Feedback Between Channel Conveyance and Flood Frequency Under Stage–Discharge Rating Curve Uncertainty: Insights From the Pulangi River (Philippines)

ABSTRACT

In alluvial rivers, changes in channel conveyance can alter flood frequency, just as floods can reshape channel morphology. These bidirectional feedback over engineering timescales may be significant enough to challenge assumptions of streamflow stationarity that underpin conventional understanding of flood behaviour. Using the Pulangi River (Philippines) as a case study, this work explores three questions around hydrogeomorphic dynamics: (1) How does riverbed instability relate to shifts in channel capacity? (2) Do floods of specific magnitudes predictably modify the channel? (3) What is the relative contribution of geomorphic drivers to flood frequency trends, independent of upstream factors? Through gauge‐based analyses of rescued hydrometric data (1983–2020), changes in channel capacity, bed elevation and flood probabilities are estimated within an ensemble framework that accounts for uncertainties in the stage–discharge relationship and propagates them to subsequent computations of trends in rating curve residuals. Results reveal significant long‐term changes: The riverbed deepened by 1.40 m, thus augmenting channel capacity by 64%, with each centimetre of annual incision corresponding, on average, to 1.27 m ³ /s of additional conveyance. However, short‐term morphodynamic adjustments (scour vs. fill, cross‐sectional area expansion vs. contraction and channel roughening vs. smoothing) cannot seem to be predicted as a function of peak‐flow magnitude alone, suggesting that other factors, such as sediment supply fluctuations and inter‐event low‐flow conditions, may exert a stronger control on channel response. Lastly, the conveyance gains amounted to a 74% reduction in flood exceedance frequency over 35 years, whereas hydrologically driven flood occurrence showed interannual variability but little discernible long‐term change. Consequently, the channel could accommodate 83% of the stationary 10‐year flood discharge by 2020, compared with only 66% in 1985, before reaching capacity. These findings exemplify how fluvial geomorphic changes may be equally, if not more, influential in explaining evolving flood risks, particularly for rivers under extreme anthropogenic pressure.