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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
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
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
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
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
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
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
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
► 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
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
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
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 et.al., 2008)

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
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
Seaton Existing Landuse
Agricultural
Natural Heritage
Urban
15

16. 16
Seaton Proposed Landuse
16
Agricultural
Natural Heritage
Residential
Parks
Commercial
Institutional

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
Existing Conditions: Generated Runoff

19. 19
Post Development: Generated Runoff

20. 20
Post Development with LID: Generated Runoff

21. 21
Existing Conditions: Cascading Runoff
Click for Animation

22. 22
Post Development: Cascading Runoff
Click for Animation

23. 23
Existing Conditions: Actual ET

24. 24
Post Development: Actual ET

25. 25
Post Development with LID: Actual ET
Bioswales

26. 26
Existing Conditions: GW Recharge
Iroquois Beach Sands

27. 27
Post Development: GW Recharge
Iroquois Beach Sands

28. 28
Post Development with LID: GW Recharge
Iroquois Beach Sands

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
Predicted GW Impacts – With LIDS
► Simulations indicate LIDS would sustain groundwater recharge and
mitigate effects on aquifer levels and stream baseflow

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