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.