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Circulation and sedimentation in LakeBenmore, New ZealandR. A. Pickrill a & J. Irwin aa Division of Marine and Freshwater Science Department of Scientificand Industrial Research , New Zealand Oceanographic Institute , P.O.Box 12 346, Wellington North , New ZealandPublished online: 24 Jan 2012.

To cite this article: R. A. Pickrill & J. Irwin (1986) Circulation and sedimentation in LakeBenmore, New Zealand, New Zealand Journal of Geology and Geophysics, 29:1, 83-97, DOI:10.1080/00288306.1986.10427525

To link to this article: http://dx.doi.org/10.1080/00288306.1986.10427525

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New Zealand Journal of Geology and Geophysics, 1986, Vol. 29: 83-97 83 0028-8306/86/2901-0083$2.50/0 © Crown copyright 1986

Circulation and sedimentation 1n Lake Benmore, New Zealand

R. A. PICKRILL J. IRWIN New Zealand Oceanographic Institute· Division of Marine and Freshwater Science Department of Scientific and Industrial Research P.O. Box 12 346 Wellington North, New Zealand

Abstract Benmore is the largest artificial lake in New Zealand. The lake has two arms which have quite different inflows in terms of volume, tem­perature, and suspended particulate matter (SPM) loadings. The Ahuriri Arm is fed by natural runoff from unglaciated catchments, whereas the Waitaki Arm receives the outflow of three large glacial-fed lakes which filter out much of the SPM and restrict the temperature range of water entering Lake Benmore. Geographical isolation of the two arms allows each to develop its own annual cycle oflake/ river interactions. The Ahuriri water, which is more responsive to seasonal temperature changes, under­flows the warmer Waitaki water in winter and overflows in summer.

Upstream lakes entrap most of the bedload from the upper catchment and much of the coarse silt fraction in the suspended load. Consequently, there has been little delta growth with most of the sus­pended load being clay, which is swept further down the lake.

Since forming in 1965, the shoreline (42%) has been altered by limnic processes, with beaches forming in sands and gravels, steep shores being cliffed, prefill subaerial slope failures being reju­venated, and bedrock and low slopes left unaf­fected. However, shoreline erosion has only contributed about 8% of the sediment infilling the lake with more than 85% originating from river­borne SPM. Sedimentation rates (3 mmjyear) are lower than in the natural lakes upstream, and sedi­ment deposition should not create any problems to the efficient running of the power station during its design life.

Keywords circulation; temperature; surface waters; suspended sediments; cores; shorelines

Received 16 October 1984, accepted 17 September 1985

INTRODUCTION

The Waitaki River was first harnessed for hydro­electric power generation in the 1930s. Since then, major power stations have been built and planned, culminating in the Upper Waitaki scheme which is still under construction. A major part of these projects has been the controlling of levels and out­flows from natural lakes and the formation of new storage lakes downstream. Siltation has been a problem for storage lakes in New Zealand (Thomp­son 1976; Ministry of Works and Development 1977; Pickrill & Irwin 1980; Phillips & Nelson 1981; Irwin & Pickrill 1983; Pickrill et al. 1984). In 1979, New Zealand Oceanographic Institute started a research programme to study the circulation and sedimentation in a lake in each of the Waitaki and Clutha catchments, with a view to applying these findings to existing and planned lakes downstream. Studies of Lakes Tekapo and Wakatipu are nearing completion (Irwin & Pickrill 1982; Pickrill & Irwin 1982, 1983) and implications for downstream stor­age lakes have been drawn (Irwin & Pickrill 1983). Consequent to these findings, research has turned to Lake Benmore to study the circulation and sedi­mentation and compare and contrast this with the natural lakes.

