Late Holocene Floodplain Processes and Post

University of Wollongong Thesis Collections University of Wollongong Thesis Collection

University of Wollongong Year 

Late holocene ﬂoodplain processes and post-European channel dynamics in a partly conﬁned valley of New South Wales Australia Timothy J. Cohen University of Wollongong

Cohen, Timothy J., Late holocene ﬂoodplain processes and post-European channel dynamics in a partly conﬁned valley of New South Wales Australia, Doctor of Philosophy thesis, School of Earth and Environmental Sciences, University of Wollongong, 2003. http://ro.uow.edu.au/theses/1931

This paper is posted at Research Online.

4. Climatic variability on the mid-north coast of NSW

CHAPTER 4 - CLIMATIC VARIABILITY ON THE MID-NORTH COAST OF NSW

4.1 INTRODUCTION

The large variability of both rainfall and river discharge is a characteristic of the Austtalian climate and hydrology (e.g. Finlayson and McMahon, 1988), with the Australian continent having some of the highest indices of both flood and rainfall variability (McMahon et al., 1992). The large variability has formed the basis for numerous authors suggesting that landscape response in Australia is driven by these climatic extremes. An extension of this argument is the inferred role of large magnitude low frequency events on channel morphology in south-eastern Australia (Erskine, 1986), with particular reference to inferred altemating flood and drought dominated regimes (Erskine and Warner, 1988, 1999) (Chapter 1). In order to examine the nature of post-European channel response in the Bellinger valley in Chapter 5 and 6 it is necessary to examine the evidence for climatic variability as specified in the third aim of the thesis:

To identify temporal trends in rainfall and discharge for the mid-north coast of NSW examining evidence for alternating climatic regimes.

In order to examine the temporal pattern of rainfall and discharge in the study area both regional and catchment scale analyses were performed to provide a broad assessment of regional climatic patterns, along with a more catchment specific investigation used in later chapters. The chapter will analyse the temporal and spatial variability of both rainfall and flood data, determine the statistical validity of the presence of any climatic regimes and attempt to determine the controls on the character of the regimes.

4.2 RAINFALL PATTERNS ON THE MID-NORTH COAST OF NSW: A REGIONAL ANALYSIS

Rainfall pattems on the mid-north coast are broadly influenced by landscape position in relation to the Great Dividing Range, with some areas being strongly controlled by orographic influences. This regional analysis draws on long-term rainfall records extending back to the mid 19"" century in the tablelands (the headwaters of many of the coastal catchments), the dissected ranges below the escarpment and on the coastal plain. The choice of gauges is aimed at examining the nature of rainfall pattems in the mid-late

170 4. Climatic variability on the mid-north coast of NSW

19'*' century as well as providing a comparison to the 20'" century data of die Bellinger catchment. Figure 4.1 shows the rainfall and discharge gauges used within tiie regional and catchment analysis, while Table 4.1 provides rainfall gauge details. Of tiie six rainfall gauges used within the study, only two are classed as high quality stations (Lavery et al., 1992,1997) suggesting a cautionary interpretation of some of the otiier stations. These high quality stations are denoted as such in Figure 4.1.

In order to assess temporal changes in rainfall pattems on the mid-north coast residual mass curves (following Kraus, 1956; and Pittock, 1975) have been used to analyse total monthly rainfall. The choice of total monthly rainfall over daily rainfall is one based on convenience, with daily and total monthly rainfall data sets yielding similar pattems. Residual mass curves (termed cusum curves from here on in) plot the cumulative departure from a given reference point, such as arithmettic mean (Reinfelds, subm.) and have been used previously to assess both rainfall (Kraus, 1956; Pittock, 1975, Riley, 1988) and streamflow (Smith, 1995; Reinfelds, subm.). This technique is often used to assess directional trends away from the given reference point in time series data and to detect break-points within such series (Kraus, 1956). Rising limbs of a cusum curve indicate a time intervals where values are consistentiy above average, falling limbs indicate time intervals where values are below average, and horizontal limbs are where values are close to average.

Previous studies have identified significant changes to annual rainfall pattems in south- eastem Australia in the mid 20"^ century (Kraus, 1955; 1958; Pittock, 1975; Cornish, 1977; Bell and Erskine, 1981; Erskine and Bell, 1982; Erskine and Warner, 1988). Kraus (1955) first identified an abrupt increase in rainfall in the mid 1940s, with Pittock (1975) identifying that the secular shift occurred in 1945 - 1946 with changes of 10 - 20% in mean annual rainfall. It was demonstrated that the shift in the rainfall pattern of the mid 20* century was related to the latitude of the subtropical ridge along the east coast of the

Table 4.1 Long-term rainfall gauges for the mid-north coast of NSW assessed in the regional and catchment analysis CATCHMENT STATION STATION LATrruDE LONcrruDE ELEVATION PERIOD OF NO. NAME A.H.D (m) RECORD Richmond 058063 Casino airport" 28'52"S 153'3"E 26 1858-present Clarence 059026 Upper Orara 30'19"S I52'59"E 155 1899-present Clarence 059013 Dorrigo P.O. 30'2r'S 152'42"E 731 1905-present Nambucca 059002 Bowraville P.O. 30'38"S 152'51"E 18 1890-present Nambucca 059018 Macksville 30'43"S 152'43"E 33 1888-present Hastings 060026 Port Macquarie" 31'27"S 152'55"E 7 1885-present Bellinger 059001 Bellingen P.O. 30'27"S 152'53"E 15 1899-present * All gauges have been analysed up until 31/12/1999. High quality rainfall gauges after Lavery et al.. (1992, 1997).

171 4. Climatic variability on the mid-north coast of NSW

TWEED QUEENSLAND

BRUNSWICK \

1 r-'-H RICHMOND 29° S-

NEW SOUTH CLARENCE WALES

A// Coffs Harbour Upper Orara o BELLINGER NAMBUCCA o

Bowraville/Macksville

^ 31° S— MACLEAY ^ HASTINGS o Port Macquarie

'III 11 Great Dividing Range Discharge gauge • Rainfall gauge © High quality rainfall gauge 200 km

Figure 4.1 Location of rainfall and flood discharge gauges used in the regional and catchment analysis, with high quality rainfall gauges denoted after Lavery et al., (1992,1997). Codes for discharge gauges are those used in the regional analysis presented in Figure 4.9.

172 4. Climatic variability on the mid-north coast of NSW continent (Pittock, 1975).

Figure 4.2 presents the cusum curves for the gauges used within die regional rainfall analysis. As can be seen from this figure, tiie pattems in the long-term deviation for tiie mid-north coast are remarkably consistent between gauges suggesting a broad regional scale control on the patterns exhibited. The longest gauges (Casino and Port Macquarie) display rising limbs between 1885/87 and 1895/1900 highlighting an above-average rainfall pattem. In all of the gauges, bar Macksville, it appears that this above-average pattem was followed by a brief period of average rainfall conditions between 1895 and 1900, before exhibiting a clear below-average pattem for the eariy part of the 20* century. All gauges in Figure 4.2 exhibit falling limbs between 1900 and 1946/47. As noted by Pittock (1975) the rainfall pattems exhibited in Figure 4.2 all show a mid-late 1940s breakpoint, with the turning point occurring over a 12 month period. While the 1900 - 1946/41 period was characterised by below-average rainfall there is a 10 - 15 year period of average conditions between the eariy 1920s and mid 1930s (shown cleariy at Bowraville, see Figure 4.2). The second half of the 20* century shows a progressive rising limb, interspersed with short fluctuations lasting 2-5 years, with peaks (i.e. points of change) in 1964, 1977 and 1990. Since 1990, rainfall gauges show either an average or slight below-average condition.

above below average above average average 4000 # JX.A, :^ average^: c I 2000 CA .jft. n

0 v\ '\.si s J- /^^^ o * . 1 CA Casino MAC , W^ i I -2000 PM Port Macquariei h'- BOW - ^ ^ Bowraville Qi MAC Macksville BOX^i f UPGR UPOR Upper Orara I -4000 - DOR "3 DOR Dorrigo S 3 -6000 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Figure 4.2 Residual mass curves (cusum) for total monthly rainfall at six gauges on the mid-north coast (see Figure 4.1 for gauge location). Casino and Port Macquarie are high quality gauges after Lavery et al., (1992,1997).

173 4. Climatic variability on the mid-north coast of NSW

What is apparent from the cumsum curves is that rainfall pattems on the mid-north coast of NSW in the second half of the 20* century have been characterised by a broader multi- decadal above-average trend but with quasi-periodic short-term fluctuations. The presence of these shorter-term fluctuations is likely to be a result of broader ocean-atmospheric patterns either in the Pacific and/or the Indian Ocean, known as ENSO (El Niiio Southem Oscillation). The cusum curves are useful in highlighting temporal pattems, however, the criteria for delineating the length of the 'climatic' period will obviously depend on the aims of the analysis, with some authors further breaking the time series into either sustained positive or sustained negative trends (Zhang and Casey, 1992; Nakken, 1999). The above-average rainfall pattem for the late 1800s shown in Figure 4.2 has also been identified by Srikanthan and Stewart (1991) and corresponds to the sustained positive trend in the Southern Oscillation Index (SOI) identified by Zhang and Casey (1992).

