Michelle Sheehan
Intro GIS – Assignment 6
November 6, 2007
Introduction
With the following analysis I am attempting to show the importance of new and
innovative stormwater management in Providence, RI. Providence does not currently
have any policies in place to deal with urban runoff but are in the process of a major
construction project to reduce the number of combined sewer overflows (CSOs). This
project is underway and is slated to take another 12-15 years to complete.
In the meantime many of Providence’s residents live in close proximity to these
overflows. According to the USEPA “Typical pollutants found in CSOs include total
suspended solids (TSS), metals, bacteria, viruses, nutrients, oxygen-demanding compounds
and other pollutants washed from city streets and parking lots. CSO impacts include adverse
human health effects often due to recreational exposure, (e.g., gastrointestinal illness), beach
closures, shellfish bed closures, toxicity for aquatic life, and aesthetic impairment.” 1
A second, but related issue is the flood vulnerability of residents near Providence’s urban
rivers. This risk is exacerbated by the lack of innovative stormwater policies that allow
rainwater to infiltrate into the ground.
Analysis: Part 1
The first part of this analysis shows the numbers of people who live near CSO discharge
points and the racial make-up of this population.
The second part deals with the 500 year flood zone along Providence’s rivers. I wanted to
identify the most vulnerable areas in the city to a major flood and to see how many of these
areas overlapped the CSO-proximate areas.
To show the population near CSO discharge points I used the “select by location” feature
and chose census blocks (from US Census 2000 (SF1) available from RIGIS) “within a
distance of 200 feet” from the CSO point locations. I created a new layer, called “blocks near
CSO” from this selection. The map below shows the blocks identified in this selection:
Sewer Overflow Points
Rivers
blocks near CSO
Census Blocks in proximity to CSO discharge points
I then used the “statistics” function to determine the population of those census blocks. I
actually did this a number of ways. I calculated the total population, the number of
housing units, and the number of owner-occupied housing units. This is because the
population of Providence is in flux. I wanted to know how many people live there now
but also how many people have the potential to be living there based on existing housing
stock. The table below shows the total housing units in the selected blocks:
There are: 1,483 housing units, 3,072 people, and 180 occupied housing units within 200
feet of a combined sewer outfall in Providence.
For my next analysis I wanted to determine what the racial makeup of this population is.
To do this I created a new field in the “blocks near CSO” attribute table, called
“Minority”. I used the field calculator to add together the numbers of the following
populations: African American, Hispanic, Asian, Native American, Multi, and “Other”.
Then I summarized the population field by African American, Hispanic, and Multi do see
how many of each group lives in each census block. Below is the summary table:
The following statistic table gives the number of minority residents that live in census
blocks within 200 feet of a CSO discharge point.
Analysis: Part 2
The second part of my analysis looks at the 500 year flood zone data for Providence
(From the FEMA Statewide Floodmap available at RIGIS). Once again I used Census
2000 blocks to show the population residing within this zone. First I used the “select by
attribute” feature and chose the flood map layer where “zone” = 500 year. I saved this
selection as a layer file titled “500 year flood”. I then used the “select by location” feature
and chose census blocks “that have their centroid in” the “500 year flood” layer. The map
below shows some of the blocks identified in this selection:
500yr flood blocks
Providence Flood
500 year
100 year
Rivers
I then calculated the total population and the total housing units in these blocks. Below is the
statistics table for total housing units:
983 people live in these housing units.
A third analysis that would be useful is to see how many residents live both near a CSO
discharge point and are in the 500 year flood zone. In the case of flood they would be in
particular risk of contamination.
Caveat
When I calculated populations near CSO discharge points I used census blocks within a
distance of 200 feet from the points. This is tricky because of course everyone within the
census block is a different distance from the discharge points. Also, I sort of picked 200
feet randomly without an analysis of how close is too close.
1
U.S. Environmental Protection Agency - October 2007
FY08 – FY10 Compliance and Enforcement National Priority: Clean Water Act, Wet Weather, Combined
Sewer Overflows, Available online at
http://www.epa.gov/compliance/resources/publications/data/planning/priorities/fy2008prioritycwacso.pdf