Welcome
Organization of the Manual
Audience
Useful Background
Planning Context
Economic Theory
Flood Damage Reduction
Flooding
Flood Problems
Introduction
Floodplains
Floods Are Natural
Floodplain Management
Causes of Flooding
Rainfall Flooding
Snowmelt Flooding
Urban Drainage (Stormwater Runoff) Flooding
Coastal Storm Flooding
Tsunami Flooding
Hurricane Flooding
Ice Jam Flooding
Dam Failure Flooding
Catastrophic Outburst Flooding
Glossary of Terms
National Flood Damages
Flood Control Act of 1936
Local Response
Flood Damages
Intangible Damages
Tangible Damages
Property Damage Categories
Flood Damage and Land Use
Buffer
Central Business District
Commercial
Neighborhood Commercial
Regional Commercial
Industrial
Heavy Industrial
Light Industrial/Office
Institutional
High-Density Residential
Medium-Density Residential
Low-Density Residential
Very Low-Density Residential
Mixed-Use
Parkland/Recreation/Open Space
Cropland
Grassland Pasture and Range
Cropland
Forest
Land Use and Flood Damages
Harris County, Texas: An Example
Land Use in the United States
Selected Flood-Related Policies
Expected Annual Damages
What is Flood Reduction Worth?
The Hydrologic Model - How Do We Think About
Flooding?
Benefit Estimation
An Example
Stage Damage
The Stage-Damage Curve
What is a Flood Damage?
Damage Survey
Primary Data Collection
Secondary Data Collection
Estimating Structure Value
Software Support
Updating Cost Estimate Help-What is Current Guidance on
How to Update?
Elevation Data
Depth-Damage Relationships
Stage-Discharge Relationships
Constructing the Stage-Discharge Curve
Discharge-Frequency Relationships
Exceedence Frequency
100-Year Event
Reaches
Without-Project Condition
Introduction
The P&G Evaluation Procedures (Standards
Section IV)
2.4.4 Evaluation Procedure: General
2.4.5 Evaluation Procedure: Step 1 – Delineate
Affected Area
2.4.6 Evaluation Procedure: Step 2 – Determine
Floodplain
Characteristics
2.4.7 Evaluation Procedure: Step 3 – Project
Activities in Affected
Area
2.4.8 Evaluation Procedure: Step 4 – Estimate
Potential Land Use
2.4.9 Evaluation Procedure: Step 5 – Project
Land Use
2.4.10 Evaluation Procedure: Step 6 – Determine
Existing Flood
2.4.11 Evaluation Procedure: Step 7 – Project
Future Flood Damages
2.4.12 Evaluation Procedure: Step 8 – Determine
Other Costs of Using
the Floodplain
2.4.13 Evaluation Procedure: Step 9 – Collect
Land Market Value and
Related Data
2.4.14 Evaluation Procedure: Step 10 – Compute
NED Benefits
Without- and With-Project Condition Scenarios
The P&G on Without- and With-Project Conditions
in Urban
Flood Analysis
2.4.3 Planning Setting
Other Conditions of Potential Interest
Historic Condition
Existing Condition
Base Year Condition
Without- and With-Project Condition Comparison
Illustrated
Without-Project Condition
What Make a Good Future Condition Scenario?
What Comprises a Future Condition Scenario?
Identify the Condition to be Forecast
Assumptions
Common Criteria
Supporting Analysis
Integration of Parts Into a Coherent Whole Condition
Narrative with Supporting Documentation
Urban Flooding Without-Project Condition
Identify the Condition to be Forecast
Assumptions
Common Criteria Set
Forecasts
Supporting Analysis
Integration of Parts Into A Coherent Whole Condition
Narrative With Supporting Documentation
Future Conditions and Uncertainity
With-Project Condition
Introduction
With-Project Condition
Flood Damage Reduction Measures
Flood Damage Reduction Examples
Expected Annual Damages and the With-Project Condition
The Stage-Damage Function
The Stage-Discharge Function
The Discharge-Frequency Function
Overall Plan Effect
Uncertainty
Introduction
ER 1105-2-101 Summarized
EM 1110-2-1619
Variability and Uncertainty
Thinking About Uncertainty
Knowledge Uncertainty
Model Uncertainty
Quantity Uncertainty
Empirical Quantities
Defined Constants
Decision Variables
Value Parameters
Index Variables
Model Domain Parameters
Outcome Criteria
Sources of Uncertainty in Empirical Quantities
Random Error and Statistical Variation
Systematic Error and Subjective Judgement
Linguistic Imprecision
Variability
Randomness and Unpredictability
Disagreement
Approximation
A Few Useful Tools and Techniques
Acknowledge Uncertainty
Assumptions
Research
Sensitivity Analysis
Scenario Analysis
Monte Carlo Process
Step One: Generate a Random Number in the
[0,1] Interval
Step Two: Transform the ri's Into Useful Values for Your Model
Probabilistic Scenario Analysis
Expected Annual Damages With Uncertainty
Stage-Damage
Stage-Discharge
Discharge-Frequency
Uncertain Expected Annual Damages
Risk Analysis in Expected Annual Damages
HEC-FDA Example
Stage-Damage Function
Structure Value
Content-to-Structure Value Ratio
First-Floor Elevation
Stage-Discharge Function
Discharge-Frequency Function
Guidance
Engineering Performance of Flood-Damage
Reduction Plans
Expected Annual Exceedance Probability
Long-Term Risk
Conditional Annual Non-Exceedance Probability
Consequences of Capacity Exceedance
Levee Failures
Benefit-Cost Analysis
Cost Estimation
Net Benefits
Nonstructural Alternatives
Introduction
Nonstructural Flood Damage Reduction Measures
Moderating Community Susceptibility to Floods
Emergency Preparedness
Flood Forecast and Warning
Flood Insurance
Flood Proofing
Information and Education
Modifying Equipment
Relief, Recovery and Rehabilitation
Reducing Hazardous Uses of Floodplains
Building Codes
Design and Location of Services and Utilities
Evacuation
Housing Codes
Public Acquisition
Relocation
Sanitary and Well Codes
Subdivision Regulations
Tax Adjustments
Urban Storm Drainage
Zoning Codes
Estimating Benefits of Floodplain Evacuation-Background
Current Guidance
The Rationale
Capital Asset Pricing
The Problem: Structure Values in the Floodplain
Current Evacuation Benefit Estimation Practices
Evaluation Procedure for Nonstructural Flood Control Projects
Implementing Section 219 (a) of WRDA of 1999
Land Use Changes
Other Benefits
Changes in Net Income
Floodplain Land and NED Benefits
Intensification Benefits
Location Benefits
Intensification Benefits
Location Benefits
Policy on Land Value Benefits
Land Development
Using This Information
Introduction
Know the Flood History
Visit the Floodplain and Study Area
Acquire Maps and Photography
Know Land Use Plans
Coordinate With Others
Identify the Floodplain
Plan Your Damage Survey
Survey Design
Statement of Objectives of the Survey
Target Population to be Sampled
Frame
Sources of Frames
Sample Design
Method of Measurement
Measurement Instrument
Selection and Training of Field-Workers
Pretest
Organization of Field Work
Organization of Data Management
Data Analysis
Generate Damage Curves
Estimate Without-Project Expected Annual Damages
Understand How Plans Work
Estimate With-Project Expected Annual Damages
Check Your Work
Expect Revisions
Forecast Land Use Changes
Calculate Changes in Net Income
Document Your Work
Welcome
Welcome to the National Economic Development (NED) Procedures Manual for Flood
Damage Reduction. This web-based manual makes use of a vast array of flood damage
reduction resources and it offers access to a variety of media. It enables you,
the reader, to go as deeply as you like into a topic or to merely skim its surface.
