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 a l . , 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 a l . , 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 10 6m 3). 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 gauging station
Clywedog to Dam*
Severn to Rbermule
Vyrnwy to Dam*
River Vyrnwy
Severn to Montford
River Tern Severn
to Bewdley
River Teme Severn
to Saxon's Lode
River Avon Severn
to Haw Bridge
Drainage area (km2)
49.
580
94.
772
2025
852
4325
1480
6850 2210
9895
0
3
Year: ADF
204
1210
363
1770
3700
636
5340
1600
8040 1490
9660
95%
Exc.
lit
150
29+
180
520
200
970
200
1480 340
1820
May to October : ADF
123
656
242
1010
2000
438
2950
790
4400 920
5400
95% Exc.
5.5+
110
16+
140
460
170
870
160
1210 300
1540
DWF
25
65
22
148
341
219
966
249
1490 253
1830
RMF
3
12
3
27
70
92
280
66
422 57
490
Annual average rainfall (mm)
i860
1260
1910
1330
1180
729
945
845
777 683
776
DWF Average annual minimum weekly recorded weekly flow (naturalized). * Naturalized flows 1968-1981. t Approximate.
flow (naturalized); RMF Minimum
340 R-C.Goodhew
TABLE 1 continued
(b) Supporting storage
Reservoir Total capacity (Ml)
Regulation capacity (Ml)
Commsd. Maximum release (Ml day'1)
Regulation yield (Ml day'1)
Llyn Clywedog 49 900 49 900 1968 Lake Vyrnwy 59 700 6 OOOf 1890 Shropshire groundwater under development from 1985
500 405
500* 25%
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 :
Gross Net
Effluent returns: Severn3 Elsewhere
Llandinam Penarth
She1 ton Ironbridge
Hampton Loade Trimpley (many)
PWS Newtown
Navigation/AWS canal
PWS Shrewsbury EPG cooling PWS Black Country PWS Birmingham spray irrigation
11
12c
39 110z
182 114
40e
2
10e
8 32
182 114
40e
gu
2U
31u
78z
410-0 0
Total supported net abstraction upstream of Control Point 336 42uy
Barbourne Upton Mythe Gloucester
Purtonx
(many)
PWS Worcester PWS Coventry
PWS Glos'shire Navigation/PWS canal
PWS Bristol spray irrigation
42 100
109
680° 168x
25e
8 20 65P
n/a 168P
25e
34
80 44
0 0 0
105L
91*
196z
43l
n/a 1341
Total supported net abstraction upstream of tidal limit 48Î 241c n/a
AWS Agricultural water supply; EPG PWS Public water supply
Electricity power generation;
a above tidal limit; b below tidal limit; c Transport Act abstraction unsupported by storage; d downstream of Control Point; e estimated 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 SOUTH ST.VRTORDSiilR
riWQSC
•MBITf-MINGf
1STERJ
SSÙJ cov
AH|
2iTt *l
1
2 3 4 5 6 7 8 9
i o 1 1
Dolwen Abermule C a e r h o w e l l L l a n d r i n i o M e l v e r l e y Mont ford C o a l p o r t Q u a t f o r d H i g h l e y S a x o n ' s Lode Haw B r i d g e
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.
Winter rainfall had been consistently below average in Shropshire during the early 1970's, so groundwater levels had fallen about a metre by 1974. Regulation releases were needed on 12 days in 1974 until wet weather increased natural flows.
THE DROUGHT
Structure of the runoff drought was complex. A severe surface water drought developed in summer 1975. Naturalized flows at Bewdley were the lowest since 1949 and regulation releases were needed on 113 days up to 3 November. The dry winter which followed failed to refill many depleted local sources although Llyn Clywedog filled by early May. More significantly, lowland aquifers were not recharged, so 1976 was predisposed to low baseflows. Another surface water drought occurred in summer 1976 which exceeded many records. River flows fell to minima of less than half previously recorded rates -or ran dry. A summary of frequency data is given in Table 2.
