Jïwfriofogy of Natural and Manmade Lakes (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 206,1991.

The hydrological studies for the Avalon Lakes scheme

A. T. NEWMAN National Rivers Authority, Wessex Region, UK P. J. HAWKER Sir William Halcrow & Partners, UK S. M. POSTLE Global Atmosphere Division, Department of the Environment, UK ABSTRACT The Counties of Somerset and Avon in south-west England contain 635 km2 of land below the level of the highest tides in the adjacent Bristol Channel, but protected from them. Some 13 km2 of these wetlands are being excavated of peat for horticultural use. The residual depressions of the land are in coherent blocks of up to 3m deep, and close to a river system with available water resources for a pumped storage scheme. The Avalon Lakes project was perceived as an opportunity to harness these resources for public water supplies, by using a series of peat excavations surrounded by clay bunds. The bunds would be constructed specifically in order to transform the former peat workings into shallow lakes. Hydrological examination of the yield of such a scheme was complicated by a number of factors peculiar to the site. Principal amongst these was the nature of land management in the wetlands, which relies upon numerous river offtakes for distribution of water between the fields of the surrounding valley. Other factors were the water retention characteristics of the underlying clays, and the need for rigorous attention to environmental constraints which governed reservoir area and storage potential. Investigations of the feasibility of the scheme included pilot scale experiments using on-site lagoons to test both leakage rates and water quality. However, the variability of acquired data left a great onus on the hydrologist to devise a robust model to predict the yield of the scheme within acceptable limits of confidence. The source of the water for the reservoirs was the River Brue, where reliable streamflow gauging was confined to the upper reaches unaffected by diversion into the wetlands. The construction of a streamflow time series for the intake site well within the wetland area had

197 A. T. Newman et al. 198

thus to recognise the large, unmeasured consumptive use within much of this area. Determination of the proportion of flow available for abstraction at the intake then had to take into account a variety of environmental and water quality considerations. A flow series representing the volume of water available for abstraction was then applied to the various storage possibilities, in order to estimate the potential yield from the lakes to supply over the critical storage period. The analysis took into account anticipated leakage and evaporation losses, including allowance for variation in the former in proportion to the depth of stored water relative to surrounding groundwater levels. Using practical assumptions, together with measured values for a range of hydrological variables, it was possible to demonstrate the limitations of the proposed scheme or the purpose of confident evaluation of the economics of the project.

BACKGROUND

This paper describes hydrological studies carried out to assess the reliable yield from a proposed pumped storage scheme in the South West of England. The scheme was known as the Avalon Lakes Project and was to make use of former peat workings, by building low bunds around them to form a series of some five shallow reservoirs. These reservoirs were to be generally rectangular and 30-40 ha in extent. They were to be used for public water supply and replenished by pumping from the River Brue some 2 km to the north of the lakes area (see Figure 1). The hydrological studies formed part of a wider investigation into the overall feasibility of the scheme carried out for the then Wessex Water Authority (WWA) by Sir William Halcrow & Partners Ltd (Halcrow) in 1986-88. The Avalon scheme has since been deleted from Wessex Water's capital programme due to its cost. It is now likely that reclamation of the former peat workings will take a different form. As one would expect with peatland areas, the local ecology is unusual and therefore of great interest. In addition, numerous archaeological finds in the course of peat removal catalogue man's activities in the area when the peat was being formed, in the period from <-. 300 BC to 400 AD. The Avalon Lakes project area was thus of very high intrinsic environmental value. Yield assessments deduced during the hydrological studies for Avalon Lakes were based upon a synthesised 1 in 50 years drought flow sequence for the River Brue at the intake site. Development of the sequence was complicated by the fact that the nearest gauging stations for which reliable flow records were available are a considerable distance upstream. Furthermore, in summer considerable quantities of water are diverted out of the river into an extensive series of open channels to sustain the water table beneath farmland 199 The hydrvlogical studies for the Avalon Lakes scheme in the intervening catchment. Environmental and other factors governing operation of the river abstraction, and of the lakes themselves were taken into account. Storage characteristics such as available volume, abstraction rate to supply and leakage and evaporation losses were also taken into account. A micro-computer based simulation model developed by Halcrow for water resource scheme evaluations was used for yield assessment.

