Marlborough District Council recently commissioned University of Canterbury researchers to map the North Marlborough area for river and stream catchments that may be susceptible to debris flows or debris floods, to improve the Council’s understanding of landslide hazards. The research became possible after the capture of large areas of LiDAR (light detection and ranging) data that delivered precise, three-dimensional information about the area’s land surface characteristics.

North Marlborough debris flows catchment map

Environmental Scientist Matt Oliver told the Council’s Environment Committee today that the study was a useful indicator of potential landslide and flood susceptibility.

“This report has produced tools which can be used to inform landowners, forest managers or prospective land developers on the potential susceptibility of downslope land to debris flows and floods.”

He noted the report’s data layer was not intended to be used as a regulatory tool or for use on property land information memoranda (LIMs).

“But it is useful information that can prompt further geotechnical investigation of a particular site.”

Mr Oliver said Marlborough had a lot of steep land that was susceptible to erosion.

“In particular, the Marlborough Sounds are prone to heavy rainfall and various forms of landslides. We have a history of local damage to infrastructure such as roads and houses, including as recently as July 2021. Landslides also deliver large amounts of sediment to sensitive receiving environments such as rivers and estuaries.”

“Debris flows and debris floods are intense water flows that occur in steep catchments when heavy rainfall causes slope failure, delivering large quantities of sediment to stream channels. This causes channel sediments to become mobilised as surges, fronted by boulders and trees. These can rapidly move down channel to fan areas, where they can be highly destructive,” he said.

“Fortunately, debris flows only occur occasionally in any given catchment, perhaps just once every few decades or centuries.”

Mr Oliver said initially the research was proposed as a way of assisting forest managers to mitigate risks of land sliding from harvested forests.

“However we realised that this methodology could also be applied and used to screen for general debris flow and flood susceptibility.”

He noted there were limitations to the ‘Melton Ratio method’ used in the report for determining debris flows and flood susceptibility.

“Our report comes with a clear proviso that the maps from the report can only be used as a screening layer; they are not a definitive geotechnical categorisation of a catchment.”

The total mapped area covers 344,873 hectares. A new Smart Maps Pro webmap will now be developed for use by trained land management professionals such as forest managers and surveyors. Further development of the webmap would include GIS layers of natural and built receiving environments and infrastructure below susceptible catchments to assist users in developing risk assessments.

The report is available on Council’s website at https://bit.ly/3w31yQB

Notes for Editors

The Melton Ratio method used in this report provides a number of advantages:

• It is fast to produce and can screen large areas
• It can produce a screening layer in the absence of evidence of previous debris flow/flood events,
• It can map catchments to less than 0.5 ha (although the capability of such small catchments to generate debris flows is not clear)
• It’s an objective method using hard digital data without relying on the subjectivity of observation-based methods.

The Melton Ratio method also has limitations, however:

• It appears reliable when catchment length is less than 2.7 km long. Above this length, debris flows may not propagate the full length of the catchment.
• It may not be a good indicator of debris flow susceptibility where the catchment has relatively low relief but steep side streams.
• The method is only applied to the LiDAR-derived surface digital elevation model. In other words, it only assesses the surface morphology of the catchment. In simple terms it assesses if the catchment can ‘catch’ enough rain and move that at sufficient speed to mobilise/transport very heavy loads of sediment and other debris. Catchments may require further investigation to determine the influence of other factors on land susceptibility.

Marlborough District Council is responsible under the Resource Management Act to manage significant risks from natural hazards. Part of the Council’s role under the Act is to control the use of land to avoid or mitigate natural hazards. In order to fulfil this, the Act requires that councils commission research to effectively carry out their obligations.

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