Main index     Proceedings main index     Proceedings #6 index

WAITOMO STREAM CATCHMENT CONTROL SCHEME

J H Simmons & A L Lohrey, Waikato Valley Authority

SYNOPSIS

Following concern over rapidly increasing sedimentation rates within the Waitomo Caves, a major tourist attraction, an upper catchment study was initiated resulting in an approved catchment management scheme. Erosion control works and protection measures can be implemented with the cooperation of landowners with the assistance of government funds. The paper sets out the Waikato Valley Authority involvement, the objectives of the scheme and progress to date.

1.0 INTRODUCTION

For the benefit of overseas delegates and those not familiar with the water and soil conservation administration in New Zealand a brief explanation is warranted.

Catchment Authorities such as the Waikato Valley Authority are regionally based with a statutory role of maintaining and enhancing water and soil values in New Zealand. In most regions the Authorities comprise an elected board and employ in the order of 30 professional staff plus administration and technical support. They operate under two primary statutes, the 1941 Soil Conservation and Rivers Control Act and the 1967 Water and Soil Conservation Act. Responsibilities extend to:

The Waikato Valley Authority was formed in 1953 by way of a specific act. The initial reason for its formation was to effect flood control in the lower Waikato, but by 1967 activities had expanded to include soil conservation works.

Initially individual property works were implemented and land development in the central plateau volcanic pumice was generating such dramatic erosion problems a catchment scheme approach was evolved as the only comprehensive way in which control of present and potential problems could be achieved.

For catchment authorities to implement catchment schemes the following procedures must be completed:

Depending upon level of grant assistance, approval may be required from NWASCA, Minister of Works and Development, or Government.

2.0 BACKGROUND AND OBJECTIVES

Whilst many of you will be very familiar with the history and particular problems associated with the Waitomo Caves a brief summation will serve to illustrate how the Authority became involved and eventually gained approval to promote the management scheme.

In 1974 the WVA received a request from NWASCA to investigate the source of sediment causing siltation of the caves, which was confirmed as being a product of road works in the upper catchment. Desilting of the caves was undertaken as a result of the sedimentation - a deteriorating situation could not be permitted as a major tourist resource was at risk.

At about the same time Professor P Williams reported the problem as having four contributory causes:

  1. County road works upgrading Caves-Te Anga Road.
  2. Feral goat populations thriving in the dry limestone formations and browsing extensively in bush areas depleting ground cover.
  3. On-farm erosion caused by lack of good land management.
  4. Weir installed within Caves causing accelerated silt deposition.

Concurrently as the above reports were being written the Authority was actively involved in soil conservation measures to mitigate some sources of sediment, namely from the road works. In the winter of 1974 8.5 hectares were fenced and planted on Fredericksen's property for road stabilisation purposes.

In June 1975 the Nature Conservation Council expressed concern over Council roading works which had resulted in massive amounts of spoil material being placed in a position where stabilisation would be extremely difficult to achieve.

Another problem was the fact that the transport time through the waterway systems would result in a very attenuated deposition period in the caves, a natural sediment trap.

With the support and encouragement of the interdisciplinary caves study group set up in 1975 the Authority commenced a detailed land use capability study of the catchment to record geology, soils, vegetation, slope and erosion (present and potential). This provides base data from which management guidelines and a catchment protection scheme could be developed.

At this stage Government was actively stimulating the rural sector to increase stock numbers and the area of farmed land by way of investment and production incentives.

To counter the risk of land clearing resulting in soil erosion and affecting river systems through increased runoff the Authority introduced a Conservation of Ground Cover By-law which requires approval of all vegetation removal from areas of land greater than 1 hectare per annum.

In the case of the Waitomo catchment only a limited number of permits have been issued, generally for small areas of scrub clearing. Nevertheless the regulatory control the By-law has is significant in terms of catchment protection. More established importantly in many respects it offers an introduction to landowners who in many instances have become involved in soil and water conservation plans for their properties.

