EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 10,193200 (1985)

SEDIMENT YIELD ESTIMATED FROM RESERVOIR SILTATION I N THE OCHIL HILLS, SCOTLAND

JOHN McMANUS AND ROBERT W. DUCK Department of Geology, University of Dundee, Dundee DDl 4HN. Scoiland

Received 28 January 1984 Revised 1 May 1984

ABSTRACT Measurements made on the floors of the temporarilydrained Glenfarg and Glenquey Reservoirs indicate that sediments with wet volumes of 63.94 x lo3 m3 and 1264 x lo3 m3 were deposited in 56 and 73 years respectively. These figures represent 2.5 per cent and 1.1 per cent losses of original storage capacity. When corrected for water, organic, and diatom skeleton contents,and reservoir trap efficiency inorganic sediment yields of at least 31.3 tonnes km-2yr-1 and of 9.0 tonnes km-2yr-1 are suggested. The difference is probably related to contrasts of land use.

KEY WORDS Sediment yield Reservoir siltation Organic content Diatoms

INTRODUCTION

Late in 1982 the water was drained from Glenfarg and Glenquey Reservoirs (Figure I), which are 16 km apart in the central Ochil Hills, Scotland, to permit civil engineering works. As a result the floors of the flooded valleys were exposed for the first time since 1926 and 1909 respectively. Although the two reservoirs differed substantially in their initial configuration, the former is a little-modified flooded valley and the latter depends for its storage capacity largely upon substantial excavations from the valley floor kame terraces, both contain similar sedimentary deposits.

PHYSICAL SITUATION

Glenfarg Reservoir Glenfarg Reservoir (NO 0161 lo), which lies in a northwest to southeast trending faultcontrolled valley

3 km west of Glenfarg village, Tayside Region, has a water surface area of 405 ha and had an initial estimated capacity of 2.586 x lo6 m'. The greatest depth of water, 168 m, is reached when the water surface level rises to 183.6 m above O.D. The natural catchment of 5.82 km2 was increased by 4.72 km2 with the diversion of waters from the Slateford Burn at the time of damming in 1926, and subsequently by a further 12.96 km2 through diversion of run-off from the Water of May in 1972 (Figure 1). Although the bulk of the flow from these two catchments is diverted into the reservoir there is a statutory requirement that compensation waters of at least 2.6 x 1 O3 m3 day- (28 per cent of the mean annual run-off) be permitted to continue through the systems. An average of 6.55 x 103m3day-' of diverted waters enter the reservoir.

01 97-9337/85/020193-O8$0 1 .OO 0 1985 by John Wiley & Sons, Ltd.

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I A

-

/ 0

GLENFARG RES. Sl i te lord Burn

0 GLEhOEVON

YETTS OF MUCKART

I00 Km. - Figure 1 . Glenfarg and Glenquey Reservoirs-location map and catchment areas

The surrounding dissected terrain rises westwards from the 200 m level to reach a peak altitude of 497 m. Arable farming dominates the east and north of the combined catchments, with patches of woodland throughout, whereas further west afforestation dominates the Tonquey Faulds tributary of the Water of May, with grazed moorlands elsewhere on the high ground. The catchment is underlain by northward dipping (10"-20") Lower Devonian basic andesite lavas, with subsidiary beds of volcanic conglomerate. Till, with erratics largely of local origin, blankets the lower ground, but elsewhere only thin soils mask the bedrock.

Glenquey Reservoir Glenquey Reservoir (NN 980027) is situated in a northeast to southwest trending fault-controlled valley on

the border between Tayside and Central Regions, some 3 km northwest of Yetts of Muckart. It occupies an area of 16.7 ha, and had an initial estimated capacity of 1.191 x 106m3. The greatest depth of water, approximately 18 m, is achieved when water surface level rises to 287.8 m above O.D. The 5.58 km2 catchment is principally of rough open moorland, rising to 643m on Whitewisp Hill (Figure 1). It is underlain by northwestward dipping Lower Devonian basic andesites interbedded with thick volcaniclastic rocks. Although till mantles most of the lower slopes, areas of peat are present to the north and south, and alluvial sand and gravel underlie the meandering Glenquey Burn southwest of the reservoir.