BACKGROUND

Lake Benmore is the largest artificial lake in New Zealand. A 110 m high and 823 m wide dam across the Waitaki valley formed the 94 km2 lake in 1965. The dam was built across the outlet of a gorge cut in Torlesse Group greywacke and argillite (Mutch 1963). Flooding the gorge produced a narrow deep lake with a complex outline that follows the incised course of the rivet channel (Fig. 1). Four kilo­metres back from the dam, the gorge bifurcates into the Ahuriri and Waitaki Gorges. Upstream from the gorges, the valleys open up into the rolling out­wash alluvium and till deposits of the Mackenzie Basin (Mutch 1963; Gair 1967). The bathymetry (Fig. 1) reflects the prefill river valley with maxi­mum depths of91 m behind the dam, narrow deep steep-sided Waitaki and Ahuriri Gorges, and shal­low broad expanses in the upper reaches (Ahuriri and Waitaki Arms), broken only by the incised course of the former river channels and flanking terraces (Fig. 1 ).

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LAKE PUKAKI

: I

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N0.8 //

......... ·" ..... . 0 I

0 I 0 I . \ . ~

~TEKAPO B I

: ! I

I

~ CONTROL STRUCTURES

.... DAMS

• POWER STATIONS

0o° CANALS .

Fig. 2 Schematic diagram of the Upper Waitaki power scheme.

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Fig. I (opposite) Lake Benmore bathymetry (m). Station positions occupied February 1982 (summer), November 1982 (spring), and August 1983 (winter), for suspended sediments and temperature observations.

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Lake Benmore is one of a series of hydroelectric power stations harnessing waters of the Waitaki catchment. In order to understand past, present, and future circulation and sedimentation in the lake, we need to understand how engineering works in the headwater catchment affect inflows to Benmore. The Waitaki catchment contains three large natu­ral storage lakes, Tekapo, Pukaki, and Ohau, which drain southward into the head of Lake Benmore (Fig. 2). By 1965, the outflow from Lakes Pukaki and Tekapo was partially controlled, but remained in the original river channels.

Further development in the upper Waitaki since 1965 has increased storage capacity, improved hydraulic load, and provided more generating capacity, all of which has affected waters received by Lake Benmore. Water from Lake Tekapo now flows via a canal and power station {Tekapo B) to a raised and larger Lake Pukaki (Fig. 2). From here another canal guides these waters to join a canal out of Lake Ohau. This flows to a power station (Ohau A), then to a new Lake Ruataniwha, and will by canal travel through two more planned power stations (Ohau B and Ohau C) before discharging into the head of Lake Benmore. Minimal flows are maintained down natural river channels to enter the lake at the head (Fig. 2).

METHODS

Twenty stations were occupied over the lake in summer, spring, and winter (Fig. 1 ). These times were chosen to coincide with periods of maximum and minimum water temperatures and to be com-

N D

25

30

"' 40 ~ Q.

" 50~ " (.) c

" 70 ~

100 " 0::

150 200 300

Fig. 3 Lake Benmore monthly water discharge and residence times.

parable with previous studies made in Lake Tek­apo (Irwin & Pickrill 1982). National Institute of Oceanography water bottles of 1.125 L capacity were used to collect water samples. The samples were filtered through preweighed 0.8 Jlm pore size Nucleopore filters to determine absolute concen­trations of suspended matter. Samples were col­lected at 3 and 5 m depths at shallow stations and at 10m intervals at deeper stations. Temperatures were measured with reversing thermometer and bathythermograph. Inflowing waters in the Tek­apo, Pukaki, Ohau, and Ahuriri Rivers were mon­itored for the 3-4 day survey period using Ryan thermographs.

Fourty-three bottom sediment samples were col­lected over the body of the lake by Lafond Dietz grab. Two short cores were collected from the southern end of the lake using a 30 mm diameter Phlegar corer. All samples were analyse£! for tex­ture using standard sieve and pipette techniques.

INFLOWS AND RESIDENCE TIMES

With a catchment area of8500 km2, Lake Benmore has a mean annual inflow of 323 m3/s, of which 290 m3/s enters over the headwater deltas of the Tekapo, Ohau, and Pukaki Rivers in the Waitaki Arm, the Ahuriri River discharges 21 m3/s into the Ahuriri Arm, and other streams contribute the remaining 12 m3/s (Ministry of Works and Development 1978). Inflow and discharge are high­est and most variable during the spring and sum­mer melt, when excess water is spilled (Fig. 3).

