Modeling of flood and retention in the Sogn allotment garden

Abstract

More frequent and severe rainfall due to climate change, on the one hand, and more impervious surfaces due to urbanization, on the other hand, are increasing the risk of urban floods. Sogn allotment garden is mainly a green area located in an urban region in Oslo that offers natural retention. However, due to the terrain and soil condition and the surrounding area's characteristics, the allotment garden is also vulnerable to stormwater inundation. This study aims to model stormwater in the Sogn allotment garden and its adjacent regions that contribute to flood inundation in the area and estimate the inundation by source (within the garden vs. adjoining areas). It also aims to map the extent of the inundation.

The hydrological analysis and flood modeling were carried out through Geographic information system (GIS) and Storm Water Management Model (SWMM). GIS was utilized for streamlines generation and subcatchment delineation and to produce parameters such as slope and width of subcatchments as input data for the SWMM model. In addition to the GIS outputs, meteorological and site boundary condition data were employed as input data in the SWMM model. The model was run based on continuous simulation for the period of 01.01.2017 to 25.02.2022.

The model simulation results provided time series of runoff, infiltration, and evaporation. The model also estimated peak runoff events and maximum water depth, which was further used to generate flood inundation maps in GIS. Based on the volume and peak of the runoff from subcatchments generated by the model and further calculations on them, the sources of stormwater runoff were identified.

The results revealed that the main source of runoff in the study area is outside the allotment garden, from where runoff flows towards the garden. The garden, due to its terrain condition and the presence of the depression, encompasses larger inundated areas than outside the garden. However, the source of the runoff leading to inundation is mainly the adjacent areas outside the garden due to more impervious land cover and hence less infiltration than the allotment garden.