Flood Slip Recovery - Wainui Hill - MWH NZ Ltd

ClientTasman District CouncilLocationNew Zealand
ConsultantMWHContractorStephen Bennett

Flood Slip Recovery - Wainui Hill

Project Summary

December 2011 brought unexpected record rainfall and slips to the Nelson/Tasman region of New Zealand. Wainui Hill in Golden Bay was just one example of the downpour’s aftermath, with 25 slips over a 2km stretch resulting in hill closure & house evacuations.

Geotech Engineers required a quick set of survey data to enable visual extents of slips and their catchments, enable profiling along testpits, to produce geotech reports/costs on options.
Roading Engineers required a DTM model showing existing carriageway and structures to enable concept road design to accompany remedial work options.

The Challenge

With the short time frame provided, and the steep, bush-clad environment, it was decided to use the Council’s existing LiDAR data. This, of course, did not include the new slips, and LiDAR point data files are huge and cumbersome (Excel would not open them for edit in one go); many software products cannot handle them, and the data is not as accurate in level.

Some existing road/recent slip asbuilt data was available in older formats, but this information differed in coordinate and level datums.

Field surveys were undertaken over four days to gain GPS control where reception was available, and robotic total station topographic surveys undertaken of each slip site as well as road edges, culverts, and structures.
LiDAR level datum was validated as good enough for purpose (prelim road design alignments) on road surfaces and good enough though less accurate for geotechnical purposes in vegetated areas above and below the actual slip sites.

The Solution

The job was divided into two parts (each side of hill) for ease of use and because one side was reported on first (though 12d Model software could easily have handled it as one project).
The robotic topographic survey traverses where easily adjusted to the GPS control using Survey Adjustments Helmert 2d (advanced). Old data was imported using the File I/O data input DXF import, then adjusted to match GPS control and converted using NZ Conversions. This created a combined actual surveyed data set of slip surfaces and road edges or CLs, culverts, etc. 12d Model’s ability to display different views with different datums all in one project, and the ease of conversion (and not having to worry about overlapping point numbers!), made this task a relative ‘breeze’.

The LiDAR data was imported using x,y,z pt id input. The ease of the format options was helpful as it enabled the team to bypass the problem of editing huge files in a third-party product, by simply ticking what columns were to be imported, and indicating in what order. These four tiles were huge, so the team employed the FENCE routine, by only bringing in the points required inside a drawn polygon (fence).

A polygon was also drawn around the extents of the merged topographical surveys. Using FENCE, this time from the Utilities menu, the LiDAR data inside the topographic polygon was easily fenced out and deleted. By turning on the remaining LiDAR data and the more accurate topographical models, the team gained a complete set of data which was used to create the TIN model. This method led to an excellent TIN and contour model without the rough edges that can occur (because of the huge number of LiDAR points) when creating super tins between data of differing qualities.

The visual representation for the geotechnical engineers was achieved by simply creating a perspective view and toggling on SHADE, then turning on the appropriate models (roads, slips, culvert strings, etc.) that show in surveyed positions on the TIN. By using TIN DRAPE tool, the legal boundaries could even be represented over the TIN. All in all, a successful presentation of data was achieved.

Profiling of slip sites was then undertaken by creating a simple line string through test pit sites and extending above and below the slip sites, then draping over the TIN (visually seen live on screen). PPFs were then edited to show appropriate string cuts, etc. Outputs to other programs were also performed, to be included in reports, and the overall model used by the road designers for design alignments.

The Result

As a relatively basic survey team still learning to use 12d Model software, and with no Visualisation module yet installed, 12d Model proved to be the ideal answer to everyone’s needs - the simple learning curve and time saving opportunities made what would have been a near-impossible task much easier.

Download this Case Study as a PDF HERE