Proposed Development on Land West of Lancaster Park, Morpeth, Northumberland. Flood Risk Assessment including Surface Water Drainage Strategy Addendum

Transcription

1 Mitford Estate LLP Proposed Development on Land West of Lancaster Park, Morpeth, Northumberland Flood Risk Assessment including Surface Water Drainage Strategy Addendum August 2015 Matthew Elliott

2 Report Control REPORT Project: Client: Job Number: File Origin: Flood Risk Assessment including Surface Water Drainage Strategy: Addendum Proposed Development on Land West of Lancaster Park, Morpeth, Northumbria Mitford Estate LLP A N:\Projects\A A095000\A090801\reports\FRA\Addendum Document Checking: Primary Author Matthew Elliott Initialled: Contributor Simbi Hatchard Initialled: Review By Tom Beavis Initialled: Issue Date Status Checked for Issue 1 04/03/2015 First Issue

3 Contents 1 INTRODUCTION PURPOSE OF THE ADDENDUM REPORT PROPOSED DEVELOPMENT SCOPE AND STRUCTURE OF THE FRA ADDENDUM GROUNDWATER FLOOD RISK DRAINAGE OF THE TRUNK ROAD SERVICE AREA (TRSA) MISCELLANEOUS ITEMS AND CONCLUSIONS APPENDICES Appendix A - Updated Surface Water Drainage Strategy (SK-01) Appendix B - NCC Response to FRA Appendix C - Meeting Record 1

4 1 INTRODUCTION 1.1 PURPOSE OF THE ADDENDUM REPORT Mitford Estate LLP has commissioned WYG Engineering Ltd to undertake a Flood Risk Assessment (FRA) in respect of a proposed development on land west of Lancaster Park, Morpeth. The FRA also includes a Surface Water Drainage Strategy. The FRA report has been prepared to accompany an outline planning application for a 36.6 ha development comprising a Trunk Road Service Area (TRSA), residential units and a country park and other community facilities. However, since issue of the FRA in March 2015 comments have been received from a number of parties including NCC in their role as Lead Local Flood (Risk) Authority (LLFA) and Highway Authority (responsible for construction of the North Morpeth By-Pass). In addition, a number of amendments to the masterplan have occurred. The amendments to the masterplan do not significantly impact on the findings of the FRA. Accordingly, this FRA Addendum does not replace the existing FRA but provides clarification and additional information in relation to the matters raised by NCC and other consultees. 1.2 PROPOSED DEVELOPMENT The land which will accommodate the proposed site has recently become open for access due to the proposed Morpeth Northern Bypass, intended to become operational by The TRSA is intended to provide services to users of the A1 and includes: 400m 2 mixed use development with commercial use at ground floor with 12 apartments above; a hotel Class C1 of around 60 bedrooms; a 550m 2 restaurant / pub adjacent to the hotel; a fuel station with 100m 2 A1 retail floor space and a 1450 m 2 amenity building. The remainder of the development includes 255 residential units, a country park and supporting infrastructure. The country park and community facilities aim to accommodate informal recreational activities, public art, ongoing farming practises and creating enhanced ecological and landscape features. The development is to be located on land currently used for agricultural purposes. As such it is defined as greenfield site in flood risk terms. 1.3 SCOPE AND STRUCTURE OF THE FRA ADDENDUM The key issues raised by the consultees and covered in this FRA Addendum are as follows: 1

5 (i) (ii) (iii) NCC has raised concerns regarding groundwater flood risk at the site and has requested that more detailed consideration be given of groundwater flood risks. This issue is considered in Section 2; NCC note that Area A (see FRA Fig 3) drains northwards and across the route of the North Morpeth By Pass. Area A captures almost the entirety of the proposed Trunk Road Service Area (TRSA). The interface of the proposals for the drainage of the TRSA with the drainage to be provided in conjunction with the North Morpeth by pass has been discussed with NCC (as highway authority) and Highways England (responsible for the A1). The outcome of these discussions is provided in Section 3; Section 4, picks up on a number of miscellaneous items not covered by the above and provides further conclusions in addition to those already provided in the FRA. As requested by NCC, drawing SK-01 (surface Water Strategy) has been revised to reflect the new masterplan and marked up to show the proposed discharge rates from areas A, B1 and B2. This drawing is included as Appendix A to this Addendum. The response from NCC as LLFA is provided in Appendix B and meeting notes following the meeting with NCC on 1 st July 2015 are provided in Appendix C. 2