It is a much more active sort of document than a traditional manual.
Hardcopy manuals have generally been written to be read linearly from start to
finish. It is likely that no two readers will ever follow the same path through
this manual. Some will read all the content and click every link, pursuing the links
until they are satisfied, others will pursue the links until there are no more links
to pursue. Some will work with the sample computation files that have been attached.
Others will study the Microsoft PowerPoint slides, a number will replay the audio files.
Several links lead to some wonderfully informative animations. In short, this web-published
Procedures Manual offers a portal to more flood damage reduction information than any one
person will ever need. So welcome to the U.S. Army Corps of Engineers' NED Procedures Manual
and the web’s world of flood damage reduction!
Organization of the Manual
The materials in the manual are organized in eight topical areas or chapters:
This Welcome includes the organization of the manual, a description of the target audience and some
useful background references as well as a glossary of acronyms used throughout the manual.
The Flooding topic provides an introduction to floodplains and the magnitude of the flood damage problem
in the United States. It introduces several different types of flood problems and offers a taxonomy of flood
damage categories for use in estimating flood damages. The chapter provides a summary description of various
land uses found in the floodplain. These are important to understand for forecasting without- or with-condition
scenarios that depend on changes in land use and for estimating benefits that result from land use changes.
The chapter concludes with a listing of some relevant Corps policy considerations for flood damage reduction.
The next three topics cumulatively provide the reader with a sound foundation in the estimation of flood
damage reduction benefits. The Corps' hydroeconomic model and its three input functions are considered in the
expected annual damages topic. Using the model to estimate expected annual damages in the absence of any flood
damage reduction measures, i.e., the without-condition, is the next topic. The with-condition topic provides
a discussion of the model’s use to represent the effects of a wide variety of flood damage reduction measures.
The most critical topic for analysts who will be responsible for estimating flood damage reduction benefits is
probably the Expected Annual Damages (EAD) topic. This topic offers a conceptual presentation of the model used
by the Corps to estimate these benefits. It provides a simple example of benefit estimation.
The topic begins by considering the nature of flood damages and proceeds through a discussion of each of the
four relationships (stage-damage, stage-discharge, discharge-frequency and damage-frequency) that comprise the
basic model. Although the Corps has developed sophisticated software to support this model the manual proceeds
using a simple Microsoft Excel model that illustrates the concepts more transparently and simply. It also enables
the interested reader to examine the calculations more carefully, if desired. The topic provides additional
focus on a few key data parameters that are generally the concern of Corps’ economists and that often surface as
concerns during the risk analysis estimate of benefits.
The Without-Condition topic begins with the relevant Principles and Guidelines (P&G) context and proceeds to
some informal definitions of the concepts of the future without any flood damage reduction measures and the future
with one or more measures in place. Recurring elements in the without-condition forecast are identified and
then integrated into the elements of an urban flood damage forecast.
The With-Condition topic begins with a discussion of the elements of a flood damage reduction investigation’s
with-condition and then shifts its focus to the consideration of a range of flood damage reduction measures. Links
provide examples of many of these damage reduction measures in use throughout the country. The chapter concludes by
examining how flood damage reduction measures affect one or more of the of the three basic model inputs to the
hydroeconomic model and how these changes affect the model output, i.e., the damage-frequency curve that is the
source of the EAD calculation.
Risk analysis is introduced as a major theme in the Uncertainty topic. It is
not introduced until after the reader has been provided the opportunity to understand
the basic workings of the EAD model and the key concepts used in estimating
urban flood damage reduction benefits. The topic focuses on the risk analysis
of flood damage reduction benefits and begins with the policy basis for this
found in
ER 1105-2-101.
The topic distinguishes variability and uncertainty, and expands the taxonomy
of uncertainty to include a wide array of concepts. The expected annual damage
model of the manual is revisited to provide a conceptual illustration of how
variability and uncertainty in EAD model inputs can be handled. The risk analysis
features of
Hydrologic Engineering Center's Flood Damage Analysis (HEC-FDA) are summarized
and the chapter concludes with a review of the project performance measures
used to identify a tolerable level of risk.
The next topic is devoted to a discussion of the special consideration of the benefits of Nonstructural measures.
The seventh topic considers location and intensification benefits. These are benefit categories that can arise as a
result of changes in Land Use. The methods for estimating these benefits can sometimes differ from the hydroeconomic
model approach.
The last topic provides an informal checklist-like discussion of the tasks an analyst is responsible for when doing
an economic analysis of a flood damage reduction project (Using this Information). It supplements the more formal checklists
found in the Corps' guidance.
Audience
The principle audience for this manual is the Corps planner, especially Corps economists who will work on the NED analysis of flood damage reduction studies. The manual is, of course, suited for anyone interested in learning more about the economic analysis of flood studies. Non-Corps readers are forewarned that the methodology of this manual is influenced and guided by the Corps' policy history and this may differ in places from pure economic theory.
