Numerical estimation of inflow flux of floating natural macro-debris into Tokyo Bay

•We numerically estimated the inflow flux of grass from rivers into Tokyo Bay.•We applied a two-way particle-tracking model and an inverse method.•We used HF radar-derived surface velocities and data on grass collected in the bay.•The inflow fluxes were significantly related to the river discharge.•We successfully reproduced an amount of debris collected in the bay.

We numerically estimated the inflow flux of terrestrial grass, which is the main floating macro-debris, into Tokyo Bay from April 2008 to March 2009 based on a two-way particle-tracking model and an inverse method applying a Lagrange multiplier. In the estimation, we used surface current velocities derived by high-frequency ocean radar and the quantity of grass collected by clean-up vessels which are operated daily in the bay. At least 2115 m3 yr−1 of the grass flowed into the bay annually, and the contribution of a flood event to the inflow flux of grass was larger than that of the inflow flux of freshwater. We show that 39% of the annual inflow flux of grass into the bay was collected, and 61% flowed out of the bay or sank to the seabed. The numerical estimation in this study will be useful to establish a system for predicting patches of floating macro-debris in the bay, and to evaluate the effects of river development or clean-up along river banks and flood plains in the upper reaches.