The secular shift from the below-average rainfall conditions in the 1940s has been shown to be a function of increased summer rainfall (Cornish, 1977; Nicholls and Lavery, 1992). More recentiy, Franklin (1999) in a thorough analysis of rainfall patterns on the NSW mid-north coast, broke the record into pre-and post-1946 and has demonstrated the nature of spatial and temporal changes in rainfall patterns. In Franklin's analysis it has been shown that the percentage change in mean annual rainfall between pre- and post-1946 was up to 32% in some parts of the mid-north coast, with rate of change being determined partly by landscape position. Figure 4.3 highlights that in general the greatest percentage change has occurred in regions with elevations that range from 200 - 600m, with the least occurring in the elevated tablelands.

35.0

.y.=.4£7QSjf..-.Q.O.klx'...t.O,059.Zx.+.12,6.86... Rl = 0.3559

600 800 1400 Elevation Figure 4.3 Percentage change in mean annual rainfall from 1900-1945 to 1946-1997 as a function of elevation (m) within the NSW mid-north coast region (after Franklin,1999).

174 4. Climatic variability on the mid-north coast of NSW

While the relationship exhibited within Figure 4.3 is only moderate (R^ = 0.36) it still suggests that both the tablelands and low-lying coastal plain on the mid-north coast have been least affected by the post-1945 change in rainfall. Franklin (1999) identified that tiie Great Dividing Range districts experienced a post-1946 change of 19%, the coastal districts a 14% change and the tablelands a change of only 9%.

Figure 4.4 highlights the nature of seasonal variability in regional rainfall, demonstrating that summer and autumn had the largest increases in rainfall post-1945 (26% and 14% increases respectively). What is apparent from this figure is that the magnitude of change exhibits considerable spatial variability throughout tiie mid-north coast region, with die western portions of the Clarence and Macleay displaying lower percentage change in mean summer rainfall. The patterns investigated by Franklin (1999) show that the western portions of the two largest catchments in terms of areal extent (Clarence and Macleay) exhibited the lowest percentage change between time periods. This is in contrast to smaller neighbouring catchments such as the Bellinger and Nambucca, which typically display larger percentage changes between the first and second half of the 20* century.

While, not conclusive, the data analysed by Franklin suggest that broad-scale regional topography directiy affects the spatial distribution of percentage changes in rainfall patterns. Indeed, the patterns presented in Figure 4.3 and 4.4 either suggest a skewed relationship due to under-representation of western rainfall stations or the presence of topographically controlled, orographically influenced rainfall patterns. If the latter is tme, catchment orientation and physiography are important controlling variables for current rainfall distribution and may have been a key determinant in shifts in 20* century rainfall pattems on the mid-north coast of NSW.

The role of the El Niiio Southern Oscillation (ENSO) and its influence on inter-annual rainfall variability is an atttibute of climatic analyses that has received much attention since Walker (1924) first identified pressure differences between the Indonesian-Australasian region and the South American-south-eastern Pacific region. The relationship between ENSO and rainfall in south-eastem Australia has shown to be related, but with significant lags (McBride and Nicholls, 1983), with the high annual variability and the magnitude of the inter-annual fluctuations being influenced by the Southem Oscillation (Nicholls, 1988; Zhang and Casey, 1992). Nicholls and Kariko (1992) have demonstrated that the mid 20* century increase in rainfall has been a function of a greater number of rain events, but a decline in their intensity. ENSO modulation has been shown to be strongest during the austtal Winter and Spring months where the Walker circulation weakens associated with

175 4. Climatic variability on the mid-north coast of NSW

So 20- c

• •••' ^--' RICHMOND

Summer Autumn Winter Spring

CLARENCE

r jf Coffs Harbour

BELLINGER NAMBUCCA

MACLEAY

HASTINGS

200 km

Figure 4.4 a) Distribution of percentage change in mean annual rainfall between 1900 -1945 and 1946 - 1997 for the mid-north coast of NSW; shaded area represents approximate tablelands region, b) Averaged mean seasonal rainfall from 1900-1945 to 1946-1997, and assocaited variability index for the mid-north coast of NSW (modified after Franklin, 1999).

176 4. Climatic variability on the mid-north coast of NSW

slackening of Pacific ttade winds. This reduction in tiie ti-ade winds results in less moisture availability in the western Pacific and sustained warming of the central and eastern Pacific producing El Nino (dry) conditions in Australia. The converse. La Nina is where there is strengthened Pacific ti-ade winds producing warmer sea-surface temperamres and wetter conditions in northern Australia, and cooling in the centtal and eastern Pacific. While El Niiio and La Nina have been shown to influence tiie extent of rainfall in south-eastern Austtalia, die relationship is also influenced by additional circulation pattems such as the Indian Ocean monsoon pattem (Tortence and Webster, 1999).

The Southern Oscillation Index (SOI) is one of many climatic indices used to correlate hydrological variables such as rainfall and discharge in Austi-alia and is the standardised anomaly of the mean sea-level pressure (MSLP) between Tahiti and Darwin. A negative SOI usually indicates El Niiio conditions while positive values indicates La Nina conditions. Figure 4.5 presents the filtered SOI after Allan et al., (1995) witii identified major El Nino and La Nina events. What can be seen from Figure 4.5 is that some of the major La Nina events are associated with above-average rainfall pattems in the cusum curves (e.g. 1950), while others 1917/1918 correspond within a broad period of below- average conditions. The 1950 La Niiia was the highest Winter/Spring recorded SOI and corresponds with the onset of large-scale flooding in south-eastern Austtalia.

20.0 -1

-20.0 - n I I 1 1 1 1 1 1 I I r 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

Figure 4.5 Filtered (18-88 month)S01 from 1876 to 1992. Solid dots and triangles indicate major La Nifia and El Niiio phases (redrawrn after Allan et al., 1995).

177 4. Climatic variability on the mid-north coast of NSW

This was followed by the occurrence of La Niiia conditions in 1955 - 1956 also producing the above-average rainfall seen within the cusum curves and a continued period of widespread flood activity on coastal rivers of NSW. Hence, while the role of SST in the Pacific and Indian Oceans has been shown to influence inter-annual variability and the number and intensity of rainfall events in south-eastern Australia (Nicholls, 1989; Nicholls and Kariko, 1992), additional longer-term processes would appear to influence the multi- decadal direction of rainfall and associated flood activity in coastal NSW.

While, the above authors have identified the role of ENSO on inter-annual variability more recent work has focussed on multi-decadal fluctuations that modulate the intensity of ENSO on a shorter time-scale. Allan et al., (1995) have shown that sea-surface temperatures (SSTs) were cooler at mid latitudes and warmer in the subtropical Indian Ocean in the periods 1900- 1920 and 1921-1941, compared witii the 1942 - 1962 and 1963 - 1983 epochs. The 1900 - 1921 and 1921 - 1941 saw an anomalous atmospheric cyclonic feature in the southern Indian Ocean, while in the latter 1963 - 1983 period a distinct anticyclonic feature was prominent in the same region in the Austral summer (Allanetal., 1995). The patterns identified in the cusum curves in Figure 4.2 correspond with a number of these trends, with the identification of below-average rainfall from 1900 - 1946/47 corresponding to the cool mid-latitude SSTs in contrast to warmer SSTs and above-average rainfall seen post-1945.

Torrence and Webster (1999) have also shown that 1875 - 1920 and 1960 - 1990 contained intervals of higher ENSO variance, while 1920 - 1960 was a time of lower ENSO variance. Their results highlight that the 2 - 7 year variance seen within the time series also contains a 12 - 20 year oscillation consisting of an equivalent 12 - 20 year modulation of ENSO and monsoon amplitude. The low ENSO variance seen within the 1920 - 1960 time interval corresponds broadly to the 20 and 34 year frequency mode identified by Kirkup etal., (2001). Using wavelet analysis Kirkup et al., (2001) have also shown that rainfall in south-eastern Australia has a 10 - 12, 20 and 31 year frequency mode. Port Macquarie (Figure 4.2) was shown by them to exhibit a strong 12 year cycle before 1900 and then very strong after 1945, with the intervening period being characterised by a 20 and 34 year mode. This change in dominating frequency modes appears to be a characteristic of decadal to multi-decadal climate regimes throughout the 20* century.

While intra-decadal rainfall variability has been linked to ENSO events, there is currentiy no robust physical explanation for the multi-decadal variance in ENSO magnitude or SST

178 4. Climatic variability on the mid-north coast of NSW anomalies. What can be ascertained from the rainfall data are the presence of above- average trends from the late 1890s through to 1900, a prolonged below-average time interval between 1900 and 1920, an average period between 1920 and the late 1930s, followed by a below-average trend up until 1946/47. Since 1946/47 the rainfall patterns have been characterised by quasi-periodic fluctuations within a broader above-average trend up until 1990.