Useful Background
There are no prerequisites for using this manual but the new Corps planner would be well advised to be familiar with the Corps' flood damage reduction planning context. A general familiarity with the Corps' planning process and with economic theory is assumed on the part of all readers. Following are a few useful references to provide background and context for this manual:
Civil
Works Engineering Regulations (ER)--a complete list of ER's available online
Planning Context
Principles
and Guidelines--the seminal document for the Corps' planning process
Planning
Manual--provides an introduction to the six-step planning process used by
the Corps
Planning
Primer--provides an interpretative summary of the larger planning manual
Planning
Guidance Notebook--this is the planner's single best resource when it comes
to Corps' Policy on planning matters
Economic Theory
Overview
Manual for Conducting National Economic Development Analysis--this is an
important source document for those looking for an explanation of the basic
nature of a NED benefit and the methods for estimating it
National
Economic Development Costs--NED costs are discussed at length in this manual
Flood Damage Reduction
National Economic Development Procedures Manual-Urban Flood Damage March 1988, IWR Report 88-R-2, U.S. Army Corps of Engineers, Water Resources Support Center, Institute for Water Resources (not available online)
National Economic Development Procedures Manual-Urban Flood Damage Volume II Primer for Surveying Flood Damage for Residential Structures and Contents, October 1991, IWR Report 91-R-10, U.S. Army Corps of Engineers, Water Resources Support Center, Institute for Water Resources (not available online)
Flooding
Flood Problems
Introduction
Flooding is a natural and common global phenomenon. It is most
often caused by an increase in stream flow beyond the point where
the normal stream channel can contain the water. Water spills
over the riverbanks and spreads out along the adjoining floodplain.
Floodwaters may occupy the floodplain for a matter of hours, as
in the case of flash floods, or for months, as occurred in some
communities during the 1993 flood of the Mississippi River (story,
photographs).
Flooding is part of the
hydrologic cycle in which water is forever recycled between
the earth and the atmosphere. A flood "problem" requires
two elements, flooding and floodplain development. These two elements
are the subject of this chapter.This chapter provides a simple
introduction to the flood problems of the United States.
The floodplain is a relatively level expanse of land carved out
by the river. It is a natural extension of the river channel.
In spite of the risks associated with flooding, people have long
been drawn to occupying the floodplain because of the many benefits
the floodplain provides. Since the dawn of civilization people
have settled and lived along the edge of rivers and lakes.
Apart from the obvious source of drinking water, the river supported
a food source, provided a means of transportation and eventually supplied power to mills. Level
land next to waterways is cheaper to build on and services are easier
to install. People continue to occupy floodplains but for many different
reasons now. Waterfront and water view property is aesthetically pleasing.
Floodplain development in the U.S. includes every kind of land use imaginable.
Historically, floodplain occupants rarely made accommodation for the river's
eventual return to the land. Consequently, the stories of floods are often
told in terms of how they affect the lives of the people in the floodplain.
This is the social context of floods.
What is the oldest flood story you know? Chances are you answered
Noah's flood, and God's Old testament warning for Noah is among
the oldest evidence of a flood forecast and warning system! For
more details on this ancient flood story see The
Search For Noah's Flood. Almost every culture on earth has
an ancient flood story (see Ancient
Flood Story). Although flooding is a natural part of the hydrologic
cycle, it is the social context of floods that is of greatest
concern in this manual.
As floodplain development increases so does the damage caused by flooding.
Communities began to try to protect themselves from floods with measures
such as levees, dams, channel improvements and diversions. These structures
have had some success, but they have also had an unintended consequence
of creating a false sense of security that in the past may have lead to continued floodplain
development.
This dramatic photograph shows the steel sheetpile floodwall
in Forty-Fort, Pennsylvania being overtopped by the floodwaters
of the Susquehanna River caused by Tropical Storm Agnes in 1972.
This was the beginning of the Nation's greatest natural disaster,
as 
measured
by flood damages, to that point in time. Tropical Storm Agnes'
flood damages have been exceeded several times since then.
The structural approach to flood protection has often appeared to have
solved a community's flood problem. But, structures are neither permanent
nor infallible. Levees, for example, can be eroded over time or become
saturated and fail during extended periods of high flow. More importantly,
no matter what level of flooding structures are designed for, there will
come a time when the flood level exceeds the design flood, as has happened
in this photograph.
Once the structural system of protection fails, for whatever reason, the flood damages will most likely be greater than if the structures had never been put in place. This is a story repeated time and time again in the 20,000 plus flood prone communities of the U.S.
This chapter begins by looking at floodplains and floods as a natural phenomenon. It identifies some of
the different causes of flooding. The social context of floods is considered in an introduction to flood damages.
This discussion of damages proceeds from the National perspective to land use and finally to the planning analyst's perspective.
Floodplains
Over 20,000 communities in the U.S. are subject
to a substantial risk of flooding. Most of them participate
in the National Flood Insurance Program (NFIP).
Some of these communities are along large rivers or smaller
streams, some border on lakes, others are coastal communities,
some are in the desert, while others are on hillsides. With
very few exceptions, most communities in the U.S.
experience some kind of flooding, when the right set of
circumstances occurs, for example, after spring rains, heavy
thunderstorms, winter snow thaws, the subsidence of land
along a body of water, or heavy storms over a large body
of water. Fires are the only disasters more common and widespread
than floods. For some basic water facts see Where
is Earth's Water.
A flood, is defined by the NFIP as: "A general and temporary condition of partial or complete inundation of two or more acres of normally dry land area or of two or more properties from:
- Overflow of inland or tidal waters
- Unusual and rapid accumulation or runoff of surface waters from any source
- A mudflow
Floods are neither mysterious nor freak occurrences. They are entirely
natural events. The most frequent cause of flooding is heavy
rain. They rarely strike without some advance warning, and
they usually occur in places where flooding can be expected,
in floodplains. A floodplain is the relatively flat land
beside a lake, river or other watercourse that is naturally
prone to flooding. The floodplain is part of the water's
"living space" which it uses periodically to stretch
and spread out. Floods are most destructive in that part
of the floodplain known as the floodway, where the water
flows fastest. View another typical floodplain map and cross
section at Key
Elements of a Floodplain.
Google
Earth is a great Internet tool that provides
a satellite or aerial view of any river on the Earth.
Many floodplains are easily imagined using this tool.
With Google Earth and a list
of U.S. rivers and other
well-known rivers you can learn a great deal about
how the world's floodplains have been developed right
from your desk. |
Floods Are Natural
Floods are often associated with property damage and loss of life. But
floods are a natural phenomenon and have an ecological context as well as a social one. They are necessary for the survival and health
of valuable ecosystems. The ecologies of floodplains, rivers and lakes
have adapted to their annual and longer term water cycles. Wetlands and
areas covered by shallow surface water especially rely on variations in
water levels caused by natural fluctuations in flows to maintain their
ecological balance and productivity. Many lands rely on sediments deposited
during flooding to remain above sea level. Long-term changes to the water
levels, through the use of reservoirs, channelization or levees, for example,
can cause a change in the long-term succession direction of existing vegetation
and habitat. For example, an aquatic habitat can move toward a terrestrial
one or vice versa.
