Hydrol. Earth Syst. Sci. Discuss., 4, 3839–3868, 2007 Hydrology and www.hydrol-earth-syst-sci-discuss.net/4/3839/2007/ Earth System HESSD © Author(s) 2007. This work is licensed Sciences 4, 3839–3868, 2007 under a Creative Commons License. Discussions Papers published in Hydrology and Earth System Sciences Discussions are under Floodplain sediment, open-access review for the journal Hydrology and Earth System Sciences 30-year flood, Ping River, Thailand S. H. Wood and A. D. Ziegler Floodplain sediment from a Title Page 30-year-recurrence flood in 2005 of the Abstract Introduction Ping River in northern Thailand Conclusions References Tables Figures S. H. Wood1 and A. D. Ziegler2 J I 1Department of Geosciences, Boise State University, Boise, ID 83702 USA 2Geography Department, University of Hawaii, Honolulu, HI 96822 USA J I Received: 1 October 2007 – Accepted: 9 October 2007 – Published: 18 October 2007 Back Close Correspondence to: S. H. Wood ([email protected]) Full Screen / Esc Printer-friendly Version Interactive Discussion 3839 EGU Abstract HESSD This paper documents the nature of flood-producing storms and floodplain deposi- tion associated with the 28 September–2 October 2005 30-year-recurrence flood on 4, 3839–3868, 2007 the Ping River in northern Thailand. The primary purpose of the study is to un- 5 derstand the extent that deposits from summer-monsoon floods can be identified in Floodplain sediment, floodplain stratigraphy A secondary objective is to document the sedimentation pro- 30-year flood, Ping cesses/patterns associated with a large contemporary flood event on a medium-sized River, Thailand Asian river. Maximum sediment depths of 15 cm were found on the river levee, within 30 m of the main channel, and at 350 m thickness was 4 cm. Sediment depth gener- S. H. Wood and 10 ally decreased exponentially with distance away from the main channel. The extent of A. D. Ziegler sediment deposition was about 1 km from the river channel. However, 72% of the sedi- ment was deposited within an oval-shaped area 200–400 m from the main channel and centered on a tributary stream, through which sediment-laden water entered the flood- Title Page plain, in addition to overtopping the levee of the main channel. Sediment concentration −1 Abstract Introduction 15 during the flood was estimated at 800–1500 mg L ; and we believe the sediment was delivered by flows of well-mixed flood water occurring over a 1–2 day period. These Conclusions References data suggest that flood-deposited strata related to 30-year recurrence floods is only Tables Figures likely to be preserved in deposits located relatively close to the main river channel where fine sand and clayey coarse silt deposits have thicknesses of at least 5–10 cm. J I 20 These relatively thick deposits would survive bioturbation, whereas more distal areas with thin clayey silt deposits would not. J I Back Close 1 Introduction Full Screen / Esc Infrequent large floods usually occur in northern Thailand late in the May–October rainy season. Although the May–October rainfall is dominated by storms of moist air moving Printer-friendly Version 25 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 Interactive Discussion 3840 EGU 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 (13–16 August) HESSD was the result of a heavy monsoon rainstorm associated with a low-pressure trough 4, 3839–3868, 2007 moving westward across northern Thailand. Chiang Dao District of the Chiang Mai 5 Province reported 200 mm of rain during this period. Flooding and mudslides affected a large area, including Chiang Mai, Chiang Rai, Phayao and Mae Hong Song Provinces Floodplain sediment, (Fig. 2). Peak flow at Chiang Mai reached 747 m3 s−1 (a stage of 4.90 m) at 18:00 on 14 30-year flood, Ping August. The river flooded again on 21 September reaching a peak flow of 485 m3 s−1 River, Thailand (3.8 m). This lesser flood was associated with tropical storm Vincente as it weakened S. H. Wood and 10 to a tropical depression, again traveling westward across Indochina from the South China Sea (20–22 September). A. D. Ziegler The third and largest flood (29 September–1 October) reached a flow of 750 m3 s−1 (4.93 m) at the Chiang Mai P1 gage. This event was a result of Typhoon Damrey, which Title Page made landfall on Hainan on 25 September and swept westward across the Indochina 15 Peninsula as a tropical storm (Fig. 3). The storm rained 55 mm on 28 September, Abstract Introduction 43 mm on 29 September, and then 200 mm on 30 September at Chiang Dao (Fig. 4). Rainfall at the Angkhang Meteorological Center near Fang measured 200 mm (Chi- Conclusions References ang Mai News, 8 October 2006). Flash