Mean bulk residence time (lake volume -;.- aver­age inflow) is 67 days, which is much lower than

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the 412-880 day residence times in the headwater lakes (Irwin & Pickrill 1983). Seasonal fluctuations of inflow produce generally shorter residence times but a wider range in spring/summer (25-200 days) and shorter residence times with a much narrower range in winter (50-150 days, Fig. 3).

LAKE/RIVER INTERACTIONS

In deep lakes with dominant headwater inflows, interactions between inflowing river water and receiving lake water is a major determinant of cir­culation and sedimentation (e.g., Hamblin & Car­mack 1978; Irwin & Pickrill 1982; Pickrill & Irwin 1982). The passage of river water through the lake is principally controlled by density differences between the water bodies. Density is largely con­trolled by water temperature and concentration of dissolved and suspended particulate matter (SPM). Three inflow conditions can occur.

(1) If river water is denser than lake water, it flows to a sharply defined plunge line, where it sinks and flows downslope along the bot­tom as an underflow.

(2) If the lake is stratified and river waters enter at densities equivalent to those in midwater depths, then the water sinks at the plunge line and flows downslope to a depth where its density matches that of the surrounding lake water. The river water spreads at this depth as an interflow along surfaces of con­stant density.

(3) If river water is less dense than receiving lake water it spreads across the surface as an overflow.

Lake Benmore differs from previously studied New Zealand lakes in having two separate arms, which have different river inflow characteristics (Fig. 4A, B). The Waitaki Arm is fed by upstream lakes that buffer normal fluctuations in river tem­perature and turbidity, whereas the Ahuriri Arm is fed by uncontrolled river flows. These differences in river regime produce different lake/river inter­actions in the two arms, which in tum produce interesting interactions at the confluence of the gorges.

Summer In summer, the lake was stratified with surface temperatures of 17.3-18SC, a well-defined ther­mocline at 15-30 m depth with a gradient of OSCjm, and bottom temperatures of 7SC.

In the Waitaki Arm, inflowing river water was at 14-18oC with 22-30 mg/L (SPM). Receiving

87

waters in the epilimnion were generally warmer (17-18oC) and less turbid than inflows, which, being denser, underflow to the base of the thermocline. Below 30 m depth, interactions are difficult to determine, although the SPM stratification with highest concentrations in bottom waters and iso­line gradients upslope to the inflows indicate that, in the past, cold high SPM underflows have extended downlake to the dam face.

In the Ahuriri Arm, inflow temperatures cover a wide range (12-21oC) and SPM levels are very low ( < 3 mg/L). As a result, epilimnion waters are cooler and clearer than in the Waitaki Arm. The data suggest the river waters overflow and inter­flow down to the base of the thermocline. The large diurnal temperature range in inflowing river water probably produces a diurnal cycle of interflows and overflows similar to that found in Lake Wakatipu (Pickrill & Irwin 1982).

In the Benmore basin, at the confluence of the two arms, the SPM distribution indicates that tur­bid water from the Waitaki Arm underflows to the dam face and extends a short distance back up the Ahuriri Arm. Similarly, clear epilimnial water from the Ahuriri overflows the Waitaki water and backs up into the Waitaki Arm.

Winter Inflows to the Waitaki Arm had temperatures of 5-?"C and SPM levels of approximately 10 mg/L. The arm was near isothermal ranging from 6oC at the surface to 5.2°C at the bottom, with some local heating at the surface to 7.4oC (Fig. 4). SPM levels are generally higher than in summer and show a complex and patchy pattern. However, the high SPM levels in bottom waters and tilted isotherms originating from the head of the arm are indicative of underflows extending 20 km down the lake.

Inflows in the Ahuriri Arm are colder (2.5-5SC) and clearer than in the Waitaki Arm. As a result, receiving lake waters are near isothermal but about 1 oc (4.2-5.0oC) colder than in the Waitaki Arm. Lake/river interactions close to the rivermouth are unclear. However, with inflow temperatures rang­ing above and below the maximum density of water (4°C), and lake temperatures just above this maxi­mum, there is pOtential for under, inter, or over­flow, although density differences are small and any flows are pro6ably weak.

