Authors: Mohammad Reza Hassani, Teemu Kokkonen, Nora Sillanpää & Harri Koivusalo
Publication type: A1
Study region
Saunalahdenranta (SR) catchment, Espoo, southern Finland
Study Focus
Urban construction phases introduce abrupt and nonstationary disturbances to stormwater systems, yet their hydrological and water quality impacts remain poorly quantified. This study investigates construction-phase changes in event-scale runoff–pollution dynamics using a paired-catchment counterfactual modeling framework. A nearby stable urban catchment, Vallikallio (VK), was used to define reference-informed buildup–washoff behavior without active construction disturbance, while the SR model was used to simulate a stable benchmark under observed meteorological forcing. Observed–counterfactual differences were evaluated using non-parametric effect sizes and load decomposition. Understanding this dynamic change improves stormwater modeling and adaptive management under nonstationary conditions.
New hydrological insights for the region
Results showed a sharp, time-bound degradation in stormwater quality, with the annual median of event-mean TSS concentrations increasing 6.6-fold from 2001 to the 2004 construction peak. Rank-biserial effect-size analysis indicated that the probability of observing higher TSS concentrations in SR than in the reference catchment increased to approximately 70–75% during peak construction. Counterfactual comparison further showed that the 2004 observed EMC median was 3.3 times the corresponding reference-informed counterfactual median, which distinguishes the broader observed temporal change from the peak-year divergence relative to stable urban conditions. Load decomposition showed that early-stage deviations were partly hydrology-related, whereas peak-construction divergence was dominated by concentration-related underestimation, consistent with elevated sediment availability. These findings suggest that construction phases can be associated with short-lived but high-impact stormwater-quality regimes not captured by static buildup–washoff parameterizations.