Hydrographic survey of Chaktomuk, the confluence of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

Abstract

and the Cambodian Ministry of Water Resources and Meteorology, completed a hydrographic survey of Chaktomuk, which is the confluence of the Mekong, Tonlé Sap (also spelled Tônlé Sab), and Bassac Rivers near Phnom Penh, Cambodia. The hydrographic survey used a highresolution multibeam echosounder mapping system to map the riverbed during April 21-May 2, 2012. The multibeam echosounder mapping system was made up of several components: A RESON Seabat TM 7125 multibeam echosounder, an inertial measurement unit and navigation unit, data collection computers, and a Real-Time Kinematic (RTK) Global Navigation Satellite System (GNSS) base station. The survey area was divided into six survey subreaches and each subreach was surveyed within 3 days along survey lines oriented parallel to the flow direction. Complete coverage of the riverbed was the operational objective; however, to obtain broad spatial coverage, gaps between parallel swaths were permitted, especially in wide, shallow areas where multibeam swath widths were narrow. The survey was referenced to two existing bench marks with known geographic coordinates by establishing a GNSS base station on the bench marks each day and using real-time corrections from the base station to correct boat navigation data. The World Geodetic System of 1984 (WGS 84) ellipsoid was used during data collection to reference height, and data were adjusted to the local datum, Ha Tien 1960, during postprocessing. The quality of hydrographic surveys was described by an uncertainty estimate called total propagated uncertainty (TPU). Calculations of TPU were completed for the hydrographic survey data resulting in the maximum TPU of 0.33 meters. The mean and median TPUs were 0.18 meters, and 99.9 percent of TPU values were less than 0.25 meters. Detailed hydrographic maps of Mekong, Tonlé Sap, and Bassac Rivers showing the riverbed elevations surveyed April 21-May 2, 2012, referenced to Ha Tien 1960 were produced. The surveyed area included a 2-km stretch of the Mekong River between the confluence with the Tonlé Sap and Bassac Rivers, and extended 4 km upstream and 3.6 km downstream from the 2,000-m confluence stretch of the Mekong River. In addition, 0.7 km of the Bassac River downstream and 3.5 km of the Tonlé Sap River (from the confluence to Chroy Changvar Bridge) upstream from their confluence with the Mekong River were surveyed. Riverbed features (such as dunes, shoals, and the effects of sediment mining, which were observed during data collection) are visible on the hydrographic maps. All surveys were completed at low annual water levels as referenced to nearby Mekong River Commission streamflow-gaging stations. Riverbed elevations surveyed ranged from 24.08 m below to 1.54 m above Ha Tien 1960.

Linking rapid erosion of the Mekong River delta to human activities

“…5b ), and have been attributed to these massive channel bed sand extractions 48 . This mining activity has generated numerous pools and pits up to 15 m deeper than the natural channel bed levels in Cambodia 49 , and especially Vietnam, where the deepest pools generated between 1998 and 2008 are up to 45 m deep 48 . The numerous pits and pools created by large-scale sand mining actively trap bedload transported downstream during the high-discharge season 48 .…”

Analysis of bed changes in the segment of the Tien river flowing through Tan Chau

“…The result is increased riverbank erosion, which is an inevitable consequence of excessive and poorly managed sand mining [13,22,24]. Studies Dietsch et al (2014) has indicated that this activity has created numerous deep channels and pits in the riverbed, some as deep as 15m, exceeding the natural channel depth in Cambodia [25], especially in Vietnam, where the deepest channel reached 45m, formed between 1998 and 2008. The pits and deep channels created during large-scale sand mining become sediment traps during downstream transport during the flood season [13].…”