Documentation for 
Parcel Scale Green Infrastructure Siting and Cost Effectiveness Analysis 

This report documents the materials and methods used to prepare the geospatial data published as “Parcel Scale Green Infrastructure Siting and Cost Effectiveness Analysis for Pittsburgh, PA.” 

1.	Background and Motivation

The Pittsburgh region is served by a combined sewer, which collects, conveys, and partially treats both stormwater and wastewater in the same infrastructure system.  During periods of heavy rainfall, stormwater can exceed the capacity of combined sewers, which causes overflow into nearby rivers and streams.  While these combined sewer overflows (CSOs) mitigate upstream flooding, they release untreated wastewater into receiving water bodies. 

Improvements to “gray” infrastructure - pipes, pumps, storage, and treatment facilities - can increase the capacity of the collection system to accommodate more severe wet weather events.  Conversely, “green” infrastructure includes features that reduce the stormwater entering the collection system by temporarily retaining or diverting stormwater.  Types of green infrastructure vary from completely natural systems, such as converting a parking lot to a park, to single purpose engineered systems, such as pervious paving. 

A comprehensive green infrastructure strategy starts with identifying broad areas appropriate for green infrastructure given variation in hydrologic (land use plus topography) and hydraulic (the capacity of pipes, pumps, and treatment) then applying a mix of additional criteria to target specific installations.  This procedure is demonstrated schematically in Figure 1.   

Technically feasible installations of green infrastructure include all possible structural installations at their maximum load ratio plus all possible non-structural installations.  The technically feasible installations are identified through hydrologic and hydraulic models performed at the sewershed scale. 

Cost optimal installations are those technically feasible projects that, taken together, minimize the net cost of our wet weather plans.  These projects are identified by applying engineering economic analysis (e.g., cost effectiveness analysis) to scale estimated project-level performance to sewershed or service area level. The cost optimal set of installations include all possible cost effective installations when considering gray plus green strategies in balance. 

Not all cost optimal installations are realistic.  The realistic potential of green infrastructure considers land use in balance and recognizes any limitations to installations on private property, where property owners significantly influence the feasibility of installation. 

Figure 1: The tiers of green infrastructure estimates, shown on the x-axis, derived through different types of analyses, shown across the top of the figure. 

Policy enabled green infrastructure projects are those that are technically feasible, cost effective, and realistic but need some supporting policy for implementation.  Examples might include projects spurred by a stormwater tax on impervious cover, decommissioning of distressed property,  or a rebate for installations in private property.  These projects can be identified through policy analysis, which recognizes all of the upstream contributions to project identification but also includes an assessment the various policy approaches for implementation.  The difference between policy enabled projects and current project installations identifies a policy gap or implementation opportunity.    

The methods and data applied herein produce location planning-level estimates aligned with the criteria associated cost optimal and realistic installations. 

2.	Data Sources, Data Manipulation, and Limitations

Three data sources were used to characterize land cover.  The data set Urban Tree Canopy [Allegheny County 2010] roughly delineates land into building, roadways, trees, open space, water, and other surface impervious cover (such as driveways, sidewalks, and parking lots). The Allegheny County Wooded Areas [Allegheny County 2011] provides additional information about denser stands of trees. The data set Building Footprints [City of Pittsburgh 2008] reflects building footprints as of 2008.  These data were harmonized by prioritizing the building footprints spatial information and reallocating any buildings misestimated by the urban tree canopy layer using nearest neighbor algorithms.

The resulting land cover data were assigned to five types: buildings, open space, trees, surface impervious cover, and roadways in the public right-of-way.  The land cover data were then unioned to parcel delineations [Allegheny County 2016] and a 10-foot building buffer. To ensure any surface green infrastructure could be readily inspected, polygons adjacent to the public right-of-way were identified by proximity to the street curb [City of Pittsburgh 2009].  The resulting database divides land cover for City of Pittsburgh into relatively small portions contiguous with with respect to land cover, parcel, proximity to buildings, and proximity to the right of way.  The average, median, and standard deviation of the resulting land portions are W, X, Y, and Z in square feet. Finally, the mean elevation and slope was assigned to each contiguous portion of land. 

