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Floodplain Analysis (A Guide for the Non-Professional)


A floodplain is the area of land on both sides of a natural waterway that becomes inundated with water during a large rainfall event when the water leaves the normal banks of the waterway.

The flooding associated with the large rainfall event is referred to as the base flood and the large rainfall event is normally the 100-year storm. A 100-year storm is the rainfall event that in theory has the probability of occurring once every 100 years. The base flood elevation (BFE) is the actual floodplain elevation that is determined through detailed studies for the 100-year storm at different locations along a waterway.

A floodplain analysis is the determination of the boundaries of the floodplain in the existing condition and in the proposed condition in the case where new construction is proposed that could impact the existing floodplain boundaries. This new construction could be a proposed land development with a layout or grading that could affect an existing floodplain. The new construction could also be a proposed road with an associated culvert or bridge that could impact the existing floodplain.

Who Should Be Concerned With Floodplains

The following are three common examples of people who should be concerned with the location of a floodplain relative to their proposed construction.

1) A land developer with a plan that has proposed grading that is encroaching into a floodplain.

2) A state department of transportation with plans to construct a new road with a culvert or bridge that would be crossing over a waterway with a floodplain that is regulated by FEMA (Federal Emergency Management Agency).

3) Individual homeowners who would like a land development plan to be developed for their property that includes a waterway or that contains a floodplain.


How to Determine If a Part of Your Property Is in a Regulated Floodplain

Probably the easiest way to find out if a part of your property is in a floodplain that is regulated by FEMA is directly through FEMA through their FIRM maps.

A FIRM map is a Flood Insurance Rate Map created by FEMA that shows the boundaries of floodplains regulated by FEMA throughout the United States. Hard copies of these maps can be ordered directly from FEMA. They can also be viewed online at the following website.

FEMA Flood Maps

You just have to type in your property address to start. You would also have the option to create for your records a PDF file of the map that includes your property.

Reasons for Doing a Floodplain Analysis

Floodplain boundaries that are shown on a FIRM map are delineated based on either detailed studies or approximate methods. Floodplain boundaries determined through detailed studies will show BFE numbers and the waterway cross section locations used for the studies. This information can be plotted on an existing conditions plan of your property to determine the limit of any existing floodplain boundary.

If the floodplain boundary on a FIRM map indicates a zone (for example, Zone A) that used approximate methods, the boundary limit to be drawn on an existing conditions plan might have to be estimated based on the lines shown on the FIRM map.

A full floodplain analysis might be required if it determined that a more accurate floodplain boundary is required for your site.

An analysis would also be required if you have a proposed land development project that involves the filling of earth in a floodplain or a driveway crossing over a creek or stream.

When to Hire a Civil Engineer

Unless your property or proposed land development project is clearly far away from any regulated floodplain map per the FIRM maps, it is likely that the services of a civil engineer will be required to delineate the floodplain boundary on a plan for your site in order to determine what kind of impact any proposed land development will have on the existing floodplain limits.

Even if your property does not show a floodplain that is regulated by FEMA, you might still want to consider hiring a civil engineer to do a floodplain analysis for your property for any existing waterway that flows through your site if you are thinking about proposing any new land development projects to help prevent any major issues with flooding.

Related: What Is a Land Development Engineer?

The Steps Involved With a Floodplain Analysis

Once it is determined that a detailed floodplain analysis is required for your property, the following are the general steps that a civil engineer would have to go through to determine either existing or proposed floodplain boundaries.

1) Determine the flow rate to the site. This usually involves what can be complicated hydrology calculations.

2) Create cross sections of the waterway to be analyzed. This step might mean having a land surveyor obtain new survey information.

3) Input flow rate and cross section information into a waterway modeling program. This creates the base of the floodplain analysis model.

4) Add any proposed condition information into the modeling program. This is especially important for any proposed waterway crossings or areas of proposed grading involving fill.

5) Run the modeling program. This step would include fixing errors and resolving issues involved with the model.

6) Plot the results. The resulting floodplain boundaries would be plotted along the cross section locations chosen for the model.


The Time Involved for a Floodplain Analysis

With the many steps involved with a floodplain analysis, a reasonable time to expect for the analysis to be completed could be at least one week. More time could be required if new survey has to be obtained for generating additional cross sections.

What Costs to Expect

The cost for a floodplain analysis could easily be in the thousands of dollars. It could even reach near $10,000 if you include the time for additional survey as well as application fees, review fees, and the time required to address any review comments for any projects that are shown to impact regulated floodplain areas.

Floodplain Management Is Important

Floodplain analysis is just a part of the important and complex topic of floodplain management.

More information can be found at the following website.

FEMA Floodplain Management Information

Posted in Engineering

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