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HYDROLOGY AND STREAM SEDIMENTS IN A MOUNTAIN CATCHMENT
VOLUME I (of three volumes)
A SUMMARY
A dissertation presented to the University of Canterbury in fulfilment of the requirements for the degree Doctor of Philosophy.
JOHN A. HAYWARD 1978 HYDROLOGY AND STREAM SEDIMENTS IN A MOUNTAIN CATCHMENT
VOLUME I (being Volume I of three volumes)
A SUMMARY page nos. ACKNOWLEDGMENTS
CHAPTER 1: 1970. The status of knowledge about hydrology, erosion and sediments in New Zealand mountain 1ands. 6
CHAPTER 2: A summary account of studies in the Torlesse stream catchment 1972 - 1977. 15
CHAPTER 3: The floods of April 1378: An example to illustrate the findings of this study. 25
CHAPTER 4: The implications for future management. 42
REFERENCES: 54 LIST OF FIGURES, VOLUME I Fo llows page No.
1. Extracts from the Christchurch Press and Christchurch Star, 1940 - 1943. 8
2. Extracts from Campbell (undated). 11
ll 3. Extract from "The Weekly News • 29 May 1963. 11 4. The Torlesse stream catchment. 17 5. Location of Torlesse stream catchment 17 6. Plan of study area showing places mentioned in text. 17
7. Area altitude relations, Torlesse stream catchment. 17
8. Soil sets, Torlesse stream catchment. 18
9. Degree of depletion, Torlesse stream catchment. 19
10. Distribution of major plant communities, Torlesse stream catchment. 19
11. Land capability assessment. 20
12. The Torlesse stream catchment. 27
13. The Kowai river catchment. ·27
14. Rainbow gully. 27
15. The Irishman stream. 27
16. Sediment accumula~ion in Rainbow gully: 28
17. Alluvial sediments stored in the Torlesse stream channel. 28
18. Synoptic conditions 14,18,29,22 April 1978. 28
19. Hyetograph and hydrograph, 16 - 18 April 1978. 32
20. Ground water emerging to the catchment surface. 32
21. Stable channel below a point of ground water emergence. 32
22. Unstable channel below a point of ground water emergence. 32
23. Movement of debris deposit in Slug Gut. 33
24. Face above Helen stream. 33
25. Failure of unconsolidated stream banks. 34 contd ..•. LIST OF FIGURES CONTD. Fo 11 ows page No. 26. The Kowai river. 36 27. Sediment accumulation upstream of the Torlesse stream control section. 36
28. Remnants of a gravel wave. 37 29. Hyetograph and hydrograph, Torlesse stream catchment, 19 - 20 April 1978. -, 38
30. Hyetograph and hydrograph, Torlesse stream catchment, 22 - 23 Ap r i 1 1978. 39 31. The Kowai river flowing alongside the Kowai scree slopes. 41
32. Rock outcrop provides stabil ity to the upper slope. 41
33. Degradation of the Foggy fan. 41
34. Degradation of the Foggy fan. 41 LIST OF TABLES, VOLUME I
Follows page No.
1. Slope classes, Torlesse stream catchment. 17 2. Soil sets, Torlesse stream catchment. 17
3. Degree of depletion, Torlesse stream catchment. 19
~. Soil loss and erosion, Torlesse stream catchment. 19 - 1 -
ACKNOWLEDGEMENTS
This study has almost become a way of 1 ife and I should like to gratefully acknowledge the many people who are a part of it.
Between 1967 and 1969 I carried out an erosion st~dy using small run-off plots at Porter1s Pass on the Southern end of the Torlesse Range. During that study I real ised that while soil erosion was active on that catch
ment's surface, little if any of this debris appeared to r~ach the stream channel. This observation caused me to doubt ~he complete validity of a presumed direct relationship between land erosion and presumed sediment accumulations in downstream channels. ~
During that period those concerned f6r soil conservation and rivers con trol throughout the high country were encouraging and promoting the retirement and rehabilitation of high altitude lands with the objectives, jnter alia~ of reducing both the rates at which sediment accumulated in streams and rivers, and the frequency and magn!tude of flooding.
It was my view that these were untested propositions.
- In .1970 I was fortunate to be able to visit the Western United States. Almost by chance I met Dr. Peter Klingerman at Oregon State University and saw his vortex tube sediment trap at Oak Creek. This device has been the central element of our Torlesse experience.
It was my view that high country erosion and sediment research should be based on whole catchments and that suth studies should be of a multi-dis-
ciplinary nature. In 1971 a group came together and submitted a succesi~ ful proposal to the Nuffield Foundation.
This group included Dr. D.J. Painter (New Zealand Agricultural Engineering Institute, Lincoln College), Dr. J.M. Soons (Geography Department, Canterbury University), Dr. A.J. Sutherland (Civil Engineering Department, - 2 -
Canterbury University) and myself. We were given support from Mr. R.D. Dick UJorth Canterbury Catchment Board} ,1Ir. D.W. Ives (then of D.S.I.R. Soil Bureau), Dr. W.McCave (D.S. I .. R.. [nstitute of i·~uclear Sciences), Mr. A. Ryan (Ministry of Transport Meteorological Services) and Dr. G.T. Daly (Plant Science Department, Lincoln College}.
Although the work reported here represents my contribution to this groupls endeavours it could not have been accompl ished without a great deal of assistance from many people. I wish.tri place on record my most sincere appreciation to all those Wil0 have contrrbuted to this physically demanding and intellectually stimulating experience.
In November 1971 we began construction of the base hut. and sediment trap. I should 1 ike to pay a particular tribute to the enthusiasm and competence of Messrs Ian Fryer and John Stevens.of tbe Tussock Grasslands and Moun- . tain Lands Institute. Without their building skills and without the help of Messrs Terry Crowhen, Hugh Orr, and Dick Martin this project would have remained an idea. r am grateful to Mr. T.D. Heiler for his assistance \tIith hydraulic design; to t1r. I Calvert for his assurances that my elementary structural design was "unl ikely to fail"; and to Hr. J.T. Johnson (Ministry of Works and Development Testing Laboratory) for his advice on how best to make concrete from river bed gravels. To the .many friends who helped us pour the concrete and build the base faci lities; thank you.
I am grateful to Mr. Alan Ryan and hls staff in the Meteorological Office at Harewood for their storm warnings.
The task of weighing sediments during storms has been exhausting, demanding and occasionally fun. To Messrs Ian Fryer, Ross Stratford, Chris Ward,
Errol Costello, Clyde Shum~-Jay, Rob Blakely, Peter Ackroyd, Doug Hilne, Gerry McSweeney, Phil McGuigan, Dave Adam, Lindsay Main, Hike Mardin and Drs. Bob and Emily Oaks, thank you for the cheerful way in which you helped weigh gravel in what \-Jas usually appalling weather. To Ian and Anne Forne I express my grateful thanks for their assistance with the study of infil- tration rates ..
In 19J4/]5 was invited to Colorado State University as a visiting Professor in Watershed Management. During that trme I met, and was profoundly impressed by, Dr. S,A. Schumm and his postgraduate group of students who were doing research into the morphology of fluvial systems. By J9]4 we had recorded, marked variabrl ity in, and periodicity of, sediment-movement, but there was little apparent pattern in this movement. It was exciting to find that from a physically very different, small, experimental sand bed catchment, some of Dr. Schumm's group were recording comparable behaviour. My association with Dr. Schumm was a major in- fluence in understanding the behaviour of the Torlesse Stream catchment.
I should also 1 ike to express particular thanks to Mr. Rob Blakely. During my absence Mr. Blakely had been employed by the Institute to help maintain the project, to carry out sediment size analyses, and to make detailed surveys of the pool-riffle morphology of the stream systems. In correspondence with Mr. Blakely I advanced the propositions that the variability in sediment yields was in part due to a pulsing or wave-l ike movement of sediments and asked that his surveys attempt to establish the
presence or absence of grave I !'waves" in the st ream channe 1. ~/h i 1e the results show that sucn "waves" do exist they are not as I had originally conceived. It was Mr. Blakely's competence as an observer, rather than hi.s survey skills, which established t:leir presence. Following his observations Mr. Blakely took a number of initiatives aimed at better understanding the storage and release of sediments within the channel system. The descriptions of gravel wave movement (Chapter 8 Vol. I I) are derived from his measurements.
I am also indebted to Hr. Blakely for the surveys of the Kowai and Torlesse Stream channels and to the invaluable help of Mr. Derry Gordon and students from the Civil Engineering Department, Canterbury University.
In ]9]6, Dr. Robert Oaks, (Department of Geology, Utah State University) spent a sabbatical leave mapping the geology of the Torlesse Stream catch- ~ 4 - ment., In addition to these studies he made some most important observations about the significance of the fine fractions to the stabil ity of colluvial deposits. These observations have been of the utmost importance in under- standing the stability of the Torlesse stream catchment.
By focussing my attentions on one catchment I have been better able to note short term and longer term changes which have provided me with insights into the processes operating in the catchment. A notable example was the study of subsurface discontinuities and I am particularly grateful to tlr. Richard Lewandowsky (Geology Department, Canterbury University) for the seismic refraction surveys reported ln Chapter 4 Vol. I I.
A second example is in the'coricept that stream energy relations play an important role in controll ing catchment stabll ity. I am most grateful to Dr. Tim Davies (Department of Agricultural Engineering, Lincoln Col lege) for his enthusiastic s[lartng of my intuitions, and for his help with the laboratory study reported in Chapter 9 Vol. I I. also express my
graditude to i~essrs Geoff Thompson and Col in Tinker for thei r part in that study.
In presenting these results and in interpreting their significance I recognise that I have received the benefit of comment and criticism from many people. Information contained in Chapter B Vol I I, has been presented at a symposium of the New Zealand Hydrological Society and some of that presented in Chapters 3, 5 and 6 Vol. II was del ivered to a D.S. I .R. Symposium on Soil Plant Water relations.
In the last five years I have accompanied many New Zealand and overseas visitors to the Torlesse catchment. I 'doubt that there has been one occasion in which I have not developed my understandings and insights as a direct result of these visits.
owe much to my' friend and colleague t1r. Terry Hei ler who tolerated my
enthusiasms and critlqued my ideas~ I should also like to express my unreserved appreciation for Dr. Kevin O'Connor, Director of the Tussock Grasslands and tlountain Lands Institute, who has given the Torlesse ~ 5 ~ project his unfailing support and been a constant source of stimulus, and support to ~Jy endeavours of the last five years. I am grateful to !1r, Ken Lefever, Dick t1arttn and Le Ba HO,ng for their competent assistance with data processing; to Pat Prendergast and Val Davies for their draught ing and illustrative work; and to Mrs. Beryl Bond and Mrs. Nan Thomson for theIr typfng of this manuscript.
To Maurice and Helen Milliken (Brooksdale Station) on whose land this project is centred I should I ike to say a sincere thank you for your hospitality, generosity and friendshfp. There have been times when you must surely have questioned our sanity and U'le value of this whole proj ect.
Finally I should 1 ike to express my appreciation for the support and encouragement given to me by my wife and family. It was their tolerance of the disruptions that storm events have brought to our family life that really made this study possible. CHAPTER 1.
1970
THE STATUS OF KNOWLEDGE AND CONSERVATION PRACTICES
IN MOUNTAIN LANDS. SUMMARY
New Zealand attitudes to soil and water conservation have their origins in
Europe and I~orth America where l.egislation for the conservation of soil and water resources preceded research by 20 ~ 30 years.
In the ]930'5 and 40 15 much New Zealand land was in a depleted and eroded condition. Those who first advocated soil conservation saw a clear need
for remedial action in preference to research. North A~erican attitudes, policies and research findings became the bases for New Zealand pol icies and programmes.
Most surveys and investigatio~s made in New Zealand mountain land were predicated on :Jorth American concern for soil surface conditions and Hortonts concept of overland flow. - 6 -
CHAPTER 1.
1970: THE STATUS OF KNOWLEDGE AND CONSERVATION PRACTICES
IN MOUNTAIN LANDS.
It is possible to search the writings of ancient philosophers and find occasional evidence of a concern for relations between land use, soil erosion and river behaviour (for ex~mple G1aken, 1958).
However contemporary soil and water conservation is generally believed to have it1s origins in 19th century Central Europe.
In the 18th and 19th Centuries the population of the Swiss Alps increased as entrepreneurs clear-cut forests for the rapidly developing down country iron and glass industries. The subsequent erosion and flooding went largely unheeded until the Vienna Congress of 1848 gave politic~l stability to the region. Disaster~ in the 1870 l s and 1880'5 convinced the Swiss Government that remedial action was necessary. In 1876 and again in 1902 the Swiss Constitution and forast legislations gave authority to the Government to undertake remedial work with Federal money.
