the tropical depression from Typhoon Damrey swept across northern Thailand delivering 100–200 mm/day at stations in mountainous areas. Peak flow from the 6355-km2 drainage area of the Ping River upstream of the city of Chiang Mai was 867 m3s−1 (river-gage of height 4.93 m) and flow greater than 600 m3s−1 lasted for 2.5 days. Parts of the city of Chiang Mai and some parts of the floodplain in the intermontane Chiang Mai basin were flooded up to 1-km distant from the main channel. Suspended-sediment concentrations in the floodwater were measured and estimated to be 1000–1300 mg l−1. The mass of dry sediment (32.4 kg m-2), measured over a 0.32-km2 area of the floodplain is relatively high compared to reports from European and North American river floods. Average wet sediment thickness over the area was 3.3 cm. Sediment thicker than 8 cm covered 16 per cent of the area, and sediment thicker than 4 cm covered 44 per cent of the area. High suspended-sediment concentration in the floodwater, flow to the floodplain through a gap in the levee afforded by the mouth of a tributary stream as well as flow over levees, and floodwater depths of 1.2 m explain the relatively large amount of sediment in the measured area. Grain-size analyses and examination of the flood layer showed about 15-cm thickness of massive fine-sandy silt on the levee within 15-m of the main channel, sediment thicker than 6 cm within 200 m of the main channel containing a basal coarse silt, and massive clayey silt beyond 200 m. The massive clayey silt would not be discernable as a separate layer in section of similar deposits. The fine-sand content of the levee sediment and the basal coarse silt of sediment within 200 m of the main channel are sedimentological features that may be useful in identifying flood layers in a stratigraphic section of floodplain deposits.

Highlights

Infrequent large floods usually occur in northern Thailand late in the May–October rainy season
The May– October rainfall is dominated by masses of moist air moving northeast from the Indian Ocean, large floods are typically associated with tropical depressions moving westward from the South China Sea (Fig. 1)
While the pattern we found at the Ban Ko study area cannot be applied to the whole Ping River floodplain, it can serve as an indicator for areas that were submerged more than 1 m for similar lengths of time

Summary

Introduction

Infrequent large floods usually occur in northern Thailand late in the May–October rainy season. The May– October rainfall is dominated by masses of moist air moving northeast from the Indian Ocean, large floods are typically associated with tropical depressions moving westward from the South China Sea (Fig. 1). Three flood events on the Ping River in August and September, 2005 flooded parts of Chiang Mai city and other floodplain areas within 1 km of the main channel. The first flood (14–16 August) was the result of a heavy monsoon rainstorm associated with a low-pressure trough moving westward across northern Thailand. Flooding and mudslides affected a large area, including Chiang Mai, Chiang Rai, Phayao and Mae Hong Song Provinces (Fig. 2). A second, smaller flood (20–22 September) was associated with tropical storm Vincente as it weakened to a tropical depression, again traveling westward across Indochina from the South China Sea

Results

Discussion

Conclusion