Basin-Scale Sediment Transport for Sustainable Sand Mining – A Case Study in Punatshangchhu Basin, Bhutan

Abstract

This case study evaluated sediment dynamics in the Punatshangchhu basin in Bhutan. Sand mining
operations in the basin commenced in 2007, with an average annual extraction rate of 0.3 × 106 m3
,
accounting for approximately 65% of the country's sand demand. However, sand mining has been
conducted without a comprehensive understanding of the basin's sediment budget. Therefore, this
study examines the sediment budget at the basin scale using numerical modeling. Two numerical
models were developed: the Rainfall-Runoff-Inundation (RRI) model simulated the 2017 flood
event, while the Rainfall-Sediment-Runoff (RSR) model computed daily sediment yields during the
monsoon season (May–September). The RSR model was simulated for two cases: Case 1
considered sediment supply from the river channel only, while Case 2 considered sediment supply

from the river channel and slopes. The model results were evaluated against daily observed stream-
flow data, suspended sediment load, and particle-size distribution. The model achieved NSE,

PBAIS, and R2 values of 0.9, -0.12, and 0.9, respectively, in calibration. The validation achieved
similar NSE and PBAIS values, with an improved R2 of 0.94. The total sediment yield in Case 2
was 34% higher than in Case 1. The model results showed balanced sediment inflow and outflow
in the basin. However, sediment extraction at the current rate is unsustainable, with riverbed
degradation at a rate of 0.26 m/year. Nevertheless, a downstream hydropower dam is expected to
create an opportunity for sustainable mining.