6 2 GROUNDWATER FLOOD RISK The application site covers an area of approximately 36.6 ha and lies west of the Lancaster Park development, Morpeth, Northumberland, as shown in Fig 1 of the FRA. The application site is currently made up of undeveloped open fields used for agricultural purposes. It can be seen by comparing the topographical information provided in support of the application (see FRA Appendix B) that the majority of areas A, B1 and B2 (see FRA Appendix G, dwg SK-01 and FRA Addendum Appendix A) are largely flat and at a level above 68.0 metres above Ordnance Datum (m AOD). As the immediate subsoil material is clay, these flat areas are subject to water temporarily standing in wet weather (as noted by the FRA prepared by 3E and dated November 2014). In an attempt to mitigate this problem, the fields have been provided with a land drainage system as indicated by the information provided in Appendix D of the FRA. However, this has not been entirely successful as evidenced by the standing water. Clayey substrates very often reduce the effectiveness of land drains, because any disturbance of the adjacent clays by ploughing will close up the available pathway to the underlying land drain system provided by permeable backfill originally provided over the land drains. However, this elevated land (at a height above 68.0 metres above Ordnance Datum (m AOD)) has lower land on all sides. For instance, on the west side of the site, the A1 cutting runs down towards the River Wansbeck; on the east the Scotch Gill valley commences as a shallow ditch becoming a deeply incised valley as it progresses downwards in a southerly direction towards its confluence with the River Wansbeck. In addition, the land falls away sharply to the south, and rather more gently towards the north. In view of the above, land at or above 68.0m AOD would only be under threat from groundwater flooding if this was on account of artesian pressures. Such artesian pressures would require a relatively unusual geological structure and such pressures would typically be evident in the steep slopes to the west and east. However, there is no evidence of this type of artesian pressure. It is therefore concluded that there is no groundwater flood risk to the land above 68.0 maod. It is possible that those parts of the site on which built development is proposed that are below 68.0m AOD (i.e the northern part of the TRSA and parts of the south east of area B2) may be subject to emergent groundwater. However, if this was a likely possibility, it would be expected that such an eventuality would be evidenced by the presence of springs or groundwater in excavations. In the site walkover, no such springs have been observed, and no undue problems of groundwater have been reported in conjunction 3

7 with excavations along the TRSA northern boundary undertaken in conjunction with the North Morpeth By Pass. However, as this risk cannot be entirely discounted from the south east of Area B2, it is proposed that as part of the detailed application, further investigation of groundwater levels will be undertaken as already proposed in the FRA (See Section 7 Conclusions and Recommendations ). In the event, that groundwater is identified as an issue in this area, it will be necessary to safely direct the emergent groundwater away from buildings and into the adjacent watercourses (i.e the tributaries of Scotch Gill). In view of the above it is concluded as follows: (i) The only location where groundwater flooding might arise as an issue is in the south eastern corner of Area B2; (ii) No existing flows from groundwater within the built development footprint have been observed or are expected other than as noted above; (iii) Should emergent groundwater be identified by more detailed investigations in the south east of area B2, such water can be directed to existing watercourses and away from buildings; (iv) On account of (i) (iii) above, there is no significant groundwater flood risk to proposed buildings within the development that cannot be readily mitigated; (v) On account of (i) (iii) above, there will be no displacement of groundwater; (vi) On account of (i) (iii) above, there will be no increase in off-site groundwater flood risk; 4