In 1902 Arnold Engler began the Emmentha1 project to study the effects of forests on stream flow. There are at least two features of the early Swiss experience which are of interest. First, the pol itica1 decision to rehabilitate the mountain lands preceded research into forest influences
by about 25 years. Second, Engler an~ later Hans Burger were experienced and practical foresters who not only believed in the Swiss Government1s policies of rehabil itation but felt responsible for them. They were apparently strong willed and determined men who engaged in science in search of support for their opinions. From their writings a reader can apparently learn much of the political climate of the time. (Keller pel's cOl/un.) - 7 -
Meanwhile settlers in the Western United States were recognising the
importance of water fo~ irrigation. Their concern for the limited avail- abil ity of water, and the prevail ing European view that forests had a favourable influence on streams, led Congress to enact legislation in 1897 to create forest reserves for the prime purpose of "securing favourable conditions of water ·flows". In.1911 the Week's Forest Purchase Act extended the "acquisition of lands for the purpose of conserving the navigabil ity of navigable waters" (USDA Forest Service 1933),
In the same year (1911) the United States Forest Service and Weather Bureau set up a co-operative paired catchment study into the effects of forests 6n stream flow at Wagon Wheel Gap in Colorado, the results of ~hich would not be rep~rted for another 17 years (Bates and Henry 1928).
However in the following year (1912) Raphael Zon attempted to enlighten the U.S. Congress and public with his report "Forests and Water in the Light of Scieritific Investigation", Forests, he reported, were not only "beneficial to stream flows, but they actually caused increases in preci pitation (Zon, 1912), Although this latter view is now discredited it was at the time an important contribution to the developing concern about forest influences.
But the concern was· not limited to forest lands. In 1917 F.L. Duley and M.F. Miller began a series of runoff and' erosion experiments on agricultural . land and showed that greatest runoff and erosion took place on bare un- cultivated soils. In contrast permanent pasture allowed little runoff or soil loss. (Duley, 1952)
In 1928 Bennet and Chapl ine produced an assessment of the erosion problem--~ in the United States and warned that
"corrective action IIIUS·t be taken soon if far gY'eateY' dconage and mOl'e difficult contl~ol aY'c to be obm:ated. OWners of Y'ange land should conside11 the use of the'il' land not only fOl' immediate gain., but still moY'c in the light of the futw'e pY'oductivity of the l'ange., the pY'otccUon of lcutel' ,supply., aHd stl'eam-floliJ reg1tlat1.:on". (Bennet and Chap1 ine, 1928). Three years later, (193J), Forsling reported results of ]5 years of measure ments on two experimental watersheds in Utah. liThe results show the inrportance oj' herbaceous vegetaHon in ;reducing rainfall runoff and floods and in controlling erosion. They also show the need for regulating grazin;; to prevent depletion of the herbaceous cover ..• F (Forsl ing 1931).
The dust storms of the 1930's had a profound influence on American and international attitudes to water and soil conservation. The tlB 1ack Dusters" of Oklahoma, Kansas and Colorado not only confirmed the fears of early conservationists but occurred in combination with a major economic depression and led to a widespread migration from the Dust Bowl.
'~pproximately 100 million acres still largely in cultivation has lost all or the greater part of its topsoil. In short~ half of the better cropland of the Nation has been affected by erosion in degrees varying from the state of incipiency to complete des truction. Tens of thousands of farmers have become subsoil farmers~ which means something very close to bankrupt farming on bankrupt landll (Bennett ·J93]; quoted by United States Department of Agriculture ]940). Bennettls magnum opus "Soil Conservationtl and Jacks and Whytes' liThe Rape of the Earth" published in 1938 were widely read and acclaimed for the clarity of the simple messages they contained.
By 1940 there was an established and recognised need for conservation of the soil and water resources of the United States.
In i~ew Zealand, IkCaskill (1973) records that between 1870 and 1930 J. Buchanan, Captain Campbell Walker, the Rev. P. Walsh, H. Hill, J.P. Grossman, J. Henderson and Ii-H. Ongley, Dr. E. Kidson, and It1alabar" all expressed concern for erosion and it's consequences. There was little public support for their views.
It is generally accepted that it was the Poverty Bay and Hawkes· Bay floods in February and April 1938 that tri.ggered public interest in river and erosion control. Newspaper articles of the day reflect a concern for urgent remedial action (Figure J). They also record that the early New Zealand advocates for soil and water conservation were very famil iar with Figure 1 follows Figure j: Extracts from the Christchurch Press and Christchurch Star 1940 - 1943.
These accounts indicate that in the 1930 ls and J940lS those who voiced concern for soi 1 and water conservation in i-Jew Zealand were famtl iar with North American attitudes, pol icies and programmes. SOIL EROSION --SOIL EROSION PROBLEM: NEED \ FOR ACTION ~/~ Investigation and Control AGAIN URf.ED,~:~;~ of the Canterbury he executive . ht decided to ,A LESSON FOR NEW ZEALAND T e last U1g . n PrOgress Leag~ ts in the C[\mpal~ continue Us e gr vernment action In for Immediate o. n Members were combating soil erOBIO d~splte the dlm (SPf' I, LI.1' WIII"'~l'1 roa Till .anl.) in agreement that war situation. arising from the d at once. I By L. W. McCASKILL.] cu Ities should be fnce ) the problem _ (Mr P. R. ClImle The mOl e one traveb ir America collected in the 77,OOO-lcre wllter,hed The secret aT) th Mlnlflter . . . 'd members that e pie) Mudylng ]'roblems of ~()II erOSIOn, Bnd the 31100-at:re lake throu,h which It dlllCharl!l<, to arrive ., pr_ctlcel olt remln~~c works (the Hon RI St~\duee forestry,. B ld wild life. conti 01 gencr the farms whleh will redUce ailt.ation o~ r;~ that he intended to (~\r~anisa- :ally. the r 101 c one feels ho v 5imilar to a mlniml'l'I'\. ~_ ~'~lnl\"n under whl~h ,_:~""I~ke the \the prohlr m> are to those in New iil~7 thet AbleMI! at Data ~U1L (.,1--1;. .1.) ...... ~ • .l.. ... , mething ltwas found quite earl}' in the: ?!'epated 8I'hefne that there Wis Inlutftcltnt data· anfwhere h the world as td the re- HOW.PROBLEM IS ,',': '," . experi~ I:ltlon between loil erOilpn and the lVe our nature of r'llnstorms. It wll!l realised .nor mou9 that odd records from the few exi6t· SOLVED IN U~S.A. ing rlllnfnll stlltionll gave little funda , ' ment.l knowledlte. Accordin81y there was set up at New PhllsdelJjhla, the headquarters of the most elaborate cli • matic research in exl.tent:e. 8ecause of the vast number of observen neces ...... ~~ I..L _ ... ,.-1"" ...... 0 ,unllln ".live bPctn dif. CONSERVATION OF RESOURCES • Steps Taken In The United States
NEED ~EEN FOR ~IMJLAR ACTlON IN DOMiNlON the preceding 30 years of experience in the United States.
Three years later 094J} the Soil Conservation and Rivers~ Control Act became law .. Thi.s Act inter aUa establ i.shed the Soil Conservation and Rivers l Control Council which had the objects of; 1. the promotion of soil conservation 2. the prevention andnritigation of soil erosion 3. the prevention of damage by floods 4. the utiUsation of lands in such a manner as will tend toward the attainment of the objec'l;s afore said. .
To attain these objectiv~s in eroded mountain lands the Council would evolve pol ides and programmes which would tnclude: 1.. the retirement of eroded high altitude land from grazing by domestic and feral animals. (Where this required fencing~ government grants would cover the total cost.) 2. the provision of alternative grazing for displaced stock. 3. the prohibition or strict control of burning. 4. the provision of firebreak access tracks. 5. the rehabilitation of eroded retired lands. (Poole 1972)
Although the reasons for these polic[es have been stated in various forms they may be summarised as a desire to create
"a more protective and stabiUsing cover of vegetation~ so as to mitigate soil erosion~ and the choking of river channels with detritus~ and to minimise flooding 11. (Anon 1961)
By 1970,530,000 ha (1.3 mill ion acres) had been, or was in the process of being reti.red from the 1.25 mi 11 ion ha O.J mi 11 ion acres} of South Island high country reckoned to be in need of retirement. (Anon ] 973) . As it was generally recognised that retirement per se would not reverse erosion processes in many areas, catchment authorities were being urged to address the issue ofrehabil itation for eroded and depleted lands. (Poo 1e 1973).
What evidence was there to support these atti.tudes, policies and programmes:
Those who read thewrttings of the New Zealanders who first promoted a - 10 - concern .for soil conservation must come to the conclusion that New Zealand attitudes were profoundly influenced by North American experience. Being biologists and agricultural ists the early writers were quick to accept such propositions as: "Any modificaUon of the plant cover and surface soil~ by cuUivaUon~ burning~ 01~ overgrazing~ induces conditions unfavorable to the opti mum development of these soil fauna and flora~ which affect the abiUty of the soil to take up wa;ter." (U.S. Department of Agricul ture 1940) "ffhen a drop of rain strikes the ground covered with a dense covering of vegetation~ it breaks into a spray of clear water which finds it way into i;lw numberless interstices and channels of the soU; but when it strikes bare soil formerly developed under a mantle of vege tation~ the force of the drop causes it to take up fine particles into suspension; it becomes a drop of mud.dy water. As it sinks into the soil the fine particles filter out at the surface to form a thick film which chokes up the surface pores of the soil. Then only a part of the drop filters into the soil~ another part is left unab sorbed and flows over the surface; the accumulation of infinite unabsorbed drops on sloping land gives rise to superficial storm· flows." (Lowdermilk, quoted by U.S. Department of Agriculture 1940)
From writings such as these, and from the condition of much New Zealand hill and high country it was obvious to the early conservationists that exploitive land use led,to a deterioration of the plant cover with con sqeuent increases in erosion and flooding. This proposition was regarded as ~ fundamental truth. But also, implicit in this proposition was the view that the restoration of plant cover meant a reduction in the rates of erosion and flooding. If such 'self evident' propositions needed the support of research then this could be found in the writings of: .:orton (1933, 1938) Baver (1937) Bennett (1933) Lowdermilk (1935) Duley & Ackerman (1934) Musgrave (1947) Kittredge (1954) and others.
Both before and after 1940, a number of New Zealand surveys gave emphasii--· to plant, soil, erosion relations. (See for example Zotov (1938) Commit tee of Inqui~y (1939) Gibbs et al. (1945) Barker (1953) Tussock Grasslands Research Committee (1954) ~raight (1960, 1963, 1967)). These studies gave impl icit (and sometimes explicit) support to the view that because good plant cover was a desirable watershed feature, it should become an - 11 - objective for land management. This view was further supported by much general writing and a number of reports commissioned or presented by visitors from overseas (see for example Ell ison (1957) Heady (1967) Costin (1962) Love (1957).
The earliest bulletins of the Soil Conservation and Rivers Control Counci 1 and other agencies ·left a reader in no doubt as to their authors l views of -the problem and its solution. Their sometimes extravagent titles, (for example "Fire - Public Enemy No. 1" (Campbell undated), "At War with a River" (Scott undated), their dramatic photographs and presentation (Fig. 2), were clearly intended to arouse public awareness of the need for positive soil and water conservation. In the 1950's and 60's editorial and feature writers in daily newspapers and magazines gave support for erosion control measures. Including the retirement and rehabil itation of eroded mountain lands (for example see Fig. 3).
That such views came to have general acceptance is illustrated by the.out .come of .submissions to a Parliamentary Select Committee considering noxious animal legislation in 1964. In evidence the New Zealand Forest Service (1964) stated "the key to control of flooding and erosion and to satisfactory water supplies Ues in the lveU being of the skin of vegetation . •• which covers the mountains". Similar views were expressed by the New Ze~land Catchment Authorities Association (1964) "There wi U {n future be an increasing importance p iaced on a continuous yield of good lL'ater from mountain catchments for domestic, agY"icultU}?al, hydro eZ,ectric and industrial use". "Vegetation depletion and soil compaction lead directly to increased rates of runoff and accelerated eros'ion". Although such views were accepted, we, with the benefit of hindsight might
well ask "\~here was the New Zealand evidence upon \vhich such views were based?"
The nDuntain lands of New Zealand have been host to much research but comparatively little of this has been directed tm'lard understanding plunt,
soil, sediment and hydrologic ~elations. The vegetation studies noted earlier, established that much mountain land \vas in a severely eroded or Figures 2 & 3 follow Figure 2a: This cartoon was first published in the United States and was reprinted in the Soil Conservation and Rivers Control Council Bulletin No.4. It was clearly intended to arouse public awareness of the need for soil and water conservation.
