EVOLUTION OF THE WEST TOKYO FLOODPLAIN AND MULTI-HAZARDS ALONG THE TAMAGAWA

In the immediate aftermath of the Second World War, Tokyo rapidly expended to the West, where some of the most expensive wards of the capital and the second international airport of Tokyo, Haneda, have emerged. This rapid expension of the city laid an apparent homogeneous carpet of concrete over the Tamagawa floodplain sediments - i.e. abandoned channels, point-bars, swamps, paddy fields and holocene marine sediments. At the same time, this post-world war reconstruction fueled intense gravel and sand mining along the Tamagawa river, creating even further heterogeneity in the subsurface.

In the present contribution, we have analysed the historical changes of the landcover and the subsurface, in order to understand the potential impacts a major earthquake would have on the city, including the immediate secondary hazards (tsunamis) and longer term secondary hazards (floods). The method is based on a hydrological, geomorphological and sedimentological analysis, as well as a comparison with the events and research carried out in Christchurch, New Zealand, in the aftermath of the February 2011 Earthquake, using the framework of "Coastal Earthquake" (term coined by Dr. D. Hart).

The results have revealed that for a major earthquake, the soils are most likely to react and accelerate with singular local variations, due to the origional heterogeneity of the subsurface, but also due to the complex human interventions. The proximity of the water-table makes the Tamagawa floodplain prone to liquefaction, and the polluted soils are a further worry of what could resurface (from mutagenic material - detected in the Tamagawa River - to radioactive material - found in Setagaya-ku in 2012). The local geomorphology also dictates where the liquefaction material may pond and be concentrated: burried - and still running - channels, paleo-channels, historic paddy fields laid with clay. As the Tamagawa River has been harnassed with a 500 m wide floodplain, results show that a major tsunami would flow upstream meeting the first major barrier at Noborito, where the river is mostly prone to flood. As the main river has a complex protection system, river floods similiar to those observed in Christchurch are most unlikely, except for the local "gerira-gou", which are very intense localised storm that often fill the storm-drains in summer.