8 3 DRAINAGE OF THE TRUNK ROAD SERVICE AREA (TRSA) The application site all drains into the North Sea via the River Wansbeck. However, the site can be divided into three catchments which are distinguished by the route by which any water falling onto and flowing across the surface of the ground would reach the River Wansbeck. This is illustrated in Fig 3 of the FRA which is provided again below. FRA Figure 1 Direction of Surface Water Flows Key Site Boundary Surface Water flow direction Catchment Boundaries 69.37m AOD Part of the upstream catchment of Benridge Burn / Mitford Dean (Catchment A) Part of the upstream catchment of Scotch Gill (Catchment B1) Part of the upstream catchment of Scotch Gill (Catchment B2) 33.62m AOD Direct run off to River Wansbeck (Catchment C) Catchment A currently drains towards the A1 and the North Morpeth By Pass. The flat nature of this area, and the possibility that some land drains may be present (although non have been reported in NCC excavations in association with the by pass works) means that the precise extent of the catchment may be difficult to establish. However, it is accepted by all parties, that part of the TRSA drains towards the A1 and the new by pass. The proposed rate of discharge (based on restricting flows to existing greenfield run off 5

9 rates) is 12.1 l/s (See FRA Table 2). Restriction to this rate of discharge will be acheived by the use of open attenuation ponds and linking swales that will provide, conveyance, contributory storage and water cleansing. Discussions with the By-Pass team have identiifed that when the A1 was constructed a culvert was provided to convey drainage from the eastward extension of the Benridge Burn catchment which was in effect severed by the A1 embankments. This culvert is being used for temporary managment of surface water flows during construction of this part of the By Pass as can be seen from the information submitted to address site surface water management issues. An excerpt of the by pass surface water managament strategy is provided below illustrating use of the existing culvert. Reviewing Ordnance Survey contours, indicates that the severed catchment extent was approximately as shown on the plan below and includes the majority of the application TRSA. The culvert allows this area to drain under the A1 embankments and to Benridge Burn which drains via Mitford Dean and eventually into the River Wansbeck. 6

10 A more comprehensive picture of the watercourses concerned has been provided in Fig 4 of the FRA, which is provided again below. 7

11 FRA Figure 2 - Locations of Watercourses in the Vicinity of the Application Site Key Site Boundary Main River (River Wansbeck) Ordinary Watercourse Tributary to Scotch Gill Proposed Waste Water Treatment Plant 8

12 It has been argued by Mitford Estates that the new by pass, effectively cuts off the Mitford Land from the culvert provided in conjunction with the A1 when the A1 was set in its curent alignment. In view of these concerns, the by pass drainage has been revised to include a conection to receive land drianage from the Mitford Estate land. This is shown on the By Pass drainage drawing MNB_AEC_Z1_D_DR_05_0501-C04 Rev C04, an excerpt of which is provided below. This indicates the provision of a 225mm diameter connection into the application site (ref S100.1/8.001/225). A 225mm diameter connection will have adequate capacity to receive a flow limited to 12.1 l/s. However, the By Pass drainage discharges to Benridge Burn through assets belonging to Highways England (HE) who are responsible for the A1. Whether or not the re-developed land will be permitted to continue to drain (at greenfield run off rates) via HE assets is still under discussion. Notwithstanding the above unresolved matter, an alternative surface water drainage solution is available for the TRSA (Area A in the FRA) and it has been confirmed by NCC (in their role as LLFA) that this would be acceptable subject to confirmation of details. The alternative scheme would require surface water to be piped back from the TRSA access onto the new by-pass roundabout to outfall into Scotch Gill via the site SUDS system, at a point where the levels will allow 9

13 such a connection. This will require the provision of additional attenuation to ensure that existing greenfield discharge rates into Scotch Gill are not exceeded. An record of the meeting held with NCC at which this was agreed is provided in Appendix C. 10

