Lesson 37: Low-Impact Urban Development

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1 Lesson 37: Low-Impact Urban Development 53:171 Water Resources Engineering Low-Impact Development (LID) LID is a site design strategy with a goal of maintaining or replicating the predevelopment hydrologic regime through the use of design techniques to create a functionally equivalent hydrologic landscape LID deals with stormwater management 1

2 Development Changes the Landscape Pre- Development Landscape Post- Development Landscape Goal: Maintain predevelopment hydrologic regime Changes in Hydrologic Regime Daily Discharge (cfs) Boneyard Creek, Urbana, IL More Urban Development Traditional SWM Larger flood discharges LID Less Baseflow Less Urban Development Exceedance Probability (% ) 2

3 Low-Impact Development: The Next Phase of Stormwater Management Low-Impact Principles Control stormwater at the source by the use of small-scale controls that are distributed throughout the site Reduce the effective impervious area Promote infiltration and groundwater recharge Lengthen flow paths and runoff time Preserve/protect environmentally sensitive features 3

4 Development Alternatives Site Tradition Low-Impact Site Design Vegetation Cover Turf Grass Deeprooted native vegetation promotes infiltration [Courtesy of Conservation Design Forum] 4

5 Driveways Traditional Design Low-Impact Design Residential Practices Compost Amended Soils Rain Garden 5

6 Residential Practices Rainwater Harvesting Green Roof Porous (Permeable) Pavement 6

7 Roadway Median Design Roadside Bioswale Low-Impact Development: Can it Replicate the Pre-Development Hydrologic Regime? 7

8 Urban Development Scenarios Pre-development High Impact Low Impact All vegetated 100% A pervious Part vegetated, Part direct impervious Part vegetated, Part connected impervious A impervious = A pervious Storm Runoff Volume Runoff Fraction Soil Type A Pervious Soil Pre-development Precip (in) Runoff Fraction Soil Type D Impervious Soil Precip (in) 8

9 Storm Runoff Volume Runoff Fraction Soil Type A Pervious Soil High Impact Precip (in) Runoff Fraction Soil Type D Impervious Soil Precip (in) Storm Runoff Volume Runoff Fraction Soil Type A Pervious Soil Low Impact Precip (in) Runoff Fraction Soil Type D Impervious Soil Precip (in) 9

10 Design Flood Hydrographs year Design Storm Pre-development Time (hr) year Design Storm Time (hr) Ralston Creek, Iowa City Design Flood Hydrographs year Design Storm High Impact Time (hr) year Design Storm Time (hr) Ralston Creek, Iowa City 10

11 Design Flood Hydrographs year Design Storm Low Impact Time (hr) year Design Storm Time (hr) Ralston Creek, Iowa City Annual Water Balance Total Volume per unit area (cm) Evapotranspiration Direct Runoff Recharge 0 Turf High Low Case Ralston Creek, Iowa City 11

12 Insights LID is most effective for small, frequent events Both the impact & benefit of infiltration practices depend on soil texture Still need to address flooding concerns for larger, less frequent events Hydrologic Design of LID Practices 12

13 Compost Amended Soils A amended A impervious Area and depth of compost amended soils Design parameters Amended area ratio A amended / A impervious Depth of amendment d Design Criteria Volume criteria (Event-based) Retain the first ½ inch of runoff from impervious areas Regime criteria (Continuous simulation) Maintain runoff from site at (or below) pre-development level Maximize groundwater recharge at site 13

14 One-Dimensional Modeling Design Example + Impervious Runoff One-Dimensional Modeling Design Example Iowa City weather + Impervious Runoff 14

15 One-Dimensional Modeling Design Example Iowa City weather Modified Van Genuchten parameters for amended soils + Impervious Runoff One-Dimensional Modeling Design Example Iowa City weather Modified Van Genuchten parameters for amended soils Continuous hourly simulation (20 years) 15

16 Water Budget A amended = A impervious Amended ratio = 1 Type B soil Amendment depth for no net increase in runoff? d ~ 10 cm Varying Soil Texture Other SCS soil types with Iowa City weather Varying ratio of amended to impervious areas Depth of Amendment (cm) B C D Amended Area ratio 16

17 Regional Variations Pacific NW Midwest Mid- Atlantic Midwest Sites Required Depths for Given Soil Type and Location MAD-ORD ORD-DSM DSM-MAD

18 Generalized Design Curves Depth of Amendment (cm) B C D Amended Area ratio Power Function fit: d min = b (A a ) m Can apply relation by soil type for Midwest and Middle Atlantic stations Pacific Northwest Sites Required Depths for Given Soil Type and Location MFR-PDX PDX-SEA SEA-MFR

19 Rain Garden (Bioretention) Rain Garden Capture surface runoff for concentrated infiltration and recharge Rain Garden Performance Optimal size is 10-20% of impervious area Storage zone thickness relatively insensitive Dussiallant et al [2004] 19

20 Some more insights... LID practices and their design The impact on hydrologic regime depends on soil properties A site s hydroclimatology plays a big role Quantity or quality? More is better Focused recharge (rain gardens) usually more effective LID can be effective in any development type Hydrologic Design Challenges 20

21 Change the Stormwater Mentality Traditional event-based stormwater management design is insufficient LID is aimed at everyday events Dependent on weather sequences Continuous simulation of water cycle is needed for design guidance Flow regime rather than event volume Hydrologic Prediction Predicting the changes in soil hydraulics Inspect/monitor as part of development? 21

22 Hydrologic Prediction Predicting the changes in soil hydraulics Inspect/monitor as part of development? Can we say how changes affect Curve Numbers (CN)? Understanding subsurface site characteristics Evaluating integrated effects of disaggregated LID control measures Let s wrap things up 22

23 Conclusions LID deals with more than stormwater management LID does not replicate all aspects of the hydrologic regime low impact, not no impact Need sound technical (hydrologic) basis for design of LID practices 23