dam rehabilitation and the national flood insurance...

Copyright © 2017 Association of State Dam Safety Officials, Inc. All Rights Reserved of 3

I. EXTENDED ABSTRACT

The National Flood Insurance Program (NFIP) was established in 1968 through the passage of the National Flood Insurance

Act with the purpose of reducing the impact of flooding on private and public structure. The NFIP requirements are found in

Chapter 44 of the Code of Federal Regulations (44 CFR). The premise is simple: if communities adopt floodplain

management guidelines and agree to regulate floodplain development, the local property owners are eligible to purchase

flood insurance. Both the flood insurance and the floodplain regulations work to reduce the socio-economic impact of

flooding.

The regulatory floodplain is defined through Flood Insurance Rate Maps and Flood Insurance Studies published by the

Federal Emergency Management Agency as part of the NFIP. For communities that participate in the National Flood

Insurance Program, critical flooding sources are analyzed through hydrologic and hydraulic analyses. These analyses focus

on the base flood, defined as the one percent chance flood or the 100-year flood. For this flood, hydraulic analyses are

completed to determine the inundation limits.

The scope and level of detail of the FEMA hydraulic analyses vary. Some streams are studied by approximate methods. The

floodplains for these streams are designated as the Zone A special flood hazard area. Other streams are studied by detailed

methods which establish base flood elevations, or 100-year water surface elevations for the reach. The floodplains for these

streams are designated as the Zone AE special flood hazard area. In some cases, additional hydraulic analyses called

encroachment analyses are completed to establish the regulatory floodway. A "Regulatory Floodway" is defined as the

channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood

without cumulatively increasing the water surface elevation more than a designated height.

Although dams are an important feature in floodplains, dams are not consistently depicted or consistently analyzed in the

development of these studies. The analyses completed to establish the floodplain and regulatory floodway were completed

over a period of approximately forty years beginning the late 1970s. The older studies may rely on outdated methodologies,

models or other input data. The level of detail incorporated into the studies varies widely. The methods used to consider

dams varies from completely neglecting the presence of the dam to incorporating detailed reservoir routing techniques and

hydraulic relationships.

For communities participating in the NFIP, the regulations for floodplain development are, at a minimum, governed by the

federal regulations contained in 44 CFR Parts 59, 60, 65, and 70. Some communities, in an attempt to further regulate

floodplain development have passed more stringent regulations than these minimum regulations.

At a minimum a community is required to:

Require permits for all proposed construction or other development within the floodplain and determine whether

proposed building sites are reasonably safe from flooding.

In areas without regulatory floodways, prohibit construction unless it is demonstrated that the proposed development

does not increase the water surface elevation of the base flood more than one foot.

Amanda J. Hess, P.E., CFM, H&H Group Manager, Gannett Fleming, Inc.

Ryan Knarr, P.E., Civil Engineer Consultant, PA Department of Environmental Protection

Dam Rehabilitation and the National Flood Insurance

Program – What Designers, Regulators and Dam Owners

Need to Know


In areas with regulatory floodways, prohibit encroachments within the adopted regulatory floodway unless it is

demonstrated that the proposed encroachment does not result in any increase in the water surface elevation of the

base flood.

To gain credits for flood insurance premium reductions within a community, many communities are imposing more strict

requirements by regulating all floodplains as floodways or by regulating floodplains to a larger flood discharge, like the 500-

year flood.

FEMA defines development as any man-made change to real estate. This includes filling, excavation and dredging. Given

that dams are most often located across rivers and streams, modifications at dams often results in development or

encroachment within floodplains or floodways, or modification of the base flood discharge. The type of flood hazard area

defined at the dam site and how or if the dam was analyzed in the development of the flood insurance study can play a

significant role in the types of dam modifications that are permitable or even possible. Engineers can overlook or ignore

these floodplain management implications and be met with unsuspected and undesired regulatory requirements during the

critical project permitting phase. Many dam safety regulatory agencies require analyses or other documentation to prove that

the proposed project is consistent with floodplain management regulations.

Recent dam rehabilitation projects that have faced regulatory hurdles due to floodplain management considerations are

discussed and potential means for addressing hurdles are highlighted. The two most common floodplain management issues

that are encountered during dam rehabilitation projects include: (1) increasing the base flood discharge, and (2) encroaching

within the floodway. A brief synopsis of each issue follows:

Increase in Base Flood Discharge. The most common dam safety deficiency for dams is insufficient spillway

capacity. Rehabilitating a dam to safely pass the spillway design flood can involve enlarging the spillway to increase

the spillway discharge capacity. An unintended outcome from an enlarged spillway can be an increase in the base flood

discharge due to the reduction in reservoir attenuation with the increased capacity. If the downstream floodplain is

regulated to prohibit base flood elevation increases, design features such as a two-stage or notched spillway can be

incorporated to address this requirement. Alternatively, detailed investigations into the FEMA hydrologic analyses

may reveal that the downstream floodplain is regulated to the without-dam base flood elevation. In this case, intentional

design features to mimic the existing spillway discharge relationship may not be required to comply with floodplain

management regulations. Ways to consider downstream flood risks in the alternatives evaluation and design process are

discussed.

Encroachment in Floodway. Dam rehabilitation projects may include modifications to the immediate downstream

area to incorporate updated features such as embankment filters or modifications, outlet works modifications, or the

construction of other appurtenances. These encroachments may result in unauthorized increases to the base flood

elevation. Analysis techniques and potential design modifications will be discussed. The effects of outdated and

approximate Flood Insurance Rate Maps will be highlighted.

The goal of the presentation is to provide designers, regulators and dam owners with the information they need to develop

successful dam modification designs that compliment FEMA floodplain regulations and avoid complications.

II. AUTHOR BIOGRAPHIES

Amanda J. Hess, P.E., CFM

H&H Group Manager, Gannett Fleming, Inc.

207 Senate Avenue

Camp Hill, PA 17011

(717)763-7211

[email protected]

Ms. Hess is a Hydrology and Hydraulics Group Manager and Senior Project Engineer in the Dams and Hydraulics Section of

Gannett Fleming, Inc. Amanda received her bachelor’s and master’s degrees in civil engineering from Pennsylvania State

University. She has over 17 years of experience working on dams, flood control projects, and water supply systems. She is

responsible for leading a team of engineers to perform hydrologic and hydraulic analyses for water resources projects and


design hydraulic structures related to flood control reservoirs, dams, bridges, and channel improvement construction,

rehabilitation, and reconstruction projects, including the design of spillways, outlet works, and stilling basins. Amanda

regularly instructs courses on hydrologic and hydraulic modeling.

Ryan Knarr, P.E.

PA DEP Dam Safety

P.O. Box 8460

Harrisburg, PA 17105-8460

(717)772-5943

[email protected]

Mr. Knarr is a Civil Engineer Consultant Hydraulic in the Division of Dam Safety at the Pennsylvania Department of

Environmental Protection. Ryan received his bachelor’s degree in civil engineering from Pennsylvania State University. He

has over 11 years of experience with the Dam Safety program. He is responsible for performing and reviewing hydrologic

and hydraulic analyses for dam projects throughout the state of Pennsylvania.

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