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.
Gopakumar et al. (Thu,) studied this question.