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

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SUDS Requirements – The Four Pillars

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SUDS Change In Legislation – Beneficial?

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

or 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?

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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 obtaining 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

offset 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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