Chapter 17 - Floodplains
- What Does this Topic Include?
- Laws, Regulations, and Guidance
- Interagency Coordination
- Determining If a Project Encroaches on a Floodplain
- The Location Hydraulic Study
- Minimal Encroachment
- Significant Encroachment
- Finding of "Only Practicable Alternative"
- Public Involvement
- Preparing a Location Hydraulic Study
- Preparing a Summary Floodplain Encroachment Report
- Preparing a Floodplain Evaluation Report
- Information Needed for Project Delivery
- Activities that May Occur During the Project Design Phase
- Activities that May Occur During the Right-of-Way Phase or During Construction
- Map Revision
- Additional Definitions
- Exhibit A Location Hydraulic Study form
- Exhibit B Summary Floodplain Encroachment Report form
This chapter discusses the requirements of Executive Order 11988 and the responsibilities of FHWA, the Department, and local agencies when projects encroach on a 100-year base floodplain.
All Federal-aid projects must make diligent efforts to:
- Avoid support of incompatible floodplain development,
- Minimize the impact of highway actions that adversely affect the base floodplain,
- Restore and preserve the natural and beneficial floodplain values, and
- Be consistent with the standards/criteria of the National Flood Insurance Program of the Federal Emergency Management Agency (FEMA).
This mandate applies to all federally-approved highway construction, reconstruction, rehabilitation, repair, or improvement projects.
- Executive Order 11988 Floodplain Management (May 24, 1977)
- Department of Transportation (DOT) Order 5650.2 Floodplain Management and Protection (April 23, 1979) (PDF) (3.34 MB)
- Title 23 Code of Federal Regulations, Part 650, Subpart A - Location and Hydraulic Design of Encroachments on Flood Plains
- 23 CFR 771 - Environmental Impact and Related Procedures
- Revised Guidance on Co-operating Agencies (FHWA Memorandum, March 9, 1992)
- Technical Advisory T6640.8A, Guidance for Preparing and Processing Environmental and Section 4(f) Documents, October 30, 1987 (FHWA)
- National Flood Insurance Act 1968
The following definitions of terms are made for the purpose of uniform application in the documentation and preparation of floodplain evaluation reports. Refer to 23 CFR 650, Section 650.105 for a complete list of definitions.
- Any highway construction, reconstruction, rehabilitation, repair, or improvement undertaken with federal or Federal-aid highway funds or FHWA approval.
- The rise in water surface elevation due to encroachment.
- The flood or tide having a one percent (1%) chance of being exceeded in any given year (100-year flood).
- The water surface elevation of the base flood.
- The area subject to flooding by the base flood.
- The peak discharge, volume if appropriate, stage or wave crest elevation of the flood associated with the probability of exceedance selected for the design of a highway encroachment. (In other words, the project will not be inundated at the design flood frequency.)
- A floodway designated by a state or local agency. California State Reclamation Board (Board) definition: A designated floodway means either: (1) the channel of the stream and that portion of the adjoining floodplain reasonably required to provide passage of a base flood or (2) the floodway between existing levees as adopted by the Board or the Legislature.
- FEMA definition: Construction, placement of fill, or similar alteration of topography in the floodplain that reduces the area available to convey floodwaters. FHWA definition: An action within the limits of the base floodplain.
- The floodplain management map issued by FEMA that depicts, on the basis of detailed analyses, the boundaries of the 100- and 500-year floodplain and the regulatory floodway.
- The statistical number of years that takes place before the recurrence of a flood of the same magnitude. (10-year flood, 50-year flood, 100-year flood, etc.)
- The insurance and floodplain management map issued by FEMA that identifies, on the basis of detailed or approximate analyses, the areas of 100-year flood hazard in a community.
- It is a report that describes and delineates the Special Flood Hazard Areas and the elevations of the community.
- Any land area subject to inundation by floodwaters from any source.
- A technical report which evaluates effects of the floodplain encroachment concerning the 6 key items identified in 23 CFR 650.111(b)(c)(d) verified by results of the Location Hydraulic Study, but in greater detail. This report is required in situations where it is uncertain or clear that a project may involve a significant encroachment. This report is to be used as a backup for the Environmental Assessment/Finding of No Significant Impact (EA/FONSI) or an Environmental Impact Statement (EIS). The risks, impacts, and mitigation measures must be summarized in the National Environmental Policy Act (NEPA) document.
- Fish, wildlife, plants, open space, natural beauty, scientific study, outdoor recreation, agriculture, aqua culture, forestry, natural moderation of floods, water quality maintenance, groundwater discharge, etc.
