Archer et al. (2025) Characteristics of gauged abrupt wave fronts (walls of water) in flash floods in Scotland

Identification

• Journal: Hydrology and earth system sciences
• Year: 2025
• Date: 2025-10-28
• Authors: David Archer, Felipe Fileni, Sam A. Watkiss, Hayley J. Fowler
• DOI: 10.5194/hess-29-5777-2025

Research Groups

• School of Engineering, Newcastle University, United Kingdom
• JBA Trust, North Yorks, United Kingdom
• JBA Consulting, Skipton, North Yorks, United Kingdom

Short Summary

This study characterizes abrupt wave front floods (AWFs) in Scotland using gauged river level and discharge data, revealing their widespread occurrence, downstream intensification, and the inadequacy of traditional flood forecasting models to represent their distinct hazard to human life.

Objective

• To present the spatial distribution of annual maximum 15 min rises in river level and discharge in Scotland, along with derived metrics to assess the severity of Abrupt Wave Front (AWF) events.
• To investigate the downstream evolution and hazard potential of AWFs, particularly whether flood forecasting models focused on peak discharge and river level adequately represent the risk posed by rapidly rising flows.

Study Configuration

• Spatial Scale: 260 gauged catchments across Scotland, ranging in area from less than 50 km² to 2861 km². Specific detailed analysis on the River Findhorn (catchment area 416 km² at Shenachie, 782 km² at Forres).
• Temporal Scale: River level and flow records from the 1950s onwards, with a median length of 33 years. Analysis focuses on annual maximum 15 min rises, primarily during the summer months (April to September). Wave celerity estimated over reaches of tens of kilometers (e.g., 49 km on the Findhorn) with travel times of several hours.

Methodology and Data

• Models used: No explicit hydrological or hydraulic models were used for the analysis. The study primarily involved statistical and time-series analysis of observed data.
• Data sources:
  • 15 min flow and level time series data from the Scottish Environment Protection Agency (SEPA) time series data service (API).
  • Station selection based on availability of both flow/level data and a National River Flow Archive (NRFA) identifier.
  • Data quality control (QC) procedure (Fileni et al., 2023) and visual inspection to validate AWF events.
  • Derived metrics:
    • Absolute maximum 15 min rise in river level (HW15abs).
    • Absolute maximum 15 min rise in discharge (QW15abs).
    • 15 min rate of rise normalized by the median annual maxima peak flow (QMED), termed QW15QMED.
    • Ratio of maximum to median 15 min annual maximum rise in discharge (QW15Ratio).
    • Proportional increase in flow from initial to peak of 15 min rise (QW15Prop).
  • Wave celerity estimated by analyzing time differences in wave onset at successive gauging stations along a river channel.

Main Results

• Abrupt Wave Fronts (AWFs) are widespread across Scotland, with concentrations in the northeast and southern Scotland, and fewer observations in the western Highlands. They are suppressed by upstream lakes and reservoirs.
• AWFs were observed on catchments up to 2861 km² in area, with a median catchment area of 201 km². Only 10.8% of observed AWFs occurred on catchments smaller than 50 km².
• The severity of AWFs is significant, with 15 min increases in level exceeding 1.4 m and/or discharge increases over 100 m³/s observed at 12 stations. The highest observed HW15abs was 1.88 m (Ruchill Water at Cultybraggan), and the largest QW15abs was 216 m³/s in 15 min (River Findhorn at Forres).
• The median QW15QMED (15 min rise normalized by QMED) was 0.36, with some stations exceeding 0.8, indicating extreme severity relative to typical peak flows.
• The median QW15Ratio (maximum to median 15 min rise) was 2.9, but extreme ratios (> 5.0) were found on catchments where the absolute rise was not necessarily the highest, suggesting unexpected severity.
• The median proportional increase in flow (QW15Prop) was 10 times the initial flow, with 11 stations showing increases greater than 30 times, and one station (Ettrick Water at Brockhope) exceeding 100 times.
• Wave celerity was estimated to range from 2.86 m/s to 3.6 m/s on rivers like the Findhorn and Tyne.
• AWFs can intensify downstream, with wave fronts steepening as they travel through lowland river reaches, as observed on the River Findhorn, supporting kinematic wave theory.

Contributions

• Provides novel, large-sample evidence and a comprehensive characterization of abrupt wave front floods (AWFs) in Scottish rivers, including their spatial distribution and downstream evolution.
• Introduces and applies new metrics (normalized, ratio, and proportional measures of flow change) to quantify the severity and hazard potential of AWFs, moving beyond traditional peak level and discharge indicators.
• Demonstrates, with real-world gauged examples, the phenomenon of flood wave steepening and intensification downstream, supporting kinematic wave theory and highlighting a critical aspect of AWF dynamics previously subject to uncertainty.
• Emphasizes that current flood forecasting models, focused on peak flows, may not adequately capture the distinct risk to life posed by rapidly rising flows, advocating for separate consideration of AWFs in monitoring, modeling, and warning systems.
• Highlights the need for improved monitoring, including sub-15 min data resolution and non-contact measurement technologies, especially in headwater catchments and downstream reaches where AWFs are generated or intensify.

Funding

• NERC-funded One Planet Doctoral Training Partnership (grant no. NE/S007512/1)
• UKRI-funded Co-Centre for Climate + Biodiversity + Water (grant no. NE/Y006496/1)
• Scottish Environment Protection Agency (SEPA)

Citation

@article{Archer2025Characteristics,
  author = {Archer, David and Fileni, Felipe and Watkiss, Sam A. and Fowler, Hayley J.},
  title = {Characteristics of gauged abrupt wave fronts (walls of water) in flash floods in Scotland},
  journal = {Hydrology and earth system sciences},
  year = {2025},
  doi = {10.5194/hess-29-5777-2025},
  url = {https://doi.org/10.5194/hess-29-5777-2025}
}