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METHODS

A period of freezing weather conditions in December 1982 and January 1983 permitted many traverses of the basin floors to be made. Observation and sampling was therefore not largely confined to re-excavated stream floors and adjacent areas as in the North Esk and Kelly Reservoirs (Love11 et al., 1973; Ledger et al., 1980).

The thickness of sediment on the reservoir floors was determined by digging through the frozen deposits, or by removing polygonal columns of frozen silt to bare either bedrock or a prominent soil layer containing heather or grasses. Although probing with survey rods enabled detection of the soil level at additional sites, the method was unreliable except over bedrock. Where doubt existed concerning probe estimated depths they were checked by excavation. Along the banks of the streams continuous exposures revealed the thicknesses of the deposits. Over 120 confirmed spot depth determinations were made in Glenfarg Reservoir and 35 in Glenquey Reservoir. All measuring sites were plotted on maps at a scale of 1 : 10000.

OBSERVATIONS

Glenfarg Reservoir The principal basin is 1.5 km in length and at most 600m wide. At its western end it divides and extends a

further 300 m due west and 700 m northwards. In the broadest part of the main basin an elongate northwest to southeast trending ridge, which normally forms an island, was present, with a cluster of smaller, lower and normally submerged mounds beyond its eastern end. Along the centre of the main basin the stream followed the course of its predecessor, with the Plains Burn, rising on Blair Hill, forming the major southern tributary.

The uppermost 6 m of the reservoir shores below top water level exhibited extensive horizontal, wave- erosion climets and were devoid of deposited material. Angular cobbles, pebbles, and granules of locally derived rocks covered these slopes.

The reservoir sediment blanket thickened downslope reaching 20-30 cm on the former flood plain area. The deposits also thickened towards the dam, at the foot of which lay a mud pool area stretching to the confluence of the two main streams. Thicknesses of 5 M O cm were measured around the periphery of this zone. In the narrow central zone of the depressions between the island and its satellite mounds and the mainland the sediments normally thickened to 40-50 cm. Elsewhere former hollows showed accumulations of 30-40 cm and restricted linear, drainage-parallel zones revealed more than 50 cm of sediment (Figure 2).

The sediments consisted of apparently non-liminated, uniform, dark greenish-grey silts. Occasional bright red chironomid larvae up to 4 cm in length, which created oxidized burrows, provided the only observed textural variations. No other infaunal representatives were recognized. Locally the surface exhibited assemblages of the dead gastropod Potamides jenkinsii.

Dominating the entire exposed surface of the muds were polygonal patterns of cracks, which had apparently originated through desiccation, but had been accentuated by later frost-heave. The dried columns varied in size directly with the depth of sediment, being 10-12cm in diameter in 5cm of sediment, 15-20cm in 20cm of sediment, and up to 35 cm across in 40 cm of sediment. Similar patterns have been recorded from Haweswater (Donovan and Archer, 1975).

Glenyuey Reservoir

The Glenquey basin extends some 600m from the dam and has a maximum width of 200m. Although occupying a flooded valley floor, the basin includes a steep-sided excavation, 200 m long by 50 m wide, cut into the kame terrace deposits along the southeastern shore. The coarse gravels removed from this area were used in construction of the earth fill dam. The westernmost area is essentially a swamped flood plain.

The lowermost ground stretching from the head of the excavation to the foot of the dam is coated with dark grey-green, apparently non-laminated sediments always more than 20 cm in thickness. The former stream bed to the north exhibited sections of silts up to 18 cm in thickness, resting upon coarsely bedded alluvial sands (Figure 3).