Inter-annual rainfall variability on the mid-north coast is influenced by ENSO, superimposed on a multi-decadal scale oscillation influenced by ocean temperatures. The physical control for the latter is yet to be fully understood, but coupling between ocean and atmospheric processes at a number of spatial and temporal scales appears to conflrm the variability seen within the record. While, the presence of the subtropical ridge may influence onshore winds and subsequent rainfall pattems in south-eastem Austtalia, as suggested by Pittock (1975), the interdependence of SSTs, wind and ENSO makes the clear distinction of cyclic rainfall patterns difficult. In conclusion, the length of record on the mid-north coast simply precludes the possibility of identifying 'cyclic' multi-decadal rainfall periods beyond those of the 20* century. While the presence of multi-decadal fluctuations may have existed for the last few centuries, if not for much of the late Holocene, their identification prior to 1880 is not possible at this stage.

4.3 RAINFALL PATTERNS OF THE BELLINGER CATCHMENT

The Bellinger catchment is located in a sub-tropical climatic region where summer rainfall predominates. Fig 4.6a highlights the seasonal distribution of total monthly rainfall, demonsttating that the region receives the greatest amount of rain per month from January to March. Long term average rainfall at Bellingen is 1525 mm but ranges from 2220 mm in the western headwaters to 3300 mm in the north-eastern headwaters. While mean annual rainfall is ~ 1500 mm, inter-annual variability is large and is shown to have varied from - 3000 to 650 mm/yr (Figure 4.6b). The following section will aim to isolate pattems within this inter-annual variability.

Figure 4.6c presents the cusum curve for total monthly rainfall at Bellingen from 1899 until 1999. What can be seen from this figure is that the trends identified in the regional analysis hold true for the study catchment. The below-average trend for the first half of the 20* century is demarcated with a falling limb until 1920, where there is an average period of rainfall through to the late 1930s. The onset of this average rainfall period, also identified in the neighbouring Nambucca catchment (Lyall & Macoun, 1999), corresponds

179 4. Climatic variability on the mid-north coast of NSW

^ 250 S a) S s 200 c 2 150

I 100 s 1 50 c CS 0 4 Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Month

^ 3500 b) § 3000 I 2500 ^ 2000 g 1500 c c 1000 I 500 1 0 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Year ^ s CS Below average average , Below Above average - quasi-periodic ii average i

•= -5000 "3 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 S Year s U Figure 4.6 a) Monthly rainfall distribution at Bellingen Post Office (stn. 059001) 1899- 1999. b) Mean annual rainfall at Bellingen Post Office 1899-1999, with 11 year moving average, c) Residual mas curves (cusum) for total monthly rainfall at Bellingen Post Office. Note the presence of quasi-periodic fluctuationspost-1947 , within a broader above-average trend.

180 4. Climatic variability on the mid-north coast of NSW to the wettest year on record in 1921. The breakpoint in the 1940s is consistent witii regional trends, occurring in October 1947. The post-1947 rainfall pattem in Bellingen, like other regional rainfall gauges on the mid-north coast, is characterised by quasi- periodic fluctuations within a broader above-average trend that extends until 1990. The post-1947 time interval is a period that contains nine out of the ten largest total annual rainfalls in the entire record. The percentage change in mean annual rainfall at Bellingen P.O. between the first and second half of the 20* century is in accordance with regional changes presented by Franklin (1999) witii post-1947 increases of between 14 and 19%. Of these changes, Franklin (1999) identified that summer and autumn months experienced the greatest change of 23 and 16%, respectively, with winter and spring changing by 3 and 4%, respectively.

4.3.1 Rainfall magnitude-frequency patterns in the Bellinger catchment

In order to assess shifts in rainfall magnitude-frequency relationships in the Bellinger catchment in the 20* century an annual series analysis was undertaken for the gauge at Bellingen post office for the period pre-1947 and post-1947 (following Erskine and Bell, 1982). Three time intervals; 1948 - 1977, 1948 - 1990 and 1948 - 1999 were chosen to compare with the pre-1947 record. The first two periods reflect significant dividing peaks on the cusum curves post-1947, while the latter tests the first half of the 20* century rainfall against the second. A normalised percentage-change analysis is shown in Figure 4.7a-c for the three time intervals highlighting that the number of rain events > 200 mm/24hours was considerably larger post-1947 for all three periods assessed (157 - 175% increase). The post-1947 trends all display consistent patterns between the three time-intervals with a decrease of 44 - 46% in the 50 - 100 mm/24hr category, with a 3 - 12% increase in 100- 150 mm and 150 - 200 mm/24hr rainfall events. Hence, it appears that in the Bellinger catchment the second half of the 20* century had a greater number of large magnitude rainfall events than did the first half.

To further investigate the proposition of a shift in the rainfall magnitude-frequency distribution an annual series was undertaken on the 48 hour maximum for each calendar year for pre-1947 and for the three above time intervals under investigation. The 48 hour rainfall event was assessed as it encompasses time of concentration (t^) at the downstream end of the study area (t^-35 hours, after Pilgrim, 1987). Figure 4.8a-c presents the annual series based on a log-normal probability for the pre-and post-1947 time intervals with their 90% probability limits. This figure highlights that all pre- and post-1947 distributions appear different. Because, however, the 90% probability limits generously

181 4. Climatic variability on the mid-north coast of NSW

r 3.00 11900-1947 ii 2.50 > 11948-1977 ii • % change <^ 2.00 o d c 1.50 (D «3 73 1.00 o 0.50

0.00

c >ii a o d c •o ii Vi

3.00 11900-1947 2.50 11948-2000 > ii -A—% change o 2.00 d c 1.50 u M 73 1.00 B Ul O 0.50 Z 0.00 50-100 100-150 150-200 >200 i. rainfall (mm/24hr) Figure 4.7 Number of 24 hour rainfall events (normalised) and percentage change at Bellingen RO.; a) 1900 -1947 and 1948 - 1977; b) 1900 - 1947 and 1948 -1990; c) 1900 - 1947 and 1948 - 2000.

182 4. Climatic variability on the mid-north coast of NSW

yuu - ? y = 105.29Ln(x) + 136.29 800 - R2 = 0.9528 Xg 700 - ^—t ex ii 600 - .. •• . ., r:..-^"'"" 1948-1977 ^^^^'"1^^^ li d 500 -

c 400 ^ ^ ..- -<• ^^—"- S^'^''''^ . w • ^ ^ -^ 'rt • -^ ^ ^ ^.^•—• — " " ^ " cx: 300 - 200 - p^>r^^ 1900-1947 y = 96.278Ln(x) + 103.44 100 R^ = 0.9731 0 * ,_ „ , , 1 , , , 1 10 100

oUU - y = 93.602Ln(x) +142.26 ^^^ 700 ' B R^ = 0.9439 ^^' B^ 600 ^ X - ••• _ .- ' " „„..-- i a. 500 - 1948-1990 ^ 1 ' ' ^^^^^^^"^*

\\d 400 - ^ ..^^- •• V'--:-^^C-^^^^^^^^^^ -'---" cd t+^ »-"'«' ^" -.-r--"^0<-<'''''''—-''''''*'' ^— -" "^ ^ - - ' " 300 - . . ..-» -V • •.^•--<^ - - " Rai n 200 - .y-^^S*^^*^^' 1900^1947 y = 96.278Ln(x) + 103.44 100 ' ^'^'^^ R^ = 0.9731

U . . , . . 1 1 1 1 10 100

cSUU y = 100.05Ln(x) + 136.94 700 - R*^ = 0.9526 ?E 600 - •' ' -^ ^ ^•*-.-' -•• "" ^ "^ x: ^ ^ ^,~" • a. 500 ^ ^ ^T-"'^""'.--^-'''''''"'^ • a 1948-1999 ^^^-;^^^^^;ll>^^^^ ^_ ^^-a 400 15 300 - ^ , ^^C-J^^^^l- -- — '"" in f cd -*••" -- "^ *-5—""•'''^ — v" " oi 200 - .a:^^'^'^ 1900-1947 y = 96.278Ln(x) + 103.44 100 - S^'*'^' R^ = 0.9731 >

VJn - . . , .,,...._ -•! -r 1 10 100 Retum period (yrs) Figure 4.8 Annual series of (calendar) 2-day rainfall frequency curves of the periods before and after 1947 at Bellingen P.O. based on log-normal probability distribution and their 90% probability limits (dashed lines); a) 1900-1947 and 1948-1977; b) 1900-1947 and 1948 -1990; c) 1900-1947 and 1948-1999. All points represent gauged data with plotting position after Cunnane (1978).

183 4. Climatic variability on the mid-north coast of NSW overiap in all three examples there is no reasonable justification to suggest that pre- and post-1947 time intervals, for the annual maximum 48-hour rainfall, are independentiy distributed.

