Monthly water situation report

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Monthly water situation report

Summary – September 2018

The Hertfordshire and North London Area received 58% of the long term average rainfall for

September and 73% of the LTA rainfall for the summer period. All of the catchments ended September with a soil moisture deficit larger than the long term average. Our river flow indicator sites ranged between normal and exceptionally low monthly mean flows with the majority having recorded below

normal flows.

All of our groundwater indicator sites recorded a fall in groundwater levels since last month.

Rainfall

It was a dry month with the Hertfordshire and North London Area (“the Area”) having received 58% of the long term average (LTA) rainfall for September. There was an average of 23 dry days (<0.2mm) with most of the months rain falling on the 22^nd which resulted in the largest daily total of 23.2mm being recorded at Central Park in the Roding catchment. The Chiltern East Colne and Lee Chalk catchments recorded normal rainfall while the remaining catchments recorded below normal rainfall. For the summer period (April-September) the Area received only 73% of the LTA rainfall and all of the catchments ended the 6 month period with below normal rainfall.

Soil Moisture Deficit/Recharge

Soil moisture deficits continued to increase since last month in all but the Chiltern East Colne catchment.

However, all of the catchments ended September with a soil moisture deficit larger than the LTA. Recharge was limited as the Area received only 61% of the effective rainfall for September. For the summer period the Area received 118% of the effective rainfall but the Chiltern East Colne and the Lee Chalk ended the 6 month period below LTA.

River Flows

The majority of our river flow indicator sites recorded below normal monthly mean flows. The exceptions to this were the River Lee at Fieldes Weir which recorded normal flows, the River Lee at Waterhall and the River Roding which were notably low and the River Ash which recorded exceptionally low flows. Our river flow indicator sites responded to the rainfall and baseflow has stabilised. No flood alerts or warnings were issued for Hertfordshire and North London Area during September.

Groundwater Levels

All of our groundwater indicator sites recorded a continued fall in groundwater level. In the Lee Chalk all of our indicator sites had normal levels with the exception of Lilley Bottom which was below normal. Groundwater levels in the Colne Chalk were below normal apart from Amersham Road which recorded notably low groundwater levels.

Reservoir Storage

The Lee Valley reservoir groups’ levels ended September close to average while the Thames reservoir groups’

levels continued to decline and ended the month well below average.

Environmental Impact

The abstraction licence flow constraints that were in force in September, out of a summer maximum of 27

:

Week Commencing 3 September 10 September 17 September 24 September

Number of Constraints 8 12 14 6

The source of the River Bulbourne moved below Northchurch, while flows on the Chess notably declined through Chesham.

Author: Groundwater and Hydrology, HNL Contact details: 03708 506 506

Hertfordshire and North London

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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).

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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.

CEH) and the Ordnance Survey (© Crown Copyright).

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River Flow and Groundwater Status Map

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

CEH) and the Ordnance Survey (© Crown Copyright). Geological map reproduced with kind permission from UK Groundwater Forum, BGS © NERC

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Colne

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

Exceptionally high Notably high Above normal Normal

Below normal Notably low Exceptionally low ─── Latest data

0 20 40 60 80 100 120

Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 Jun-18 Jul-18 Aug-18 Sep-18

Rainfall (mm)

Chilterns - East - Colne (N) - Rainfall

0 10 20 30 40 50 60 70 80

Effective Rainfall (mm)

Chilterns - East - Colne (N) - Effective Rainfall

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

30-Nov-17 31-Jan-18 31-Mar-18 31-May-18 31-Jul-18 30-Sep-18 30-Nov-18

Daily Mean Flow (cumecs)

RIVER VER AT COLNEY STREET (HANSTEADS) Ranking derived from data for the period 01/10/1956 to 31/12/2012

0.0 0.5 1.0 1.5 2.0 2.5

RIVER GADE AT CROXLEY GREEN Ranking derived from data for the period 01/10/1970 to 31/12/2012

0 2 4 6 8 10 12

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

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

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

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Colne Groundwater

75 76 77 78 79 80 81 82

Dec-15 Apr-16 Aug-16 Dec-16 Apr-17 Aug-17 Dec-17 Apr-18 Aug-18

Level (mAOD)

AMERSHAM ROAD OBH

Ranking derived from data for the period Nov-1991 to Dec-2012

108 113 118 123 128

Level (mAOD)

ASHLEY GREEN STW OBH

Ranking derived from data for the period Sep-1987 to Dec-2012

124 126 128 130 132 134 136 138 140

Level (mAOD)

BALLINGDON FARM

Ranking derived from data for the period Jan-1975 to Dec-2012

48 49 50 51 52 53 54 55 56 57

Level (mAOD)

WAPSEYS WOOD OBH

Ranking derived from data for the period Mar-1988 to Dec-2012

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Upper Lee

0 10 20 30 40 50 60 70 80 90

Rainfall (mm)

