hydrology Article Multi-Objective Model-Based Assessment of Green-Grey Infrastructures for Urban Flood Mitigation Carlos Martínez 1,2, * , Zoran Vojinovic 2 and Arlex Sanchez 2   Citation: Martínez, C.; Vojinovic, Z.; Sanchez, A. Multi-Objective Model-Based Assessment of Green-Grey Infrastructures for Urban Flood Mitigation. Hydrology 2021, 8, 110. https://doi.org/10.3390/ hydrology8030110 Academic Editors: Pierfranco Costabile and Luis Cea Received: 13 June 2021 Accepted: 15 July 2021 Published: 27 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Program of Civil Engineering, Faculty of Engineering, Universidad del Magdalena, Carrera 32 No. 22-08, Santa Marta 470004, Colombia 2 Environmental Engineering and Water Technology Department, IHE Delft Institute for Water Education, Westvest 7, 2611 AX Delft, The Netherlands; z.vojinovic@un-ihe.org (Z.V.); a.sanchez@un-ihe.org (A.S.) * Correspondence: cmartinez@unimagdalena.edu.co Abstract: This paper presents the performance quantification of different green-grey infrastructures, including rainfall-runoff and infiltration processes, on the overland flow and its connection with a sewer system. The present study suggests three main components to form the structure of the proposed model-based assessment. The first two components provide the optimal number of green infrastructure (GI) practices allocated in an urban catchment and optimal grey infrastructures, such as pipe and storage tank sizing. The third component evaluates selected combined green-grey infrastructures based on rainfall-runoff and infiltration computation in a 2D model domain. This framework was applied in an urban catchment in Dhaka City (Bangladesh) where different green- grey infrastructures were evaluated in relation to flood damage and investment costs. These practices implemented separately have an impact on the reduction of damage and investment costs. However, their combination has been shown to be the best action to follow. Finally, it was proved that including rainfall-runoff and infiltration processes, along with the representation of GI within a 2D model domain, enhances the analysis of the optimal combination of infrastructures, which in turn allows the drainage system to be assessed holistically. Keywords: coupled 1D/2D model; green-grey infrastructure; infiltration; multi-objective optimiza- tion; urban flood mitigation 1. Introduction Retrofit solutions for the management of urban infrastructure have been successfully applied in cities worldwide [1]. They have been proven to be a cost-effective solution to manage flood risk, whilst also delivering a range of other benefits [2–4]. These solutions include constructed structures such as treatment facilities, sewer systems, stormwater systems, and storage basins, which are known as grey infrastructure. A strategically planned network has also been used as an approach that projects, restores, or mimics the natural water cycle, also known as green infrastructure (GI). Previous implementation of these practices suggests that the combined green-grey measures turned out to be more effective than the grey-only option [5,6]. Projects attempting to enhance the performance of retrofit solutions in urban catch- ments have discovered significant improvements, focusing on: (i) overcoming uncertainty and barriers using blue-green infrastructures for risk management [7,8]; (ii) proposed frameworks to assess green infrastructure to mitigate urban flood hazards [9,10]; (iii) mod- elling the interference of underground structures by groundwater flow and potential remedial solutions for this [11]; and (iv) integrating strategies to improve the microclimate regulation of green-blue-grey infrastructures in specific urban forms [12]. The results of these studies have produced, among others, a comprehensive evaluation of the integration of green-grey practices. Hydrology 2021, 8, 110. https://doi.org/10.3390/hydrology8030110 https://www.mdpi.com/journal/hydrology