Hydrologic Assessment of using Low Impact Development to Mitigate the Impacts of Climate Change Chris Jensen, AScT Master of Science Thesis Bowker Creek Initiative April 12, 2012
Outline 1. Future Impacts a) Rainfall & Land Use b) Hydrology 2. Adaptation a) Rain Gardens b) Green Roofs c) Enhanced Top Soil 3. Results 4. Implications Source: waterabalance.ca
Research Problem Increased rainwater runoff from urbanization + Higher intensity rainfall from climate change = INCREASED RISK OF FLOODING
Case Study: Bowker Creek Watershed Downtown Victoria 1018 ha. watershed 7.9 km long channel 37% open channel Watershed is 50% impervious Soils are predominately clay with some rock outcroppings Total average annual precipitation is 608mm
1a. FUTURE IMPACTS: RAINFALL Key research challenge: Generating future scenarios for shortduration extreme rainfall. Method: Step 1) Use Global Climate Models to generate monthly precipitation changes Step 2) Use regression equations to calculate relationship between monthly rainfall volume and short-duration rainfall intensity Step 3) Adjust the Intensity-Duration-Frequency (IDF) Curve used for hydrologic modeling (Holm and Weatherly, 2010)
Future climate projections generated using an ensemble of 15 Global Climate Models. Parameters: Timeframe: 2050s (2040-2069) Emissions scenario: A2 Time of year: Result: Future Rainfall Nov, Dec & Jan (three wettest months) % change in monthly rainfall volume Median 75th percentile Nov 10% 20% Dec 6% 14% January 4% 11% Average 7% 15%
Future Rainfall Scenarios Two future rainfall scenarios developed based on November projections Scenario 1 = median projection Scenario 2 = 75 th percentile projection % change month % change 24-hour event Rainfall 24-hour 25-year (mm) Historic 88.8 Scenario 1 10% 5.4% 93.6 Scenario 2 20% 10.8% 98.4 The Master Drainage Plan increased 25-year event by 20%. The District of Saanich s Climate Change Adaptation Plan estimates a winter precipitation increase (mm/day) of 5% for the 2050s and 11% for 2080s.
Future Land Use Impervious Area Assessment: Each of the 21 subcatchments were assessed for future land use changes and associated increase in impervious area. Assumes no mitigation measures Existing impervious area: 50% 2050s impervious area: 59%
1b. HYDROLOGY Hydrologic model created with XP-SWMM Widely used software best suited for accurate prediction of flow rates from smaller urban catchments Builds on model developed for Master Drainage Plan (KWL, 2007)
Precipitation Intensity (mm/hr) Hydrologic Model Hydrologic response generated using a single event-based approach Focus on the local level of service: 24-hour 25-year event Used Soil Conservation Service Type 1A synthetic design storm 25 20 2050s Scenario 2 Historic Climate 15 10 5 0 3:00 2:00 1:00 0:00 7:00 6:00 5:00 4:00 11:00 10:00 9:00 8:00 Time (hh:mm) 15:00 14:00 13:00 12:00 23:00 22:00 21:00 20:00 19:00 18:00 17:00 16:00 SCS Type 1A hyetograph for the 24-hour 25-year rainfall event showing precipitation intensity for the historic and future climate scenario 2. 24:00
Precipitation Intensity (mm/hr) Synthetic vs. Real Storms The SCS Type 1A synthetic design storm has a narrow peak that may overestimate flows compared to the distribution of some storm events. For example: December 11/12, 2010 rainfall event Estimated 10-year return period Flat rainfall distribution compared to design storm 20 Dec 11/12 2010 18 SCS Type 1A 16 14 12 10 8 6 4 2 0 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 1:00 Time (hh:mm)
Dec 11/12 2010 rainfall event: Bowker Creek Park Image Credit: Taylor Davis
Dec 11/12 2010 rainfall event: Monterey Ave Image Credit: Taylor Davis
LiDAR data used to generate: Hydrologic Model Digital Elevation Model (ground) Digital Surface Model (above ground: buildings, vegetation, etc). High resolution and comprehensive data supports accurate flood modeling (e.g. overland flows)
Hydrologic Response to Climate Change and Future Land Use Current Land Use Future Land Use Historic Climate Climate Scenario 1 Climate Scenario 2