Lee - Chalk - Rainfall

0 10 20 30 40 50 60

Lee - Chalk - Effective Rainfall

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

RIVER MIMRAM AT PANSHANGER Ranking derived from data for the period 01/12/1952 to 31/12/2012

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

RIVER ASH (HERTS) AT WARESIDE (MARDOCK) Ranking derived from data for the period 03/06/1980 to 31/12/2012

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

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

0 5 10 15 20 25 30 35

RIVER LEE (MIDDLE) AT FEILDES WEIR Ranking derived from data for the period 01/10/1883 to 31/12/2012

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Upper Lee Groundwater

92 93 94 95 96 97 98 99

Level (mAOD)

LILLEY BOTTOM OBH

Ranking derived from data for the period Jul-1979 to Dec-2012

85 86 87 88 89 90 91 92

Level (mAOD)

CRESCENT COTTAGES

Ranking derived from data for the period Aug-1968 to Dec-2012

71 76 81 86 91

Level (mAOD)

THERFIELD RECTORY

Ranking derived from data for the period Jan-1883 to Dec-2012

68 69 70 71 72 73 74 75 76 77 78

Level (mAOD)

HIXHAM HALL

Ranking derived from data for the period Jun-1964 to Dec-2012

84 86 88 90 92 94 96

Level (mAOD)

CAVE GATE

Ranking derived from data for the period Jun-1966 to Dec-2012

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Lower Lee

0 10 20 30 40 50 60 70 80 90 100

Rainfall (mm)

Lower Lee - Rainfall

0 10 20 30 40 50 60

Lower Lee - Effective Rainfall

0 5 10 15 20 25 30 35

RIVER LEE (LOWER) AT LEE FLOOD CHANNEL (LOW HALL) Ranking derived from data for the period 01/01/1980 to 31/12/2012

0 5 10 15 20 25

RIVER LEE (LOWER) AT LEA BRIDGE Ranking derived from data for the period 18/09/1992 to 31/12/2012

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North London

0 10 20 30 40 50 60 70 80 90

Rainfall (mm)

North London - Rainfall

0 5 10 15 20 25 30 35 40 45 50

North London - Effective Rainfall

0 1 2 3 4 5 6 7

RIVER CRANE AT CRANFORD (CRANFORD PARK) Ranking derived from data for the period 03/04/1978 to 31/12/2012

0 1 2 3 4 5 6 7 8 9

RIVER BRENT AT BRENT (MONKS PARK) Ranking derived from data for the period 01/12/1978 to 31/12/2012

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Roding

Reservoir Storage

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

0 20 40 60 80 100 120

Rainfall (mm)

Roding Catchment - Rainfall

0 5 10 15 20 25 30 35 40 45 50

Roding Catchment - Effective Rainfall

0 2 4 6 8 10 12 14 16

RIVER RODING AT REDBRIDGE Ranking derived from data for the period 19/01/1950 to 31/12/2012

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

RIVER INGREBOURNE AT UPMINSTER (GAYNES PARK) Ranking derived from data for the period 01/11/1970 to 31/12/2012

0 10 20 30 40 50 60 70 80 90 100

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Percentage of live capacity

Thames Water - Lower Thames Reservoirs - Lower Thames

0 10 20 30 40 50 60 70 80 90 100

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Percentage of live capacity

Thames Water - Lee Valley Reservoirs - Lower Lee

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Flows in the chalk fed rivers – September 2018

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Summary of rainfall, effective rainfall and soil moisture deficit

Rainfall and Effective Rainfall – September 2018

Rainfall (mm) Effective Rainfall (mm)

Area Total

(mm)

LTA (mm)

% of LTA

Total (mm)

LTA (mm)

% of LTA

Chilterns- East - Colne 40 60 68 5 7 65

Lee - Chalk 34 54 63 4 6 65

North London 24 55 44 0 0 0

Lower Lee 29 55 53 0 0 0

Roding Catchment 31 51 60 0 1 0

Hertfordshire and North

London Area Average 32 55 58 2 3 61

Soil Moisture Deficit (SMD) – September 2018

Area End of Month

SMD (mm)

End of Month SMD LTA (mm)

Chilterns- East - Colne 128 89

Lee - Chalk 127 97

North London 138 100

Lower Lee 138 97

Roding Catchment 128 98

London Area Average 132 96

Rainfall and Effective Rainfall – summer total for period 1 April 2018 to 30 September 2018

Rainfall (mm) Effective Rainfall (mm)

Area Total

(mm)

LTA (mm)

% of LTA

Total (mm)

LTA (mm)

% of LTA

Chilterns- East - Colne 228 340 67 42 46 91

Lee - Chalk 243 313 77 37 38 98

North London 237 312 76 21 12 169

Lower Lee 217 312 69 18 11 161

Roding Catchment 232 295 78 22 11 201

London Area Average 231 314 73 28 24 118

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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