14 4 MISCELLANEOUS ITEMS AND CONCLUSIONS NCC has raised a number of additional queries in their formal response to the FRA which is provided in Appendix B. By way of clarification the following are confirmed: (i) the series of swales and ponds along the northern boundary of Area B1 provide the surface water drainage outfall to area B1. (ii) Area C does not include any significant impermeable areas and therefore does not require a formal surface water drainage system. (iii) The site systems will retain all water on site up to equivalent 1 in 100 year greenfield run off rates. Whilst it is noted that concerns on this point have been raised by the Morpeth Flood Action Group, in that this return period is a lesser return period than that provided by the recent Morpeth Flood Alleviation Scheme, the following should be noted: (a) post development discharge rates include a 30% allowance for climate change over a 100 year period; (b) the critical return periods associated with the site discharges are typically no more than a few hours at most, whereas the critical return period for the Morpeth Flood Alleviation Scheme is very much longer (bearing in mind the size of the Wansbeck catchment upstream of Morpeth). As a result, the type of event that is critical to the development is likely to be a high intensity low volume event, whereas the events critical to the Morpeth Flood Alleviation Scheme are likely to be of much longer duration and higher overall volume. It is noted that water quality concerns have been raised in relation to discharges to Scotch Gill. It is confirmed that all discharges will be provided with two levels of treatment prior to discharge to Scotch Gill in accordance with best practice set out in the SUDS Manual. Further details are provided in Section 5.5 of the FRA. This level of treatment will ensure that water in Scotch Gill will not be subject to deterioration on account of the proposed development. The primary threats to water quality in Scotch Gill are on account of existing unattenuated traditional surface water outfalls from the existing development to the east. In conclusion it is considered that the Addendum provides adequate further information on points where clarification has been requested. Although a number of items remain to be finalised in each case it can be seen that deliverable solutions are available indicating that these matters could be dealt with via appropriate conditions. 11

15 Appendix A: Updated SW Strategy Drawing 12

16 DO NOT SCALE: CONTRACTOR TO CHECK ALL DIMENSIONS AND REPORT ANY OMISSIONS OR ERRORS KEY SURFACE WATER PIPE SWALES LOW FLOW PIPE DETENTION POND CHECK DAM ORDINARY WATERCOURSE: SCOTCH GILL AND TRIBUTARIES OUTFALL ADDITIONAL NOTES AREA A: DISCHARGE AT 12.1 l/s TO EITHER: (i) HIGHWAY NETWORK (N. MORPETH BYPASS) OR (ii) TO SCOTCH GILL NORTHERN OUTFALL OR SOUTHERN OUTFALL AREA B1: DISCHARGE AT 12.2 l/s 50% TO NORTHERN OUTFALL (SCOTCH GILL) 50% TO SOUTHERN OUTFALL (SCOTCH GILL) AREA B2: DISCHARGE AT 20.5 l/s TO SCOTCH GILL SOUTHERN OUTFALL AREA C: NO IMPERMEABLE SURFACES NORTHERN SCOTCH GILL OUTFALL - TOTAL 6.1 l/s SOUTHERN SCOTCH GILL OUTFALL - TOTAL 26.6 l/s FILENAME : N:\PROJECTS\A A095000\A090801\ACAD\DWGS\SURFACE WATER DRAINAGE STRATEGY V2.DWG PLOTTED BY : SIMBI.HATCHARD PLOTTED DATE : 04 August :28:50 THE REVISED PLAN INCORPORATES THE REVISED MASTERPLAN, HOWEVER, AREAS A, B1 AND B2 REMAIN AS STATED IN THE FRA IN VIEW OF THE MINOR NATURE OF THE CHANGES FROM THE MASTERPLAN DEVELOPMENT REV DESCRIPTION BY CHK APP DATE MITFORD ESTATE LLP ARNDALE COURT HEADINGLEY LEEDS LS6 2UJ TEL: +44 (0) FAX: +44 (0) leeds@wyg.com Project: PROPOSED DEVELOPMENT ON LAND WEST OF LANCASTER PARK, MORPETH, NORTHUMBERLAND Drawing Title: CLARIFICATION OF SURFACE WATER DRAINAGE STRATEGY A0 1:1000 Project No. A Drawn Date Date Approved Date Checked SH MSE Revision Office Type Drawing No. 21 CS SK-01 P2 C WYG Group Ltd.