The surface contact between the cold, clear Ahu­riri water and warmer turbid Waitaki water is marked by a front midway through the Ahuriri Gorge. Waitaki water overflows back up the Ahu­riri Gorge and overflows to the dam, whereas the cooler clearer Ahuriri water underflows the Wai­taki water at the front and continues down to the dam face.

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Dam

Ahuriri Arm Waitaki Arm

Spring Temperatures (DC) Spring Temperatures (DC)

--------8-------------/

Summer Temperatures (DC) Summer Temperatures (DC)

Winter Temperatures (DC) Winter Temperatures (DC)

10 15 20 oc_ ______ _,_s _______ 1_,_o ______ __.15 ______ ___.2o ______ ___J25 (km)

Fig. 4A Lake Benmore seasonal water characteristics: temperature.

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Summer suspended sediments (mg/L)

Winter suspended sediments (mg/L)

0L_ ___ L5 ____ 1L0 ____ 1L5 _____ 2L0------~25(km)

Fig. 4B Lake Benmore seasonal water characteristics: Suspended sediments. Arrows indicate inferred flow, broad lines mark fronts between Waitaki Arm and Ahuriri Arm water masses.

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Spring

In the Waitaki Arm, inflow temperatures ranged between 10 and 16·c, with SPM levels of 4-10 mg/L. Thermal stratification had begun with epi­limnion waters at 10-12·c at the head of the arm. The SPM distribution is more complex than during other seasons, with pockets of turbid water in upper waters midlake and at the dam in mid and bottom waters. This situation is possibly due to the sam­pling period being spread over four days when inflows were variable; other surveys covered two consecutive days. Higher SPM concentrations in bottom waters and inclined isotherms in the epi­limnion are indicative of underflows. However, inflows were 4-1 o·c warmer than bottom waters, suggesting the temperature and SPM pattern in the Waitaki Arm is preserved from earlier cooler underflow events.

In the Ahuriri Arm, inflow temperatures ranged beween 7 and 160C with very high concentrations of SPM (53-127 mg/L). The SPM distribution in the arm suggests this sediment-laden river water interflows along the top of the forming thermoc­line, although the general pattern further downlake is one of increasing SPM concentration in bottom waters indicating a recent period of underflow.

The interaction of water from the two arms in spring represents an intermediate position between winter and summer. Ahuriri water still underflows Waitaki water, although these flows are now weaker than in winter. Ahuriri water also begins to over­flow Waitaki water at the surface. Waitaki water shows up at intermediate depths as an interflow. As the season progresses and the thermocline strengthens, Waitaki water is forced progressively deeper, to enter the lake in summer as an under­flow, thus completing a seasonal cycle of interac­tion. This cycle is driven by differences in inflow characteristics between the uncontrolled fluvial sources of the Ahuriri Arm and the more stable sources in the lakes feeding the Waitaki Arm.

SEDIMENTATION

Surface sediment texture In Lake Benmore, limnic sediments have been deposited as a drape over the former tussock­covered hillslopes, river terraces, and riverbeds. At the head of Waitaki Arm, the Tekapo, Pukaki, and Ohau Rivers have deposited silty sand close to the river mouths (Fig. 5). From the rivers, sediments grade downlake into sandy silt, silt, mud, and finally clay off Falstone Creek 5 km downlake. The per­centage clay fraction continues to increase until approximately 10 km downlake where the clay content stabilises at 85-90% in the lower 15 km of

[:/i:J Silty Sand

J:.):·l Sandy Silt

II? I silt

~Mud

Gclay

b~~Sandy Mud

o 1 2 3 km

Fig. 5 Lake Benmore surface sediment distribution. Dots mark sample positions.

lake behind the dam (Fig. 6). The Ahuriri Arm shows a broadly similar pattern, grading from silt to mud off the Ahuriri River mouth ana clay mid­way through the.Ahuriri Gorge. The only exception to this trend is two pockets of sandy mud (Fig. 5). One against the southern shore of the Ahuriri Arm is probably associated with outflow from creeks, a second in the Ahuriri Gorge probably has a sand source in the nearby steep subaerial slopes.

