Performance of a Major River Regulation Resource System Under Design Conditions
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Optimal Allocation of Water Resources (Proceedings of the Ivxeter Symposium, July 1982). lAHSPubl.no. 135. Performance of a major river regulation resource system under design conditions R, C. GOODHEW Severn-Trent Water Authority, Malvern, Worcestershire, UK ABSTRACT Rarely is a major innovation in water control tested beyond its design capacity during the first decade of operation. The drought of 1975-1976 provided such a test for the water resource system which depends on the regulated flow of the River Severn in Wales and England. Prior operating experience was invaluable in conserving resources and in reducing waste. Analysis shows that, whilst the principle of river regulation is sound on this river, care is needed in operation and in future design to minimize losses. Since 1976 improvements have been made to the hydrological design to avoid some tactical problems. The system is being enhanced with underground regulating storage. Wise husbandry of the asset depends on a basin wide, integrated approach to control. INTRODUCTION Papers have previously been presented which describe: (a) promotion of the Severn scheme and structural design of Clywedog dam (Fordham et al., 1970); (b) operating problems encountered in regulating the River Severn in the early 1970's (Kitson, 1973); (c) place of the River Severn in the overall resource strategy of Severn-Trent Water Authority (Sharp, 1976); (d) forecasting techniques used in the Severn basin during the drought (Bailey & Goodhew, 1978); (e) perspective on UK river regulation losses (Jackson & Bailey, 1979), and (f) current developments in flow forecasting at Severn-Trent Water Authority (Jones, 1980; Manley et al., 1980). This paper seeks to establish significant parameters for efficient regulation of the River Severn during a drought. Features of the system are identified which merit careful consideration in the promotion of similar schemes. DESIGN AND USE In the late 1950's a consortium of water engineers from various authorities jointly sponsored in the basin of the River Severn the first large scale regulated river resource scheme in the UK. It was designed to meet increasing demands for potable supplies during the next twenty years. 337 338 R.C.Goodhew Design data Abstractions Gross abstraction required was 525 Ml day-* (Risbridger, 1961). This was reduced to a net rate of 336 Ml day-1 upstream of the Control Point after allowing for effluent returns (CRJA, 1963) . Summer peak demands were also considered. River flows The longest flow record was at Bewdley (1921). This included a minimum flow of 500 Ml day- in 1949. In I960 a minimum acceptable flow of 727 Ml day- had been proposed to serve the needs of river users (MoHaLG, I960). Later calculations showed that an equivalent "acceptable" flow of 1140 Ml day-1 would occur at the Gloucester tidal limit if: (a) a minimum maintained flow (MMF) of 727 Ml day- was supported at a Bewdley Control Point; (b) river abstraction rates did not exceed design values above or below the Control Point; (c) natural river flows did not fall significantly lower than 1949 rates at Bewdley or Gloucester. These assumptions implied that a "balance" of natural flows should enter the Severn above and below Bewdley to ensure adequate river conditions downstream of the Control Point. Storage The 1949 Bewdley flow hydrograph (Fig.l) was adopted to calculate the size of support storage to maintain 727 Ml day during net upstream abstractions of 330 Ml day- . 10% was added for operating "losses" to give a total reservoir size of 50 000 Ml (50 x 106m3). This volume was notionally allocated as 65% for abstractors, 30% for river interests and 5% for flood alleviation (Hill & Sheppard, 1962). The project was financed on this basis and Clywedog dam was built in 1965-1968 to provide the storage needed. FIG.l Flow hydrograph of River Severn at Bewdley, 1949. Performance of a river regulation system 339 The resource system See Table 1 for basic data. River flows The River Severn rises in the mountains of mid Wales and receives a number of upland tributaries above Montford gauging station. These tributaries include the reservoired areas of Llyn Clywedog and Lake Vyrnwy. Downstream of Montford the Severn crosses the lowland Shropshire Plain where it receives steadily flowing streams arising on the Bunter sandstone aquifer. Bewdley Control Point is in the middle reaches where about half of the drainage area is sandstone. Only 3% of the area is reservoired. TABLE 1 River Severn water resource system; (a) river flows standardized to period 1965-1981 (Ml day l) River Drainage Year: May to DWF RMF Annual gauging area ADF 95% October : average station (km2) Exc. ADF 95% rainfall Exc. (mm) Clywedog to Dam* 49. 