THE RIVER BRUE

Understanding the river system was fundamental to developing flow sequences for the proposed intake site. The River Brue rises at the western edge of Salisbury Plain and flows westwards between the Mendip Hills to the North and the Polden Hills to the South, crossing the Somerset Moors before flowing out to sea in Bridgwater Bay. The catchment area upstream of the proposed intake site amounts to some 300 km2; this area includes the principal tributary, the River Sheppey, which rises at the southern edge of the Mendips and joins the Brue immediately upstream of the intake (see Figure 1). The catchment is predominantly pastoral in character and consists mostly of grassland supporting dairy and beef cattle. The lower Brue basin around the project area is low lying, flat and very prone to winter flooding. Efforts by man over the centuries to manage the area to his greater advantage have resulted in an intricate system of interconnecting channels, known locally as

FIG. 1 Location map. A. T. Newman et al. 200

rhynes. These are managed for effective drainage in winter, and for retention of high water tables to stimulate grass growth on the pastures in summer. They also act as 'wet fences' in summer to deter cattle from straying between fields. The latter two objectives are achieved by closing sluice gates to pen up the main river between April and November, so that it has sufficient hydraulic command to feed water into the rhynes system at need. Significant quantities of water are diverted out of the river in this way, so that actual river flows at the chosen intake site are difficult to determine. On site measurement is complicated because the relationship there between river stage and discharge is not constant. Neither are predictions based on records from gauging stations further up the catchment straightforward, because of the numerous diversions into the rhynes system which occur over the intervening length of river and which cannot be accurately measured.

APPROACH TO HYDROLOGICAL STUDIES

The scheme analysed comprised : ® a river intake on the Brue at Westhay; © phased development of up to five lake areas; o water abstraction from the lakes for public supply. The pattern of flows in the Brue is such that, depending upon the chosen river flow value below which abstraction is assumed to be not permissible in the interests of protecting the river system downstream (prescribed minimum flow), there will be extended periods every summer during which pumping from the intake cannot take place. Thus lake replenishment will not occur and, because of continuing leakage, evaporation and abstraction for public supply, the water levels in the lakes will decline. The drought reliable scheme yield (DRY) is a function of the period during which river abstraction cannot take place, and of the available storage in the lakes. There is no scope to increase summer yield above the DRY by reducing demand over the winter. Accordingly, it was assumed for the purposes of the study that the scheme would operate continuously, producing a steady daily yield to public supply via a new water treatment works. The fundamental approach to the hydrological studies was to : (a) deduce realistic flow sequences for the River Brue at Westhay, particularly for the critical 2% (1 in 50 year return period) drought ; (b) identify and apply suitable abstraction control rules to safeguard the river system downstream and to limit the risk of pumping poor quality water into the lakes; (c) determine the safe sustainable yield from the lakes at each phase of development, using a drought inflow sequence based on (a) and (b) above, and taking into account leakage and evaporation losses. The analysis was based upon the critical period approach, which involves : • selecting the critical drawdown period for the scheme; obtaining a drought volume and appropriate flow pattern for the critical period; 201 The hydrological studies for the Avaion Lakes scheme

simulation of the scheme using a specifically developed mathematical model of the flow sequence for water abstracted at the intake, reservoir storage and the gross lake outflow sequence including public supply, leakage and evaporation. Earlier studies demonstrated that the critical period for the Avaion scheme based on the River Brue is of six months duration.

RIVER FLOWS AT THE INTAKE SITE

No long term flow record exists for the River Brue at the intake site, because gradient and flood risk precluded the construction there of a conventional gauging structure, and because backwater effects from penning structures downstream complicate the stage discharge relationship at the site. Gauging stations with continuous records stretching back to the mid I960's do however exist for the River Brue and the River Sheppey at sites upstream of the intake. These have catchment areas of about 135 km2 and 60 km2 respectively, compared with the 300 km2 estimated catchment area at the proposed intake site. As part of the Avaion Lakes project studies, an electromagnetic flow gauge was installed near the intake. This was completed in early 1987, and it was hoped that some 18 months data (including two summers) could be obtained and compared with the corresponding upstream records. Unfortunately, commissioning problems with the new gauge prevented it from recording reliable flow data until June 1988 ; the available record was thus insufficiently long for a useful comparison to be made with the records from the older gauges. In addition to the new station at the intake, an existing weir some 10km upstream was modified and a flow recorder installed in early 1987. The data gathered were used to estimate seasonal distribution of penning demand. The flow sequences used in the yield assessment were derived as follows : (a) theoretical flows were based on long term flow records for the upstream gauges which were factored down to the intake site in accordance with the catchment area ratios ; (b) a 2% drought six month flow volume was estimated, and a seasonal flow pattern was applied to it; (c) seasonal flow volumes for diversions to the rhynes system and for licensed abstractions were estimated and were subtracted from the theoretical seasonal pattern derived in (b) to obtain the residual 2% drought flow series to be expected at the intake ; (d) a similar procedure was followed to obtain the average year summer month flow sequence. The seasonal flow pattern used for the six month critical drought was that observed at the upstream gauging stations in April - September 1976. Earlier studies had suggested that this drought period had a 2% (1 in 50 year) return probability. By the same logic, the 1977 flow sequence was used for average year estimates. Penning demand - the amount of water diverted from the river to maintain levels in the rhynes system - was by far the most critical aspect of flow estimation at the intake. This has always evaded practical methods of measurement. However, it is believed that the A. T. Newman et al. 202

