Welsh Davies Melton Ratio NZ

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Landslides (2011) 8:183194DOI 10.1007/s10346-010-0238-4Received: 4 January 2010Accepted: 20 July 2010Published online: 3 August 2010 Springer-Verlag 2010

Andrew Welsh I Tim Davies

Identification of alluvial fans susceptible to debris-flowhazards

Abstract We describe and test a method for identifying alluvial fans

likely to be affected by debris ows. It is based on identifying

catchment parameters by geographical information system inter-

rogation of a digital elevation model, using the Melton ratio as the

discriminating parameter. The method was calibrated using data from

debris-ow-generating catchments in Coromandel and the adjacent

Kaimai Ranges, North Island, NZ,and tested against data from the rest

of New Zealand. The procedure is remarkably (but not completely)

reliable for identifying debris-ow-capable catchments, and thus fans,

across the wide range of climates and lithologies in New Zealand

mountains. A case study illustrates the potential of the method for

avoiding future hazards and emphasises the need for a precautionary

approach when eld investigations do not detect evidence for past

debrisows.

Keywords Debris-ow hazards . Hazard area identication .

GIS . Catchment parameters . Geomorphology

Introduction

Alluvial fans are commonly used as sites for development in

mountainous regions because they are gently sloping, with good

views, well-drained and above the ood range of major rivers.

However, many small, steep fans in active mountains are occasionally

affected by debris ows (Jakob and Hungr 2005); these events are

characterised by violent surges of high sediment concentration, often

carrying large boulders and tree trunks, which can be very destructive

and affect any part of a fan surface. Evidently, any fan affected by such

events carries a high risk of severe damage to structures and thus tolife (Davies and McSaveney 2008). It is therefore essential, in

sustainable development of fan areas, that the possibility of debris

ows is investigated.

Such investigations are not simple; ideally, the magnitude-

frequency spectrum of debris-ow occurrence would be inferred

from geomorphic data and the risk judged accordingly (Davies and

McSaveney 2008). However, the rst step is to decide whether a

specic fan on which development is proposed is likely to have

suffered debrisows at all in its history; if it has not, no geomorphic

investigations are needed. We present herein a method for preliminary

assessment of likely debris-ow occurrence, based on existing

empirical knowledge of the characteristics of catchments known to

give rise to debris ows (e.g. Wilford et al. 2004). This uses ageographical information system (GIS) to identify catchments with the

required characteristics in a digital elevation model (DEM). Catch-

ments have much greater relief than fans and are thus able to be

characterised much more accurately from a relatively coarse DEM

than are the more gently sloping fans. The procedure we outline is

suitable for routine use in preliminary assessment of development

proposals.

Background

Herein, we summarise the major outcomes of Welsh (2008). That

study developed a GIS methodology for identifying catchments

with specic morphological parameters relevant to debris-ow

occurrence. Fieldwork tested the ability of some published

discriminators to successfully identify catchments that had been

affected by debris ows.

Alluvial fan hazards

The risks attending development on alluvial fans are described by

Davies and McSaveney (2008). In particular, debris ows pose a

frequently overlooked hazard because of the following:

1. They are not well known outside the research community, and

their risks are often not considered in routine assessment of

development proposals.

2. They occur infrequently in any given drainage (a few times

per century or even millennium), so they may not feature in

local knowledge.

3. Their behaviour is so different from normal streamow that

their consequences are often difcult to believe.

Further, the geomorphic evidence of past debris ows may be

removed by subsequent streamow erosion and sedimentation,

so even expert eld inspection may reveal no sign that debris

ows have occurred.

Debris flows

The causes of debris

ows, and their behaviour once initiated, areknown in outline but poorly understood as yet (e.g. Klubertanz et al.

