Evaluating the Impact of Build-out on
Stormwater in Concord, MA
Rhiannon Ervin
May 9, 2011
Introduction
Stormwater runoff is generated when precipitation flows over land or impervious
surfaces without percolating into the ground. As the runoff travels over these surfaces, it
accumulates debris, chemicals, sediment and/or other pollutants that could adversely affect water
quality if the runoff is discharged untreated (U.S. EPA). Generally, the quantity of stormwater
runoff is proportional to the amount of impervious surface. As cities develop, the density of
impervious surface increases, leading to increased stormwater flow.
One way to estimate increased impervious surface from new development is to conduct a
build-out analysis. A build-out analysis employs current zoning regulations to estimate
conditions at some future time when all parcels are built to the maximum extent allowable. The
amount of impervious surface at build-out can be estimated and used to calculate future
stormwater volumes. This allows cities to develop plans for managing the increased stormwater
flow. One option for stormwater management is to change zoning regulations to require Low
Impact Development (LID) technologies to control the additional stormwater created by
development.
The objective of this poster is to conduct a build-out analysis of Concord, MA, with a
particular focus on stormwater management. This analysis will be used to estimate the increased
amounts of development, impervious surface and stormwater volume at build-out. Additionally,
possible locations for LID technology implementation are highlighted.
Background
Similar analyses to that proposed herein have been conducted in the literature. Most of
the papers focus on one or more of the three key factors: build-out analysis, stormwater analysis,
and LID implementation. Often, a more traditional build-out analysis is conducted without
incorporating GIS. These papers often state that the best method for displaying their analysis
would be to use GIS. More complicated engineering analyses incorporate decision support
system (DSS) models into a GIS analysis to determine the effect of putting best management
practices (BMPs) in specific locations.
Chester et al. (1996) wrote a review of using impervious surface as an environmental
indicator. When natural surfaces are paved, the natural hydrologic cycle changes in both the way
water moves and the way water is stored, usually degrading the water resources in the area.
Previous research had shown that water degradation can be divided into categories, each relating
to percent impervious surface in a watershed. The three categories are: protected, < 10%
impervious; impacted, 10%-30% impervious; degraded, > 30% impervious. These categories are
general and are not the perfect criteria for every watershed, but are useful for estimating impacts
over a broad range of locations. Additionally, the amount of pollutants washing off an
impervious surface depends upon the use of that specific surface. Therefore, watershed
degradation can be tied to both impervious surface and land use. Chester et al. (1996)
demonstrate how maps of the current built state and the future build-out state can be used to
show stakeholders the degradation of the watershed in both instances. Maps were created using
satellite imagery, from which general land-cover categories and estimates of impervious surface
were taken. The authors then suggested that the maps be used to aid regional and local planning,
as well as modify regulations. They suggest that from this rough build-out analysis planners
should revisit zoning requirements and emphasize the minimization of impervious surface in
future development.
In their 2005 paper, Conway and Lathrop discuss the use of local scale watershed
management to “bridge the gap between land use planning and natural resource management”.
They particularly focus on build-out analysis for examining future landscapes, before permanent
changes occur. To present the utility of the build-out analysis, the authors conducted four buildout scenarios for the Bamegat Bay Estuary in New Jersey: 1) Current regulations scenario:
based on current zoning and environmental regulations; 2) Down zoning scenario: minimum lot
size is increased; 3) Large buffer scenario: down zoning with an increased buffer around all
freshwater; 4) Open space scenario: down zoning scenario with a more aggressive plan to
protect open space. For each scenario maps were created to show future land use and percent
imperviousness. The authors then proposed that these maps be shared with regulators to help
them understand the impact of current and future zoning and regulations.
Sample et al. (2001) demonstrate the utility of combining GIS with water resources
decision support systems (DSS) for local scale stormwater management. In general, DSS
incorporate hundreds of variables and use complicated algorithms to solve the optimization
problem. Sample et al. (2001) state that “the strength of GIS is that it can be used as a common
ground between specialists and nonspecialists to help them communicate effectively.” To this
end, the authors examine a developed mix use neighborhood to determine what best management
practices (BMPs) could be added to the neighborhood to return stormwater characteristics to
predevelopment conditions. The analysis used GIS data layers to calculate, the curve number,
which assigns different levels of change to the natural hydraulic system depending on land use
and impervious surface. Then, the amount of land that should be dedicated as BMP locations to
make up for the change in the hydraulic system is calculated. At this point, an optimization
routine can be run to determine the lowest cost locations to implement BMPs.