system fulfils an irrigation function, making up soil moisture deficits arising from évapotranspiration of crops. Critical, factors are thus the peak rate of water use, its seasonal variation, and the area fed. Available data to help evaluate these factors included : • earlier studies of consumptive water use in the Somerset moors ; a few weeks of records from a pumping station feeding a discrete rhynes system on a small section of the moor; the 18-month flow record from the modified weir 10km upstream of the intake ; évapotranspiration records compiled by the British Meteorological office. The first two items provided some information on peak levels of water demand on the rhynes system. However, the range of estimates was very wide. The second two items helped to determine seasonal variation in demand. The area fed by rhynes taking water from the river upstream of the intake was deduced by means of walkover surveys and discussion with drainage board engineers. It was found to be about 125km2, of which 97km2 are peatland areas, and the remainder clay soils. The earlier studies suggested a peak water use rate of 0.3 - 1.2 Ml/d/km2 - that is, up to 150 Ml/d when applied over the whole area. This upper limit in fact exceeded the estimated amount of available water in the river in drought periods. The pumping records suggested a peak water use of 0.3 - 0.6 Ml/d/km2 in the peatland, and about 0.15 Ml/d/km2 in the clay areas. In the event, iterative flow synthesis analyses using alternative peak rates of water use, with seasonal variation deduced from the new gauging station at the modified weir, suggested a peak rate of 0.4 Ml/d/km2 for the peatland areas. Flow regimes synthesised on this basis agreed well with actual records from the modified weir. As a sensitivity check, Westhay flows were also computed on the basis of a 0.6 Ml/d/km2 nominal peak rate of water use in the rhynes system. However, comparison with such reliable records as exist for the site suggest that synthesised flows based on a penning rate of 0.4 Ml/d/km2 more accurately reflect the situation there. Adjustments were made to the synthesised flows for licensed abstractions upstream of Westhay. These adjustments were based upon the purpose for which the licences were granted, and the conditions governing them. It is interesting to compare consumptive water use in the Brue valley with an assessment of water use in the lower Great Stour in Kent, which has a similar system of rhynes. Here spray irrigation is widely practised, and field observations suggested that percolation losses from the rhynes were minimal; it was suggested that water levels in rhynes which were not being kept topped up from the river fell no faster than would be expected from consideration of evaporation. Even so, peak consumptive use in the lower Stour as deduced from an analysis of spray irrigation and stock-watering activity was found to be in the range 9-16 mm/ month, or 0.3 - 0.5 Ml/d/km2 .

INTAKE OPERATION

Having deduced flow hydrographs for Westhay as described above, 203 The hydrological studies for theAvalon Lakes scheme

certain rules were developed and applied to the operation of the intake, in order to determine the amount of water abstracted from the river to replenish the lakes. These rules address : • prescribed minimum flow in the river; • water quality; • pump performance. These are discussed in turn below.

Prescribed Minimum Flow

When river flow at Westhay falls below the prescribed minimum flow (pmf) pumped abstraction would not be permittee. The objectives in specifying such a threshold are to : ® avoid abstracting poor quality water which may result from higher concentrations of effluents at low river flows; ® safeguard the river biota and amenity value downstream of the intake ; ® meet any abstraction licences operating downstream. Comparison of water quality data and flow records showed that at low flows the only significant flow dependant variable was phosphate concentration, which is itself indicative of treated effluent dilution. However, WWA indicated that, in view of the dilution and other benefits accruing during storage of abstracted water in the reservoirs, they would not wish to restrict scheme yield by imposing a pmf to guard against quality problems associated with low flows which would in practice only occur for a few days per year. River biota/amenity considerations are less easily resolved. Fisheries studies revealed that most of the species present (with the exception of Chub) are also commonly found in still waters; thus there was some argument for considering the river downstream of the intake as an elongated pond, to which little inflow would be necessary to sustain the existing species. On the other hand, the ability of the river to sustain its biota may relate closely to the low flows which have prevailed in the past. However, the absolute values of low flows deduced as described above are theoretical and may thus not truly reflect the present situation. In particular, they are very sensitive to assumptions made about the effects of penning. Even if the pond argument were scientifically correct, there is a subjective amenity argument which cautions against setting a pmf close to zero if such a low flow is historically rare for the stretch of river in question. In the event, biota/amenity pmf values were deduced by analysing the synthesised Westhay flows for an average summer six months. A flow duration curve was produced, and that flow identified which was exceeded 90% of the time (SQgp) . This is equivalent to the flow which is theoretically equalled or exceeded on 164 days out of the 183 day (six month) period. The exercise was carried out for Westhay average summer flows based upon an assumed nominal penning rates of 0.4 Ml/d/km2. The resulting value was :

