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Poole Harbour Habitat Creation SchemeRidge - Groundwater Preliminary Evaluation
2
Introduction and objectives
• This presentation is intended to set out our current understanding of drainage and groundwater beneath Ridge and the relationship these may have with our proposed scheme
• We have adapted and developed our investigations according to concerns raised by the community
• This presentation offers a sneak preview of the work carried out and represents an insight into our studies and site investigations to date
• We are continuing to gather data and our interpretation may be expected to evolve further
Surface Drainage
• Hydrological catchment
• Natural “off slope” drainage
• Surface water drainage impacts/ intercepts natural overland flow
Surface water drainage catchments
• Road and roof drainage
• Soakaways to western part at top end of SW catchment
• Soakaways capture surface water runoff and recharge underlying shallow aquifer
Area drained primarily by soakaway
Geology- summary of underlying deposits Bedrock - Poole Formation
Unit Occurrence
Parkstone Clay Member Not present
Parkstone Sand Member To West of Ridge
Broadstone Clay Member [BRTC]
Widespread beneath Ridge and Arne Moor
Broadstone Sand Member [BROS]
Beneath Arne Moor May be interbedded and undifferentiated Oakdale Clay Member [OAKC]
Oakdale Sand Member
Superficial deposits
Unit Occurrence
Topsoil Thin, widespread
Made Ground [MGR]
Embankments etc some “disturbed ground” beneath Ridge
Head [HEAD] “In situ” product of weathering and erosion… beneath Ridge
Peat [PEAT] Beneath Arne Moors only
Tidal Flat Deposits: granular, non cohesive [TDFG]
Beneath Arne Moors
Tidal Flat Deposits: cohesive [TDFC]
Beneath Arne Moors
Hydrogeology - Regional perspective • Poole Formation - minor
aquifer • Succession of interbedded
sands and clays, variable thickness and depth
• Recharge across outcrop to south and within the upper catchment of the Furzebrook
• Discharge northward toward Poole Harbour
• Clay layers “confine” lower parts of the sand aquifer such that water in the lower parts is under pressure
• When clays are “punctured” by boreholes, water under pressure rises to surface
Parkstone Sand
Broadstone Sand
Broadstone Clay
Recharge area
Discharge area
Parkstone Sand
Parkstone Clay
Ridge
Superficial Deposits
Bedrock
Site topographic sections
G
G’
B
B’
A A’
C
C’
D
D’E’
E
F’
F
Lines of geological cross sections
Initial conceptual understanding (schematic section)
1 1
0 0
-1 -1
-2 -2
-3 -3
-4 -4
-5 -5
-6 -6
-7 -7
-8 -8
-9 -9
Chainage (m)
Offset (m)
Elevation (mAOD)
0.00
16.8
8
151.
78
271.
68
407.
63
544.
16
666.
39
712.
83
726.
11
24.6
2
29.6
3
60.4
0
17.1
9
9.19
26.8
3
21.7
9
0.94
0.36
0.63
0.41
0.65
0.00
0.38
0.40
0.90
1.50
3.70
5.00
10.00
BH15
TOP
TDFC
TDFG
TDFG
BRTC
BROS
0.35
0.90
1.20
2.72
5.12
CPT
13
TOP
TDFG
0.20
1.70
3.80
4.20
6.00
10.00
BH16
TOPTDFC
TDFG
BRTC
BROS
BROS
0.150.30
1.201.20
2.15
2.35
3.42
4.92
CPT
15
TOP TDFC PEAT
0.20
0.70
3.00
5.65
9.00
10.00
BH17
TOPTDFC
PEAT
TDFC
BROS
OAKC
0.20
0.40
3.50
TP20
TOP
0.150.20
TDFC PEAT
1.00
5.60
9.00
10.00
BH20
TOP MGR TDFC
PEAT
BROS
OAKC
Cross Section B – B’ - S-N, West of Arne Moors Site
TDFG
TDFG
TDFGTDFG
TDFG
PEAT
Project Title: Poole Harbour Habitat CreationLocation: Arne MoorsClient: Environment Agency
Vertical Scale: NTSHorizontal Scale: NTS
Legend KeyTOPSOIL
TDFC
TDFG
PEAT
BROS
BRTC
Water Strike level after 20 minutes
Water Strike level during boring
OAKC
MGR1
Project Title: Poole Harbour Habitat CreationLocation: Near Wareham, DorsetClient: Environment Agency
Vertical Scale: NTSHorizontal Scale: NTS Engineer: CH2M
9 9
8 8
7 7
6 6
5 5
4 4
3 3
2 2
1 1
Chainage (m)
Offset (m)
Elevation (mAOD)
0.00
25.7
7
345.
5135
2.27
685.