At all sites outside the excavated area and away from the stream a prominent soil horizon bearing roots, grasses and heather lay beneath a veneer of reservoir floor silts. The silts thinned up slope and were absent

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Plains Burn 500 m. /.-

2G50 cm sediment; black area, greater than 50 cm sediment

/ 0 500 n -

Figure 3. Isopachyte map of Glenquey Reservoir. Blank area, no sediment deposited; dotted area, &20 cm sediment; diagonal lined area, 20-40 cm sediment

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within 6 m of the upper water level. Within the bare, upper zone gravel ridges and workmens’ shovels were apparently undisturbed despite 73 years of submergence. Polygonal fracture patterns of desiccation followed by refrigeration occurred throughout the basin.

LABORATORY ANALYSES AND RESULTS

More than thirty grain-size analyses were made of reservoir sediments from cores and surface samples, using the pipette method (British Standard 1377, 1977). The sediments are without exception silts or sandy silts (Folk, 1954). By contrast the underlying soils are of sands or silty sands. Median grain sizes of the deposits vary between 9.4 and 17.6pm.

From the measured sediment thcknesses isopachyte maps were constructed for both reservoirs (Figures 2 and 3). In Glenfarg, 15.74 ha had a mean sediment thickness of 0.15 m, 9.63 ha one of 0.30 m and 2-08 ha one of 0.55 m, giving a total wet sediment volume of 63-94 x lo3 m3 in a surface area of 40.5 ha. In Glenquey 4-49 ha had a mean sediment thickness of 0.15 m and 2.36 ha one of 0.25 m, giving a total wet sediment volume of 12.64 x lo3 m3 in a surface area of 16.7 ha.

Laboratory analysis revealed that polygonal monoliths and cores of sediment had average water contents of 68 per cent (16 analyses) and 63 per cent (six analyses) by weight respectively. From a knowledge of the weight loss on drying known volumes of material, it was calculated that one cubic metre of sediment as collected from the reservoirs contained 430 kg of dry material. Thus in Glenfarg a total of 27494.2 tonnes of dry sediment was present, and in Glenquey a total of 5435.2 tonnes. In each case this figure includes sediment and organic materials washed into or produced within the reservoirs. Determination of organic content of the sediment by weight loss on ignition (Ball, 1964) yielded mean figures of 25.34 per cent and 253 per cent for Glenfarg and Glenquey Reservoir respectively.

DISCUSSION

The calculated volume of 63.94 x lo3 m3 of wet sediment retained in Glenfarg Reservoir represents a 25 per cent loss of storage capacity over the total impounding period of 56 years. The loss of capacity in Glenquey Reservoir amounts to 1.1 per cent in a period of 73 years.

It would be incorrect to equate the reduction in reservoir capacities with the sums of material eroded from their catchment areas. In order to assess the sediment yields from the catchments it is necessary to make allowances for factors such as the water and organic contents of the deposits and the trap efficiencies of the reservoirs.

The measured water contents (63 per cent and 68 per cent) are lower than mean values obtained from bed sediments of Loch Tummel(700 per cent from 72 analyses), Loch Earn (724 per cent from five analyses), Loch Marlee (74.9 per cent from eight analyses), and Loch Butterstone (86.4 per cent from eight analyses) recorded by Duck (1982). The reduced water content probably resulted from dryingafter completion of drawdown, and corresponds closely with the 68.4 per cent water content recorded from the North Esk Reservoir by Lovell et al. (1973). The average of 51 per cent water content of sediment cores from the Kelly Reservoir (Ledger et al., 1980) may provide evidence of compaction and dewatering in the thick deposits encountered.

Determination of the readily-oxidizable organic matter content through loss on ignition (Ball, 1964) gives the most reliable readily obtainable estimates (Dean, 1974). The 25 per cent by weight from Glenfarg and Glenquey Reservoirs is similar to the upper ranges of values obtained from Lochs Butterstone, Marlee, and Tummel (Duck, 1982) but is substantially above the 10 per cent by weight quoted for the Kelly Reservoir (Ledger et al., 1980). Since the percentage of organic matter by volume greatly exceeds the percentage by weight, the 10 per cent by volume in the North Esk Reservoir sediments (Lovell et al., 1973) represents no more than 4 per cent by weight, an exceptionally low figure for any other than shoreline areas on the other lochs examined. Taking account of the organic matter content reduces the masses of sediment trapped on the reservoir floors to 20,527.2 tonnes and 4,049.2 tonnes respectively.

