Post on 06-Aug-2020
Promoting Low Impact Development in Pima County Through Site Planning and Watershed Management
Evan Canfield
01-08-20
Pima County LID Policies
Pima County LID Guidance
4
Table 1 – Best Use of Rainwater and Stormwater Described in this Paper
Built Environment Future
Development Regional Watercourses Recharge X X Capture X X Tributary Watercourses Recharge X X Capture X X Neighborhood Drainage Capture X X Lot Scale Capture X X
Regional Watercourses: Santa Cruz River, Rillito Creek, Pantano Wash, Tanque Verde Creek, Canada Del Oro Wash, Brawley Wash, Black Wash Tributary Watercourse is a tributary to a Regional Watercourse
Recharge vs CaptureRecharge - Infiltrate to
the regional aquifer for future use.
Capture – store for use in the near term.
6
Measure all the Rain falling on a watershed over years
Depth x Area = Volume‘Harvestable’ =
Measure all the Runoff out on a watershed over years
Volume
Volume Runoff------------------Volume Rain
Santa Rita 128 years (4.88 acres)
16.5 inchs---------------- = 4.3%387 inches
7Graph updated from City/County Water Study Stormwater as a Supplemental Water Source, May 2009
Notes: Area = 230 sq Miles, Potable Water Sales from 2014, 27% Outside Water UseRainfall =11.3 inch/yr, Harvestable Stormwater assumes 30% Impervious at 83% Harvestable
0
20,000
40,000
60,000
80,000
100,000
120,000
140,000
160,000
CAP Potable Sales Outside WaterUse
Avg AnnualRainwater
HarvestableStormwater
Tucson Water Service AreaWater in Acre-Ft
10
Value Unit Source
Tucson Area 236square miles
Stormwater Harvesting and Management as a Supplemental Resource (2009)
151,040AcresRainfall 11.3inches
0.94FeetTotal Rainfall Volume 142,229Acre-Ft of Rainfall
Impervious Area 30%Impervious Common in TSMS HEC-1 Files45,312Acres
Harvestable 83%Harvestable off Impervious Surface
Stormwater Harvesting and Management as a Supplemental Resource (2009)
35,415Acre-Ft of Harvestable Water
Low Impact Development andGreen Infrastructure
Low Impact Development (LID)
‘A comprehensive stormwater management and site-design technique. . . the goal of any
construction project is to design a hydrologicallyfunctional site that mimics predevelopment
conditions…’
Green Infrastructure -
‘As a general principal, Green Infrastructure techniques use soils and vegetation to infiltrate,
evapotranspirate, and/or recycle stormwaterrunoff…’
EPA Green Infrastructure Homepage - Glossary
• Used variables from Jeff Kennedy’s KINEROS model to create a SWMM model.
• Uses Green Ampt infiltration based on Jeff Kennedy’s tensiometermeasured infiltration data and parameters calibrated to runoff data at La Terraza.
• Urban soils at La Terraza – optimal Ksat = 2.5 mm/hr (0.10 in/hr)
0
10
20
30
40
50
60
70
80
0 5 10 15 20 25 30 35 40 45Observed Qp (cfs)
Pred
icte
d Q
p (c
fs)
R2 = 0.79
RMSE = 4.6 cfs
n = 59
Modeling the urban runoff from rainfall data and using grassland runoff data as upstream inflow:
100% distributed 50% distributed 0% distributed
LID and Stormwater Harvesting are particularly effective for small events. However, what are the impacts on the 100-yr event?
1-hr, 100-yr Storm applied to 12 cases Varied catchment scale (2%, 5%, 10%, 16%) Area of stormwater harvesting (SWH) relative to developed
area diverted to SWH basin Varied distribution in urban watershed 100% distribution (each lot has SWH basin) 50% distribution (1/2 at lot, ½ at outlet) 0% distribution (all SWH at outlet)
0
50
100
150
200
250
0:00 0:15 0:30 0:45 1:00 1:15 1:30 1:45
Out
flow
(cfs
)
Time (hours)
No Stormwater Harvesting
10.3% Volume Retained
25.7% Volume Retained
51.4% Volume Retained
85.7% Volume Retained
Modeled 100-yr Outflow Hydrographs
Distribution of SWH basins has a large effect on runoff volume and peak discharge.
