Assessing the impact of plastic waste accumulation on flood events with citizen observations

Abstract

This research assesses the impact of plastic waste accumulation on flood events with citizen observations in Kumasi, Ghana. In Kumasi, the fastest growing city in Ghana, flooding events have become more intense due to rapid urbanisation and land use intensification. Additionally, rapid population growth, urbanisation and lack of organised waste collection, brings along tonnes of solid (plastic) waste on the streets, riverbeds and -ways. Plastic waste creates blockages in drains and rivers, which causes flooding of open areas, streets and houses. To what degree plastic waste accumulation aggrevates flooding in Kumasi remains poorly understood. Therefore, this study investigates to what extent plastic accumulation impacts flood severity, in response to rainfall events in the city of Kumasi, Ghana. This is done by modelling different rainfall scenarios in a 1D-2D model developed with the software HEC-RAS. This model was forced with static and dynamic data available from open source databases and collected during fieldwork. Data collection took place during six consecutive weeks in May-June 2019 and consists of bathymetry and discharge measurements, a flood- and plastic waste survey performed together with citizens. The impact can be determined using different model scenarios varying in rainfall magnitude and in obstruction of riverways, serving as plastic accumulation. Flood severity was analysed in terms of extent, depth, velocity, local hydraulic impact and flow distribution. The model has been validated with two events, one during which velocity and stage was measured, which results in the same order of magnitude, modelled slightly lower than measured in the field. The second event was a historic rainfall event, which could be validated with community reported flood depths. From this validation it became clear the flood extent of the event was underestimated and depths close to the streams were overestimated. The results show that the selected rainfall events without blockage cover the studied area between 11-25% with water depths higher than 10 cm. The inundated areas are adjacent to the river streams and inundate flat and low lying areas. Comparing this to the scenarios where bridges where obstructed to 33% and 67% individually, the total inundated area remains more or less the same, varying between 0-3% in inundated area compared to the scenario without blockage. However, in terms of flood distribution, areas around the blocked bridge are impacted more, thus inundation areas shift within the observed study area. Comparing this to the rest of the area, upstream or downstream less flooded areas arise, making the change in total flood extent only change slightly. Furthermore, the two bridges that are obstructed have different impact on the flow in the study area, as the scenarios of obstructing the second bridge with 33% produce a numerical instability and therefore incorrect results. From the plastic waste survey it followed that plastic waste is very much abundant in the study area, thus the occurrence of plastic accumulation at the bottleneck locations seem inevitable. The results of this research have shown the main contribution to flooding is flow from upstream areas exceeding river capacity, while local blockages mainly aggravate flooding around the blocked locations.