Allegheny County’s original parcel identifier (original field “PIN”) was modified (modified field “PINnew”) to better reflect the land cover associated with condominiums. Currently, Allegheny County codes common land for all condominiums as “COMMON.” As a result, parcel level analysis aggregates all common land for all condominiums across the City.  Additionally, many condominiums with more than one floor include separate PINs for each unit. Reflecting each unit separately would significantly misrepresent the land cover at each condominium parcel.  As result, all common areas were given a new PIN by concatenating the County’s map and block identifier with the word “COMMON.”  For example, the common area for condominium on map 0001, block G00, would be labeled, “0001G00COMMON.”  PIN’s for the building cover of condominiums were arbitrarily selected from the onsite condominiums. 

The land cover, building footprint, and street curb data are outdated. Any changes to buildings since 2008, land cover since 2010, and street curbs since 2009 are not properly reflected in the maps. In addition, the various land cover information, while the best available at the start of this study, is spatially approximate.  For example, many buildings are indicated as crossing property lines and extending into the public right-of-way. The land use data are taken and represented on an “as is” basis. 
3.	Objectives, Limitations, and Intended Uses 

The objectives of this project are to estimate (1) locations on each parcel in the City of Pittsburgh where rain gardens, downspout disconnects, pervious pavement, and green roofs could feasibly be installed given highly generalized siting and design criteria and (2) estimate how cost effective each installation is with respect to reducing stormwater runoff (or inflow into the combined sewer system).  It should be emphasized that these estimates are highly imperfect given limitations in the underlying land use data, applications of generalized and often crude sizing and siting criteria that may not represent unique site conditions, the selection of a simplified design storm, and cost estimates that may not represent specific site conditions.  

The estimated siting and cost performance data are intended to partly, but not completely, identify planning level cost optimal and/or realistic potential allotments of green infrastructure. As discussed in Section 2, there are several limitations to the underlying data and methods. In no way should these data be considered a replacement for site specific design and analyses. 

Example applications of the resulting data and maps could be to identify vacant or public property with a high inflow reduction potential, contrast the published parcel level green infrastructure installations with higher level sewershed estimates, elicit variation in cost effectiveness driven by variation in site characteristics, identify variation in wet weather performance by property type or demographic for subsequent policy analyses, and/or identify locations where parcel aggregation towards a particular installation is sensible. 

A demonstrative application of the data is shown in Figure 2, which presents a inflow mitigation “supply curve.”  For each feasible installation, the inflow mitigation supply curve plots the rank-ordered cost effectiveness (y-axis) against the cumulative inflow mitigated.  Those installations estimated to be more cost effective with respect to reduce inflow into the stormwater system (stormwater runoff) are shown on the left and the cost effectiveness of installations decreases moving right. Variation in the cost effectiveness is a function of the site specific hydrologic conditions and the cost the technology. See Section 6 for more details regarding the cost effectiveness assumptions.  

Figure 2: Inflow mitigation curve for 138,000 installations of green infrastructure estimated using the materials and methods documented herein. 

4.	Green Infrastructure Sizing and Siting Criteria

For this study, ineligible parcels for the installation of rain gardens, downspout disconnects, pervious pavement, and green roofs are assumed to be public parks, cemeteries, railroad right-of-ways, and parcels with where impervious surfaces represents less than 5% of the total parcel coverage (n = 15,200 out of 121,600 total parcels). 

The green infrastructure sizing and siting criteria a summarized Table 1. 

Roof types labeled as either “rolled” and “rubber” in Allegheny County’s property tax database [Allegheny County 2016] were to be flat roofs, and the remaining roofs with completed records were assumed to be sloped.  However, most roof descriptions in Allegheny County’s property tax database are missing. Thus the roof type (flat or sloped) was assumed based upon the property use as described in the Supplemental Information. 

Table 1: Green infrastructure sizing and siting criteria applied to each eligible parcel. 

Criteria	Green infrastructure type				
	Rain Garden	Pervious
Pavement	Green Roof	Downspout disconnect 
(without downstream retention)	
Affected land cover	Open space	Impervious cover not in right-of-way	Buildings	Buildings	
Size criteria	At least 20 square feet of contiguous open space				
Slope criteria	Existing grade less than 15%				
Building buffer	At least 10 feet from building	At least 10 feet from building			
Curb proximity	Adjacent to curb	Adjacent to curb		At least 10 feet from all curb boundaries	
Elevation	Below impervious cover	Below building		Conforming, contiguous open area below building skirt	
Load ratio	1:5			1:1	
Limitation	1 per parcel on largest conforming area	1 per parcel on largest conforming area	Flat roofs greater than 10,000 square feet only as per assumptions summarized herein	The maximum of either half or all  of the building footprint given the assumed load ratio	

R code was developed to applying the sizing and siting criteria, roof type assumptions, and property eligibility criteria to each parcel in the City of Pittsburgh, resulting in the selection of land portions representing potential locations for the indicated green infrastructure technologies. The selection locations are indicated in the fields “disconnect,” “raingarden,” “greenroof,” and “permpave.”

