Subsidence has resulted in increased flood risk, as Bangkok is already prone to flooding due to its low elevation and an inadequate drainage infrastructure, often compounded by blockage from rubbish pollution (especially plastic waste). The city now relies on flood barriers and augmenting drainage from canals by pumping and building drain tunnels, but parts of Bangkok and its suburbs are still regularly inundated. Heavy downpours resulting in urban runoff overwhelming drainage systems, and runoff discharge from upstream areas, are major triggering factors. Severe flooding affecting much of the city occurred in 1995 and 2011. In 2011, most of Bangkok's northern, eastern and western districts were flooded, in some places for over two months.