Population growth and economic activity have caused pressure to alter
the flow regime of surface water systems and the landscape of the floodplains.
More predictable water flows were a goal of many watershed management
programs during the 20th century. "Flood control," a term no
longer in vogue, described the attitude of earlier generations that sought
to control this natural occurrence through the construction of dams, levees,
walls, diversion channels, channel dredging and realignment as well as
the drainage of wetlands. These efforts to control floods, although perhaps
beneficial to economic activity in some cases, have resulted in the decline
of fish and wildlife habitats and the disruption of entire ecosystems.
Wetlands have been eliminated, shoreline erosion has increased and the
sediment filtration capabilities of the floodplains are among the ecosystem
resources that have been lost or altered. A more enlightened approach
to inundation damage reduction takes these ecosystem values into consideration
when planning and designing solutions to problems.
Floodplain Management
There are many impressive local initiatives to address floodplain management problems around the nation. Some of these involve partnerships with the Corps, others do not. For a sample of what local governments are doing to address these issues and to involve their citizens you may want to look at a few of these web sites.
Association
of State Floodplain Managers
Floodplain Management
Association
Floodplain
Management Plan, Clearwater, FL
Floodplain
Management Plan, Los Angeles
National
Association of Flood and Stormwater Management Agencies
National
Watershed Coalition
Wellington
(NZ) Regional Council
Causes of Flooding
The most common cause of flooding is when the volume of water exceeds the capacity of the river or stream channel. Rivers are natural drainage channels for surface waters. Surface waters comprise two components: runoff and base flow. Runoff is that part of precipitation that flows toward the rivers or streams on the ground surface or within the soil (subsurface runoff or interflow). Base flow is the part of stream flow that enters the stream channel from groundwater.
Stream flow is affected by a number of factors (The Corps' Hydrologic
Engineering Center (HEC) offers Hydrologic
Engineering for Planning a hydrology course for non-hydrologists
for those interested in more details than are provided here).
The most important of these for the purposes of this manual are
the amount and type of precipitation, the nature and condition
of the drainage basin and climate. During a rainstorm, the amount,
intensity and duration of the rain as well as the area of the
storm and its path, all determine the surface water runoff that
reaches a stream.
The amount, intensity and duration of rain affect the ability of the land to absorb the precipitation, which further affects the rate of runoff. The area and path of the storm in relation to the size of the watershed determine the area contributing runoff. The runoff rate and the area affected together determine the volume of water that will pass a given point downstream. The volume of water moving through the channel and the channel’s dimensions and conditions determine the nature and extent of the flood.
The shape, size, soil type and topography of the drainage basin are other factors that can affect the quantity of water reaching the stream and the timing with which it arrives. Although some of these factors are constant, some (like the absorptive or shedding properties of the soil) vary with vegetation cover, season and previous rainfall.
Climate can also influence the relationship between precipitation and runoff. Frost makes most soil impenetrable if the soil contains moisture. Parched soil can also influence runoff rates. A large part of the year's precipitation may be stored in the form of snow in the Northern U.S. during winter. Heavy ice formation on rivers can also influence flooding.
Floods may result from one or more of the following causes:
- Rainfall
- Snowmelt runoff
- Urban stormwater runoff
- Coastal storms, tsunamis, cyclones, hurricanes
- Ice jams and other obstructions
- Dam failure or the failure of some other hydraulic structure
- Catastrophic outbursts
Rainfall Flooding
As noted above, rainfall is the most common cause of
flooding in the U.S. The volume of water
in the stream or river’s channel simply exceeds
its capacity to convey the water. As a result water
begins to spill out of the channel onto the adjoining
lands of the natural floodplain, which may have been
significantly altered by human activity. |
Floods can rise slowly or quickly. In many areas they may develop over a period of days. Flash floods can be extremely dangerous. Unanticipated, they usually happen on small watersheds as a result of a torrential downpour, often caused by heavy thunderstorm activity. In a flash flood, stream flow peaks within hours of the rainfall. Estimating damages due to rainfall floods is now a straightforward process.
Snowmelt Flooding
|
During winter in some parts of the U.S., most of the precipitation may be
stored as snow or ice on the ground. As temperatures
rise huge quantities of water are released. These floods
are most common in spring but can occur as a result
of sudden winter thaws. Heavy runoff can result from
the rapid melting of the snow under the combined effect
of sunlight, winds and warmer temperatures. If the
ground is frozen, the water produced by the melting
snow is unable to penetrate and runs off into streams
and lakes. Flooding becomes even more severe if the
snowmelt runoff is compounded by runoff from concurrent
heavy rainfall. The later the spring thaw, the greater
the risk of this compound flood problem. Snowmelt explains
the prevalence of heavy spring runoff and flooding in
some parts of the country. |
 |
Urban Drainage (Stormwater Runoff) Flooding
|  Urbanization
drastically alters the drainage characteristics of the land.
The slanted roofs, downspouts, storm gutters and stormwater
conveyance systems increase the volume and rate of surface
runoff. The urban runoff from intense rainfall can exceed
the carrying capacity of the sewer system, creating a backup
in the system. This backup often causes flooding of basements
and low lying roads. Urban stormwater runoff can also cause
local rivers to flood as well as the urban area itself.
Although the impact on a major river may be minimal, the
carrying capacity of small streams can be quickly exceeded,
causing localized flooding and erosion problems.
|
Coastal Storm Flooding
High winds and wave action have created flood conditions on the seashores as well as on the shores of the Great Lakes and other large water bodies throughout the U.S. A related cause of flooding includes the interaction between high estuarine flows and tides. Storm surge or seiches occurring simultaneously with high waves can cause shoreline flooding. Every body of water has a set of natural periods of oscillation at which it is easy to set up motions called seiches. Surges are caused by sudden changes in atmospheric pressure and by the wind stress accompanying moving storm systems.
 |
Storm systems occur frequently and some have the
potential to cause abnormal water levels at coastlines.
Determining water elevations during storms is a complex
problem. It involves interactions between wind and
water and differences in atmospheric pressure. Erosion
damage can be a significant category of losses in
these kinds of floods. For examples of erosion damage
on the Great Lakes see Great
Lakes Issues. This makes estimating damages for
such events complex and difficult.