Whilst allowances have been made for the soft tissues and organic carbon, may freshwater organisms produce persistent skeletons. Silica-secreting diatoms abound in most lakes and reservoirs. The siliceous

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materials, which are largely grown within the reservoir water, are deposited among introduced sediment particles. Locally diatom frustules may dominate the lake floor sediments so that particles generated and grown within the stored waters may provide a significant contribution to the sediment budget.

Scanning electron microscopy of surface and core sediments from Glenfarg and Glenquey Reservoirs confirms the presence of both pennate and centric diatom skeletons. Visual estimates of the diatom content varied from 10 per cent to 40 per cent, with an average of 20 per cent. This appreciable proportion must be recognized before any estimates of catchment yields based on reservoir sedimentation may be made. Thus corrected figures for the masses of inorganic sediment in Glenfarg and Glenquey Reservoirs are 16,421.8 tonnes and 3,239-4 tonnes respectively.

Although in previously reported studies account has been taken of the water content and the presence of readily oxidizable organic matter in the deposits (Cummins and Potter, 1967; Love11 et al., 1973; Ledger et al., 1980) we are not aware of any previous British reservoir studies in which attention has been paid to the contribution from diatoms.

The ratio of the sediment retained by a reservoir to the total inflow of sediments is its trap efficiency. This is normally estimated from the ratio of reservoir capacity to the mean annual water inflow (Brune, 1953). The initial capacity of Glenfarg Reservoir was 2.586 x lo6 m3 and its annual inflow estimated by Fife Regional Council is 7.615 x lo6 m3, giving a capacity/annual inflow ratio of 0-34. The initial capacity of Glenqucy Reservoir was 1.191 x lo6 m3 and its annual inflow 3.500 x lo6 m3, also coincidentally giving a ratio of 0.34. Using the curves of Brune (1953) the trap efficiency in each case lies between 90 and 97 per cent. Thus up to 10 per cent of the sediment entering the reservoirs may have been lost through the outflows. From this estimate a further maximal correction to the masses of inorganic sediment which have been carried into the reservoirs is required, giving figures of 18,064 tonnes for Glenfarg and 3,563.3 tonnes for Glenquey Reservoir.

On dividing the above mass of sediment which has passed into Glenquey Reservoir (3563-3 tonnes) by the age of the water body (73 years), and then by its catchment area (5.58 km2) minus its water surface area (16.7 ha), an inorganic sediment yield of 9.0 tonnes km-2 yr-' is obtained for the catchment.

For the combined catchments draining into Glenfarg Reservoir, computation of sediment yield is more complex. If all the inorganic sediment accounted for (18,064 tonnes) was derived exclusively from its natural catchment, the inorganic sediment yield, on the above basis, is 59.6 tonnes km-2 yr-'. However, waters diverted from the Slateford Burn and the Water of May also enter the reservoir. Whereas in broad deep rivers there are both vertical and lateral gradients of suspended sediment concentration, both of these boulder- strewn water courses are narrow, relatively shallow, and have steep bed gradients; conditions which encourage high turbulence and homogenization of suspension concentrations. In consequence, it is assumed that the suspended sediments are approximately uniformly distributed throughout the waters. Although the bed levels of the streams fluctuate slightly through time, on average the intakes for the water are situated 0.5 m above the beds. Thus the waters extracted are believed to carry representative proportions of the suspended sediments.

The intake from the Water of May flows directly into Glenfarg Reservoir but that from the Slateford Burn passes through a silt trap from which approximately 5 tonnes of sediment are removed annually. Therefore over the 56 year impounding period%pproximately 280 tonnes of silt has been prevented from entering the reservoir. This increment must be taken into account when calculating sediment yield from the catchments.