5.	Hydrologic and Hydraulic Modeling

For each parcel, baseline stormwater runoff and runoff reduced by green infrastructure was estimated using the Environmental Protection Agency’s Stormwater Management Model (SWMM).  Runoff reductions were estimated by simulating discrete green infrastructure interventions for each parcel.  Thus the runoff reductions from combinations of interventions are not additive.  By simulating each parcel separately, overland flow across parcels is not modeled.  The assumptions regarding land cover for each parcel are presented in Section 3 and are included in the Supplemental Information. The remaining hydrologic modeling parameters and the simulated design storm are also summarized in the Supplemental Information.  

6.	Cost Effectiveness Analysis

Cost effectiveness was estimated as installed cost per gallons of inflow reduced for each discrete installation.  The assumed unit costs are summarized in Table 2.  As of January 2017, only the middle installed cost estimate is published online. Users interested in modeling variation in cost can download the model results as described in Section 7.  

Technology	Installed Cost 
($ per sq ft. drainage area)			
References	
	Low	Mid.	High		
Pervious Pavement	8.00	12.00	15.00	3RWW 2016;  City of Lancaster 2011	
Green Roof	15.00	25.00	40.00	Roseen et al. 2011; GSA 2011;  EPA 2007;
3RWW 2016;City of Lancaster 2011	
Rain Garden	10.00	18.00	27.00	3RWW 2016; EPA 2007	
Downspout disconnect	0.20	0.30	0.50	3RWW 2016; EPA 2007

7.	Published Data and Model Output

The data and model output published at CartoDB are freely available for download. Metadata for the data published at CartoDB are included in the Supplemental Information.   

8.	Contact Information

For more information about the materials, methods, and model output, please contact Mike Blackhurst at mfb30@pitt.edu.

REFERENCES

Allegheny County. “Allegheny County Contours.” Division of Computer Services Geographic Information Systems Group, 2006. http://www.pasda.psu.edu/uci/MetadataDisplay.aspx?entry=PASDA&file=AlleghenyCounty_Contours2006.xml&dataset=1200.
———. “Allegheny County Urban Tree Canopy.” Division of Computer Services Geographic Information Systems Group, 2010. http://www.pasda.psu.edu/uci/MetadataDisplay.aspx?entry=PASDA&file=AlleghenyCountyUrbanTreeCanopy2010.xml&dataset=1203.
———. “Allegheny County Wooded Areas.” Division of Computer Services Geographic Information Systems Group, 2011. http://www.pasda.psu.edu/uci/MetadataDisplay.aspx?entry=PASDA&file=AlleghenyCounty_WoodedAreas2011.xml&dataset=1228.
City of Pittsburgh. “Parcels.” Geographic Data, 2015. http://pittsburghpa.gov/dcp/gis/gis-data-new.
———. “Street Curbs.” Geographic Data, 2015. http://pittsburghpa.gov/dcp/gis/gis-data-new.
EPA (US Environmental Protection Agency). “Storm Water Management Model (SWMM).” Version 5.1.011. www.epa.gov/water-research/storm-water-management-model-swmm 
3RWW (3 Rivers Wet Weather). “Green Solutions.” Accessed January 10, 2017. http://www.3riverswetweather.org/green/green-solutions.
City of Lancaster, PA. “Green Infrastructure Plan,” 2011. http://cityoflancasterpa.com/sites/default/files/documents/cityoflancaster_giplan_fullreport_april2011_final_0.pdf.
EPA (US Environmental Protection Agency). “Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices,” 2007. https://www.epa.gov/sites/production/files/2015-10/documents/2008_01_02_nps_lid_costs07uments_reducingstormwatercosts-2.pdf.
GSA (US General Services Administration). “The Benefits and Challenges of Green Roofs on Public and Commercial Buildings,” 2011. https://www.gsa.gov/portal/mediaId/158783/fileName/The_Benefits_and_Challenges_of_Green_Roofs_on_Public_and_Commercial_Buildings.action.