Flood Extent VIDEOS 1. 25-year event, Historic Climate, Existing land use 2. 25-year event, Climate Scenario 2, Future land use 3. 100-year event, Climate Scenario 2, Future land use
Flood Extent (m 2 ) Flood extents: Linear increase in flood extent No major topographic barrier Flood Extent 120000 100000 80000 Scenario 1 Historic Scenario 2 Existing Land Use Flood Area (m2) Historic Climate Scenario 1 Scenario 2 35,300 42,800 52,800 % change 21% 50% 60000 40000 Future Land Use Existing Land Use Future Land Use Flood Area (m2) 43,200 52,800 60,900 % change 22% 50% 72% 20000 0 60 70 80 90 100 110 120 130 Future increase in impervious surfaces has a similar impact as the median climate change projection 24-hour Rainfall Amount (mm) 24-hour 25-year rainfall events highlighted
Flood Volume (m3) Flood Volume 200,000 180,000 160,000 140,000 120,000 100,000 80,000 60,000 Return Period 5-Year 10-Year 25-Year 100-Year 40,000 20,000 0 Historic 1 2 Climate Scenario
Flood Volume Bowker Creek by Monterey Ave. Present Land Use Future Land Use Historic Climate 10-year Return Period 25-year 100-year Historic Historic Scenario 1 Scenario 2 Climate Scenario 1 Scenario 2 Climate Scenario 1 Scenario 2 Volume (m3) 5,500 11,900 26,300 39,200 54,400 72,900 115,600 149,100 184,000 % change 114% 376% 39% 86% 29% 59% Volume (m3) 10,200 21,300 35,100 53,200 67,700 87,500 125,600 159,100 197,800 % change* 84% 285% 535% 36% 73% 123% 9% 38% 71%
Peak Flow Rate (m 3 /s) Peak Flow Rates Peak flows limited due to conduit capacity. Flooding occurs. 30 25 20 Scenario 1 Historic 15 10 5 30 50 70 90 110 24-hour Rainfall Amount (mm) Scenario 2 Monterey - Future Land Use Monterey - Existing Land Use Trent - Future Land Use Trent - Existing Land Use
Peak Flow Rate (m3/s) Peak Flow Rates with Climate Change 25 20 15 10 S2 peak flow increases by 19% S1 peak flow increases by 8% Scenario 2 Scenario 1 Historic 5 0 4:48 6:00 7:12 8:24 Time Peak flow rate at Monterey Ave. for 24-hour 10-year rainfall event under historic climate for future climate scenario 1 and 2 9:36 10:48 12:00
Potential Solution: Low Impact Development What is it? LID relies on runoff management measures that seek to control rainwater at the source by retaining and infiltrating rainwater. Key Principles Minimize the creation of impervious surfaces Manage rainwater as close to its origin as possible Use source controls to manage rainwater runoff LID is not promoted for flood control of major storm events. But can it help? 2. ADAPTATION
Research Project Can LID mitigate the increase in flood risk posed by climate change? LID mitigation objective Q 25-yr (FUTURE) Q 25-yr (HISTORIC) How much LID is needed? What types are best? Effective under what future climate scenarios? Expected Changes in Extreme Event Frequency Curves (Arisz and Burrell, 2006)
Low Impact Development Research focus Source: waterbalance.ca
A rain garden is a planted depression that allows runoff from impervious areas to infiltrate into the soil. Basic Components Growing Medium Vegetation Rock Trench Perforated Pipe Overflow Reduces runoff by: Detention/Retention Infiltration Evaportrasporation 2a. RAIN GARDENS
Locating sites for Rain Gardens Use Digital Elevation Model to identify local runoff patterns and potential locations LiDAR can be used to generate micro-scale runoff patterns
Locating Sites for Rain gardens LiDAR data used to delineate drainage vectors and runoff collection points Trent Street Rain garden
Specifications: Rain gardens Designed to capture rainfall from the 24-hour 6-month event (35mm) Overflow/under drain connected directly to drainage system Maximum applicable area of watershed (%) 1.26 Ratio of impervious area to rain garden area 10:1 Growing medium depth (mm) 500 Growing medium infiltration rate (mm/hr) 40 Native soil infiltration rate (mm/hr) 0.8 Ponding storage (mm) 150 Rock pit depth (mm) 800 Rock pit storage capacity (%) 35 Maximum available water storage capacity (mm) 385
Bowker Creek Image Credit: Taylor Davis
