Drainage Strategy Report

Transcription

1 CONSULTING ENGINEERS Report No: Date: C1014 Aug 2017 Project: Drainage Strategy Report Danygraig Works, Risca Client: P.D. Edenhall This report is provided for the benefit only of the party to whom it is addressed. CB3 Consult Ltd do not extend responsibility to any third party for the whole or any part of the contents. CB3 Consult Ltd owe no duty of care in relation to this report to any third party. CB3 Consult Ltd Report prepared by: Graeme Tulip BEng(Hons),CEng, MICE Signature: Report status: FINAL Approved issue: for Dylan Gravell BEng(Hons), PgDIP MIStructE, FFB CEng, Signature: Date: 11/08/2017 CB3 Consult Ltd 7 St James Crescent Swansea SA1 6DP

2 Issue History Date Revision Revision Notes 11/08/2017 First Issue INDEX 1.0 Introduction 2.0 Existing Drainage Arrangements 3.0 Proposed Site Drainage Strategy 4.0 Summary Appendices A B C D E Topographic Survey DCWW sewer map / NRW flood maps Proposed Development Layout Drainage Strategy Plan Outline Microdrainage Storage Calculations Date: Aug 2017 Page 1

3 1.0 Introduction 1.1 Brief and Objectives The drainage strategy report has been prepared to support a planning application for residential re-development of industrial land, currently the Danygraig Brickworks at Danygraig Road, Risca, Newport NP11 6DP. The main objectives of this Report are: To undertake an assessment of the drainage infrastructure available to the site. To identify any constraints which will need to inform the design process. To develop a strategy for surface water and foul water disposal for the future site in accordance with TAN15 and other relevant requirements and best practice. 1.2 Limitations This drainage appraisal report is prepared for PD Edenhall for the purposes of supporting a planning application only and should not be used for any other purpose. The information, views and conclusions drawn concerning the site are based, in part, on information supplied to CB3 Consult Ltd by other parties. CB3 Consult Ltd has proceeded in good faith on the assumption that this information is accurate. CB3 Consult Ltd accepts no liability for any inaccurate conclusions, assumptions or actions taken resulting from any inaccurate information supplied to CB3 Consult Ltd from others. Date: Aug 2017 Page 2

4 2.0 Existing Drainage Arrangements 2.1 General Description The site is a brickworks off Danygraig Road, just west of the A467 at Risca, Newport. The National Grid Reference for the site is ST A location plan is shown in Figure 1. An aerial photograph of the Site is shown in Figure 2. Figure 1: Site Location Plan 2.2 The site is bounded by Danygraig Rd and the A467 to the east and north, woodland rising to the west, and Danygraig Bungalows and residential development at Tir y Cwm Lane to the south. The site has a long history of continual industrial use dating back to the early 1800s. 2.3 A topographical survey of the site undertaken in May 2017 is included in Appendix A. This shows a largely flat site totalling approximately 0.66ha of positively drained hardstanding and roof area. The site measures some 160m in the north-south direction by 50m east-west at the south, narrowing to 30m east-west at the northern end of the site. Date: Aug 2017 Page 3

5 Figure 2: Aerial View of Site 2.4 Site Hydrology and Hydrogeology Land drainage in the vicinity flows generally in an easterly direction towards the River Ebbw (main river) which is located some 30m to the east of the site. 2.5 Existing unnamed watercourses are shown on OS mapping on the hillside rising to the west of the site. It is not known whether there is any culverted watercourse through the site. This will need to be checked when any existing drainage at the site is removed to make way for the proposed development, and if necessary any alteration or other associated works discussed and agreed with the Lead Local Flood Authority. 2.6 An unnamed stream runs between nos 6 7 Tir y Cwm Lane some 65m south of the site. This becomes culverted beneath the road at the junction of Tir y Cwm Lane and Danygraig Road. 2.7 No intrusive site investigation information is available at this stage, however a desk study and preliminary environmental risk assessment has been undertaken by Earth Science Partnership (ref. ESP.5481b.2925 dated July 2017). This indicates that the superficial glacial deposits at the site are classed as unproductive strata, whilst the underlying sandstone bedrock is classed as a principal aquifer. As such the bedrock Date: Aug 2017 Page 4

6 may be highly fractured, sensitive to pollution and may support water supply or river base flow on a strategic scale. 2.8 Public Sewerage The sewerage undertaker (DCWW) public sewer map for the area is included in Appendix B. This shows no public sewers onsite and the nearest public sewer on Danygraig Road in front of no1 Danygraig Bunglaows just south of the site. The existing onsite drainage system will very likely discharge to this sewer, and as such the sewerage up to the first manhole within the developer s site curtilage will be classed as public sewer since the sewer transfer in Flood Risk According to the NRW flood maps, also included in Appendix B, the site is not located in an area of fluvial or tidal flooding. There is no risk of flooding from surface water at the site as shown on the s.w. flood map, with the exception of a small area of low flood risk in front of the existing building at the south of the site. Date: Aug 2017 Page 5

