DisasterMap.net’s Analysis of the National Levee Database and 2010 Census

Posted on August 14, 2015 By

US Levees and Bad Levees

Introduction

Our nation depends on levees and floodwalls to protect our communities, our homes, and our vital economic assets. America has nearly 15,000 floodwalls and almost 22,000 levees, according to our interpretation of a database maintained by the Army Corps of Engineers. These structures protect over 85,000 sq. miles that include 1,800 named Census places with a total population of 65 million people. However, not all of these levees are in good shape, and an estimated 6.8 million people live in a named place that has one of the 392 levee and floodwalls that have been rated “unacceptable.”

This project combined data from a nationwide database on levees and floodwalls with population and housing data to explore the depths of this dependence. We’ve found that millions of Americans depend on these levees to protect their homes. However, not all of these levees are “good levees,” and millions of people and homes are left at risk due to our nation’s “bad levees.”

The National Levee Database documents the nation’s levees, the areas they protect, and their condition. This project combines this database with 2010 Census population and housing data to examine the people that the good levees protect and the people at risk due to the bad levees. The project consists of a web visualization for all levees and one for the bad levees. Each webpage consists of an interactive web map, interactive graphics, and basic summary statistics. This blog post summarizes our findings, while a technical note provides details on the data analysis. Finally, we also created this static map showing the levees throughout the United States.  Together, we hope these tools help citizens, journalists, and others to better understand America’s relationship with it’s levees from national, regional, and local perspectives.

Data and Analysis

The US Army Corps of Engineers maintains the National Levee Database (NLD) and an associated public access website. The website includes an interactive web map viewer and tools for custom reports. For this project, the levee centerlines, the floodwalls, and the protected areas layers were obtained from NLD Web Feature Service in January 2015 and then processed with the desktop QGIS program.

We also obtained the unacceptable levee list using the NLD’s report tool. From the report tool, we downloaded a spreadsheet that included the results of the last inspection. We then filtered the spreadsheet for those that were rated “unacceptable” after their last periodic inspection. We then mapped the approximate location of these bad levees. Our technical note describes limitations to the accuracy of the location recorded for the bad levees.

To examine the demographic context of these levees, population and housing data were obtained from the 2010 US Census. A downloadable geodatabase provides data on total population, total homes, and occupied homes at the county, place, and tract levels. A series of desktop GIS processing steps (described in the technical note) extracted the counties, places, and tracts that have a levee protected area or a bad levee within its bounds. From there, these datasets were used to derive basic statistics, described in the next two sections, summarizing the people and houses adjacent to levee protected areas and near the bad levees.

Of note, the different aggregation levels (county, place, and tract ) for the census data correspond to different geographic units of analysis. Unfortunately, none of the geographies provide the spatial detail needed to precisely estimate the number of people living in the levee protected areas. Still, each of the three spatial units have different strengths in assessing the relationship between levees and settlement patterns.

Tracts likely provide the closest measure of the population and homes within the levee protected areas, though these still likely overestimate the true number. The tracts are a well defined Census unit that provides complete coverage of the U.S. with units drawing based on “an optimum size of 4,000 people.” To minimize variation in the population size, Census tracts vary greatly in their geographic size. Dense, urban tracts are small and tightly spaced, while rural tracts are large and spread out. Because most of the population residing in leveed areas are also in urban areas, the tracts provide the most accurate measure of the population and homes directly within the protected areas.

Places represent communities that are defined through social bounds and economic interactions. Unlike counties and tracts, this unit does not provide continuous nationwide coverage, though the “unnamed” areas contain a small percentage of the total population. Large places, such as Sacramento and Memphis, can include many tracts, while small places such as Willow Brook, KS can cover only a small portion of a large rural tract. Finally, counties are well defined economic units with specific direct and indirect benefits/impacts tied to the levee protected areas, such as employment rates and property tax revenue.

While none of the Census geographies correspond directly with the outlines of the levee areas, it is important to keep in mind the impacts of levee extend beyond just the people and houses located within the protected areas. Levees and floodwalls also provide indirect benefits to a community, particularly for areas where waterborne commerce is a major part of the economy.

Figure 1 below illustrate the different Census geographies with respect to the NLD layers.

