Water Situation Report Hertfordshire and North London Area

Monthly water situation report Hertfordshire and North London Area

Summary – September 2014 September was a dry month with little rainfall. Much of the month’s rainfall fell in heavy showers on the 18th, 19th and 23rd. River flows in the Chalk catchments declined but remained healthy, while rivers in the Clay and urban catchments suffered from the low rainfall. Groundwater levels were normal or higher, with groundwater levels at several of our indicator sites above normal.

Rainfall The Hertfordshire and North London Area (the “Area”) had a dry September with only an average of 17mm of rainfall (31% of the long term average (LTA) monthly rainfall). Much of the rain fell in heavy showers on the 18th, 19th and 23rd. The biggest daily rainfall total of 21.8mm was recorded on the 19th at Stevenage on the Lee Chalk with 13mm of this falling within a 15 minute period. September was the end of the hydrological summer and year. During the summer (April to September), the Area had near average rainfall with 89% of the summer LTA rainfall. There was a split across the Area, with the Lee Chalk and the Roding recording normal rainfall, while the Colne-Chilterns-Chalk, North London and Lower Lee all experiencing below normal summer rainfall. With the heavy rainfall in December, January and February, the Area ended the hydrological year with above normal rainfall totals, with the North London and Roding recording notably high rainfall totals.

Soil Moisture Deficit/Recharge The soil moisture deficit (SMD) remained high during September and the Area ended the month with an SMD of 122mm (the LTA end of month SMD is 96mm). With the low rainfall during September, there was little effective rainfall available for recharge or river flow in impermeable catchments.

River Flows River levels across the Area declined during September. The rivers in the Chalk catchments remained healthy, but continued their seasonal decline as groundwater levels fell. There was a clear split between the Chalk catchments where flows in all our indicator sites were normal or higher, and the urban and Clay catchments which suffered from the low rainfall. Our indicator sites on the River Colne, River Misbourne, River Ash and the River Lee at Feildes Weir all had above normal flows for September. The River Roding recorded below normal September flows, while the River Brent was notably low. Our indicator site on the River Crane recorded exceptionally low flows with only 38 % of the LTA September flow. No flood alerts or warnings were issued for the Area during September.

Groundwater Levels Across the Area, groundwater levels continued their seasonal decline. Groundwater levels were normal or higher at all of our indicator sites. Groundwater levels were above normal at Wapseys Wood, Lilley Bottom, Crescent Cottages, Therfield Rectory, Alswick Hall Farm and Hixham Hall.

Reservoir Storage The two reservoir groups of the Lower Thames and the Lower Lee both maintained higher than average levels.

Environmental Impact The table below shows the abstraction licence flow constraints that were in force in September, out of a summer maximum of 30:

Week Commencing 1 September 8 September 22 September 29 September Number of Constraints 1 2 2 2

The Chalk rivers of the Colne-Chilterns-Chalk continued to flow along much of their length. The River Bulbourne flowed from Northchurch, the River Gade from Great Gaddesden, the River Ver from below Markyate, the River Chess from above Chesham, while the River Misbourne flowed from below Great Missenden.

Author: Groundwater, Hydrology & Contaminated Land Team Contact details: 03708 506 506

All data are provisional and may be subject to revision. The views expressed in this document are not necessarily those of the Environment Agency. Its officers, servants or agents accept no liability for any loss or damage arising from the interpretation or use of the information, or reliance upon views contained herein. www.gov.uk/environment-agency © Environment Agency 2014

Rainfall Map

Total rainfall for hydrological areas across the Hertfordshire and North London Area for the current month, classed relative to an analysis of respective historic totals.

Data source: Rainfall calculated using the Environment Agency, South East Soil Moisture Model.

Some features of this map are based on digital spatial data licensed from the Centre for Ecology and Hydrology, (© CEH) and the Ordnance Survey (© Crown Copyright).

www.gov.uk/environment-agency © Environment Agency 2014

Rainfall Map

Total rainfall for hydrological areas across the Hertfordshire and North London Area for the current month, the last three months, the last six months, and the last 12 months, classed relative to an analysis of respective historic totals.

Data source: Rainfall calculated using the Environment Agency, South East Soil Moisture Model.

Some features of this map are based on digital spatial data licensed from the Centre for Ecology and Hydrology, (© CEH) and the Ordnance Survey (© Crown Copyright).

www.gov.uk/environment-agency © Environment Agency 2014

River Flow and Groundwater Status Map

Groundwater site status based on end of month level. Surface water site status based on mean monthly flow.