Effects on rivers
Flows reduced very rapidly in June 1976 after a month when normal rainfall disguised the low baseflows. Temperatures exceeded 30°C causing increases in abstraction for domestic and agricultural use. By the end of June releases exceeded 500 Ml day- and reservoir depletion was 2% per dag. As natural flows from the upland areas dwindled and the unusually low baseflows took over, a real threat existed that design conditions were being substantially exceeded.
Abstractions downstream of the Control Point had increased faster than those upstream. This imbalance and the basin-wide flow deficit contributed to very severe conditions in the lower reaches. Attempts to measure flows near Gloucester were accelerated - with mixed success. This development indicated that design assumptions were being exceeded and that Gloucester and the associated supply for Bristol were the most vulnerable to regulated flow deficits.
Performance of a river regulation system 343
Conservation measures
Very large releases continued during July amidst hot, dry weather
whilst procedures were agreed to reduce the MMF at Bewdley to
545 Ml day This came into force in mid August by which time the
consequences of the drought were severe - especially in Wales where
TABLE 2 Severity of 1975-1976 drought in the Severn basin:
(a) Rainfall over drainage area to Control Point
% average (1916-1950) 1975 1976 Design Lowest on year record
24 months to 8/76 16 12
9 6
3
2 years 1 year
(b) Runoff
Recurrence interval Mid Wales
---68 72 61
-73
(years): Upstream from Bewdley
80 55
60
59 51
28
80 87
_ -
80
73 77 57
104 93
--70
59 47
28
79 70
Recurrence interval
Mid Wales
(1921)
(1929) (1921)
(1929)
(1933/4) (1921)
(years): Upstream
from Bewdley
24 months 18 16 10
9
70 40 40
9 8
30
30 45
7 7
15
6 months 5 4 3 2
1 month
70 70 45
2 OOf 2 OOf
100
35* 60*
100*% 2 OOf* 2 OOf* 200f*
Natural runoff to Bewdley (Ml day 1975 1976 1949 1933-1934
4 months 2
1075
975
800 435%
1120 815 750
Flow duration below low flow thresholds (% time): 1975-1976 1949 1949
(Design (Reprocessed data) in 1975)
Less than 1000 Ml day 30
545 Ml day"1 16 28 <1
21
0
344 R.C.Goodhew
TABLE 2(b) continued:
Natural drought flows at
Calendar year April-September June-August
Minimum mean daily
1975: 570 1949 (design data).
Bewdley
natural
: 489
as % of 1975
73 81
100
1949 :
flow at Bewdley (Ml
1976: 280 1949 (reprocessed
flows:
day'1
data)
1976
80 56 45
) :
: 600
Downstream of Bewdley Control Point natural runoff for all durations up to 4 months has a recurrence interval in 1976 of less than once in a hundred years. * Lowest on record. t More than. § About 1 in 30 in 1949 design year. 1Ï Minimum maintained flow demand is 3.5 times that in
1949.
(c) River abstractions
Net June-Sept June-Sept. May-Oct. May-Oct. abstraction 1975 1976 1975 1976
(Ml day-ij (Ml day~J) (% design] (% design)
Upstream from Bewdley Control Point 190 195 49 Downstream from Bewdley Control Point 235 280 75 Total upstream from tidal limit 425 475 63
Gross abstraction (June-Sept. 1976) (Ml day A)
Shelton Ironbridge Power Station Hampton loade Trimpley Irrigation upstream from Control Point
Total upstream from Control Point
Irrigation downstream from Control Point Barbourne
50
79
65
20 2m
121 18 40*
220
25* 28
Performance of a river regulation system 345
TABLE 2(c) continued:
Gross abstraction (June-Sept. 1976) (Ml day )
Upton 76 Mythe 90 Gloucester 332']' Burton (147)1
Total downstream from Control Point 551
* Estimated II Net rate t Into canal § From canal
(d) Support storage requirement (% regulating capacity)
1949 1975 1976
Total 100 36 126* for abstractions 81 28 58* for river flows 19 8 68*
* If flow had been maintained at 727 Ml day all season.
farmers "poached" supplies from the regulated rivers as local sources ceased.