In their 2010 paper, Viavattene et al., create a GIS-based BMP model to examine the
reduction in stormwater runoff through BMP implementation. They say that one strength of this
model is that the GIS interface allows it to be used by “a range of local authority/municipal,
federal/state regulatory agencies, drainage engineers/consultants and other interested
stakeholders in the development and evaluation of stormwater drainage infrastructure contained
within stormwater management plans.” Viavattene et al. (2010) demonstrate the effectiveness
of their tool using a case study of the urban regeneration project in the city of Birmingham, UK.
This city was chosen because (as of 2007) a large part of the sewer network surcharged during 5year rainfall events. The user is able to use the “ADD BMP Tool” to add appropriate BMPs to
data layers. Running the model before and after BMP implementation showed that the same
number of sewer surcharges occurred, but that the flow of each surcharge was greatly reduced
due to the BMPs. The tool showed the user that the specific selection of BMPs helped reduce
the magnitude of the problem, but that more modifications would be necessary to completely
eradicate the problem.
Data Layers
Data Layer
Boundary Polygon
Source
M Drive City of Concord
Hydrography Polygon M Drive City of Concord
Wetlands Polygon
M Drive City of Concord
Open Space Polygon
Roads Polygon
Mass GIS
M Drive City of Concord
Buildings Polygon
M Drive City of Concord
Easements Polygon
Zoning Polygon
Parcels Polygon
Edge of Pavement
line data
Soils Polygon
M Drive City of Concord
Mass GIS
M Drive City of Concord
M Drive City of Concord
M Drive City of Concord
Source Scale
unknown
Digitized from aerial
photography
Digitized from aerial
photography
1:25,000
1:1,200
Digitized from aerial
photography
unknown
unknown
1:1,200
Digitized from aerial
photography
1:24,000
Year Created
1996
1996
1996
2005
1996
1996
2005
2007
1996
2007
2005
Methods
Examine current conditions
Separate zoning layers for the residential (R1, R2, R3 and R5), light industrial, general
industrial and limited business zoning regulations were created. This was done to facilitate the
multiple following steps requiring the selection of an object lying within each specific zoning
designation.
The impervious surface area of the current conditions was calculated using the building
footprint polygon, the roads polygon and the edge of pavement dataset. For both the polygon
data sets, the “statistics” tool was used to measure their area. Since the roads polygon does not
always include smaller roads and driveways, the additional amount impervious surface for
limited business, light industrial and general industrial was calculated using the “measure” tool
on the edge of pavement line data. The residential areas are large and have too many small
driveways to measure each individually. Therefore, total additional impervious surface was
calculated by measuring the amount of additional impervious surface in a representative area, for
each residential zoning type and assuming that the rest of the zoning designation had similar
impervious characteristics.
Create layers of already built-out land
For residential zoning designations, the “select by location” tool was used to select
parcels whose centroid lies within the zoning layer. Then, the selection was narrowed to all
parcels having a shape area less than twice the minimum lot size shown in Table 1. The rational
here is that any lot greater than twice the minimum lot size could be split in half and further
developed. New layers of built-out land were created from these selections.
Table 1. Minimum residential lot size and selection criteria for built-out land
Residential Zoning
Designation
R1 (>80,000 sq. ft.)
R2 (40,000-79,999 sq. ft.)
R3 (20,000-39,999 sq. ft.)
R5 (5,000-14,999 sq. ft.)
Min Lot Size
(sq. ft.)
80,000
40,000
20,000
10,000
Selection Criteria
(sq. ft.)
< 160,000
< 80,000
< 40,000
< 20,000
The same method was not applicable to industrial and limited business parcels because
though most of these parcels are larger than the minimum lot size, they are covered with large
impervious surface areas. Since impervious surface is the principal component of this particular
build-out analysis, these lots already covered with impervious surface are considered built-out.
The area of Concord zoned for Light Industrial, General Industrial, and Limited Business is
small, which allowed these areas to be examined parcel by parcel to determine buildable areas
for creation into new layers.
