This document summarizes an assessment of low impact development (LID) strategies using integrated surface water and groundwater models. The assessment evaluated various LID configurations for a proposed new development in Ontario to determine which strategies should be used and where to best preserve wetlands and aquifers. Modeling indicated that unmitigated development could lower aquifer levels but that LIDs like bioswales and infiltration galleries could sustain groundwater recharge and mitigate impacts, helping the municipality and conservation authority evaluate LID scenarios.

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