Lake flooding can be complicated by the fact that it is often a weir flow that can last for extended periods of time in areas afflicted by high lake levels. |
Tsunami Flooding
Tsunami is a Japanese term for “harbor wave.” A tsunami, also known as a tidal wave, is the most spectacular coastal flooding event. A tsunami actually has nothing to do with the tides. An undersea movement such as an earthquake or a landslide causes a disturbance that gives a vertical motion to the water column resulting in a tsunami.
An earthquake of 7.0 on the Richter scale can generate a series of waves. In the Pacific Basin these waves have been known to travel at almost 570 mph over long distances with little loss of energy. Crests can be several hundred miles apart. As the wave approaches the coast it grows as it slows down. The mass of water that hits the shore can have both tremendous velocity as well as force behind it.
Estimating damages from these kinds of floods is very
difficult because tsunamis are unique with respect to location,
amplitude of waves and time between troughs. Because the
source of the wave is always unknown, modeling these events
remains a crude approximation. For an overview of recent
tsunami events see Recent
Tsunami Events. The December 2004 tsunami in the Indian
Ocean is well documented. See NOAA
and the Indian Ocean Tsunami for a starting point. Informative
publications can be found at After
the Tsunami: Human Rights of Vulnerable Populations
and Hope
for Renewal: Photographs from Indonesia After the Tsunami.
Several informative animations are also available on-line
including: Savage
Earth Animation, USGS
Earthquake & Tsunami and NOAA.
Hurricane Flooding
The following materials were taken from the FEMA
Hurricanes site.
A hurricane is a tropical storm with winds that have reached a constant speed of 74 mph or more. Hurricane winds blow in a large spiral around a relative calm center known as the "eye." The "eye" is generally 20 to 30 miles wide, and the storm may extend outward 400 miles. As a hurricane nears land, it can bring torrential rains, high winds and storm surges. A single hurricane can last for more than two weeks over open waters and can run a path across the entire length of the eastern seaboard. August and September are peak months during the hurricane season that lasts from June 1 through November 30. Hurricanes are called "typhoons" in the western Pacific Ocean, while similar storms in the Indian Ocean are called "cyclones."
Moving ashore, they sweep the ocean inward while spawning tornadoes and producing torrential rains and floods. Even more dangerous than the high winds of a hurricane is the storm surge, a dome of ocean water that can be 20 feet at its peak and 50 to 100 miles wide. The surge can devastate coastal communities as it sweeps ashore. Nine out of 10 hurricane fatalities are attributable to the storm surge.
|
Heavy rains and ocean waters brought ashore by strong winds can cause flooding. The runoff systems in many cities are unable to handle such an increase in water because of the gentle topography in many of the coastal areas where hurricanes occur. Hurricanes are capable of producing copious amounts of flash-flooding rainfall. During landfall, a hurricane rainfall of 10 to 15 inches or more is common. If the storm is large and moving slowly, less than 10 mph, the rainfall amounts from a well-organized storm may be even greater. To get a generic estimate of the rainfall amount (in inches) that can be expected, divide the storm's forward motion by 100, i.e., Forward Speed/100 = estimated inches of rain. Tropical Storm Claudette (1979) brought 45 inches of rain to an area near Alvin, Texas, contributing to more than $600 million in damage.
|
 |
Estimating damages for hurricane floods is more difficult than for fluvial floods. Estimating wave damages, for example is one problem,
separating out wind damage from water damage is another
challenge. Nonetheless, hurricane flood damages are estimated
routinely. To see some of the latest advances in this area
see the Corps' Storm Damage Reduction Model.
Ice Jam Flooding
Ice jams are a major concern in some cold region parts of the country.
Jams form during both the freeze-up and breakup periods
of ice formation. They result from the accumulation of ice
fragments that build up in a logjam fashion to restrict
the flow of water. The jams act as a temporary obstruction
to stream flow. The mechanics of ice jam flooding can be
quite complex, for more information see the Ice
Jam and Ice Flooding Clearinghouse. A brief overview
is provided below.
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Ice floes left behind by floodwaters
Ice jam
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During freeze-up ice jams usually form where floating ice slush or blocks, formed by frazil ice, encounter a stable ice cover. The beginning of the ice jam is the toe and the upstream end is the head. The stable ice is usually frozen to the banks or is restricted from moving by the channel configuration. Generally, incoming ice fragments either submerge and deposit under the stable ice cover, pile up behind it, or both. Bridge piers, islands, bends, shallows, slope reductions and constrictions can increase the likelihood of a jam forming. Ice jams in the spring result from accumulated ice from the breakup of the upstream ice cover.
Ice jams cause flooding for two reasons. First, ice jams can be very thick, many feet thick in some cases. Second, the underside of the ice cover is usually very rough. In an open stream the streambed is the only source of friction retarding the flow of water. The rougher the streambed, the greater the depth required to pass a given stream discharge. With an ice jam in place frictional resistance is greatly increased and the flow depth has to be much greater than for open water. Add the depth of water needed to float the ice jam to the depth required to maintain the discharge and extremely high water levels can occur, even at relatively small discharges.
When an ice jam suddenly is released it produces a surge of flow that can move at very rapid speeds. This surge can carry and deposit chunks of ice as large as automobiles, presenting a significant increase in damage potential for these kinds of floods. Estimating damages for ice jam floods is made difficult by the fact that it is very difficult to estimate the frequency of occurrence of an ice jam and the significance of damages caused by floating ice floes.
Dam Failure Flooding |
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Flooding can result from
the failure of dams or other hydraulic structures.
These failures can result in a wall of water being
released in a surge down the river channel. The suddenness
and magnitude of such an event can have disastrous
results. |
Catastrophic Outburst Flooding
Outburst floods are more common in western Canada and other parts of
the world than they are in the U.S. An outburst flood occurs
when lakes dammed by glaciers or moraines suddenly drain and tons of water,
mud and debris are released. The resulting floodwaters can pick up large
quantities of sediments and transform into destructive debris flows. The
random and often unpredictable nature of these kind of events make the
estimation of damages resulting from them as difficult as estimating damages
from dam failures.
Glossary of Terms
For a glossary of flood-related terms used by the U.S.
Army Corps of Engineers see Glossary.
A Glossary
of Lake and Water Words is available from the North
American Lake Management Society. A Water
Words Dictionary has been prepared by the Nevada Division
of Water Resources.