Determine buildable land
Starting with the zoning polygon, use the “erase” tool was used to remove the built-out
layers created above. Then the “erase” tool was also used to remove roads, protected open
space, hydrography, wetlands, floodplains and easements.
Estimate the area of impervious surface at built-out
The “summarize” tool was used to calculate the land area of each zoning type in the
build-out scenario. Using the built-out data layers, the ratio of impervious surface to land area of
built-out parcels was estimated for each zoning type. The buildable land area for each zoning
type was multiplied by its corresponding ratio to determine the amount of impervious surface at
build-out.
Calculate the effects of build-out on stormwater flow
A general rule of thumb used in stormwater analysis is that approximately 90% of
pollutants in stormwater are found in the first half inch of rainfall for each storm. After the first
half inch has fallen, most of the pollutants are expected to be already washed from the
impervious surfaces. Therefore, a one stormwater management criterion is to capture and
control this “first flush”. This analysis used 0.5 in. of rainfall to calculate the amount of
stormwater caused by impervious surfaces that needs to be managed. To do this, the area of
impervious surface at both current and build-out conditions was multiplied by 0.5 in to calculate
a volume of stormwater.
Assess possible locations for LID implementation
In order for LID technologies that discharge stormwater back into the ground to
effectively manage stormwater, they must be built on soils that can transmit the water at a high
enough rate. If the soils cannot transmit the water, flooding will occur. Therefore, the soils
polygon was used to find locations that where the soil drainage is moderately well drained or
higher.
Results
Current Conditions
Concord is currently zoned to be 94.7% residential, 1.0% limited business and 3.7% industrial.
Analysis of the building footprint polygon, roads polygon and edge of pavement polygon were
used to determine that approximately 7.3% of the land surface is impervious. Examination of
parcel data determined that 25.3% of Concord is already built to the maximum extent allowed by
the zoning regulations. The light industrial and high density residential zoning designations have
the highest percentage of their land already built-out, at 42.9% and 48.8%, respectively. Land
designated as limited business is only 1.3% built-out. However, as limited business is only 1%
of the total land area, additional construction in these districts is unlikely to have a large impact
on stormwater.
Build-out Analysis
As stated previously, the land available for development was determined by erasing the
already built-out land, surface water and protected areas from the total area of Concord. This
build-out analysis shows that 29.5% of the land area is still available for development. At buildout, 54.8% of the land in Concord will be developed. As shown in Table 2 only 25-30% of the
land designated as residential is available for development, while 50-65% of the land designated
as business or industrial can be developed. However, since the total amount of land set aside for
business and industrial development is small compared to the size of Concord, the largest
amount of land available for development (22%) is zoned as low density residential. Once buildout is reached, approximately 12.7% of the Concord land surface will be impervious.
Table 2. Buildable land statistics
Buildable Land
Zoning Designation
5,000-14,999 sq. ft.
20,000-39,999 sq. ft.
40,000-79,999 sq. ft.
> 80,000 sq. ft.
Limited Business
Light Industrial
General Industrial
% Of Total
Land
3.9
12.7
28.2
49.9
1
1.7
2
% of Zoning
Designation
24.7
28.9
25.8
29.4
51.8
60.2
65.0
% of Total
Land
1.0
3.7
7.3
14.7
0.5
1.0
1.3
Stormwater and LID Analysis
One of the most important components of a stormwater analysis is to determine the
volume of stormwater created in the first half inch of rainfall. Because this “first flush” carries
approximately 90% of the stormwater pollutants, capturing and treating this water can
significantly reduce the impact of development on the water quality of the area. Currently, the
1.9 square miles of impervious surface in Concord produces 2.2 million cubic feet of stormwater
during the first 0.5 in of rainfall. At build-out, the additional 1.4 square miles of impervious
surface will add another 1.6 million cubic feet of stormwater.
According to the environmental indicator developed by Chester et al. (1996) the increase
of impervious surface from 7.3% - 12.7% will move Concord’s water quality from “protected” to
“impacted”. However, if zoning regulations are changed to require developers to capture and
treat the volume of stormwater created due to their development, Concord could continue to have
an effective impervious surface of 7.3%. This would keep the watershed in the “protected”
category. One of the best ways to treat this additional stormwater would be to incorporate LID
technologies into the new developments. Unlike traditional engineering solutions for managing
stormwater, LID systems can be incorporated into development without ruining its visual appeal.
