Discussion on “performance of sustainable drainage capillary barrier systems for climate change adaptation in temperate climates”

This discussion critically examines the methodological approaches employed in Holmes et al.'s (2025) study on sustainable drainage capillary barrier systems (SuDS-CBS) for temperate climates. While acknowledging the study's valuable contributions to climate-adaptive infrastructure, several fundamental limitations in hydraulic characterization are identified that may compromise the reliability of barrier design and analysis. The primary concerns include: (1) incomplete soil-water characteristic curve (SWCC) determination using HYPROP 2 limited to 0-100 kPa suction range, inadequate for water treatment residual (WTR) amended soils that exhibit significant hydraulic behavior at higher suctions; (2) insufficient consideration of tensiometer response time and equilibration requirements, potentially introducing systematic errors in suction measurements; (3) questionable breakthrough mechanism interpretation based solely on hydraulic conductivity convergence rather than suction equilibrium, particularly relevant for fine-grained WTR systems; and (4) absence of long-term performance evaluation under cyclic wetting-drying conditions and biodegradation effects of organic-rich WTR amendments. The discussion emphasizes the necessity for multi-method SWCC approaches, proper hydraulic equilibration protocols, and suction-based breakthrough criteria. These methodological improvements are essential for accurate hydraulic characterization and reliable prediction of capillary barrier performance in climate-adaptive applications.