flooding and mudslides occurred in the same Tables Figures provinces that were hit in August, and additionally in the Lampang and Phrae Provinces 20 (Fig. 2). Scattered small slope failures occurred during the storm along many of the J I roads. The cities of Chiang Mai and Lampang were partly flooded. The 3-day rainfall in and around Chiang Mai was less than 15 mm, indicating that the main storm passed J I north of this area. Heavy rainfall apparently occurred along a 100-km wide swath of Back Close the west-traveling storm path. 25 The 28–30 September storm is characteristic of storms producing major floods with Full Screen / Esc greater than 10-year recurrence in northern Thailand. Heavy rainfall occurs along a westerly-travelling storm path after landfall of a South China Sea typhoon. These Printer-friendly Version storms are limited in their north-south extent, and often occur in August and Septem- ber. The 14 September 1994 flood on the Ping River, for example, was associated Interactive Discussion 3841 EGU with the westerward traveling storm from Typhoon Harry. Kidson et al. (2005) indicate that the 12 August 2001 storm produced a 16-year recurrence flood peak on the Mae HESSD Chaem River, and the BBC news (2001) reported this storm caused disastrous flood- 4, 3839–3868, 2007 ing in Vietnam and in the Petchabun Province of Thailand associated with Typhoon 5 Usagi. Such storms, however, are not restricted to the late summer monsoon. The 21–23 May 2006 storm was a low-pressure system that produced heavy rainfall, disas- Floodplain sediment, trous flooding and mudslides in the Dannang Province of Vietnam and the Uttaradit and 30-year flood, Ping Sukothai Provinces of northern Thailand (Asian Disaster Preparadness Center, 2006). River, Thailand This weather system occurred 5 days after the eye of Typhoon Chanchu changed its S. H. Wood and 10 track from NW to NE about 1000 km off the coast of Vietnam, apparently a result of unsettled atmospheric conditions at the onset of the summer monsoon. A. D. Ziegler Because sediment concentrations during large flows in the Ping River typically ex- ceed 500 mg L−1 (Royal Irrigation Department, 1995, 1996, 1997), the potential for sub- Title Page stantial “silt” deposition during flooding is high – although this phenomenon has never 15 been documented in detail. Most studies of sediment deposition during individual big Abstract Introduction floods has been on European and North American large rivers that commonly have suspended sediment loads less than 500 mg L−1 (Asselman and Middelkoop, 1995; Conclusions References Gomez et al., 1995). Understanding sedimentation from infrequent large floods on Tables Figures moderate-sized SE Asian rivers is of interest because sediment concentrations can 20 be quite high. Depositional evidence of these events, however, is often short-lived on J I account of lush vegetation and frequent cultivation of the floodplain areas. It is intuitive to think that the depth of floodplain sedimentation is related to the duration of the over- J I bank stage and the concentration of suspended sediment at the time of inundation. In Back Close this work we measured the thickness of mud sediment deposited on the floodplain of 25 the Ping River following 29 September 2005 flood event. The purpose of our study was Full Screen / Esc to understand the nature of storms producing 30-year recurrence floods, to investigate phenomena affecting the sedimentation patterns, and to describe the sediment to aid Printer-friendly Version in interpretation of floodplain stratigraphic studies in the area (e.g. Wood et al., 2007). Interactive Discussion 3842 EGU 2 Study area HESSD The Ping River drains a mountainous area of northern Thailand with steep hills up to elevation 1500 to 2000 m, and valleys at 330 to 500 m (Fig. 3). The Ping River basin 4, 3839–3868, 2007 is underlain by older Paleozoic gneissic granites, Paleozoic sediments and volcanics, 5 Mesozoic granitic rocks, and Tertiary continental basin-fill sediments (Hess and Koch, Floodplain sediment, 1979; Rhodes et al., 2005). The lowlands are underlain by alluvial fan, terrace, and 30-year flood, Ping floodplain deposits (Margane and Tatong, 1999). Upland areas and older terrace and River, Thailand fans have deep weathering profiles of saprolite one to tens of meters thick overlain by red-yellow argillic soil horizons one to several meters thick. Surface soils are dark S. H. Wood and 10 brown loams up to 25 cm thick. Valley bottoms are mostly clayey silt with gleyed soils A. D. Ziegler in the paddy areas. It was estimated that 70 per cent of northern Thailand highlands were covered by subtropical forest in 1960.