2009; Jakob and Hungr2005; Iverson1997). However, it is clear that

occurrence of a debris ow requires large volumes of sediment to be

available, either on slopes or in a stream channel, and steep slopes to

allow rainfall and/or streamow of sufcient intensity to mobilise

the sediment. Thus, we expect that debris ows will occur in

catchments that are (1) steep and therefore (2) generally rather small,

(3) where erosion is active and (4)where intense rainfall or snowmelt

can occur from time to time. The geomorphic signatures of debris

ows include lobate fan deposits (by contrast with the very even fan

surfaces generated by uvial processes), channel-side levees,

anomalously large boulders in channels and on the fan, and scarring

high on streamside and fan trees.A number of investigators have differentiated between debris

ows and debris oods (also referred to as hyperconcentrated

ows; e.g. Pierson2005), in terms of their behaviour and hazard

potential. Both differ from normalood ows in streams by having

very high concentrations (30% by weight; Davies 1988) of

suspended ne sediment, but only debris ows carry large boulders.

In the present work, our focus is on debris ow hazards.

Catchment characteristics

A number of studies have sought to identify catchment and fan

topographic discriminators for debris ows (e.g., Jackson et al.

Landslides 8 & (2011) 183

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1987; Kostaschuk et al. 1986; deScally et al. 2001; de Scally and Owens

2004; Wilford et al.2004; Rowbotham et al. 2005). In particular, a

handful of morphometric variables including basinarea (Kostaschuk

et al. 1986; de Scally and Owens 2004), Melton ratio (an index of

basin ruggedness; equal to basin relief divided by the square root of

basin areaMelton1965; Jackson et al. 1987; de Scally and Owens

2004; Wilford et al. 2004) and watershed length (the planimetric

straight-line distance from the fan apex to the most distant point on

the watershed boundaryWilford et al.2004) have been identied

as capable of identifying and differentiating debris-ow and non-

debris-ow basins and their respective fans.

Field studies

We have used catchments in the Coromandel and Kaimai

Ranges of North Island, New Zealand (Fig. 1), to test Melton

ratio and watershed length against geomorphic evidence for

debris-ow occurrence. This area has in the past been affected

by a number of damaging debris-ow events (e.g. Fig. 2 shows

Fig. 1 Location of the Coromandel/

Kaimai region study area; Matata

(lower right) is the location ofAwatariki and Waitepuru streams,

which are discussed later

Fig. 2 Devastation and destruction as a result of debris flows in the small town of

Te Aroha, New Zealand, in 1985 (Encyclopaedia of New Zealand2000)

Original Paper

Landslides 8 &(2011)184


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the result of an event in which 50 homes were damaged and

three people died; Montz 2007).

The study area lies in the northeast of North Island; its mountain

ranges rise to almost 1,000 m above sea level (asl) and cover a total

area close to 6,500 km2. The basement rock is late Jurassic greywacke,

overlain by Oligocene sediments and late Cenozoic sub-aerial

volcanics. Volcanism rst occurred about 20 million years ago near

the northern tip of the Coromandel Peninsula and spread southward

over the next 18 million years to form a chain of volcanoes, theremnants of which now comprise the Coromandel and Kaimai ranges.

A well-developed NNWand NNE to ENE fault block pattern is evident

in the Coromandel/Kaimai region (Christie et al. 2001; Figs.3 and4).

The Coromandel and Kaimai regions are characterised by a diverse

range of vegetation, from low-lying grassland and mixed indigenous

scrub to indigenous and exotic forest (Newsome1987).

Weather patterns in the Coromandel and Kaimai regions

(and New Zealand in general) are dominated by the easterly

movement of frontal systems from the Tasman Sea (Environ-

ment Waikato 2007; Jane and Green 1984; Maunder 1970).

Annual precipitation often exceeds 3,000 mm in the ranges,

with maxima up to 4,500 mm (Environment Waikato 2007).

Extreme rainfalls in the ranges often result from summer

tropical cyclones; winter storms bring heavy rainfall and

occasionally snow to the ranges. Mean annual temperatures

vary between 12C and 14C (515C in winter months and 1525C

in summer); the region receives between 1,800 and 2,000 h of

sunshine per annum (National Institute of Water and Atmospheric

Science2007).The main population centres include Thames (population 6,500),

Paeroa (3,700), Te Aroha (3,700), Waihi (3,700), Whitianga (2,500) and

Whangamata (2,500). Tourism, horticulture, farming, shing, forestry

and mining are the main commercial activities in the region; many

smaller settlements associated with these activities occupy locations

close to small streams.