Figure 2b: The cover of this bulletin indicates that in the 1940's the priorities for soil conservation were public awareness and action programmes. Research is the 10th in a I ist of 10 IIfirst steps in soil conservation". j~ I:
CONTROL GRAZING AND p~~~ROY AVOID BURNING
WHAT HAS HAPPENED APPRECIATE TO VEGETATION - TO SOILS
BECOME AWARE P~~:~E~ ~ ~ '. ~""- •• • --~ -<7' ,d1L,. ~~.. ' -~ 'I! . : .. .~ ...... ,. ~...... --. ,,-~ i '.pii-~'~~' .... " - .. .. ' .." . .. -:. '. ", - , "" ,,,--' ~. .-" -.- . ,"l: ':._ .' .. - ":,.::; --_"'1a • .- ~~ ~ ....• ."~h·1':::,,- " ~ .~ ..... - .-. . .-~ ~::
GOING THE INDIAN ONE BmER Fig. I. An American impression, from Conservation of Natural Resources, U.S.A. Figure 3: Extract from The Weekly News 29th Hay, )963. THE WEEKLY NEWS
Whh whh:h h, I'H~rpo'd'l!d THE N!W ZEALANb ~RE£ lANC~ AUCKlAND, WEDNESDAY, MAV~? I9IJJ
Forestry the answer to erosion problems If v;lhHl""' f,mnlanlls in tI,,· lower I'all"vs "nd lin lh,· plilin' "i'e [0 l)e "ave" f r n Jll serious lioodlng "'lit 1'('811Itun[ dl'posits 1)1' ('o"r$t shingle. leth nj'ltle,; must be fOllnd fu)' Ihl' xa\'ing of mouutiun v(.·.~('tatinn ilnd illi' r""fforesta I j01l or I·(·.t.!n!~stllg 01 deHutird twrl PJ' {':"h'n~iv~' ('l'osion ill IJH' \It hb ft':..!iolt ~Il Oil' ('(Inrlw,'ll(,{-, or till' :\ \' (I {' II and Jl.Hlwr Hh'l~r~, TJlf' riv{~r,:..; WlIltf tori IJousl)' through tl nl!~ln1tH1t· of t"iHHIIH+ls l~IIJ~~l(;H;~~:.III~!,~d ht';:l~~ L'illIlltf'~ lti!.;(:lIH'l' .. - 12 - depleted condition. In response, OICOnn0r (1962, 1967) Dunbar (1970, 1971) Nordmeyer (J97~ and others found species that would be suited to revege tation and establ ished the fertility needs for a range of sites. The benefits from the widespread application of these findingswer~not clear. Vork by Grad\'/ell (1955,1957,1962) Soons (1968, 1971), and Butterfield (1970) confirmed that frost and wLnd were active agents of erosion. Not ~tithstanding these studies, soil surveys and soil process studies, Cutler (1961) reported that although erosion in South Island high country had long been a controversial and frequently acrimonious subject of debate, there were no valid data on the stability of soils on steep slopes. Subsequent work by Molloy (1962) showed that instability had been a periodic feature of at least one New Zealand mountain range (Torlesse). Geologic mapping (for example Grindley et at. 1961) has provided much in- formation about New Zealand's complex geologic history. However the emphasis on mapping time - stratigraphic units has not been particularly useful in understanding rates and mechanisms of erosion. O'Loughlin's (1969) study of the geology and geomorphology of a mountain catchment was an important contribution towards such understanding. From his investi gations he concluded that a modern phase of accelerated erosion (associated with- the depletion of plant cover) had substantially modified stream bed- forms. He suggested that the morphology_of the stream bed and channel was controlled by infrequent catastrophic events, and called for longer term studies to verify his findings and provide more information on sources of bed load. Prior to 1970 there was information about sediment loads in lm'lland rivers (see for example Jones (1968)) but Johnson's (1970) preliminary study in the Craigieburn Range was the only contribution to at:! understanding of nnuntain stream sediments. In the decade before 1970 rapid advances were made in understanding the tharacter of mountain cl imate, and the temporal and spatial variabi lity of temperature and precipitation. (See for example Coulter (1964,1967) - 13 - Grant (1966, 1969a, 1969b) Greenland (1969) Hutchinson (1969) Mark (1965) Morris (19£5) Rowe (1968, 1970)). Investigations by Archer (1969, 1970) Chinn (1969) Chinn & Bcllcllny (1970) Gi 11 ies (196 11) Morris (1965) O'Loughl in (1969a~ 1969b) established that snow made a significant contribution to total precipitation above about 1,500 m. Toebes (1970) reviewed hydrolo'gical research relating to land management, and noted that because experimental basins (Anon (1965, 1969)),. begun in the mid 1960's, required three to four years for both calibration and evaluation, few results were available. Although results on the hydro logic impact of land use were available for Moutere (Scarf 1970a, 1970b) and 11akara (Vates (1964, 1965, 1966, 1971) Toebes et ala (1968)), there was rio comparable information for high country catchments. Some information was available about some hydrologic processes. For example ~tudies of interception had been made by Aldridge & Jackson (1968), Blake (1965) Fahey (1964) and Keller (1964). Work by Rowley (1970) led Mark & Rowley (1969) to conclude that a natural undisturbed cover of tall tussocks provided for the maximum yield and control of water from low alpine snow tussock grasslands. Infiltration studies by Nordbye & Campbell (1951) Gillingham (1964) Selby & Hosking (1971) confirmed earl ier findings from the United States that infiltration rates decreased with an increasing intensity of land use. However, results from Gill ingham & Selby were less clear-cut than those of Nordbye & Campbell. The fate of infiltrated water and the significance of soil water to a mountain catchment's hydrologic response were largely matters of speculat~on. Gradwell & Jackson (1970) had not~d the broad differences in hydrologic behaviour that could be expected as a consequence of.variations in soil pore sp~ce. Jackson (1966) and Grant (1966, 1969) had provided some information abo~t soil mass st~bil ity under saturated conditions ass6ciated with heavy rainfalls. - 14 - Those who read the pre 1970 literature relating to the conservation of soil and water resources in mountain lands must come to the conclusion that valuable as they were, the studies referred to here were largely incidental to our attitudes, pol icies and programmes in mountain lands. Holloway (1954) in his major work on forests and cl imate in the South Island "caut i oned /I hypotheses themselves are not facts~ though they may sometime be proven to be suhstantiaUy founded on fact. II By 1970 high country land management in New Zealand was based on a large number of hypotheses . .. CHAPTER 2. A SUMMARY ACCOUNT OF THIS STUDY. 2.1. Introduction. 2.2. The Torlesse Str~am Catchment. 2.3. A summary of work done 1972-1977. SUm1ARY This chapter describes the Torlesse stre~m catchment and outlines investi gations carried out between 1972 and 1977. This is an integrated study of the hydrologic and hydraulic processes Hhich determine the output of water and sediment from an eroded mountain catchment. It began with the design and construction of a sediment trap and the measurement of bed load and suspended load .sediments. It evolved to include studies of other processes which appeared to be significant to an unde~standing of catchment behaviour. The studies reported in volume I I may appear discrete but Here carried ,. out as interrelated components of a whole system, in which findings from one study I ed to the formul at i on of hypothese.S for the next. The concept of the Torlesse catchment as a system is an important charac teristic of this study. - 15 - 2. 1 • ,I NTp,ODUCTI ON In the late 1960's the National Development Conference focussed attention on New Zealand's future needs. It became clear that if the country's population doubled by the turn of the century there would be as much agricultural and industrial development in the follO\'Jtng 30 years as in the preceeding century. There would be greater competition for, and more intensive use of, the country's soil and water resources. The mountain lands were an acknowledged source of much of the country's water, but the suitability of some of this water for some purposes (such as hydro electric power generation, irrigation) was lirnited by the ~ediments it carried. The preamble to thIs study's successful 1970 application for ~ research grant from the Nuffield Foundation noted: '~e8pite the widely acclaimed~ but rarely studied severity of erosion in high country and the sediment laden condition of most streams derivedthere~ curiously little is known about the re- lationships between land erosion and stream sediment. Despite the publicity of erosion in the South Island high country~ re liable data are lacking on the stability of soils on steep slopes. The world literature on river sedimentation is almost without exception based on the behaviour of sand bed l'ivers. As such it has limited applicability for New Zealand's predominantly gl'aveZ bed streams. Traditionally it has been thought that a simple and direct rela-_ tionship existed between land erosion and stre.arn sediment. How ever~ the causes and sources of stream sediment may be considerably more complex than fomel'ly thought~ and reducing sediment yields may involve mOl'e or less than the rehabilitation of whole er'oded catchments. The practices of soil conservation and rivel' control have developed for many yem.'s without comllensupate resea}'ch in these fields. :J.'here is an urgent need for a nationally sponsored pl'ogramme of erOS1.:on research. As a contribution to such a pl'ogl'CV17Jne this study aims at undC1 1 standing the n.'lture of erosion and stream sediment in one catclwJent. II It is important to note that the aim of this study ,,,as to develop an under standing of the character of, and relations between, catchment hydrology and stream sediments in an eroded mountain catchment. To achieve this aim the study took four primary objectives: 1. To determine stream flow responses to precipitation. 2. To determine the character and origins of stream sediments. 3. To determine the character of relations which were presumed to exist between land erosion and stream sediments. 4. To infer downstream responses to changes in the conditfon of upper catchments. This aim and these objectives constrained the choice of study method. Earl ier experience with runoff plots (Hayward 1969) had shown that although they had been used widely in the United States they would be, in this context, entirely unsatisfactory. Because the aims and objectives required information about integrated catchment responses the basic unit of study had to be a catchment. However the experimental catchment method (Hewlett et aZ. ]969) was best suited to comparative studies of toe affects of land management. It was not intended that this study would involve the appl (cation of land management treatments. The most appropriate study method was judged to be an "observational " study (Boughton 1958),of a whole catchment. This method would be supplemented by rndividual process studies' if there was reason for bel ieving that the understanding of a process was important to an under standing of whole catchment behaviour. ... J7 ""' 2.2. THE TORLESSE STREAM CATCHMENT Several catchments were considered as possible study areas and judged against the following criteria: 1. The area had to be a moderately to severely eroded mountain catchment; 2. The area had to be close enough to Christchurch to allow gauging parties to reach the catchment in time to make observations during storms and to record sediment discharge during floods~ 3. Access had to be certain in times of flood; 4. The catchment had to be small enough to - Ca) allow the hydrol6gic effects of a variety of surface conditions to be studied and (b) produce manageable peak discharges; 5. Tfte catchment had to be large enough to allow studies of channel behavi.our; 6. The catchment outfall had to be stable and suitable for a gauging and sediment measuring station (i.e. the construction of a control section would not alter the bydraul ic behaviour of the channel}. A catchment on Brooksdale Statton of south~east aspect on the Torlesse Rang~ was chosen as the one most closely meeting all criteria, (see Figures 4 & ·~l. This. basin, named after Mt. Torlesse its highest point, drains into the Kowai river and thence intq the Waimakarirt system.. It has an area of 335 ha and is 80 km from Christchurch. Figure 6 is a plan of the basin and shows the location of features mentioned in this study. Access is gained via State Highway 73 to the foot of Porterts Pass and then by 4 km of track suitable for only four wheel drive vehicles. The basin is steep. Within about 4 km, altitude rises from ]60 m at the outlet to about 2/000 m at the highest po[nt. Figure] shows a mean alti.tude of about j /300 m. Table J shows that most land surfaces slope 0 0 between 26 and 35 , Figures 4, 5/ 6, 7, Tables 1,2 follow Figure 4; The Torlesse Stream Catchment. figure 5; Location of Torlesse Stream Catchment. ~ Torlesse Stream Catchment o 10 20 I I I km Figure 6; Plan of Torlesse Stream Catchment showing places mentioned in the text. ISo, 1400 1']00 '<0, r 00./ ._._0. ..>"-- --__..... ,0 T·- \ C n .,..._.-._.- 16 .._r... ' .....---" L' Gle 16860...... '\ )) '"j--_._. .JI44e- 01 ,.1 .- .-r'--' /";;~/\\ f -,,'Papa~ui f Ro d ._. .,.. ,,,_/ " .....,. ,,',f.... \, I, \ , a -! I r-"" ./ , ,\ \ 1700 ,. /" - ' ,- \' \ 'I \' f .I ( ' " )"' / / / ". / __\.J" '-:' r""-/ I I I t' I '; ., I ... \ • I " X- \ \ \, ,'" I I ~ /" \ I' I i I ", I .... ' '< \. \ I I ,'" I' I 7--." " " I ~ I 1800 ,. '\ I """ I ,I It ~ ,...... "". '. I · \ I ~ '. '~"_/;~~-~/ \ ,~.d I \ I (...... ' l (..1'····· \ \ ) \ ! \,If" ,-+.' I I " -.',". ~y----- / \ , \ \ I 1·~_1 \ ...... >-·--1 ' I . .. __ .. _ ...... _.:::.-....._ ... " .• / " -('".>-. .. I . I .. _.. ~···· .. ·· .... ·---.... T ...... V' \ --.. ·, \I\ I I, I --"./>C,' / ' )...... -_•. .• , I· I \ ,/' V 1/"', ,· \\ " \ \ " X / ~,,,,,,.' , . - -~nO';;' /,,' I ,'- J _ ' \ -- v ,- f [\ I /-< --'/ --.. ' ' \ ,',,<' ' 1900! •I Mil, \'I \" ",.. ", \ ,...... ,,-.,< '\ ,_ ....-,".,.. \' __l-_.- ---)I \\....\ ~-----' '\ ,\<:o",e ~'I' t /-0/ I 1963ta::'..j.0rlesSe', _,_ \' '--.~'.....--- / J-' ...... J.l' ,a,p.'J S"e> '-- <; _,<;;..,.."",ed./ ~\Vql::5 , ._ ' ... I < 0" /'_ . __,j."