In summary, the rainfall data highlight that there has been a recognisable increase in post- 1947 mean annual rainfall totals as a function of increased summer precipitation (Franklin, 1999). However, while the number of large magnitude 48-hour rainfall events for the Bellinger catchment appears to have increased post-1947, their probability distributions suggest that they are not significantiy different.

4.4 FLOOD VARIABILITY ON THE MID-NORTH COAST OF NSW

This section draws together the flood discharge data for the mid-north coast region over the 20* century, while developing a more catchment specific dataset in which to utilise in later chapters. As shown in the previous sections, rainfall throughout the late 19* and throughout the 20* century was characterised by distinct above and below-average periods of rainfall. This section investigates to what extent these rainfall patterns are exhibited in the flood record. The term Bellinger basin is used in context of the designated hydrological basin (Department of Land and Water Conservation, Basin 206) in which both the unconnected Bellinger and Nambucca catchments fall. This is in contrast to the Bellinger catchment, which refers specifically to the Bellinger catchment area incorporating the two major arms, the Bellinger River and the Kalang River.

4.4.1 Temporal trends in flood discharge on the mid-north coast of NSW

There has been very little systematic analysis of regional flood records for the mid-north coast of NSW, with government departmental studies focussing on flood-risk assessment and academic research on catchment specific datasets (e.g. Bell and Erskine, 1981) or broader-scale hydrographic relationships (Pilgrim et al., 1987). It is only in the last two decades that both the inter-annual and inter-decadal variability in flood discharge in south- eastem Australia has received attention (Erskine and Wamer, 1988,1998; McMahon et al., 1992, Kirkup et al., 1998, 2001; Franks, 2002a, b; Kiem et al., 2003). Flood records for coastal NSW, like many parts of south-eastem Austtalia, are extremely limited with many gauges only providing a continuous record from the mid 20* century onwards. In some larger regional centres longer flood-records are available through historical stage-height measurements which can extend the records back to the late 19* and eariy 20* centuries. However, the post-European changes to channel geometry, in addition to altered catchment boundary conditions, makes the comparison of historical to current flood heights difficult.

184 4. Climatic variability on the mid-north coast of NSW

As such, many basins have poor quality records tiiat may only contain 20 - 40 years of continuous data.

In order to assess the long-term temporal trends in discharge of the mid-north coast eight gauges, both north and south of Bellinger catchment, were selected for analysis. Gauge selection was based on lengtii of record rather than attempting to derive a spatially consistent distribution of gauges. The longest record for the mid-north coast extends back to 1909 with seven of the eight gauges extending beyond 1950, thus providing a measure of discharge pattems for the inferred drought dominated regime (DDR) of the eariy 20* century (Table 4.2 provides gauge details, while Figure 4.1 presents gauge locations).

Cusum curves have been used to analyse daily discharge data for the gauges presented in Table 4.2, normalised as percentages (shown in Figure 4.9a). The regional cusum curves show a distinct similarity in form for all eight gauges, with the major trends being a below- average discharge for the first half of die record, a steep rising limb from the mid 1940s through to the late 1970s, followed by a consistent below-average discharge until present.

The patterns observed in Figure 4.9a are similar to those presented in preceding sections for long term rainfall trends in Figure 4.2, and reinforce the notion of a distinct secular change in rainfall and discharge in the mid 20* century (sensu Bell and Erskine, 1981; Erskine and Bell, 1982; Erskine and Warner, 1988, 1998). There is some departure from the rainfall cusum curves shown in Figure 4.2 where above-average rainfall extended through to 1990, in contrast to the breakpoint seen within the discharge cusum curve that shows the second breakpoint occurring in 1977. The breakpoints for the various time intervals display some intra-regional variability, with the first breakpoint occurring over a two year time period between 10/6/1945 and 2/5/1948, with the most southern gauge in tiie Macleay catchment registering the latest breakpoint. Interestingly all basins, despite the

Table 4.2 Long-term flood discharge gauges for the mid-north coast of NSW assessed in the regional analysis CATCHMENT STATION RIVER STATION AREA ELEVATION PERIOD OF NO. NAME NAME (km^) A.H.D (m) RECORD Richmond 203002 Coopers Ck Repentence 62 45 1920-present Clarence 204002 Clarence R. Tabulam 4550 115 1909-present Clarence 204007 Clarence R. Lily dale 16690 5 1922-present Clarence 204008 Guy Fawkes Ebor 31 1285 1923-present Clarence 204004 Mann River Jackadagery 7800 100 1919-present Clarence 204001 Nymboida Nymboida 1660 175 1908-present Clarence 204025 Orara River Karangi 135 105 1950-present Macleay 206001 Macleay Jeolga 163 810 1918-present * Ail gauges hav e been analysed up until 31/12/01

185 4. Climatic variability on the mid-north coast of NSW

DDR ADR DDR. FDR ADR FDR rs- 400000 DDR ae 300000 ii s 200000 s 100000 Is o 0

-a -100000 -200000 es -300000 5-400000 o O O O O O O O o o o TI o NO o 00 ON O ON ON ON ON ON ON ON ON ON O

34 -'^T •T^ I I I I I I r ^ ^ b) ^ oi Post-1945 ^ g^3.2 / y^ ^^^^ -' / ^ --- ^- ii /^ iX ^-^' ^- ^ I 3 .. '-' ' yy^<-' ^ y^'\^^ v ' I 2.8 • /y \ '//' y-'fy^,'-' \ C8 .,'' Pre-1945 I 2.6 •'k/'it^' 'OD mg / ii /' ' / &IPO~0 Pi 2.4 . • • 10 IC 10 100 average retum interval (yrs) average retum interval (yrs)

2.0 1 11 i d) 1.5

1.0

0.5

2 0

-0.5 _

-1.0

-1.5

-2 1900 1920 1940 1960 1980 2000 Year Figure 4.9 a) Normalised cusum curves for mean daily discharge on the mid-north coast of NSW, highlighting the breakpoints in the late 1940s and in the late 1970s; b) Log-normal expected percentiles and their 90% probability limits (dashed lines) in a regional analysis of pre and post 1945 discharge trends for NSW, after Franks (2002b); c) Log-normal expected quanriles and their 90% probability limits (dashed lines) for a regional index for NSW under the positive and negative IPO phases, after Kiem et al., (2003); d) The Inter-decadal Pacific Oscillation (IPO) from 1900-1999, after Kiem et al., (2003). 186 4. Climatic variability on the mid-north coast of NSW varying catchment areas, display a similar form and similar mid-century breakpoint (within two years). The timing of this turning point concurs with results presented for discharge for catchments on the north and south coast of NSW (Erskine and Bell, 1981; Franks, 2002; Reinfelds, subm.). Indeed, the similarity in trends between Figure 4.9a and those presented by Reinfelds (subm.) for the south coast of NSW suggests two major turning points of discharge in south-eastern Australia in the 20* century; 1945/48 and 1976/77.

The breakpoint that delineates the below-average discharge from the late 1970s also displays some intra-regional variation for the timing of the onset of the current 'DDR'. All gauges, bar one, exhibited a breakpoint between April 1976 and June 1977. The Richmond River, the most northeriy of the gauges assessed, exhibits a peak in June 1977 and a larger peak in June 1990. All gauges examined display a peak in the eariy 1990s, with both the Richmond and Orara River gauges displaying larger magnitude peaks than the other gauges (also reflected in their residual mass curves for rainfall presented in the preceding sections).

What is apparent from Figure 4.9a is that the time interval between 1945 - 1948 through to 1977 - 1978 is characterised by more pronounced short peaks than the first half of the record. These peaks occur at around 1955 and 1965 and have been identified on the south coast of NSW by Reinfelds (subm,), with Nakken (1999) referring to these shorter intervals as sustained positive or sustained negative trends. Whether these shorter time intervals are further used to break the time series for comparative analysis has been debated by Kirkup et al., (1998). The patterns exhibited in Figure 4.9a are similar to the rainfall cusum curves, in that both exhibit 'average' conditions. These have been termed average dominated regimes (ADRs) and occurred between the early 1920s through to the late 1930s. A second ADR is present, but less well identified, in the second half of the 20* century. The time period between 1956 and 1967 is a period of 'average' conditions in all but one of the gauges (see Orara River on Figure 4.9a). Between 1967 and 1977 the gauges exhibit a continued upswing until the onset of the current DDR in 1977

Reinfelds (subm.) identified that since July 1976 that there has been a decrease in the frequency of La Nina conditions and a concurrent increase in the frequency of El Niiio conditions, witii a cusum curve of Troup's SOI showing the most significant departure throughout the record. Following on from Bell and Erskine (1981) and Erskine and Bell (1982), Franks (2002b) has suggested that the most likely transition year in the first half of the 20* century is 1944 based on assessment of 40 gauges (both in coastal and inland NSW). The work by Franks demonstrates that the flood frequency distributions pre- and

187 4. Climatic variability on the mid-north coast of NSW post-1945, are statistically different with the presence of multi-decadal discharge regimes (sensu Erskine and Wamer, 1988). This is exemplified in Figure 4.9b which shows the difference in frequency distributions for the stratified regional index assessed by Franks (2002). This normalised regional index highlights the significant difference in pre- and post-1945 recurrence intervals, with no overlap at the 90% probability limit.