2b. GREEN ROOFS Green roofs are building roofs which are purposely covered with vegetation. Typically a green roof consists of a layer of foliage planted in soil or an engineered growing medium. The foliage rests on top of a synthetic, waterproof membrane. CRD Headquarters
Locating Potential Sites for Green Roofs Methods: % of watershed 1. Use LiDAR to delineate rooftops into GIS polygons 14.5 2. Select roof areas 300m 2 4.1 3. Slope Analysis: Identify roof areas with slopes <3% 2.6
Green Roof Analysis: Area Hillside Mall Oak Bay Rec Centre
Green Roof Analysis: Area Hillside Mall Roof areas >300m 2 Oak Bay Rec Centre
Green Roof Analysis: Slope Slope > 3% 3% Roof areas >300m 2
Green Roof Specifications: Max Applicable Area: 2.6% Growing medium depth: 150mm Infiltration rate: 50mm/hr Max Water Storage Capacity : 45mm Drainage layer with underdrain
2c. ENHANCED TOP SOIL An absorbent topsoil layer reduces peak flows as it serves as a sponge when it is raining. Improving or adding an amended topsoil layer to a site is one of the simplest and easiest source controls to implement. Source: Green Infrastructure Partnership, 2010
Enhanced Top Soil Enhanced tops soils reduce runoff by increasing infiltration rates Current Soils Enhanced Soils Infiltration Rate by Soil Group/ Texture Infiltration Rate by Compaction/Moisture
Locating Sites for Enhanced Top Soil Methods: 1. Use GIS to remove all impervious surfaces, bedrock outcrops, etc 2. Slope Analysis: Identify pervious areas with slopes <2% 3. Manual analysis of orthophotographs Specifications: Maximum applicable area: 5.4% Amended top soil depth: Infiltration rate: Max water storage capacity: 400mm 20mm/hr 80mm
3. RESULTS Hydrologic modeling results for: Rain Gardens 12.8 ha Green Roofs 26.4 ha Enhanced Top Soil 55.0 ha All LID 94.2 ha All LID = 9.3% of total watershed area Source: waterbalance.ca
% Change in Peak Flow Rate (m3/s) Peak Flow Rate 15% 5-Year 10% 5% 0% -5% Existing Land Use -10% Top Soil -15% Green Roof -20% Rain Garden -25% -30% Historic (64.8 mm) Scenario 1 (67.2 mm) Scenario 2 (69.6 mm) All LID Rainfall
% Change in Peak Flow Rate (m3/s) Peak Flow Rate 10-Year 15% 10% 5% 0% -5% -10% -15% Existing Land Use Top Soil Green Roof -20% Rain Garden -25% -30% Historic (74.4 mm) Scenario 1 (79.2 mm) Scenario 2 (84 mm) All LID Rainfall
% Change in Peak Flow Rate (m3/s) Peak Flow Rate 25-Year 6% 4% 2% 0% -2% -4% -6% -8% All LID can mitigate a 9% increase in rainfall for the 25-year return period (97mm event). Existing Land Use Top Soil Green Roof Rain Garden All LID -10% Historic ( 88.8 mm) Scenario 1 (93.6 mm) Scenario 2 (98.4 mm) Rainfall
% Change in Peak Flow Rate (m3/s) Peak Flow Rate 100-Year 4% 3% 2% 1% 0% Existing Land Use Top Soil -1% -2% -3% Top soil amendments provide the greatest peak flow reductions under the 100-year event Green Roof Rain Garden All LID -4% Historic ( 108 mm) Scenario 1 (115.2 mm) Scenario 2 (122.4 mm) Rainfall
Peak Flow Rate (m 3 /s) Peak Flow Rate Rain gardens for smaller events. Top soil amendment for large events 24.0 22.0 20.0 18.0 16.0 14.0 All LID Dominant influence 12.0 changes from rain gardens to top soil amendments at 102mm rainfall event 10.0 65 75 85 95 105 115 125 24-hour Rainfall Amount (mm) Existing Land Use Top Soil Green Roof Rain Garden
Flood Volume with Climate Change & LID 25-year 100.0% 80.0% 60.0% 40.0% 20.0% 0.0% -20.0% -40.0% Existing Land Use Top Soil Green Roof Rain Garden Full LID -60.0% -80.0% Historic ( 88.8 mm) Scenario 1 (93.6 mm) Scenario 2 (98.4 mm)
Flood Volume with Climate Change & LID 100-year 70.0% 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% -10.0% -20.0% Existing Land Use Top Soil Green Roof Rain Garden Full LID -30.0% -40.0% Historic ( 108 mm) Scenario 1 (115.2 mm) Scenario 2 (122.4 mm)
Flood Volume (m3) Flood Volume Rain gardens for smaller events. Top soil amendment for large events 200000 180000 160000 140000 120000 100000 80000 60000 40000 20000 Existing Land Use Top Soil Green Roof Rain Garden Full LID 0 60 70 80 90 100 110 120 130 Rainfall Amount (mm)
