Determinations of the Slope of the Chao Phraya Riverbank and Manning’s Roughness Coefficient Utilizing Precise Point Positioning (PPP) Measurement Technique

Flooding in Thailand, particularly in the Chao Phraya River basin, is a recurrent challenge exacerbated by seasonal monsoons. The catastrophic 2011 floods highlighted the urgent need for accurate flood prediction and management strategies. This study aimed to enhance flood modeling through precise determination of riverbank slopes and Manning’s roughness coefficients using Global Navigation Satellite Systems (GNSS) and Precise Point Positioning (PPP) techniques. Elevation data were collected along the riverbanks from Ayutthaya to Bangkok using various GNSS positioning methods, with CSRS-PPP identified as the most reliable technique for height difference measurements. The study determined the river’s slope to be approximately 0.00003, translating to a gradient of 1:33,333. Manning’s roughness coefficients were calculated using observed discharge data and were found to range between 0.035 and 0.045 across different river stations. The derived roughness coefficients were validated against actual discharge measurements, revealing seasonal variations in flow resistance. The study further established empirical relationships between river depth, cross-sectional area, hydraulic radius, and discharge, enabling more accurate hydrological modeling. The results contribute to improved flood prediction and mitigation strategies by offering a reliable methodology for terrain elevation assessment and flow rate estimation in flood-prone regions.