Fig. 6 (opposite) Percentage clay in surface sediments, Lake Benmore. L1030, L1042 refer to core sample positions.

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Fig. 7 Cores from Lake Benmore (Ll030 left, Ll042 right; station positions shown in Fig. 6). Pale limnic clays overlay prefill soils.

Sediment thickness All surface samples were planned to be collected by Phlager corer. However, after numerous attempts at different sites around the lake, only two cores were retrieved (Fig. 6, 7). At most sites, the limnic sediment cover was too thin to be retained in the corer. The corer frequently returned to the surface with a badly damaged cutter, suggesting it had hit rock, although mud around the head of the cutter was indicative of an overlying veneer of mud. At station L1 042, 4 km behind the dam, a 52 em core

was recovered with 6 em of clay overlying prelake silty soil. The clay was unstratified. A similar sequence with 1.5 em of overlying clay was recovered at the outlet of the Ahuriri Gorge.

The Lafond Dietz grab takes a relatively undis­turbed sample 5-7 em thick from the lake floor. Most samples from Lake Benmore were very stiff and the stratigraphy was well preserved in the grab. In 17 out of 43 samples, the grab penetrated through the limnic mud to sample the underlying soil, river sand, or gravel. Immediately behind the dam, limnic sediments are 5-6 em thick. Soil was not recovered in the gorges, suggesting sediment cover may be greater than 5-7 em whereas beyond the gorges, in the main lake, sediments thin to 3-5 em. Off the rivers at the head of the Waitaki Arm, in depths out to 10-20 m, the grab failed to penetrate through the limnic sediments. The Ahuriri Arm shows a similar pattern with approximately 5 em of sediment cover, thickening westward up the delta slope.

Sediment thickness on the deltas at the head of the main lake is difficult to evaluate. Bathymetric surveys in 1978 (Fig. 1) show the prefill river chan­nels have not been infilled, but rather draped with a veneer of sediment. When flooded in 1965, the Pukaki and Tekapo Rivers were controlled but still carried all of the outflow from their lakes. Since 1979, much of this outflow has been diverted into the Ohau River. The increased flows down the Ohau have scoured gravel over 20 km of alluvial river channel, thus accelerating growth of the Ohau delta. Surveyed sections show the outer face of the delta to be prograding at about 200 mjyear and depositing approximately 660 000 m3jyear between 1979 and 1981 (S. M. Thompson pers. comm., Ministry of Works and Development, Christ­church). These changes are exceptional, and accel­erated deposition will cease with the commissioning of Ohau B and C power station. Over the same period, low flow in the Tekapo and }>ukaki Rivers has probably reduced deposition on these deltas to negligible volumes.

Shoreline characteristics and postfill alterations The unaltered slopes surrounding the lake were mapped in the field. Three slope groups and five material classes were recognised (Table 1). The slope categories were later confirmed from NZMS I topographic maps.

The sparsely vegetated land of the Mackenzie Country makes up about one-half of the shoreline length, with mixed soil-covered and rocky slopes contributing another 25% (Fig. 8). The remaining shoreline is bedrock (13%) and fluvial sand and gravel (13%). Almost all the riverbed sand and gravel is contained in the headwater rivers in the

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Table 1 Lake Benmore shore-line characteristics (km). Sand & gravel

Bare soil/ Mixed soil/ River Slope vegetated rock Rock Terraces bed Total

Fig. 8 Lake Benmore, steep rocky shoreline with thin soil and vegetation cover, little modified by limnic processes.

Fig. 9 Lake Benmore low cliffs cut into low-angled, well vege­tated fan deposits with a sand/ gravel beach forming at the base.

Steep ( > 30°) Moderate (15-30°) Low ( < 15°) Total:

Waitaki and Ahuriri Arms; only minor contribu­tions come from side streams around the rest of the shore. Most of the low-angle vegetated slopes are confined to the Waitaki and Ahuriri Arms whereas the two gorges are moderate to steeply sloping and contain most of the rocky shore and mixed shoreline. In most areas, the overburden is thin, and scree slopes are common on the steep faces.

