Surface Water Drainage Latest policy and practice in respect of surface water drainage and delivery of competitive solutions August 2013 Claire Cornmell

2 Aim: To know more than we need to know

3 Some topics for discussion Flood and Water Management Act Draft National Standards Permeable Paving Detention Basins

Slide 4 Exceptions to the requirement for approval: - internal drainage boards - highway authority roads - a railway by Network Rail - does not apply to construction work (a) unless the construction work requires planning permission; or (b) if the construction work does not require planning permission, unless it involves the construction of a building or other structure covering an area of land of 100 square metres or more Flood and Water Management Act

Slide 5 Draft National Standards The Local Planning Authority may set more stringent local requirements. The SAB is responsible for adopting and maintaining new SuDS that serve more than one property and have been constructed as approved and function as designed. I. Principles II. Standards with design, construction and maintenance requirements for SuDS

Slide 6 Draft National Standards Part I – Principles a.Managed at source b.Managed on the surface c.Public space is used and integrated with the drainage system d.Design is cost-effective to operate and maintain over its design life e.Account for climate change and changes in impermeable area over the design life of the development Affordability – compared to conventional system Functionality – 8 weeks following inspections

Slide 7 Draft National Standards Part II – Section A – Runoff Destination 1. Ground - unless the rate of runoff is greater than the rate of infiltration, in which case as much of the water as reasonably practicable must be discharged by infiltration - unless ground instability, mobilising contaminants, water quality (SPZ1, BAP etc), groundwater flooding, leakage of groundwater to combined sewer 2. Surface water body – unless not reasonably practicable to convey, pumping required or unacceptable risk of flooding 3. Surface water sewer or local highway drain 4. Combined Sewer

8 Draft National Standards Attenuation Storage Interception Losses Long Term Storage Approach 1 Approach 2 2 l/s/ha Q1 GF Q100 GF Q1 GF or 2 l/s/ha Qbar GF or 2 l/s/ha Part II – Section B – Peak Runoff Rate and Volume

Slide 9 Draft National Standards Previously developed land a)An approach as close to Approach 1 as is reasonably practicable b) The flow rate discharged from the site must not exceed that prior to the proposed development for: - the 1 in 1 year event; and - the 1 in 100 year event c) The volume of runoff may only exceed that prior to the proposed development where the peak flow rate is restricted to 2 l/s/ha.

Slide 10 Long Term Storage Decide whether unpaved areas contribute or not Formula assumes 80% run-off from paved areas, but can adjust to 100% for conservative approach. Uses 100 year 6 hour rainfall event Vol = RD.A.10 [ ( α 0.8) + 1- (β.SPR) – SPR ] Where pervious areas are landscaped so they do not enter the network: Vol = RD.A.10 [ 0.8 – SPR ] Where all pervious areas are assumed to continue to drain to the network: Vol = RD.A.10 [ 0.8 – SPR ] 100 PIMP 100 PIMP 100 PIMP 100 PIMP 100 PIMP

Slide 11 Draft National Standards Part II – Section C – Water Quality

12 CBPP (Concrete Block Permeable Paving) Clay pavers Permeable Macadam Permeable Resin-bound aggregate Gravel Grass paving Permeable/ No-fines concrete Self-binding golden gravel Permeable Paving

13 The sub-grade layer of a pavement is, essentially, the underlying ground. Geo-textiles are permeable sheets, usually non-biodegradable. Geo-membranes are impermeable and usually non- woven, i.e. a film or solid sheet. The sub-base layer is the main load- bearing layer. The interlock between adjacent particles of the sub-base material works by distributing a point load over a larger area to the sub- grade below. Permeable Paving

14 Laying course: 3-6mm grit (2–6.5mm) Sub-base: Type 4/20 (4mm minimum and 20mm maximum particle size) coarse graded aggregate (and/or hydraulically bound CGA in more heavily trafficked areas). Typically mm deep.

15 Permeable Paving Service Corridors Construction A bitumen macadam may be installed with holes punched through on a 750mm orthogonal grid. Sloping Sites Check dams Terracing Geocellular at low end Increase sub-base thickness

16 Permeable Paving Pollutant Removal Removes 60 to 95% of total suspended solids (i.e. silt) and 70% to 90% of hydrocarbons. Heavy metals remain within structure. Interception Storage Interpave – Jan 2010 Edition 6 Maintenance

17 Resources: Interpave: Permeable Paving

18 Side slopes Depth Invert level of Outlet Structure Freeboard / Design Exceedance Detention Basins