Marlborough Trails Audit – Wither Hills Farm Park (Blenheim) and Victoria Domain (Picton)
The Marlborough District Council (MDC) engaged Tasman Trails Ltd to provide an independent opinion on the state and sustainability of mountainbike trail assets in the Wither Hills Farm Park, and Victoria Domain.
This report provides feedback based on the 20th June 2014 inspection.
Wither Hills Farm Park
A well-established MTB trail network is in place in the designated MTB area of the Wither Hills Farm Park.
Mountainbike Marlborough carries out significant volunteer led trail development and maintenance on an ongoing basis, with MDC Reserves resource also a key part of managing this area.
The Wither Hills Farm Park presents a challenging landscape for trail development (or any type of man made soil disturbance for that matter), having a predominantly highly porous loess base. The often-unique characteristics of this fine blend of clay/sand/silt/calcium carbonate can present naturally occurring episodes of vertical capillaries, sinkholes and under-running drainage ducts.
All of these characteristics of a loess substrate were visible in many areas around the Park.
It was also clear, that in general any man made trails that have been cut in the Wither Hills Farm Park have been undertaken with a good level of understanding of how the loess substrate performs in both wet and dry conditions. This understanding also seems to be a good mixture of experiential (namely from the Mountainbike Marlborough trail crews) and more formal (from MDC Reserves).
Specific feedback on the existing Mountainbike Park trails is as follows:
The use of outslope as a means of sheeting water off trails is used, but on an intermittent basis. Those trails that have outslope built into the tread surface show a markedly increased level of integrity and a muchreduced level of scouring. Where outslope hasn’t been factored in, water scouring is clearly an issue, in some cases tending to exacerbate the natural tendencies of the loess substrate to fracture and undercut.
- Outslope of a minimum 5% utilised as a key tool to manage erosion on all trails, with an increase of
up to 10% where appropriate.
- Maintenance of existing trails carry an increased focus on correcting outslope as part of regular maintenance programmes
2. Grade Reversals
Forcing water to drain off trails by reversing grades is a technique that has been well used in few places around the Park. However, there are also many sections of trail where long, straight runs have been utilised which give little opportunity for water to shed if it tends to pool on the trail tread. Based on the type of loess substrate in the Wither Hills Farm Park, a maximum tread length of approximately 5-6m on a trail having an average of 10% would be a good benchmark.
- For any new trails, the grade reversal technique should be designed in as a key component to managing erosion. This design technique will also enhance the playfulness of these trails.
- For existing trails, it can be difficult to retrofit grade reversals however the effort to do so will reward in terms of lowering maintenance load and enhancing the user experience
- Where it might be difficult to retrospectively implement grade reversals, wide drainage nicks and scallops could be utilised especially in those places subject to seepage across the tread.
3. Trail Gradient
Most trails in the Park are constructed within good gradient boundaries respective to what they are intended for. Climbing trails especially fit within appropriate gradient specifications. Issues with erosion, surface scouring and a degradation of tread surface were apparent in many areas around the Park, but typically on descending trails, and generally only where trail gradient had exceeded what appears to be a natural limit respective to the loess substrate.
- Develop a matrix of trail gradient specification for this Park, covering average and maximum
allowable gradients for grade 2-4 trails.
- I suspect that a maximum sustainable trail gradient (for short segments of trail) may be much less for this fragile loess substrate than anticipated – there is evidence in many areas around the Park that this maximum gradient could be as little as between 5 and 10%.
- Manage the desire for and informal construction of ‘steeper’ tracks out of the Wither Hills Farm Park, to a more appropriate area. This area may simply be unsuitable for steeper trails of this nature.
- Combine outslope technique with reverse grade design to allow for trail grades of a slightly steeper nature (but within the boundaries of what the loess substrate can handle)
4. Bench Blending
In general, bench cuts haven’t been blended well to encourage water sheet flow. It isn’t unusual for bench cuts to be mostly vertical in MTB trail environments (as blending is often an overlooked, labour intensive job), however an awareness of the benefits of blending these benches to allow water to sheet across and off the trail in a gentle manner will help with maintenance load. Effective blending will also help with aesthetics,
and give a perception of increased trail width.
- Blend all bench cuts at approximately 30 to 45°.
5. Corner Design
Climbing corners have been generally constructed well, with some easily rectified legacy issues in places on trails such as Skyline. Some corners on Skyline are ‘stacked’ with no opportunity for water collecting on the trail to find any egress point – resulting in water flowing down the tread for long distances, scouring and worsening erosion issues.
Descending corners would benefit from more widespread application of the reverse grade design philosophy (on corner entrance and exit) to help manage water flow through the corner, and also to minimise user generated erosion through unnecessarily hard braking.