However to afford the required protection to the Caves catchment a comprehensively planned scheme was required. Given the prevailing trends in rural land development a scheme was designed which offered landowners the option of afforestation, rather than pastoral development, tree planting to control slip and slump erosion; road works stabilisation; and fencing to retire bush areas from stock intrusion. Government grants and a contribution from the Tourist Hotel Corporation, together with a l5% cost input from landowners would fund the Scheme.

The WVA adopted the Scheme in November 1979, and referred it to NWASCA for approval. An economic evaluation indicated an internal rate of return of 16.9%.

The approval process as mentioned earlier, can be prolonged. In the case of the Waitomo Scheme it took four frustrating years, finally being approved in July 1983 at a total cost of $200,000 with a Government grant of 70%.

In the interim the scheme objectives and guidelines had been successfully incorporated into the Waitomo District Council District Scheme to give due recognition to the importance of catchment protection for this area.

3.0 RESOURCE ASSESSMENT

3.1 Aim

The scheme report was prepared in response to a request from the Tourist Hotel Corporation and the Waitomo Caves Scientific Research Group, for slope stability information on the catchment upstream of the Glowworm Cave. The cave was noted to have suffered severe silting from years 1973-78 with the associated problems of reduced cave access and effects on the glow worm habitat.

The report aimed to put forward a commentary on the stability of the catchment and devised works proposals to reduce silt levels in the Waitomo River.

3.2 Technical Aspects

The following factors were studied as background to preparation of the report.

3.2.1 Catchment Description

The catchment largely comprises steep to very steep hill country with small areas of both flat alluvial valley bottoms and gently rolling slopes. Vegetation cover is about 50% grass under pastoral farming and 50% native bush or scrub.

A large proportion of the Waitomo Valley is limestone and calcareous sandstone and it is these lithologies that give the catchment its characteristic topography with numerous enclosed depressions, sinkholes, bluffs and rock outcrops.

The caves catchment covers some 48km2 and is drained primarily by the Waitomo River and the Okohua Stream.

3.2.2 Geology

Geology within this catchment has the major effect on land stability and becomes the technical aspect of most significance as background to this report. The caves catchment area is faulted north/south giving rise to a variety of lithologies. In general tertiary group sandstones, limestones and siltstones occur in individual blocks over the catchment. These overlie a base of argillite which appears at the surface near the Mairoa fault to the west of the catchment. These rock types are capped in places with volcanic ignimbrite - hard and stable and Mairoa ashes up to 5 metres deep, which although prone to surface erosion are relatively stable to block movement

The location and natural stability of sedimentary rock types determines likely sediment sources. The limestones and calcareous sandstones within the catchment are stable formations and of less importance to this study. The Mahoenui formation siltstones and sandstones to the north of the catchment and the large block of sandstone to the middle of the catchment are inherently unstable and offer the major potential source of sediment to the caves.

3.2.3 Soils

Soils within the catchment are derived from both volcanic ash showers and on steeper areas from the underlying lithology where the ashes have been lost. Soils are of lesser importance to this report as land stability is largely determined by geology.

3.2.4 Drainage Characteristics

Drainage of the Waitomo Valley is very complex because of the nature of the underlying geology. Subterranean streams are common and hence the topographic divide does not always equate with the drainage divide. Two major streams drain the valley; the Waitomo Stream and the Okohua Stream which joins the Waitomo River at the Ruakuri resurgence. The Waitomo River originates as a subterranean system to the north and west of the catchment but after the Waitomo River emerges at the foot of the Piopio fault it remains on the surface draining through the area of unstable siltstones with its side streams draining the area of sandstones to the middle to the caves catchment. The Okohua Stream has its head waters in the Waipuna Scenic Reserve then sinks and flows through the Otorohanga Limestone and reappears at the foot of the Piopio fault. The stream flows along the Okohua fault in a series of caves to re-emerge at the Ruakuri Tourist Cave.

