U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004-3060

June 2004

Mapping a Flood...Before It Happens

Printed on recycled paper

Most flood maps today are not maps of 

real-life floods; they are maps of an imaginary 

flood used to help communities get an idea of 

where especially flood prone areas probably 

are. Sometimes these are called “100-year 

flood maps,” although that phrase is a little 

misleading because it is based on statistical 

probabilities for some specific location, not 

for a region. There’s a good chance that a 

“100-year flood” will occur somewhere in 

your state every year.

Flood forecasts (like the ones seen on TV 

newscasts) are made by the National Weather 

Service for storms days in advance of the 

actual flooding. These forecasts estimate the 

highest level the river will get, based mainly 

on how much rain is expected. Unfortunately, 

the forecasts are made only for a few specific 

places; they don’t predict flood levels for anywhere except those specific places. So, unless 

you live near one of these “forecast points,” 

the predictions are good only as a rough 

measure of how large the flood is predicted to 

be. They don’t tell you whether your house, 

or a school, or your local sewage treatment 

plant is in danger of being flooded. Even if 

you do live near a forecast point, the forecast 

is still only an elevation describing the highest 

expected river level. It doesn’t mean a lot to 

you unless you know your elevation compared 

to the reference elevation, or “datum,” of the 

forecast point. What you want if you live in 

a floodplain, whether you live near a forecast 

point or not, is a map that shows where flooding is expected.

When the NWS issues a forecast flood 

hydrograph (plot of flow vs. time) for a particular location, the USGS uses the hydrograph 

to create a flood-inundation map. The USGS 

has developed methods to make maps of these 

forecast floods for long stretches of a river 

near forecast points. These maps fill a critical 

information gap in flood forecasting: information on when and where floodwater from 

an approaching storm will arrive. The maps 

also show how deep the water is expected 

to be all across the floodplain. In essence, 

these methods combine to make a somewhat 

obscure “peak forecast elevation” into a map 

with flooded areas, times-of-arrival, and flood 

depths.

A number of new technologies and 

methods make the creation of flood forecast Figure 2. Flood waters washing out bridge.

Figure 1. Time-to-arrival for flood, November 23, 1986, Snoqualmie River, Washington 

maps possible. First is the ability to get very 

accurate elevations throughout the floodplain 

quickly and affordably. This is done with 

“LIDAR” technology (see more below). 

Second is a computer program (TRIMR2D) 

that can simulate flood flows all across the 

floodplain and many, many miles downstream 

from the forecast point. Third is spatial analysis software (GIS) that turns the model results 

into maps and overlays them on other maps, 

like a map of a neighborhood, or even onto an 

aerial photograph. Last is software (IMS) that 

makes the maps available on the Internet in a 

flexible and user-friendly way. (You can see 

a demonstration of the system on the Web at 

http://wa.water.usgs.gov/projects/pugethazards/urbanhaz/MappingNWS.htm.)

LIDAR-Based System Collects 

Data Quickly, Accurately

Light Detection And Ranging (LIDAR) 

technology collects high-accuracy elevation 

data (better than 1-foot accuracy) for very 

large areas very quickly and at lower cost 

than traditional methods. The concept is quite 

simple: it’s essentially a laser rangefinder in 

an airplane, but it’s no ordinary laser rangefinder. 

LIDAR systems use lasers that pulse tens 

of thousands of times a second. To turn a 

laser-determined distance into the elevation of 

a point on the ground requires sophisticated 

hardware and software. First, you need to 

know the location of the airplane to within 

less than an inch at all times. This is done 

with a high-precision Global Positioning 

Extending the National Weather 

Service Forecast Flood

What’s missing from flood forecasts? 

Maps—The only maps generally 

available today are maps used for 

planning. They are maps of theoretical 

floods, not maps of flooding forecast 

for an approaching storm. The U.S. 

Geological Survey (USGS) and the 

National Weather Service (NWS) have 

developed a way to bring flood forecasting 

and flood mapping together, producing 

flood maps for tomorrow’s flood today...

and getting them on the Internet in time 

for those in harm’s way to react.
Figure 3. A geographic information system (GIS) takes LIDAR elevations data and information from the 

TRIMR2D model to build maps of areas that likely will be inundated, based on the National Weather Service 

forecast flood.

Figure 4. Typical Light Detection And Ranging 

(LIDAR) typical system used to collect high-accuracy elevation data needed to make flood maps.

System (GPS). Next, you must know the 

orientation of the airplane (nose up or down, 

wings level or not) with similar precision. 