One very good example of a corner that has the right mix of these desired characteristics can be found in the middle of the Deviation trail, with many other corners on this trail almost there, but suffering at various degrees from a straight in-straight out alignment approach.
The top 200m of the Skyline trail provides a great example of a mix of good design technique – frequent reverse grades are built in, with the use of trees as strong anchors to define the changes of trail attitude.
There are clearly sections of this area where the loess also contains a mix of gravel and natural rock (eg parts of Skyline) – those areas show an immediate and noticeable improvement in integrity and durability, and may be able to be maximised in terms of what sort of trail could be sustainable and appropriate on that mixed substrate. Given the much-improved durability of these areas, trail armouring with a suitable imported
rock could be considered as an integral part of all trail development and general maintenance as a matter of course across the whole MTB Park area.
Pamplona as a grade 4 trail has been well built in places with good line choice and line of sight.
The approach with this trail has been to use a steeper gradient to provide an increased level of difficulty, and while this has been achieved it has come at the cost of an increased load in managing those steeper parts of the trail, especially towards the bottom. Steeper sections of this trail may be greater than the maximum sustainable gradient the loess base can handle, and if there are options available to lessen trail gradient at these key points this approach could lessen erosion and maintenance load. Regardless of whether rerouting options are available on Pamplona, making sure all corners have a substantial drainage feature before and after each radius (in lieu of reverse gradients being built-in) will help with erosion caused by a steep grade.
Additionally, retrospectively built and frequent reverse grades will help on all parts of this trail as well.
The new Sweet As trail utilises a different approach to providing an increased grade of trail difficulty, and that is by cutting a narrow width and using a more technical design philosophy to provide a higher challenge level. And this is done while running the trail across the contour of the hill at a sustainable grade. Technical difficulty of the trail will likely increase over time, especially with the trail crossing exposed root systems on the downhill side of many pine trees.
The on-hold Pete’s Trail, in the northernmost corner of the Park is an example of a combination of unsustainable design and poor build technique, where a contour approach and minimum impact bench cut philosophy would have been far more appropriate. Remediation work will be required to correct severe water pooling, potential vertical fracture and trail steepness – however this could also be an opportunity to apply sound design technique in a young area of pine plantation to give a great long-term result.
The development of the mountainbike trail network in the Victoria Domain has largely been led by local groups on an informal basis, and can be separated into the north side and south side of the Domain ridge.
The native bush is regenerating at a rapid rate, and over most of the Domain is well established and vibrant.
Substrate is typically a heavy clay base with a rich organic soil overlay.
In general, mountainbike trail design and construction has been undertaken with an appreciation of the values of this type of native bush environment, and as such tends to be minimalist, narrow and of a higher technical challenge than a Wither Hill Farm Park style trail resource.
The newest traverse, on the north side of the Domain ridge has been constructed at a good contour line,
with only a few sections where a steeper grade may be considered inconsistent with the overall nature of the trail. While these steeper sections do tend to interrupt user flow, they don’t seem to provide too much of a barrier to trail use, but will tend to be a barrier to more recreational cyclists. This traverse also reflects a drier aspect of the Domain, and utilises many trees as anchors and pinch points to manage speed and enhance the technical nature of the trail. Reverse grade has generally been used to good effect on this traverse, but
an increased emphasis on outslope would ensure a low maintenance load and improved sustainability.
On the south side of the Domain ridge, several challenges become immediately apparent. Due to the southern aspect, this face of the Domain will always hold more water and requires a much tighter approach to trail development as a result.
The descending trails running down to Ranui Street are constructed at grades that are probably too steep, and in many places are showing the effects of erosion due to water running down long sections of trail.
These trails are narrow, with generally poor corner design and impeding side vegetation, which could easily be a safety issue. Some effort has been made (recently) to realign some lower sections of this trail to lessen gradient, however steepness is still an issue with these sections. Reverse grade and outslope haven’t been utilised, and with the current gradient of the trail tread length sections shouldn’t exceed approximately 7-8m.
The rest of this mountainbike trail network that climbs back up to the Domain ridge road utilises a combination of legacy type machine cut tracks, and informal hand cut mountainbike tracks. The legacy tracks are all pitched at gradients that are unsustainable from both user and erosion management perspectives – to correct these rutted and scoured sections of trail would require significant realignment and rehabilitation of the old, together with implementation of well proven, sustainable design techniques.
The informal mountainbike tracks that feed off these legacy tracks are steep, narrow and unrideable in manyplaces. Corners are generally too steep and tend to be barriers to maintaining any type of climbing momentum. Some trails appear to be no exit trails as well!
The newest traversing trail on the north side of the Domain ridge provides a fairly good example of staying within defined gradient limits and with a contour approach – this design philosophy will also benefit trails on the south side of the ridge. However significant realignment and rehabilitation of retired trails (and sections of trails) would be required to make this work.