Figure 1: Maps depicting NLD layers over the the different Census geometries. Sacramento is a large named place that is mostly inside a levee protected area, and most of its Census tracts are completely within levee protected area. Memphis is another large Census place but only a small portion is protected. Only a few of it’s tracts are within protected areas. Hutchinson City, KA is a small Census place that is partially levee protected. Unlike the dense urban areas, most of it’s tracts are only partially protected.
NLD&CensusBoudaryExamples

Results: All Levees

This section describes the basic summary statistics for the levees, floodwalls, and leveed areas and then discusses our results from overlaying these NLD layers with the three Census layers.

According to the NLD, there are over 21,000 levee reaches and 14,000 floodwall reaches in America. The levees total over 13,000 miles, while the floodwalls span only 80 miles. Combined, they protect an estimated 85,000 sq. miles. The largest levee is 233 miles, while the largest floodwall is 7 miles. The largest protected area is 10,500 sq. miles. See Table 1.

Table 1: Summary of levees, floodwalls, and leveed areas.

Levees (miles)

Floodwalls (miles)

Leveed Area (sq mi)

Count

21,772

14,459

2,530

Maximum value

233

7

10,566

Sum

13,400

80

85,097

Mean value

0.62

0.0055

33.64

Out of 74,000 total Census tracts, there are 5,359 Census tracts that have a levee or floodwall, and they have a total population of 22 million along with 9 million total housing units of which 8 million are occupied.Census named places comprise any named place (incorporated or not) registered with the US Census bureau. These can be a large city or a simple intersection with a few dozen nearby houses. Out of a total of 30,000 named places, there are 1,820 named Census places that have a levee. The total population of these places is 65 million. There are 27 million total housing units, of which 24 million are occupied.Finally, out of 3,200 counties in the US, 761 counties have a levee or floodwall within their bounds. They have a total population of 149 million, 62 million total housing units, and 55 million occupied housing units.

Table 2: Summary Statistics for Counties, Places, and Tracts

 

Count

Total Population

Total Houses

Occupied Houses

Tracts

5,359

22,051,440

9,124,252

8,093,411

Places

1,820

66,446,370

27,378,771

24,832,481

County

761

148,801,127

61,858,004

55,489,195

 

Table 3: Top 20 highest population places with a levee protected area.

City

Levee System

Population

Total Houses

Occupied Houses

New York city

Oakwood Creek West Bank

8,175,133

3,371,062

3,109,784

Los Angeles city

Ballona Creek 1

3,792,621

1,413,995

1,318,168

Houston city

Lynchburg Pump Station

2,099,451

892,646

782,643

Phoenix city

Glen Harbor LB Downstream

1,445,632

590,149

514,806

San Diego city

Tijuana River 3

1,307,402

516,033

483,092

Dallas city

Kaufman LID 6 East Fork Trinity RB

1,197,816

516,639

458,057

San Jose city

Guadalupe River – LB

945,942

314,038

301,366

Indianapolis city

Indianapolis Levee System

820,445

379,856

332,199

Columbus city

West Columbus, OH, LPP

787,033

370,965

331,602

Fort Worth city

Waterworks Levee Clear Fork RB

741,206

291,086

262,652

El Paso city

Central El Paso Fort Bliss Diversion, Levee

649,121

227,605

216,894

Memphis city

Memphis – Wolf River Backwater Levee System

646,889

291,883

250,344

Washington city

Anacostia

601,723

296,719

266,707

Nashville-Davidson

Nashville, TN – Metro Center

601,222

272,622

249,002

Louisville

Louisville Metro Levee System

597,337

270,928

246,438

Portland city

Multnomah – East

583,776

265,439

248,546

Oklahoma City city

North Canadian Waste Water Treatment Levee

579,999

256,930

230,233

Albuquerque city

Alb. Middle Rio Grande, West Levee

545,852

239,166

224,330

Tucson city

Tucson Diversion Channel 4

520,116

229,762

205,390

Sacramento city

American River FCD – Dry Cr, NEMDC, Arcade Cr

466,488

190,911

174,624

 

Results: Unacceptable Levees

There are 392 levee reaches that were rated “unacceptable” during their last inspection. The total length of the levees is over 3,000 miles, and they protect over 13 million acres. Another 1,033 levee reaches that protect 15 million acres were rated “minimally acceptable.” Only 116 (7.5%) of the levees that had been inspected were rated “acceptable.” Of note, nearly 40% of the levee reaches did not have an inspection result listed, so the number of people and houses at risk due to bad levees is almost certainly much higher.

