a technical presentation to IMechE Staffordshire Branch, April 2022, about the water cycle in general and specifically the Derbyshire River Derwent catchment. Rainwater collection, river water pumping, raw water storage and distribution to cities in the East Midlands, UK. Also drought and flood planning

1. The hydrology of Carsington Water
EurIng Ian Jenkinson CEng FICE FIMechE CMgr MCMI

2. Carsington Reservoir

3. Carsington Reservoir 9th largest in England
• 36,331 ML (7,800 million gallons)
• 10% of STW’s water storage
• Sufficient for half million people for 1 year
• 300 hectares (741 acres), 31 metres deep
• 8 miles circumference (3 mile long, 1 mile wide)
• Filled by pumping 131m up through a 10Km tunnel
• Gravity release to Ogston Reservoir or back to River Derwent

4. A word about units and numbers
• some very BIG numbers and ‘odd’ units
• ML/d (mega litres/day), Cumecs (cubic metres/second)
• ‘Odd’ terminology: % stored, SAAR (Long Term Average)
• For context 1 ML/d is 12 litres per second (bathtap 15 litres /minute)
• A typical household uses 120 litres / person / day
• “Target 100” aims to reduce use to 100 litres/person/day by 2040

5. Gentle reminder about the Hydrologic cycle

6. Pumped Storage because limited catchment
• Catchment 1580 Ha (6 sq mile)
• SAAR rainfall = 927mm
• 1580 x 0.927 = 14,646 ML/year
• To fill 36,331ML = 2.5 years
• BUT hydrologic losses >50% pa
• also Compensation Flow
1,508ML/year (10% of rainfall)
• Hence pumping 320 ML/day
(116,800 ML/year or 6% per
week)

7. 2019 a period of heavy rain, saturated ground, zero pumping or release
6 Jun-7 Sept rain=197.9mm & 2-7 Sep level +480mm
12 Sep-14 Oct rain=161.8mm & 28 Sep-12 Oct levels +270mm
Hence at best 1” rain ~ 2” of water depth; at worst 1” rain = 1” of level rise
0
200
400
600
800
1000
1200
1400
15-May 04-Jul 23-Aug 12-Oct 01-Dec 20-Jan
Rainfall and water depth
Rain mm Water Depth mm

8. Just one bit of a very complicated system

12. For Carsington Sailing Club water is crucial

13. But the shoreline is also critical

14. As levels change the beach area grows and shrinks

15. Low water is a problem for boat launch & recovery

16. Club website reports

17. Water resources 2009 to 2022
50%
60%
70%
80%
90%
100%
01/12/2009 01/12/2010 01/12/2011 01/12/2012 01/12/2013 01/12/2014 01/12/2015 01/12/2016 01/12/2017 01/12/2018 01/12/2019 01/12/2020 01/12/2021
Carsington percentage capacity
(mud level, limit of centre slipway, rent reduction level)

18. Abstraction Permits
• Max pumping 320ML/day (7%/week) up to half of Derwent flow
• BUT only if flow is sufficient (flow gauge at St Mary’s Bridge Derby)
• If flow is greater than 680ML/d then unrestricted pumping
• If flow less than 680ML/d then only 15ML/d taken
• If flow lower than 340ML/day then no water is taken
• Energy cost of pumping about £1m pa
• Max release 206ML/day (4%/week) 1/3rd of daily use in 3 Counties

20. Major drought depletion
• STW Drought Plan suggests infrequent depletion, low probability
“i.e. a likelihood of one summer in approximately 40.”
• Range of measures from hosepipe bans to ‘abstraction reductions’
• Triggers based on regional EA Water Situation Reports
• Medium term = Leakage reduction
• New reservoirs seen to be “politically difficult”
• Long term ‘Target 100’ by 2040 = 100litres/person/day

21. Derwent Reservoirs & Carsington & Ogston
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
31-Jan-08 31-Jan-09 31-Jan-10 31-Jan-11 31-Jan-12 31-Jan-13 31-Jan-14 31-Jan-15 31-Jan-16 31-Jan-17 31-Jan-18 31-Jan-19
Stored Capacity Megalitres
Derwent ML Carzy+Ogston ML

22. Flood prevention ?

23. Flood prevention - divert flow to Carsington?
• For a 100 year flood event
• a 450mm flood water reduction in Derby
• requires 30% of flow in Derwent to be diverted and stored
• Storage required approximately 4,000 ML
• Carsington = 36,331 ML so 11% of total capacity required
• But maximum capacity of Ambergate Pumping Station 320ML/day
• So 13 days of 24/7 pumping

24. Reservoirs and flood prevention ?
• Derwent Flow 500 - 2000 ML/d
• “Above Normal” > 2500 ML/d
• Max Abstraction 320 ML/d (13%)
• So for serious flooding the
impact is marginal at best

25. Toddbrook, what if Carsington is breached ?
• The map demonstrates
the impact of a breach
at Carsington Water,
which would extend
through Nottingham
• STW Raw Water
Reservoir Surveillance
Training Manual
• https://britishdams.org/assets/documents/We
b%20Ref%20Docs/2019_03_04_Maintaining%
20Reservoirs_%20Redesign.pdf

26. More information https://www.carsingtonsailingclub.co.uk/
• https://www.stwater.co.uk/about-us/reservoir-levels/
• https://nrfa.ceh.ac.uk/data/station/spatial/28103
• https://www.gov.uk/government/publications/weekly-rainfall-and-
river-flow-reports-for-england
• https://www.gov.uk/government/collections/water-situation-reports-
for-england