The work by Franks (2002b) and Kiem et al., (2003) is in strong accordance with tiiat presented by Allan et al., (1995) and Power et al., (1999), who have demonstrated that the mid 1940s was a time which corresponded to a change in both sea-surface temperature anomalies, as well as circulation patterns. Allan et al., (1995) have shown that the Indian Ocean Sea-surface Temperatures (SST) were cooler at mid-latitudes and warmer in the subttopical latitudes in the periods 1900 - 1941 when compared with the period 1942 - 1983. In addition to SST variations in the 20* century, Allan et al., (1995) found similar anomalies in surface winds concluding that the ''semi-permanent anticyclones in the mean flow field of the atmosphere over the southern Indian Ocean in the austral summer was weaker in the first 42 years of the 1900s". Hence, it is now more apparent that regional pattems in discharge for the mid-north coast of NSW are modulated by large- scale climatic modes such as SST. Power et al., (1999) have demonstrated that individual ENSO events (i.e. El Niiio, La Niiia) had stronger impact across Australia during the negative phase of the Inter-decadal Pacific Oscillation (IPO). Kiem et al., (2003) used the IPO, an index of multi-decadal variability derived using principal components analysis of Pacific SST, to define anomalous warming and cooling in the Pacific Ocean. The IPO is the temporal mode of variability in the Pacific SST and it has been demonstrated that the modulation of the IPO produces significant differences in discharge regimes.

Figure 4.9d shows the IPO from 1920 - 2000 presented by Kiem et al., (2003). This diagram and Figure 4.9a shows a correlation between positive IPO values and below- average discharge in both the first half of the record and post-1978. Indeed, Kiem et al., (2003) identify two positive IPO phases (IPO > 0.5) between 1924 - 1943 and 1979 - 1997, and a negative phase (IPO <-0.5) from 1946 - 1976, excluding the years 1958 - 1967 when the absolute value of the IPO index was less than 0.5. The first negative phase of the IPO between 1946 - 1958 corresponds with the onset of the steep rising limb in the regional cusum curves and correlates to a period of high flood activity on coastal rivers in the eariy 1950s. The time interval of 1967 - 1976 also corresponds with an above-average discharge period documented for coastal NSW and relates to the last upswing in the regional discharge cusum curves. The occurrence of the IPO neutral phase between 1956 - 1967 also coincides with the ADR on the regional discharge cusum curves. As shown in

188 4. Climatic variability on the mid-north coast of NSW the following section, however, this ADR still produced a period of flood activity in die study area. Kiem et al, (2003) have further demonstrated tiiat IPO modulates both tiie magnittide and frequency of La Nifia events, with tiie IPO negative phase associated witii a significant proportion of enhanced La Niiia events. This results in decadal/multi-decadal periods of elevated/reduced flood frequencies, witii the negative IPO associated witii a significantly higher flood risk than the other IPO phases, irrespective of individual ENSO events (Figure 4.9c).

Regional Discharge Summary

This section has assessed the regional trends for discharge patterns on the mid-north coast of NSW, with the data suggesting multi-decadal variability in discharge regimes witiiin tiie 20* century. While the work by Kirkup et al., (2001) highlights the changing intensity of certain frequency modes of discharge within the 20* century, the above data demonstrate that the eariy part of last century, up until the eariy 1920s was a period of below-average discharge. This was followed by a period of average discharge (ADR) between 1920 and the late 1930s, followed by another below-average period up until 1945/48. The flrst half of the last century, previously identified as a DDR by Erskine and Warner (1988, 1998) corresponds broadly to regional sea-surface anomalies highlighted by Allan et al., (1995). This section, however, has highlighted the decadal variability within this period and its association to the IPO.

The next time interval for regional discharge is cleariy the 1945/48 through to 1977/78 characterised by above-average discharge regime - termed a flood-dominated regime (FDR). This period is also characterised by shorter climatic episodes reflected in both the discharge records and the IPO, with the time period between 1956 - 1967 being characterised by ADR conditions, coinciding with the IPO neutral phase. The regional discharge records all exhibit a below-average discharge regime (DDR) from 1977/78 until present. This time period coincides with positive IPO, decreasing from 1992 onwards and with the attainment of negative IPO values from 1999 until present, potentially indicating another shift to an increased discharge regime.

4.4.2 Measures of flood potential on the mid-north coast of NSW

Jansen (unpublished data) has drawn together the flood records and assessed both area- discharge relationships and measures of flood variability for six catchments north of the Hunter River (Figure 4.1). To date, this compilation of data remains the most

189 4. Climatic variability on the mid-north coast of NSW comprehensive synopsis for the mid-north coast of NSW and is summarised in Table 4.3. It highlights both a measure of flood potential using the Flash Flood Magnitude Index (FFMI), developed by Beard (1975) and the flood peak variability index (FPVI), developed by Stevens et al., (1975,1977). The former was used by Patton and Baker (1977) and Baker (1977) to relate flood charactersitics to river behaviour and channel and floodplain morphology, and is defined as the standard deviation of the logarithms of the annual maximum flood peaks. The flood peak variability index is calculated as the ratio of the maximum flood of record to the mean (geometric) annual flood.

The FFMl and the FPVI and their measure of flood variability have also been utilised by Erskine (1993, 1994a, 1996) to relate the extreme flood variability of south-eastern Australia to the episodic changes documented within the historical record of coastal rivers of NSW. Inherent within this premise is that rivers with large flood variability will be more prone to episodic channel and floodplain modification and may display a non-equilibrium form. As can be seen from Table 4.3 the mid-north coast of NSW displays a FFMI which is on par with the Australian average, with the Bellinger catchment displaying the highest index of the six basins assessed with an index of variability of 0.57. This table also highlights that Australia has twice the index of variability for the world data set assessed by McMahon et al., (1992).

Values for the FPVI for the mid-north coast all fall under 10, with a regional mean of 6. The Bellinger/Nambucca catchments fall under the regional mean with a mean FPVI of 5.33. According to Erskine (1986) flood discharges that exceed 10 times the mean annual flood (i.e. where FPVI is > 10) have a large potential to cause catastrophic stripping. It has

Table 4.3 Summary attributes of flood potential for the basins of the mid-north coast Basin No. of Catchment FFMI FFMI FPVI FPVI gauges Area (km^) (med) (mean) (med) (mean) Richmond 26 241 0.39 0.42 3.81 4.40 Clarence 49 1221 0.47 0.47 5.98 6.55 Bellinger/Nambucca 6 332 0.57 0.53 5.71 5.33 Macleay 20 1939 0.47 0.54 7.93 9.36 Hastings 9 369 0.41 0.40 4.69 4.89 Manning 18 927 0.38 0.39 5.27 5.49 Hunter" 24 1077 - 0.65 - - MID-NORTH COAST 128 996 0.44 0.46 5.58 6.00 AUSTRALIA 280 - - 0.45* - - WORLD 651 - - 0.21* - - After Jansen (unpublished data); Area refers to mean area of gauges used within the analysis; ~ Hunter data after Erskine and Livingstone (1999); * Australian and worid FFMI taken from McMahon et al., (1992). FFMI = V'[2(log xi-logx)-/n-l] where logx is the mean of the logarithms of the (x) values and (n) is the number of events, after Beard (1975); FPVI = Qmax/Qa where Qa is calculated as the average annual peak- discharge, after Stevens et al., (1975, 1977);

190 4. Climatic variability on the mid-north coast of NSW

been suggested that rivers with large FPVI > 10 may exhibit non-equilibrium channel characteristics (Stevens etal., 1975,1977; Erskine, 1993,1994a, 1996), and where FPVI > 40, rivers are said to be susceptible to floodso f a 'cataclysmic' magnitude, where alluvium can be stripped from the valley trough (Erskine and Peacock, 2002). As Table 4.3 highlights none of the mid-nortii coast catchments have FPVI > 10 which would infer tiiat none are susceptible to catasttophic stt-ipping. Chapter 1 highlighted tiiat numerous smdies have demonstrated the occurrence of catasttophic sttipping in these mid-north coast catchments (e.g. Nanson, 1986; Wamer, 1997). Hence, it would appear tiiat die FPVI is not a sensitive index for predicting large-scale morphological response. Jansen (unpublished data) highlighted that the FPVI is sensitive to flood record lengtii and gauge location, with many records derived from headwater locations producing large area- discharge relationships.

Figure 4.10 highlights the flood envelope, based on the flood of record from 73 gauges on the mid-north coast of NSW. As can be seen from this flood envelope the mid-north coast of NSW plots below the dataset of Costa (1987) and Crippen and Bue, (1977) witii tiie former stemming from arid or semi-arid locations. Thus, it would seem that on world standards mnoff relationships for the temperate and sub-ttopical latitudes of the mid-north coast are lower than other world maximum flood envelopes, derived in landscapes more conducive to producing extteme discharge per unit area relationships.