Many of these technologies directly input the stormwater back into the ground. For this
to work and not flood the surrounding area, the LID technology needs to be built on soils that are
able to drain quickly and transmit the volume of stormwater directed into the ground. Therefore,
a soil drainage map was created to visualize the drainage characteristics of the buildable land.
Any soil moderately well drained or higher is deemed a good location of LID implementation.
In Concord, 84.8% of the buildable land is well drained enough to support these technologies.
Therefore, LID implementation seems like a reasonable method for handling stormwater due to
increased development in Concord.
Conclusions and Caveats
A GIS based build-out analysis shows that developed land in Concord, MA will increase
from 25.3% to 54.8%. Most of the development will occur in low density, residential areas.
Many assumptions were made to complete this build-out analysis. One of the key difficulties
was that the data sets range in creation date from 1996-2005. Additionally, a rigorous
assessment of the accuracy of each data set was not performed. The build-out analysis
conducted does not represent the most built-out Concord could become because of the method
used to determine parcels that are already built-out. Parcels less than twice the minimum lot size
were determined to be already built-out. It is likely that many of these parcels could add more
development and impervious surface while still following zoning regulations.
Impervious surface of the current conditions was estimated by combining building
footprint, road and edge of pavement data. Since the edge of pavement data was a line data set,
the area of this data set could not be calculated using ArcGIS. The additional impervious surface
from the edge of pavement data was estimated using the measure tool for a small section in each
zoning type and then assumed to apply to the entire zoning type. If the impervious surface to
land area ratio is not homogeneous throughout a zoning designation, this method could easily
underestimate or overestimate the amount of impervious surface. Additionally, these same ratios
were used to estimate the impervious surface area of future developments. Though this could
make both current and future estimates incorrect, the analysis is consistent for both time frames.
The stormwater analysis conducted for this project was very preliminary. Though a “first
flush” assessment is good for handling some stormwater pollutants, it does not apply to all
pollutants of concern. Additionally, the other principal problem with growing development and
impervious surface is flooding from increased stormwater flow. Flooding was not addressed at
all in this assessment. To look at the effects of build-out on flooding, one would need to create
impervious surface connectivity data sets, look at local slopes and use other design storms in
addition to the first half inch. However, the assessment done in this project would be adequate
preliminary work to set up for a flooding analysis.
Lastly, the LID assessment conducted in this project was also very preliminary. Whether
or not LID should be implemented at all, as well as the various types of LID that should be used
depend on much more than soil drainage data. Among other things they depend on land use,
land slope and proximity to sensitive environments. All of these things could be examined using
GIS, but ultimately site visits are necessary to select acceptable locations for LID
implementation.
Overall, this build-out analysis and stormwater assessment of Concord, MA is an
adequate preliminary assessment. The visuals created would be helpful for planning and
community meetings to demonstrate what the area could look like under the current zoning
conditions. They also give planners an estimate of the increased stormwater that will need to be
managed in the future. However, any future designs and changes of zoning regulations would
require a more in depth analysis of all areas touched on in this project.
References
Arnold Jr., Chester L. and Gibbons, C. James. 1996. Impervious Surface Coverage: The
Emergence of a Key Environmental Indicator. Journal of the American Planning Association, 62:
2, 243 — 258.
Conway, M. and R.G. Lathrop. 2005. Alternative land use regulations and environmental
impacts: assessing future land use in an urbanizing watershed. Landscape and Urban Planning
71: 1–15.
Sample, D.J., Heaney, J.P., Wright, L.T., and R. Koustas. 2001. Geographic Information
Systems, Decision Support Systems, and Urban Storm-water Management. Journal of Water
Resources Planning and Management, May/June 2001
Viavattene, C., Ellis, J.B., Revitt, D.M., Seiker, H and Peters, C. 2010. The application of a GISbased BMP selection tool for the evaluation of hydrologic performance and storm flow
reduction. Proc. 7th Int.Conf. Sustainable Techniques and Strategies for Urban Water
Management; NOVATECH10. 28 June – 1 July 2010. GRAIE, Insa de Lyon. Lyon, France.