mesocosm

This device generates a spatially homogenous wall shearing stress, by controlling two parameters simultaneously:

The device illustrated consists of a circular test chamber 30cm in diameter (A), with a removable lid housing the stirring disc, and water input and output. This enables a single lid to be used with a number of chambers. The device is small enough to be deployed in the field. The disc is positioned 7.5 cm above the sediment bed; it is 20 cm in diameter and has a 5 cm skirt. The disc is rotated by a 30V motor powered by a 30V battery pack in the field or from a mains adapter in the laboratory. A 12V pump (C) draws water from the chamber through the central rotating axis via 10 mm clear plastic tubing and returns it to the top of the chamber.

This fluid recirculation path enables the inclusion of additional measurement and processing units, for example to control and monitor the amount of suspended material retained in the eroding fluid.A circular hole in the lid enables measurements to be taken directly inside the chamber. Water samples can be taken from the chamber, allowing the measurement of various water properties such as suspended particulate matter (SPM), carbohydrate concentration, chlorophyll a and salinity. A three-way adapter on the tubing sited after the pump allows water samples to be collected from the water pumped from the chamber. Sediment traps and other devices can also be connected to the tubing. The stress over the bed in the chamber is determined by the discharge rate from the pump, and the rpm of the plate. This device was calibrated for a series of pumping rates and rpm necessary to achieve a series of U* values using constant temperature anemometry.

The microcosm system can be used to study the erosion and deposition of sediments

The microcosm chamber is placed onto the sediment surface and carefully pushed into the sediment so the disc is 7.5 cm above the bed.
A circular piece of plastic bubble wrap of the same diameter as the chamber is carefully placed on the sediment surface inside the chamber. Water isotonic with the pore water is carefully poured onto the bubble wrap until the chamber is almost full. The bubble wrap protects the sediment surface and as the chamber fills with water it floats away from the sediment.
When the chamber is almost full of water, the lid is placed on top of the chamber and screwed down. The chamber is then topped up with water.
The nephelometer is set up inside the chamber; the pump and motor are attached and set to the desired starting levels. The microscosm is now ready for an erosion test to be started.
Nephelometer readings are taken every 30 or 60 seconds. After 9 minutes, the nephelometer output is noted and a 20 ml sample of water is taken from the chamber to measure the SPM and calibrate the nephelometer. Any other samples that are required (e.g. flocs) are also taken at this time.
After 10 minutes the rpm of the disc is increased and the pumping rate changed if necessary.
At the end of an erosion test, nephelometer readings can be taken to investigate the deposition rates of the eroded sediment.