1000003 ^dBi^: v9rr)

10000;

BO 1000; rel • X ,• Vio

T3 • 100 ; / • North coast NSW ea k 0. O NSW max Q (after Erskine, 1996) United States (after Costa, 1987) • • • I' I I r' I i^riT 0.1 10 100 1000 10000 100000

Catchment area (km ) Figure 4.10 Envelope curves of maximum floodsrecorde d on the mid-north coast of NSW (after Erskine, 1996; Jansen, unpublished data) and the United States (after Crippen and Bue, 1977; Costa, 1987).

191 4. Climatic variability on the mid-north coast of NSW

4.5 BELLINGER RIVER FLOOD FREQUENCY ANALYSIS

Flood frequency analysis for the Bellinger catchment draws on historical flood peak data, along witii flood discharge records that have been recorded from approximately 1955 onwards. It draws on cusum curves for daily discharge, annual and partial series analyses for the longest gauges in the catchment, in addition to area-discharge relationships developed by Reinfelds et al., (subm.). The latter has been extended by bringing these area-discharge relationships up to date to include data until the end of 2001.

4.5.1 Historical flood stage-heights in BeUingen: 1870-2001

The historical flood data available for the catchment is essentially limited to stage-height records at Bellingen Bridge. This location is at the tidal limit and is downstream of the study reaches, and has undergone changes in channel capacity, slope and sediment supply, yet provides the only location where flood height has been consistentiy recorded. While there are limitations to using stage-height data, it is essentially the only means to assess patterns of change in magnitude-frequency relationships between time intervals. Therefore this following section draws on the stage-height data set as a proxy to discharge, however also attempts to address the potential limitations of such data.

The Public Works Department (PWD) (1980) in a comprehensive compilation of written and anecdotal records assessed the flood record at Bellingen Bridge. This site has been the location of measuring floods since settlement (1842), although levels have only been referred back to a fixed point since 1912 when the first bridge was built (PWD, 1980). Bridge location has changed three times since 1912 within 100 m of the original location, with flood levels more consistentiy recorded after the current bridge was built in 1953. Hence, all flood levels prior to 1912 have been taken from landmarks near the bridge based on anecdotal evidence collated by PWD (1980). The flood-stage evidence at Bellingen Bridge has been supplemented by post-1979 stage-height data recorded at the same location, maintained by State Emergency Services (Figure 4.1 la). This figure shows an annual series of floods over a threshold deemed 'significant' for public safety and infrastructure (PWD, 1980). The presence of seven floods over this threshold prior to 1900 and nine floods over the threshold between 1947 - 1977, in contrast to only four between 1900 and 1947 led Wamer (1987, 1992, 1993) to suggest that these time intervals corresponded to periods of above and below-average discharge regimes (FDRs and DDRs). It has been further suggested that the Q233 - Q20 essentially doubled between 1901 - 1945 and 1946-1979. Wamer (1987, 1992, 1993) utilising the combination of

192 4. Climatic variability on the mid-north coast of NSW

a) Flood dominated regime Drought dominated regime Flood dominated regime Drought dominated regime FDR - (?) DDR - (?) FDR - (?) DDR - (?) 12^^ *-^ >-4 • 11

o O o o o o o o o o o o o t^ 00 o orH N Tt iTi ^0 t^ 00 0\ 00 ON en o 00 00 0\ 0\ 0\ a\ 0\ ON ON ON 0\ ON o b) 12^^ •>-•- -t-4- ->-*- 11 10 9 bc o I ' o 6 " 5 o O o O O o o o o o 00 M n -* >on VO r^ 00 ON o 00 00 ON ON 0\ 0\ ON ON 0\ 0\ o

Year Figure 4.11 Historical floodso n the Bellinger River at Bellingen Bridge after PWD (1980). a) Annual series of stage-heights over 8 m at Bellingen Bridge; b) Partial series of stage-heights over 6 m at Bellingen Bridge.

193 4. Climatic variability on the mid-north coast of NSW

Table 4.4 Number of flood events over the 6 and 8 m threshold in the various time intervals and estimates of discharge at Bellingen (656 km') 1864-1900 1901-1947 1948-1977 1978-2001 FDR DDR FDR DDR No. of events > 6m 10 16 28 11 No. of events > 8m 8 5 9 2 Q 2.33 (m'/s) 7 350 800 QsCm'/s) 1200 700 1500 963" Qio(m^/s) 2300 1200 2300 1317" Oon(mVs) 4200 1600 3000 1648" Flood thresholds after (PWD, 1980) and discharge estimates after (Wamer, 1992, 1993). R = values taken from annual series area-discharge relationships based on catchment area of 656km- (after Reinfelds et al., subm.). Time interval used in Reinfelds et al., (subm.) incorporates both pre- and post-1977. the historical stage-height data and three flood gauges in the basin presented estimates of discharge at Bellingen for the various flood and drought dominated regimes (Table 4.4). It has been specified that stage-height has been correlated to the three gauges, however, few other details are presented for the derivation of these estimates.

Table 4.4 also presents a summary of the number of events greater than 6 and 8 m at Bellingen Bridge. What is cleariy apparent from this summary in addition to Figure 4.1 lb, which shows a partial series of events > 6 m in stage, is the period of high flood activity from 1948 - 1977 with 28 events greater than 6 m, including the third largest flood on record in 1950. The 1978 - 2001 time interval is comparable in events > 6 m to 1864 - 1900, however this earlier time period was characterised by a larger proportion of floods > 8 m, but as will be discussed in the following paragraphs, may well be a function of changed reach-averaged channel geometry at the gauge location. The partial series of flood stage-heights, shown in Figure 4.11b, highlights the clustering of floods not just in the 1948 - 1977 time interval, but in the previous time interval around 1920 - 1921, 1937 - 1939 and in the following time interval around 1988 - 1990.

The flood-stage record presented in Figure 4.11 is immediately downstream of a chute- channel cutoff that was finally abandoned in the late 1970s (expanded upon in Chapter 5). As such, stage-height would have been directiy affected following the final abandonment as a result of changes in channel capacity and increases of 15 - 20% in 'reach-averaged' bedslope (Figure 4.12a-c). Bankfull stage at this enlarged location 250 m upstream of the gauge is ~8 m (AHD), the threshold used to identify 'significant'floods. The final abandonment of this cutoff would therefore have reduced stage-height for a given discharge from the late 1970s onwards, magnifying the identification of this time period as a drought dominated regime. Figure 4.12a also shows the dimensions of the channel at the enlarged location in 1943 derived from

194 4. Climatic variability on the mid-north coast of NSW

a) Bellingen palaeochannel 15

a 10

ii -5 100 200 300 400 500 Distance (m)

'SMi<-^2000

500m

d) Bellingen gauge cross-section

12 10 I Pleistocene Q 8 terrace X 6 < c 4 2 > ii left-bank right-bank 0 -2 -4 50 100 150 200 Distance (m) Figure 4.12 Post-European dimensional changes at and upstream of the Bellinger River gauge at Bellingen. 1943 cross-sections are photogrammetrically derived (expanded upon in Chapter 5). a) Note the partially cut chute-channel in 1943 which was abandoned in the late 1970s and the change in channel dimensions between the palaeochannel and the current chaimel. d) Bar accumulation at the current gauge location.

195 4. Climatic variability on the mid-north coast of NSW photogrammetric data (expanded upon in Chapter 5). This cross-sectional data show that tiie chute-channel has been progressively cut since 1943. Hence, it would appear that all flood levels post-1943 have been subject to progressively changing channel dimensions immediately upstream of the gauge location, and such, their value is questioned.

It is likely that the post-European dimensional and planform changes that have occurred elsewhere in the catchment (expanded upon in Chapter 5) have also occurred in the upper tidal reaches, found downstream of the gauge location at Bellingen Bridge. While the gauge location at Bellingen Bridge is confined on one side by a Pleistocene terrace, it is not possible to rule out dimensional changes at the gauge location itself. Estimates of changes in channel capacity at the gauge location is exemplified in Figure 4.12d which highlights bar accumulation of 2 - 4 m at the gauge location. Therefore, the stage-height data are potentially subject to variations in channel capacity both at Bellingen Bridge and immediately up and downstteam.

Quantifying the influence of the planform and dimensional changes on stage-height, however, is not possible due to the lack of more extensive survey data in the reach of the gauge location. The above cross-sections are simply presented to highlight the problems of assuming constant reach-averaged channel capacity over the time of the historical flood record. Therefore it is not possible to rely on this dataset to identify the flood and drought dominated discharge regimes or estimate bankfull discharge at this location in the various time intervals, given the potential limitations of the site. It is assumed that the highest recorded flood stages in 1870 and 1875 may be related to smaller channel dimensions and may not be directiy comparable to events of equivalent discharge in the last 50 years. While there is no dispute of the presence of large magnitude floods in the late 19"' century, prescribing significance to their stage is problematic given the nature of the documented changes in channel capacity.

