assessment of low impact design (lid)
TRANSCRIPT
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.
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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
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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
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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?
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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
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USGS-GSFLOW
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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)
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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
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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
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► 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)
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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)
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Centralized LID Features
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Centralized LID Features are larger scale features that receive water from upslope impervious sources or 3rd-pipe roof runoff
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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
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(Markstrom et.al., 2008)
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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
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Seaton Lands - Hydrogeologic Conditions
► Complex hydrogeology: 3 Aquifers day-lighting along Duffins Creek
► Extensive wetland connectivity and riparian zones
A A’
A
A’
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Seaton Existing Landuse
Agricultural
Natural Heritage
Urban
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Seaton Proposed Landuse
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Agricultural
Natural Heritage
Residential
Parks
Commercial
Institutional
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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.
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Existing Conditions: Generated Runoff
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Post Development: Generated Runoff
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Post Development with LID: Generated Runoff
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Existing Conditions: Actual ET
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Post Development: Actual ET
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Post Development with LID: Actual ET
Bioswales
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Existing Conditions: GW Recharge
Iroquois Beach Sands
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Post Development: GW Recharge
Iroquois Beach Sands
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Post Development with LID: GW Recharge
Iroquois Beach Sands
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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
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Predicted GW Impacts – With LIDS
► Simulations indicate LIDS would sustain groundwater recharge and mitigate effects on aquifer levels and stream baseflow
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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