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Papers and Proceedings of the Royal Society of Tasmania, Volume 121, 1987 23 STREAMFLOW CHARACTERISTICS OF NORTHEASTERN TASMANIA: I. REGIONAL FLOOD FLOWS by A. D. Knighton (with four tables and seven text-figures) KNIG HTON, A.D., 1987 (30:vi): Streamflow characteristics of northeastern Tasmania: 1. Regional flood flows. Pap. Proc. R. Soc. Tasm., 121: 23-33. ISSN 0080-4703. Department of Geography, University of Sheffield, Sheffield, United Kingdom SID 2TN. Based on streamflow records from thirteen stations, regional equations are developed which enable the estimation of mean annual discharge and various flood flows at ungauged sites along rivers in northeastern Tasmania. The area around Swansea is shown to be hydrologically distinct, at least as far as flood discharges are concerned, and the analysis subsequently focuses on the rest of the region. Despite the wide range of climatic and physiographic conditions there, relationships are highly significant, with drainage area explaining more than 97% of the variation in flood discharge. Rates of change are not only relatively high but increase with flood magnitude, suggesting rapid downstream transmission of flood waters. Network magnitude may be a viable alternative to drainage area as an estimator. Its usc has the advantage that the downstream pattern of flow addition can be readily charted for major rivers, as illustrated for the Ringarooma and George, Key Words: Tasmania, stream flow, drainage area, flood discharge. INTRODUCTION The basic variable used to express the size Macquarie River. Regional How analyses are a and erosive effectiveness of natural rivers is dis­ compromise to a certain extent: between a larger charge, which is generally measured at established sampled area with more gauging stations and a gauging stations. However, the number of mea­ smaller one in which flow conditions are likely to surement sites within an area is usually quite small be more homogeneous. Thus enlargement of the and rarely does a sequence of gauging stations exist sampled area to augment the data base increases along an individual river. Consequently, in order to the degree of heterogeneity and the attendant risk extend a spatially limited data base and provide of sampling from different hydrogeographic popu­ estimates of flow characteristics at ungauged sites, lations. Such proved to be the case here. some form of regional analysis is necessary. The The area can be divided into three main estimates so derived can be used for water manage­ sections. That to the north of the South Esk basin ment and river engineering purposes at regional has rivers draining to the east and north coasts and and basin scales, as well as for routing water and to the Tamar estuary, so that they come under a sediment through individual catchments. Indeed wide range of climatic and physiographic influences. the latter provided the initial stimulus for this study Mean annual precipitation is at a maximum (> 1600 in that reliable flow estimates were required as a mm) to the north and northeast of Ben Nevis, spatial series along the Ringarooma and George where many rivers (notably the North Esk, Rivers in order to model the movement patterns of Ringarooma, George and South Esk) have their material introduced from mining sources. Although headwaters. The first three rivers listed provide a so inspired, the results presented below have impli­ west-east sample across this northern section, the cations for a wider range of purposes. pattern of their mean monthly discharges being The study falls into two parts, the first of distinctively seasonal (fig. 2) and typically temperate which is concerned with flood flow estimation and oceanic (Shaw 1983). Despite the broadly similar the second with hydraulic geometry relationships. regimes with July-September and January-March Both deal with rivers in northeastern Tasmania, a being respectively the maximum and minimum somewhat arbitrarily defined area bounded in the flow periods, small but possibly significant west by a line along the Tamar estuary, South Esk differences are apparent in the monthly runoff and Macquarie Rivers (fig. I). The southern limit is patterns, particularly between the George and the defined by the southern-most sweep of the other two rivers. Even when dealing with averaged 24 A.D. Knighton Gauging Stations T Tomahawk River GC Goatrock Creek lC Lau rlston Creek GM Great Musselroe River MA Macquarle River CU Curries River R RingaroorQa River SE L South Esk, Llewellyn p Pipers River C Cascade Ri ver SEp South Esk at Perth arid River G George River NE North Esk B N GF Great Forester River A Apsley River RA Ransom River S Swan River ME Meredith River t ,,- .... ,, \ " I I I I ~...... ,. .... _'" ".. , I I I I '1.;,1 o 10 20 30 40 50km ....I FIG. 1 - Distribution of gauging stations in northeastern Tasmania. Streamflow Characteristics of NE Tasmania, 1. Regional Flood Flows 25 flows