Although water use was cut by a fifth in response to official requests, the reduction in abstraction demand was marginal when compared with the natural flow deficits in rivers. A second Drought Order was promptly approved under emergency powers. This Order abandoned the principle of a maintained flow and substituted a release based on 2% of the remaining storage adjusted for economies in abstraction rates. This radical development illustrates the speed with which conditions can change in a large regulated river resource system.
The drought broke in the south at the end of August. Sporadic rain further north was just sufficient to keep river flows above rates defined by the second Order. Conditions at Gloucester improved slightly and the full effects of the second Order in a month were not seen. Very heavy autumn rains rapidly refilled local sources but did not reach mid Wales so the overdrawn storages of Clywedog and Vyrnwy required local Orders to reduce compensation releases into 1977.
Reservoir releases Total volume released from Llyn Clywedog was 39 OOO Ml during the 121 day regulation season to 15 September 1976. Maximum release was 520 Ml day- . Maximum depletion was to 22% full capacity. Effect of releases on river flows at Bewdley Control Point is seen in Fig.3.
346 R.C.Goodhew
RELEASES FROM
LlYN CLYWEDOG
SfRAY IRRICATiON
iRONBHIDGE
ABSTRACTIONS UPSTREAM
OF SEWDLEY
HAMPTON LOADE 0 '
RiVER FtOW
AT BEWDtEY
A A . GAUGED FLOW
M NATURAL FLOW <•
WilMOUT ABSTRACTIONS
OR SEiEASES
700-
600-
500-
400-
300-
200-
FIG.3 Releases, abstractions and river flows (Ml day ) , River Severn, summer 1976 (STWA, 1977).
HYDR0L0GICAL CONTROL TO REDUCE WASTE
Flow forecasts Control of releases to maintain a steady minimum flow at the Control Point is exacting. Forecasting of low flows at Bewdley is difficult because of four areas of uncertainty.
Meteorological uncertainty Short term changes in weather conditions produce variable runoff response over the 4325 km drainage area. Tributary flows often increase while a change of release is passing from the reservoir to Control Point.
Hydrological uncertainty On some rivers rates of flow recession reduce abruptly as the interflow component tends to zero. The
Performance of a river regulation system 347
change of slope is usually encountered in the 650-900 Ml day flow range at Bewdley when changes of release are becoming established. Better correlations are being sought with groundwater parameters.
Hydrometric uncertainty Four main areas of difficulty occurred in the drought. Firstly, measurement of releases was unreliable from Llyn Clywedog because the two measuring weirs were too close to the dam and are only accurate in the low (compensation) and high (spillage) flow ranges. Secondly, measurements at intermediate sites appeared to indicate a "loss" of water from the channel between reservoir and Control Point. Thirdly, operational flows at the Control Point were uncertain because of weed growth. Fourthly, the urgent measurements needed at Gloucester were hampered by tides, lockages, boats and very low velocities.
Artificial uncertainties Regular and irregular changes in flow during dry weather which could not be attributed to conventional processes complicated the plotting of flow recessions. These changes became more pronounced as natural flows fell. Subsequent analysis revealed a long list of causes which interacted to produce a level hydrograph of considerable complexity (Fig.4).
The causes of artificial fluctuations in the River Severn during summer low flows proved to be (STWA, 1980):
(a) variable diurnal pumping rates by abstractors;
(b) variable pumping rates, purge releases and maintenance
discharges from the 25CO Ml day internal cooling system at
Ironbridge Power Station;
(c) lockages by pleasure boats (lower reaches);
(d) sewage effluent diurnal fluctuations;
(e) sluice operation during land drainage maintenance or amenity
lake fishing;
River Level Feet A.O.D.
59.25 r
8th SEPTEMBER 1975
FIG.4 River level fluctuations at Bewdley during low flows, summer 1975 (STWA, 1977).
348 R.C.Goodhew
(f) spray irrigation diurnal fluctuations. Tributaries are sometimes affected by diurnal transpiration from riparian meadows and photosynthesis effects on water weed buoyancy. Most of these problems are being overcome by better hydrometry, liaison and forecasting techniques. Specific areas of operating experience which helped to reduce over releases during the drought are outlined below.
