The last couple of weeks we have been on the river every week testing our equipment. On Wednesdays we did our test runs looking for the methods to sample litter from the river. The thing we really wanted to find out was the mesh of the nets we should use in the winter season.
Basically we sample on two places:
a) skimming the surface with a surface net and
b) sampling the water at a depth of 10 – 60 cm with the suspension net.
So far we have been sampling with improvised nets made from do-it-yourself materials to get some clues about the mesh we should choose.
sample nets, lifted suspension net on the right, surface net on the left
The surface net has a width of 1.35 m and the suspension net has an opening of 1,0 m x 0,5 m.
Test run 1 was done with only a suspension net with a mesh of 12,5 mm. After sampling more than 8000 m3, nothing was found.
Clean suspension net (mesh 12,5 mm)
That day was a beautiful day after a long period of dry weather. The throughput of the river was very low and there was no wind at all. All litter stayed peacefully on the banks or was trapped in bushes along the river. The water in the Meuse was or seemed “clean”.
litter on riverbank (on the right of the picture)
Small fragments and pellets waiting on the bank for further transportation.
One week later, test run 2 was done with both a surface net (mesh 6,25 mm) and the old suspension net with a mesh of 12.5 mm.The results were quite different from the results the week before. We caught quite a lot of litter on the surface net, not so much on the suspension net.
surface litter caught on a windy day
What was the difference with test run 1 the week before? First of all there was a lot of wind now and the water had sometimes waves of about 50 cm high.
fishing in turbid water
Litter resting on the banks was washed in the river together with a lot of leaves and branches. We skimmed about 22.000 m2 and the result is shown here.
collected river litter without the organic parts
The suspension net (having a mesh of around 12 mm) showed results different from the week before.We caught mainly flexible pieces of plastics which were floating under the surface, pulled there by the turbulence of the wind-agitated water. Comparing the catch with the surface net was not possible because of the different mesh-sizes.
The third week we tried on both nets a mesh size of 2 mm, but here we had no success. The surface net collapsed by the pressure of the water and the suspension net was completely clogged with algae and silt.
surface net pushed back and leaking
suspension net clogged with algae and silt
- It makes a lot of sense to distinguish between measuring the surface and to measure below the surface. Plastics float in calm waters, but are pulled below the surface as soon as turbulence occurs. The division between “floating” and “in suspension” is very much related to the turbulence in the water.
- The nets should have the same mesh and the optimum seems to be somewhere around 3-4 mm. We can then sample microplastics (pellets and little fragments, without the nets having clogged too soon with algae and silt).
- It is vital to take into account the different conditions when we do the sample runs. The influence of the wind, the throughput of the river and the meteorological situation in the days or weeks before in the entire catchment area will have a big influence.
- One other observation is the effect of lakes connected to the river. We noticed a kind of plume of litter coming out of the lake pushed out by the wind.
Opening from lake to the Meuse
We will now concentrate on building the final nets and prepare for measuring campaigns in winter where there will be water speeds of up to 15 km/hr. So far we have been measuring at maximum water speeds of only 1 or 2 km/hr!
The last couple of weeks we have been busy to test the equipment to measure the amount of litter in the Meuse. There are some elements worth mentioning:
The ship de Blauwe Reiger (The Blue Heron) is a 14.95 mtr long “barkas” build in 1952 in Germany as a service vessel for all kinds of jobs. The skipper (Twan) rescued her from a scrapyard and rebuilt her almost in her original state as a basic recreational vessel. The present owners (Yamila and Hergat) keep Twan using and servicing the boat and go out frequently together just for the pleasure of sailing on the Meuse.
When they heard of our interest in using the Blauwe Reiger for measuring purposes, they immediately agreed to cooperate and they are ever more enthusiastic about the purpose of this whole venture. No other boat owner would allow us to drill holes in the hull or install masts and gear on their ships, but they do… and we cannot be grateful enough for their cooperation!
de Blauwe Reiger , Yamila and Hergat
The catamaran shaped pontoon, carrying the net between the floatation bodies was designed to be towed alongside the boat to prevent measuring in disturbed water from the propellor. As the tests showed, the floatation bodies and the shape they have, just leave the water flowing between them almost undisturbed and the net can cleanly cut through a surface of 1m x 0.5 m water. After the first test this was not yet perfect, but after modification it worked out beautifully. The net operates at a depth of approx. 10 cm. and catches the materials which are in suspension. As a hypothesis we assume this 10 – 60 cm depth will be representative for the rest of the water column, especially in turbulent conditions, but this has to be proven.
The pontoon can be equipped with a net skimming the surface to catch the materials that really float on the surface like bottles, branches and other products with a low specific surface (surface/volume ratio).
The rigging needed to tow the net next to the ship (picture)
rigging en catnet
The current speed sensor with the mounting took also some attention. Normally this sensor is used to be mounted in more spacious environments with less chance to get covered with litter or in deeper water. It took some tests to get rid of the vibrations that started to occur in the first runs. We have clear readings now that give us accurate information of the watervelocity we have when towing the net through the water. It allows us to calculate with great accuracy the volume of water we have sampled.
Current speed sensor (Aquadopp)
The nets gave us quite some headache. The supplier had send us the nets THREE TIMES !!! and they kept getting lost in the logistic mess of the parcel delivery guys. Okay, we did the testruns with a improvised net I build in my backyard under the watchfull eye of my dog and we learned what we needed to learn.
The improvised net has a mesh of 24 mm. The real nets will be 18 mm and 1,1 mm. These should give us the information we are looking for.
improvised net, approved by Blister
The basic procedure of the test is as follows:
surface (m2) x velocity (m/s) x duration (s) = volume of sampled water.
Checking the caught litter and categorise it with the OSPAR list, will give us the concentration of litter per cubic meter of river water. This is what we want to know as a first objective.
Further analyses of the data and the samples will give us information on the materials, the sources, and the position in the river where the samples are collected.
Every Wednesday we will be on the Meuse for measuring. Feel free to contact me if you want to join us (call +31 6 53693382 or reply to this blog)