in neighbouring basins, flow behaviour can RINGAROOMA RIVER 2 be variable. Mean annual The South Esk basin dominates the area with discharge:::; 8'71 m3 5- 1 the main river describing a horseshoe-like course z « (fig. I) along which mean annual precipitation ill :2 varies from 1600 mm in the upper reaches to less than 600 mm south of Perth. The main tributary, r::' o 1 the Macquarie River, has a southern origin and f= « enters the South Esk downstream of Perth, the site a: of the gauging station with the largest drainage area used in this study. In effect the South Esk basin separates the northern section from a southern one represented by a cluster of gauges in the o~--------------------------------~ neighbourhood of Swansea (fig. 1). There, mean annual precipitation ranges from 650 to 900 mm. GEORGE RIVER As the following analysis will show, this southern Mean annual z 3 1 section has streamflow properties which are distinct « discharge =6'12 m 5- .----..-.--1 ill from those further north. :2 o f- REGIONAL ANALYSIS g «f­ The estimation of flood frequency or hydro­ a: graph parameters at ungauged sites commonly relies on statistical relationships between river flow and catchment characteristics. The latter can be o - broadly grouped into climatic, surface cover (soils and land use) and drainage basin properties, cate­ I NORTH ESK RIVER gories which include a potentially large number of 2 r Mean annual 3 operational variables. Ease and reliability of ! discharge = 5 43 m s-l measurement, and likely success in prediction are appropriate criteria in the choice of suitable o variables. From the point of view of the first f- criterion, drainage basin properties are probably '2 the most suitable provided reliable topographic f­« maps are available at a large enough scale. Those a: properties can be further subdivided into: (i) size variables - drainage area, mainstream length, total channel length; Oi) slope properties of the channel and hillslope o M A M systems; and A SON 0 (iii) channel network variables drainage density, FIG. 2 - Flow regimes of the Ringarooma, stream frequency, network magnitude; George and North Esk Rivers expressed by the with drainage area and network magnitude being ratio of mean monthly to mean annual discharge. relatively easy to measure. As regards the second criterion, drainage area has consistently been an the most consistent out of 16 independent variables effective variable, one reason for its success being since it was the only one to appear in all ten that it subsumes a great deal of relevant hydro­ analyses. Given also that drainage area has been logical information. In probably the most compre­ successfully used to predict the median flood (Q2) hensive study of streamflow variation yet completed in western Tasmania (Watson 1975, Watson & (NERC, 1975), drainage area proved to be the Williams 1983), its choice for this study seems single most important variable influencing the obvious, especially since this study is largely of an mean annual flood (Q233) at both national and exploratory nature. Following standard practice, regional levels of the British Isles. In assessing ten least-squares regression analysis is used to derive regional flood-frequency analyses from the United the required relationships which are assumed to States, Riggs (1973) found that drainage area was have the typical log-linear form. 26 A.D. Knighton Mean Annual Discharge achieved by the inclusion of mean annual precipita­ tion as an additional independent variable. Never­ Mean annual discharge is simply the arith­ theless, most of the residuals above the regression metic average of all daily mean flows, with a line have higher than average precipitation, while frequency of 29, 32.5 and 30% respectively at the those below have lower than average. That the two North Esk, Ringarooma and George gauging sta­ South Esk stations plot as negative residuals can be tions. In most humid areas it is closely related to partly attributed to the precipitation effect, drainage area and mean annual precipitation. although channel storage may be a contributory Here, its relationship to drainage area is highly factor at these large drainage areas. However, significant with a coefficient of determination (p2) exclusion of those stations to give of 0.98 and a standard error of estimate of 0.11 02 (fig. 3). Basins of less than 30 km2 rarely figure in Qm. :: 0.01 A/· (I) such plots but there are three in this area and they has obviously little effect on a well-defined regional plot well relative to other points. trend which is maintained over three and a half cycles of logarithms. oc -----r-r--, 1 - ---------'1 --~J r- ! Flood Discharges and Drainage Area "S~p SEc The main part of this paper is concerned with 10 R. higher magnitude flows which are more relevant G. hydrologically and morphologically. Flood fre­ NE- .5 quency analysis is a procedure for estimating the GF.

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