6569
7.26
5.33
0.40
0.79
8.78
3.83
6.28
Legend Key
0.90
1.301.45
1.90
2.402.552.70
3.00
3.55
4.00
WS1
7
MGR
HEAD
BRTC BROS
BRTC
BROSBRTC BRTC
BROS
BRTC
0.15
0.35
2.002.10
2.45
3.00
WS0
1
MGR HEAD
HEAD
BRTC BRTC
BRTC
0.200.300.40
1.30
1.90
2.85
4.00
WS1
6
TOP
HEADHEAD BRTC
BRTC
BRTC
BRTC
Water Strike level after 20 minutes
Cross Section G-G’ – South West to North East (Line 1) and North to South (Line 2) in Ridge Village
A
A
B
B
C
CSW NE NLine 1 – South West to North East Line 2 – North to South S
TOPSOIL
HEAD
BROS
BRTC
MGR1
Legend KeyTopsoil
Sands (Head deposits)
Sands (Broadstone sands)
Clay (Broadstone clay
Made ground
Sands and gravels (Tidal Flat Deposits)
Clays (Tidal Flat Deposits)
Deep sands (Broadstone sands member)
Clay (Broadstone clay member)
Sands and gravels
Clays (Head deposits/Alluvium)
Sands
Clay
Ground Investigation -Monitoring well locations
Installed water level loggers
Well Hydrograph-Ridge • Logger recorded
groundwater levels response
• 3 months data
• We are continuing to capture data
• Water levels within 0.3m from surface
0
0.2
0.4
0.6
0.8
1
1.2
7
7.2
7.4
7.6
7.8
8
8.2
8.4
8.6
8.8
9
13/0
1/20
18
20/0
1/20
18
27/0
1/20
18
03/0
2/20
18
10/0
2/20
18
17/0
2/20
18
24/0
2/20
18
03/0
3/20
18
10/0
3/20
18
17/0
3/20
18
24/0
3/20
18
31/0
3/20
18
07/0
4/20
18
14/0
4/20
18
21/0
4/20
18
28/0
4/20
18
05/0
5/20
18Ra
infa
ll m
m
Wat
er L
evel
(mAO
D)
Date
Ridge VillageLocation: WS17
Ground Level 8.78 mAOD
Typical groundwater recession curve
Well Hydrograph Ridge vs Swanage rainfall (mm) Smaller rainfall events (< c.5mm) little impact
Larger or cumulative events. Rapid response in groundwater levels
Significant and sudden rise, little rainfall…. Snowmelt
Groundwater response vs, tidal cycles Spring and neap tidal cycles exert no apparent pattern of response in the well hydrograph
Spring Neap
Ground Investigation - well hydrographs- detail (one week of data)
• Is there a diurnal influence at Ridge
• Wareham tidal range typically 1- 1.2m
• No evident tidal effect at Ridge WS17
-0.4-0.2
00.20.40.60.8
11.2
17/03/2018 18/03/2018 19/03/2018 20/03/2018 21/03/2018 22/03/2018 23/03/2018 24/03/2018 25/03/2018
Tidal variation Wareham
Ground Investigation - well hydrographs- detail (one week)
• Is there a tidal effect further on to Arne Moor?
• Possible minor tidal effect (1- 2 cm?) at BH16
• Relationship appears to be inverse (i.e. a tidal low results in an increase in water level)
• This is suggestive of a pressure response not direct connectivity
• BH16 close to tidal channel/ harbour
• Response zone at 2-4mdepth in “upper aquifer”
Ground Investigation - well hydrographs- detail (one week – Arne Moor)
• Is there a tidal effect elsewhere on Arne Moor?
• Data over 1 week suggests little influence on any of the well hydrographs (vs a tidal range of 1- 1.2m)
• Response zones are:
BH 04 0.5-9.5m
BH 13 1.5 – 10m
BH 18 3.5 – 9m
Note: All these are in the “lower” aquifer. A separate upper aquifer does not occur in these locations
Initial conceptual understanding section (schematic)
Schematic- detailWe are examining this understanding, gathering more detailed information on the groundwater regime
Summary (1)
• Our investigations at Ridge are ongoing, these are preliminary findings only • The geology beneath Ridge broadly comprises a layered system of
superficial “Head” deposits overlying Broadstone Clay of the Poole Formation, in turn overlying Broadston sand
• Younger units (Parkstone Sand Member) of the Poole Formation are exposed in the high ground West of Ridge village
• Beneath Arne Moors, layered geological systems also occur comprising near surface superficial deposits of Tidal Flat deposits (both clays and sands/ gravels), with some peat further into the Moor
• Poole Formation strata (Broadstone Clay over Broadstone/ Oakdale Sands) occur beneath the superficial deposits beneath Arne Moors
Summary (2)
• Groundwater beneath Ridge is locally recharged by rainfall, flow from the higher ground, soakaways and possibly leaking drainage
• Groundwater occurs in the superficial deposits of “Head” at shallow depth
• Its downward movement is constrained by layers of clay and it tends to move laterally, perhaps discharging at the foot of slopes (e.g. to the north where) it may be collected by surface drains
• Connectivity with the deeper aquifer beneath Ridge is limited by the underlying Broadstone Clay
Summary (3)
• Groundwater monitoring at Ridge shows groundwater at shallow depth, with a rapid response to rainfall recharge
• Groundwater beneath Arne Moors, in part, comprises a two layer system (upper, lower) separated by clay and/or peat
• The lower (Broadstone sand) aquifer, confined by the overlying clay is “pressurised” from regional recharge
• When penetrated by boreholes the groundwater rises, sometimes to above the surface (artesian conditions)
• The groundwater response of both upper and lower aquifers beneath Arne Moors is markedly more muted than the response at Ridge
• There is no currently no significant evidence to suggest that groundwater beneath Ridge is hydraulically “connected” to groundwater beneath Arne Moors
• We are gathering further evidence (continued monitoring with loggers, water quality data etc.) to further clarify our understanding of the groundwater and drainage regime
Thank You
Mike Barker
Principal Hydrogeologist