0 204 lit 123 5.5+ 25 3 i860 Severn to Rbermule 580 1210 150 656 110 65 12 1260 Vyrnwy to Dam* 94. 3 363 29+ 242 16+ 22 3 1910 River Vyrnwy 772 1770 180 1010 140 148 27 1330 Severn to Montford 2025 3700 520 2000 460 341 70 1180 River Tern 852 636 200 438 170 219 92 729 Severn to Bewdley 4325 5340 970 2950 870 966 280 945 River Teme 1480 1600 200 790 160 249 66 845 Severn to Saxon's Lode 6850 8040 1480 4400 1210 1490 422 777 River Avon 2210 1490 340 920 300 253 57 683 Severn to Haw Bridge 9895 9660 1820 5400 1540 1830 490 776 DWF Average annual minimum weekly flow (naturalized); RMF Minimum recorded weekly flow (naturalized). * Naturalized flows 1968-1981. t Approximate. 340 R-C.Goodhew TABLE 1 continued (b) Supporting storage Reservoir Total Regulation Commsd. Maximum Regulation capacity capacity release yield (Ml) (Ml) (Ml day'1) (Ml day'1) Llyn Clywedog 49 900 49 900 1968 500 500* Lake Vyrnwy 59 700 6 OOOf 1890 405 25% Shropshire groundwater under development from 1985 225% * 100 day season at maximum release. f Maximum § At average release. IT Full development. (c) River Abstractions (Ml day x - as applicable in 1976) Intake Function Abstractions : Effluent returns: Gross Net Severn3 Elsewhere Llandinam PWS Newtown 11 2 gu Penarth Navigation/AWS canal 12c 10e 2U She1 ton PWS Shrewsbury 39 8 31u z Ironbridge EPG cooling 110 32 78z Hampton Loade PWS Black Country 182 182 410- 105L Trimpley PWS Birmingham 114 114 0 91* (many) spray irrigation 40e 40e 0 Total supported net abstraction upstream of Control Point 336 42uy 196z Barbourne PWS Worcester 42 8 34 Upton PWS Coventry 100 20 80 Mythe PWS Glos'shire 109 65P 44 43l Gloucester Navigation/PWS canal 680° n/a 0 n/a Purtonx PWS Bristol 168x 168P 0 1341 (many) spray irrigation 25e 25e 0 Total supported net abstraction upstream of tidal limit 48Î 241c n/a AWS Agricultural water supply; EPG Electricity power generation; PWS Public water supply a above tidal limit; b below tidal limit; c Transport Act abstrac tion unsupported by storage; d downstream of Control Point; e esti mated during drought; f allows for effluents from abstractions above Control Point; g from river gravels; p partly unsupported by storage; t to River Trent; u upstream of Control Point; x from canal; y excluding power station; z typical rates on full load. Performance of a river regulation system 341 Downstream of Bewdley the Severn is joined by two large tributaries: the River Teme which drains a steep, upland area to the west and the River Avon which drains a lowland, clay basin to the east. The Severn discharges to the sea at Gloucester where tidal effects inhibit reliable flow measurement. Reservoired area to Gloucester is 1.5%. Flow measurement at the Bewdley Control Point depends on a rock bar for rating stability and on a separate current meter rated section for velocity gauging. River levels are continuously recorded and are available in near real time at the regulation control office via telephone and satellite links. Artificial controls From 1 May each year all storage in Llyn Clywedog is available to regulate river flow. Changes of release take 3.5 to 4 days to reach the Control Point. At Lake Vyrnwy only compensation water is available for regulation; changes take three days to reach Bewdley. There are 10 main abstractions from the River Severn (Fig.2). Most are for public water supply, two feed canals and one is at an electricity power station. The water supplies serve about six million people from mid Wales through the West Midlands to Bristol. Some sewage effluent from these areas returns to the Severn for re-use. SHROPSHIRE GROUNDWATER WOLVERHA.MPTP"! i 1 Dolwen SOUTH ST.VRTORDSiilR 2 Abermule 3 Caerhowell 4 Llandrinio •MBITf-MINGf AH| 5 Melverley 6 Montford 7 Coalport riWQSC 1STERJ 8 Quatford 9 Highley io Saxon's Lode SSÙJ cov 2iTt *l 11 Haw Bridge A Llandinam B Penarth C Shelton D Ironbridge E Barbourne F Gloucester frPISTOIj FIG. 2 River Severn water resource system (STWA, 1981). 342 R.C.Goodhew Design capacity Prior to the drought the resource system was assumed to be able to provide collateral support for many smaller local sources. "If our source fails the Severn will see us through" was often heard in the early 1970's. Operating experience The reason for this attitude was that Llyn Clywedog was little used until 1975. Baseflows did not fall low because the sandstone aquifer started the 1970's well charged from above average rainfall. Experience in setting regulation releases was gained by assuming a temporary higher MMF at Bewdley. This showed a need for better hydrometric coverage to detect enhanced runoff and for better rainfall forecasts. Net abstractions were forecast from daily predictions provided a week in advance. The main operating problems related to unexplained diurnal flow variations, to very rapid flow recessions in hot weather and to the inability of abstractors to predict accurately.