/' ,'l0" '"& "- ' I' l' 1-. # T . +, Lf -1 l, SLOPE AREA % OF CATCHMENT less than 25° 4] ha 12 % 26° ~ 35° 300 ha ]8 % Steeper than 35 ° 38 ha JO % 385 ha JOO % TABLE 1 Slope classes, Torlesse.Stream Catchment. Source: D.H. Saunders peT's comm. SOIL SET . AREA Ha % OF CATCHHENT Cass ]0 2.6 Tekoa 46 ]2 ,J Puketeraki 34 3.7 Ka i koura J63 42 .. J Alpine J48 38.5 River Bed 4 1.0 385 JOO,O TABLE 2 Soil sets, Torlesse Stream Catchment (provisional) Source: D.H. Saunders peT's COrml. The bas~ment rocks are sandstones, 5llt~stones, mud stones and cherts similar to those reported from other parts of the Torlesse supergroup (Andre\1s 09]4), Bradshaw (19]2), Blair CJ9]2}), The sandstones usually form massive beds which can be very thick or quite thin, while the silt stones are interbedded with each other, coarse and fine varieties alternating. There is a thick chert unit of various colours in the north west of the area and two basaltic dykes have been found, one of which feeds a small doleriteintrusi.on into the Irishman Stream (Main ]9]5) . The catchment occupies part of a major fault zone and the base rocks have been intensely faulted. Adjacent blocks of rock frequently bear no relation to each other. Jespite a history of active faulting the stream channels do not appear to be closely related to fault movements (Main 19751. One important consequence of faulting, is the micro-fractures or jointing that are common features of the bed rocks. Exposed surfaces disintegrate rapidly into 1 em to 30 em particles. These particles form the scree deposits and rock debris slides of the basin. (Baldwin ]972). The geomorphic features of the basin are a product of its geology, and its tectonic and glacial hi~tories. Although the Kowai valley displays abundant evidence of late Pleistocene glaciation (Mardin ]976) glacial effects are not well developed in the Torlesse basin. (Main 1975) .. Saunders (pers comn.) has provided a provisional map of the distribution of soil sets of the basin. (Figure 8, Table 2). All soils are upland and High Country Yellow Brown Earths and have some general features common to this group. They are weakly weathered but strongly leached and contain poor suppl ies of nutrients essential for plant growth. Topsoils are marginally more fertile than subsoils. Textures are stlty to sandy loams and all profiles contain rock fragments. The soils drain freely but soil moisture rarely falls below wilting point (Leamy J9711- While soil prof tIes do not have illuvial pans, some show gradational texture differ ences down the profile which may in part be due to i.lluviation (Harvey Figure 8 fo 11 ows Figure 8: Soil sets Torlesse Stream Catchment. Source: O,H. Saunders peps comm. '0 }> :A ""0 (') D Cl C ~ Cl 7\ 7\ (J) 7\ ~ (t) 0 (J) c:9 (t) 0 ~ Cl Q C..., (t) po 0--__ (J) ..., I () Cl Cl (') ..., (J) 7\ Cl (t) (J) ~ (t) po (J) \ (t) I po (t) (J) :r.. ':.. D ~ G) (t) '-" "0 Cl (t) "0 0 c ~ D \. 0...... <6 Cl ..:.. '\.. (t) -" ell ~ (\) (J) () .... 0 0 ]9741. Other local ised discontinuities in the profile are found where topsoil has been overwhelmed and burled by material of upslope origin, These buried soils give support to Molloyts (1962) view that instabil ity has long been a feature of New Zealand steeplands. The modern phase of erosion has been described by Saunders (peX's comm.) usi'ng the methods outl ined in the Land Use Capability Survey Handbook (SC & RCC. 3969). Tables 3 and 4 and Figure 9 show that about 80% of the catchment is in a severely eroded and depleted condition. In pre-Polynesian times Nothofagus forests covered the catchment up to about 1,400 m with Chionocloa grasslands, rock and scree above this alti- tude. During the period of Polynesian occupation the forests were destroyed by fire and replaced by DX'acophyllum scrub_and .Chionocloa grass lands. (Molloy 1964). The burning and grazing of the early European period of occupation brought about further dramatic changes in the dis tribution of the main species resulting the pattern of vegetation shown in Figure 10. With the except[on of the rarnfall record from nearby Mt. Torlesse Station, there were no long term climatic records which could give a reliable indi- cati~n of the catchment~s climate. [t was thought that the climate would be comparable with the drier eastern Canterbury high country. Lnformation gathered during the study period is presented in Appendix 1 Vol. III. The catchment is part of a· block that was traditionally used for summer sheep grazing. In recent years half ~red ewes grazed the catchment from mid summer unti I early autumn. Appendix VIII (Vol. III) presents an account of the manner in which animals distributed themselves throughout the basin. Based on a·land cClpabi I tty assessment (Fi:gure Jj), and under . a soil and water conservation plan a~Jinistered by the North Canterbury Catchment Board, the.bastn, as part of a larger area was retired from sheep graz i I1g oetween ] 97J and J 9]5. The reasons for th i s ret t rement Fig u res 9, 10, Tab 1e s 3 I 4 follow Figure 9: Degree of depletion Torlesse Stream Catchment Source: D.H. Saunders peT's corron. OJ ;- 0 -0 0 .-.... c 0- :::J 0 I..... t- c.9 ([) '- ~0 ~0 ~ ~ ~ CO 0 [l) 0 0 0 0 ----Q N "-.;t -<> 0 ...... 00> 0 ,...... 'c!( N "-.;t -<> C 0 U) 0 '- W N M "-.;t II) -Q OJ) i Fi gure 10: Distribution of main plant communities Torlesse Stream Catchment. Source: D. H'. Saunders pers comm. ! DEGREE OF DEPLETION % OF CATCHMENT I (% OF BARE GROUND) AREA i I 1 ..,. 10% J] 4.5 J ] ~ 20% J] 4.3 21 ..,. 40% 32 8.,4 43 ~ 60% J4 3.] 60% + 300 ]] .,8 Bush 5 J,3 385 JOo.,O TABLE 3 ) Degree of depletion, Torlesse Stream Catchment. Source: D.H. Saunders pers comm. SOIL LOSS AREA Ha % OF CATCHMENT Up to 75% topso i 1 22 5.7 75% topsoi 1 ~. 25% subsoil 22 5.7 25% to 75% subsoil 30 ].8 More than 75% subsoil 306 ]9.5 Bush 5 J .3 385 ]00 .. 0 '---- TABLE 4 Soil loss and erosion, Torlesse Stream Catchment. Source: D.H. Saunders pers comm. 140 ISO 0 1'...... '0' .... - 16 ...... ' "._._._' ,. 00 ,.,/ ._.-. .>'-- ...... 1686 .'"\ -/,' \ \ 5.""" /" ' I ...r...:----"-./" \ I (_/ I 1 I j 900 1700 .i I / ,/J 1// • (I I SOO I / ( \ \ I . 1 ...... I wef /k I \. I 1800 ; '\ I ,...... I I· \ \ I r/" ...... I· \\ I \ II ...... + ..... I I I· \\ \ I ·...... YI ...... · .. ·.... ·.. ·t'·_I .. ·.. · , . I I \ I ...... I I \ I I I I ,. \ \ I \ \ \ \ "- d~ ·, \ \ \ '\.( CO I /-L[\ I \ '-, ... /.,/. , 1900 .I \ \ \' \. , ...... ":-"'" "'.' I 1963 ...... I (\ ' \.. ..-- ...... < \ -... " - I ) ,,/,"7'-'-)--'-' I Fescue Tussock with (t.~.-." _>._ " \ /'..... ),...... (J,cS' ...... )..( '-.- o Raolia subsericea & Noto ~cS' ,.,.C) <::P setif( ,,0 /" ._..-+. .J' I "'<, '!" 00' ,,'V ~ Manuka ~ 00 j If' {I ,C?3 0 ,t5 ~(j Oracophyllum Chionochloa macra & C. flavescens, some Podocarpus nivalis Oraco- phyllUi Mountain Beech -+ -+- included; "To decrease soil erosion on the property where it occurs .. and to encourage where possible .. a denser vegetati.ve cover on the ground surface. This will in turn assist a reduction in the rate of surface runoff and the quantity of silt and debris travelling down the gullies and stl'eams. (I (D.icket al. .. pel'S carom.) The area behind the retirement fence fs still used for I imited winter cattle grazing. Figure 6 shows the location of the base hut ~ vortex tube sediment trap and meterological equipment. The hut rs well appointed for wet weather work, and a portable generator provides electric I ight for night work at the sediment trap. figure 11 follows CP ~ 0 .... 0 0 0 0 ~ ~ ~ ~ "...... ~ "- \ Q) \ M M Q) Q) ------Q N N N Do> Q) Q) Q) ..c CI) ::J Q) Q) Q) CO > > > :,' .:. : " I I ... o ~ :,' . "" .: . . :. . ,,'. -Q ': ... D~ . . '/ : ':". .: " .::; ,: . ,,: . .. .:: . \ I \-0 R Figure ll: Land capability assessment Torlesse Stream Catchment. Source: D.H. Saunders pers carom. 2.3. A SUMMARY OF fiNDINGS AND WORK DONE BETWEEN J972 AND 1977. The Torlesse stream catchment was chosen as the study area in the spring of ]970. The following summer a channel survey was made of the Torlesse. The problems of selecting representative sites through which to establ ish cross sections, drew attention to the pool-riffle morphology of the channel. Subsequent investigations, (reported in Chapter 9 Vol. II) showed that: the Torlesse stream channels are generally well ordered sequences of pools and riffles. two patterns are discernable. A "major '. pattern of riffle steps (or boulder steps) separates regions of lower velocity stream flow. A 'minor' pattern of boulder steps is found within the major riffle steps. These boulder steps are separated by pools. relations between step height and length are found to be consistent and vary with grade. it is thought that the major pattern is determined by low frequency events with return periods in the order of 50 ~ lOa years. it is thought that the minor pattern is determined by more frequent events in the order of 'l - 5 years. although channels are well ordered, they include some disordered segments. It is thought that these are in the process of adjustment toward a more ordered state. it is thought that the concept of dynamic equilibrium may be more appropriate to the Torlesse stream channel than the appearance of the channel might at first indicate. During the first recorded floods in 1972 it appeared that the stabil ity of some riparian lands was strongly influenced by flows in the stream channels. Subsequent field and laboratory investigations showed that: the pool riffle morphology is a most significant mechanism for dissi pating stream energy. when pools are submerged by high flow rates or inundated by sediments, stream energy is increased sharply. although streams such as the Torlesse have been loosely described as mountain torrents, this description is accurate only when a channel's pool riffle morphology is subme.rged. The sediment trap and base hut were built during the summer of 1971/72 but it was not untfl August .1972 that the sediment trap became fully oper- ... 22 - attonal. from the beginning sediment yields and transport rates showed great yariabi ltty both within and between storms. During the study period information was derived from 8J storm events to show that; al though the 'rorlesse stream catchment is commonly described as 'severely eroded' annual sediment yields during the study period are among the lowest values reported for New Zealand and overseas mountain lands. sediment yield is determined more by the supply of available sediment I than by the transporting capacity of stream flow. sediments move down channel as I'waves't. a majority of a longer term sediment yield will be derived from a few low frequency events. suspended sediments contribute less than lO% to total annual sediment yield. estirnates of bed load using conventional estimating equations consis tently over-estimated yields by up to several orders of magnitude. Bagnold's (1966) concept of stream power and the proportion of stream power utilised in bed load transport was found to show close agreement with measured values. The real isation that sediment yield was controlled by sediment supply rather than the stream's transport capacity focussed attention on the source areas of sediment. An attempt to identify source areas of bed material usin~ X-ray spectra failed because of high levels of within sample variabil ity (Appendix 9 Vol. Ill). Field obs~rvations showed that: a majority of bed sediments were derived from the disintegration of limited areas of intensely faulted exposed rock surfaces in the Irishman and Rainbow gullies. 1!:he visually dominant coarse block scree fields of the upper catchment were comparatively stable. Geological studies by R.Q. Oaks (to be reported elsewhere) suggested that - the stability of a colluyial rock deposit was largely dependent on the proportion of fine material that they contained. As the failure of such deposi"ts in a riparian zone could introduce Ilslugs" of sediment into the river, it was thought that this might be one explanation for the recorded 'varfabil tty' of bed load transport rates and yields. field surveys confirm a wave like movement of sediments and show that pools within the stream channel also periodically store and release gravel. (Chapter 8 Vol. II). suspended sediments were also observed to move as 'waves '. or 'clouds' but less frequently than bed load. Observations made during a storm in J972 led to studies which have altered our understanding of the hydrologic responses of this catchment to rainfall. In that storm 135 mm of rain fell in 36 hours .. However water which was poured by hand onto an eroded subsoi 1 was observed to soak directly into it. This expefience confirmed the experience of many hydrologists that overland flow was a rare phenomenon in mountain soils such as these. An infiltration study (Chaper 3 Vol. rl) snowed that; infiltration rates varied with the degree of surface cover depletion but that the lowest recorded values were in excess of most rainfall intensities. This finding raised the question , that if overland flow did not exist , by what mechanism was the 'overland flow'· component of the hydrograph generated? The partial contributing area study (Chapter 5 Vol. II) is an attempt to apply that concept to the Torlesse stream catchment. That study over-simplifies the conversion of rainfall to runoff but indicates that for this catchment at least; stream channels and adjacent riparian lands are more important source a·reas of flood flow than has been generally recognised. In the course of several storms water was observed to flow from some eroded areas. It was reasoned that a subsurface disc?ntinuity such as bedrock was forcing ground water to the surface and transforming it into surface flow. A siesmic refraction study (Chapter 4 Vol. II) confirmed that: some erosion features are associated with bedrock which is close to the catchments surface. return flow (Dunne & Black ~970) is one process by Ylhich rainfall is converted to runoff. From rainfall and stream flow records obtained over the study period it has been possible to esttmate the proporti6n of catchment precipitation which is Yielded as stream flow. rt has also been possible to consider the producti,on ,of summer low flows from the bastn. That study (Chapter 6 VoL Lil t n d j' cat e s that: between 80% and 90% of catclunent precipitation is returned as water yield. from published North American data, land lUanage:rnent influences on water yield have been recorded when .lO% ,.., 60 90 of precipitation is yielded as stream flow. preliminary results from three other South Island catchments suggest that land lUanagement for water yield may be lUore realistic in areas other than the Torlesse stream catchment. The studfes