- To design and construct a project to keep floodwaters out or to reduce the effects of floodwaters.
- The channel of a river or other watercourse, plus any adjacent floodplain areas, which is designated a floodway by a public agency, that must be kept free of encroachment so that the 100-year flood discharge can be conveyed without cumulatively increasing the water-surface elevation more than one foot above the BFE. (Since the one-foot is already accounted for, no increase of any amount in the BFE is allowed in the floodway.)
- The portion of the 100-year floodplain that is not within the floodway and in which development and other forms of encroachment may be permitted under certain circumstances.
- The preliminary investigative study to be made of base floodplain encroachments by a proposed highway action. (This study must be performed by a registered engineer with hydraulic expertise.)
- An encroachment that is parallel to the direction of flow. Example: A highway that runs along the edge of a river is, usually considered a longitudinal encroachment.
- A floodplain area that is reserved in an open manner by federal, state or local requirements i.e., unconfined or unobstructed either horizontally or vertically, to provide for the discharge of the base flood so that the cumulative increase in water surface elevation is no more than a designated amount (not to exceed 1 foot as established by FEMA for administering the National Flood Insurance Program). (Since the 1 foot is already accounted for, no increase more than 0.00 feet is allowed.)
- An economic comparison of design alternatives using expected total costs (construction costs plus risk costs) to determine the alternative with the least expected cost to the public. It shall include probable flood-related costs during the service life of the facility for highway operation, maintenance, and repair, for highway-aggravated flood damage to other property, and for additional or interrupted highway travel.
- An economic and/or non-economic assessment of the impacts associated with the floodplain encroachment(s). It is meant to be more general in detail than a risk analysis. The format and content of the Summary Floodplain Encroachment Report form is the minimum required for a risk assessment.
- A highway encroachment and any direct support of likely base floodplain development that would involve one or more of the following construction- or flood-related impacts:
- A significant potential for interruption or termination of a transportation facility which is needed for emergency vehicles or provides a community's only evacuation route.
- A significant risk [to life or property], or
- A significant adverse impact on natural and beneficial floodplain values.
- The areas delineated on an NFIP map as being subject to inundation by the base (100-year) flood.
- It is a floodplain assessment report which addresses the 6 key items identified in 23 CFR 650.111(b)(c)(d) verified by results of the Location Hydraulic Study. If it is determined that a project does not have a significant encroachment, this form can be used as the documentation for a Categorical Exclusion (CE) determination. For federally-funded projects on the State Highway System (SHS), the Department's Project Engineer will sign the Summary Floodplain Encroachment Report. For local assistance projects, this report must be filled out and signed by the local agency Project Engineer, with concurrence signature by the District Local Assistance Engineer (DLAE).
Additional definitions not specifically used in this chapter are located at the end of this chapter.
Local, state, and federal water resources and floodplain management agencies must be consulted if a proposed action encroaches on a 100-year base floodplain. Coordination also may occur in order to obtain current information on development and proposed actions in the effected watersheds. The Department, or in the case of local assistance (off SHS) projects, the DLAE, is responsible for initiating early coordination meetings to discuss potential floodplain encroachments.
If there are potential impacts to endangered species or wetlands, and/or if a 404 permit is required, the federal and state agencies with jurisdiction and permitting authority should be identified early in the environmental process. The Department and local agencies are responsible for early and on-going coordination with the U.S. Fish and Wildlife Service regarding technical information and standards for mitigation (as necessary) and with the U.S. Army Corps of Engineers regarding permit requirements.
The Department evaluates any action which involves a floodplain encroachment and is responsible for making the floodplain finding. In the case of a significant encroachment, FHWA must concur in an "Only Practical Alternative Finding."
Coordination with FEMA must occur when floodplain studies indicate that any of the following four conditions pertain:
- A proposed encroachment on a regulatory floodway (BFE increase) would require an amendment to the floodway map. (However, it is not likely that any increase would be allowed by FEMA.)
- A proposed encroachment on a floodplain where a detailed study has been performed but no floodway designated and the maximum 1 foot increase in the base flood elevation would be exceeded.
- A local community is expected to enter into the regulatory program within a reasonable period and detailed floodplain studies are underway.
- A local community is participating in the emergency program and base flood elevation in the vicinity of insurable buildings is increased by more than 1 foot.
