Sunday, December 29, 2013

What's wrong with Rapid Creek?

(Written in November but left on the shelf for about six weeks I thought I'd post this just to clear the blog)

Recently local residents and others who value Darwin's Rapid Creek gathered to challenge the  potential threat to clear mature trees and riparian vegetation by the Department of Planning and Infrastructure along the banks of Rapid Creek.

At the time people noticed that trees had been marked with pink spray paint, commonly used to indicate trees for removal.


A public meeting was held at the footbridge across rapid creek, which was attended by local residents, members of the landcare group and various other stakeholders.
See Rapid Creek Landcare website: HERE

(29.12.13 There is now a Save Rapid Creek facebook group)

Many issues were raised, including zero consultation with the community, zero coordination with Landcare group, lack of scientific foundation to support removal of riparian vegetation etc...

Members of Parliament were contacted and invited to take part in the discussion but they declined. Three members of the department of Lands Planning and Infrastructure turned up to answer our questions. We were told that the markings were only part of preliminary work on the creek, that it didn't mean any trees were going to be cut down and that it's all part of a flood mitigation program. (Some of the trees were even marked PRUNE)

It is obvious that Rapid Creek is under a lot of environmental pressure. There are now massive alterations to tributary water courses and man made drainage lines. Creation of a large paved carpark at the Airport has effectively means that the land it is on is no longer able absorb water and recharge groundwater. The excess water is directed into wide open channels which feed directly into the creek an obvious contributor to increased velocity of flow. I found their answers to be contradictory and somewhat dishonest. It was obvious to everyone who was there that it was the departments full intention to cut those trees.


The conversation followed about flooding of McMillans Rd and the potential threat to property when the creek floods.

It concerns me that the Department has employed 'Engineers' to manage this delicate ecological zone, with the single objective of flood mitigation. Their linear approach to the management of a waterway borders on criminal and from an environmental perspective is insane! There are multiple contributors to flooding in Rapid Creek. The presence of trees in or around the creek is hardly one of them! Natural vegetation is fundamental to reducing erosion in waterways and reduces destructive velocity of fast flowing water. Most of the elaborate water calming devices that engineers introduce into their man made water course are poor attempts to immitate natural obstructions to water velocity such as trees and vegetation offer naturally.

During the meeting people had to explain to the Department representatives other contributing factors to flooding that absolutely dwarf any impact few trees might have on causing water to back up.

Potential contributors to flooding in Rapid Creek:

Storm surge: If there is excessive rain during a high tide it is extremely likely that fresh water will back up along the rapid creek catchment. What is the elevation of the creek? How many metres above sea level?
Siltation: The removal of vegetation along the catchment has lead to increased erosion which in turn has caused culverts to become blocked, water takes longer to flow through the culverts and so backs up in the creek and man made open drains that contribute to Rapid Creek.

Erosion at McMillans Rd drainage 5/11/13
Increased flow: Land clearing and hardening of surfaces to make carparks and increase infrastructure at the Airport causes larger amounts of water to be shed from the catchment soon after rain. This is likely to have two long term affects. One, I suspect that the incidence of flooding will increase since there will be higher quantities of water entering the creek whenever it rains. The second negative side effect of creating hardened surfaces is that it will reduce the long term flow of the creek and reduce groundwater recharge. Instead of water slowly filtering through the landscape and eventually running into the creek it is shed immediately after rain.

Mc Millans road drainage 5/11/13

Cutting trees will not address any of these issues and in the case of the second and third points is a major contributor to flooding!

At the moment what concerns me most is the excavation work taking place on the Airport side of Mc Millans Rd. A huge amount of earth has already been washed into the open drains. The immediate results of this are obvious and plain to see. Large scale siltatation of the drainage channels, blocked culverts, increased water flow. Longer term the consequences are predictable increased turbidity of the creek, siltation of the creek, blocking of culverts, potential flooding etc...

I believe that due to the storm surge factor there may be no way of mitigating against flooding... although this may not stop the departments engineers from trying to enclose the whole creek in a concrete drain. (Actually confirmed by a current Department of Infrastructure and Planning report 29/12/13)
As far as the health of the creek is concerned I believe that the irresponsible upstream developments within the catchment are the major cause for concern and will likely contribute heavily to future risk of flooding. Yet the department appear to have chosen not to address negligence of developers, instead their engineers have turned their eye to elimination of the creek's remaining natural defense against extreme hydrological conditions downstream. Our creek is already stressed and threatened by irresponsible upstream development, by removing the riparian vegetation that holds the creek bank together the Department would be responsible for delivering the death blow to our creek!

Silt buildup at McMillans Rd drainage 5/11/13
If the creek were a living organism it's riparian vegetation would be like it's immune system. It Absorbs the shock of extreme weather event's such as flooding, holds moisture in times of drought, prevents erosion, provides habitat and creates a micro climate which facilitates the existence of many species of plants and animals  etc... Vegetation helps the creek maintain it's form, filters water entering the system and so on. Removing riparian vegetation would be something like introducing a virus to the organism that has the capacity to destroy the creeks entire life cycle. 

To any ecologist or anyone schooled in land management this seems like madness. But I guess not so to an engineer. There are many reasons why we need the help of engineers to develop and manage our environment... However they should never have full power of decision making when it comes to natural systems!    




Clogged culvert, what happens when you let engineers design watercourses.

3 comments:

GreenBici said...

Happy New Year, David!
Peace :)

Anonymous said...

I also live in Rapid creek, but not subject to these problems. Decreased upstream infiltration is a MAJOR, MAJOR issue, and can be partially rectified. All development could use permeable paving to radically reduce run off. Use of organics to filter and allow better infiltration is also possible. A solution will be multi faceted, but should it be as radical as now apparently proposed by government, and could they afford it? Oh.... I forgot..... they will be getting kickbacks from their real estate mates!

David J said...

It must be incredibly frustrating for an engineer to have to deal with natural processes interfering with their impeccable designs...
The wide drains along Osgood drive used to have a reasonable amount of natural vegetation, grasses etc... holding the soil together. Then they went and scraped it bare and tossed in a few hay bale silt traps to prevent the culverts from clogging up. So what they've done is remove a natural biological erosion inhibitor and attempted to stop the resulting erosion from traveling too far. The final image shows just how effective this method has proven to be... Next step I'm sure will be to lay bitumen across the whole thing!