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.
ABSTRACT Riverine sediment, a critical element for biogeochemical cycles, is subject to the interacting controls of climate, vegetation, cryosphere processes and anthropogenic infrastructure. This study established a comprehensive attribution framework to quantify various drivers' contributions to suspended sediment flux (SSF) changes in the Upper Lhasa River Basin and analyse synchronous and asynchronous occurrence probabilities, thereby elucidating the profound impact of anthropogenic regulation on sediment transport mechanisms. Results indicate that reservoir commissioning coincided with a critical regime shift, transitioning sediment transport from a climate‐dominated to a regulation‐dominated state. Compared with the pre‐dam period (1982–2005), mean annual SSF in the post‐dam period (2006–2022) decreased by 78.8%, decoupling from the 14.8% reduction in discharge. Double mass curve analysis indicated that anthropogenic influences and precipitation variability accounted for 90.1% and 9.9% of the SSF reduction, respectively. Reservoir regulation disrupted longitudinal sediment connectivity and promoted an increasingly supply‐limited regime downstream. This transition overrode the effects of increased precipitation, discharge, and thaw depth, reversing the climate‐driven increase in SSF (42.5% 10 year −1 ) into a significant decline (−21.3% 10 year −1 ). Concurrently, anthropogenic regulation, particularly reservoir operation, weakened the responsiveness of SSF to environmental variability by attenuating the transmission of precipitation signals to sediment transport. Accordingly, precipitation–SSF synchrony declined from 63.4% to 41.9%, while the joint probability of high precipitation–high SSF decreased from 26.83% to 5.52%, providing indirect evidence consistent with sediment‐peak attenuation under reservoir regulation. These findings highlight the reshaping impact of reservoir regulation on sediment driving mechanisms and transport modes in alpine basins and provide a novel perspective for watershed management.
ABSTRACT To assess the long‐term sustainability of soil retention in erosion‐prone karst regions, investigating historical soil erosion processes and their mechanisms in Southwest China, a typical karst‐dominated region, is essential. While soil erosion processes over the last ~60 years have been widely examined in Southwest China, longer term records remain rare. Here, we examined soil erosion processes in two karst depressions over the past ~250 years in Chongqing Municipality, located in the northern part of Southwest China, utilizing 137 Cs, 210 Pb and AMS 14 C dating on sediment profiles. These results were then compared with published soil erosion rate data to identify how soil erosion processes in Chongqing are similar to or different from those in other parts of Southwest China. The results exhibited high soil erosion rates from ~1750s to ~1935–1945 ad at both depressions, related to the Hu‐Guang Fills Sichuan Migration, Zea mays cultivation expansion, and elevated precipitation. Erosion rates declined after ~1935–1945 ad , likely due to wartime impacts. Slight soil erosion peaks appeared during ~1950–1960 ad , linked to the Great Leap Forward and the Great Steel Campaign. Subsequently, a long‐term decline in soil erosion rate occurred from ~1960s to 2018 ad . A compilation of published data reveals that this declining trend was in fact a region‐wide phenomenon in karst regions across Southwest China. This period of soil erosion decline is possibly linked to reduced erodible soil stocks due to earlier severe erosion and cessation of the Great Steel Campaign and hence reduced deforestation, combined with the large‐scale ecological restoration projects implemented since the 1990s. This study provides a scientific basis for projects of improving sustainability of karst ecological restoration.
ABSTRACT Traditional field methods rely on tape measures to record boulder dimensions, whilst modern handheld LiDAR and Structure from Motion photogrammetry approaches are increasingly used to obtain dimensions and volumes with potentially greater accuracy. Although drone‐based remote sensing remains underutilised for boulder studies, it shows promise for rapidly collecting large datasets by digitising boulder outlines on orthomosaics of aerial photographs. This study assesses the accuracy of using boulder statistics extracted from digitised perimeters and LiDAR generated 3D models, and tests whether the results are statistically comparable with manually collected field data. These methods were tested on boulder ridges at the Grind o' da Navir, Shetland, Scotland. When compared with traditional field measurements, LiDAR‐derived volumes are up to 37% smaller, suggesting that manual methods may systematically overestimate boulder size. When validated on the same boulders, the 2D digitisation method yielded mean differences of just 0.05 m for the A‐axis and 0.02 m for the B‐axis, with similar histogram distributions to the traditional method. A‐axis orientation results also show broadly similar distributions between the two methods, implying the digitised perimeter method can be used to reveal flow directions of the causative storm waves. These findings demonstrate that the boulder digitisation method is suitable for investigating entire deposits of thousands of boulders, quicker than with traditional field methods, with good accuracy at population level.
ABSTRACT Beach cusps have been widely studied due to their distinct patterns and prevalence across many regions. However, mesoscale behaviour and spatial distribution of beach cusps have received limited attention, leaving a knowledge gap in the comprehensive understanding of the conditions conducive to beach cusp development. This study examined spatial and temporal dynamics in beach cusps associated with wave conditions and alongshore beach volume changes. Monthly to bi‐monthly uncrewed aerial system (UAS) surveys were conducted for 19 months along a 2.7 km stretch of Wonpyeong Beach, an intermediate beach on the east coast of South Korea. The quantity of beach cusps was significantly related to Cumulative Average Wave (CAW) indices (significant height, peak period and runup height) 2 and 3 days preceding the observation. The range of cusp elevation observed in each survey was significantly correlated with the ranges of wave indices between consecutive surveys, indicating the influence of multiple event‐scale wave conditions on the multi‐level beach cusp development. Previously unexplored, 74.3% of the beach cusps were present in an accretionary environment related to megacusp horn development or embayment infill. Several erosional cases, associated with beach nourishment and storm events, support that post‐event sediment‐abundant foreshores may facilitate beach cusp development. The study findings collectively suggest that event‐scale wave conditions and mesoscale nearshore dynamics influence beach cusps. Future research on mesoscale surf zone dynamics and sediment transport mechanisms is necessary to systematically understand the relationship between beach cusp development and nearshore processes.