17 Appendix B: NCC Response to FRA 15

18 15/01285/OUT Lancaster Park, Morpeth LLFA Comments Paragraph 103 of the NPPF states When determining planning applications, local planning authorities should ensure flood risk is not increased elsewhere and only consider development appropriate in areas at risk of flooding where, informed by a site-specific flood risk assessment following the Sequential Test, and if required the Exception Test, it can be demonstrated that: Development is appropriately flood resilient and resistant, including safe access and escape routes where required, and that any residual risk can be safely managed, including by emergency planning; and it gives priority to the use of sustainable drainage systems. The proposed development is classed as a major development. In accordance with Schedule 4 of the Development Management Procedure Order the Lead Local Flood Authority are a statutory consultee in the planning process. In response to the above a FRA from 3E consulting and a Drainage Assessment from White Young Green have been submitted with the planning application. After reviewing these documents, we require further information on groundwater flooding and the disposal of surface water from the water. Until we have received these details we object to the planning application. Technical Comments Groundwater flood risk Paragraph 10 of the Planning Practice Guidance states For the purposes of applying the National Planning Policy Framework, flood risk is a combination of the probability and the potential consequences of flooding from all sources including from rivers and the sea, directly from rainfall on the ground surface and rising groundwater, overwhelmed sewers and drainage systems, and from reservoirs, canals and lakes and other artificial sources. Looking through the submitted FRA from 3E consulting there appears to be some groundwater issues that are present on site. Paragraph 4.7 of the FRA states another potential source of flooding to the site is the potential for rising of groundwater which could impact the proposed development. A ground investigation has not yet been undertaken but during the site visit the ground was waterlogged at field entrances. 16

19 The NPPF and Planning Practice Guidance require Flood Risk Assessments to be appropriate to the nature and scale of the development. However they should also look at all sources of flooding and where local considerations dictate, investigate further. Therefore we will require a more detailed analysis of groundwater flows from the site. In particular any assessment will need to: Look at the sources of where groundwater appears; Assess the existing overland flows arising from groundwater; Assess the impact of the development on these flows; Determine that the new dwellings / commercial units on site will be safe and will not flood as a result of groundwater; Determine that groundwater will not be displaced as a result of the development; Determine that the risk of groundwater flooding will not increase off site. Disposal of surface water Looking through the Drainage Assessment and the Surface Water Drainage Strategy (SK-01) we are pleased to see that the use of SuDS will be implemented throughout the development. In addition we are content that the hierarchy of disposing surface water throughout the development has also been adhered to. However, looking through the strategy and the site masterplan we have some further questions of which we hope can be addressed. Area A The existing catchment for area A is in a northerly direction and to the Benridge Burn which is to the west of the development. The Drainage Strategy suggests that a connection to the highway for the Morpeth Northern Bypass will be used to convey these flows. At present no confirmation from Northumberland County Council Highways or Highways England has been received demonstrating that they are happy with a connection and confirming the allowable discharge rate. For an outline planning application we need to ensure that the principles of surface water drainage can be achieved. Therefore for area A we need to ensure that a connection is available and that the relevant Highway Authority can accept these flows. Area B1 Looking at the topography of the development site, it appears that the ponds and swales shown north of area B in the surface water drainage strategy drawing cannot be accessed for any development. Therefore what is the intention for how will water in area B be attenuated? Area B2 Will any development in the southern and western parts of area B2 have a positive gravity fall to the tributary of the Scotch Gill? 17

20 Area C How will water in this area be drained? Will it be through infiltration and evaporation? Will there be a connection to the river Wansbeck? Proposed discharge rates Discharge rates into the Scotch Gill and other networks need to be established at this point of the planning process and clearly displayed on a drainage masterplan. Therefore can the proposed discharge rates for areas A, B1, B2 and C (if applicable) please be added to Surface Water Drainage Strategy (drawing SK-01). 18