Methodology

Arc View (V.9.2; ESRI 2006) was used to identify catchments

and calculate their parameters (Welsh 2008). The catchment

Fig. 3 DEM of the Coromandel Range

region showing the location of specific

stream sites surveyed in the fieldinvestigation



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extent was delineated by the watershed area contributing to

streamow at the location of the most upstream building in

the valley bottom located on the 1:50,000 map of New Zealand

and the corresponding 25-m DEM. Tools (Arc GIS geoprocess-

ing commands) within the spatial analyst toolset in Arc

Toolbox (the user interface used for accessing, organising and

managing geoprocessing tools, models and scripts; ESRI 2006)

were used to investigate the hydrologic characteristics of the

Coromandel/Kaimai study area. These tools were linked within

the Spatial Analyst Model Builder (the interface used to build

and edit geoprocessing models in ArcGIS; ESRI 2006) to

construct a systematic ow diagram or model, enabling the

delineation of watersheds in the study area and extraction of

morphometric parameters associated with them. Input data

were entered as a list into each tool to enable model iteration

and hence processing of large amounts of data automatically

and simultaneously.

The watershed model thus formulated was divided into

two parts: the rst was used to extract a study area subset

from an existing DEM of New Zealand and to delineate a

drainage network for the study region; the second was used to

delineate watersheds in the study area and extract the

morphometric parameters associated with them. Once devised

and run to produce outputs, the extracted morphometric

parameters were exported into a Microsoft Excel spreadsheet

where Melton ratios and watershed lengths for each of the

watersheds were calculated and prepared for subsequent

analysis. Further details of this procedure are presented in

Welsh (2008).

After some preliminary trials, the parameters Melton ratio (R=

Hb/Ab, whereHbis basin relief andAbbasin area, after Melton1965)

and watershed length (WL, based on the model proposed by Wilford

et al. 2004) were selected as combining ease of use with adequate

discrimination.

Field investigations involved identication and quantication of

a range of geomorphic and sedimentary discriminators (listed later) in

Fig. 4 DEM of the Kaimai Range

region showing the location of

specific stream sites surveyed in the

field investigation

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the stream channel. Aerial photograph interpretation involved

searching for the same discriminators on stereo pairs; this was

ultimately unsuccessful due to the inadequate resolution (1 m) of the

available images.

Hazard thresholds for Melton ratio and watershed length parameters

Melton ratio (R) values for each stream watershed were allocated into

three categories, corresponding to the thresholds for debris ow,

debris

ood and

uvial phenomena, de

ned by Jackson et al. (1987)and Wilford et al. (2004):

& R0.30The threshold below which conventional uvial

processes are generally dominant in a watershed

& 0.305)

& Narrow channel, small width-to-depth ratio

& Semi-circular to U-shaped channel

& Sinuous terraces formed byow margins, sloping away from the

channel

& Channel scoured to bedrock

& Lobate areas of even-age vegetation younger than the

surrounding growth

& Old bark scars high on trunks and branches of trees

& Coarse deposits beyond the channel on the fan

& Substantial depositional lobes in the channel and on the fan surface

& Levees of coarse material aligned along the stream on upper fan

& Boulders rolled against trees on the channel banks or lodged

high above stream channel

& Isolated boulders in the channel and on the fan surface

& Unstratied deposits with no structure or imbrication

& Angular to sub-angular clasts

& A-axes of clasts oriented randomly or parallel to ow

& Poorly sorted and matrix-supported deposits

Debris-oods:

& Channel has medium to large width-to-depth ratio

& Bars, sheets, fans and splays notable at local scale in the channel

& Bouldery deposits beyond the channel on the fan

& Moderate to poor sorting of deposits

& Clast-supported deposits

& Deposits have mixed clast orientations: A-axes of large clasts

perpendicular to ow, pebbles and small cobbles parallel to

ow direction

Table 1 Category combinations ofR and WL

Category Combination

A R0.30; WL>2.7 km

B R0.30; WL2.7 km

C 0.30


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& Weakly imbricated deposits and collapse packing in coarser

sediment fraction

& Poorly stratied deposits, comprised of loose mixtures of

coarse gravel and sand

& Clasts rounded to sub-angular

Fluvial oods:

& Meandering or braided channel pattern

& Large width-to-depth ratio

& Moderate to well-sorted clast-supported well-imbricated deposits

& Presence of sedimentary features (e.g. bars, rifes)

& Presence of stratication and layering, cross-bedding, ame

structures

& Clasts sub-rounded to rounded

& A-axes of larger clasts perpendicular to ow

& Deposits rare beyond the channel on the fan

& Bars, sheets, fans and splays notable at local scale in the channel

The evidence recorded for debris ow, debris ood and

uvial processes was quantied by the percentage of the total

noted featurescorresponding to each criterion for each stream in

each category A, C and D, and F (Figs. 5,6and7).