National Flood Damages
Floods are the most common natural disaster in the U.S. Every state and virtually
every U.S. territory has experienced floods. According to the Federal Emergency
Management Agency (FEMA),
part of the U.S. Department
of Homeland Security,10 million households in this country are located in
areas of significant flood risk. Even so, 20 to 25 percent of all flood insurance
claims are paid to people who live in low to moderate flood risk areas. The
Great Midwest Flood of 1993 (1993
Great Midwest Flood: Voices Ten Years Later, Midwest
Flood of 1993: Weather, Climate, and Societal Impacts), which inundated
large parts of nine states for up to four months, was one of the worst floods
in U.S. history. It destroyed or heavily damaged 49,000 homes, caused at least
$16 billion in property damage, and took 50 lives. Tropical Storm Allison (Tropical
Storm Allison Recovery Project), although it never reached hurricane status,
caused $5 billion in property damage and 43 deaths in 2001. Hurricane Katrina
(2005) is likely to be the worst storm/flood disaster in U.S. history, in terms
of property damage. FEMA
provides a list of events since 1978 along with the NFIP losses paid. (If you
access the FEMA list, note that losses paid is not the same as total damages.)
Nationally, from 1929 through 2003, annual flood damages measured in
1995 dollars have ranged from a low of $18 million in 1931
to over $17 billion in 1993. On an annual basis national
flood damages have averaged $2.4 billion annually. In half
of those years damages have been $1.3 billion or less. Total
damages have exceeded $171 billion (data are not available
for 1980-1982). To examine the data set from which these
figures were taken simply access the attached data file
National
Flood Damages or visit it at its original source at
Flood
Damage Data.
To put the damages into a more personal context consider
the figure below. If we think of flood damages as a head
tax imposed by nature on the United States that tax has
varied from a low of $0.15 (1995 dollars) per year to a
high of $66.16 and has averaged $11.42 for every man, woman
and child in the U.S. since 1926. If these average
"taxes" were funds available for flood damage
reductions it would amount to about $3.4 billion a year
in 1995 dollars based on a current U.S. population of about
295 million US
Population.
For a comprehensive discussion and history of flood damage
data in the United States see Flood
Damage in the United States 1926-2003.
Flood Control Act of 1936
The damage data above clearly establishes the severe
magnitude of the Nation's flood problems. In 1936, as a
result of several devastating floods in the Midwest and
the Northeast United States a National flood control policy
was enacted by the Flood Control Act of 1936. This law created
the program the USACE executes to
this day. It is the seminal legislation for the urban flood
damage reduction initiatives this manual addresses. The
first four sections of the Act, which established flood
control as a National policy, can be seen at Flood
Control Act of 1936. The last sentence of Section 1
is particularly important because it establishes the use
of benefit-cost analysis for the evaluation of flood control
projects, a criterion later extended to other water resource
projects as well.
The 1936 Flood Control Act has been revised and expanded many times since its passage. The estimation of flood control
benefits, a term more recently replaced by inundation damage reduction benefits, began with the passage of this law. This
manual summarizes the Corps' current best practice approaches to flood damage estimation.
Local Response
For two excellent examples of how local governments are
responding to local flooding problems and how the Internet
is being used to inform and involve people in finding workable
solutions to these problems see King
County, Washington Flooding Topics and for perhaps the
single best flood-related web site anywhere see Harris
County, Texas Flood Control District.
Flood Damages
The 1936
Flood Control Act established the requirement for a
benefit-cost analysis of flood control projects that continues
to this day. Flood damage reduction is usually the major
category of benefits for a flood protection project. Flood
damages can be classified in a variety of ways, one of these
is to differentiate tangible and intangible flood damages.
Tangible flood damages are those that can be quantified in monetary or other (e.g., acres, lives, structures, linear feet) reasonable terms. Intangible flood damages are those that cannot be readily or reliably quantified.
Intangible Damages
Flooding imposes many intangible costs to a community. Some of the more common examples include the following:
- Injury
- Peace of mind or trauma
- Inconvenience
- Isolation
- Evacuation from home
- Stress and anxiety
- Disruption of life
- Health issues
These intangible damages are not easily quantifiable and have not been included in the monetary assessment of flood damages.
Tangible Damages
Tangible damages are usually quantified and measured as monetary losses. Some tangible flood damages may be quantified in other terms. For example, we may speak of potential lives lost during a flood, or people at risk who live in a particular floodplain. Damages may be measured in acres, linear feet or other non-monetary metrics.
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Flood damages may be direct or indirect. Direct damages result from the
actions of floodwaters, inundation and flow, on property and structures.
Indirect damages arise from the disruptions to physical and economic activities
caused by flooding. For example, if a plant that produces canned tomato
products is flooded it suffers direct flood damages. If, as a result of
the flood, local tomato growers far removed from the flood waters lose
a buyer for their product they suffer indirect flood damages. Likewise,
the can manufacturer located out of the floodplain that produces cans
for the tomato plant also suffers indirect flood damages. Indirect damages
are a negative spillover effect (externality) of flooding. |
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Tangible damages may be categorized in many ways in addition to direct and indirect damages. It is common to collect flood damage information using one type of categorization and then to report it using another. For example, analysts may identify and collect content damage, structure damage, flood fighting costs, surplus losses and recovery costs information then aggregate it and report it as the broad land uses damage categories of residential, commercial, industrial, public and other damages.
Property Damage Categories
Property is often categorized as real or personal property. Real property is land and appurtenances, i.e., anything of a permanent nature such as structures and other improvements, trees, minerals, and the interest, benefits, and inherent rights to use these things. Personal property is any property that is not real property. Personal property can be tangible (e.g., furniture, equipment, automobiles and clothing) or intangible (e.g., business interest, stocks and bonds). Flood damage can accrue to many kinds of real and personal property. The most common categories of flood damages to property usually include:
- Structure damage
- Content damage
- Infrastructure damage
- Damage mitigation or flood fighting costs
- Costs associated with evacuation
- Net income losses
- Traffic disruption
- Clean-up and recovery costs
Structure Damage |
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Content Damage |
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Flood Fighting |
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Traffic Disruption |
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These categories are useful when collecting and organizing damage data. Damage data are collected in post flood surveys, when investigators seek to document actual flood damages or in damage estimates when investigators estimate relationships between flood depths and flood damages in order to forecast flood damages.
The general working distinction between structure and content damage is if the average person would leave the item behind when moving it is structural. Otherwise, it is content. More formally, structure damage accrues to real property, content damage to personal property. The building, its utilities, permanent improvements to it like paneling, cabinets and the like as well as wall-to-wall carpeting and similar improvements are all part of structure damage. Damage to outbuildings, swimming pools, landscaping and so on are also part of structure damages.
