Consulting Civil & Structural Engineers SAB Seminar - - - PowerPoint PPT Presentation

HCE Limited – Ioan Howells Consulting Civil & Structural Engineers

SAB Seminar - Consultant’s Perspective 16th May 2019

Introduction

 SUDS Requirements – The Four Pillars  SUDS Change In Legislation – Beneficial?  Impact Upon Stakeholders



Homeowners/Self Builders



Small Developers (2-10 unit Typical Developments)



Larger Developers



Agricultural Users (e.g. Farmers)

 Case Study 1 – Single Unit Development  Case Study 2 – Mixed Use Development  Case Study 3 – Agricultural Development  Summary 1

SUDS Requirements – The Four Pillars

2

SUDS Change In Legislation – Beneficial?

3



Since 07th January 2019 developments with construction area of 100m2

• r more will require sustainable drainage to manage on-site surface

water.



There has been a growing concern by approving bodies in England and Wales that not enough is/has been done historically at the masterplanning stage of developments. Wales has taken the lead via the recent change in legislation thereby enforcing consideration of SUDS at the planning stage.



Government/advisory bodies are concerned about increase in flooding events, pollution from uncontrolled run-off and the lack of groundwater recharge due to the increase in urbanisation with positive drainage systems not replicating natural greenfield drainage systems.



Concern by some stakeholders that the change in regulation will significantly increase costs for development.



At the moment we are in the early stages of implementation (first 5 months) so the true effect and impact is still being assessed.

Need For Balance?

4

End User/Client Concerns

 CONCERNS RAISED – REAL OR PERCEIVED?  Another Additional Cost To Be Borne  Increase In Development Risk  SUDS Drainage Solutions Cost More Than Traditional Solutions  SUDS Require Large Areas Of Land Use Required  Disproportionate Burden For Small Developments/Single Residential Units

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Impact Upon Stakeholders

Single Dwelling/Self Builders



No Net additional cost to development but increase financial risk due to design costs being required upfront, prior to securing Planning.



For small single unit developments not cost effective for drainage design to be separated. Therefore developer incurs full design costs early!



Due to size of development and available land use – SUDS solutions can be limited but can also driven by client.



Cost Savings from SUDS solution limited due to scale of development

Minor Developments (<10 Units, Small Commercial)



No Net additional cost to development but increase financial risk due to design costs being required

• upfront. Although proportionally this is much less compared to single unit developments.



Full array of SUDS solutions are usually available. The challenge is changing perception/mindset of developers to consider wide array of SUDS solutions and potential benefits that good schemes bring.



A SUDS solution is usually cheaper than a traditional drainage system (10% Considered

Achievable).



Large Areas Of land Use Required – No net increase in land use required. However there has been a general increase

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Impact Upon Stakeholders

Major Developments (>10 Units, Large Commercial)



No Net Additional Cost For End User.



Minor Increase In Development Risk – Yes but considered much less impact as a result of the scale of typical developments. Larger developers usually have available funds set aside for

• btaining planning.



SUDS solution can be cheaper than a traditional positive drainage system (10% Considered Achievable).



Implementation of a good SUDS solution should not necessarily require additional areas of land

• use. A more balanced wider array of SUDS solutions are possible on larger developments and

should be reviewed/implemented. Change in legislation is considered positive for such development ensuring early implementation during the masterplanning phase. This will benefit large developments by enhancing the communal areas and improve bio-diversity whilst improving water quality and groundwater re-charge.

Agricultural Developments



Additional Engineering Design Costs Incurred To Secure Planning & Increase In Risk



Larger Impact Upon End Users - Agricultural Developments have historically had less planning restrictions compared to residential/commercial developments.



Agricultural buildings are getting bigger! A typical 400 cattle shed can equate to similar roof drainage runoff from around 25-30 houses. Managing SW runoff off is considered key to reducing localised flooding and fluvial contamination.



Additional Capital Costs For Constructing Suitable Drainage Solutions, however these can be

• ffset by implementation of RWH and Reuse. On farms there is always a need for water!

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• St. Anne’s Vicarage

Case Study 1 – Single Unit Development



Design Challenges



Impermeable Site with Heavy Clay



No Nearby Watercourse



Within Burry Inlet Area



Discharge To Combined Public Sewer Accepted But with Low Discharge Rate 

SUDS Design Features



WAVIN Aquacell Attenuation Storage Cells With Flow Control Meter Used To Control Peak Volume



Permeable Paving To Help Attenuate and Improve water quality of SW run-off



Rain Gardens for bio-remediation to improve quality of amenity space 

Lessons Learned



Even without source control small scale SUDS solutions can be used to improve habitat/bio-diversity, water quality, slow run-off and reduce run-off volumes

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Cwrt Canna, Llangan – 13 Residential, 3 Commercial Units Case Study 2 – Mixed Use Development



Design Challenges



3-4m cross fall across site



Impermeable Soils At Shallow Depth



Small Permeable Band at Circa 3-4m depth



Site Area Quite Tight 

SUDS Design Features Incorporated



Permeable Paving Used Throughout Except For Main Access Into Site



Pond Used To Enhance Bio-Diversity & Water Quality But Also Attenuate Run-Off From Commercial Development Areas Up To 24hrs for extreme events



Swales Used To Convey Water Through Development



Filter Strips Used To Convey Water Around Boundary



Deep Borehole Soakaways For Infiltration



Green Roofs For Residential Units



Rain Gardens To Be Used To Enhance Bio-Diversity & Water Quality



Tree Pit At Centre of Residential Square 

Lessons Learned



Early Design Engagement Essential To Achieve Quality SUDS Solution



Consideration of All Aspects Of SUDS Solutions Will Create Flexible Design Solutions To Manage SW Run-Off



Encouragement of RWH and Grey Water Schemes – Still Not Much Take Up

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New 122x32 Cattle Shed Carmarthenshire Case Study 3 – Agricultural Development



Design Challenges



3-4m cross fall across site (maximising Soakaway and attenuation Efficiency)



Permeable Soils At Shallow Depth Located in adjacent field



Volume of Run-Off- Roof Area ~0.6Ha equivalent to ~30-35 domestic roofs.



No Obvious Surface Attenuation Systems & Client Did Not Want to Consider RWH.



SUDS Features



Ground Permeable At Shallow Depth. Therefore reasonable For Infiltration



Use of weirs within soakaway to ensure (maximising infiltration and attenuation Efficiency achieved).



Discharge to watercourse limited to less than 2l/s/ha



Roof Area ~0.6Ha equivalent to ~30-35 domestic roofs



Lessons Learned



Peak Volume Control For Extreme Events Will Require Management Of Large Volumes



RWH and re-use for such developments should be considered as part of holistic approach to SW run-off management.



RWH can be quite cost effective with a reasonable payback period achievable. An accurate cost/benefit analysis can be carried out.

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Summary



No Net Additional Cost To Stakeholders More Timing Issue.



Some Increase Financial Risk Due To Design Costs Being Required Upfront Prior To Securing Planning.



Single Dwelling/Self Builders Impacted By Change In Legislation. Design costs for development are proportionally higher than for larger schemes and costs are incurred upfront resulting in increased risk.



Minor & Major Residential Developments No Cost Impact By Change In Legislation. It is Considered Change Will Likely Improve The Quality Of Schemes Considerably By Early Engagement!



Agricultural Sector Considered Most Impacted By Legislation Due To Historical Issues Related With Planning Process/Obtaining Planning Approval.

Any Questions?

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