2 SQ90 (0.4 Ml/d/km penning) - 48 Ml/d. A. T. Newman et al. 204

Another environmental consideration taken into account in relation to intake operation was the dependence on winter flooding of a series of meadows downstream of Westhay, frequented by migratory wading birds. Analysis suggested that flows of about 2000 Ml/d are needed to cause such flooding, and it was concluded that the maximum intake capacity of 50 Ml/d would be unlikely seriously to affect the frequency with which it occurs. An investigation of abstraction licences downstream of Westhay showed that, after allowing for return to the river of used water, only about 1 Ml/d is committed at present. Accordingly, a pmf of 49 Ml/d was adopted, based upon synthesised Westhay flows assuming a penning rate of 0.4 Ml/d/km2. To test sensitivity of scheme yield, analyses were also carried out assuming a pmf of 31 Ml/d; this corresponds to the lowest flows at Westhay as synthesised for an average year, and compares well with the lowest measured flow at the new gauge in summer 1988, of 34 Ml/d.

Water Quality

The principal water quality variables which are flow dependant, and can thus be guarded against by intake control rules, are seen as the high turbidity and (seasonally) high nitrate concentrations which are characteristic of the "first flush' off the catchment at the onset of a flood. Accordingly a mechanism in the simulation model compares successive days' river flows and automatically suspends abstraction for 24 hours when the previous day's flow is exceeded by more than 100%. Fluctuations at low flows were not expected to cause quality problems, so this rule was waived for river flows not exceeding 100 Ml/d. No further allowance for suspension of pumping was made to reflect excessive seasonal nitrate concentrations, or pollution incidents. Dilution in storage and, if required, the eventual addition of a denitrification stage to the treatment works would control nitrate levels in water entering public supply. Pollution incidents which would necessitate intake closure were estimated to occur less frequently than once a year.

Pump Performance

To reflect the impracticability of all available water being abstracted at flows in the range (pmf) - (pmf + intake installed capacity), the simulation model applies a pump 'catch factor' of 90% to such available water.

STORAGE CHARACTERISTICS

Available storages in the reservoirs at successive phases of development were estimated as follows : 205 The hydrological studies for theAvalon Lakes scheme

Lake Storage Volumes (Ml)

Phase 1 2

Gross Storage 1222 1770 1797 906 Available Storage (80% of gross) 978 1416 1438 725 Cumulative available storage 978 2394 3852 4557

Losses from storage comprise : evaporation leakage abstraction for public supply. Available (usable) storage was confined to top 80% to maintain an acceptable depth of water on the reservoir floor to suppress colonisation by rooted vegetation. The simulation model automatically accounted for evaporation by reference to the lake stage/area relationship and a set of month-by- month evaporation rates ranging between 0.19 mm/day (December) and 3.79 mm/day (June and July). Leakage rates were based on the results of extensive the geotechnical studies. Leakage for each lake at full supply level was estimated using the appropriate bund lengths and median values of leakage, based on the observed local piezometric heads in association with laboratory and in situ measurements of permeabilities. The values were built into the simulation models using a stage/leakage relationship, to reflect the reduced rate of loss associated with drawdown. Abstraction for public supply equates to the computed scheme yield, and was assumed to be a steady daily amount throughout the year.

SCHEME YIELDS

On the basis of the foregoing analyses, reliable yields from the Avalon Lakes scheme in a 1 in 50 year, six month drought were estimated to be as follows for the various phases of the scheme :

Phase 2% Drought Reliable Yield Range (Ml/d)

1 5-7 2 11-15 3 18-22 4 21-25

In all cases, a penning rate of 0.4 Ml/d/km2 has been assumed. To ensure that the lakes refill over the following winter, an intake pump capacity of 50 Ml/d is required. The above ranges of values reflect uncertainties both in the hydrology relating to available river flow at the intake and in predictions of leakage losses. A. T. Newman et al. 206

CONCLUSION

Estimation of the Avalon Lakes scheme yield was complicated by- uncertainties relating to both river flows and reservoir leakage rates. Through a systematic approach of adopting practical assumptions and testing sensitivity to these, a narrow band of yield predictions were obtained which were satisfactory for use in assessing the feasibility of the overall scheme.