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Rhyl 3-16 Catholic School – Drainage Strategy Report 1

Technical Note

Project: 3-16 Catholic School Rhyl To: Kier Construction

Subject: Drainage Strategy Report From: Ben Scott (Atkins)

Date: 20.10.17 Ref: 5157258/DSR/002

1. Site Description and Location

The Rhyl Catholic School site is in Rhyl (Denbighshire) at an approximate grid reference SJ 01722 80687. Redevelopment of the existing school site has been proposed, where an extent of the existing school areas will be retained while the new proposed development is constructed in a 2-phase plan. Ultimately all existing school buildings will be demolished. The site is currently occupied as a separate high school and primary school named as Blessed Edward Jones High School and Ysgol Mair RC Primary School.

The 3.5-hectare site is bordered to the east, west and south with residential buildings and a small business park to the north. Existing topography suggests that the site ranges from 10.82mAOD (south) to 4.85mAOD (north). The new building will be located in the centre of the site. The proposal is a 2-storey teaching facility which will consist of classrooms, Design and Technology rooms, Nurture, LRC/ICT, group rooms, halls, sports hall, chapel, dining room, music and music practice rooms, HUB, kitchen and ancillary spaces. With external car park facilities, MUGA and sport pitches.

Proposed development site location is included in Appendix A.

A proposed preliminary drainage layout has been completed (Appendix E). However, ground investigation results are pending, to confirm if ground conditions are suitable for infiltration, together with the results of a CCTV investigation. The results from the investigations mentioned above could affect the proposed preliminary drainage layout. A proposed site plan has been completed by DEP Landscape Architecture (Appendix B), however, no landscape levels have been finalised at this stage. This document also provides information relating to the water conservation statement.

Figure 1 – Proposed development site plan


Technical Note

2. Existing Conditions

2.1. Ground Conditions

No ground investigation (GI) has been undertaken yet. Infiltration testing will be undertaken in due course to confirm if infiltration will be suitable for this site, this will be in accordance with BRE Digest 365. However, from anecdotal evidence that it is unlikely that infiltration will be feasible.

2.2. Proximity to Watercourses

The site is in the proximity of a small pond named Pwll Brickfields Pond and New Cut watercourse. Flood probability information taken from the Natural Resources Wales flood map does suggest the presence of any watercourses within the immediate vicinity of the site.

2.3. Public Sewer Networks

The proposed school site is close to a Welsh Water owned public combined sewer, as shown in their sewer records (Appendix C). The public sewers adjacent to the site are:

• Combined water sewer (Unknown diameter) – along the northern boundary

• Surface water sewer (Unknown diameter) – along the eastern boundary

2.4. Existing Drainage on Site

A GPRS (Ground Penetrating Radar Survey) drainage survey has identified that the site is served by two separate systems consisting of a combination of surface water system and foul water system shown on the drawings in Appendix D. These systems consist of:

• 2x Combined drainage systems: ultimately serving all foul and surface water flows from the impermeable areas and foul drainage within the school site.

• Surface water drainage system: serving some hardstanding drainage within the site.

• Foul water drainage system: serving some foul drainage within the site.

The contents of the initial drainage survey have been reviewed and further investigations are to be completed. These further investigations will consist of a CCTV survey to confirm that the two separate combined water systems outfall to the Welsh Water combined sewer. The GRPS survey suggests this to be the case, however, the survey was terminated at the site boundary just short of the area where the Welsh Water combined sewer is located.

It is assumed from the GPRS drainage survey that all drainage within the site boundary only serves the 2 existing schools and associated hardstanding areas and does not receive flows from any of the surrounding residential properties.


Technical Note

2.5. Pre-development Surface Water Discharge Rates

The site currently has an existing impermeable area of approximately 2.1ha which is assumed to be discharging to the Welsh Water combined sewer via 2 x combined drains (1x 300mm diameter and 1x 150mm diameter) existing the site. The generated runoff from the site can be estimated with the Rational Method (Lloyd-Davis) approach as summarised below:

Q = 2.78 C i A

Where Q = run-off in litres per second (l/s)

A = Area being drained in hectares (2.07ha)

i = rainfall intensity in millimetres per hour (50mm/hr in line with guidance from Building Regulations Part H)

C = non-dimensional catchment coefficient, which is dependent on the catchment characteristics (a figure of 1.0 is appropriate)

Therefore, using the above equation, existing peak runoff generated on site is calculated as 287l/s. However, the invert levels of the Welsh Water combined sewer are not yet known and a gradient for the two combined drains exiting the site cannot be calculated at the present time. Once the Welsh Water combined sewer invert levels are confirmed via a requested CCTV survey, it can be established if the 2x combined drains exiting the site have the capacity to discharge 287l/s.

It is expected that Welsh Water will require a 30% reduction on the existing surface water peak flow rate for the proposed development.

3. Proposed Drainage

The proposed development is not classed within the hierarchy of flooding from rivers and sea in the Natural Resources Wales Flood Risk map (High, Medium, Low and very low). Therefore, the proposed development is at a very low risk of flooding.

The new school building, car park and associated sports facilities will be served by a number of drainage systems as follows;

• Foul drainage – Serving toilets and other sanitary appliances.

• Surface water drainage – Serving roofs, footpaths, hardstanding areas, games areas and the new car park.

It is envisaged that no land drainage will be required on this site. This will be confirmed once landscape levels are finalised.

A proposed drainage layout is illustrated in Appendix E.