7.5 24.7 11.2 7.4 40.8

35.2 16.0 5.0 56.2

16.5 9.5 26.0

59.2 30.7 16.2 7.4 9.5 123.0

Approximately 42% of the shoreline has been modified by limnic processes that include wave cutting of cliffs (30%) and beach formation (12%) (Fig. 9). Cliffs range in height up to 4 m, with most about 1-2 m. Higher cliffs are found on the steep shores which are naturally unstable prior to under­cutting at the toe. Erosion at the toe of steep slopes regenerates screes and triggers new rotational slumps and slides which extend upslope beyond the

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94 New Zealand Journal of Geology and Geophysics, 1986, Vol. 29

normal reach of cliffing. Cliff erosion is still active. Debris cones are at the"base of many cliffs and the sediment is reworked during times of high lake level. In places, the soil cover has been completely removed and the underlying bedrock exposed. On low to moderate slopes, beaches have formed at the foot of the cliff whereas, on steeper slopes, sedi­ment has been deposited in deep water.

Terraces in the Waitaki and Ahuriri Arms have been reactivated by erosion at their bases, with sediment accumulating in a narrow beach at the foot of the cliff (Fig. 10). Terraces are. typically 2-l 0 m high and play an important role in supplying sediment to the largest beaches around the lake. There are few other beaches around the lake and these form only where there is an adequate sedi­ment supply, near the mouths of streams, at the foot of cliffs cut in low-slope shores, or in re-entrants downdrift from erosional sections of shorelines. Nearly all beaches are mixed sand gravel, reflecting the texture of source sediment in the terraces, cliffs, and streams, rather than any size selective trans­port in the littoral zone.

Two types of shoreline have remained substan­tially unaltered by formation of the lake - bare rock exposed in the gorges and well-vegetated very low slopes in the main lake and Ahuriri Arm. On low slopes, wave energy is dissipated over a wide zone and appears to be insufficient to erode the vegetated surface. Similar stable slopes, but vege­tated by aquatic plants, have been noted around the shores of Lakes Manapouri and Te Anau (Johnson 1972a, b). The stability of these slopes around Lake Benmore for the last 18 years suggests they will remain vegetated.

Fig. 10 Lake Benmore, rejuven­ated river terraces on northwest shore of Waitaki Arm. A mixed sand/gravel beach has formed at the toe of the cliff by undercutting and cliff collapse.

Detailed maps of shoreline characteristics and postfill alterations have been compiled and are available from the authors.

SEDIMENT BUDGET

In artificial lakes such as Benmore, a critical engi­neering problem is the prediction of the rate of sediment infilling, the nature of the sediment, it's sources, and areas of deposition. In Lake Benmore, we have put together a sediment budget to answer some of these questions.

Sediment enters the lake from two main sources, rivers and the shoreline. Once in the lake, the river bedload is usually deposited close to the point of entry, whereas the suspended load may remain in suspension for long periods and either J:>e deposited in the calmer deeper waters of the lake or be drawn off at the outlet and continue downstream to be lost to the lake. The trapping efficiency of the lake is a measure of this volume of sediment lost from the budget.

From the map of shoreline alterations, volumes of sediment eroded from the shoreline since 1965 have been estimated. Cliffs around the lake were assumed to be vertical. From the cliff height, slope of the shore, and percentage of shoreline affected, an estimated 211 300 t of sediment have been eroded (Table 2). Even though some of this sedi­ment has remained in beaches, we assumed it all contributes toward lake infilling. Mean annual input is 11 750 t but presumably erosion was highest immediately after filling and will continue to decrease as the shoreline stabilises.