Linear regressions were carried out to test the relationships

between observed evidence for debris ows, debris oods and

uvial processes and the parameters R and watershed length.

The range in evidence observed for uvial, debris-ow and

debris-ood processes at stream locations in the study area

differs substantially between categories (Figs.8,9and10; Welsh

2008); we conclude that the categorisation chosen as the basis of

the GIS analysis has succeeded in matching the eld parameters

indicating the dominant processes.

Application to other known debris-flow streamsIn addition to the watershed data derived for stream locations in

the study area, Melton ratios and watershed lengths were also

derived for 18 other stream locations in New Zealand. These were

selected because they are known (e.g. McSaveney et al. 2005; de

Scally and Owens 2004) to have experienced debris ows in the

past (Table3).

All of these apart from the Awatariki stream and

Waitepuru stream (see Fig. 1) are in the Southern Alps of the

South Island, New Zealand (Fig. 11). R values for the stream

watersheds range from 0.17 at Awatarariki Stream in Matata,

Bay of Plenty, to a maximum of 1.31 at Candys Creek, Otira,

and WL values from 1.15 km at Candys Creek, Otira, to 4.4 km

at Bullock Creek, Fox Glacier. A scatter plot of R against WL

for these stream watershed locations is shown in Fig. 12. The

majority of stream watersheds (11/18; 61%) plot in category F.

The next highest frequencies are observed in categories C (2/18;

11%) and E (2/18; 11%). The R and WL values derived for

0

10

20

30

40

50

60

70

80

90

100

Otutu

ruSt

ream

Otoh

iStre

am

Waio

tahiS

tream

Wah

ineStre

am(2)

Gordo

nStre

am

Stan

leyStre

am

Category C & D stream watershed locations

Observedcriteria(%)

Debris-flow

Debris-flood

Fluvial

Fig. 6 Evidence observed for all processes at each stream location in categories C and D

0

10

20

30

40

50

60

70

80

90

100

Waiw

hang

oStre

am

TePuru

Stre

am

Taru

ruSt

ream

Waito

kiStr

eam

Putan

giSt

ream

Matut

uStre

am

Category A stream watershed locations

Observedcriteria(%)

Debris-flow

Debris-flood

Fluvial

Fig. 5 Evidence observed for all processes (debris flows, debris floods and fluvial

processes) at stream watershed locations exhibiting R values 2.7 km (category A)

Original Paper



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debris-ow watersheds outside the study area compare well

with the study area results (Fig. 12), despite a wide variation in

geological, lithological and meteorological settings. A few ofthe watersheds, however, display lower R values and higher WL

values than expected. In particular, Awatarariki (R =0.17) and

Waitepuru (R=0.25) stream watersheds in Matata (Bay of

Plenty, North Island) display abnormally low R values which

fall well below all thresholds identied in the literature and are

consistent with that dened for uvial watersheds (R


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3.18 km; Waterfall CreekWL 3.15 km; and Pipson Creek

WL 3.11 km) exhibit longer watershed lengths than 2.7 km. They

also, however, exhibit high R values (e.g. 0.52, 0.94 and 0.88,

respectively). In light of this, it is possible that a slightly higher

threshold of WL may exist for differentiating debris-ow from non-

debris-ow watersheds in the Southern Alps. In practice, this

complication can be avoided by focusing primarily on the Melton

ratio and ignoring the watershed length.