3.1. Proposed Foul Drainage

Foul water flows from the site are proposed to be discharged to the existing 300mm diameter combined drain within the site via a new chamber. This combined drain is assumed to ultimately outfall to the Welsh Water combined sewer north of the site. Based on the existing site levels on the site, it is assumed that a connection by gravity will be possible, however this is subject to confirmation of sewer levels.

A Pre-development Enquiry will be sent to Welsh Water in due course so the drainage design can be accepted in principle.

The estimated peak foul flows from the proposed development, based on the unit discharge rate calculation using BS EN 12056 – 2:2000, is: -

• Total foul flows are 9.7 l/s.

The foul flows calculation sheets are included in Appendix G.


Technical Note

3.2. Proposed Surface Water Drainage

The proposed storm water drainage layout is included in Appendix E. The impermeable area from the proposed new development is approximately 1.76ha.

Method of discharge

The Building Regulations - Approved Document H (2002) details a hierarchy of potential methods for disposing of surface water as shown below in order of preference:

• Discharge via infiltration

• Discharge to watercourse

• Discharge to sewer

Discharge via Infiltration

Infiltration testing in accordance with BRE 365 is due to be undertaken on the site. However, from anecdotal evidence and that there is no evidence of any existing types of SUDS system on the existing site it is unlikely any infiltration will be feasible.

Discharge to a watercourse

The closest watercourse to the site is New Cut watercourse located approximately 200m north of the site. Also, there is a small pond (Pwll Brickfields Pond) which is approximately 400m east of the site. It is assumed that a connection to the pond or the watercourse is not a feasible option due to the distance from the site.

Discharge to Sewer

Given the above information, the only available method for surface water discharge is to the Welsh Water public sewer, subject to the results of the site investigation (CCTV confirmation of the 2x existing combined drain connections to the Welsh Water combined sewer). The surface water discharge will be limited by the existing peak flows imposed by Welsh Water. It is proposed to restrict the discharge to a 30% reduction of the existing discharge rates.

The calculated peak discharge will be in the order of 201l/s with the assumption of a 30% reduction on the existing peak discharge rate (as calculated in section 2.5). The initial proposed point of connection will be upstream of the private 300 diameter combined drain within the site which is assumed to outfall to the Welsh Water combined sewer north of the site. This strategy will be pending the response from a pre-development enquiry to Welsh Water and confirmation of the levels of the Welsh Water combined sewer.

Surface Water Drainage Strategy

Assuming that the Site Investigation confirms that the discharge via infiltration is not feasible, the proposed drainage layout illustrated in Appendix E can be summarised as follows:

• Flows from the new development (roofs, access road, car park, games areas and hardstanding areas) will be collected and discharged into the 300mm diameter combined drain on site via a new connection at an overall discharge rate of 201l/s. If the Welsh Water combined sewer levels are not satisfactory to convey 201l/s through the pipe (the length of 300mm diameter combined drain between the new chamber and the assumed connection into the Welsh Water sewer) at a self-cleansing velocity of 1m/s and the gradient is not steep enough accommodate the proposed flow, a direct new connection into the Welsh Water combined sewer will be proposed (S106 new connection application will be applied for to Welsh Water).

• A flow control device will be provided to reduce the discharge to a maximum total of 201 l/s for the 1 in 100-year storm (+ 40% climate change).


Technical Note • Peak surface water flows in excess of 201 l/s during storms of up to 1 in 30 years will be temporarily

attenuated below ground on site via a combination of underground structures (geo-cellular storage and pipes). The design of any surround to the tank will depend on further geotechnical investigations at the location of the tank.

• Peak surface water flows in excess of 201 l/s during storms in excess of 1 in 30 years (up to and including 1 in 100 year with a 40% allowance for climate change) will be attenuated on site in below ground geocellular storage. The estimated size of the underground geocellular storage is 500m3.

• The detailed design of the attenuation systems will depend on additional geotechnical investigations and designs which will assess the possibility of achieving some surface water disposal by infiltration from the below ground tank.

• Overland flow routes as a result of exceedance or failure in the system will be controlled within the site with landscape levels to protect the building and adjacent properties against surface water flooding.

The proposed drainage layout is illustrated in the layout plan included in Appendix E.

Storm Drainage Design Parameters

The design parameters for surface water and roof drainage will be:

Greenfield runoff rate IH 124 Method Flows and infiltration As generated by Microdrainage XP Solutions Pipe roughness coefficient 0.6mm Standards of service No surcharge in system from storm of 1yr return period

No flooding from storm of 30yr return period No flooding of occupied premises from storm of 100yr period with 40% allowance for climate change

3.3. Outstanding investigation

The following investigations are required before proceeding with the detailed design:

• Infiltration testing in accordance with BRE Digest 365 should be undertaken on the site.

• Confirm the levels, location and conditions of the 150mm diameter combined drain and 300mm diameter combined drain exiting the site.

• Confirm the levels and exact location of the Welsh Water combined sewer north of the site.


Technical Note

Appendix A.- Development Site Location


Technical Note Appendix B.- Proposed Site Plan


Technical Note


Technical Note Appendix C.- Welsh Water Sewer Records


Technical Note


Technical Note Appendix D.- Existing Drainage Systems


Technical Note


Technical Note Appendix E.- Proposed Drainage Layout


Technical Note


Technical Note Appendix F.- Attenuation Storage Calculations


Technical Note Appendix G.- Foul Drainage Calculations

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