Several navigable channels lead through these shoals to the mouths of the Sungai Kuala Kualu and the Sungai Panai.
Between Tanjung Siapiapi and the drying banks off the mouth of the Sungai Panai, the flood sets into Teluk Piai Geul and follows the direction of the channel.
The ebb sets on to Tanjung Siapiapi. Off Tanjung Pertandangan, the ebb sets to the E.
Both off and within the river mouths, for some time after the flood begins, the surface water continues to run out while an undercurrent sets inward. Outside the 10m curve N of the banks, the flood sets SE. The greatest rate of the current outside the 10m curve is 2 knots; in the channels and rivers, the greatest rate is 3 to 4 knots. There is very little slack water at springs.
Outside, each current runs for about 6 hours, but farther in the ebb runs for 7 hours and the flood for 5 hours. The currents turn about 45 minutes after HW and 30 minutes after LW.
Off Tanjung Prapat and Tanjung Datu the flood sets towards the coast while the ebb sets strongly to the NW.