Some features of this map are based on digital spatial data licensed from the Centre for Ecology and Hydrology, (© CEH) and the Ordnance Survey (© Crown Copyright). Geological map reproduced with kind permission from UK

Groundwater Forum, BGS © NERC

www.gov.uk/environment-agency © Environment Agency 2014

Colne

Monthly total rainfall (mm) Long term average rainfall (mm) Chilterns - East - Colne (N) - Rainfall Chilterns - East - Colne (N) - Effective Rainfall 180 180

160 160

140 140

120 120

100 100

80 80

60 60 Rainfall (mm) 40 40 Effective Rainfall (mm)

20 20 Nov Dec Nov Dec Aug Sep Aug Sep May May Jul Jul Jan Jun Jun Jan Mar Mar Feb Feb Oct Oct 0 Apr 0 Apr ------14 14 ------13 13 14 14 - - 14 14 14 14 14 14 14 14 13 13 13 13 14 14 14 14 14 14 RIVER VER AT COLNEY STREET (HANSTEADS) RIVER GADE AT CROXLEY GREEN Ranking derived from data for the period 01/10/1956 to 31/12/2012 Ranking derived from data for the period 01/10/1970 to 31/12/2012

2.5 3.5

3.0 2.0

2.5

1.5 2.0

1.5 1.0

1.0

0.5 Daily Mean Flow(cumecs) Daily Mean Flow(cumecs) 0.5

0.0 0.0 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14

RIVER MISBOURNE AT DENHAM LODGE RIVER COLNE AT DENHAM Ranking derived from data for the period 01/07/1984 to 31/12/2012 Ranking derived from data for the period 01/10/1952 to 31/12/2012

1.8 25.0

1.6

20.0 1.4

1.2 15.0 1.0

0.8 10.0 0.6

0.4 5.0 Daily Mean Flow(cumecs) Daily Mean Flow(cumecs) 0.2

0.0 0.0 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14

Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data

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Colne Groundwater

AMERSHAM ROAD OBH ASHLEY GREEN STW OBH Ranking derived from data for the period Nov-1991 to Dec-2012 Ranking derived from data for the period Sep-1987 to Dec-2012

81 128 80 123 79

78 118

Level (mAOD) Level 77 (mAOD) Level 113 76

75 108 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14

BALLINGDON FARM WAPSEYS WOOD OBH Ranking derived from data for the period Jan-1975 to Dec-2012 Ranking derived from data for the period Mar-1988 to Dec-2012

57 139 56 137 55 135 54

133 53 52 131 51 Level (mAOD) Level 129 (mAOD) Level 50 127 49 125 48 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14

Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data

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Upper Lee

Monthly total rainfall (mm) Long term average rainfall (mm) Lee - Chalk - Rainfall Lee - Chalk - Effective Rainfall 160 140

140 120

120 100

100 80 80 60 60

Rainfall (mm) 40 40 Effective Rainfall (mm) 20 20 Nov Dec Nov Dec Aug Sep Aug Sep May May Jul Jul Jun Jan Jan Jun Mar Mar Feb Feb Oct Oct Apr 0 Apr 0 ------14 14 ------13 13 14 14 - - 14 14 14 14 14 14 14 14 13 13 13 13 14 14 14 14 14 14 RIVER MIMRAM AT PANSHANGER RIVER ASH AT WARESIDE (MARDOCK) Ranking derived from data for the period 01/12/1952 to 31/12/2012 Ranking derived from data for the period 03/06/1980 to 31/12/2012

2.5 12.0

10.0 2.0

8.0 1.5

6.0

1.0 4.0

0.5

Daily Mean Flow(cumecs) Daily Mean Flow(cumecs) 2.0

0.0 0.0 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14

RIVER LEE AT HOWE GREEN (WATER HALL) RIVER LEE AT FEILDES WEIR (Naturalised) Ranking derived from data for the period 01/04/1959 to 31/12/2012 Ranking derived from data for the period 01/10/1883 to 31/12/2012

9.0 100.0

8.0 90.0

80.0 7.0 70.0 6.0 60.0 5.0 50.0 4.0 40.0 3.0 30.0 2.0 20.0 Daily Mean Flow(cumecs) Daily Mean Flow(cumecs) 1.0 10.0