Any other physical losses to property situated within the structure or on the grounds that are not part of the structure caused by the flood are content damages. By this definition damages to vehicles, inventory, CD collections, furniture, appliances, business equipment and so on are examples of content damages.
In some analyses it may be useful to identify specific subcategories of structure and content damages. An investigation might, for example, report damage to landscaping (structure) or vehicles (content) separately if they are significant types of damage.
Infrastructure damage, although sometimes considered a type of structure damage is more often a separate category. Floods may cause extensive damage to the social infrastructure. This includes physical damages to roads, gas and electric power, telephone, water supplies and conveyance systems, storm water and sewage systems, utilities, public health, public safety, education, flood control structures and other critical social infrastructure.
Damage mitigation or flood fighting costs include the value of all resources used immediately prior to the flood in an effort to minimize or limit the extent of flood damages. These include the costs of moving or removing personal property, sand bagging, securing property, rescue work, preventive maintenance and so on. Labor resources are usually a great part of flood fighting costs.
Not all damage to real or personal property is physical. Other losses that can be monetized have come, for better or worse, to be called net income losses. When normal economic activities are disrupted by flooding, businesses may lose profits. To include such losses among the tangible flood damages two conditions must hold. First, the profits must be lost and not simply postponed or transferred to other firms. Second, the profits must be economic profits and not accounting profits (see figure).
Consumers
can incur income losses if they are out of work for
some period of time as a direct or indirect result of
flooding. The loss is the value of the labor resource,
which is the associated income in a competitive market.
As with the loss of economic profits, the analyst must
assure that the loss is not a simple postponement or
transfer. A second issue is the avoidance of double-counting.
If the employee's income loss has already been accounted
for by the employer it cannot be counted again.
Traffic disruption can be a major source of tangible damages. The 1993 Flood of the Mississippi River disrupted rail, barge and highway traffic for an extended period of time. The costs of traffic disruptions caused by flooding is the value of the resources required to use alternative modes of transportation or routes for the disrupted traffic. This can include increased fuel costs, increased wear and tear on equipment and the value of the time spent in longer routes.
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Temporary Housing for Floodplain Evacuees
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Clean up and recovery costs include all labor and materials associated with cleaning up flood debris
and damage, repairs, replacing evacuated and moved property, emergency food, water, shelter and medical
expenses, policing and securing damaged areas, clearing roads; disposing of debris and like expenses.
ER
1105-2-100 in Section 3-3 has the following to say about
flood damages:
Types of flood damage. Flood damages are classified as physical damages and nonphysical damages. Each activity
affected by a flood can experience loss in one or both of these classes.
Physical damages. Physical damages occur to residential, commercial, industrial, institutional and
public property. Damages occur to buildings, contents, automobiles and outside property and landscaping.
Physical damages include the costs to repair roads, bridges, sewers, power lines and other infrastructure
components. Physical damages also include the direct costs and the value of uncompensated hours for cleanup after the flood.
Nonphysical flood losses. Nonphysical flood losses include income losses and emergency costs. Income losses
are the loss of wages or net profits to business over and above physical flood damages that usually result from a
disruption of normal activities. Estimates of these losses must be derived from specific independent economic data
for the interests and properties affected. Prevention of income losses result in a contribution to NED only to the
extent that the losses cannot be compensated for by postponement of an activity or transfer of the activity to other
establishments. Emergency costs include those expenses resulting from a flood that would not otherwise be incurred.
For example, the costs of evacuation and reoccupation, flood fighting and administrative costs of disaster relief;
increased costs of normal operations during the flood; and increased costs of police, fire or military patrol.
Emergency costs should be determined by specific surveyor research and should not be estimated by applying arbitrary
percentages to the physical damage estimates.
The table below reflects the classification of flood damages by land use category
and damage type contained in ER
1105-2-100.
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Physical Damages |
Nonphysical Damages |
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Buildings |
Contents |
Automobiles |
Outside property |
Landscaping |
Infrastructure |
Cleanup |
Income losses |
Emergency costs |
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Flood Damage and Land Use
The previous sections presented evidence of a significant
and ongoing flood damage problem in the U.S. Here
we begin to examine the nature of the flood damages that
form the basis for the most significant category of inundation
damage reduction benefits, physical damages to property.
The problem is quite simple at its most basic level. Lands
that are naturally subjected to periodic inundation by floodwaters
and storms are also attractive to humans for a variety of
uses. Floodplain lands have been and are put to every kind
of land use imaginable. The use of land by humans places
plant, animal and human life and health at some risk from
flooding. In addition substantial amounts of property are
exposed to the existing flood hazard, resulting in the damages
described previously.
One of the most important elements of a flood protection
benefit analysis is a good knowledge of the existing land
use. This includes land at direct risk of flooding and land that
is dependent on the activities that take place in flooded
areas. More will be said about these issues in subsequent
sections of the manual. For now, it is sufficient to understand the different
kinds of use to which land can be put, because each type
of land use has its own unique damage characteristics. When land use changes in the watershed or floodplain have the potential for affecting the flood problem under investigation, it will be necessary to project changes in future land use by category. Some
typical land use categories follow.
Buffer
A transitional area used to separate land
uses that are not naturally compatible. Buffers are often
green space, but it can also be structures, e.g., neighborhood
commercial land uses may effectively transition from industrial
to residential land uses.
Central Business District
The Central Business District (CBD) or downtown is often a specially
designated land use, characterized by a mix of dense urban
development. It includes high density office, high-rise
office and commercial services buildings in the heart of
the city as well as a variety of retail, institutional,
tourism-related and residential uses which provide services
to the entire city and the metropolitan region. These areas
may also serve important national and international functions.
Commercial
There are several categories of commercial land use activities
found in the U.S. and the language used to describe these categories varies from
location to location. The terms used below are suggestive of some commonly found
commercial land uses.
Neighborhood Commercial
This category includes small-scale retail or service operations
that serve the surrounding residential area and have limited
impact on the surrounding area in terms of traffic, parking
and hours of operation. Examples of neighborhood commercial
include convenience
stores, barber and beauty shops, general retailers, specialty
shops, boutiques, art galleries, small grocery stores, gas stations, pharmacies, drug stores, banks, bakeries,
specialty food, restaurants, sandwich shops, coffee houses,
movie theatres, entertainment spots, hotels/motels, health
and fitness clubs, personal services, print/copy shops, video
rentals, dry cleaners, auto dealerships and the like.
