Poole Harbour Habitat Creation Scheme Ridge - Groundwater Preliminary Evaluation
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 2
Surface Drainage • Hydrological catchment • Natural “off slope” drainage • Surface water drainage impacts/ intercepts natural overland flow
Surface water drainage catchments • Road and roof drainage Area drained • Soakaways to primarily by western part soakaway at top end of SW catchment • Soakaways capture surface water runoff and recharge underlying shallow aquifer
Geology- summary of underlying deposits Superficial deposits Bedrock - Poole Formation Unit Occurrence Unit Occurrence Topsoil Thin, widespread Parkstone Clay Member Not present Made Ground Embankments etc some [MGR] “disturbed ground” beneath Parkstone Sand Member To West of Ridge Ridge Broadstone Clay Member Widespread beneath Head [HEAD] “In situ” product of weathering [BRTC] Ridge and Arne Moor and erosion… beneath Ridge Broadstone Sand Member Beneath Arne Moor Peat [PEAT] Beneath Arne Moors only [BROS] May be interbedded and undifferentiated Tidal Flat Deposits: Beneath Arne Moors Oakdale Clay Member [OAKC] granular, non Oakdale Sand Member cohesive [TDFG] Tidal Flat Deposits: Beneath Arne Moors cohesive [TDFC]
Hydrogeology - Regional Parkstone Sand Ridge perspective Broadstone Clay • Poole Formation - minor aquifer Broadstone Sand • Succession of interbedded sands and clays, variable thickness and depth Recharge across outcrop to • Parkstone Sand south and within the upper catchment of the Furzebrook • Discharge northward toward Parkstone Clay Poole Harbour • Clay layers “confine” lower parts of the sand aquifer such that water in the lower parts is Recharge area under pressure • When clays are “punctured” by boreholes, water under Discharge area pressure rises to surface
Superficial Deposits
Bedrock
Site topographic sections
Lines of geological cross sections F’ B’ C’ E’ D’ F B E A’ A D C G G’
Initial conceptual understanding (schematic section)
Project Title: Poole Harbour Habitat Creation Vertical Scale: NTS Location: Arne Moors Horizontal Scale: NTS Client: Environment Agency Cross Section B – B’ - S-N, West of Arne Moors Site BH15 BH17 BH16 CPT15 CPT13 1 1 BH20 TOP TOP TOP 0.40 TP20 TDFC 0.20 0.20 Legend Key TDFC TDFC TOP TOP TOP 0.15 0.15 TDFC MGR 0.30 0.20 0.90 PEAT 0 0.35 TDFC 0 TOPSOIL TDFG 0.70 TDFG TOP PEAT 0.20 TDFC 0.40 PEAT 0.90 1.50 TDFG 1.00 TDFC PEAT 1.20 1.20 PEAT TDFG 1.20 -1 -1 1.70 TDFG TDFG 2.15 TDFG PEAT 2.35 -2 -2 TDFG 2.72 3.00 TDFC TDFG BROS 3.70 BRTC -3 -3 3.42 TDFG 3.80 BRTC BRTC 3.50 4.20 BROS -4 -4 5.00 OAKC BROS 4.92 5.12 MGR1 -5 5.65 -5 BROS 5.60 BROS 6.00 BROS Water Strike level after 20 minutes -6 -6 Water Strike level during boring -7 -7 -8 -8 9.00 OAKC 9.00 OAKC -9 -9 10.00 10.00 10.00 10.00 151.78 271.68 407.63 544.16 666.39 712.83 726.11 16.88 0.00 Chainage (m) 24.62 29.63 60.40 17.19 26.83 21.79 Offset (m) 9.19 Elevation (mAOD) 0.94 0.36 0.63 0.41 0.65 0.00 0.38
Project Title: Poole Harbour Habitat Creation Vertical Scale: NTS Location: Near Wareham, Dorset Horizontal Scale: NTS Client: Environment Agency Engineer: CH2M Cross Section G-G’ – South West to North East (Line 1) and North to South (Line 2) in Ridge Village SW NE N S Line 1 – South West to North East Line 2 – North to South A C B WS17 9 9 MGR 8 8 0.90 HEAD 1.30 BRTC 1.45 BROS 7 7 1.90 BRTC WS16 2.40 BROS TOP 2.55 BRTC 2.70 0.20 BRTC HEAD 6 6 0.30 HEAD 0.40 3.00 BRTC BROS Legend Key 3.55 BRTC 5 5 1.30 TOPSOIL BRTC 4.00 HEAD 1.90 WS01 BRTC 4 4 BROS MGR 0.15 HEAD 0.35 2.85 HEAD BRTC BRTC 3 3 MGR1 4.00 Water Strike level after 20 minutes 2 2 2.00 BRTC 2.10 BRTC 2.45 BRTC 1 1 3.00 A B C 345.51 352.27 685.65 697.26 25.77 0.00 Chainage (m) Offset (m) 5.33 0.40 0.79 Elevation (mAOD) 8.78 3.83 6.28
Legend Key Topsoil Made ground Sands (Head deposits) Clays (Head deposits/Alluvium) Sands and gravels (Tidal Flat Deposits) Clays (Tidal Flat Deposits) Clay (Broadstone clay Sands Sands (Broadstone sands) Clay Sands and gravels Clay (Broadstone clay member) Deep sands (Broadstone sands member)
Ground Investigation - Monitoring well locations Installed water level loggers
Ridge Village Location: WS17 Water Level (mAOD) 1.2 9 Well Ground Level 8.78 mAOD 8.8 Hydrograph- 1 8.6 Ridge 8.4 0.8 • Logger recorded 8.2 Rainfall mm groundwater levels 8 0.6 response 7.8 • 3 months data 0.4 7.6 We are continuing • to capture data 7.4 0.2 • Water levels within 7.2 0.3m from surface 7 0 13/01/2018 20/01/2018 27/01/2018 03/02/2018 10/02/2018 17/02/2018 24/02/2018 03/03/2018 10/03/2018 17/03/2018 24/03/2018 31/03/2018 07/04/2018 14/04/2018 21/04/2018 28/04/2018 05/05/2018 Typical groundwater recession curve Date
Well Hydrograph Significant and sudden Ridge vs rise, little rainfall…. Swanage rainfall Snowmelt (mm) Smaller rainfall events (< c.5mm) little impact Larger or cumulative events. Rapid response in groundwater levels
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 Tidal variation Wareham WS17 1.2 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 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
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 2-4m depth 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- detail We 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
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