The methodology developed thus appears capable of providing

an initial estimate of the potential for debrisows to occur on any

fan whose watershed parameters can be analysed via a commonly

available DEM. With the exception of streams such as Awatariki and

Waitepuru, it appears that any stream with a Melton ratio greater

than about 0.5 should be considered as a potential source of debris

ows. The dashed line representing this criterion in Fig. 12

successfully identies all the South Island debris-ow streams, as

well as all the category F streams studied in Coromandel and Kaimai(Table2).

Case studyStony Creek, Westland, New Zealand

The alluvial fan of Stony Creek, near Franz Josef, Westland, New

Zealand, is presently undergoing extensive development as part of

the rapid growth of tourism in the region (Fig. 14). Prior to

development, it was recognised as a potential source of debris

ows based on the work of de Scally and Owens (2004) because of

its location in a region of high and intense rainfall, with fault-

weakened rock in much of the catchment (Grant 1998; Fig. 13).

Figure12 conrms this potential on the basis of the present work.

Unpublished eld investigations undertaken by professional

geologists and engineers, however, reported no evidence for

debris ows on the fan or in the streambed, and on that basis

development on a small part of the fan was consented. During

this development, excavations revealed that large boulders were

present in signicant quantities (Fig. 14).

Development has since proceeded, and utilisation of much of

the rest of the fan area as a tourist resort is now proposed (see

http://www.franzalpineresort.co.nz/opport.htm).

Table 3 R and WL derived for stream locations outside the study area

Stream watershed location R WL (km) Rock Climate

1 Pipson Ck, Makarora 0.89 3.11 S M

2 Waterfall Ck, Lake Hawea 0.94 3.15 S M

3 Bullock Ck, Fox Glacier 0.57 4.46 S W

4 Yellow Ck, Fox Glacier 1.18 1.53 S W

5 Stony Ck, Tatare 0.86 2.63 S W

6 Greyneys Ck, Arthurs Pass 1.07 1.80 G M

7 Halpin Ck, Arthurs Pass 0.52 3.18 G M

8 Unnamed Ck, Turiwhate 1.17 1.48 Gr M

9 Turiwate Ck, Turiwhate 1.06 1.58 Gr M

10 Grahams Ck, Turiwhate 0.88 2.33 Gr M

11 Carew Ck 1, Lake Brunner 1.02 1.77 Gr M

12 Candys Ck, Otira 1.31 1.15 G M13 Unnamed Ck 1, Boyle River 0.92 2.05 G M

14 Unnamed Ck II, Boyle River 1.08 1.97 G M

15 Bullock Ck, Mt Thomas 0.58 1.51 G D

16 Kowhai R, Peel Forest 0.69 2.32 G D

17 Awatarariki Stm, Bay of Plenty 0.17 3.68 V D

18 Waitepuru Stm, Bay of Plenty 0.25 2.38 V D

Rock type: S schist, G greywacke,Grgranite, V volcanic; annual rainfall: Mmoderate (4,000 mm),W wet (>5,000 mm), D dry (


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Discussion

The criterion R>0.5 appears to be reasonably successful in

identifying the catchments that are capable of generating debris

ows across wide varieties of lithology, climate and vegetation.

This is an unexpected result, given the fact that debris-ow

occurrence is known to depend on rainfall intensity and sediment

availability, and these can vary dramatically with lithology, uplift

rate and climate. In particular, there is evidence that a threshold

of rainfall intensity must be exceeded to initiate a debris ow (e.g.

Caine1980) and that this threshold varies widely with topography

and lithology (Baum and Godt 2010). One would expect a

successful predictor to involve all these factors and to be

correspondingly complex. We appear to have identied two

phenomena (the occurrence of debris ows and R >0.5) that both

depend in a similar way on these controlling factors and thus are

closely (but perhaps not causatively) correlated to each other.

Certainly, it is difcult to develop a convincing mechanical

argument that the occurrence of debris ows depends only on R.

The method reported herein failed to identify the two

Matata streams as likely sources of debris ows. This is a

signicant concern and needs to be borne in mind when using

the method. It appears that the Matata debris ows must be

different in some respect to those that occur in Coromandel/

Kaimai and in the Southern Alps, but the nature of the

difference remains unclear; in the sense of the preceding

paragraph, perhaps R and debris-ow occurrence at Matata are

differently related to the governing variables. We therefore

recommend that the present method be applied only in areas

in which the topography resembles that of Coromandel and the

South Alps, which are dominated by approximately angle-of-

repose slopes and sharp or rounded peaks and ridge crests,

rather than in quite different topographies such as the low-

elevation (300 m asl) at-topped coastal plateau into which

Awatariki and Waitepuru (Matata) streams are incised (Fig. 15).

