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

45

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