reported in Volume I I should not be viewed as isolated pieces of work. While the manner of presentation may suggest that each was a separate and self contarned unit, they are part of a whole system. They evolved in response to observations, questions and understandings about the rnter-relations~rps which determine the behaviour of the Torlesse stream catchment. CHAPTER 3 THE FLOODS OF APRIL 1978 - AN EXAMPLE TO ILLUSTRATE THE FINDINGS OF THIS STUDY - 25 - SUt1MARY The floods of April 1978 are not included in the study period but an account is presented here to illustrate some findings of this study. A low pressure system of tropical origin produced four flood events in 9 days. The response of the catchment to each event was quite different. Throughout most of the storm sequence sediment transport rates exceeded the capacity of the sediment trap. The estimates of sediment yield are based on extrapolations of former experience, with the occasional confirmation of a recorded transport rate. The first event was small and insignificant from the point of view of this study. The second was a major storm of 175 mm. In the 12 month flood-free antecedent period, sediments had accumulated in the channel and riprian lands and the catchment was in an loversupplied l condition. Sediment yield from this event was estimated at 600 tonnes. This may be compared with the total yield of 564 ionnes for the 5 year period 1972-77 to indicate the importance of low frequency events to long term sediment yields. At the time of the third event sediment supply conditions were assumed to be 'averagel and the yield was estimated at 200 tonnes. Thus within five days the Torlesse stream is estimated to have delivered nearly twice as much sediment as in the preceding five years. By the fourth event the supply of sediment was virtually exhausted. The response of the Torlesse and Kawai Rivers was to degrade their channels. In the Kawai River this also involved undercutting the toe of a scree which subtends the stream channel. This action caused rejuvenated eroslon--~ on the slope above the channel. As in other storms rainfall intensities were much lower than recorded infiltration rates and overland flow was observed only on limited areas of rock and saturated riparian lands. Overland flow did not occur on other sites regardless of vegetation type or degree of erosion. - 26 - Sediments were derived from limited areas of riparian land. The extensive scree fields which dominate the upper catchment remained stable and did not contribute sediment. The most Important source areas were limited areas of intensively faulted argill ite and greywacke, and volcanic intrusives. As these fine textured sediments flowed into the main stream channels they inundated the channel's pool riffle morphology, and the water surface slope steepened to approximate valley slope. This 3-5 fold increase in slope conferred additional stream power and allowed sediments to pass rapidly down channel in a series of 'waves'. At the end of the second storm evidence of remnant waves could be found throughout the Irishman and lower Torlesse stream channels. These remnants were remobilised in the third event and by the end of the fourth flood almost all evidence for their existence had been removed. As in other storm events less than 30% of incoming precipitation appeared as flood discharge (quick flow). The remainder was detained in the catchment to be released subsequently as low flow. The implications of these and other findings are discussed in Chapter 4. · ~ 27 - THE FLOODS OF APRIL 1978 The studies reported in Volume' I derive from data collected between 1972 and 1977. Although the floods of April 1978 are outside the study period they are referred to here as they illustrate some of this study's findings. The reconstruction of these storms is based on hydrograph and hyetograph records and field observations at the time. It is also supplemented by experience gained during the study period, and reported in Volume I I. Places mentioned in this chapter are shown in Figures 12 and 13. Antecedent Conditions The 12 months preceding Apri"l 1978 were notable for the lack of significant storm events. In this respect the antecedent period was similar to a 10 month flood free period in 1972-73 (Figure 39, Chapter 8, Volume I I). This periodicity of drought and storms has been suggested by Grant (1969) and reported by Tomlinson (1976). (Because climatic conditions fluctuate about long term means, there must always be an element of uncertainty about the long term significance of results from short term periods of study.) During the 'drought ' of 1977 there was continual disintegration of surficial material of the intensely faulted exposed volcanic intrusives in the Irishman stream, and the crushed sandstones and siltstone in Rainbow Gully (Figures 14, 15). In summer this disintegration was due mainly to diurnal temperature changes causing unequal expansion and cQntraction within the joint~ assisted ~he disintegration process. Because these rocks have been intensely faulted and crushed, they disintegrated into particles ranging from coarse sand to medium gravels. Elsewhere on the catchment, exposed - rock which was less jointed and fractured, disintegrated at a slower rate, and into larger particles. Figures 12, 13, 14 & 15 follow Figure 12: The Torlesse stream catchment Mt Torlesse ~ ------f \\1\1 I l(.i//: //,/ ;;;;; . ,i ~t '''0,~\~~, ~ \ ~\ "" !!il\' ! /"I ,/ A';;4C~~ . 7 Jtfl/-f ~ G:, \\i;, ~\'!\' I ,;' i} / ~;&J3:i:P~",,' I , !f/ I/" \, \\, 1\1 11- ~, ",.~ ~qr.:;/ , '. , "Y?'l/:?<.'\I\i~\\\I\IIIIIII!IN,I/ .', "', .'11,,, ,,', / '1((1 I,//' ,/ \\,0'~"'~ ~\III/,~ , \ Figure 14: Rainbow Gully Figure 15: The Irishman stream - 28 - By April 1978 up to 1 m depth of fine gravels had accumulated in the channel of Rainbow gully (Figure 16). Elsewhere, alluvial and collovial deposits were perched along the riparian zone (Figure 17). By April 1978 the Torlesse stream channel had been provided with a larger quantity of gravel than normal. This condition is described as loversupplyl (Figure 43, Chapter 8, Volume I I). During the Idroughtl, stream flows had been low. Stage heights at the control section varied betwe~n 0.05m - 0.08 m. These depths correspond to flow rates of between 0.06 and 0.12 m3sec-1 • The bed of the Torlesse stream had become armoured and showed no indications of movement during the few minor freshets of the preceding 12 months. This experience is in line with Kellerhall1s (1967) laboratory study which showed that channel pavements form only at low bed load transport rates. ~t~orological Conditions On the 6th April 1978 the Australian Meteorological Service reported that a tropical depression had formed in the Gulf of Carpenteria. This depression (named HAL) was drawing moist air down from the tropics. It filled and was lost as it passed over the Cape York peninsula, but an air floltl on its eastern side persisted to draw very moist equatorial maritime air southward into the north Tasman Sea. Between the ;{Sth and .10th Apri I, a shallow depression formed off the Queensland coast and moved in a S.E. direction. By the 13th April, an upper atmospheric depression formed over S.E. Australia. This began to supply colder polar air into the western side of the tropical depression. The colder air was mixed with the warmer moist equatorial air mass, and condensation occurred. It was the latent heat released by this condensation which allowed the depression to deepen rapidly in the next two days. Figures 16, 17 & 18 follow Figure 16: Rainbow gully. Prior to April 1978, ] m depth of sediment had accumulated at this site. Figure J7: Colluvial sediments stored in the riparian zone of the Torlesse stream channel. ~Igure 18: Synoptic conditions April 14-22, 1978. - 29 - On the 14th, New Zealand's weather was dominated by a westerly air flow and a front embedded in this flow passed over the country. This produced the 29 mm rainfall recorded at the study area. On the 16th, the now deep depression was centred on the North Tasman Sea and air was flowing into it across the isobars. That is, the moist air of tropical origin which was drawn southward on the depression's eastern side was now being drawn into the depression from the south east. The South Island's east coast still further deflected these winds to the south west. Thus although the surface winds were from the south west the air mass 0 originated in the tropics. There were relatively high temperatures of 13 - 14°C at Christchurch and 18°-200 e at Hokitika. Onthe 16th and 17th most parts of Canterbury experienced heavy rainfall and widespread flooding. In 42 hours from 1600 hours on the 16th, 175 mm of rain were recorded at the study area. On,the 19th, the depression was centred off Cook Straight and pressures were low off the Kaikoura coast. By 0900 hours on the 20th, the depression had reformed east of Kaikoura. The 85 mm of rainfall recorded on the 19th and 20th derived from the depression reforming off the Kaikoura coast. Although the air mass was still of tropical origin it was beginning to cool as the depression moved further south. Onthe 21st and 22n~ remnants of the depression which had been left in the Tasman sea linked with the now stationary reformed depression, and central New Zealand was covered by a convergent air flow. In this time a further 89 mm was recorded at the study area. This came in two periods, each associated with a remnant low pressure cell reforming with the main depression. There were therefore four phases of activity in this §ingle synoptic condition:- Phase 1 April 14 - a cold front crossed the study area and produced 29 mm rainfall. - 30 - Phase 2 April 16-18 - a warm moist airflow moved on to the Torlesse range from the south west-south east and produced 1 75 mm r a i n fall . Phase 3 Apr i 1 19-20 - the depression reformed off the Kaikoura coast and a cooler S.W. airstream flowed on to the Torlesse range. This produced a further 85 mm rainfall. Phase 4 April 21-22 - central New Zealand was covered by a zone of convergence as remnants of the depression moved eastward to join with the reformed depression. Rainfall was continuous but two heavier showers were recorded. The total rainfall for this phase was 89 mm. At the study area a total of 378 mm of rainfall was recorded in nine days. At Mount Torlesse Station the total rainfall was 230 mm. The concept of a return period has little significance to an event such as this. Each autumn, low pressure systems of tropical origin can be expected to produce significant rainfall. The unusual feature of this system was that it deepened rapidly about the 15th (Kingan, pers comm). The return period for a synoptic condition that yields a total storm rainfall of 230 mm at Mount Torlesse Station is about once in 40 years (Chapter 1, Volume I i). The return period for the second phase might be .~ : 2 years. Phases 3 and 4 would be annual events. ~ .... From the 16th there was a strong likel ihood that this synoptic condition would produce a significant flood. Experience during the study period had shown that although storms froman easterly quarter are less common than I those from the. south there is a greater probability that they will produce---,c a major flood (Chapter 1, Volume I I). - 31 - Catchment Responses Phase 1 14th ApSi 1 1978 From 1400 hours, April 14, to 0800 hours, April 15, a total of 29 mm of \ . rain falls on the study area. In response, stream flow rates increase from 0.06 m3sec- 1 to 0.08 m3sec- l • This is the largest 'event' recorded since December 1977 but has no significance to either sediment movement, channel or catchment stability. Phas_e_2___ 1_6_-_18_t,,-,-,°ho Aer i I 1978 Total rainfall: 175 mm Peak f low: 2.1 m3sec-1 (approx.) Estimated sediment yield: 600 tonnes Rain begins to fall at 1600 hours (April 16,1978) and will continue at·a steady rate of about 4 mm h- I for the next 14 hours. These rates are in keeping with the generally low rainfall intensities common to most of New Zealand and are substantially less than infiltration rates for all sites within the basin (Chapter 3, Volume I I). At about 1900 hours (the 16th) stage height at the control section begins to rise in response to 10 mm of rain falling directly on the stream channel. At 0400 hours on the 17th, 0.10 - 0.12 m depth of water flows through the control section, and bed load begins to move at rates in the order of 10 - 20 kg min-I. These transport rates are higher than normal due to a more than adequate supply of sediment. At 0600 hours stage height has increased rapidly to 0.15 m in response to 55 mm of rainfall. Rainfall Intensity increases to 7 mm h- 1 and w'lll ·be maintained at this rate for the next 10 hours. This will be the maximum rainfall intensity for this storm. As the lowest recorded infiltration rates in this catchment are in the orqer of 10-80 mm h- 1 all rainfall will - 32 - infiltrate into the catchment 'surface regardless of ground cover or condition. The only exceptions are limited areas of impervious bed rock, and the saturated channel zone. Rain which falls on the stream channel and the now saturated riparian zone may travel between 4,000 and 8,000 metres in the next 24 hours. In comparison, rain which infiltrates may travel only 1 to 10 metres in the same period. Because Infiltrated water moves so slowly, most of it will be delivered to the stream channel after this flood. At 0900 hours stage height is 0.20 m and a gravel wave passes through the control section. This is the first of six such waves which will move through the trap in the next 15 hours. These waves cause violent fluctuations in water level and make it difficult to establish 'true' water depths. It is probable that this first wave has come from the release of gravels stored in pools immediately upstream of the sediment trap. 1200 hours. 