4.5.2 Gauged flood data in the BeUinger Basin

Gauge record length within the Bellinger basin is extremely variable with many of the gauges being discontinued (Table 4.5). As such, there is very littie continuous quality data that exist to assess the differences between inferred flood and drought dominated regimes. As can be seen from this table only three of the 15 gauges remain operational with none of the gauges having data for the first half of the century > 5 years in length. Hence, it would seem improbable to derive a meaningful comparison for flood discharge between pre- and post 1945/48 within the basin, based on these records.

196 4. Climatic variability on the mid-north coast of NSW

Table 4.5 Flood gauges within the Bellinger basin (Basin No. 205) Station Station Name Period of Station Catchment Elevation Continuous data No. Record status Area (km^) A.H.D. (yrs) for LP 111 (m) 205001 Never Never R. @ Gleniffer 1925-28 Disc. 23 NA - 205002* Bellinger R @ Thora 1955-present Current 433 25 39 205003 Never Never R @ Slingsby Rd 1948-58 Disc. 12 595 - 205004* Kalang R @ Scotchman 1959-74 Disc. 165 25 12 205005 Bellinger R @ Boggy Ck 1959-66 Disc. 622 5 - 205006* Nambucca R @ Bowraville 1959-present Current 539 5 43 205007 Woolgoolga Ck @ Woolgoolga 1960-94 Disc. 11 NA 16 205008* Taylors Arm @ Greys Crossing 1970-94 Disc. 319 5 13 205009* Warrel Ck. @ Warrel Ck 1970-85 Disc. 205 5 14 205010 Bellinger R ® Upper Thora 1971-82 Disc. 407 35 8 205011 Warrel Ck. @ Barrage No data available 5 - 205012 Corindi R @ Upper Corindi 1975-85 Disc, 55 15 8 205013 Kalang R. @ Kooroowi 1974-84 Disc. 161 25 - 205014 Never Never R. @ Gleniffer Br. 1982- Current 51 25 6 205416 Nambucca R. @ Macksville No data available NA - * used in the area- discharge analysis

In order to assess temporal patterns in the discharge records two gauges within the basin have been used to construct basin cusum curves. The Bellinger River (205002) at Thora and the Nambucca River at Bowraville (205006) have the longest records available, witii the former providing a location within the Bellinger catchment to assess any changes in magnitude-frequency distribution pre- and post-1977/79. Figure 4.13 presents the cusum curves for daily discharge on both the Bellinger and Nambucca Rivers. Both gauges exhibit similar patterns throughout the record length matching the regional discharge cusum curves presented eariier. Oscillating above-average trends are shown from the start of the record with a steep rising limb from 1971 until the breakpoint of 1977. This is followed by a below-average trend from 1977 until present with peaks in 1990 (matching the daily rainfall peak) and again in 2001.

In order to assess differences between the inferred discharge regimes an annual and partial series analysis was carried out for the two gauges that contain data from 1956 onwards (Figure 4.14). Missing years on the Bellinger River were filled via a least squares regression with the Kalang River (205004 - R^ = 0.75) and the Nambucca River at Bowraville (205(X)6-R^ = 0.71). The correlation between neighbouring gauges provides a crude estimate of peak instantaneous discharge in missing years and yields a more complete time series in which to make comparisons between pre- and post-1977. A comparison was made between 1956 - 1977 and 1978 - 2001 and is shown in Figure 4.15 and summarised in Table 4.6. A two parameter log-normal distribution provided a more appropriate fit to the short annual maximum series, while a log Pearson Type III distribution was fitted to the partial series.

Table 4.6 highlights tiiat there has been an estimated reduction of predicted discharge between the two time intervals on the annual series of 11 - 26% on the Bellinger River and 35 - 48% on the Nambucca River. The partial series on the Bellinger River shows a

197 4. Climatic variability on the mid-north coast of NSW

1977 2000000 a) Bellinger River at Thora u e ii 1500000 •M e 1000000 4-o1 a o 500000 > ii T3 ( (U > td -500000 3 s 3 -1000000 •T) o u o >n O in o U-) o m o «n >o vo r- t~- 00 00 ON a\ o o\ ON ON ON ON ON ON ON ON

1977 2000000 -| b) Nambucca River at Bowraville

J3 1500000

O c 2 '> 0) D _> S "3 S 3 o lO lO O in m u »n o VO O 0o0 00 OoN ON o ON O^N ON Or~N~ ON 0\ ON ON ON o

Figure 4.13 Residual mass curve (cusum) for mean daily discharge (Ml/day) at a) Bellinger River at Thora (205002) and b) Nambucca River at Bowraville (205006). Note the above average trend up until 1977, followed by a below average trend from 1977 until present.

198 4. Climatic variability on the mid-north coast of NSW

>—s OKCif^ -a) Bellinger River at Thora (205002) •' ^ -' ^ 2000 -

£? A - 'A X 1500 1 partial!serie s * ^ \ 4 Ui A 4 ^ 1000 J \ • 4^ C 1 ca ^—* Vi B. 500 1 am lua series ns - 1 1 CL. 0 J 10 100 1 Return period (yrs)

b) Nambucca River at Bowravi le (205006)

,-• E , • w 2000 -

£P • X 1500 - partial ser ies » * * •^ A k*< ' ?• ^ 1000 ^ c .^cd* CO .M- ^ 500 ; r^ .^r\ ! ann ual serie s 03 ^ 0 J 1 10 1 00 Return period (yrs) Figure 4.14/ 'k.nnual maximum series and annual exceedance series (k=n) <3n the Bellinger Flive r at Thora (205002) and the Nambucca Riv er at Bowraville ( 205006) from 1956-2001. Log Pearson III distriburion fitted with expectec quantile shown.

199 4. Climatic variability on the mid-north coast of NSW

•• S'SQ fe =3o ^^^^1^^ > c/1 Q O 0 1 ,

I \ L »; I 1 li R i \ \ i. 1

\ \ \ 1 M r seri e \ 1 \ \ ^ 1978- 2 \ \ ^' \\ 1 \\ 1 T \ ^4 fo r

1 « annua l e s \ '\ Mambucc a i I S\ * I \ ^.^^ \ I i i J3 .k-i \ 1 i^ VI 1 \ "^ ^ >^ \ O +-' k-

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200 4. Climatic variability on the mid-north coast of NSW

Table 4.6 Annual and partial series for total record and pre- and post-1977 ANNUAL Bellinger River @ Thora Nambucca River @ Bowraville IJTCRTL 2-y ^-y 10->"" 20-yr 2-yr 5-yr 10-yr 20-yr ___^ (m^/s) (mVs) (mVs) (m^/s) (mVs) (m^/s) (mVs) (m^/s) 1956-2001 LPHI 331.9 844.7 1188.7 1477.4 376.6 1004.9 1387.1 1673.2 1956-19772PLN 251.4 855.1 1621.4 2750.4 329.5 1416.4 3035.7 5697.1 1978-2000 2PLN 252.9 759.3 1348.7 2167.7 212.8 822.7 1668.1 2990.5 PARTIAL 2-yr 5-yr 10-yr 20-yr 2-yr 5-yr 10-yr 20-yr (mVs) (m'/s) (m'/s) (mVs) (m'/s) (m'/s) (m'/s) (m'/s) 1956-2001 LPIII 469 3 3779 23544 1711.2 636.8 1046.85 1371.6 1723.9 1956-1977 LPIII ^g^i 9973 i288.2 1573.9 791.9 1262.7 1569.4 1853.5 1978-2001LPIH 3793 7992 1334.1 2180.8 4746 818.9 1129.5 1501.9 2PLN - 2 parameter log-normal; LPIII - log Pearson Type III

reducrion of 54-25% post-1977 for the 2 and 5-year retum period and increases of 4 - 40% for the 10 and 20-year return period, while the Nambucca River shows a reduction of 40 - 19% in esrimated discharge for the four return intervals. While the tabulated results concur reasonably well with the expected magnitude of changes between flood and drought regimes as presented in estimates by Warner (1992, 1993), shown previously in Table 4.4, all results fall within the 90% probability limits for respective time intervals (Figure 4.15). This figure highlights a generous overiap in the probability limits of both the annual and partial series on the Bellinger and Nambucca River. While, the partial series on die Nambucca River highlights some independence for more frequent intervals, the overiap in all of the graphs would suggest that discharge in the two time intervals are not independently distributed. If the pre-1977 records contained the additional 6-8 years of data, which would include the period of high flood acdvity in the eariy 1950s, then the distributions seen witiiin Figure 4.15 may highlight more independence. As such, the identification of the various discharge estimates presented in Table 4.4 for the FDR and DDR regimes is essentially statistically inaccurate and not possible given the above distributions and limited record length.