While the nature of the mountainbike trails in the Victoria Domain, especially those on the south side of the Domain ridge may represent the interests of the volunteer groups who have constructed them, the wider appeal of these trails to a broader range of rider ability must be in question. Potential to cater for a much wider range of interests is certainly present, with scope to connect mountainbike tracks up to the marina and Picton end of the Domain.
Wither Hills Farm Park
Long, straight pitch.
Lack of any reverse grade.
Evidence of grass/outside edge bund build-up defeating any outslope.
Insloped scouring/vertical fracturing.
Informal jump construction (to left)
Rutted, seeping corner
Originally built as an insloped corner, has been steadily scraped clean over time due to seepage issues in the apex, sinking lower and lower into the loess.
Lack of bench blending together with uphill seepage causing constant degradation and slumping of the bench.
Fairly steep ramp on the corner exit as a result of the steady sinking.
This corner was rebuilt as part of the June 21st Sustainable Trails Workshop – the shown corner was retired and the trail rerouted on a larger radius curve around it. Utilising a slightly
raised crown approach in the apex, the new top leg incorporates a reverse gradient, and an insloped tread to drain off the outside of the corner. Requires more work to build the
width and radius of the lower tread up.
The following photos show initial repair scoping, with the corner reroute forming.
Nappy Change Junction
Example of vertical benching, and flat tread surface.
Natural gravel component mixed with the base loess, however signs of water scouring down the tread surface and scrubbing fines away.
Typical example of vertical capillaries on both sides of the tread and an undercut drain formed beneath.
Looking down a section of trail on stacked corners.
Water scouring can be seen generating from the corner (at bottom of shot) and running all the way down the trail to flow into and through the next right hand corner.
This section of trail would benefit from wide scalloping at the entrance and exit points of each corner, some work performed on possibly insloping the corners at a shallow angle and some form of reverse grade between the corners to isolate any water tending to still flow down the trail.
Unauthorised, digger built track, now on hold pending remediation. Photo shows a deep,
horizontal cut with all spoil laid proud on the outside edge of the tread. Tread surface is wet, soft loess, holding water with the bench degrading and breaking away. Trail drops away at a very steep pinch at the far left of shot.
Typical steep section of trail on the south side of the ridge.
Very little, if any outslope preventing sheet flow across the tread.
Heavy organic material layer holding moisture and causing tread surface to degrade through user damage.
Typical trail section on the south side of the ridge.
While reasonably harmonious with the surroundings, width is an issue through these areas, impinging on trail users.
Trail sections like this would benefit from reverse grade sections at frequent intervals.
Boggy area solution, bottom of trail above Ranui Street.
Plastic pallets utilised to raise the trail tread above a very soft, boggy area. While not very aesthetically pleasing, it is an effective solution.
Would benefit from being retained and anchored securely i.e. treated timber outside edges running the length of the structure, with pallets fixed to those edges.
Currently the pallets are tied together with zip ties, and are able to settle and move at will.
New, steep section of trail, above Ranui Street.
Gradient approximately 15%, and too steep to be sustainable, showing signs of concave scouring.
Legacy type trail, between Ranui Street and QC College
Very steep, virtually unrideable and with a well developed rut running right through the middle of the tread.
Steep, uphill corner below the Harbour View area of Victoria Domain.
Tight radius, and a large height difference between the upper and lower legs makes this corner a major challenge to all but very experienced riders.
Would need to be realigned to increase the radius, with the potential to create an insloped switchback likely the preferred approach.
Useful Contact Details
Trail Specific Tools:
Trail Fund NZ are now supplying MTB specific trail tools on behalf of Hilton Taylor (Hawkes Bay) and Gareth Hargreaves (Dunedin)
Hilty Hoe (small trail hoe)
Trail Boss 5 in 1 (multi rake, combination tool)
Trail Rake (heavy duty earth mover)
Weapon of Mass Creation (four sided trail hoe, wooden handle)
Atlas spike and/or axe blade mattocks are tradesman quality grubbers, available ex stock from Placemakers
Suunto manufacture a high quality clinometer, which is the industry standard, however price is often a barrier at approximately $350.
Many smart phone applications are available, however the following is almost free (NZ 0.99c) and works very well in the field:
Trail Solutions – IMBA’s Guide to Building Sweet Singletrack
Managing Mountain Biking – IMBA’s Guide to Providing Great Riding
Natural Surface Trails by Design – Physical and Human Design Essentials of Sustainable, Enjoyable Trails All available through Ground Effect (contact Cherie Rusbatch firstname.lastname@example.org) - a set of the manuals are usually provided free of charge to MTB Clubs, with additional copies available for purchase.