0
50
100
150
200
250
0:00:00 0:28:48 0:57:36 1:26:24 1:55:12 2:24:00 2:52:48
Time
J20
(cfs
)
100% dist50% dist0% dist100-yr Base model at J20
0.0%
5.0%
10.0%
15.0%
20.0%
25.0%
30.0%
35.0%
40.0%
0.0% 2.0% 4.0% 6.0% 8.0% 10.0% 12.0% 14.0% 16.0% 18.0%
Stormwater Harvesting Catchment Scale
100-
yr V
olum
e R
educ
tion
0% Distributed50% Distributed100% DistributedAverage Value
𝑄𝑄𝑄𝑄𝐺𝐺𝐺𝐺 = 𝑄𝑄𝑄𝑄𝑉𝑉𝐺𝐺𝐺𝐺𝑉𝑉
Equation 4-9
y = -0.38x2 + 1.47x - 0.133R² = 0.95
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 20% 40% 60% 80% 100%
Mod
eled
Red
uctio
n in
Pea
k D
isch
arge
Modeled Stormwater Harvesting Basin Volume relative to Runoff Volume
La Terraza SWMMResults
Commercial SiteSWMM Results
RMSE = 6.9%
Pima County:Detention-Retention Manual
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0% 20% 40% 60% 80% 100%
Dai
ly P
reci
pita
tion
(inch
)
Percent
85th Percentile Rainfall 0.48 inch
Replace Retention Requirement with a‘First Flush’ Retention Requirement
(data U of A Daily rainfall 1895-2000)
Classification of Watershed vs Proposed Use
Riparian/High Permeability, Proposed Impervious Area 1815 0Nonriparian/lLow Permeability, Proposed Impervious Area 1440 2.100 3024Riparian/High Permeability, Proposed Disturbed Area 245 0.300 74NonRiparian/Low )Permeability, Proposed Disturbed Area 140 0Remaining Undisturbed Area, Pre-Developed Watershed (Info Only) 0.000Total Required First Flush Volume 3098
Volume ft3/ac
Table 2.1
Area of Proposed Use (ac)
First Flush Required
Volume (ft3)
Pima County LID Policies
Low Impact Development Benefits
Site Planning(Avoidance and Prevention)
Green Infrastructure(Structural)
• Flood Control• Stormwater Management•Pollution Prevention•Energy Efficiency• Supports Landscape Amenities
•Preserve Natural Flow Paths•Minimize Impervious area•Reduce Disturbance
•Rainwater and Stormwater Harvesting Features
•Naturalized Conveyance Features
LID TechniquesBeneficial Alternatives to Traditional Practices
Example Commercial Site
39 cfs
12 cfs
20 cfs71 cfs
Existing Adjacent Commercial
Redundant Entrance
SITE PLAN SUBMITTAL
Offsite and Roof Flows to Storm Drain
Detention in Underground
Chambers
Minimum Volume in Distributed
Basins
Number of Parking Spaces Exceeds Required Number
Driveway Spacing Exceeds Minimum
Low Impact Development Benefits
Site Planning(Avoidance and Prevention)
Green Infrastructure(Structural)
• Flood Control• Stormwater Management•Pollution Prevention•Energy Efficiency• Supports Landscape Amenities
•Preserve Natural Flow Paths•Minimize Impervious area•Reduce Disturbance
•Rainwater and Stormwater Harvesting Features
•Naturalized Conveyance Features
LID TechniquesBeneficial Alternatives to Traditional Practices
Offsite Flows to Vegetated Swale with
Check Dams
• Saves $ on Storm Drain• Landscape Buffer for
Residences to East• Supplements Irrigation
Pavement and Roof Flows to Distributed
BasinsReduced Parking = Basin Area
Driveway Spacing Reduced by 20 feet =
Area on East for Swale
• Saves $ on Underground Chambers
• Provides Shade for Parking• Supplements Irrigation
Making this Project More LID-Friendly
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Applying these planning principles to larger-scale residential projects results in preservation of flow corridors and riparian
habitat, both associated with reduced flood risk
Parcel Existing Conditions Traditional Maximized Grading Concept
Lot Yield Comparison When Flow Corridors are Preserved
Loss of 169 Lots??????
$$$ $2,675 x 559 Lots = $1.5 Million$$$ Reduced Grading Costs$$$ Reduced Cost of Constructing Drainage Channels$$$ Reduced Cost of Landscape Installation$$$ Reduced Cost of Detention and Other Flood Works$$$ Reduced Cost of Riparian Habitat Mitigation$$$ Reduced Cost of Salvaging and Relocating
Protected Species such as Saguaros
Take a Look at Cost Offsets
Final Construction Merged Lot-Yieldand Open Space Drivers
Plat Boundary Expanded
Modified Lot Sizes
Not All Flow Corridors Preserved
Lot Yield = 953 Lots = + 225 Lots
1. Residential Parcels: ~1/3 of available landscape for selected parcels delineated as rain gardens. Included streetside basins if appropriate for the space.