If all of the suspended sediment carried by the streams entered the intakes it is possible, taking into account the periods of diversion, to derive an inorganic sediment yield of 26.3 tonnes kmW2 yr-'. This value must, however, be regarded as an absolute minimal estimate of inorganic sediment yield for the combined catchments. Because not all of the waters are diverted into the reservoir it may be assumed that some of the suspended materials continue past the intakes. There is no way of estimating the precise proportion of sediment transported into the reservoir annually. However, since no more than 72 per cent of the waters are diverted it is not unreasonable to suggest that an equivalent maximum percentage of the silt leaves the streams. Thus, rather than the minimal estimate given above, the authors believe that the greater figure of 31-3 tonnes knC2 yr- ', calculated on the assumption that this percentage has been constant throughout the stream diversion periods, should be considered a more realistic minimum estimate of the overall inorganic sediment yield.

The contrast in yields estimated from the Glenfarg (31.3 tonnes kmW2yr-') and Glenquey Reservoir (90 tonnes km-2 yr-') catchments may result from the much greater proportion of land devoted to arable

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farming in the Glenfarg area. The Glenquey Reservoir catchment is entirely used for rough grazing and forestry.

The rates of soil erosion estimated from computation of reservoir siltation elsewhere are varied, ranging from 25 tonnes km-’yr-’ in the Hopes Reservoir of East Lothian (Ledger et al., 1974), 25.4 tonnes km-’yr-’ in the Cropston Reservoir of Leicestershire (Cummins and Potter, 1967), and 25.6 tonnes km-’ yr-’ in the North Esk Reservoir of Midlothian (Love11 et al., 1973) to 41 tonnes km-’yr-’ in the Kelly Reservoir of Strathclyde (Ledger et al., 1980) and 127 tonnes km-’ yr- ’ in the Strines Reservoir of Yorkshire (Young, 1958). Thus the Glenfarg Reservoir catchment yield is similar to the majority of these whereas the Glenquey result of 9.0 tonnes km-’yr-’ is the lowest known to the authors.

The very high yield suggested by Young (I 958) for the Strines Reservoir is the result of volume estimation on the drawndown reservoir. No corrections seem to have been applied for factors such as water and organic content or reservoir trap efficiency. Assuming a 68 per cent water content, a 25 per cent organic content and a reservoir trap efficiency of 90 per cent the corrected catchment yield is the substantially lower figure of 39.4 tonnes km-’yr-’. This adjusted figure conforms closely with the 2 M O tonnes krn-’yr-’ range of yield suggested as characterizing upland catchments in upland Britain (Ledger et al., 1980).

The reservoirs from which these estimates are derived have catchments individually dominated by sedimentary rocks of Precambrian to Triassic age, whereas the catchments of Glenfarg and Glenquey Reservoirs are entirely on volcanic rocks. In all cases the bedrock is thickly mantled with till or sand and gravel. The strong similarity between yields estimated from four reservoirs in very disparate geological situations suggests that the drift may serve as the principal supplier of detritus and that the nature of the underlying bedrock is largely immaterial to the rates of sediment yield.

The estimates of sediment yield derived from reservoirs are mostly substantially lower than those obtained from sediment transport in rivers. Increasing interest in sediment transport in the Scottish rivers has revealed solids losses of 42.4 tonnes km-’yr-’ in Galloway (Kirkby, 1967) and 56.2-169-9 tonnes km-’yr- ’ from several tributaries of the River Clyde (Fleming, 1970) whereas the rivers of Tayside Region have suggested losses of 17.344.9 tonnes km-’yr-’ on the River Almond, 89.2-225.2 tonnes km-2 yr-’ on the River Earn and 439-201.4 tonnes km-’yr-’ on the River Tay (Al-Jabbari et al., 1980).