4.0 SCHEME DESIGN

As noted the scheme was designed with the main aim of reducing the sediment loading to the Waitomo Stream and to the caves.

Catchment condition was defined using the available technical information as well as undertaking a field survey of the catchment area concentrating on likely problem areas.

4.1 Present Erosion

A map of present erosion was prepared identifying erosion present and assigning each area in a 3 class severity rating. The following factors were noted from the field survey.

4.1.1 Soil creep and soil slip erosion are the most common forms and occur on moderately steep to steep slopes throughout the catchment. The limestone is stable except for some soil slip erosion in the red clay soils covering it.

4.1.2 The Caves-Lemon Point Road area located on the Mahoenui siltstone geology to the north of the catchment was noted as the major site of active erosion. In 1979 the road area was described to have collapsed during winter with deep rill and gully erosion and continued earth flow movement in large areas of road filling.

4.1.3 Stream bank erosion was noted as minimal occurring mainly during storm events.

4.1.4 Wild goats were a contributing factor in 1979 toward catchment erosion, through over-browsing of surface vegetation within the native bush. Goats are still a major problem and it is difficult to control numbers. In 1979 goats had little value other than for some control of scrub weeds. Currently feral female goats are worth more than sheep hence farmer reluctance to undertake goat culling operations within the bush area - a traditional means of keeping numbers under control.

4.2 Potential erosion

Two areas were seen as potential erosion problems affecting the caves.

4.2.1 The central part of the valley on Horotea sandstone is steep to very steep and is stable under its present cover of native bush or scrub. However if this cover is removed potential for moderate to severe erosion of slopes exists. The scheme report noted that bush cover should be retained on these slopes.

4.2.2 Road Works. Further problems will occur where road works are carried out with the fill being dumped over the road edge. This is particularly a problem on the siltstone portion of the catchment (Caves-Lemon Point Road), where careful planning and implementation of works is vital to mitigate the risk of erosion.

4.3 Silt Sources

The report noted that the Waitomo Stream appeared to carry a greater silt loading than the Okohua Stream as would be expected both due to the greater bush cover and more stable geology within the Okohua catchment. Considerable quantities of silt were noted to be lost from the road works area on the Caves-Lemon Point Road. The initial major siltation problems in the caves were also noted to coincide with the major Caves-Lemon Point Road works in 1974.

5.0 MANAGEMENT PROPOSALS

From the available information as outlined, a programme of physical works and management proposals were prepared.

5.1.1 Bush Retention/Retirement Fencing. To maintain areas of scenic reserve within the catchment.

5.1.2 To maintain and fence off areas of steep country that are in native forest or scrub. This comprises mostly the large block of sandstone to the centre of the catchment. This land can be unstable and clearing is likely to increase surface erosion contributing greater sediment loadings to the Waitomo Stream. Also the steep slopes below the Caves-Lemon Point Road were designated for the retention of existing vegetation.

5.1.3 Planting/Retention Structures/Retirement. Afforestation of the road fill areas below the Caves-Lemon Point Road was recommended. Also afforestation of some steeper land near the middle of the catchment was recommended to reduce soil slip and surface erosion forms.

Debris dams were proposed to retain silt within the catchment of origin.

5.2 The original works proposed were as follows:

  1. Protect steep slopes and retain water storage resources by maintaining native scrub and forest on VIIe and VIIIe land. Total area of 300ha
  2. Protection planting of steep Class VII land with an alternative for partial grazing on some areas. Total area of 277ha
  3. Block planting for road stabilisation. Total area of 23ha
  4. Intensive pole planting (50 poles/ha) Total area of 20ha = 1000 poles
  5. Open pole planting (25 poles/ha) Total area of 68ha = 1700 Poles
  6. Fencing of bush areas: Total length estimated = l0,000m
  7. Construction of debris dams
  8. Bank Protection on Waitomo Stream

6.0 PROGRESS TO DATE

The Catchment Control Scheme was prepared for 1979 conditions however a number of factors have effected the rate of planned progress since that date.