Table 4: Summary of the Last Inspection Rating based on the NLD system report downloaded on January 9, 2015.Last Routine Inspection Rating

Last Routine Inspection Rating

Count

Total Length (Miles)

Total Leveed Area (Acres)

ACCEPTABLE

116

288

212,050

MINIMALLY ACCEPTABLE

1,033

6,448

15,244,311

UNACCEPTABLE

392

3,295

13,201,183

not listed

994

4,854

8,983,234

Total

2,535

14,885

37,640,778

In total, there are over 5,000 Census tracts with bad levees with nearly 22 million people living inside them. They contain 9 million total houses and 8 million occupied houses.There are 774 counties that haves bad levees with a total population of nearly 40 million, 16 million total houses, and nearly 15 million occupied houses.Among named Census places, there are 127 that have bad levees. They have a population of 6.8 million, nearly 3 million total houses, and 2.7 million occupied houses.

Table 5: Bad levee summary statistics for state, county, place, tract

 

Count

Total Population

Total Houses

Occupied Houses

Tracts

5,289

21,792,356

9,008,954

7,988,001

Places

129

6,789,111

2,972,890

2,653,076

County

176

39,024,536

16,362,823

14,764,095

 

Table 6: Top ten highest population Places with Bad Levee

City

Levee System

Population

Total Houses

Occupied Houses

Fort Worth city

White Settlement Levee West Fork RB

741,206

291,086

262,652

Memphis city

Ensley Levee System

646,889

291,883

250,344

Washington city

Anacostia

601,723

296,719

266,707

Cleveland city

Euclid Creek, Cleveland, Ohio – Local Flood Protection

396,815

207,536

167,490

Bakersfield city

Kern River left bank – Bakersfield

347,483

120,725

111,132

Stockton city

Littlejohn Creek left bank – Unit 1

291,707

99,637

90,605

Toledo city

Point Place, Maumee Bay/Ottawa River

287,208

138,039

119,730

Irving city

Irving Flood Control District Section-1 East Levee

216,290

91,128

82,538

Arlington CDP

Arlington

207,627

105,404

98,050

Shreveport city

Red River – West Agurs

199,311

88,253

80,651

Augusta/Richmond County

Augusta Levee

195,844

84,427

75,208

Providence city

Fox Point HSPP – Providence, RI

178,042

71,530

62,718

Sioux Falls city

Sioux Falls – Diversion Channel LB – South

153,888

66,283

61,707

Rockford city

ROCKFORD, IL – ALPINE DRY RESERVOIR

152,871

66,700

59,973

Alexandria city

Alexandria

139,966

72,376

68,082

Ponce zona urbana

Portugues West

132,502

57,856

49,861

New Haven city

West Riv RB – New Haven, CT

129,779

54,967

48,877

Hartford city

N&S Br Park Riv, Park Riv Conduit Sys-Hartford, CT

124,775

51,822

45,124

Waterbury city

Naugatuck Riv LB – Waterbury & Watertown, CT

110,366

47,991

42,761

Palm Bay city

Upper St. Johns River Basin, North

103,190

45,220

39,482

 

Conclusion

The relationship between levees and settlement patterns is complex, but undeniable. This exploratory research took at a first stab at exploring this relationship using recent data from the the USACE and the US Census. While preliminary, these results do show that levees and floodwalls are important to American communities.Some preliminary conclusions drawn from this exploratory analysis include:

  • Over 1,800 named Census places with tens of millions people and houses have a leveed area within its bounds.
  • Nearly 800 counties throughout America receive tax revenue and job opportunities from their leveed areas.
  • Nearly 7 million people scattered across 129 Census places are at risk because of a bad levee.
  • Some 176 counties with 39 million people must worry about how these bad levees could impacts their revenue and employment situation.

Until verified, these results should be taken with a grain of salt. Further study and analysis using the rich data provided by the Corps with the National Levee Database should examine this relationship in greater detail.We want these tools to be resources for anyone, whether they are researchers, work for government, and citizens concerned about the levees around them. The statistics here just touch the surface of what’s available.Where your interests are national, regional, and local our two websites give you the ability to interact with the data and to explore your own interests. If nothing else, type in your zip code in the address search box (upper right corner) to see what levees and floodwalls or what bad levees and floodwalls are near you.

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