The lack of conformity of soil erosion estimates derived from rivers and reservoirs has been attributed to an underestimation of the trap efficiency of British reservoirs (Rodda et al., 1976). If the trap efficiency of Glenquey Reservoir had been as low as 50 per cent, the estimated inorganic sediment yield would still have been less than 20 tonnes km-’yr-’, which is at the lower end of the ranges estimated from the rivers. The reservoir figures may also appear low because they are long term averages, whereas most rivers have been examined over a period of no more than a few years. In view of the improbability of gross underestimation of the trap efficiency of the reservoirs the discrepancies may stem from the inexact methods of estimating sediment transport in rivers, and in particular in the currently-used techniques for the direct determination of suspended loads. The indirect methods of load estimation are notoriously difficult to select and results within one order of magnitude of directly estimated loads are normally accepted as of good quality. Hence the authors believe that the methods of estimation of sediment yield from reservoir siltation are far more satisfactory than those used in most rivers.

ACKNOWLEDGEMENTS

We wish to express our thanks to the Water Division of Fife Regional Council for permission to visit the sites and for provision of numerical data on the reservoirs and their inputs. In particular we are indebted to Mr D. W. Jack and Mr J. 0. D. ODonnell. The work was carried out during receipt of financial assistance from N.E.R.C.

REFERENCES

Al-Jabbari, M. H., McManus, J., and Al-Ansari, N. A. 1980. ‘Sediment and solute discharge into the Tay Estuary from the river system’, Proceedings of the Royal Society of Edinburgh, 78B, 15-32.

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Ball, D. F. 1964. ‘Loss-on-ignition as an estimate of organic matter and organic carbon in non-calcareous soils’, Journal ofsoil Science, 15,

British Standard 1977. Methods ofTesting Soilsfor Civil Engineering Purposes, B.S. 1377, British Standards Institution, London. Brune, G. M. 1953. ‘Trap efficiency of reservoirs’. Transactions of the American Geophysical Union, 34, 40741 8. Cummins, W. A. and Potter, H. R. 1967. ‘Rate of sedimentation in Cropston Reservoir, Charnwood Forest, Leicestershire’, Mercian

Dean, W. E. 1974. ‘Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition:

Donovan, R. N. and Archer, R. 1975. ‘Some sedimentological consequences of a fall in the level of Haweswater, Cumbria’, Proceedings of

Duck, R. W. 1982. Physical Sedimentology of Some Lake Systems, Unpublished Ph.D. Thesis, University of Dundee, 359p. Fleming, G. 1970. ‘Sediment balance of the Clyde Estuary’, Journal oft he Hydraulics Division, Proceedings ofthe American Society ofcivil

Folk, R. L. 1954. ‘The distinction between grain size and mineral composition in sedimentary rock nomenclature’, Journal of Geology, 62,

Kirkby, M. J. 1967. ‘Measurement and theory of soil creep’, Journal ofGeology, 75, 359-378. Ledger, D. C., Lovell, J. P. B., and McDonald, A. T. 1974. ‘Sediment yield studies in upland catchment areas in south-east Scotland, Journal

Ledger, D. C., Lovell, J. P. B., and Cuttle, S. P. 1980. ‘Rate of sedimentation in Kelly Reservoir, Strathclyde’, Scottish Journal ofGeology,

Lovell, J. P. B., Ledger, D. C., Davies, 1. M., and Tipper, J. C. 1973. ‘Rate of sedimentation in the North Esk reservoir, Midlothian’, Scottish

Rodda, J. C., Downing, R. A., and Law, F. M. 1976. Systematic Hydrology, Newnes-Butterworth, London. Young, A. 1958. ‘A record of the rate of erosion on Millstone Grit’, Proceedings of the Yorkshire Geological Society, 31, 149-156.

84-92.

Geologist, 2 , 3 1-39.

comparison with other methods’, Journal of Sedimentary Petrology, 44, 242-248.

the Yorkshire Geological Societ y, 40, 547-562.

Engineers, 96, 2219.2230.

344359.

of Applied Ecology, 11, 201-206.

16, 281-285.

Journal of Geology, 9, 5 7 4 1 .