Runoff (mm/hr) Rain Garden Performance Rain garden capacity: Historic 25-year event: saturates soon after storm peak Climate change: saturates during storm peak 60 50 40 30 20 10 0-10 -20-30 0 10 20 30 40 50 60 70 80 90 Rainfall Intensity (mm/hr) -40 100 11:00 10:00 9:00 8:00 7:00 6:00 5:00 4:00 3:00 2:00 1:00 Time
Runoff (mm/hr) Rain Garden Design Storm Rain gardens often sized for volume control or water quality objectives (i.e. not flood control). For example: 6-month return period (35mm) 2-year return period (48mm) If adaptation is the objective, then may want to design for 2-year event. 60 0 50 10 40 30 20 10 0-10 -20 20 30 40 50 60 70 80 Rainfall Intensity (mm/hr) 2-year design 6-month design Rainfall -30 90-40 100 1:00 2:00 3:00 4:00 5:00 6:00 7:00 Time 8:00 9:00 10:00 11:00
Watershed Scale 4. IMPLICATIONS Land-use results in more prominent flow changes in smaller scale watersheds. LID more effective in smaller urban watersheds ( 10km 2 ) Bowker Creek Impact of land use and climate variability on hydrological response as a function of scale (Bloschl et al., 2007).
Implications: Bylaws Municipalities are already adopting bylaws that require onsite rainwater management (e.g. using LID techniques). For example: In light of climate change, Local Governments may want to further advance the implementation of onsite rainwater management. Tools such as the Water Balance Model can support bylaw requirements. WBM has a new climate change module that allows users to model and mitigate future impacts.
Implications: Provincial Goals Provincial goals, policies and programs are increasing the level of support for low impact practices and climate action. Example: Living Water Smart - BC s Water Plan Adapting to climate change and reducing our impact on the environment will be a condition for receiving provincial infrastructure funding. Incorporating LID can help communities become more resilient to climate change and ensure eligibility for senior government funding.
Implications: Asset Management An Asset Management Program identifies the useful life of infrastructure and establishes longer term maintenance and financing for renewal and replacement. The time for renewal of drainage infrastructure can coincide with necessary upgrades to address climate change. LID can be used as a strategy to help maintain the current level of service until necessary upgrades can be made (may be 50+ years away)
Preliminary cost estimate: Green Roof Implications: Costs Low Med High incremental cost per m 2 $100 $200 $300 $26,468,000 $52,936,000 $79,404,000 Rain Garden cost per m 2 $150 $300 $450 $19,392,900 $38,785,800 $58,178,700 Enhanced Top Soil cost per m 3 $10 $25 $40 $2,198,880 $5,497,200 $8,795,520 Master Drainage Plan Class D cost estimate for upgrades range from $22m to $46m (2007 dollars). The MDP addresses all flooding, not just climate change.
Summary 1. The model indicates that climate change is projected to increase flood extent by 21% to 50% for the 24-hour 25-year event. 2. The unmitigated increase in impervious surfaces poses similar flood impacts as the median climate projection. 3. Green roofs: Minimal mitigation benefit for all modeled events. 4. Rain Gardens: Mitigation benefits up to ~25-year return period. 5. Enhanced Top Soil: Main mitigation benefits occur during large rainfall events. 6. All LID can mitigate a 9% increase in rainfall for the 24-hour 25-year event. 7. LID provides multiple benefits beyond flood control (e.g. water quality improvements)
Conclusion LID provides an incremental and flexible approach for climate change adaptation. LID reduces flooding (with or without climate change). Relatively extensive use of rain gardens and top soil amendments are required to mitigate the full impact of climate change. Onsite rainwater management should be considered in adaptation planning. Modeling results indicate that in the Bowker Creek watershed, LID can help reduce the adverse impacts of climate change.
Funding Support THANK YOU
Questions? CHRIS JENSEN 250-356-8737 Chris.Jensen@gov.bc.ca