Because the GIS technology required for this methodology

is widely available and simple to set up and so can be used by

Fig. 11 South Island locations and

allocatedR/WL categories for streamwatersheds outside the study area

known to have produced debris flows



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persons untrained in geomorphology, it has the potential to

overcome the two primary problems in identifying alluvial fans

with potential debris-ow hazardsthe fact that few people

are skilled in recognising the geomorphic signatures of debris

ows and the fact that evidence may not obviously be present

though the hazard is.

T he m et ho d d oe s n ot g iv e a ny i nd ic at io n o f t he

magnitude-frequency distribution of debris ows in identi-

ed catchments. That would require much more intensive

investigation and indeed may be neither feasible nor

worthwhile. This follows from the deep conundrum ofdebris-ow hazard mitigation; debris ows occur relatively

rarely in any given catchment, but they can occur during

any intense rainstorm, and when they do occur they are

potentially lethal. Debris-ow hazard mitigation is not,

fundamentally, a matter of risk management because the

level of acceptable risk to life from natural hazards is 105

106 per year (Finlay and Fell 1997; Gillon 2000) , a nd a

debris ow is always a risk to the life of any person in its

path. If a debris ow occurs, say, every 100 years or so on a

given fan, and affects say 10% of the fan area, then the risk

to life of a person anywhere on that fan is 103 per year of

occupancy; this is a least two orders of magnitude greater

than acceptable. Structural countermeasures for debris ows

are recognised as both expensive and unreliable (Davies

1997; Takahashi 1981), and avoidance is always both cheaper

and more reliable. It follows that if a fan is identied as a

debris-ow fan, development should not occur under

rational decision-making systems.

The development of Stoney Creek fan described in thecase study raises a number of issues:

& Lack of evidence for debris-ow activity did not in this case

mean that no activity had occurred; it later became apparent

that sub-surface evidence existed but was not found by the

investigation. This is always a possibility; the present method

may act as an incentive to look more closely if debris-ow

susceptibility is indicated.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

Melton's R

Watershedlength(km)

A

B

C

D

E

F

5

17

18

Fig. 12 Scatterplot of Melton ratio

and watershed lengths for stream

watersheds outside the study area

known to have experienced debris

flows.5 = Stony Creek, 17= AwatarikiStream, 18 = Waitepuru Stream.Dashed line is R =0.5

Fig. 13 Catchment and fan of Stony

Creek, Westland, New Zealand,

March 2009. The approximate fan

area now proposed to be developedisoutlined in yellow. Dashedlines indicate approximate locationof the Alpine fault surface trace.

View is to the south

Original Paper



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& Development pressures make it difcult for local authorities to

constrain, delay or decline consent applications on the basisthat hazards may be present if no evidence has been found on

the ground. Thus, by default, hazards are assumed to be absent

unless denite evidence for their presence can be found. Given

that lives may be at risk when the next debris ow occurs, this

policy is difcult to justify, especially in comparison with the

precautionary principle commonly used to avoid environ-

mental impacts from developments. Again, methods such as

the one described herein could be an incentive for more

searchingeld investigations.

Fans already developed are, of course, a common, different

and much more difcult matter. Relocation is always politically

difcult and often socially unacceptable too, so structural

measures may have to be utilised in such cases, although the

design criteria are exceedingly difcult to specify reliably (Davies

1997), to the extent that their likely effect is difcult to quantify.

With the advent of weather radar able to provide advance

warning of intense rain, warning-evacuation systems might be

feasible in some cases (e.g. Davies and Hall1992), though the time

taken to reliably evacuate a substantial number of people in bad

weather must not be underestimated and neither must the

problem of resistance to evacuation resulting from a series of

false alarms.

The method presented herein, then, appears to have some

potential value as a hazard identication tool. It does not mitigate

the hazard, but until a hazard is identi

ed it cannot be mitigated, soit is an enabling device. Its success in picking out debris-ow-prone

catchments in a wide range of lithologies and climate zones in New

Zealand mountains may indicate potential value elsewhere in the

world.