4.5.3 Unit-discharge relationships for the Bellinger Basin

The Bellinger basin contains 16 gauges shown previously in Table 4.5. Due to short record lengths and die small number of gauging stations for most individual catchments the entire hydrological basin has been used to investigate regional discharge characterisrics. Cohen et al., (1998) and Reinfelds et al., (subm.) derived unit-discharge power functions for the Bellinger Basin utilising six gauge records > 10 years in length (Table 4.5). This extended basin-wide dataset has been chosen to improve the accuracy of flood modelling undertaken in Chapter 6. Using data outside the Bellinger catchment, but within the same hydrological basin assumes that hydrological characteristics of individual catchments are comparable. Such an assumption forms a standard technique for assessing

201 4. Climatic variability on the mid-north coast of NSW discharge characteristics of ungauged catchments (Pilgrim, 1987). This assumption is considered reasonable given the similarities between the Nambucca and Bellinger rainfall and discharge characteristics shown in preceding sections.

The annual series power functions derived by Cohen et al., (1998) and Reinfelds et al., (subm.) have been updated to include the 2001 flood events, while additional partial series area-discharge functions were calculated for each of the six gauges. The probability for the annual maximum series was determined by fitting a log Pearson Type III distribution by the metiiod of moments, using FLIKE, a Bayesian flood frequency program (Kuczera, 1999). The annual exceedance series (partial series) was extracted where k = n (after McDermott and Pilgrim, 1982), where independence of flood peaks was determined by an examination of hydrographs and delineated by 2 x tcmax, plus a 90% reduction in peak discharge within that time period. Tcmax refers to time of concentration at the gauge with the largest catchment area (~ 47 hours). Flood peaks that did not meet both criteria were eliminated from the analysis and considered part of a multi-peaked event.

Annual and partial area-discharge functions are shown in Table 4.7, in addition to regional relationships based on the analysis of 73 gauges on the north coast of NSW by Jansen (unpublished data). There is good consistency within the Bellinger basin relationships with r^ that range from 0.99 to 0.66 for the annual series and from 0.96 to 0.84 for the partial series (see Appendix 4.1 for area-discharge functions). Both the annual and partial series show better relationships at the more frequent return intervals, such as the Q2 - Qio and in keeping with observations of flood magnitudes from river basins elsewhere (e.g. Baker and Costa, 1987; Costa; 1987), rates of runoff for the Bellinger basin decrease sharply with increasing catchment area. All three relationships will be used in the stage- discharge assessment and flood modelling in Chapter 6.

Table 4.7 Bellinger basin and regional unit-discharge relationships Return Interval Regional - north coast Bellinger basin Bellinger basin ANNUAL ANNUAL PARTIAL y=ax y=ax'' y=ax'' Q2 flood 6.1 area °^^ 0.67 2.61 area °-'^ 0.99 6.86 area °*' 0.96 Q5 flood 11.4 area °*' 0.74 7.89 area °-'* 0.95 10.96 area °-^° 0.94 QIO flood 14.8 area °-^^ 0.77 16.29 area °*^ 0.90 13.64 area °-^' 0.92 Q20 flood 18.7 area °-^^ 0.79 39.74 area °-^^ 0.82 16.11area°^- 0.89 Q50 flood 21.5 area °'^ 0.79 71.20 area °^' 0.75 19.16 area °-^^ 0.87 QlOO flood 24.1 area °«* 0.78 126.15 area °-^' 0.66 21.32 area °'' 0.84 Qmax flood 21.7 area"** 0.79 25.85 area °*^ 0.85 25.85 area °-^ 0.85 area = catchment area; regional relationships derived from flood gauge datasets with >20 years of flood record (after Jansen, unpublished data).

202 4. Climatic variability on the mid-north coast of NSW

4.6 ALTERNATING CLIMATIC REGIMES IN THE BELLINGER BASIN? A SUMMARY

This chapter has presented a regional and catchment assessment of rainfall and discharge patterns on the mid-north coast of NSW. The regional rainfall data cleariy show that the end of the 19"" century was an above-average period for rainfall, while 1900 through to 19z|6/47 was generally a below-average period. There is, however, evidence to suggest a period of 10- 15 years of average rainfall conditions between the eariy 1920s through to the late 1930s, coinciding witii an IPO positive period. Regional rainfall cusum curves all exhibit above-average conditions from 1946/47 through to 1990, witii pronounced peaks in 1964, 1977 and 1990. The data confirm previous studies highlighting a 14% increase in mean annual rainfall after the 1946/47 breakpoint, with the results of Franklin (1999) suggesting the areas in the Great Dividing Range experiencing the greatest post-1946/47 change in mean annual rainfall and the elevated tablelands changing the least. These changes in mean annual rainfall have been shown to be a function of increases in summer and autumn totals. The regional trends in rainfall also display some correspondence to previously reported multi-decadal SST anomalies, suggesting that rainfall on the mid-north coast is influenced by climatic processes operating on the decadal scale along with shorter fluctuations such as ENSO.

The Bellinger rainfall patterns exhibit similar attributes to the regional trends with a 1947 breakpoint and a 14 - 19% increase in mean annual totals as a function of equivalent increases in summer and autumn rainfall. While on initial inspection the number of rain events > 200 mm/24hours has increased by 150 - 175% post-1947, there is littie evidence to suggest that 48-hour maximum rainfall stems from independent distributions between pre- and post-1947 time intervals. Hence, the regional and catchment rainfall analysis has highlighted some important temporal trends in rainfall that have occurred throughout the 20* century. The identification of alternating multi-decadal rainfall regimes prior to 1880 is simply not possible on the mid-north coast of NSW and therefore it cannot be assumed that what was observed in the 20* century is similar to that which was experienced in the 19* century.

The regional trends presented for discharge essentially concur with the rainfall patterns for the mid-north coast, with a drought-dominated regime (DDR) prior to 1920, an average- dominated regime (ADR) between 1920 and the late 1930s, followed by another DDR up until 1945/48. A flood-dominated regime (FDR) can be identified from 1945/48 through to 1977, but witii an intervening ADR between 1956 and the late 1960s. Catchment analysis has shown that the flood stage-height data set, while providing a guide to the

203 4. Climatic variability on the mid-north coast of NSW timing and clustering of floods cannot be used to estimate shifts in the magnitude- frequency distribution between the inferred flood and drought dominated regimes. Indeed, while the Bellinger and Nambucca cusum curves are similar to the regional discharge patterns, distinguishing the statistical difference between time-intervals is less clear.

The data within the second half of the 20* century demonstrate that while there has been a reduction in predicted discharge for a given return period of up to 54% post-1977, the overiap in probability would suggest that they are from similar populations. As such, the identification of alternating multi-decadal discharge regimes in the Bellinger catchment, while visually apparent in the cusum curves, is more difficult to statistically differentiate. The presence of a post-1947 FDR and a post-1977 DDR appears convincing, but it must be stressed that within the post-1947 FDR there is also the presence of an ADR. While not statistically distinguishable in the Bellinger catchment, Kiem and Franks (in press) have shown that these multi-decadal periods are more readily identified in other coastal catchments. The identification of alternating FDRs and DDRs beyond the 20* century based on the regional and catchment data presented within this chapter is simply not possible. Given the limitations of the data and the problems delineating 'independent' climatic regimes it would seem prescribing geomorphic significance to these inferred alternating regimes as extremely problematic and potentially statistically inappropriate.

It would appear that the secular shifts in rainfall and discharge in 1945/48 and 1977 are intrinsically linked to broad-scale ocean-atmposheric interactions, with the indices such as the IPO providing a reasonably good measure of association between SST and periods of flood activity seen on the mid-north coast of NSW. The IPO has been shown to modulate ENSO's influence on Australia, with significant correlations when the IPO is negative (i.e. reduced tropical Pacific SST), but with poorer correlations when the IPO is positive (Power et al., 1999). This has been further substantiated by Kiem et al., (in press) who have shown that when the IPO is negative the frequency at which La Niiia (wet conditions) occurs is significantiy higher. Conversely, they have also shown that there are a greater number of neutral ENSO events when the IPO is positive. This has led Kiem et al., (in press) and Kiem and Franks (subm.) to suggest a higher rate of occurrence of the ENSO extremes when the IPO is negative. Thus, it appears that ENSO extremes are dependent on the multi-decadal climatic state, with La Nifia events being enhanced (i.e. wetter) when the IPO is negative and less severe when the IPO is positive. The identified ADRs within this chapter correspond to either positive or neutral IPO conditions.

204 4. Climatic variability on the mid-north coast of NSW

Identifying the presence of alternating multi-decadal regimes will only come about through a better understanding of the interactions between climatic variables operating at a number of spatial and temporal scales, while also improving the proxy data such as coral reef records to examine the longevity of climatic regimes. Understanding the potential influence of such altemating climatic regimes on landscape processes in south-eastern Australia must also attempt to separate the accelerated responses associated with anthropogenic disturbance. This will be attempted in Chapter 5.

205