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Assessment of Low Impact Design (LID)


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Assessment of Low Impact Design (LID)

  1. 1. Assessment of Low Impact Design (LID) Strategies using Integrated and Distributed Surface Water/Groundwater Models Presented to: IAH Conference October 2, 2013 Dirk Kassenaar, M.Sc. P.Eng. M.A. Marchildon, M.Sc. P.Eng.
  2. 2. 2 Land Development Impacts ► “They paved paradise and put up a parking lot…” Assessing the impacts of land development is certainly important! ► SW assessments have focused on peak flows and, more recently, on how Low Impact Development (LID) can mitigate storm sewer “end of pipe” flows. ► Recent work indicates that a more holistic approach is needed, including assessment of the whole flow regime (not just peak flows) and impact to GW levels and baseflow discharge to wetlands
  3. 3. 3 Low Impact Development (LID) Strategies Local LID Features: - A local LID feature captures and attenuates storm water - e.g. bioswales, permeable paving, rain barrels, green roofs, soak-away pits, etc. A bioswale can attenuate pavement runoff by enhancing ET and GW infiltration
  4. 4. 4 Assessment of Low Impact Development ► Low Impact Development strategies offer significant benefits ► Not all LID strategies will work in all locations. Need to consider:  Soil and surficial geologic conditions (infiltration capacity)  Depth to water table (possible rejected infiltration)  Other factors such as terrain, slope accumulation, and pervious/impervious configuration ► SW-only models are focussed on end of pipe sewer flows and stormwater ponds:  Cannot predict if hydrogeologic conditions are suitable for a specific LID design  Cannot predict if ecologic and hydrogeologic benefits will actually be achieved. ► GW-only models cannot predict the complex change in 3D recharge ► Only an integrated GW/SW model approach can assess all aspects of a LID implementation  Which LID is optimal and where? Will the ecological benefits be achieved?
  5. 5. 5 Integrated Water Systems Modelling ► Integrated GW/SW modelling involves:  Groundwater: Flow through the subsurface  Hydrology: Vegetation, land use and soil zone  Hydraulics: Flow in streams, wetlands and lakes ► “Fully-distributed” modelling approach  Study area is subdivided into millions of cells  Soil zone hydrology and groundwater processes simulated in each unique cell  Streamflow simulated in a linear channel network that accepts cascading overland runoff and pickup (or loss) from the aquifer systems
  6. 6. 6 USGS-GSFLOW 6 Integrated Ground-Water and Surface-Water Flow Model Based on the Integration of the Precipitation-Runoff Modeling System (PRMS) and the Modular Ground-Water Flow Model (MODFLOW)
  7. 7. 7 GSFLOW Hydrology: Sub-Cell Processes ► Each upper layer model cell has both pervious and impervious areas and processes. Impervious areas & Depression storage Pervious area Tree canopy (interception) Micro-topographic depressions Parking Lot Rooftop
  8. 8. 8 Conceptualization of LIDs in GSFLOW ► A Manabe (1969) Reservoir was added to each cell to represent the local LID feature ► The LID Reservoir can receive water from the impervious area and, depending on the E, Q and D parameters, attenuate and infiltrate that water Impervious areas & Depression storage Pervious area Tree canopy (interception) Micro-topographic depressions Parking Lot Rooftop LID Reservoir Parameters: E =Evaporative loss Q=Overflow D =Drainage
  9. 9. 9 ► Bioswales  E>0, Q>0, D=K ► Green Roofs  E>0, Q>0, D=0 ► Retention Ponds  E>0, Q=0, D>0  (Smax=∞) E =Evaporative loss Q=Overflow D =Drainage ► Detention Ponds  E>0, Q>0, D>0 ► Infiltration Galleries  E=0, Q>0, D=K ► Rain Harvesters  E=0, Q>0, D=D(t) GSFLOW Manabe Reservoir - One reservoir available per model cell - Parameters adjusted to represent a variety of LID features (Figures from CVC & TRCA, 2010)
  10. 10. 10 Additional LID Conceptualization: Permeable Pavement Simulated by decreasing the (effective) percent imperviousness Roof Downspout Disconnection Simulated by routing impervious runoff to (same-cell) pervious area (CVC & TRCA, 2010) (CVC & TRCA, 2010)
  11. 11. 11 Centralized LID Features 11 Centralized LID Features are larger scale features that receive water from upslope impervious sources or 3rd-pipe roof runoff
  12. 12. 12 Modification of Cascade Network for Centralized LIDS ► A cascade network is used to route overland flow and interflow ► Segments of the network can be changed (red arrow) to direct a portion of locally captured water to a Centralized LID 12 (Markstrom, 2008)
  13. 13. 13 Seaton MESP LID Assessment Objectives ► Proposed new development for 70,000 residents north of Pickering, Ontario ► Simulation Objectives:  Evaluate overall cumulative effects of various LID configurations ► Which LID strategy (or combination) should be used, and where?  Will the ecological function of the wetlands and ponds be preserved? ► Will buffers around the NHS lands be sufficient?  Can the impacts on the underlying aquifers be mitigated through LIDS? ► Issues:  Commercial-industrial land use planned for high recharge Iroquois Beach sands  Need for quantitative comparison of alternatives
  14. 14. 14 Seaton Lands - Hydrogeologic Conditions ► Complex hydrogeology: 3 Aquifers day-lighting along Duffins Creek ► Extensive wetland connectivity and riparian zones A A’ A A’
  15. 15. 15 Seaton Existing Landuse Agricultural Natural Heritage Urban 15
  16. 16. 16 Seaton Proposed Landuse 16 Agricultural Natural Heritage Residential Parks Commercial Institutional
  17. 17. 17 Implemented LIDs ► Employment areas: Rooftop capture and 90% of the overflow being redirected to bioswales ► Residential, recreational and school areas  Roof-to-lawn routing of impervious runoff (amount dependent on roof coverage as a proportion of modelled cell); ► Unlined (leaky) storm water management ponds ► Infiltration gallery for commercial developments on the Iroqouois Beach ► Road side ditches along rural cross sections as opposed to serviced roadways.
  18. 18. 18 Existing Conditions: Generated Runoff
  19. 19. 19 Post Development: Generated Runoff
  20. 20. 20 Post Development with LID: Generated Runoff
  21. 21. 21 Existing Conditions: Cascading Runoff Click for Animation
  22. 22. 22 Post Development: Cascading Runoff Click for Animation
  23. 23. 23 Existing Conditions: Actual ET
  24. 24. 24 Post Development: Actual ET
  25. 25. 25 Post Development with LID: Actual ET Bioswales
  26. 26. 26 Existing Conditions: GW Recharge Iroquois Beach Sands
  27. 27. 27 Post Development: GW Recharge Iroquois Beach Sands
  28. 28. 28 Post Development with LID: GW Recharge Iroquois Beach Sands
  29. 29. 29 Predicted GW Impacts – No LIDS ► Simulations indicate unmitigated development would cause up to 4 m of aquifer drawdown and a corresponding decrease in baseflow discharge to streams
  30. 30. 30 Predicted GW Impacts – With LIDS ► Simulations indicate LIDS would sustain groundwater recharge and mitigate effects on aquifer levels and stream baseflow
  31. 31. 31 Seaton LIDS Analysis: Conclusions ► Integrated modelling identified the unique and site specific recharge functions in the Seaton Lands MESP area ► Detailed cell-based simulations were able to represent site specific LID implement issues and benefits ► Modelling provided a framework for comparison of LID scenarios, and facilitated discussions with the Municipality and TRCA