Elizabeth Agyeman-Budu City & Regional Planning Department, Cornell University April, 2015

USING GIS TO IDENTIFY THE VULNERABLE DEMOGRAPHY IN THE CITY OF ITHACA’S 100-YEAR FLOODPLAIN

AREA UNDER STUDY: CITY OF ITHACA, NY

The City of Ithaca is a relatively small jurisdiction, covering only 1.2 percent of Tompkins County’s total land size area.

However, under the existing condition of the flood management channel, a sizeable area city is more prone to be

inundated in an incident of a 100-year or 500-year flood within the county, as 33 percent of the county’s flood zone

parcels are located within this jurisdiction. The dense concentration of flood prone areas within the city also raises

issues of safety, as an estimated 30 percent of the County’s population live within this vicinity.

Tompkins County covers a land area of 474.65 square miles and has an estimated population of 103,617 (US Census Bureau, 2010) .

Map Credit: : Army Corps of Engineers

The Army Corps report proposes three hazard reduction dredging scenarios that the municipal government could adopt, to reduce flood risk to the surrounding low lying area. As such, under the prevailing conditions of the flood management channel, any disastrous flood event has the potential to affect a higher spatial

concentration of people, properties and businesses within the county.

285 Property Parcels Affected 899 Property Parcels Affected

1981 FEMA 100-year Boundary 2011 Army Corps 100-year Boundary

The 2011 Army Corps of Engineers hydraulic study of the Cayuga Inlet, revealed a 70% reduction in the

inlet’s capacity to hold a 100-year flood due to significant sedimentation in the 3-mile long flood

management channel. The city is located in a low lying watershed, drained by three main creeks which all

drain into the Cayuga lake at the northern end of the city. The flood management channel was therefore

constructed to contain a 100-year or 50-year flood. However, under the channel’s current condition, the

1981 FEMA mapped areas for flood Insurance rate purposes has its boundaries now extended to

encompass new areas not designated as flood prone.

—Area to be Dredged

Each Dredging Scenario that the municipal government decides to adopt, has impacts some given section

of the resident population in the City. This research therefore seeks to identify the implications of each

scenario the city adopts by identifying the vulnerable demography within the flood zone in any incident of a

100-year flood. The results derived is intended to inform:

1.  How many households will be displaced in a case of a 100-year flood under each dredging scenario

2.  In terms of rescue efforts, what number of vulnerable resident population is expected to receive the

most help, under each dredging scenario

SS

METHODOLOGY Steps Used in

Generating Maps

and Results

Step 1: Joined Tables to Parcel Shapefile with the Join Tool

a.  Data Used: Excel File of list of affected property parcels (Source: City of Ithaca, GIS Program) Tompkins County Parcel Shapefile (Source: CUGIR)

b.  Layer Used to Join:

Swiss Print Key Process:

1. Join & Relates > Join> Selected ‘SWISPRINTK’ in the Choose the field in this layer that the join will based on: drop down menu >Keep only matching records > OK

2. Export joined table to derive the flood zone shapefile

Excel File

Step 2: Calculated the area of flooded region (sq. miles) To calculate for household distribution with the flood plains (later in the analysis) the area of the affected parcels is calculated first and foremost Process:

Create New Field>”Flooded_AREA>calculate geometry (sq. miles) Step 3: Intersect Parcel and Block Shapefiles with the Intersect Tool To derived the number of households within the affected zones in each scenario by intersect the parcel shapefile with the Tompkins County census block shapefile. a.  Data Used:

Flood Zone parcel Shapefiles Tompkins County Census Block Shapefile (Source: CUGIR)

Usefulness of the Intersect Tool: The attribute table of the intersect output shapefile retains the census data as well as the data from the parcel shapefile which will be used for all further analysis

Parcel Shapefile

(without Demographic data)

Census Block Shapefile

(with Demographic Data)

Step 4: Calculate for Households Affected The analysis takes into consideration that, some blocks are unevenly flooded. The fringe or boundary blocks are most at times partially flooded as such, it is essential to calculate for the number of housing to be affected in an event of a flood. To do this:

a.  The Household Density is first calculated Density= Housing

Area_Flooded b. Households affected is then calculated

Affected Households= Flooded Area * Density Step 5: Calculate for Vulnerable Population Process:

a.  Generate a new variable known as Vulnerable Population Vulnerable Population = Population Under 5 + Population Above 65

b. Taking into consideration that not all blocks are evenly flooded and assuming that the

vulnerable population is evenly distributed within a block;

•  Calculate for Vulnerable Population Density Density= Vulnerable Population Area_Flooded

•  Calculate for Affected Vulnerable Population Affected Vulnerable Population= Flooded Area * Density

Partial Flooding at Fringes of Flood Boundary

RESULTS •  FINDINGS

•  MAPS

RESULTS : Count Reduction

Significant reduction in affected households and vulnerable population in scenario 2 and 3

RESULTS: Spatial Reduction

Even though the physical count of the vulnerable population decreases, the blocks with higher concentration of

vulnerable population in the southern section of the flood plain are still impacted in all scenarios.

CRITIQUE, CONCLUSION & RECOMMENDATION 1.  There is a bias in the assumption that, the distribution of the affected households and vulnerable population are

evenly distributed within a block when calculating for affected demography within partially flooded fringe blocks.

2.  The demography of the flood plains constantly change and as such, for the purpose of disaster risk

management this analysis has to be constantly updated to reflect the current demographic distribution within the

flood plain

3.  The census data used are based on estimates and as such are subject to some margin of error. Therefore

analysis is preceded with caution.

However, even with the above listed limitations, the results derived are still useful for informing policy decision that

the City makes.

CONCLUSION AND RECOMMENDATION

•  The City of Ithaca stands to reduce the flood risk within the floodplains if it adopts either dredging scenario 2 or

scenario 3.

•  However in adopting either scenario 2 or 3, it has been identified that the vulnerable population within the

southern section of the city (within the Titus Flat neighborhood) will still be greatly impacted in the incident of a

100-year flood as opposed to other areas within the flood plain.

•  In any severe flood incident, emergency response and rescue efforts within the specified area should be

reinforced to cater for the vulnerable population.