0.0 0.0 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14

Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data

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Upper Lee Groundwater

LILLEY BOTTOM OBH CRESCENT COTTAGES Ranking derived from data for the period Jul-1979 to Dec-2012 Ranking derived from data for the period Aug-1968 to Dec-2012

100 93

99 92

98 91

97 90

96 89

95 88 Level (mAOD) Level (mAOD) Level 94 87

93 86

92 85 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14

THERFIELD RECTORY HIXHAM HALL Ranking derived from data for the period Jan-1883 to Dec-2012 Ranking derived from data for the period Jun-1964 to Dec-2012

78 77 91 76 75 86 74 73 81 72

Level (mAOD) Level (mAOD) Level 71 76 70 69 71 68 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14

ALSWICK HALL FARM Ranking derived from data for the period Jul-1982 to Dec-2012

Level (mAOD) Level 79

77 Dec-11 Apr-12 Aug-12 Dec-12 Apr-13 Aug-13 Dec-13 Apr-14 Aug-14

Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data

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Lower Lee

Monthly total rainfall (mm) Long term average rainfall (mm) Lower Lee - Rainfall Lower Lee - Effective Rainfall 160 160

140 140

120 120

100 100

80 80

60 60

Rainfall (mm) 40 40 Effective Rainfall (mm) 20 20 Nov Dec Nov Dec Aug Sep Aug Sep May May Jul Jul Jun Jan Jan Jun Mar Mar Feb Feb Oct Oct Apr 0 Apr 0 ------14 14 ------13 13 14 14 - - 14 14 14 14 14 14 14 14 13 13 13 13 14 14 14 14 14 14 RIVER LEE AT LEE FLOOD CHANNEL (LOW HALL) RIVER LEE AT LEA BRIDGE Ranking derived from data for the period 02/10/1981 to 31/12/2012 Ranking derived from data for the period 18/09/1992 to 31/12/2012

100.0 40.0

90.0 35.0 80.0 30.0 70.0 25.0 60.0

50.0 20.0

40.0 15.0 30.0 10.0 20.0 Daily Mean Flow(cumecs) Daily Mean Flow(cumecs) 5.0 10.0

0.0 0.0 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14 18-Nov-13 18-Jan-14 18-Mar-14 18-May-14 18-Jul-14 18-Sep-14 18-Nov-14

Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data

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North London

Monthly total rainfall (mm) Long term average rainfall (mm)

North London - Rainfall North London - Effective Rainfall 180 180

160 160

140 140

120 120

100 100

80 80

60 60 Rainfall (mm) 40 40 Effective Rainfall (mm)

20 20 Nov Dec Nov Dec Aug Sep Aug Sep May May Jul Jul Jun Jan Jan Jun Mar Mar Feb Feb Oct Oct Apr 0 Apr 0 ------14 14 ------13 13 14 14 - - 14 14 14 14 14 14 14 14 13 13 13 13 14 14 14 14 14 14 RIVER CRANE AT CRANFORD PARK RIVER BRENT AT BRENT (MONKS PARK) Ranking derived from data for the period 03/04/1978 to 31/12/2012 Ranking derived from data for the period 01/12/1978 to 31/12/2012

10.0 14.0

9.0 12.0 8.0

7.0 10.0

6.0 8.0 5.0 6.0 4.0

3.0 4.0

2.0 Daily Mean Flow(cumecs) Daily Mean Flow(cumecs) 2.0 1.0

0.0 0.0 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14

Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data

Roding

Monthly total rainfall (mm) Long term average rainfall (mm) Roding Catchment - Rainfall Roding Catchment - Effective Rainfall 160 160

140 140

120 120

100 100

80 80

60 60

Rainfall (mm) 40 40 Effective Rainfall (mm) 20 20 Nov Dec Nov Dec Aug Sep Aug Sep May May Jul Jul Jun Jan Jan Jun Mar Mar Feb Feb Oct Oct Apr 0 Apr 0 ------14 14 ------13 13 14 14 - - 14 14 14 14 14 14 14 14 13 13 13 13 14 14 14 14 14 14 RIVER RODING AT REDBRIDGE RIVER INGREBOURNE AT UPMINSTER (GAYNES PARK) Ranking derived from data for the period 19/01/1950 to 31/12/2012 Ranking derived from data for the period 01/11/1970 to 31/12/2012

