Litter Boom

Reducing the amount of plastic rubbish and litter entering the estuary and the sea is important for the health of many chains, including our own. We can start by improving the collection of litter in our own river. The means to do so is right on hand.

The Woolston Tidal Barrage sits apparently idle for most days of a year, its closed gates invisibly protecting the reaches upstream of the barrage from the ingress of saltwater . It does this by forcing the incoming tidal flow to travel around the full 3.24km of the Woolston Loop before reaching the upstream side of the barrage. By the time the saltwater reaches its maximum intrusion, the tide turns and the saltwater retreats on the outgoing flow.

When the Woolston Tidal Barrage was installed to correct the ecological disaster created by the Woolston Cut, collecting floating debris and rubbish was probably one of the least thoughts in the engineering designers’ minds. Nonetheless, the four upstream intake bays of the barrage, closed off normally by the barrage gates, create quite efficient accumulators of all the flotsam and jetsam that the river brings down on its flow on the out-going tide.

Much of what floats into the arms of the barrage is natural materials – weed released by water fowl or the council weed clearing operations, and sticks. The rest is the detrius of life in a city – plastic bottles and packaging and general litter plus the occasional armchair and ball.

It would be unrealistic to expect that a river that winds through industrial and residential suburbs of a significant city would not pick up at least some litter – largely wind-blown accidental release material. Given the huge amount of packaging that accompanies just about every item that is manufactured and sold, it is sometimes surprising that there is not more rubbish released. We could all do better at minimising the packaging that we consume and discard.

Polystyrene beads  One of the more insidious items of litter that make it into the river are very small beads of expanded polystyrene – the type that you find in bean bags and which make up larger blocks of foam used in packaging and within house foundations. These beads have the potential to cause enduring issues for wildlife. The plastic is long-lasting, the bead is made mobile by wind and water, and to wildlife the bead is indistinguishable from a fish egg. It has no nutritional value and can cause intestinal issues with obvious negative outcomes.

The increased use of expanded polystyrene in building foundations may account for the increase in the amount of polystyrene ending up in the river. Certainly, during the weeks while a foundation is being prepared, wind can easily cause carelessly stored and/or discarded pieces of polystyrene to become mobile and escape from within any containment on the building site.

Clearing the gates  City Care has a contract with the City Council to clear the rubbish from the front of the barrage gates on a weekly basis. They do this by utilising a truck-mounted hiab with a clamshell grab bucket. It is a slow and cumbersome operation which, unfortunately, leaves a significant proportion of the finer rubbish, including polystyrene beads, uncollected. The council staff acknowledge that the operation is far from efficient, but then, the barrage was never designed for this in the first place.

Then it rains  When a significant rainfall raises river levels past a trigger point, the automated system opens one or more gates in the tidal barrage, allowing the increased stormwater flow a direct route to the sea through the Woolston Cut and thereby reducing the likelihood of flooding in the lower-lying parts of Christchurch drained by the river. Whatever rubbish has accumulated behind the gates is immediately released by their opening, including the release of thousands of polystyrene beads all heading for the estuary and the sea and the wildlife that live there.

A solution There is always a solution and in this case it is of three parts, one of which already exists and all of which could be put in place a minimal cost.

Litter barrier  The first part of the solution is to prevent the tidal barrage collecting any rubbish at all. That can be achieved by slinging a floating barrier at an angle across the upstream face of the barrage to force all floating debris into the Woolston Loop. This floating litter barrier would be a permanent installation. (See the diagram for its positioning and shape.)

Litter Boom The second part of the solution is to place a litter boom in the Woolston Loop in such a position that it collects all the floating debris gathered by the litter barrier in front of the tidal barrage. By great good fortune, the Christchurch City Council already is the proud owner of exactly the correct type of litter boom for the task. At the moment, it is in storage for servicing and repair.

Litter Boom placement Critical to the efficient collection of the rubbish is the correct siting of the litter boom so that, on the outgoing tide, the flow carries floating material into the gathering arms of the litter boom. For this to happen, the boom must be moored in the correct position. Mooring a floating object to ensure that it is always in the right place takes experience and training, the correct equipment and vandal-proof moorings. These are all achievable and almost by design, a position for unloading the boom with adjacent paved truck access is available immediately within the Woolston Loop, the position created when the Cut was built.

Emptying the Litter Boom Efficiency in emptying the boom is important particularly if the operation, repeated one or more times each week, is not to damage the boom. Lifting rubbish out by clamshell grab is fraught with issues and the boom is currently in need of expensive repairs largely due to this means of emptying. A far more efficient method would be to have mesh “baskets” that can be lifted out of the boom by hiab to remove the contained rubbish onto a truck. See the photo of this method being employed. Removing polystyrene beads, however, will mean one more stage in the emptying process whereby fine-mesh scoops are employed by hand to remove this fine material from within the boom where it has been trapped.

Proposal The solution outlined here has been put to the Three Waters Unit in writing in the hope that when the litter boom is repaired and ready for return, the whole system can be put in place to improve the outcomes for the river, the estuary and its wildlife. In the meantime, we can all play a role in reducing the amount of rubbish that needs to be removed from the river by ensuring that our litter gets to a bin, preferably the red one that all households have available. Become part of the PickUp5 team and report to the CCC any obvious sources of polystyrene in the environment using the Snap, Send, Solve app.