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fluvial inputs are dominated by the headwater rivers. Side streams have extremely low flows, small catchments, and presumably low sediment inputs. The Ahuriri River has been gauged and specific suspended sediment yields calculated to be 43 000 t/year (Griffiths 1981). Bedload has been estimated at 10% of suspended load (Griffiths 1981). Sedi­ment volumes entering the main lake are difficult to determine. Outflow from the three headwater lakes feeding the Tekapo, Pukaki, and Ohau Rivers are well documented (Ministry of Works and Development 1978), but little is known ofthe sedi­ment loadings. However, suspended sediment loads in the three rivers probably vary little from con­centrations in their respective headwater lakes. The long residence time of these lakes (412-880 days, Irwin & Pickrilll983) means that (1) all ofthe bed­load and much of the suspended load settles out, and (2) seasonal and shorter period fluctuations in sediment concentration are absorbed. In Lake Tek­apo, the seasonal range of SPM concentrations is 6-10 mg/L (Irwin & Pickrill 1982). No suspended sediment data are available for Lakes Pukaki and Ohau. However, comparisons of secchi disc measurements from the three lakes suggest con­centrations in Pukaki are about twice those in Tek­apo whereas in Ohau they are about one-half the Tekapo levels. Estimates of suspended load enter­ing the head of Lake Benmore have been calculated based on these figures. The Pukaki River, with both flows and suspended sediment concentrations dou-

Table 2 Lake Benmore sediment budget. FLUVIAL INPUT:

Flow SPM (mgJL) (m3/s) in headwater

lakes

Tekapo River 79 6-10 Pukaki River 132 15-30 Ohau River 79 6-10 Ahuriri River 21 Others 12 Total: 323

SHORELINE EROSION: Total since filling = 211 300 t Annual input = II 750 t/year

ANNUAL SEDIMENT BUDGET: (tjyear 000)

TOTAL

SPM at 70% trap efficiency = 108-166 Bedload = 9-15 Shoreline erosion = 12

This study = 129-193 Average deposition = 1.4-2.0 (mmjyear)

95

ble any other source, contributes about 62 000-125 000 t/year of the total suspended load, with a further 15 000-25 000 tjyear from the Tekapo and 10 000-20 000 tjyear from the Ohau catchment.

fluvial bedload entering the Waitaki Arm origi­nates from channel erosion in the short reaches of the Tekapo, Pukaki, and Ohau Basins between Benmore and the upstream lakes. Under normal flow conditions this probably involves small volumes of sediment, assumed to be less than 2.5% of the suspended load. When flow was diverted down the Ohau River for two years, 660 000 m3jyear of scoured channel gravel was deposited on the delta. These conditions were exceptional and not representative of loads during the preceding 16 years.

The sparse data base permits only a general esti­mate of the sediment volumes, the relative impor­tance of different sediment sources, and the life of the lake. This budget suggests 175 000-264 000 t of sediment enter the lake annually (Table 2) of which fluvial suspended load probably makes up more than 85%, bedload 7%, and shoreline erosion 8%. Just downstream from Benmore, Jowett (pers. comm.) estimated deposition behind Waitaki dam to yield 90 t/km2 a year for the period 1934-1959. This has been used to determine a second estimate for the sediment budget, approximately double that derived from our studies (Table 2).

All bedload sediment is deposited in Lake Benmore. Trapping efficiency of the suspended

SPM yield Total SPM Total bedload (t/km2 input input

per year) (t/year 000) (tfyear 000)

15-25 } 62-125 2.1-8.5 10-20

98 43 4.3 98 24 2.4

154-237 8.8-15.2

Estimated sediment thickness since filling = 25-36 mm

Jowett (pers. comm.) = 302 = 3.2 mmjyear

=58 mm

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96 New Zealand Journal of Geology and Geophysics, 1986, Vol. 29

sediment is probably about 70% (Jowett pers. comm.), leaving 30% to carry on downstream. Assuming sediment settles evenly over the lake floor, inputs from the budgets would deposit 2.5-5.8 em since filling in 1965. The coring and grab survey showed the sediment cover to be within this range over most of the lake, suggesting net depo­sition predicted from the budgets is similar to that on the lake floor. In the 18 years since filling, sedi­mentation has resulted in a 0.09-0.29% loss in stor­age capacity, giving a projected reservoir life of 6200-18 500 years. Clearly there will be no signifi­cant loss of storage capacity during the life of the hydroelectric scheme and siltation is unlikely to be a problem in the future.