Low flow hydrometry
Changes in flow rating were spotted by daily flow gauging and inspection of flow controls. Mean daily flows were computed from frequent levels to obtain a good average. Hourly data from abstractors were plotted against level hydrographs to update release effects. Monitoring of levels at intermediate stations helped to distinguish between natural and artificial changes at Bewdley. Lockage effects were avoided by night time current metering.
Control of reservoir releases
Tactics As releases increase a small part of the enhanced flow soaks into the banks until a steady state is reached. These operating "losses" are reduced by minimizing the number of increases in release. Efficient control aims at the steady state with "fine tuning" of plateau release rates as baseflows fall. In practice two modes' of operation occur:
(a) During freshets releases change frequently in response to
forecast flows. The aim is to forecast enhanced recession flows
which permit cutbacks in release (Fig.5). During rainy weather,
excess releases are unavoidable since the rapid flow recessions
arriving from the mountains would otherwise send flows below the
statutory limit at rates of change exceeding 100 Ml day-2".
(b) During well-established dry weather, rain has little effect on
runoff. The system tends to a steady state and Bewdley flows reduce
at about 10 Ml day"2.
Dual source control Pulse releases of 180 Ml day were released from Lake Vyrnwy for four days each month under an 1880 Act. They helped to reduce the drawdown at Clywedog but at the expense of unsteady conditions in the river. Experience of these pulses prior to the drought helped to reduce overlap excesses When dovetailing releases.
Balancing storage Two intakes above Bewdley have bankside reservoirs to maintain supplies during river pollutions. Over lOOO Ml of this storage was made available to permit "secondary" regulation of flows, i.e. the rate of abstraction into the reservoir was varied to trim river flows more precisely. This mitigated the effects of any excess or inadequate releases from primary storage. These bankside reservoirs were invaluable to reduce river flows promptly at the first Drought Order. They also permitted close control of flows entering the lower Severn so that enough water reached the intakes when tributary flows were unreliable and while Clywedog releases were limited by the second Drought Order.
Performance of a river regulation system 349
Regulation Release.
'Is» % Release
Lost. • 200-300 Ml/d per 24 h.
5 0 - M 0 0 % ^ F t o w Def ic i t—< 15%
due to surface rursoff-*| due to forecast boseflow J uncer ta inty . >
when baseflo
FIG.5 An example of excess releases due to flow forecasting uncertainty.
SYSTEM PERFORMANCE
Definitions Failure Conventionally, a system fails when it is subjected to
conditions beyond the design assumptions. This happened in 1976 on the River Severn. The effects on reliability of water supplies were minimal thanks partly to consumption economies but mostly because river water quality remained high. A few salmon died at Gloucester probably from high temperatures, but generally oxygen levels stayed up due to good aeration at weirs. Cool reservoir releases helped fish in the middle reaches and navigation was not hindered. Although the system "failed" theoretically, only locally below Clywedog when compensation water was withheld did the river fail in its normal role to fence in sheep! It is an open question whether a more sustained period of very low flows would have revealed more serious problems.
Performance A better measure of achievement is to compare performance of the system with an unregulated river. On this count the principle of river regulation was vindicated for it provided both a means of cheap transfer of supplies as well as water-based activities for over 250 km where other streams and sources had run dry.
350 R.C.Goodhew
Efficiency This is measured by comparing releases with volumes of river flow which exceed the MMF at the Control Point during releases. In the case of the Severn, allowance has to be made for:
(a) travel time from storage to Bewdley (3 to 4 days),-(b) releases from reservoirs for other purposes; (c) difference between perceived and actual flows; (d) errors in predicted abstractions above Bewdley.
Residual errors in regulation are allocated to either excess releases or operating losses.
Excess releases When reservoir releases are made to maintain the flow but prove to be unnecessary if flows exceed the MMF, these volumes are termed "excess releases" (STWA, 1977).
Operating losses If water is released which does not reach the Control Point - other than by accounted abstractions - it is a "loss" to the resource system when supporting storage is in use. Excess releases, on the other hand, benefit the river and abstractors even if the MMF is exceeded.
