From rooftop pilots to campus-scale stormwater modelling: assessing green roof performance for urban flood mitigation in Hanoi

ABSTRACT

Graphical abstract: three bar-and-marker charts of green roof runoff volume reduction and peak flow attenuation, with peak delay by irrigation type, runoff delay by soil thickness, and peak delay by rainfall depth, above a map of the Hanoi University of Civil Engineering campus drainage network and a green roof cross-section.

Urban flooding is an increasing challenge in dense cities where impervious surfaces accelerate runoff and overload drainage systems. This study evaluates green roofs as a distributed source-control measure for stormwater management at Hanoi University of Civil Engineering (HUCE), Vietnam, using an integrated experimental–modelling framework that combines pilot-scale roof observations with SewerGEMS-based campus-scale drainage simulations. Green roof scenarios were assessed by varying antecedent moisture conditions, planting soil thickness, rainfall magnitude, and rooftop coverage. The results showed that antecedent soil moisture strongly controlled green roof performance. The non-irrigated green roof achieved the highest runoff reduction, peak attenuation, and peak delay, while irrigated systems performed less effectively due to reduced available storage. Increasing planting soil thickness from 80 to 250 mm improved runoff retention, with the 250-mm configuration providing the greatest hydrological benefit. At the campus scale, green roof implementation reduced runoff volume and peak discharge, with a maximum runoff reduction of 84.55% under full rooftop coverage. Water-level analysis at MH09, the most flood-prone drainage manhole in the campus stormwater network, showed that green roofs mitigated drainage stress under moderate rainfall but were insufficient as a standalone solution under extreme storms. The findings supported integrating green roofs into multi-layered urban stormwater strategies.