35.0 5.0

4.5 30.0 4.0

25.0 3.5

3.0 20.0 2.5 15.0 2.0

10.0 1.5

1.0 Daily Mean Flow(cumecs) 5.0 Daily Mean Flow(cumecs) 0.5

0.0 0.0 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14 30-Nov-13 31-Jan-14 31-Mar-14 31-May-14 31-Jul-14 30-Sep-14 30-Nov-14

Exceptionally high Notably high Above normal Normal Below normal Notably low Exceptionally low ─── Latest data

Reservoir Storage

Thames Water - Lower Thames Reservoirs - Lower Thames Thames Water - Lee Valley Reservoirs - Lower Lee 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 Percentage of capacity live Percentage 10 of capacity live Percentage 10 0 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Below minimum Above maximum ─── Average ─── Latest data

Flows in the chalk fed rivers – September 2014

Summary of rainfall, effective rainfall and soil moisture deficit

Rainfall and Effective Rainfall – September 2014

Rainfall (mm) Effective Rainfall (mm)

Total LTA % of Total LTA % of Area (mm) (mm) LTA (mm) (mm) LTA Chilterns- East - Colne 21 60 35 2 7 30 Lee - Chalk 17 54 32 1 6 24 North London 19 55 34 0 0 - Lower Lee 16 55 30 0 0 - Roding Catchment 11 51 21 0 1 - Hertfordshire and North 17 55 31 1 3 26 London Area Average

Soil Moisture Deficit (SMD) - September 2014

End of Month End of Month Area SMD (mm) SMD LTA (mm) Chilterns- East - Colne 127 89 Lee - Chalk 107 97 North London 135 100 Lower Lee 126 97 Roding Catchment 113 98 Hertfordshire and North 122 96 London Area Average

Rainfall and Effective Rainfall – Summer total for period 1 April 2014 to 30 September 2014

Rainfall (mm) Effective Rainfall (mm)

Total LTA % of Total LTA % of Area (mm) (mm) LTA (mm) (mm) LTA Chilterns- East - Colne 256 340 75 19 46 42 Lee - Chalk 323 313 103 28 38 75 North London 254 312 82 0 12 0 Lower Lee 265 312 85 0 11 0 Roding Catchment 293 295 99 0 11 0 Hertfordshire and North 278 314 89 9 24 40 London Area Average

Glossary Term Definition Aquifer A geological formation able to store and transmit water. Areal average rainfall The estimated average depth of rainfall over a defined area. Expressed in depth of water (mm). Artesian The condition where the groundwater level is above ground surface but is prevented from rising to this level by an overlying continuous low permeability layer, such as clay. Artesian borehole Borehole where the level of groundwater is above the top of the borehole and groundwater flows out of the borehole when unsealed. Cumecs Cubic metres per second (m3s-1) Effective rainfall The rainfall available to percolate into the soil or produce river flow. Expressed in depth of water (mm). Flood Alert/Flood Warning Three levels of warnings may be issued by the Environment Agency. Flood Alerts indicate flooding is possible. Flood Warnings indicate flooding is expected. Severe Flood Warnings indicate severe flooding. Groundwater The water found in an aquifer. Long term average (LTA) The arithmetic mean calculated from the historic record, usually based on the period 1961-1990. However, the period used may vary by parameter being reported on (see figure captions for details). mAOD Metres Above Ordnance Datum (mean sea level at Newlyn Cornwall). Naturalised flow River flow with the impacts of artificial influences removed. Artificial influences may include abstractions, discharges, transfers, augmentation and impoundments. Recharge The process of increasing the water stored in the saturated zone of an aquifer. Expressed in depth of water (mm). Reservoir gross capacity The total capacity of a reservoir. Reservoir live capacity The capacity of the reservoir that is normally usable for storage to meet established reservoir operating requirements. This excludes any capacity not available for use (e.g. storage held back for emergency services, operating agreements or physical restrictions). May also be referred to as ‘net’ or ‘deployable’ capacity. Soil moisture deficit (SMD) The difference between the amount of water actually in the soil and the amount of water the soil can hold. Expressed in depth of water (mm). Categories Exceptionally high Value likely to fall within this band 5% of the time Notably high Value likely to fall within this band 8% of the time Above normal Value likely to fall within this band 15% of the time Normal Value likely to fall within this band 44% of the time Below normal Value likely to fall within this band 15% of the time Notably low Value likely to fall within this band 8% of the time Exceptionally low Value likely to fall within this band 5% of the time