7 3.0 Proposed Site Drainage Strategy 3.1 A proposed development layout is included in Appendix C. The development totals 28 dwellings comprising mix of 2 bed apartments and 2/3 bed homes, served by an adoptable estate road with off-road parking and landscaping. 3.2 It is anticipated that none of the existing on-site industrial drainage will be suitable for re-use. The existing pipework exiting the site onto Danygraig Road may be suitable for re-use subject to condition survey and agreement with DCWW. 3.3 The proposed development will be served by separate surface water and foul drainage systems. 3.4 Foul Drainage A new DCWW-adoptable foul drainage system will serve the future development. It is anticipated that this will be located in the proposed estate road as shown indicatively on the drainage strategy plan in Appendix D. 3.5 Connection to the existing public sewer in the road fronting no.1 Danygraig Bunglows is anticipated. If the existing sewer leaving the site connects to the public sewer in front of the Bunglaows then this will be a public sewer and the connection would be made at the site entrance. DCWW will need to confirm capacity in their infrastructure at this connection point, although this is expected to be a formality assuming the existing industrial site connects here. 3.6 Given the gentle fall in ground level of approximately 1.3m over the length of the site, it is anticipated that a gravity system should be achievable, however the existing invert level at the site entrance or of the connecting manhole (ref. MHST ) will need to be confirmed and site levels designed accordingly. Alternatively an adoptable pumping station would need to be incorporated within the site layout. This could be located at the end of the hammerhead between units Surface Water Drainage The primary objective of any surface water drainage strategy is to mimic the drainage characteristics of the existing site as far as practicable in the future development. The proposed surface water system and associated flows should discharge by one of the following mechanisms, shown below in order of preference; Infiltration / soakaways on site Discharge to the nearest watercourse or land drainage culvert Discharge to the nearest surface water sewer or highway drain Date: Aug 2017 Page 6

8 3.8 As such, infiltration methods (e.g. porous paving, infiltration strips, soakaway tank would be suitable at this site) should therefore be integrated into the detailed site design as far as practicable and subject to favourable ground conditions and measured permeability rates. However given the absence of intrusive site investigation and soakaway testing data at this stage, and the potential identified in the GI desk-study (para. 2.7 of this report) that infiltration may be unsuitable, for the purposes of this report to confirm a viable alternative s.w. disposal method, a strategy of attenuated offsite connection to watercourse is presented. 3.9 An indicative surface water design is shown on the plan in Appendix D. This includes an offsite connection to the culverted stream at the Tir y Cwm Lane junction south of the site. It is anticipated that this will require a new manhole connection on the culvert, although an existing manhole cover is visible on the road at this location which may prove a suitable connection point. This will need to be confirmed the Lead Local Flood Authority (Caerphilly Council) and an Ordinary Water Consent may be required for the connection It may be possible to connect to an existing surface water drain at or closer to the site entrance, but this would be subject to drain condition survey and discussion / agreement with the appropriate receiving authority A flow-control device (hydrobrake or similar) will be located near the site entrance to control offsite to discharge to an agreed flow rate. The discharge rate to the offsite culvert will need to be agreed with the LLFA as part of the detailed design process post-planning For the purposes of estimating indicative storage requirements for this strategy report, the existing site is assumed as a brownfield site with a rule of thumb discharge rate of 25 lit/sec/ha. For an existing drained hard area of 0.66ha this equates to 16.5 l/s. Assuming a 20% betterment factor on this figure as a limiting discharge figure for the proposed development, this would be 16.5 x 0.8 = 13.2 l/s A 13.2 l/s maximum permitted discharge rate would require approximately 200m 3 of onsite storage capacity. If there are known existing flooding problems downstream of the site, a lower discharge rate may need to be agreed. A typical greenfield rate of 5 l/s for example would necessitate approximately 300m 3 of on-site storage. Both of these examples are to hold the critical 100yr+30% design storm onsite without flooding. Outline storage sizing calculations for these figures are included in Appendix E. Date: Aug 2017 Page 7

9 3.14 For the purpose of demonstrating viability and sufficient land availability on the layout, a storage volume of 200m3 is shown indicatively on the drainage plan as a cellular tank located in the landscaped area between the parking bays and Danygraig Road. This plan area of space would be required assuming a 1.2m tank depth. Storage could alternatively be provided in hard engineering form as concrete box culvert sections or oversized pipes in the estate road, or potentially as a surface swale Exact arrangements for storage form, size and location would be agreed at detailed design stage and subject to agreement with the relevant adoption authorities. However for the purposes of presenting a viable option for planning, it is anticipated that the surface water system including cellular tank option as shown on the plan, serving both the plots and the adoptable estate road, would be adoptable by DCWW Land drainage will be required at the western boundary of the site to intercept runoff from the woodland rising to the west. Any existing land drainage system here may be renewed, or else and new system installed. Date: Aug 2017 Page 8

10 4.0 Summary 4.1 The future site will be drained by separate gravity foul and surface water drainage systems. 4.2 A foul connection to the public sewer in the road fronting no.1 Danygraig Bunglows is anticipated. Capacity should not be an issue assuming the existing site connects here. Foul pumping is not envisaged although the invert of the connecting manhole will need to be confirmed and site levels design accordingly. 4.3 Ground infiltration methods for surface water disposal should be employed as far as practicable as a matter of course into the drainage design. At this stage with no ground condition information available, a brownfield-attenuated connection to culverted watercourse offsite is presented to demonstrate a viable alternative should infiltration prove problematic. 4.4 A maximum offsite s.w. discharge rate of 13.2 lit/sec is suggested for the purpose of this strategy report, requiring approximately 200cu.m of stormwater storage on site. Sufficient space for this is shown to be available in the landscaped strip fronting Danygraig Road. For the purposes of this report the storage is shown as a cellular tank which is anticipated would be DCWW-adoptable. Full design details including agreement of discharge rate with the receiving authority would be expected to be agreed as part of a planning condition. Date: Aug 2017 Page 9

11 Appendix A Topographic Survey Date: Aug 2017

12 Appendix B DCWW & NRW Maps Date: Aug 2017

13 Appendix C Site Layout Plan Date: Aug 2017

14 Appendix D Drainage Strategy Plan Date: Aug 2017

15 Appendix E Outline Storage Calcs Date: Aug 2017