The implementation of Executive Order 11988 in transportation projects is addressed by 23 CFR 650 Subpart A entitled "Location and Hydraulic Design of Encroachment on Floodplains." When SHS transportation improvements encroach on a base floodplain, the Department is responsible for the preparation of a Location Hydraulic Study to assess the risk involved. For local assistance projects that encroach on a base floodplain, the local agency is required to prepare a Location Hydraulic Study. The Location Hydraulic Study is summarized and appended to the environmental document for the project. When the Location Hydraulic Study indicates a significant encroachment within the base floodplain, as defined by 23 CFR 650.105, FHWA must approve the encroachment and concur in the finding that the preferred alternative is the "only practicable alternative." This information must be included in the final environmental document/determination. For further guidance, please see SER Chapter 38, "NEPA Assignment."
When a local agency's Preliminary Environmental Study (PES) or the Department's PSR and/or the Preliminary Environmental Analysis Report (PEAR) indicates that there are no floodplain impacts, the environmental document must include this information and the basis for this conclusion must be documented in the project record. The determination may be accomplished with an informal study as long as it provides adequately information to support the determination.
It should be noted that the absence of National Flood Insurance Program or State Department of Water Resources (DWR) base floodplain maps does not necessarily indicate that there is no base floodplain in the area. Where no state or federal data is available, the local agency or the Department is responsible for examining other data regarding recent flood locations and developing adequate information and analysis to support the conclusions presented in the technical report and the environmental document.
When a floodplain encroachment is anticipated, the Department or local agency prepares a Location Hydraulic Study. The Location Hydraulic Study is a preliminary study of base floodplain encroachments and must be performed by a registered engineer with hydraulic expertise.
If an increase in the base floodplain elevation (BFE) is anticipated, a hydraulic computer model must be run to determine the amount of increase in order to determine the floodplain encroachment impacts.
The minimum required content of the Location Hydraulic Study is prescribed in 23 CFR 650 Subpart A, Section 650.111(b)(c)(d) and it must include the following:
- National Flood Insurance Program (NFIP) maps or information developed by the highway agency, if NFIP maps are not available, are used to determine whether a highway location alternative will include an encroachment.
- Location studies also must include evaluation and discussion of the practicability of alternatives to any longitudinal encroachments.
- Location studies must include discussion
of the following items, commensurate with the significance
of the risk or environmental impact, for all alternatives
containing encroachments and for those actions which
would support base floodplain development:
- The risks associated with implementation of the action,
- The impacts on natural and beneficial flood-plain values,
- The support of probable incompatible flood-plain development,
- The measures to minimize flood-plain impacts associated with the action, and
- The measures to restore and preserve the natural and beneficial floodplain values impacted by the action.
- Location studies must include evaluation and discussion of the practicability of alternatives to any significant encroachments or any support of incompatible floodplain development.
- The studies required by Sec. 650.111 (c) and (d) must be summarized in environmental review documents prepared pursuant to 23 CFR 771.
- Local, state, and federal water resources and floodplain management agencies should be consulted to determine if the proposed highway action is consistent with existing watershed and floodplain management programs and to obtain current information on development and proposed actions in the affected watershed.
The Location Hydraulic Study provides the minimum documentation necessary to support a finding in regard to floodplain encroachment impacts.
If the study concludes that there is no encroachment and/or minimal impact, a Summary Floodplain Evaluation Report is completed. In the case of a local assistance project (off the SHS), this form is filled out and signed by the local agency Project Engineer, and concurred by the Department's DLAE. In the case of Department projects on the SHS, the Summary Floodplain Evaluation Report is signed by the Department's Project Engineer. This form, and its supporting study, provide sufficient documentation for a Categorical Exclusion (CE). The Location Hydraulic Study and the Floodplain Evaluation Report are retained in the project file along with any supporting documentation.
When the Location Hydraulic Study concludes that a proposed action:
- May result in a significant encroachment (as defined by 23 CFR 650.105) and/or
- May be inconsistent with existing watershed and floodplain management programs, thereby resulting in incompatible floodplain development, or that
- The impacts of the project on the floodplain are unclear
a Floodplain Evaluation Report is prepared. This is a self-sufficient technical report, which is used to support the conclusions of an EA/FONSI or an EIS. It contains all the information from the Location Hydraulic Study and is similar to the Summary Floodplain Encroachment Report but is generally more detailed. It should include a project description, a strip map delineating the base floodplain with all project encroachments identified, and minimization and/or mitigation measures, as well as a discussion of avoidance alternatives to each potential encroachment and whether the alternatives are practicable. The level of detail in this report will vary depending on the individual project, but should be commensurate with the complexity of the project, its risks, and impacts. The risks, impacts and mitigation measures described in this report form the basis of discussion in the environmental document. For further guidance, please see SER Chapter 38, "NEPA Assignment" - Floodplains.
