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 Northstowe Longstanton Phase 1 • Kingfisher Pond located in west of site Longstanton Kingfisher Pond
Topography Eastern area: Western area: • Underlain by Ampthill Clay Formation • Underlain by River Terrace (ACF) Deposits (RTD) • Ground elevations fall to southeast • Little topographic variation • Low of 5.5mAOD in southeast corner • 8.5-9.0mAOD
Pre-development surface water catchments Western area: Southern and eastern area: •Small catchment •Two main ditches – Greenway A and B – drain •Flow west into Longstanton Drain most of the site •Flow east into Reynolds Drain
Boreholes and historical water level monitoring • Comprehensive ground investigations 81 geology boreholes o 198 trial pits o 30 groundwater o monitoring locations • Historic groundwater monitoring data 2006-2008 o 2014-2016 o • Pond level monitoring 2011 o 2015-2017 o
Surface Geology River Terrace • River Terrace Deposits (RTD) Deposits in west of site Ampthill Clay North – south channel o Formation Cut down into Ampthill Clay o Formation Sand and gravel o Stores water o • Ampthill Clay Formation (ACF) in east of site Kimmeridge Clay Mudstone o Relatively impermeable o Formation
Groundwater flow
Site drainage Drain elevation reference points Ref. Location note Drain Approx. point elevation groundwater ID (mAOD) elevation (mAOD) DR-A Shallow extension of 8.4 7.5 Greenway A near Kingfisher Pond DR-B Western extent of Greenway 8.0 8.0 A close to BH116 east of the school DR-C Drain south of the Kingfisher 8.0 7.8 Pond DR-D Western extent of Greenway 6.0 8.0 B close to the geological boundary DR-E Drainage junction close to 4.8 5.2 Key design principal was to maintain the base attenuation pond of the Greenways above the water table in the DR-F Eastern extent of Greenway A 4.4 4.0 River Terrace Deposits
Geological cross-sections
Rainfall – long term and recent Average Monthly Rainfall Cambridge NIAB Meteorological Station (1961-2017) Unusually dry second half of Month Average Monthly Rainfall 2016/2017 Monthly Rainfall 2016 and Spring 2017: (mm) (mm) January 46.1 48.0* February 33.5 43.4* 70% of long-term average • March 37.2 29.8* rainfall (LTR) in the last 6 April 40.8 16.0* months off 2016 May 46.4 64.8* June 49.6 66.4** July 45.8 18.2** • 59% of LTR in March and August 53.6 44.0** April September 48.2 45.2** October 52.2 19.6** Low rainfall limits aquifer November 52.5 56.8** December 47.6 25.4** recharge TOTAL 553.5 477.6 Notes: * 2017, ** 2016
Regional groundwater levels and river flows Regional groundwater levels Regional river flows • Continual decline in groundwater • River Great Ouse flows were below levels since mid-2016 average in October and November • Low groundwater levels likely to 2016 and again in March and April persist into the summer due to 2017 long-term rainfall deficits
Pre-dewatering groundwater elevations • RTD groundwater levels show seasonal RTD variation (approx. 1m) • ACF groundwater ACF levels fluctuate rapidly in response to rainfall
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 o – 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 o 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 o Regional river flows were approximately 50% of long-term average in April 2017 o • 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
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