6.1 Economic Climate

The government land development loan available in 1979 along with the optimistic attitude in the farming sector was leading to large areas of bush being cleared in the Waitomo area. It was feared that a large area of bush in the middle of the catchment would be lost if clearing controls and permanent retirement of some bush was not undertaken. It is now unlikely that this land will be cleared in the forseeable future due to the change in economic climate.

Large areas that were expected to be lost are still uncleared hence fencelines that were proposed as a limit to bush development may at present be up to 1km inside existing bush blocks. There is no point in erecting fences through bush expecting the remainder to be cleared. Hence much of the retirement fencing proposed will not be complete until if or when the bush is cleared. It would be desirable to retain all the native vegetation present however as the land is in private ownership strong justification is necessary to retire land from grazing under the provisions of the Water and Soil Conservation legislation.

6.2 Scheme Approval

The scheme was approved in 1983, 4 years after preparation, thus reactiviation of farmer interest is necessary before significant works programmes can be organised.

6.3 Works Complete

Remedial planting and silt detention works have been carried out on the areas of road fill below the Caves-Lemon Point Road since the problem was initiated in 1974, under an agreement with the owner of the area Mr J D Frederickson. Recent works have been initiated under the Waitomo Catchment Control Scheme.

Works include pole and seedling planting, with the aim of stabilising the fill in the medium to long term. Oversowing and topdressing was carried out early on with the aim of surface stabilisation of the fill. A detention structure was recently built with the aim of retaining larger fractions of sediment within the dam assuming sediment not settling in the dam would be in suspension and would not settle in the caves. Permanent retirement of the area below the Caves-Lemon Point Road proposed for this work is complete. Feral goat control is an on-going work necessary to maintain the positive effect of the retirement work. This operation has been considerably more difficult to achieve since the rise in value of female feral goats in the last 18 months. Farmers are now reluctant to destroy any goats in retired areas, however, effective means of capturing a large percentage of these goats are not available

7.0 FUTURE OBJECTIVES

The major future objective is to ensure that the initial aim of the scheme (that is water quality and siltation control) is achieved.

Works initially planned have to be modified as circumstances affecting the land change. The Waikato Valley Authority aims to promote and complete all major works proposed in the scheme plans as and when completion becomes appropriate. In addition to works completion a continuing programme of feral goat control is necessary to ensure the effectiveness of land retirement. Maintenance of soil conservation works is necessary to ensure their long term effectiveness of this is an on-going programme with all Waikato Valley Authority Soil Conservation Works.

Future aims also include:

  1. A continuing control on land clearing operations within the caves catchment with any necessary controls being implemented under the Waikato Valley Authority Conservation of Ground Cover By-law 1977.
  2. Ensuring that land owners and the Waitomo District Council are aware of the siltation problems and its effect on the caves viability. Farming practices including particularly farm tracking, stream crossing and stream damming activities are thought to have a major effect on sediment loadings in the streams draining pasture. Care by property owners in siting and building tracks, and by the Waitomo County during road building, is essential in the long term control of human induced erosion and therefore sedimentation of the caves system.

8.0 SUMMARY

The means to substantially gain protection of the Waitomo Stream catchment is in place. Considerable works have been done to reduce the major sediment source but as you will see on the field trip the nature of the material and topography make this task difficult to achieve completely. In terms of land management sustained agricultural production and catchment protection are compatible providing the necessary controls are implemented. More work is needed in the area of farm track runoff control, bush protection and goat control. Afforestation offers a viable alternative to land clearing for pastoral use

A greater sense of awareness is necessary when road works are contemplated, with adequate provision for stabilisation works in cost estimates. It is in everyone's interest to ensure this catchment is managed in a careful manner.