Rhiannon Ervin
Assignment 7
April 21, 2011
Introduction
In many towns and cities in the United States, zoning laws were created many years ago and have
not been significantly modified since. Consequently, many of the newer ideas of smart growth, low
impact development (LID) or watershed management best management practices (BMPs) are not
incorporated into zoning ordinances. One way to illustrate the impact of leaving zoning regulations as
they are is to conduct a build-out analysis. A build out analysis takes the current zoning regulations and
maps the future look of an area if all parcels were built to the maximum extent allowed in the zoning
laws. A build-out analysis can also be used to estimate the additional amount of resources required (e.g.
water-use) to support build-out. With this knowledge, planners and regulators can discuss what zoning
laws and regulations should be added or modified to protect both the natural and the living
environment of a particular location.
For my final GIS project, I propose to conduct a build-out analysis of Concord, MA, with a particular
focus on stormwater management. First a conventional build-out analysis will be done using current
zoning laws. The increase of impervious surface and stormwater flow will be calculated for this
scenario. Then the analysis will be conducted again assuming a percentage of impervious surface is
reduced using LID technologies. This percentage and the type of LID treatment suggested will be
dependent on land use. Additionally soils data and elevation contour data will be used to ensure that
LID technologies can be used in these areas. The analysis will be general, but will be used to
demonstrate the appeal of using GIS to present complicated land use planning and stormwater
modeling projects.
Data Layers
The following data layers will be used for this analysis.
Data Layer
Boundary Polygon
Source
M Drive City of Concord
Source Scale
Year Created
unknown
1996
Hydrography Polygon M Drive City of Concord
Digitized from aerial
photography
1996
Wetlands Polygon
M Drive City of Concord
Digitized from aerial
photography
1996
Open Space Polygon
Mass GIS
1:25,000
2005
Contours Polygon
M Drive City of Concord
unknown
2005
Roads Polygon
M Drive City of Concord
1:1,200
1996
Buildings Polygon
M Drive City of Concord
Digitized from aerial
photography
1996
Easements Polygon
M Drive City of Concord
unknown
2005
Zoning Polygon
Mass GIS
unknown
2007
Parcels Polygon
M Drive City of Concord
1:1,200
1996
Edge of Pavement
line data
M Drive City of Concord
Digitized from aerial
photography
2007
Soils Polygon
M Drive City of Concord
1:24,000
2005
Catch Basin Points
M Drive City of Concord
Digitized from aerial
photography
1995
Methods
The following steps will be carried out to complete the analysis
I.
Create separate zoning layers for Residential 1 (R1), Residential 2 (R2), Residential 3 (R3),
Residential 5 (R5), Light Industrial, General Industrial and Limited Business
a. Using the “select by attribute” tool, select each zoning layer and export separately
II.
Calculate impervious surface area of the current conditions
a. Calculate the total building surface area using the “statistics” tool on the entire building
footprint data set
b. Calculate the total surface area of the roads using the “statistics” tool on the roads
polygon
c. Since the roads polygon does not always include smaller roads and driveways, estimate
the amount of additional impervious surface for Light Business, Light Industrial and
General Industrial using the “measure” tool on the edge of pavement line data
d. The residential areas are large and have too many small driveways to measure each
individually. Therefore, additional impervious surface will be calculated by relating the
amount of additional impervious surface in a representative area, for each residential
zoning type
i. Pick a representative area for each zoning category
ii. Using the “measure” tool, estimate the additional amount of impervious surface
in that representative area
iii. Divide this measured area by the area of the road polygon to give the amount of
additional impervious surface per sq. ft. of road
iv. Multiply this ratio by the total amount of road in each zoning category to
calculate the additional impervious surface area for each residential zoning type
e. Sum all impervious surface area and divide by the total area of Concord to determine
the percent of impervious surface in the town
III.
Determine built-out land
a. For residential zoning designations, use the “select by location” tool to select parcels
whose centroid lies within the zoning layer
b. Select from the above selection all parcels that have a shape area less than twice the
minimum lot size. (The rational is that any lots greater than twice the minimum lot size
could be split in half and additional construction allowed on part of the lot).
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
c.
IV.
V.
Create a new layer of all parcels that are less than twice the minimum lot size, for each
residential zoning area
d. The same method is not applicable to Light Industrial, General Industrial, and Limited
Business because most of the parcels in these locations are much larger than the
minimum lot size, but are covered with large impervious surface areas. Since
impervious surface is the most important part of this particular build-out analysis, these
lots cannot be built out further if they are already covered with a parking lot.
e. As the area of Concord zoned for Light Industrial, General Industrial, and Limited
Business is small, these locations will be examined parcel by parcel to and buildable
areas will be selected manually and created into new layers
Determine buildable land
a. Starting with the zoning polygon, use the “erase” tool to remove the built layers created
in step III
b. Continue using the “erase” tool to also remove, roads, protected open space,
hydrography, wetlands and easements
Estimate the amount of impervious surface in the built-out scenario
a. Use the “summarize” tool to calculate the land area of each zoning type in the build-out
scenario
b. Using the built-out data layers created in step III, estimate the amount of impervious
surface at built conditions for each zoning type
c. Divide this impervious surface by total area of the built land, for each zoning type
d. Multiply the amount of buildable area by the above ratio, for each zoning category
e. Divide the amount of impervious surface in the build-out scenario by the area of
Concord to determine the percent of Concord that will be impervious in this build-out
analysis
VI.
Calculate the effects of build-out on stormwater flow
a. Use the 2-year design storm to estimate the amount of stormwater for the current
situation
b. Use the same design storm to estimate the amount of stormwater for the built-out
scenario
c. Estimate the effect of controlling the stormwater from 20% of the built-out impervious
surface using LID technique
VII.
Generally suggest areas of Concord that would be best suited for LID implementation
a. Use the soils polygon to find locations that have high water storage soils
b. Use the contour lines to find less sloped areas
Deliverables
-
Map of the current conditions in Concord
Maps after each feature discussed in step IV is erased
Map of buildable areas
Map of possible LID locations
Estimate of total stormwater for the current condition, the build-out analysis and the build-out
analysis with LID implementation