Manly Dam Trail Works Update: June '11 Description

As discussed in the previous update on 27/6/11 the Manly Dam rangers arranged for a meeting at the dam to look at sheduled works on the Allambie side.

Overview

NoBMob and Manly Warringah club reps took a few hours off work to meet yesterday afternoon until well after dark to talk over the area commonly known as the 19th hole and a section near Cootamundra Drive. One of these reps was our own in house riding Geomorphologist (expert on soil erosion and design solutions).

The main points of discussion were:

• Making sure everyone is aware of the main lines that people take
• Ideas for track surfaces and controlling water runoff
• How to bring the track back to single track width
• Ensuring any reroute still provides room to access more advanced lines

19th Hole Discussion

Key issues at the 19th Hole are:

• Water channeling (blue) down the track and newish 4wd track punched in by the Electricity Board
• The track currently leads people straight to the old A-line drop (5-6 foot drop)

The alignment to the A-Line drop is perhaps less of an issue now as the vegetation has grown over the last 4 years, previously the sight line took you straight off it resulting in many people not familiar with the dam locking up at the end of the single track to avoid it. However the track is here to stay and vegetation is not the most permanent feature so this is still a good thing. The B-Line is also not a drop that requires any speed so changing the alignment should be no issue.

This water flows down the face of the old A-Line and B-Line, building up speed causing immediate erosion and also track widening and further erosion down the trail after the landing area. This area turns into a waterfall and stream after heavy rain.

The area that has eroded beyond the landing area in the photo below used to be single track with vegetation either side.

19th Hole Remedy

It was agreed by all that the progressively widening single track above the 19th Hole is far from ideal from a trail planning perspective. It is basically fall line trail with very little outslope and so channels water. However we are left with a legacy trail that was:

• Initially power line access
• Became a walking track
• Then a mountain bike track

Discussion with rangers indicates that we would all like to look at better options or putting more time into drainage along this trail at a later date with better budget and/or volunteer work.

In the shorter term there will be a minor reroute for the last 10m or so before the 19th Hole. After discussion with rider reps, Rangers and the Contractor this will comprise of:

• Initial grade reversal on existing track prior to reroute to shed water before it gets to 19th Hole
• Subject to drainage details, possible berm to turn into new segment.
• More natural track on reroute than existing board walk and will comprise of a buried armoured surface
• Rocks along track sides to assist holding armour in place. There is very limited topsoil in this segment and it needs to be protected.

The grade reversal to control water and the minor reroute will start approximately as indicated below (about 10m before 19th Hole) and rejoin the track as indicated. This will assist with water runoff and also stop leading newbies to the old A-Line drop.

We have emphasized that where this rejoins the existing C-Line provision needs to be made in the alignment so that the more advanced lines can be easily accessed otherwise the new segment will widen at the end after construction as people take the different options.

The water run off from the newish Electricity Board 4WD track is a big issue. Rangers are looking at how this can be addressed either by Council or Electricity Board. Unfortunately the Electricity Board has some powerful access rights. Drainage on this new track will need to be resolved before issues in the landing area can be fully resolved.

In the landing area it was agreed by all the need to preserve the B-Line and C-Lines and to also bring the track back to single track width. The old A-Line was always problematic with the immediate landing never retaining soil and also the alignment of the runout down the track. It was also generally better suited for bigger bikes than ridden around the dam.

The best solution identified when considering drainage was as follows.

Cootamundra Drive Discussion

At Cootamundra Drive key issues are:

• Boggy ground conditions with minimal outslope for drainage
• Tread creep / Trail widening

This area is shown below.

It was getting a little late for photos in this region. The bog is difficult to address, based on levels and the vegetation any reroute in the immediate area would still have similar issues.

For the areas with tread creep and gradual widening we looked at trail flow, considered the slightly off camber rock shelf, vegetation and the rocky climbing segment at the exit of this section.

Cootamundra Drive Remedy

This area will require further discussion to find a permanent solution. In the shorter term this will involve:

• Rocks to reduce trail widening, location to be marked in yellow by rider reps
• Patch to address boggy area

Timing and Contacts

These works are weather dependent and will start very soon. During this time please observe any trail re-routes, hopefully the remainder of the trail can remain open for much of this work.

There are many other areas of the dam riders would like to see fixed up. We have been discussing with staff how we can arrange for a process by which riders can put forward trail maintenance suggestions and also be involved through volunteer work.

Again the Mayor should be our last port of call not the first. We have re-established communication with the ranger and management of the dam and they are listening (thanks to you all for the letters over recent times). Lets keep the momentum positive and forward.