Regional Commercial
This category includes large-scale retail or service operations
that draw from outside the neighborhood and potentially bring
heavier impact in terms of traffic, parking and hours of
operation. Examples of regional commercial include shopping
centers, downtown commercial districts, large department
stores, grocery stores, big box stores, factory outlets and
the like.
Industrial
This land use category describes uses of
land devoted to manufacturing, processing, warehousing,
packaging or treatment of products. The category is usually
divided into sub-categories differentiated by the intensity
of operations on the land.
Heavy Industrial
This category is characterized by manufacturing
and processing operations that produce relatively high levels
of noise, vibration, dust, smoke or pollution or that include
outdoor storage.
Light Industrial/Office
This category is characterized by warehouses, distributors,
research and business support services and light manufacturing that does not
produce high levels of noise, vibration, dust, smoke or pollution and does
not include outdoor storage or intensive activity. Examples of office
and light industrial uses include general offices, light manufacturing, warehousing
and distribution, research laboratories, prototype and production plants, automotive
repair and bodywork, trade schools, auto dealers with display and lot storage/inventory
and so forth.
Institutional
This category covers public operations such as schools, colleges,
hospitals, daycare centers, government buildings, major sports facilities, churches,
places of worship, cemeteries, hospitals, water treatment facilities, community
centers, libraries, municipal buildings, post offices and so on.
High-Density Residential
Examples of this use include single-family attached dwellings (such as townhouses) as
well as multifamily condominiums and apartments, at
densities of 8 or more units per acre, typically about
2 to 5 stories. Densities above 18 to 25 units per acre may
require some reliance on structured parking to achieve
the density. These uses often include some amount
of central outdoor public space for their residents,
such as a pocket park. |
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Medium-Density Residential
Examples of this use include housing densities between 3 and
8 dwellings per acre and can include a mixture of dwelling types including single-family
detached and semi-detached units, single-family attached units, patio homes,
duplexes and triplexes, and townhouses. Multifamily housing is possible when
using a clustered/conservation development design that preserves large portions
of the site as permanent open space, although the overall density should not
exceed 8 dwellings per acre. |
Low-Density Residential
Examples of this use include single-family
detached residential dwellings with density ranges
from 1 to 3 dwelling units per acre and lot sizes
typically ranging from approximately 10,000 square
feet to 1 acre. Smaller lot sizes and perhaps even
single-family attached housing are possible when using
clustered/conservation development designs that preserve
large portions of the site as permanent open space,
although the overall density should not exceed 3 dwellings
per acre. |
Very Low Density Residential
Examples of this use include single-family
detached residential dwellings having lot sizes of
one acre or more. |
Mixed-Use
This is a hybrid land use category that
encourages a flexible mix of residential, commercial and
certain light industrial uses. |
Parkland/Recreation/Open Space
These land uses include green space, parks, playgrounds,
public waterfront areas, neutral grounds, recreation areas, golf courses,
open spaces, provision of public car parking, ancillary buildings and
structures required for operating and maintaining the park or open space,
and land reserved for outdoor open space. |
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Cropland
This use includes cropland used for crops, idled and pasture. |
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Grassland pasture and range
This use includes cropland pasture, forest land grazed and grassland
pasture and range. |
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Forest
This use includes private forested land as well as forest in parks, wildlife
and related areas.
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Land Use and Flood Damages
Land use is the most significant determinant of the damages that will occur as a direct result of a flood.
It is important that analysts understand the nature and extent of the various land uses in the floodplain.
Some damage estimation techniques are based on damages estimated for a typical unit of area for a particular land
use. In other words, some studies will estimate damages per acre (or part of an acre) for the various commercial,
industrial and residential uses found in the urban floodplain.
Harris County, Texas: An Example
Harris County,Texas is the third most populous county in the U.S. with
3.5 million people. It encompasses the City of Houston. The Harris
County Flood Control District provides one of the most informative
web sites available for local flooding problems. The site is well
worth a little time for anyone who is relatively new to the study
of floods and flooding.
The maps below show the beltway highway systems that encircle the City
of Houston, so all of the urban land uses discussed above are found in
ample measure in Harris County. The map on the top illustrates the different
kinds of flood plains, hence flood problems, that affect Harris County.
The map on the bottom illustrates the extent of floodplain land in the
county. These maps of just one county in the U.S. provide an excellent illustration
of the extent of the flood problems in the U.S.
Tropical Storm Allison in June, 2001 dumped as much as 80 percent of
the area's average annual rainfall over much of Harris County. The flooding
that resulted directly affected more than 2 million people. The storm
and its flooding caused 22 fatalities, 95,000 damaged automobiles and
trucks, 73,000 damaged residences, 30,000 stranded residents in shelters
and over $5 billion in property damage in Harris County alone. Before
Allison was finished 31 counties in Texas, 25 parishes in Louisiana, 9
counties in Florida, 5 counties in Mississippi and 2 counties in Pennsylvania
were declared disaster areas. Allison was the costliest tropical storm
in the history of the U.S.
For a good introduction to some of the alternatives available for flood damage
reduction see Flood
Damage Reduction Tools, which is nicely illustrated with photographs of
channel modifications, detention reservoirs and other flood damage reduction
measures. Be sure to see the Floodplains
Explained section if this is a new concept for you. These and other resources
are available in the site's Learning Center. The District's Photo Gallery provides
one of the best sources of visual information about floods available on the
web. The
Floodplain Types provides an excellent animation that explain the types
of flooding that will be encountered in many urban areas.
Land Use in the United States
Most of the United States' nearly 2.3 billion acres are not urban land uses.
The table below summarizes major land use categories for the U.S. Forty-six
percent of the miscellaneous land use is urban or rural residential. About 70
million acres of land are in urban usage. (Source: Major Uses of Land in the
United States, 1997. By Marlow Vesterby and Kenneth S. Krupa. Resource Economics
Division, Economic Research Service, U.S. Department of Agriculture. Statistical
Bulletin No. 973.)
Land Use |
Acreage |
Percentage |
Total |
2.3 Billion |
100 |
Forest |
642 Million |
28 |
Grasslands, Pasture and Range |
580 Million |
26 |
Crop |
455 Million |
20 |
Miscellaneous |
300 Million |
13 |
Special |
286 Million |
13 |
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Most land in the U.S. is privately-owned but government holdings are substantial.
For more information on U.S. land usage see
Major Uses of Land. Floods and land use differences give rise
to substantial flood damages throughout the U.S.
Selected Flood-Related Policies
Civil
Works Engineering Regulations
Flood
Damage Reduction Measures in Urban Areas
Planners'
Resource Web
Planning Community of Practice
Planning
Guidance Notebook