Transit losses In 1976 a comprehensive river gauging programme was mounted to establish the extent of losses from the Severn. Analysis showed that over 90% of releases were reaching the Control Point. In many intermediate reaches, however, observed flows were lower than expected. Down valley flow was assumed to be occurring in the gravel swathe around the visible channel. This is the only explanation which reconciles increases in water table during releases (STWA, 1976) with an overall consistency in flows between reservoir, releases and Control Point (Table 3). Suspicions of major transit losses had arisen because of over measurement of releases but
TABLE 3 Computation of transit losses on River Severn in 1976 (Ml day"1;
Gauging site Gauged flows: Changes upstream from gauging site: Severn Trlbs. Ungauged Efflu- Abstr- Net Compd.
tribs. ents actions gain flow
Clywedog GS Dolwen Abermule GS Caerhowel1 Llandrinio Melverley Montford GS Coalport Quatford Highley Bewdley GS
490 536 488* 544 551
680 656* 918 984 727
838
-14 20 20 26
139
139 312
317 319 326
-3 7
10 18
30 35 70 75 80
85
-1 5 5 6 6 7
35
43 43 43
-0
24 24 24 24 24
66 66
228
266
-17
8
11 26
151 157
351 369 214
188
490 507 498 501 516 641 647
841 859 704
678
GS gauging station * Apparent losses
Performance of a river regulation system 351
under measurement of river flows at intermediate sites and at Bewdley.
Drought data In 1975 excess releases and operating losses accounted for 66% of total releases. This was due to intermittent rainfall and to the process of gaining operating experience. Resources were not stretched that year. In 1976 "wastage" was 17.5%, whilst less than 2% was an operating loss. The 15.5% was released due to errors in forecasting, in flow measurement and in abstraction predictions (Fig.6). Procedural improvements outlined below are reckoned to reduce the residual operating allowance to 10%.
Importance of control procedures
If prior experience in operational regulation had not been available during the drought higher losses would have led to restrictions on water supplies in much of the Midlands and fish mortality could have been widespread. As it was, the emptying of used water onto gardens actually decreased the resources by denying re-use of effluent returns to downstream abstractors.
Although the system failed technically, river regulation was viable even under stress and provided a vital complement to less reliable sources. A caution is, however, needed. The larger the regulated area, the more that extremes of rain or drought are reflected in natural flow changes at the Control Point.
Regulation storage attempts to "top up" the cumulative volume to a flow threshold. This volume is highly sensitive to areal rainfall deficits. Draft on storage may not start until summer, but the depletion rate is proportional to the drought-affected drainage area and is relatively insensitive to reservoir inflows. Support storage for river regulation needs to be carefully managed to avoid undue volumes maintaining unnecessarily large flows.
Release timing Operational releases increase slowly. Transit losses are much smaller than occur in pulse type test releases. The regulation controller needs to be sensitive to the balance between regulated river levels and adjacent gravel water tables so that the rise in water table level is not exaggerated by sudden changes in release. When experience has been gained this "bank" storage can be used in a positive way to enable release cutbacks to be made sooner.
CONCLUSIONS
Departure from design assumptions
The resource system did not perform exactly as assumed in design. The drought was much more severe than 1949; the pattern of abstractions was biassed to the lower reaches; the runoff deficit was greatest in the lower reaches. Reprocessing of "design" data (STWA, 1975) has shown that the 1949 flow sequence is notably less severe than the assumed 1 in 50 year recurrence interval. Furthermore, the 1940's winter rainfall deficit was less than the
352 R.C.Goodhew
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Performance of a river regulation system 353
1970's - implying higher groundwater levels in spring 1949. The 1976 river flow support demand far exceeded a 1 in 50 year volume even though abstractions above Bewdley were well down on design rates. This demonstrates the importance of achieving winter refill of the support storages every year.
Improvements implemented
Control rules The 1975/1976 flow sequence has been used to re-assess system control rules. These now incorporate a maximum regulation release of 500 Ml day" and an agreed drought control line to initiate demand or MMF restrictions according to date and storage level (STWA, 1981).
Variable maintained flow The Bewdley MMF has been redefined as 850 Ml day over a five day period. This will reduce some operating problems and will provide an improved level of support for abstractors from the lower reaches.