17th April 1978. Rising stage. Since this phase began, 100 mm of rain have fallen in 20 hours. IDill.trsLtJon~ra-te5-are_still in excess of rainfall intensity. There is no overland flow. At sites such as those depicted in Figure 20, ground water is emerging to the surface where bedrock or other subsurface discontinuities intersect with the surface. The significance of the conversion of ground water into surface water (return flow, Dunne & Black (1970)), depends on the nature of the land downstream from the point of emergence. Where this is stable (Figure 21), the water will move to the main stream channel without entraining sediment. However, if the downslope land is unstable (Figure 22), erosion occurs as soil particles are entrained by the surface flow. The mass of soil removed from these sites will not be great but it will give spectacular colour to the Torlesse stream. Figures 19, 20, 21 & 22 follow Figure 19: Hyetograph and hydrograph Torlesse stream catchment . 16-18th April 1978. o~u III 2·0 nIl II 'If III III IIII 111'111111111" 0; 4·0 B4' - 6·0 E E 8·0 o78 - 1(}O .E12·0c ·014·0 0::: o· 72 1&0- 18{) o· 36 2(}n. 0 1 ;0 o· )4 en ~o· ~ -en ~2 ':0..c. Q) I ~o· ~6 o· JO ~ ~ A~r~\~~~~ o 24 ! I '/ Actual trace ' .... , ..... :' II .... , i I ------Reconstructed trace ; I o 18 !, ~,'l'f'- iI /1 J/ /1 12 ~ o ...,... /,"' .-- o· )6 ...... ,. 0- n ~ P P P P P P 12·4·78 13'4'78 14·4·78 15·4·78 16·4·78 17-4·78 18·4·78 Figure 20: Ground water emerging to the surface of the Torlesse stream catchment. Figure 21: Stable land below a point of ground water emergence. Figure 22: Unstable surface conditions below a point of ground water emergence. - 33 - The response of the alluvial and colluvial debris deposits to this rainfall is variable. Water drains rapidly into and through the coarse block fields and scree depos i ts wh I ch domi nate the upper catchment. These show no indication of movement. The fine gravel deposits respond quite differently. Because transmission rates for water are lower than in coarse deposits, pore water pressures increase and the deposit becomes more fluid. The deposit in 'Slug gut' will move three metres down stream in the next 10 hours. Debris on its leading face will move in a series of amoeba-like advances (Figure 23). As sections of leading face collapse a small flow of water entrains debris from the new channel and from the body of the deposit and transports it rapidly down the Qversteepened face. The stream continues to cut through the unconsolidated debris until its oversteepened sides collapse inward and divert its flow back Into the body of the deposit. A quiesent period will follow until a new section of leading face attains a fluidity that will cause it to slump and repeat the process. Fine gravel deposits which have accumulated on mid slopes above steep channels will move rapidly as debris flows. The stability of such deposits appears to depend on their proportion of fine gravels and sands, and the rainfall amount and duration. Experience in earlier storms would suggest that these deposits remain stable until a threshold value is exceeded. Because we know little about these threshold conditions it is difficult to predict their behaviour in this storm. We do know however that following failure" their movement can be rapid. For example in a previous storm, debris moved 200 m from a point of accumulation, to Helen stream in 5 to 10 minutes (Figure 24). About 20 minutes later a 'cloud' of suspended and colloidal sediments moved through the sediment trap. The fine gravels which had accumulated during the antecedent period in Rainbow gully have become a semi-fluid mass and have moved as a gravel and water slurry Into the Irishman stream. The pools in the Irishman stream have been filled and additional Inputs of gravel submerge the riffles. In this storm the influx of gravel will cause the stream bed to be raised by up to 1.0 m above its mean bed level. The most Important consequence of this is that the slope of the water profile is Increased from about 0.03 through the Figures 23 & 24 follow Figure 23: Movement of the debris deposit in Slug gut. The right-hand photograph shows that sediment has been transported from the body of the deposit and deposited on the leading face. Note that the snow patch has been almost totally overwhelmed. The time interval between the two photographs was about 3 minutes. ... Figure 24: Face above Helen stream. - 34 - former pools, to the channel slope of between 0.10 - 0.J5. Because of this increase in slope, there Is an Increase in stream power and a consequent Increase in sediment transport •. (Chapter 9, Volume II). In addition, the now elevated stream has access to unprotected and unconsolidated side slopes. Within the next few hours several sections of stream bank 'tlill be eroded (Figure 25). Because such erosion reduces the sheer strength of the side slope the probability of additional side slope failure is Increased. 1300 hours. 17th April 1978. Rising stage. A second gravel wave passes through the sediment trap. Bed load transport rates are in the order of 200 - 400 kg min- 1 and the water depth is about 0.30 m. The chaotic nature of the water surface and the violent fluctuations in bed load transport rate make it difficult to estimate either with any pretence of accuracy. The capacity of the vortex tubes has now been exceeded. Estimates of sediment yield can only be made by extrapolation of former experience and records (Chapter 8, Volume I I) and compared with the few transport rates recorded at this time. A gravel wave Js mov.ing through the lower Irishman stream. The pools and riffles are totally inundated. Stream flow is shallow and fast with the slope of the water surface close to channel slope. The channel appears to be adopting a sinuous form. This is possible since it is now not confined to its former channel. The ~ading edge of this gravel wave arrives at the Forks control section. In the next hour the bed will rise 0.7 metres and force the stream to flow around the control section. 1400 hours. 17th April 1978. Risi.ng st.age/Peak. At the Forks, the Torlesse stream is now flowing bank to bank. It is wide, shallow and fast as it flows over gravels which have Inundated Its former Figure 25 follows Figure 25: Fai lure of unconsolidated stream banks following entrainmeht of toe deposits by stream flow. The lower photograph shows that sediments inundated the channel and raised the stream above its former level. In this manner, the stream gained access to its unconsolidated side slopes. - 35 - bed~ Wi thi,n the next hour the stream wi 11 begin to down cut through these gravels suggesting that the supply rate is being reduced and that the bulk of this wave has moved through this section of stream channel. It is probable that this wave represents the bulk of sediment which was stored in Rainbow gully. It is also probable that it began to move between 0900 and 1000 hours and has moved through to 0.7 km of stream channel in the preceding 4-5 hours. Although the catchment of the true right branch of the Torlesse stream is three times larger than ihe catchment of the adjacent Irishman stream, it contributes the same volume of water to the flood. This illustrates the important detention effect of the coarse scree deposits which dominate its catchment .. In addition the true right stream is clear carrying neither bed nor suspended load. This reflects the stability of the coarse screes. 1500 hours. 17th April 1978. Peak flow. Storm rainfall is now 121 mm. A further 54 mm will fall in the next 19 hours but intensities will be less than 3 mm hour-I. Although the evidence. (Chapter 5, Volume I I) for a partial contributing area (Hewlett 1961) suggests that this is an app~opriate concept for runoff generation in this basi~ it is not possible to define the extent of the contributing zone. Water can be observed making its way to the stream channel as saturated over land flow (Hewl ett 1961) and saturated through-flow (Hew1 ett and Hibbert 1967) in the riparian zone. Return flow (Dunne and Black 1970) is evident from many mid and lower altitude sites. In addition, water can be heard moving under scree slopes in what must be subsurface stream flow (Weyman 1973). In the lower channels, bed levels have been raised by up to 0.5 m. In miny reaches the pool riffle morphology is completely "inundated by the Influx of gravel from upstream. Other pools not inundated by sediment have been 'drowned ' by the increase In stage height. Low flow velocities through these pool sections are usually less than 0.5 m sec-I. Velocities - 36 - recorded 'at this time are in the order of 1.5 - 2.5 m sec-I. Froude numbers are within the range 0.8 - 1.4 suggesting that shooting flow,which is normally found only through riffles, now occurs through some pools. Under these ·conditions it Is appropriate to describe this channel as a mountain torrent. Below the sediment trap the Kowai River is in full flood and very turbid. Sediment brought down from the upper catchment is contributing to the rise in water level. Because of ~ggradation and increased stage height the river has been raised out of Its usual channel and now has access to the full width of its bed. It is migrating toward the Kowai screes which form its true left bank (Figure 26). Rainbow gully has been swept clean of all but the largest sediments. 1800 hours - 2400 hours. 17th April 1978. Recession flow. The gravel wave which moved through the Forks at 1400 hours moves through the sediment trap with peaks at 2000 hours and 2200 hours. When the wave arrives at the control section aggradation occurs upstream to the height of the true left wall. (This' was designed as a side spill way.) Water overflows, for although the control section can discharge stream flow, it cannot cope with the gravel wave (Figure 27). Daylight observations will subsequently show that the channel 150 m upstream of the control section was inundated by up to Ow5 m .gravel. 0000 - 0600 hours. 18th April 1978. Recess ion flow. By 0600 hours bed load transport rates through the sediment trap are reduced to 10 kg min-I. This material derives from the down cutting of the stream as It returns to its former bed level. Field observations during other storms have shown that pools continue to fill or receive sediment until the upstream supply is exhausted. When an upstream pool ceases to Figures 26 & 27 follow Figure 26: Looking upstream in the Kawai River below its confluence with the Torlesse stream. Figure 27: Upper The Torlesse stream in flood flow. Lower About 0.5 m depth of sediment accumulated above the control section causing water to spillover the true right hand wall. .' 37 - scour, the next pool downstream will scour until it reaches an equilibrium with the flow. In this manner bed movement ceases, beginning first in the upper catchment and extending downstream. 0600 hours - 1200 hours. 18th .t\pril 1978. Recession flow. Bed load transport rates are In the order of 5-10 kg min-1 • Unlike the pUlsing movement earlier in the storm, bed transport rates are steady as the stream reforms its bed. During the storm many riffles were destroyed when their boulders were dislodged. Localised movement is taking place throughout the stream as the pool riffle morphology reforms. This process will not be completed in this recession flow but will continue in future freshets. Remnants of gravel waves can be found throughout the channel systems of the Irishman and lower Torlesse streams (Figure 28). There is no evidence of disturbance or instability in the channel of the true right branch. Phase 3 19-20 Apri I 1978 Total storm rainfall: 85 mm Peak. di scharge: 2.0 m3sec- 1 (Approx.) Estimated sediment yield: 180 tonnes For the first time in the study period it is not possible to gain access to the Torlesse catchment. The Kowai River is In full ·flood and impossible to cross. This account is inferred from rainfall and stream flow records, from observations made across the Kowai River and from subsequent inspections of the upper catchment. 1200 hours. 19-20th April 1978. Rising stage. Rain begins to fall again 1400 hours (19th). The still saturated riparian zone responds quickly and stream discharge increases within an hour. Figure 28 follows Figure 28: Remnants of gravel waves, Torlesse stream channel. - 38 - Rainfall will be continuous for the next 12 hours but 5 more intensive periods of rainfall will produce 5 observable stream flow responses (Figure 29). 1200 hours. 20th April 1978. A small gravel wave passes through the control section. This is the first of eight such waves that will be recorded in the next 18 hours. 1900 hours. 20th April 1978. Peak discharge occurs about now but it Is difficult to separate the influence of rainfall from sediment on stage height. Subsequent observations will indicate that few if any new sediments are introduced to the stream, channel. Sediments delivered in this storm are derived from a reworking of channel deposits which are almost entirely remnants of gravel waves left by the Phase 2 flood. The Torlesse stream is clear, in marked contrast to the turbfd Kowai River. 1300 hours. 21st Apri I 1978. Recession flow. Sediment transport rates are measured at between 10 kg min- 1 and 20 kg min-I. This supports the opinion that there ,were.only average supplies of sediment available for transport during this storm (see Chapter 8, Volume 1 I). Although peak discharge was comparable with the Phase 2 flood, sediment yield is estimated at 30% of the yield from that storm. This is because of the limited amount of sediment available to be transported by this flood flow. Because the stream is clear, bed load transport can be easily observed. Sediments are not moving over the whole width of the control section but are confined to a 0.5m - 1.0 m mid section. The location of this ribbon of sediment varies in response to changes In upstream approach conditions. Figure 29 follows Figure 29: Hyetograph and hydrograph Torlesse stream catchment 19-20th April 1978. 0·9 cTT I' 1'1 Will 1\ 2·0 W 11111111-11111 WT 'I /,1 ~I 4()' I, -- -~ ------0·8 .~ 6'()' E S·(} E 0·7 :\-1--;; 10·0 .E 12·0 .ij 14'()' 0·7 ~a::16·()' 18·(} 0·6 I-- 20·0 ---~------~ 0·60 -VI 0·5 Ql L- a; 0·48 E Actual trace ------Reconstructed trace :!: 0·1.2 Ol 'Qj I 03 ) ~ J.I'~' I 0-3 ) I ~ , ~ 02 , ~ '..I ...... - ~ .. ~ ) I tt 01 J ~~ --- - 0·1 ) ~~ -- 0·6 J" 0·0 ) . P P P P P P P Rm 18·4·78 19·4·78 20·4-78 21-4·78 22·4·78 23-4·78 24·4·78 25·4·78 - 39 - 1800 hours. 