Model representation On-the-ground potential practice
10% & 25% Scenario: Green Stormwater Infrastructure Retrofits
Valencia Residential
Drainage Area:7 Acres
Evaluation of Flood Reduction in RuthrauffBasin from Installation of GI/LID Only in Right of Way
Impact of GI/LID on Flood Peak Reduction in Ruthrauff Basin
-16%
-14%
-12%
-10%
-8%
-6%
-4%
-2%
0%10-yr 25-yr 100-yr
Perc
ent R
educ
tion
in F
lood
Pea
k 10% of Parcels
25% of Parcels
10% of Parcels10-yr 25-yr 100-yr
Mean -4.0% -3.4% -2.0%Min -15.5% -11.4% -11.1%Max 17.6% 20.3% 25.7%Outflow Volume Reduction 2.5% 1.9% 1.3%
25% of Parcels10-yr 25-yr 100-yr
Mean -14.3% -10.4% -6.9%Min -33.3% -28.9% -22.3%Max 0.0% 1.4% 7.1%Outflow Volume Reduction 6.1% 4.7% 3.3%
Evaluating a Silverbell Road Green Infrastructure Retrofit
Analysis: • Evaluate Green vs Gray
Drainage
• Evaluate Multiple Benefits
Tool: AutoCASE®
(Envision ® Rating)
GI Feature Added: BioretentionWater Harvesting basinsTreesTraffic Calming
The Triple Bottom Line Framework (e.g. for a road project)
Project CashImpacts
Revenue;Operational
Savings
Capital Costs; O&M
Costs
Water Quality
Green House Gases
Flood Risk Reduction
Water Quantity
Property Value Uplift
Shadow Wage
Benefit
Non-Cash Impacts
Heat Island
Mitigation
Value of Time
Criteria Air Contaminants
Health & Safety OtherRecreation
Value
Financial Return
Sustainable Return on Investment
Probabilistic Assessment produces more resilient projects
F = f (A, B, C, D, ..)
Reduced Energy Demand(kwh/yr)
Runoff Flooding (# Events/yr)
Energy Savings($/kwh)
Property Value ($/Flood Event)
Value of Investment
($/yr)
Jointly Determined Probabilities
• Risk analysis is the systematic use of available data to determine how often specific events may occur and what the magnitude of their consequences is.
• Probability distributions account for uncertainty in key drivers
• Monte Carlo simulation integrates uncertainties to reveal comprehensive perspective
Risk-adjusted outcomes
Sustainable NPV -incorporates all costs and benefits in the model, including impacts on the local economy, society,and the environment.
Direct Financial NPV -direct costs and benefits such as capital expenditures, revenues, etc.
The difference between the curves is the (net) non-market or societal benefits (externalities) such as lower carbon emissions, less urban heat island effect and other impacts.
Sustainable Net Present Value Benefits
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Reduced Electricity
Costs
Reduced Flood RiskReduced Heat Stress
Mortality
Reduced CO2 Emissions
Reduced Air PollutionDirect
Costs of Water
Social Costs of Water Use
Traffic Calming -Roundabouts
and Curb Extension
Other Benefits
1. Improved Safety through Traffic Calming (36%)
2. Financial and social benefits of reduced water use (25%)
3. Improved air quality (20%)
4. Energy Savings (10%)
5. Reduced Flood Risk (6%)
Conclusions• Pima County, Pima County Regional Flood Control District, City
of Tucson and Stakeholders have been evaluating Green Infrastructure (GI) and Low Impact Development (LID) to determine it’s value in:•Flood reduction•Reduction of potable water use•Value of co-benefits
• LID/GI is integrated into new drainage development standards• Pima County, Pima County Regional Flood Control District, City
of Tucson and Stakeholders have supported regulatory standards with Guidance•Green Infrastructure Manual•Case Studies
Acknowledgements
Tamara MittmanJame Piziali
Martina FreyJason Wright
Lynn OrchardSandy BolducAnn MoynihanMarie LightJennifer Becker
Irene OgataGary Wittwer
Mead Mier
Akitsu Kimoto
Ian Sharp John WilliamsJohn ParkerRyan Myers
Thank you!
Questions?
Evan Canfieldevan.canfield@pima.gov