Northstowe Community Forum, 19 July 2017 Alex Gallagher Technical - - PowerPoint PPT Presentation

Northstowe Community Forum, 19 July 2017 Alex Gallagher Technical Director, Hydrogeologist, CGeol

Northstowe location

• Northstowe Phase 1
• Situated on the former

Cambridge Golf Course and former agricultural land

• Located to the east of

Longstanton

• Kingfisher Pond located in

west of site

Kingfisher Pond Longstanton Northstowe Phase 1

Topography

Western area:

• Underlain by River Terrace

Deposits (RTD)

• Little topographic variation
• 8.5-9.0mAOD

Eastern area:

• Underlain by Ampthill Clay Formation

(ACF)

• Ground elevations fall to southeast
• Low of 5.5mAOD in southeast corner

Pre-development surface water catchments

Western area:

• Small catchment
• Flow west into Longstanton Drain

Southern and eastern area:

• Two main ditches – Greenway A and B – drain

most of the site

• Flow east into Reynolds Drain

Boreholes and historical water level monitoring

• Comprehensive ground

investigations

• 81 geology boreholes
• 198 trial pits
• 30 groundwater

monitoring locations

• Historic groundwater

monitoring data

• 2006-2008
• 2014-2016
• Pond level monitoring
• 2011
• 2015-2017

Surface Geology

River Terrace Deposits Ampthill Clay Formation Kimmeridge Clay Formation

• River Terrace Deposits (RTD)

in west of site

• North – south channel
• Cut down into Ampthill Clay

Formation

• Sand and gravel
• Stores water
• Ampthill Clay Formation (ACF)

in east of site

• Mudstone
• Relatively impermeable

Groundwater flow

Site drainage

Drain elevation reference points Ref. point ID Location note Drain elevation (mAOD)

Approx. groundwater elevation (mAOD)

DR-A Shallow extension of Greenway A near Kingfisher Pond 8.4

7.5

DR-B Western extent of Greenway A close to BH116 east of the school 8.0

8.0

DR-C Drain south of the Kingfisher Pond 8.0

7.8

DR-D Western extent of Greenway B close to the geological boundary 6.0

8.0

DR-E Drainage junction close to attenuation pond 4.8

5.2

DR-F Eastern extent of Greenway A 4.4 4.0

Key design principal was to maintain the base

• f the Greenways above the water table in the

River Terrace Deposits

Geological cross-sections

Rainfall – long term and recent

Average Monthly Rainfall Cambridge NIAB Meteorological Station (1961-2017) Month Average Monthly Rainfall (mm) 2016/2017 Monthly Rainfall (mm) January 46.1 48.0* February 33.5 43.4* March 37.2 29.8* April 40.8 16.0* May 46.4 64.8* June 49.6 66.4** July 45.8 18.2** August 53.6 44.0** September 48.2 45.2** October 52.2 19.6** November 52.5 56.8** December 47.6 25.4** TOTAL 553.5 477.6

Notes: * 2017, ** 2016

Unusually dry second half of 2016 and Spring 2017:

• 70% of long-term average

rainfall (LTR) in the last 6 months off 2016

• 59% of LTR in March and

April Low rainfall limits aquifer recharge

Regional groundwater levels and river flows

Regional groundwater levels Regional river flows

• Continual decline in groundwater

levels since mid-2016

• Low groundwater levels likely to

persist into the summer due to long-term rainfall deficits

• River Great Ouse flows were below

average in October and November 2016 and again in March and April 2017

Pre-dewatering groundwater elevations

• RTD groundwater

levels show seasonal variation (approx. 1m)

• ACF groundwater

levels fluctuate rapidly in response to rainfall RTD ACF

Groundwater elevations during construction

Phase 1A dewatering

• Groundwater elevations similar

before and after

• Drop in water level reported in

Kingfisher Pond during dewatering

• Water levels in pond recovered

afterwards (temporary impact)

Phase 1B dewatering

• Recovery of water levels in Kingfisher

Pond afterwards (in early 2017)

• But low rainfall limited recharge and

stopped recovery. Decline observed between March and May 2017

Recent monitoring

• Data loggers installed in

three monitoring boreholes in March 2017

• General decline in

groundwater elevations

• Response to heavy

rainfall on 18 May, slight recovery of groundwater levels before continuing to fall in late May

Summary and conclusions

• Comprehensive ground investigations provided accurate information upon which to base the

drainage strategy

• The River Terrace Deposits (RTD) form a discrete aquifer that is separate to the surrounding Ampthill

Clay

• Dewatering of the RTD for the construction of greywater drainage trenches had no long lasting effect
• A permanent change to the groundwater system could have occurred if:
• Grey water drainage trenches (constructed below the water table) provided a conduit to the Greenways

– but site inspections show that no water is seeping from the trenches which contain hydraulic barriers

• The main drainage ditches extended below the water table in the RTD – comparison of earthworks levels

with groundwater contours show that this is not the case

• The Autumn of 2016 and the Spring of 2017 were unusually dry:
• Groundwater levels did not recover as expected during winter and spring
• Regional river flows were approximately 50% of long-term average in April 2017
• Climatic factors are very likely to be the cause of low water levels in the Kingfisher Pond and RTD

aquifer

• Groundwater monitoring will continue and the situation will be reviewed in

6 months time

Questions?