21st April J978. Recession flow. An inspection of the upper catchment shows that most of the sediment left in the channel by the flood on 17/18 April has been removed. Although there have been many changes to individual channel features the general character and morphology is comparable with pre-flood conditions except that the supply of sediment available for transport has been almost totally exhausted. Pools have been scoured and the most accessible riparian deposits have been removed. The Torlesse stream is now in an undersupplied condition (Chapter 8, Volume I I). Phase 4 Total storm rainfall 89 mm Peak discharges (2 peaks): 1.5 m3sec-\ 1.9 m3sec Estimated sediment yield: 10 tonnes Throughout the April 21, light rain continued to fall at an average rate of about 1 mm h- 1 • (Figure 30). 2200 hours, April 21. 0500 hours, Apr!l 22 1978. Rainfall intensities increase to maximum rates of 9-10 mm h- 1 • Stream stage height shows an immediate response to a peak 0.26 m (1.5 m3sec- 1 ) at 0600 hours. Three small pulses of sediment move through the control section. (These are discernable from the stage height record as abrupt changes in stage height.) This sediment is derived from the release of gravels held in upstream pools. Sediment availability is very low and the yield for this first peak is estimated at 4 tonnes. This estimate may be compar~d with an estimated 100 tonnes which could be delivered by a flood discharge of this magnitude (see Chapter 8, Volume 1 I). Figure 30 follows Figure 30: Hyetograph and hydrograph Torlesse stream catchment 22nd-23rd April 1978. O-g J ~o 'II 11111111"11111 11" II I I I Ijlllllllll TIIIIIIIIII I "-- ~o. ... _.. __ .______. ______0-8 • &o. E 8'o. 0-7 ~f-- ~10-01.!----.------I~------ .E 12-0 -a 14-0 0-7 ~~~16-o.~------18-o. 0-6, )-1--- 20·0)~------·---·------..-.------.---. 0-61 -\I) 0-5 • ....Q) a; 0-48 E --- Actual trace ------Reconstructed trace :E 0-[,2 C'l -iii :r: 0-3 ~ /}'-'~ 0-3 , ~ r'( 02 J ')~, ) ~- -~~ 01 0-1 ------~ - - 0-6 0-0 1~~~~~~~~~~~~~~~~~~~-r~~~~~~~~~~_r~~Ir~~~~~~~~ P P P P am lLp P 1Zp 12 12pm 18-4-78 19-4-78 20-4-78 21-4-78 224-78 23-4-78 24'4-78 25-4-78 - 40 - 1200 hours - 21100 hours. 22nd April J978. Between 0500 hours and 1200 hours (22 April), rainfall rates ease to less than 1 mm h- 1 • B,eginnlng at 1100 hours rainfall intensity increases to an average 3 mm h- 1 for the next 13 hours. In response, stage height rises to a peak of 0.29 m (1.9 m3sec-1 ) and two small waves pass through the control section. The estimated sediment yield associated with this peak flow rate is 6 tonnes. When these two events are compared with the floods recorded between 1972 and 1977 they are major events. Although they transport little sediment, their recession flows aid in the reformation of the pO.ol riffle morphology of the Torlesse stream channel. The last remnants of sediment stored in pools has been scoured. A portion of this sediment is redeposited in the riffle zones of reforming pools and the remainder is transported through the control section. The Torlesse stream chann~l includes many large boulders and bed rock channel sections. Together these elements prevent channel degradation by flow which is undersupplied with sediment. The response of the Kowai River is, however, altogether different. In the first and second phases, sediments from the confined channels of the upper Kowai catchment were transported into the less confined channel below the confluence with the Torlesse stream. Channel sedimentation caused an increase in Kowai River stage height and gave the ri'ver access to the whole width of its bed. During the second phase the river migrated to the true left bank. In the third phase, sediment supplies to the Kowal were reduced and the river began to entrain debris from within its channel. This process of downcutting in response to a reduced sediment supply becomes most active In this fourth phase. Channels down cut 0.5 - 1.0 metres. - 41 - FigLJre 31 shows that a 50 m length of the Kowai River is flowing alongside the scree deposits which form its true left bank. As well as degrading its channel the river entrains about 200 m3 of sediment from the toe of Kowai screes. The most significant consequence of this removal is that a 20,000 m3 wedge of up slope gravels has now lost its toe support. Potential slope instability has been Increased because of the erosion of these riparian sediments. Slope failure above the undercut toe can already be observed. In the next 12 months or so there ",!lll be significant down slope movement of surface deposits. Re-activation will extend upslope, perhaps to the ridge 500 m above the channel. As the river erodes toe slope deposits and degrade~ it exposes a bed rock outcrop (Figure 32) which prevents further bank erosion at that site. It is interesting to note that there is a remnant tongue of vegetation upslope of this protective feature. The inescapable conclusion of the river's behaviour at this reach is that the depleted condition of the Kowai screes is in large part maintained by periodic undercutting of the slopes. Stability of both vegetation and soil is very dependent on protection against stream bank erosion of lower slopes. Further downstream at the Foggy stream confluence, degradation of the channel by an undersupplied river is illustrated by dramatic entrainment of Foggy fan sediments (Figures 33 and 34). By 1200 hours on the 24th, the river will have eroded 1000 m3 of sediment from this fan. Downstream at State Highway 73 there has been little evidence of variation in sediment supply. Throughout all phases of the storm the river is thought~ to have transported sediment at rates whIch would vary only in response to . changes in discharge. Thus while there have been obvLous and sometimes spectacular changes in channel morphology in response to flow rate and sediment supply in the upper catchment, these effects have not been observed in the lower catchment, at least in this storm. Figures 31, 32, 33 & 34 follow Figure 31: The Kowai River flowing alongside the Kowai scree slopes. Arrows mark the region from which 200 m3 of sediment were eroded by stream action. Figure 32: The rock outcrop exposed by toe slope erosion and dO\>.Jncutting of the Kowai River provides stability to upslope land (see also Figure 31). Figure 33: Degradation of the Foggy fan by an undersupplied Kowai River, 20th April 1978. Figure 34: Degradation of the Foggy fan by an undersupplied KowaiRiver, 20th-22nd April 1978. CHAPTER 4. IMPLICATIONS FOR FUTURE LAND MANAGEMENT, RESOURCE USE PLANNING AND RESEARCH. - 42 - SUMMARY Some of the findings of this study have important implications for the future management of the Torlesse stream catchment. The application of these findings to other areas of NewZealand high country is untested. Flood control, water yields, low flows and sediment yields are found to be unreal istic objectives of land management. If it was necessary, most sediments could be prevented from entering stream channels by the effec- tive treatment of less than 1% of the total catchment. Howeve r in consequence the channel would degrade its channel and riparian lands. This may give rise to more serious problems than were Ilsolvedll . Stream channels are found to be more important to land stabil ity than has been generally recognised. Strategic management of riparian lands may be the most cost effective method of inducing upper slope stabi lity. Findings from this study support the view that land management proposals should avoid slopes which might fail as a consequence of land use. Such failure may initiate a cycle of erosion which could be impossible to arrest. This study makes a major contribution to our understanding about the re- vegetation of eroded lands. It establ ishes that because the area of land in need of treatment is much less than is generally believed, (from the point of view of stream sediment suppl ies) the cost "per unit area is less important than was formerly thought. Two issues relevant to resource use planning in mountain lands are dis cussed and some directions for future research are outlined. - 43 - FUTURE LAND MANAGEMENT I n chapter] it was suggested that some of our approaches to the management of high a It it ude 1and have been cond i t i oned more by ~Jorth Ame r i can and European views than by New Zealand evidence. However results of this study call in to question some conventional attitudes and practices to· v'" " high country land management and the directions of some contemporary research and investigation. Land management for water yield. (Ref. Chapter 6 Vol II) The estimates of evapotranspiration and the proportions of precipitation returned as stream flow cast serious doubt on the val idity of improvement of water yield as an objective of management for this and comparable catchments. The results suggest that in very dry season~ 80% of preci~i- tation will be returned as str~am flow while in wet seasons the recovery is about 90%. Such values are in sharp contrast to the 10% to 60% recovery rates recorded in North America from where most of our attitudes to land management for water yield have been derived. It is difficult to conceive of any high country management practice which could,significantly alter yields in the order of 90% of precipitation and it is problematic if any management could be practiced with an assurance of altering yield in any chosen direction, For example afforestation of grasslands will lead to increased water consumption only if water is available. In view of the free-draining nature of the soils of the Torlesse catchment , increased consumption may occur only on damper riparian sites. But it can be postulated that, because a forest canopy can alter the energy balance of a site, the conversion of riparian grasslands to forests wi lJ lead to cooler surface temperatures and reduced evaporation. Evaporation from- the Torlesse stream and adjacent riparian lands may make a significant contribution to evapotranspiration, Therefore It can be speculated that a dramatic change tn plant type might produce higher rates of transpiratJon, but lower rates of evaporation, with, no net change in water yield. - 44 - Even if such speculation cannot be substantiated it must be remembered that water yield was found to be heavily dependent on precipitation. Therefore minor variations in yield due to vegetation management will be masked by the more dominant variations in precipitation. Land management for low rIm" regulation, (Ref. Chapter 2 Vo 1 II) A case, comparable to that proposed by advocates of land management for water yield, has sometimes been advanced for land management to enhance summer low flows. Results from the study sugg~sts this to be an unreal- listic objective for management. Low flows have long been recognised as a function of precipitation, catchment size, geology and geomorphology. Thrs study ~as found that summer months average 10 raindays per month and that the longest average period without rain is only eight days. The study area receives regular summer rainfall and also has the abiltty to store water and release it only slowly. For example, it was estimated that in the driest periods, low flows would halve in about ]0 days. The impact of land management on low flows will be greatest in catchments which have thin soils overlyi:ng bedrock of 10\tJ hydraulic conductivity. Although the soils of the Torlesse catchment are frequently shallow, the scree deposits are bel ieved to be deep and primarily responsible for sustaining low flow. Land management for flood control~ (Ref. Chapter 5 Vo 1 II) The impact of land management on floods was once a 1 ively topic of debate ---0 (see for example Leopold & Maddock 1954) but it is now generally recognised to· be g reates. t in s.ma 11 catchmen ts and in sma 11 5 to rms {see for examp Ie Burton ]969). In addition to these constraints, an understanding of the partial contributing area concept allows probable responses to be more clearly stated. ~ 45 - The concept postulates thClt a catchment can be divided into hydrologically active and passive zones, and that runoff is generated from a relatively small active zone. For flood generation, the active zone will be deter- mined by storm duration and by flow velocities through the soil. For short duration storms the actfve zone will be the stream channel. As storm duration increases, the active zone will be extended headwards into ephemeral channels and laterally into riparian land. The impact of land management on floods will therefore depend on the changes invoked in the active zone. Practices such as roading or urban development which increase a basints drainage density are liable to produce significant increases in flood flows. This opinion is in agreement with a large body of North American experi ence which has repeatedly found l:ncreases l:n flood flows \'/here forest clearfelling has involved track construction or skidder logging (for example see Harr et aL J975}. However, experiments in which timber has been felled but left in place have shown only very minor increases in flood flows (see for example Hewlett & Helvey ]970). On the other hand practices such as the rehabil itation of eroded land in the passive zone wIll have no effect on flood flows. Although the concept of a partial contributing zone is simple and may be , readily supported by field evidence,the task of defining the contributing zone is more difficult. The contributing zone is known to vary both within and between storms and seasons (Dunne et aZ. ]975). Before the concept can be useful for flood prediction, water qual ity management, land planning etc. it will be necessary to develop methods for recognising and predicting runoff producing zones. This study has not attempted that task. Although the concept of ~ partial contributing area has been developed with respect to flood flows there is no logical reason why it should not be appl ied to characteristics such as low flows, water quality or water yield. - 46 - Land management for sediment ~ontrol. (Ref. Chapter 8 Vo 1 I I) It has been implicitly assumed, and sometimes expl icitly stated (see for example HayvJard 196] p. J]O) that a reduction in stream sediment yields should be an objective of high country land management, However four findings call to question the val ldity of this objective in at least this catchment. First, sediment yields were found to be Jess than historic or long term rates and were amongst the lowest recorded values for mountain catchments. Second, major (low frequency) events were found to dominate