Success is not often measured by the thickness of a layer of dirt. However, following trials at a construction site in Nelson of a new sediment control product called the ‘K-Log’, we have been really pleased by the amount of sediment captured in the dirty water channels . K-logs are 2m long mesh tubes filled with bark chips and a granular anionic poly acrylamide (PAM) flocculant. In a 50m section of channel, with a shallow grade and flat invert, we pinned jute (coconut husk) matting and placed a series of K-logs diagonally across the base.

Coastal View channel with K logs

After two weeks and several rain events totalling >75mm rainfall, we revisited the site and found a thick layer of entrapped sediment along the entire 50m length of jute matting. The sediment layer was up to 35mm thick and spongy and sticky to touch. Assuming an average of 25mm depth and 500mm width, the 50m length of channel captured more than 500Kg of sediment.

Coastal View sediment from K logs

EnviroCo is planning another trial that will investigate methods to remove, dry and dispose of the sediment/flocculant mixture. The sediment layer is cohesive allowing disturbance and removal without the risk of the resuspension of sediment particles.

A key reminder from this trial is that, wherever possible, the dosing of flocculant or coagulant should be as close to the sediment source as possible. This maximises the time available for sediment to become bound and settle out . The result of this is a reduced volume of dirty water requiring treatment in sediment retention ponds (SRP), potentially allowing for a smaller construction phase SRP on the construction site!