MEKONG RIVERBANK EROSION OF THE 14th CENTURY CITIES OF CHIANG SAEN NOI, THAILAND AND SOUVANNAKHOMKHAM, LAOS, PDR

Steep silt banks, collapsed large trees, and crumbling brick ruins of the 14^th Century Buddhist monuments indicate river bank erosion on the Thai and Laos banks along reaches of the Mekong River in the Golden Triangle area of SE Asia. At the ancient city of Souvannakhomkham, Laos a large stucco and brick ruin of a 14^th–Century Buddha statue lies in the river bed, 180 m from the bank, indicating an minimum bank retreat of 0.3 m/a. At Chiang Saen Noi, Thailand, on the opposite bank, the remains of a 14^th Century brick well, originally dug at the floodplain level (10 m above the bed) is now in the riverbed gravel 20 m from the bank, indicating a minimum bank retreat of 0.033 m/a. River channel here is 600-m wide, with 10-m banks of silt overlying cobble gravels and sand of the former bed of the river. These minimum long-term rates are lower than the 0.8 – 1.0-m/a rates determined by Kummu et al (2007) for the more recent 1961-2005 period at a reach 600 km downstream at Vientiene, where the channel is about 700-m wide. Rates reported here for the Golden Triangle area are lower because they are minimum rates, or perhaps because of a recent change in river regime. The upper strata of the 10-m silt bank on the Laos side shows 1.1 m of floodplain silt deposited on the 14^th century cultural layer of pottery shards. This uppermost 1.1 m of silt is comprised of 3 individual silt layers of a flood, 0.2- 0.4-m thick, each with a thin soil, indicating that a least 3 overbank floods occurred over the ancient city of Souvannakhomkham within in the past 600 years. The most recent flood layer may have been deposited by the 1966 flood of 23,500 m³/s, considered to be a flood of slightly greater than 100-yr recurrence.