long term sedi.ment yields. l1anagement practices, such as the rehabi litation of eroded land, will be less effective under these condit~ons than in smaller or more moderate storms. There is a popular view which assumes that spectacular and visually domi nant scree fields contribute sediment dfrectly to stream channels. However in the Torlesse catchment the scree fields were found to be coarse- textured and stable. Of this 385 ha basin, only 0.5 ha contributed bed load sediments to the stream over the 5 year study period. These sites were found to be finely shattered exposures of bed rock which would be difficult if not impossible to rehabilita~e. Nevertheless this study has shown that a majority of stream sediments could be denied access to the stream system if treatment of less than 0.1% of the total catchment could be effect i ve. Such treatment may not however reduce stream sediments. It was found that in the absence of adequate sediment loads the streams tend to down cut and derive sediments from their own beds. Where such down cutting produces oversteepened banks, there is an increased risk of riparian slope failure and a new supply of sedi,ment to the stream channel. Management for sediment control should be put into effect only when the - 47 - i.mpl ications of such management are understood. Sediment transport is a natural river function. Management schemes It/hi.ch alter this function may induce down stream instabi.l ity and create a more serious "problem" than they attempted to control. The management of mountain lands for the maintenance of veg~tation. Whereas the findings from this study question the val idity of some con ventional attitudes toward the rehabil itation of this mountain catchment they should not be interpreted as support for more exploitive practices. The reverse in fact is true. Findings from this study suggest that there are a complex series of inter- related down stream responses to a sudden influx of sediment. If practices such as logging or roading result in slope failure and an influx of sediment to a stream channel, the sediment will move down channel in a wave-like form. As it moves, it will submerge pools and riffles and make them less efficient dissipators of potential and kinetic energy. With an increase in velocity (and stream power) there will be an increase in the capacity to transport sediment. While most sediments will be derived from the sediment 'wave' some may be entrained from unstable banks subtending the stre~m. This leads directly to an increased risk of further slope failure. Although downstream responses to failure at one site are at present poorly understood, the implications for mountain land management are clear. Avoid slopes which may fai 1 as a consequence of land use for such fal lure may induce instability further down stream and initiate a cycle of erosion which may be impossible to arrest. Land management for erosion control. (Ref. Chapters 3, 4 and 8 Va 1 I I) Findings from this study sug~est a more discriminating approach to revege tation than has been generally advocated, The infiltration study showed that regardless of plant cover or degree of depletion, infiltration was - 48 - • poss i b,l e on all so i 1"vegetation assodations wi th i n the expectedra.nge of rai,nfall intensities. However the stl1dy of subsurface' discontinuities showed that the condition of vegetation below a point of ground water emergence had an important influence on down-slope stability. It is evident that the control .of soil eros ton by water needs a different emphasis. The revegetation of eroded soils at ~oints of water entry is relatively unimportant, as this will have no impact on catchment hydrology, or the rate of soil erosion by water. However the rehabilitation of depleted land below points of water emergence has significance to both on-site erosion and down stream se~iments •. In the Torlesse stream catch ment such sites represent less than J% of all depleted lands. The signi ftcance of this new emphasis is, therefore, that effective treatment of less than 1% of a catchment could yield results which might not be inferior to treatment of the whole catchment. Dunbar (1971) and Nordmeyer (J976) have developed methods and found suItable species for the rehabil itation of eroded mountain lands. Holloway (J970) reported that the main problem "'las one of reducing the cost of treatment. This study is a major contri- but ion to that objective for it establ [shes that cost per unit area treated is relatively unimportant if a majority of total benefits can be obtained from the effective treatment of very I imited areas. This study also establ ishes that rivers have a greater influence on slope stability than is generally recognised. Early advocates of ~oil conser vation drew attention to slope instabil ity following forest clearance, and recommended afforestation of steep slopes (~ee for example Campbell 1945) . However it has been shown that some slopes in the Torlesse and Kowai river catchments are maintained in a depleted condition by periodic river erosion of toe slopes (Chapter 3 Vol 1). Even if afforestation of upper slopes were feasible~ it is doubtful that, under contemporary channel conditions, it would be successful. The findings of this study establ ish that in this catchment at least, stable hi 11 slopes are associated with stable rtver channels. Land management programmes which give greatest pri,ority to the stabil ity of 1 imited areas of riparian land might be more cost effective than those which give priority to upper slope man~gement. ,/ - 49 - IMPLICATIONS FOR RESOURCE USE PLANNLNG. Relations between erosion, depletfon'and stream sediments. Land inventory and capability mapping have long been a basis on which to develop management plans for the conservation of soil and water resources (see for example Greenall & Hami:1ton J954). Although such surveys may be appropriate for some purposes, thts study found erosion, depletfon and capability mapping to be unreal fable indicators of source aieas of stream sedi.ments. The surveys by Saunders et al. (pers comm) followed conven- tional procedures and showed that nearly 80% of the catchment had lost all topsoil and 25% ... ]5% of tts subsoil. They also showed that nearly 80% of the catchment supported more than 60% bare ground. Based on this and other physical and biological features, 85% of the catchment was classified as class VII r land (land not suitable for crops, grazing or commercial forestry ... recommended for watershed protection, (Anon 1964)). This study found that from 1972 to ]97] most stream sediments were derived from less than 0.]% of the whole catchment. It therefore becomes clear that if stream sediment management is to be an objective of land management a more discriminating method of source area assessment is required. The findings of this study give support to the more recent approaches of, Cuff (1974, 1977) Brougham (1978) and others. Sediment yields for resource use planning. (Ref. Chapter 8 Vo I II) Management plans for river gravels need information about sediment yields in order that extraction rates not exceed supply rates. Proposals for dam construction need information about sediment yields as the rate of sediment accumulation can have an important influence on the economic life of a res~voi:r. This study has a number of impl ications for these and other resource use plans which involve estimates of stream sediment yields. [n the I ast few decades much tnformat j'on has been presented about sed iment - 50- transport in flumes and lowland rivers. However this study has demonstrated that because mountain streams have differences in stream energy and sedi ment supply, the appl ication of lowland findings to mountain systems is inappropriate. This study has shown that the visual appearance of a mountain catchment may be a poor indicator of I ikely sediment yields. The Torlesse stream has been described as severely eroded but measured rates were found to be amongst the lowest reported values. Conventional bed load estimating equations were found to over estimate sedi ment yields by several orders of magnitude. Bagnold~-s (J966) stream power approach to sediment estimation was found to produce reliable estimates for maximum sediment yields. However t~is method needs further testing before it can be recommended with conf i deneJ. Suspended sediments were found to contribute less than 10% to Torlesse stream annual sediment yields. This is a reversal of conditions which usually apply in lowland river~. Therefore, the technique which estimates bed load on the assumption that it represents up to 20% of suspended load is not val id. The most reliable method of estimating sediment yields is to resurvey mater ial trapped in reslvoirs and to compare annual specific yields with estimates from "similar" environments. If an investigation resorts to field sampl jng .. its information will only be reI iable if the investigation takes adequate account of the spatial and temporal variability of sediment transport. Field studies therefore need to be both comprehensive and systematic. There can be I ittle confidence in I'one-off" estimates. Not withstanding the contributions that this ~tudy has made to an under standing of mountain stream sediments, reI iable information for planning can only be derived from taking several dtfferent approaches to sediment yield estimation and comparing results. With the present siate of our knowledge, order of magnitude estimates are acceptable, - 51 - LMPLlCATlONS paR FUTURE RESEARCH AND lNYESTlGATlON, Research into the hydrologic impllcatlons of land use, This study has shown that it ts inappropriate to apply the Hortonian concept of runoff generatton by overland flow to this catchment. Instead, the concept of a parttal contributing area is presented as a more real istic alternative. This impl ies that future research should place less emphasis on understanding hydrologic relations on a catchment's surface in favour of those which attempt to understand the subsurface processes by which rain- fall is converted to runoff. Particular attention should be paid to water movement through unsaturated soils. Studies of non-Darcian flow, translatory flow & return flow warrant further attention. The partial area concept may have relevance for other catchment characteristics such as water quality or water yield and the manner in which these are affected by land management. Emphasis should therefore be given to studies which attempt to better define the extent, character and variabil ity of, the partial contributing area. , The question of where such work should be ~arried out should in itself be the topic of study, Critical analyses and careful interpretation of rainfall and stream flow information already held by many t~ew Zealand agencies would indicate where future studies of hydrologic processes might be most cost effective. Studies of existing data could also indicate the relative importance of individual hydrologic processes to particular management objectives. Erosion research Some aspects of erosion as it (s related to stream sediments warrant further study. Thts study has shown erosion to be an" episodic phenomenon. It has Deen suggested""that slopes and debris deposits may remain stable until a critical or threshold condition is exceeded, Future studies might well give emphasis to the concept of threshold condttions for instabi lity. [nstabll ity"involves I ithologtc, geomorphic, mechanical and hydraulic ~ 52 - parameters and processes. Those studies which are co-ordinated and multid[scipl inary wil I be most I ikely to make signific~nt advances in our understandings. Stream sediment research This study has demonstrated the importance of low frequency events to long term sediment yields. It has been shown that although the vortex tube trap performed effectively in smaller events it became less effectIve with increasing storm magnitude. Therefore studies of sediment yield should concentrate on the direct measurement of sediments deposited in resevoirs or on fans. Such surveys should be compl imented by studies of trapping effi~iency. The problem with [nst~1 lations such as the Torlesse stream sediment trap [s that they are confined to one river. Future studies might well give emphasis to bed and suspended load samplers. These devices have the advantage of be~ng able to provide information about relations between sol id and fluid flows in a range of rivers. By using information pre- sented in thi.:s study it should be possible to design sampling programmes which could adequately account for spatial and temporal variabil ity of sediment transport and there by provide reliable information. The responses of downstream channels and riparian lands to a sudden influx of sediment are poorly understood. Future studies might weI I give emphasis to a better understanding of the movement of sediment within stream channels and its implications to the stability of riparian lands. In this respect, the complex response model of Schumm and Parker (1973) should be critically assessed for its application 'to mountain channels. Hydraulic Research This study has suggested that changes in channel morphology can have a signtf[cant influence on stream energy ·Cstream power), sediment transport and hfll slopestabil ity. At present relations between channel morphology - 53 - and stream energy are poorly understood, The significance of channel morphology in channel management i.s therefore largeJy a matter of specu~ 1atlon. further studies should be made on the dissipation of energy in steep channels so that channel management can become a real istic objective fo~ land management. REFERENC ES REFERENCES Aldrt.dge~ R,; Jackson, R.J.; 1968, lnterception of rainfall by roanuka (LeptospermWl1 scopa~f.icwn) at Taita , I~ew Zealand.. N.Z. J'ourna l of Science .1.1: 30] - j .7 '. Andrews, P,B.; 1974. Deltaic Sediments,Upper Triassic Torlesse Super group, Broken River, North Canterbury. N.Z. Journal . Geology and Geophysics .17(4}; 881-905. Anon, 1961. Objecti.ves of the Tussock Grass lands and Mountain Lands lnstit.ute. Tussock Grasslands and Mountain Lands Institute Review. No, .1" p. 5 , Anon. 1964. Land capabfl ity classifications explained. 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