problems in the distribution of the float using Fdx, Fdy Fdz

[zhaoyun]

Dear all,
Currently, I use the Lagrangian float to track the flow. Some parameters are as following (Picture 2, Fx0, Fy0 is in grid units, and Fz0 is in meter). I throw 10 floats in each grid, and the Fdx is 0.1 (grid units). I choose 'initial horizontal coordinate type' as 0 (grid units). Now there is a problem. Please see the first picture. It is the initial distribution of the float. Since the vertical level is in S co-ordinate, Fdx adds 0.1 grid, although Fdz is 0, there is a small increase in depth. In fact, the floats are all expected to be at -17 m, but the depth of the 2nd to 10th floats are not at -17 m. Could anyone help me to solve this problem? Thank you very much.