| 17 Aug 2026
Why has flood risk increased in a millennia-old mountainous town? Engineering-induced transformation of compound flood pathways in Baifusi, China
Abstract. Baifusi, a long-inhabited mountain-river town in China, coexisted with floods, yet damaging inundation has become concentrated in the recent engineering era. This study asks why flood risk has risen markedly over the last decade. We combine hydrological monitoring, field floodmarks, historical satellite images, drainage calculations, reservoir-operation rule learning, operation logs, surveyed terrain reconstruction, one-dimensional–two-dimensional coupled hydrodynamic scenario modelling, threshold analysis and interpretable surrogate-model evidence. The evidence shows that rainfall magnitude alone is insufficient: the 2016 event had a rare 24 h rainfall return period but a moderate flood-peak return period, while the 2023 event combined high Najitan release, Qiedao tributary inflow, Jinlongtan downstream backwater and local drainage blockage. Matched scenarios show a persistent 1.56–2.08 m tributary increment, whereas the downstream-backwater increment decreases from 1.67 m in the 5-year scenario to 0.03 m in the 100-year scenario. Relative to the 357.01 m hospital-entrance threshold, effective protection capacity decreases from about 42 years under the mainstream-only boundary to about 17 years with tributary inflow and 10 years under the full compound boundary. Archival images indicate a shift from sparse riverfront occupation in 2009 to a dense riverfront building belt by 2023. Pump-station calculations show that internal drainage capacity is 2.62–3.00 m³ s⁻¹, so ponding and backflow become critical when river stage is high. Flood-risk management in engineered mountain towns should move beyond a single return-period defence standard towards a process-based system that jointly manages upstream releases, tributary timing, downstream stage, drainage outlets and exposure control.
Overall Evaluation
This manuscript addresses the recent increase in flood risk in Baifusi, a long-inhabited mountainous river town. The study develops a process-oriented framework that links reservoir operation, tributary inflow, downstream backwater, terrain constraints, limited drainage capacity, and expansion of exposure in low-lying areas. The topic is well aligned with the scope of NHESS, particularly its focus on natural hazard processes, risk mechanisms, and disaster-risk management. Overall, the manuscript presents a relevant case, a clear scientific question, and useful implications for compound flood-risk governance. I recommend acceptance after revision.
Major Comments
1. The abstract should emphasize clearer quantitative and process-based insights. The authors should reduce generic background statements and highlight the main hydrological findings for the study area.
2. The introduction should define the research gap more explicitly. In particular, the limitation of single return-period, single-peak, or rainfall-only flood assessment should be clearly linked to the specific objectives of the paper.
3. The methods section should provide more detail on model configuration, including mesh setting, roughness zoning, boundary conditions, downstream-stage treatment, vertical datum harmonization, wetting-drying treatment, and scenario construction.
4. The S1, S2, and S3 scenarios are central to the manuscript. Their physical meaning, boundary-synchronization assumptions, and limitations should be explained more clearly.
5. Model validation should be strengthened. The authors should report the number and spatial distribution of floodmarks, the error-calculation method, and the boundary conditions used for the 2023 calibration and 2024 validation events.
6. The comparison of typical flood events could be improved. The manuscript should more clearly distinguish the dominant mechanisms of the 2016, 2021, 2023, and 2024 events, such as rainfall dominance, tributary contribution, backwater amplification, and external-mainstream flooding.
7. The evidence for exposure expansion in low-lying areas remains mainly qualitative. If possible, the authors should add semi-quantitative indicators, such as building expansion, riverfront-buffer occupation, low-lying development intensity, or representative cross-section changes.
8. The discussion should further develop the implications for return-period-based flood protection. A key message of the paper is that the same nominal return period may lead to very different impacts under different engineering-boundary combinations.
9. The conclusions should be more focused. They should be written as clear concluding statements aligned with the research objectives, rather than repeating detailed results and discussion.
Language and Formatting Issues
1. Units should be consistent throughout the manuscript, such as m³ s⁻¹ and events yr⁻¹.
2. Some figure captions are too long and should be shortened.
3. Table layout, column width, and line breaks need further adjustment.
4. Equation numbering, variable definitions, and symbol formatting should be checked carefully.
5. Maps, remote-sensing images, and UAV images should clearly indicate data source, date, and authorization.
6. Some long sentences should be divided to improve readability.
Recommendation
The manuscript addresses a relevant hydrological and natural-hazard problem, fits the journal scope, and provides useful insights for compound flood-risk management in mountainous towns. I recommend acceptance after minor revision.