The Problem.. SuDS: Controlling the Flow. LANDF RM Technical delivery of SuDS 15/10/2013. Bridget Woods Ballard HR Wallingford 1.

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1 SuDS: Controlling the Flow 15th October 2013 Bridget Woods Ballard The Problem.. Page 2 Bridget Woods Ballard HR Wallingford 1

2 15th October 2013 Landform Event Page 3 HR Wallingford 2013 The Aim of SuDS. Page 4 HR Wallingford 2013 Bridget Woods Ballard HR Wallingford 2

3 The Aim of SuDS 15th October 2013 Landform Event Page 5 HR Wallingford 2013 The greenfield baseline Rainfall 40% evapotranspiration 10% runoff 25% deep infiltration 25% shallow infiltration Recharge groundwater Support river baseflow Page 6 Bridget Woods Ballard HR Wallingford 3

4 The development challenge Rainfall <30% evapotranspiration >55% runoff 10% shallow infiltration 5% deep infiltration Page 7 The dense urban challenge Rainfall >85% runoff? Page 8 Bridget Woods Ballard HR Wallingford 4

5 The National Standards Objective 1: Prioritise Infiltration in order to: Support flood risk management Control water pollution Protect water resources and river baseflows Page 9 Infiltration as a priority, unless: Low permeability Runoff destination Risk of geohazards Risk of groundwater contamination Not practicable Risk of surface water contamination Pumping required Hydraulic connectivity with combined sewer LFRMS precludes Not practicable Surface water body, unless: Surface water sewer, highway drain unless: Combined sewer Foul sewer Page 10 Bridget Woods Ballard HR Wallingford 5

6 The National Standards Objective 2: Mimic greenfield response characteristics in order to: Protect the ecology and morphology of the receiving waterbody Minimise the impact of the development on flood risk associated with the receiving waterbody Page No runoff from small events Nr of events > 50% Rainfall depths (mm) Page 12 Bridget Woods Ballard HR Wallingford 6

7 The hydrologic functions of streams change with as little as 5 10% imperviousness and they change profoundly when imperviousness approaches 25%. Johnston, et al., 2006 Why? Eroded/incised channel Non functional floodplain Pollution Sedimentation Depressed water table, dry riparian soils Page 13 How do we do that? > Infiltration systems > Rainwater harvesting systems (provided regular demand) > Green roofs > Permeable pavements > Swales, filter strips, trenches > Detention basins Page 14 Bridget Woods Ballard HR Wallingford 7

8 2. Control peak flow rates Control 1 in 1 year to greenfield equivalent > 1 in 1 year event = approximately bank full (morphological control) Control 1 in 100 year to greenfield equivalent > 1 in 100 year event = extreme flood flow rate Page 15 Why? Runoff rates (l/s/ha) Return period (yrs) Page 16 Bridget Woods Ballard HR Wallingford 8

9 How do we do that? Qin Qin > Qout Attenuation storage Qout Page 17 Where can we do that? Page 18 Bridget Woods Ballard HR Wallingford 9

10 Where can you put storage? 15th October 2013 Landform Event Page 19 HR Wallingford 2013 Evidence Page 20 Bridget Woods Ballard HR Wallingford 10

11 3. Control volume of runoff Control 1 in 100 year runoff volume to the greenfield equivalent > 6 hour suggested as suitable event duration to reflect events that would impact on receiving catchment response Page 21 Why? Page 22 Bridget Woods Ballard HR Wallingford 11

12 How? Long Term Storage = Development runoff volume Greenfield runoff volume (for 100 year, 6 hour event) Page Designing for exceedance Manage risks from events that exceed the system design capacity (people, property, the environment) Some areas may be allowed to flood at lower return periods than others depending on use etc For example: You may elect to flood landscape areas during period of extreme rainfall Or protect landscape areas but route exceedance flows to designated site storage facilities Roads / car park areas could be allowed to flood during extreme events Take account of benefits of multi functionality in delivering development value and viability 15th October 2013 Landform Event Page 24 HR Wallingford 2013 Bridget Woods Ballard HR Wallingford 12

13 > Simple, feasibility level Tools to help you > Greenfield runoff rate estimation > Storage volume estimation > RWH design for stormwater control > Site evaluation > Costing. (later) Updated EA report: RF-RO management for new developments W5-074 Page 25 Page 26 Bridget Woods Ballard HR Wallingford 13

14 Page 27 Page 28 Bridget Woods Ballard HR Wallingford 14

15 Thank you 15th October 2013 Landform Event Page 29 HR Wallingford 2013 Bridget Woods Ballard HR Wallingford 15