The effects of forestry on sediment production in the Coalgate Forest, Malvern Hills, inland Canterbury

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Geology
Degree name
Master of Science
Publisher
Journal Title
Journal ISSN
Volume Title
Language
English
Date
2001
Authors
Carlton, Josephine Ruth
Abstract

The Coalgate Forest is located approximately 55 km west of Christchurch, in the Malvern Hills, inland Canterbury. In this project the effects of forest operations on sediment production were studied in a small catchments, referred to as the "unnamed tributary". Before forestry was established in the Malvern Hills area during the mid 1960's, coalmining, clay products, and farming were dominant factors in the local economy. Forestry and farming now dominate. The small unnamed tributary of approximately 78 ha was chosen for this project because of the harvesting and replanting activities currently being undertaken by the Selwyn Plantation Board Limited, who own the Coalgate Forest and several others in the Canterbury region. From the limited existing data available and field observations, sediment monitoring and rainfall information collected from the catchment, the effects of forestry on sediment production were interpreted. Rill erosion, overland flow, rainsplash erosion, mass movements, freeze and thaw action and gullying processes occurring in the catchment were monitored using erosion plots, mass movement volume estimations, and erosion stakes and pins. During the 1O months of sediment monitoring the largest long-term contributor of sediment in the catchment was the replanted hillslopes with approximately 85t of sediment per annum. Although the replanted hillslopes produced a low average sediment yield of 0.4 kg/m2 , this area makes up 90% of the forested area in the catchment. An old track leading off the old log landing produced the largest average sediment yield at 6.05 kg/m2 , however this area only contributes 0.6% of the total sediment yield in the catchment. During the course of the sediment monitoring programme a 50 year return period rainstorm event occurred, and produced large amounts of short-term erosion on the cut bank of the access track. An estimated 6 000 t of sediment was mobilised, which contributed 98% of the total sediment yield of the catchment during the 10 month period of monitoring. From this it is clear that storms of this magnitude generate significant amounts of sediment in the short term, while long-term erosion can produce similar amounts of sediment to the short-term erosion. The August 2000 produced the highest Erosivity Index at 30 000 J/m2 . mm/hr from all of the storms recorded during the 12 month period. This high Erosivity Index is reflected in large soil losses recorded for the storm. By comparing the Erosivity Indexes for long duration, moderate intensity and short duration, high intensity rainstorms it is apparent that both of these types of rainfall have similar influences on soil erosion. The radionuclides lead-210 and caesium-137 were used to analyse a sediment core collected from a small pond at the outlet of the catchment, this analysis initially indicated that sediment deposition exceeded Scm/yr because the 2 1 0 Pb did not exceed its parent radium-226, in the top section of the core. Further interpretation of the radionuclides indicated an average sediment accumulation of 1.6 cm/yr in the catchment between 1973 when the catchment was first planted and 1996 when harvesting commenced, while the average sediment accumulation rate between 1996 and present day had increased 10 fold to an average of 1O cm/yr. This is attributed to track upgrading in 1996 and subsequent harvesting activity. It can be concluded from the geotechnical properties of the material tested in the catchment that erosion in the Coalgate Forest is the result of the climate and forestry management, and not the result of unusually erosion-prone rock and soil materials. Various measures have been identified to reduce the impacts of sediment generation, of which improved engineering design of the access track is the most important.

Description
Citation
Keywords
Sedimentation and deposition--New Zealand--Selwyn District, Soil erosion--New Zealand--Selwyn District., Forest influences--New Zealand--Selwyn District
Ngā upoko tukutuku/Māori subject headings
ANZSRC fields of research
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All Rights Reserved