This is done with Inertial Navigation Units 

(INUs) so advanced and accurate they are 

considered military secrets and must be 

licensed by the government. 

The LIDAR system collects billions of 

elevation values, but commonly most of the 

laser reflections are off of tree tops, shrubs, 

bridge decks, vehicles, even telephone poles. 

Sorting through all these points to find the 

ones that are really “on the ground” requires 

complex and often tedious computer processing. But as difficult as all this sounds, it’s still 

less expensive, faster, and more accurate than 

anything that was available before.

What does LIDAR mean for flood mapping? It means that the computer programs 

(flow models) can simulate floods over the 

entire floodplain, rather than for just a few 

dozen cross-sections. In the past, elevation 

data was collected manually in the field, and 

because that’s very expensive, only cross-secequations not only for large areas, but also 

for problems that have large and fast flow 

changes. 

The stability of TRIMR2D is due to a 

solution method that separates the more stable 

terms in the equations from the less stable 

terms, and then solves them in a manner that 

minimizes the effect of the less stable terms. 

In modeling terms, TRIMR2D is called a 

semi-explicit, semi-lagrangian, finite-difference, two-dimensional, depth-averaged 

hydraulic model. 

GIS Draws the Map

A Geographic Information System, GIS, 

is a state-of-the-art database that includes 

a location with each piece of information. 

A GIS is used to manipulate, calculate, and 

process information that is inherently spatial 

in nature—all of the data is related to some 

point on the ground. Elevation data is a good 

example; all points on the ground have their 

own value of elevation. A GIS also can make 

maps of these data; they are so good at making maps that some people mistake them as 

merely mapping tools. 

For making flood forecast maps, the GIS 

uses one of several common processing 

methods—cell-based, or raster, calculation. 

This method is well suited for processing the 

solution provided by TRIMR2D, which is in a 

raster format (raster means all data is for locations that are evenly spaced on the ground, 

like a checkerboard). 

The GIS uses the model results to make 

maps of the entire floodplain showing areas 

that are likely to be flooded and how deep the 

water may get, when the floodwater is likely 

to first arrive, and when the flood will crest. 

Internet Map Server Delivers the 

Information

To deliver these information layers in a 

useful manner to emergency responders and 

the public, an Internet Map Server (IMS) is 

employed. This system allows the IMS author 

to make a variety of data layers available to 

the end user so that they can customize the 

map to their needs by selecting or turning off 

various information or reference layers. The 

information available and the appearance of 

symbols are scale dependent, allowing a great 

amount of flexibility to both the author and 

the user.

For more information, contact:

Joseph L. Jones

USGS Washington Water Science Center

1201 Pacific Ave., Suite 600

Tacoma, WA 98402

253-428-3600, ext. 2684

253-428-3614 FAX

jljones@usgs.gov

http://wa.water.usgs.gov/

tions were measured. Flow models therefore 

could simulate flow in one dimension through 

these cross-sections. With elevation data 

available for the entire floodplain, flow can be 

simulated everywhere. This type of simulation, two-dimensional, gives us a much more 

detailed picture of where water will go during 

a flood.

TRIMR2D Computer Model is 

Exceptionally Stable

Flow models are computer programs 

that attempt to solve equations that describe 

the physics of fluid flow. Because the set of 

equations is very large, a critical feature of all 

models is the method they use to arrive at a 

solution. The larger the area being modeled, 

the larger the set of equations, and the more 

difficult it is to arrive at a solution. So the 

size of area that can be simulated is limited 

by the solution method. Two-dimensional 

models, which simulate flow throughout the 

entire floodplain, are much larger than onedimensional models, so the solution method is 

especially important. 

The ability of a model to successfully 

solve large problems is referred to as “stability.” This is because of the iterative way 

the model attempts to solve the problem, 

closing in gradually on the solution. For very 

difficult problems, the model will perpetually overshoot the answer and never reach 

a solution—a condition called “numerical 

instability.” TRIMR2D is a two-dimensional 

model that uses a unique and especially stable 

solution method, so it can solve much larger 

problems—for much larger areas—than other 

two-dimensional models. Stability limits not 

only the size of the area that can be simulated, 

but also the ability to solve flow predictions 

that involve very large or fast changes in flow. 

TRIMR2D has demonstrated that it can solve 

LIDAR

GIS

3 4 5 6

IMS

TRIMR2D NWS forecast data Digital elevaton map

Flood map

TWIN ENGINE 

AIRCRAFT

LASER

SCAN WIDTH

FLIGHT DIRECTION

OVERLAPPING SWATHS