Vyrnwy water bank Compensation pulses from Lake Vyrnwy are now "banked" and released when needed for regulation support or for flood drawdown. The volume available is about 4000 Ml by mid summer and provides an equivalent yield increase of 25 Ml day .
River flows A new measuring weir is now installed at Llyn Clywedog. Improvements have been made to the weir at Lake Vyrnwy. Ultrasonic river flow meters are being installed at key points to improve accuracy of real time low flow data.
Outstation scanner A flexible automatic telemetry device has improved real time data recovery from rain, reservoir and river level/flow gauges.
Forecasting techniques Computer modelling of runoff is expected to reduce excess releases by improving confidence in flow forecasts.
Radar rainfall Weather radar used in the Severn basin since 1976 has been enhanced to provide hourly subcatchment rainfall data to help forecast runoff during droughts and floods.
Pumping control Liaison with river abstractors nas optxmxzed diurnal pumping rates to minimize flow fluctuations.
Wastage control Transit losses have been reduced by improving liaison to minimize excess flows entering a gravity-fed canal intake. Abstractions from river-fed gravels and for spray irrigation will be explicitly accounted in future droughts.
Extra storage This has been approved to provide a third source of support for the River Severn. The Bunter sandstone aquifer in Shropshire is being developed in a phased programme to yield an extra 225 Ml day- from 1985. The rate of borehole commissioning depends on abstraction demand increases.
354 R.C.Goodhew
Lessons for other schemes
Data (a) Design river flow data should be drawn from tributaries as
well as the main river. Long records are needed - possibly
simulated.
(b) Real time data should include basin-wide rainfall, reservoir
levels and river flows.
(c) Groundwater and riparian water table data should be monitored
to identify flow recession parameters and transit loss processes.
Groundwater data could give early warning of excessive demand.
(d) Low flow measurement techniques should be explicitly
specified (e.g. mm resolution) to ensure highest data quality at
Control Point.
(e) A river gauging survey should be mounted to identify reaches
where operational data do not represent typical flows.
(f) Utility of radar data depends on size and proportion of
unreservoired, regulated catchment; on reliability of rainfall; on
rapidity of runoff and on responsiveness of system controls to
runoff changes.
Control point (g) Control point should be located to protect the most vulnerable
reach. This is not necessarily the lowest reach because sewage
effluent is a most reliable source in a drought.
(h) Flexible flow criteria should include reductions for drought
seasons in large catchments. Steady statutory flows are unsuitable.
(i) Schemes for large catchments should incorporate intermediate
control points to adjust releases for transit loss.
(j) The need for balancing storage depends on the flexibility of
statutory flow provisions, travel time of releases and the extent
of water use downstream of the control point. Storage is advisable
downstream of pollution risks and adjacent to intakes where saline
intrusion may occur.
Management (k) Basin-wide management of all water services affecting river
flows is vital if system control is not to be negated by alien
artificial sources.
(1) Careful management of the system is important for even in
gravity-fed schemes regulation storage droughts develop rapidly.
Wise husbandry of gravity storage also reduces pumping costs from
underground reservoirs.
(m) Contingency allowance in design for excess releases and
operating losses should take into account all the above points and
the circumstances (e.g. evaporation) of the river concerned. The
viability of pumped schemes in particular is highly sensitive to
the level of operating losses.
ACKNOWLEDGEMENT This paper is published with the approval of the
Director of Operations, Severn-Trent Water Authority. The material
draws extensively on the results of cooperative effort between
colleagues both within the Authority and in other authorities.
Performance of a river regulation system 355
The views expressed do not necessarily reflect those of the Authority.
REFERENCES
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Bailey, R.A. & Goodhew, R.C. (1978) Forecasting for droughts. Presented to Instn Wat. Engrs, Midlands Section, Shrewsbury, UK, 12 September 1978.
Fordham, A.E., Cochrane, N.J., Kretschmer, J.M. & Baxter, R.S. (1970) The Clywedog Reservoir project. J. Instn Wat. Engrs 24(1), 17-76.
Hill, S.M. & Sheppard, M.A. (1962) Clywedog regulating reservoir -report on the fifth apportionment of costs between the public authorities benefitting, for the River Severn Water Resources Committee, October 1962.
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