Source-to-sink sediment fluxes and budget in the Chao Phraya River, Thailand: A multi-scale analysis based on the national dataset

Abstract

In this paper, we provide a holistic view of the hydro-sedimentological regimes of the Chao Phraya River (CPR) Basin, the fifth largest basin in Southeast Asia. Our analysis of daily discharge and sediment data from 42 major gauge stations showed high seasonal variation in the suspended sediment discharge (Qs), with the maximum discharge occurring in October on an inter-annual average. At Nakhon Sawan, the river discharges 304x104 tons of sediment every year, 60% of which is transported in the peak flooding months (September to November). The peak sediment discharge in October is dramatically attenuated at a station 150 km downstream from Nakhon Sawan, mainly due to discharge diversion to irrigation canals, distributaries, and branches, as well as seasonal flooding over the Central Plain.

Sediment yield (SY) calculated at major stations showed spatial variability across the basin, generally decreasing in a downstream direction (i.e. as drainage area increases) (R2 = 0.55). Some stations in the upper basin’s watersheds showed SY as high as >700 tons/km2/yr, which is comparable to upstream catchments of other large rivers with high sediment production. Two significant sediment sinks were identified in this study: the Bhumibol and Sirikit dams (each on Ping and Nan Rivers) – which trap ~236 × 104 tons of sediment each year (~90% trapping rate) – as well as the floodplain of the CPR which stores 176 × 104 tons annually along the 150 km downstream reach from Nakhon Sawan (~60% of Qs at Nakhon Sawan). By extrapolating the floodplain sediment budget, we estimate that around 300 × 104 tons/yr of suspended sediment can be stored downstream of Nakhon Sawan (to the Gulf); comparable to the total annual storage of the two mega dams upstream. Although these dams have been previously reported to cause substantial sediment starvation in the CPR Delta, this study is the first to recognize the important role of lowland storage on the CPR’s basin’s sediment discharge to the Gulf of Thailand, and how it contributes a similar degree of threat to the shrinking delta.