Summary and conclusions

We have developed and tested a simple GIS-based method for

identication of catchments likely to generate debris ows. Catch-

ment area is dened as the area contributing water ow to the

farthest upstream building on the valley oor. The method uses

Meltons R>0.5 as a discriminator; this was based on study of 18

catchments in the Coromandel and Kaimai Ranges, North Island,

New Zealand, for which the value of R was compared with

geomorphic and sedimentary indicators of the occurrence of past

debris ows, debris oods and water oods. These comparisons

showed that anR value greater than 0.5 indicated accurately those

catchments showing signs of debris-ow occurrence. We also tested

this criterion on two catchments at Matata in theNorth Island and 16

catchments in the Southern Alps, South Island of New Zealand,

known to generate debris ows and which had lithologies and

climates different from the Coromandel and Kaimai catchments;

only the two Matata catchments (Awatariki Stream and Waitepuru

Stream) hadR 0.5.

The reasons for the anomalously small R values for the Matata

catchments are unknown, but we note that both have morphologies

distinctly different from those of the rest of the catchments

investigated. The Matata catchments are incised into a low-lying

coastal plateau, while the others are set in mountain ranges

characterised by long slopes at about the angle of repose and sharp

or rounded ridge crests. We thus recommend that the methodology

presented should be applied only in the latter type of topography.

We illustrate the value of the technique using a case

example from New Zealand. A fan area was approved for

development on the basis that eld investigations yielded no

evidence for debris-ow occurrence, such as large boulders in

the fan. However, once development started, the site excavation

revealed numerous large boulders; these were not interpreted

as evidence of debris ows, and development is proceeding as

if the debris-ow hazard was not present. This catchment has

R =0.81, so it clearly meets the criterion for possible debris-

ow occurrence. The use of the present methodology at an

early stage would have indicated the need for more thorough

site investigation, which would have demonstrated the

existence of the hazard and allowed the development plans to

be modied to avoid or mitigate the hazard. In other words,

the hazard could have been signicantly reduced.

We conclude that:

1. The method presented is simple to use and appears reasonably

reliable in identifying mountain catchments liable to generate

Fig. 14 Large boulders (1-m diameter; arrowed) in spoil heaps created byexcavation during development of Stony Creek fan

Fig. 15 Matata, Bay of Plenty, New Zealand (Google Earth image post-2005),

looking NW. The distance between the two named streams is about 1.5 km. Note

the large number of slope failures in the stream catchments



12/12

debrisows from time to time, in a wide range of climates and

lithologies.

2. It did not identify as debris-ow-prone the two Matata

catchments that generated debris ows in 2005, so it may be

unreliable in terrain that differs in topography from that of

the Coromandel and Southern Alps.

Acknowledgements

We gratefully acknowledge support for this project from the

Earthquake Commission of NZ; in-kind assistance and enthusi-

astic cooperation from Environment Waikato, NZ; and the

geomorphic advice of Dr. M J McSaveney, GNS Science, Lower

Hutt, NZ. Clive Sabel and Tom Cochrane of the University of

Canterbury both provided invaluable technical advice.

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A. WelshGeoconsult Pty Ltd., Brisbane, Australia

T. Davies ())

Geological Sciences,

University of Canterbury,

Canterbury, New Zealand

e-mail: [email protected]

Original Paper

http://www.niwascience.co.nz/edu/resources/climatehttp://www.niwascience.co.nz/edu/resources/climatehttp://dx.doi.org/10.1007/s10346-009-0177-0http://www.ew.govt.nz/enviroinfo/profile/environment/climate.htmhttp://www.ew.govt.nz/enviroinfo/profile/environment/climate.htmhttp://www.teara.govt.nz/http://www.teara.govt.nz/http://www.ew.govt.nz/enviroinfo/profile/environment/climate.htmhttp://www.ew.govt.nz/enviroinfo/profile/environment/climate.htmhttp://dx.doi.org/10.1007/s10346-009-0177-0http://www.niwascience.co.nz/edu/resources/climatehttp://www.niwascience.co.nz/edu/resources/climate

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