DISCUSSION

Because Lake Benmore has been New Zealand's largest artificial storage reservoir for 18 years, it is useful to compare the circulation and sediment­ation with that in the natural lakes upstream. As in the natural lakes, circulation and ensuing sedi­mentation in both arms of Benmore is largely con­trolled by the dominant headwater inflows. In the Ahuriri Arm, the seasonal cycle of lake/river inter­actions is identical to that found in Lakes Waka­tipu and Tekapo. In the Waitaki Arm, lake/river interactions are weaker, because seasonal temper­ature differences between river and receiving lake waters are much smaller. Benmore has a much shorter residence time, and consequently the seasonal temperature range is greater than in the natural lakes, despite being much lower in altitude. Interaction between water in the two arms of Lake Benmore is probably typical of any situation where two inflows have different temperature ranges. The water more responsive to seasonal temperature changes will underflow in winter and overflow in summer.

As in the natural lakes, deposition of sediment is dominated by the headwater inflows with sedi­ment thickness and grain size decreasing downlake from the sources. However, because the SPM is filtered through the upstream lakes, the basin sedi­ments in Lake Benmore are finer. For instance, typical basin sediments from Lake Tekapo are made up of 44% clay and 66% silt; by comparison, most basin sediments in Benmore have more than 90% clay. Sediments in the natural lakes are varved. The long residence time in these lakes, dominance of underflows, and outlets at their distal end allows the silt fraction of the SPM to settle out forming the light-coloured summer half of the varve. Any seasonality in SPM size and concentration that might generate varves in downstream lakes is

therefore removed from the outflowing water which is predominantly clay rich SPM, leading to depo­sition of unstratified massive clay beds in Lake Benmore.

In the natural lakes, the bed morphology con­trols the direction and distance travelled by under­flows and thereby controls the thickness of sediment deposited in different parts of the lake (Irwin & Pickrill1982; Pickrill & Irwin 1983). In Lakes Tek­apo and Pukaki, underflows are ponded at the dis­tal end of basins and higher sedimentation rates result. In Benmore, the drowned river courses channel underflows to the dam face where sedi­ments are thicker than further uplake.

Despite large inflows to Lake Benmore, sedi­mentation rates are much lower than in the lakes upstream. Basinal sedimentation rates in Benmore are approximately 3 mm/year compared with 10 mm/year in Tekapo and 14 mmfyear in Pukaki (Ditchbum & McCabe 1977; Pickrill & Irwin 1983). In the upstream lakes, the silty summer part of the varve couplet accounts for about 70-80% of the total basin sedimentation. Most of this silty sedi­ment is removed from the water column and is unavailable to downstream lakes. The remaining clay fraction can stay in suspension for long periods without settling. The clay-rich SPM and short resi­dence time in Lake Benmore produce a much lower trapping efficiency and lower sedimentation rate than in the upstream lakes. The efficiency of Lake Benmore as a sediment trap is also a function of the circulation. The bulk residence time is only a crude measure of the throughput of water in the lake. Water is drawn off from near the surface and, depending on lake/river and arm/arm interactions, outflow may draw off relatively old SPM barren water being replenished at depth by underflows, or relatively new SPM rich/barren water overflowing at the surface. Similarly the water could originate from either the Waitaki or Ahuriri Arm.

In the natural lakes, deltas occupyapproximately one-third of the lake area (Pic krill & '1rwin 1983). Postdepositional slumping and sliding of deltaic sediment plays a major role in redistributing sedi­ment downlake and presents a potential hazard to downlake structures (Irwin & Pickrill 1983). The paucity of bedload entering the Waitaki Arm inhibits the formation of an extensive delta, post­depositional movement is not an important pro­cess, and there are no potential hazards.

ACKNOWLEDGMENTS We would like to thank W. deL. Main for assistance with fieldwork and E. Arron and J. Mitchell for analyses ofthe data. The manuscript was improved by useful comments from L. Cart~¥ and R. Heath. The figures were drafted by C. Heaton and text typed by G. Marsden.

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