If a significant floodplain encroachment is identified as a result of floodplain studies, FHWA will need to approve the floodplain encroachment and concur in the "Only Practicable Alternative Finding," which must be prepared by the Department for actions involving a significant unavoidable encroachment (see Technical Advisory T6640.8A, Guidance for Preparing and Processing Environmental and Section 4(f) Documents, October 30, 1987).
The floodplain "Only Practicable Alternative Finding" must be included in the final environmental document. The finding must refer to Executive Order 11988 and to 23 CFR 650, Subpart A. The finding must be supported by the following:
- The reasons why the proposed action must be located in the floodplain
- The alternatives considered and why they are not practicable
- A statement indicating whether the action conforms to applicable state or local floodplain protection standards
For local assistance projects, the DLAE is responsible for coordinating with FHWA to obtain the "Only Practicable Alternative Finding" concurrence for local agency projects.
A copy of the finding must be made available to the appropriate state and area wide clearinghouse pursuant to 23 CFR 420. For Department projects this is the State Office of Planning and Research (OPR), and for local agency projects, the Office of the County Clerk.
Executive Order 11988 requires that when the only practicable alternative requires encroachment into a floodplain the public must be given the opportunity for early review and comment. It also requires that the risk assessment be filed with the State Clearinghouse. Existing procedures for public involvement can be used to meet these requirements. A reference to encroachments on the base floodplain must be included in public notices and any encroachments must be identified at public hearings (Executive Order 11988). See PDPM Appendix HH for public notice format.
Location hydraulic studies are performed to evaluate the base (100 year) flood and the proposed action's impact(s) on the base floodplain. This study involves the use of Flood Insurance Study data and NFIP maps for baseline, and a hydraulic computer model to determine the new BFE. The items in the Location Hydraulic Study form below are a suggested minimum for required documentation:
- Name of stream
- Bridge number
- Geographical reference
- Description of proposed action
- Hydraulic Data:
- Base flood (Q100) Q = ____________ cuffs
- Water surface elevation for base flood: ___________________
- Flood of record (when available) Q = __________________ cuffs
- Flood of record water surface elevation (if available): ____________ feet
- Map of Floodplain, with Base Flood (Q100) and Flood of Record (when available), or the information developed, if NFIP maps are not available, to determine whether an alternative will include an encroachment.
- Base Flood (Q100) Backwater Potential
- Other Buildings
- Approximate Traffic Volume
- Emergency Supply Or Evacuation Route
- Emergency Vehicle Access
- Practicable Detour Available
- School Bus or Mail Route
- Approximate Duration of Traffic Interruption for Base Flood (Q100)
- Approximate Value of Base Flood (Q100)
Damages (Required only for the rare situations of moderate
or high risk.)
For all alternatives containing encroachments or which would support base floodplain development, the following seven items must be evaluated commensurate with the severity of the risk or environmental impact:
- The risk associated with implementation of the action
- the impacts on natural and beneficial floodplain values
- The support of probable incompatible floodplain development.
- The measures to minimize floodplain impacts associated with the action
- The measures to restore and preserve the natural and beneficial floodplain values impacted by the action
- The practicability of alternatives to any significant encroachment
- The practicability of alternatives to any longitudinal encroachment.
Local, state, and federal water resources and floodplain management agencies may need to be consulted in the preparation of the Location Hydraulic Study to determine if the proposed action is consistent with existing watershed and floodplain management programs and to obtain current information on development and proposed actions in the affected watersheds.
The person performing the hydraulic study for the Location Hydraulic Study must have hydrologic and hydraulic engineering expertise and must be identified in the technical report and in the environmental document. An environmental specialist should be consulted regarding social and ecological floodplain impacts.
The Summary Floodplain Encroachment Report is based on results of the Location Hydraulic Study and relies on assessment of the key items stated in 23 CFR 650.111(b)(c)(d), which are reiterated below. The Summary is completed by the Project Engineer after consultation with the Hydraulic Engineer and an environmental specialist. The responses to the key items can trigger other actions as follows:
- Is the proposed
action a longitudinal encroachment of the base floodplain?
- If the answer is "yes", there needs to be an evaluation and discussion of the practical alternatives to any longitudinal encroachments to supplement this form.
- Are the risks
associated with the implementation of the proposed
- This answer must be "no" in order to process this project with a CE.
- Will the proposed
action support probable incompatible floodplain development?
- This answer must be "no" in order to process this project with a CE.
- Are there any
significant impacts on natural and beneficial floodplain
- This answer must be "no" in order to process this project with a CE.
- Routine construction
procedures are required to minimize impacts on the
floodplain. Are there any special mitigation measures
necessary to minimize impacts or restore and preserve
natural and beneficial floodplain values?
- If the answer is "yes" explain it and supplement this form with the explanation.
- Does the proposed
action constitute a significant floodplain encroachment
as defined in 23 CFR 650.105q?
- This answer must be "no" in order to process this project with a CE.
- Are Location Hydraulic
Studies (same as Figure 804.7A Technical Information for Location Hydraulic Study located in chapter 804 of the Highway Design Manual) that document the above answers on file?
- This answer must be "yes" in order to validate the answers to the above items.
The Location Hydraulic Study and the Summary Floodplain Encroachment Report and their supporting documentation provide the basis for a CE. For further guidance, see SER Vol. 1, Chapter 38, "NEPA Assignment."
This determination as well as the Location Hydraulic Study and the Summary Floodplain Encroachment Report should be attached to the CE/CE form.
An EA or EIS require additional documentation (see below - Preparing a Floodplain Evaluation Report).
For local assistance projects this form is to be filled out and signed by the local agency Project Engineer, and concurred by the Department's DLAE.
The Floodplain Evaluation Report is prepared when there is a significant encroachment and/or impact in a base floodplain requiring the preparation of an EA/FONSI or an EIS.
When an environmental document is to be prepared for a project which has alternatives that have permanent features that encroach on a floodplain, the District Environmental Branch will take the lead in preparing the Floodplain Evaluation Report. The technical hydraulic requirements must be evaluated by the District Hydraulic Engineer. If floodplain encroachments are significant, then alternatives must be developed jointly among Design, Hydraulics, and Environmental.
The detail in the report is to be commensurate with the significance of potential encroachment, and risk of economic loss, private or public. The report should include the following:
- Project Description, including a strip map of the project showing the base floodplain and all project encroachments.
- Alternatives to Encroachment. A discussion of the practicality of alternatives that would avoid longitudinal or significant encroachments or the support of incompatible floodplain development.
- Risk Assessment. This is prepared by the District Hydraulic Engineer as part of the location hydraulic and/or design studies as called for in 23 CFR 650, Subpart A. Risk information is provided for each individual encroachment throughout the project.
- Impacts of the project including the support of probable incompatible floodplain development and impacts on natural and beneficial floodplain values.
- Measures to minimize floodplain impacts including measures to restore and preserve floodplain values affected by the action.
The individual performing the Floodplain Evaluation Report must be a registered professional engineer or be qualified for registration as a professional engineer under California law.
The individual determining the natural and beneficial floodplain values of the floodplain should be a qualified biologist or wetland ecologist.
Floodplain studies should be undertaken as early as possible, as the requirement to consider all practicable alternatives to encroachment upon the base 100-year floodplain may necessitate a broader study area or significant modifications in preliminary design proposals.
The Location Hydraulic Study is requested simultaneous with other technical reports, and will determine if further studies are needed.
For local assistance, it is commenced after completing the PES Form. The Location Hydraulic Study and Summary Floodplain Encroachment Report forms to be used for local assistance projects are available on the Local Assistance Program's Local Environmental Issues page.
For Department projects, the Location Hydraulic Study is undertaken upon receipt of the request to "Begin Environmental." The Location Hydraulic Study and Summary Floodplain Encroachment Report forms to be used for Department projects are available on the SER Forms and Templates page.
This information should be included in the Preliminary Environmental Analysis Report (PEAR) prepared as part of the Project Initiation Document (PID).
The "Guidelines for the Preparation of Project Study Reports" dated November 3, 1999 stipulate that PSR's and project study report equivalents contain an "inventory of environmental resources, identification of potential environmental issues and anticipated environmental processing type. Potential mitigation requirements and associated costs should also be identified."
For projects off the SHS, complete the PES form (PDF) (2.18 MB). The information required for the PES satisfies the environmental requirement for the PSR equivalent.
Instructions for determining whether the project construction area is located with a regulatory floodway or within the base floodplain (100-year) elevation of a watercourse or lake are provided in the Local Assistance Procedures Manual (LAPM), Chapter 6, Exhibit B, #13 (PDF) (477 KB).
For projects on the SHS, the following level of information is recommended to fulfill the requirements of the guidelines:
Verification of information from Regional Transportation Plan (RTP) stage and:
- Identification of 100-year base floodplain(s) within the project area using National Flood Insurance Program (NFIP) maps.
- Identification of any potential floodplain encroachments by the proposed project and construction activities.
This information should be documented in the Environmental Impact Report (EIR) prepared for the RTP and serve as a building block in subsequent decision-making.
An RTP requires the preparation of a California Environmental Quality Act (CEQA) environmental document, normally a program or master EIR. The Department encourages the MPO/RTPA's to include the following information, as appropriate, in the environmental document for the plan:
- Identification of floodplains within the transportation corridor or sub area using National Flood Insurance Program (NFIP) maps available online at FEMA's Flood Mapping Hazard page.
This information should be presented in the draft environmental document or used as supporting documentation for a Categorical Exemption/Categorical Exclusion, as appropriate:
- Verification of all information from RTP stage and PID stage and,
- Identification of floodplain encroachments and potential project impacts to base floodplain in a Location Hydraulic Study and Floodplain Evaluation Report.
This information should be presented in the final environmental document.
- "Floodplain Only Practicable Alternative Finding" if appropriate.
- Cost estimates for mitigation measures, as defined, for the Mitigation Cost Tracking System
After the environmental process is completed, detailed design studies of the encroachment are required as a part of the final design process. If the design selected involves a significant encroachment, it must be supported by analysis of design alternatives with consideration given to capital costs, risks, and to other economic, engineering, social, and environmental concerns. The detail of the studies shall be commensurate with the risk associated with the encroachment and with other economic, engineering, social, and environmental concerns. The required contents of the design studies are specified in 23 CFR 650.117.
Following the approval of an environmental document and prior to proceeding to request major approvals or grants for the project phase, the Department must determine if the environmental document remains valid. Any major changes in floodplain incursion or effects from the project at these phases may initiate a reevaluation. For more information, refer to the SER, Vol. 1, Chapter 33, "Reevaluations."
It is implied in the Federal-Aid Policy Guide (non-regulatory supplement for 23 CFR 650A) that a map revision is only required when a project results in any increase in the BFE in a floodway or increases the BFE by more 1 foot in a base floodplain. This contradicts the FEMA requirement to submit a map revision for any increases in the BFE in a base floodplain. In order to be consistent with the NFIP, it is advisable to consult with the California Department of Water Resources (DWR) and/or FEMA to determine if a map revision is necessary for increases less than 1 foot.
The map revision is a very complex process. Hence, it is highly recommended that DWR and/or FEMA be contacted for technical assistance for this process. A basic outline of the information that needed is listed below:
- FIS backup data (hydrology, hydraulics and mapping) from FEMA for pre-project hydraulic model.
- Pre-project survey of existing cross-sections of the stream or watercourse at the proposed project site.
- Design must meet local, state, and FEMA criteria and all other permit requirements.
- Original FIS models (HEC-2, HEC-RAS or WSPRO, etc.) must be rerun with new data to reflect the new base floodplain boundaries.
Map revision data must be submitted to FEMA within 6 months of project completion. However, the coordination/consultation for the revisions should begin during the design phase of a project.
It is not likely that any increases of elevation in the regulatory floodway will be allowed by any of the regulatory agencies.
- A factor of safety usually expressed in feet above a flood level for purposes of flood plain management. "Freeboard" tends to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a flood frequency and floodway conditions, such as wave action, and floating debris under bridge openings. Definition from 23 CFR 650: The vertical clearance of the lowest structural member of the bridge superstructure above the water surface elevation of the overtopping flood.
- The federal program that enables property owners to purchase flood insurance.
- The flood frequency at which a flow to occurs over the highway, over the watershed divide, or through structure(s) provided for emergency relief. Definition from 23 CFR 650: The flood described by the probability of exceedance and water surface elevation at which flow occurs over the highway, over the watershed divide, or through structure(s) provided for emergency relief.
- Capable of being done within reasonable natural, social, or economic constraints.
- To avoid modification to the functions of the natural floodplain environment or to maintain it as closely as practicable in its natural state.
- To reestablish a setting or environment in which the functions of the natural and beneficial floodplain values adversely impacted by the highway agency action can again operate.
- A river-like area.
- An encroachment that is perpendicular to the direction of flow. (Example: A bridge encroachment on the floodplain is normally considered to be transverse encroachment.
(Last content update: 02/04/2015: JH)