Making Sediment Traps
Domestic quality PVC Drainpipe can be easily cut to any size with a bandsaw or jigsaw.
For many pipe widths, custom made caps are available to use as bottoms, or flat pieces of PVC can be cut and glued to the bottom using PVC cement.
Traps can be any size, but the ratio of Height to Weight should be between 2:1 to 3:1 to avoid vertical flux, presuming normal currents will be no more than 15 cm per second (9 metres per minute).
In higher current areas, make the trap taller.
Trap size will depend on expected sediment load. 5cm diameter is fine for offshore reefs, larger pipe is suggested for muddy river estuaries.
Setting Sediment Traps
Example of trap set up (English et al, Survey Manual for Tropical Marine Resources)
Traps have to be securely fastened to a permanent post, with at least a 20 cm gap left between the sea bed and the trap, to avoid
Ideally three traps should be set at each point to provide replicate samples.
One way to do this is to hammer pieces of iron rebar into the reef bed, or cement them into concrete flags that can then be placed in flat areas.
In fact I found Gaffer (Duct) tape often comes undone, so I use cable ties as following.
- Thread two cable ties through the 4 holes in the side (white ties in picture) and fasten to form 2 small loops.
- Thread a larger cable tie (black tie in picture) through loops to fasten trap to rod.
- The large cable tie (black in picture) is used to initially fasten the trap to the rod.
- At least three more large cable ties (white ties in picture) are fastened around both the trap and the rod to stop movement.
Concrete slab is set in area so that the trap will be submerged even at low tide.
Sediment traps can also be set on permanent marker such as navigational reef beacons.
Recovering Sediment Traps
Sediments can be collected at any regular interval over the experimental period. Monthly is effective.
- Take small strong scissors (Manicurist’s cuticle scissors work very well) and cut off the cable ties fastening the trap to the post,
- CAREFULLY bring the trap to the surface, being sure not to shake or spill the contents.
- Once on the surface SLOWLY, CAREFULLY pour off the clear water. As soon as you see sediment colour in the water, stop pouring.
- Cut the cable ties fastening the baffle into the trap. Empty the trap into a strong zip lock bag, returning any marine life to the sea (I have found fish, shrimps, hermit cards, sea squirts in traps that have only been down a few weeks).
- CAREFULLY set the bag down where it will not spill.
- Use a stiff scrubbing brush to clean off the inside of the trap, and the baffle.
- Use 8 new cable ties to replace the baffle.
- Use more large cable ties to re-tie the trap back in place.