21 Appendix C: Meeting Notes 1 st July

22 I thought it might be useful to all to have a note of the key points arising from the meeting on Wednesday last week (1 st July) attended by Vivienne, James, Leyton, Gary, Laura, Sandra and myself; so I propose the following: 1. Surface Water Interface with A1/North Morpeth ByPass (i) (ii) (iii) (iv) (v) (vi) All understand and accept that part of the Mitford Estate site (part of the TRSA) drains northwards and, prior to the construction of the A1 on its present alignment, would naturally drain into Benridge Burn. When the A1 was constructed, a culvert was provided under the A1 to allow this natural routing to remain. A piped extension onwards to Benridge Burn was provided. It would appear that the specific right for land drainage to continue to pass down this route only refers to the Brown s land (is that the correct name?). If this is correct, then it would appear that Mitford Estates rights may not have been appropriately considered at this time. The North Morpeth by pass proposals do allow for land drainage to be picked up from part of the Mitford Estate land but currently this enters the same system as the highway drainage. Inasmuch as the North Morpeth highway drainage will need to pass under the A1, it will pass through pipes that will become Highways England assets. There is a memo of understanding between NCC and HE regarding this and the Mitford Estates team asked to see a copy of the memo. It was agreed that a meeting be arranged with NCC and HE to go through the above (along with a number of other related matters) to make sure that this matter is satisfactorily resolved. It was agreed that one solution would be to provide a separate pipe under the A1 as part of the North Morpeth by pass works for the Mitford Estate land drainage. Any such solution would need to consider the implications of the fact that the current agricultural land may be developed so the surface water drainage would no longer be agricultural land drainage (although, in line with current practice, peak rates of discharge would be restricted to current agreed greenfield rates. (vii) An alternative solution would be to seek to drain all of the TRSA into Scotch Gill. This would require some deep pipework and is generally contrary to normal approaches to surface water drainage. The Mitford Estates team are mindful that there is local concern in relation to flood risk and direct routing of discharges into Scotch Gill, instead of observing the natural lie of the land, might be construed negatively as it may be perceived as reducing the time for water to reach the centre of Morpeth. 20

23 2. NCC Comments on SW Strategy (i) (ii) (iii) In the event that agreement cannot be reached for a surface water solution for the application that continues to observe the natural lie of the land NCC as LLFA would accept the principle of flows being directed to Scotch Gill provided that overall peak discharge rates are not increased relative to existing greenfield run off rates. The Mitford estates team acknowledged receipt of the NCC (LLFA) comments and agreed that these would be addressed by an Addendum to the FRA and SW Strategy which would be issued once agreement had been reached in relation to the interface with the A1/North Morpeth By Pass. NCC confirm that their SUDS Local developer guidance is due for issue in September/October; NCC intend to adopt suitable SUDS but details of acceptable adoptable standards are not yet available. 3. Foul Drainage (i) (ii) (iii) It was again noted that the Mitford Estates preference is to connect foul drainage from the site to an extended public sewer network for North Morpeth provided that this is delivered and that associated costs are reasonable. NWL agreed to chase up provision of the approximate costs for a requistioned connection for Mitford Estates. The letter containing the proposed NWL response (attached) on the application and proposed condition which references the revised foul drainage strategy was agreed as being a suitable way forwards. It was agreed that a draft of the revised foul drainage strategy would be provided to NWL for their review and comment prior to finalisation and issue. The presence of white clawed crayfish may influence the consent terms for the outfall to the Wansbeck for a package treatment plant (should this be required due to collapse of the requisition negotiation) I think this covers the main points. If I have inadvertently missed something or incorrectly understood what was agreed, please let me know, Best Regards, 21

24 Matthew Elliott Director WYG Arndale Court, Headingley, Leeds, LS6 2UJ Tel: Fax: Mob: WYG Engineering Limited. Registered in England number: Registered Office: Arndale Court, Otley Road, Headingley, Leeds, West Yorkshire LS6 2UJ VAT No: