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Continuing scourge of gully erosion

Gullies are the most significant of the sediment-producing erosion processes that have in the past and continue to contribute the largest proportion of sediment delivered to river channels throughout this region.

In the early 1960s, escalating costs associated with the protection of the Poverty Bay flats from increasing amounts of gravel and the heightened threat of flooding, prompted the then-government to purchase large tracts of eroding farmland in the headwater reaches of the Waipaoa, Uawa and Waiapu catchments for reforestation.

These early attempts at reforestation proved to be a practical and inexpensive treatment option in hill country impacted by shallow landslides, earth flows, and particularly for gully erosion.

Early assessments of the successes and failures of reforestation in controlling gully erosion were, however, largely anecdotal.

Furthermore, little was known of the number or location of gullies elsewhere within this region that also likely required treatment.

Capturing this information involved the digitising of gullies from aerial photography flown in 1957, before planting commenced in this region, and again in 1997 after 40 years of reforestation efforts.

The information gathered showed the number and size of gullies within forests established earliest had decreased due to the influence of the forest cover.

By contrast, in those parts of the region that had remained in pasture, the area affected by gullies increased, particularly as many of these gullies had remained untreated throughout this measurement-period.

Unfortunately, scattered throughout this region there are also gullies that are too large to be treated successfully.

Each of these “badass” gullies had by 1997 individually increased in size, and there were more of them than existed 20-years earlier.

Since the completion of this study there has been no further evaluation of the effectiveness of these early forest plantings or of the additional 20,000ha of forest planted since 1997.

With the availability of aerial photography flown in 2017, Dr Mike Marden and Mrs Anne Seymour realised the importance of compiling an up-to-date database of gullies with the aims of firstly, reassessing the effectiveness of established forests in the remediation of gully erosion over this longer 60-year (1957-2017) time frame, and secondly to determine the location, number, size and activity of all gullies that remain untreated.

Preliminary results indicate that while gullies within forests planted before 1997 have shown a significant decrease in erosion activity as trees have become older and more established, there are now a greater number of gullies present within forested areas than 20-years ago, and largely a legacy of gullies that existed on 20,000ha of farmland before it was planted and have yet to stabilise under a forest cover.

Additionally, more than 200 new gullies have developed in the past 20 years, mostly on farmland, and particularly in areas of the region identified as the “worst-of-the-worst” eroding land (Land Overlay 3A district plan rule) in the Gisborne District Council Regional Plan, and for which there is a requirement to establish an effective tree cover before the expiry of funding through the Erosion Control Funding Programme (ECFP) in 2020.

The key to preventing further expansion in gully erosion will require a concerted and continuing treatment programme, whether by reforestation, pole planting or reversion, with the aim of targeting existing gullies and new gullies at their inception.

However, regardless of the pace and scale of future remediation efforts, the longer the delay in implementing treatment, the greater is the probability that the most active of gullies will enlarge, some of which will become “badass” gullies, and therefore beyond remediation.

Furthermore, the longer that gullies remain untreated, the greater their disproportionate effect will be on this landscape.

That is, gullies, singularly and collectively render once productive hill country unproductive, generating increasing amounts of sediment in excess of that derived from all other hill slope erosion processes combined.

Region-wide, while afforestation has been the most effective of available treatment options in reducing the amount of sediment generated by gullies, there remain more than 1800 actively eroding gullies, 38 percent of which occur within forests and 62 percent on farmland and/or in areas of reverting scrub.

Unsurprisingly, 42 percent of all eroding gullies, including the highest proportion of “badass” gullies are found in the Waiapu catchment, making the restoration goals set for this catchment that more challenging.

Unless central and local government agencies prioritise future funding efforts towards the remediation of gully erosion, gullies are destined to impact disproportionately on future land use options, the management of this regions river systems, and put increasing pressure on already strained infrastructure.

Dr Mike Marden

Dr Marden, who has a doctorate in geology, has been involved in documenting and publishing papers on land use issues, particularly erosions and erosion mitigation facing the East Coast region.

Most of his research is forestry sector-focused, including documentation of the stabilising influence of planted exotic forest on erosion-prone landscapes and the benefits of reducing the amount of sediment getting into each of the three main East Coast rivers.

Dr Marden has informed district and regional councils, government ministries, private consultants and other crown research institutes.

He has published more than 60 scientific papers, eight book chapters, 50-plus contract reports, numerous newspaper items and articles for popular magazines, the occasional radio interview and a TV appearance.

Mrs Anne Seymour

Mrs Seymour started work for the New Zealand forest service at the age of 16.

In the forestry industry she moved from draughting to cartography and then to computer based “Geographic Information Systems”.

She worked for the Ministry of Agriculture and Forestry between 2000-2005 at which time she digitised the gullies Dr Marden delineated on prints of aerial photos to create the 1997 gully database.

Mrs Seymour has seen many changes in the forest industry and in recording the various land management initiatives.

Computers have made it possible to record visually and alpha-numerically all aspects of forest work, from planting to clear felling, in a way that could not have been envisioned when I started work in 1957.

‘Badass' Mangatu Gully: Scattered throughout the region are gullies that are too large to be treated successfully. For scale, note the three people standing in the channel. Picture supplied by Dr Mike Marden