Murphys Creek Background
Marlborough District Council is currently consulting with residents along Murphys Creek about a proposal to discharge stormwater at the culvert under Middle Renwick Road.
See map of bounding properties along Murphys Creek
This consultation is being carried out prior to any application for resource consent being finalised and lodged.
Overview
The following link is to a series of photos taken in a progressively downstream direction and which look downstream. These show how the creek appeared in the spring of 2009. Of note is both the clarity of the water and the amenity the creek provides to those who live near it. The water that is flowing in the creek is almost 100% spring water. This spring water is outflow from the underground water resource which underlies the Springlands area. The same resource from which Blenheim draws its municipal supply.
The first slide shows the way in which flow increases down the creek. The second slide is key to the locations of where the succeeding series of photos were taken.
First briefing for residents
On Thursday 20 June 2013 a first briefing of residents was held. In excess of 50 invitations were sent to residents who bordered the creek from the Middle Renwick Road culvert downstream to the creek's outlet to the Taylor River.
Council staff made a presentation to the meeting, which was interspersed with questions and answers.
A copy of that presentation is included below.
Meeting record - in brief
A summary of the briefing of 20 June 2013 is included below:
Safeguarding environmental quality is a major theme
The briefing covered both the quantitative effects of a discharge (ie; flows, flow rates, flow depths and durations and bank velocities) and likely qualitative effects (general water quality, fish life and habitat plus vegetation - in the stream and on the banks of the creek).
While there were some concerns expressed about likely flows, flow levels and bank stability, these were overshadowed by concerns expressed about environmental quality. A feature of the commentary offered on this theme was that it was the present day situation that was highlighted and not so much the likely effects of a design flood event passing through the system.
The design flood
What was not made especially clear at the briefing was what the 'design flood' might look like as it passed through the creek.
But first a few notes on what a design flood is.
When Council is planning its stormwater infrastructure, it adopts design standards that will deliver the level of service expected by the community.
The design standard adopted in this instance is called a 50 year flood.
A 50 year flood is the flood event that is likely to occur once every 50 years.
However - and just to make matters a little more complicated - there is a chance that this 50 year flood event could happen in any year.
The chance is small, at 2%, or 1 in 50, but there is still a chance.
The Murphys Creek stormwater catchment
The following link is to a series of images describing the Murphys Creek stormwater catchment.
In particular the extent of the current catchment for urban stormwater from Murphys Creek is shown, along with the location of the pipeline and the various inputs that go to make up the amount being proposed as a discharge.
Stormwater from this catchment area is carried away from property and roads via a network of pipes and is then discharged to Murphys Creek.
The amount of flow that is actually discharged is controlled by the size (diameter) of the stormwater pipes that are in the ground.
A smaller pipe allows less than a bigger pipe. If the amount of rain that falls is too great for the pipe to carry, then water backs up within the network of pipes.
Within the stormwater catchment there are three ways the amount of stormwater that is discharged can increase.
Increase the size of the pipes in the network, increase the areas contributing - ie; increase the catchment size, or increase the density of housing within the catchment. Of course there can be any combination of these as well.
What Council is proposing is a combination of all three. That is; upgrading the existing pipe network, allowing for new development and allowing for what is called 'infill' development.
Design floods at photo location E
The following link is to two photographs at photo location E. These show the height and extent that flood waters would likely reach at that point during two different 'design floods'.
The first design flood is the one that would occur if it happened tomorrow. That is with the existing stormwater pipe network in place and the present levels of development, Westwood included.
The second design flood shown is one that has been calculated after upgrades to the stormwater pipe network, all new development and any likely infill development.
As you will see from the photograph the effect of the second design flood means an extra 300 mm in flow depth at photo location E over and above that which would occur if a design flood passed through tomorrow.
The amount the creek would rise from normal flow conditions during this second design flood would be 1.25 metres.
