#### STATEMENT OF FINDINGS FOR EXECUTIVE ORDER 11988

**(Floodplain Management)**

#### 

#### Lindsey Creek, Threet Creek, Lauderdale County Road 85, & Highway 13

#### 

#### Bridge Replacement

#### 

#### Natchez Trace Parkway

#### Lauderdale County, AL and Wayne County, TN 

**STATEMENT OF FINDINGS**

**INTRODUCTION**

This project proposes the replacement of the Natchez Trace Parkway
bridges over Lindsey Creek, Threet Creek, Lauderdale County Road 85, and
Tennessee Highway 13. The Lauderdale County Road 85 and Tennessee
Highway 13 bridges cross over roads and are not located in the 100-year
floodplain, therefore this statement of findings will only address the
bridges over Lindsey Creek and Threet Creek.

The present bridge over Lindsey Creek is a three-span 104-foot long
two-lane bridge built in 1956. It has a continuous monolithic concrete
slab superstructure supported by concrete abutments at each end and two
piers built on pile foundations in the creek.

The Federal Highway Administration last inspected the bridge over
Lindsey Creek in April of 2005. The bridge was determined to be in poor
condition, and major rehabilitation or replacement recommended. At
present, the north bank of Lindsey creek is located behind the north
abutment for the Natchez Trace Parkway Bridge. As a result, Lindsey
Creek is scouring away at the foundation of the north abutment and
damaging the abutment itself. Extensive serious cracks were found in the
west side of the deck and minor cracking throughout the structure. Heavy
efflorescence damage was found on the piers and abutments, with minor
efflorescence throughout. Spalling was also found on the deck and the
abutment corners. This deterioration was determined to be the result of
alkali-silica reactivity between the cement and aggregate in the
concrete. The bridge load limit was reduced from 15 tons to 13 tons due
to this deterioration. If not replaced, the bridge will continue to
deteriorate rapidly, resulting in further loss of load-bearing capacity
and eventual failure.

The existing bridge over Threet Creek is a three-span 104-foot long
two-lane bridge built in 1956. It has a continuous monolithic concrete
slab superstructure supported by concrete abutments at each end and two
piers built on pile foundations in the creek.

The Federal Highway Administration last inspected the bridge over Threet
Creek in April of 2005. The bridge was determined to be in serious
condition, and major rehabilitation or replacement was recommended.
Widespread moderate cracks were found in the concrete slab underside and
deck. Heavy efflorescence was found on both the deck and the abutments
of the bridge. Very poor concrete was found in the South span of the
bridge. Moderate to severe spalling was found throughout the structure.
This deterioration was determined to be the result of alkali-silica
reactivity between the cement and aggregate in the concrete. The bridge
load limit was reduced from 15 tons to 13 tons due to this
deterioration. If not replaced, the bridge will continue to deteriorate
rapidly, resulting in further loss of load-bearing capacity and eventual
failure.

The floodplains upstream and downstream of the Natchez Trace Parkway
bridges over both creeks are approximately 1,000 feet wide. The Natchez
Trace Parkway was built upon an embankment; as a result the floodplains
are abruptly constricted to 104' wide at both of the current bridges
across Lindsey and Threet Creeks. Because both Lindsey Creek and Threet
Creek are in FEMA 100-year floodplains, the increase in the water
surface elevations is limited to one foot.

The current design standard requires the bridge girders be two feet
above the fifty-year storm event water elevation of approximately 6.2
feet for Lindsey Creek and approximately 8.5 feet for Threet Creek.
Neither of the existing bridges meets the current design standards for a
flood event. The bottom of the current bridge over Lindsey Creek would
be 1.66 feet above the water level for a fifty-year storm. The
fifty-year water level would be 0.08 feet *above* the bottom of the
current bridge over Threet Creek.

This Statement of Findings has been prepared in accordance with the
guidelines in NPS Director's Order Number 77-2, *Floodplain Management*
and the accompanying Procedural Manual Number 77-2. The purpose of this
Director's Order is to establish NPS polices, requirements, and
standards for implementing Executive Order number 11988 "Floodplain
Management". The objective of this Executive Order is to avoid, to the
extent possible, the long and short term adverse impacts associated with
the occupancy and modification of floodplains and to avoid direct or
indirect support of floodplain development wherever there is a
practicable alternative.

## FLOODPLAIN EXTENT

Federal Emergency Management Agency (FEMA) Flood Insurance Rate maps
show that the Lindsey Creek and the Threet Creek bridges are within
100-year-flood floodplains.

## THE PREFERRED ALTERNATIVE

Alternative B would remove the existing bridges over Lindsey Creek,
Threet Creek, Lauderdale County Road 85 and Tennessee Highway 13. New
bridges would then be built in these locations. All four of the proposed
bridges would be similar in appearance to the existing bridges. Each
bridge would have an approximate total width of 34 feet.

The vertical profile of the bridge over Lindsey Creeks would be raised
and the vertical clearance would be reduced due to the increase in
bridge depth. The vertical profile of the bridge over Threet Creek would
be raised and the vertical clearance would be increased as well. The
bridge over Lindsey Creek would be lengthened to four spans, for a total
length of 144 feet. The bridge over Threet Creek would remain 104 feet
long.

The detour around the Lindsey Creek, Threet Creek and Lauderdale County
Road 85 bridges would utilize existing local roads to form one
continuous detour. The detour around Tennessee Highway 13 would utilize
local roads to form a separate detour. Depending on the availability of
funds, and the feasibility of constructing four bridges simultaneously,
the Tennessee Highway 13 detour may or may not be in place at the same
time as the Lindsey Creek, Threet Creek, and Lauderdale County Road 85
Detour. The Tennessee Highway 13 detour may be in place during a
different time frame; and possibly after the conclusion of the Lindsey
Creek, Threet Creek, and Lauderdale County Road 85 Detour.

Alternative B would have few, if any, temporary impacts to the
floodplains of Lindsey and Threet Creeks. Alternative B would have a
minor permanent impact on the floodplains of Lindsey and Threet Creeks.
Because of modern design requirements, the freeboard of the bridge over
Lindsey Creek for a 50-year-flood would be reduced to 0.82 feet. There
would be a backwater increase of 0.43 feet in the Threet Creek
floodplain; this is within the permitted limits.

There would also be a beneficial impact on the floodplains of Lindsey
and Threet Creek. The lengthening of the Lindsey Creek bridge reduces
the constriction of the floodplain by 40 feet. The proposed bridge over
Threet Creek would provide 0.87 feet of additional vertical clearance
for a fifty-year flood.

**GENERAL CHARACTERISTICS OF FLOODING IN THE AREA**

Flooding along the Natchez Trace Parkway generally occurs during high
rainfall events in the winter months of the year, and occasionally in
the spring. Floodwaters seldom overtop the banks of the roadway, and
have not historically caused any closures of the Parkway.

**JUSTIFICATION FOR USE OF THE FLOODPLAIN**

**DESCRIPTION OF SITE-SPECIFIC FLOOD RISK**

An average of 960 vehicles per day use the 444-mile Natchez Trace
Parkway, which commemorates an ancient trail that connected southern
portions of the Mississippi River to salt licks in today's central
Tennessee. This project is proposed to eliminate a health and safety
risk associated with deteriorating bridges. The project proposed to
replace existing bridges, therefore there are no additional feasible
alternative sites.

**DESIGN OR MODIFICATIONS TO MINIMIZE HARM TO FLOODPLAIN VALUES OR RISKS
TO LIFE AND PROPERTY**

In October 2002, preliminary bridge hydraulics found that the existing
bridges did not provide two feet of freeboard for the 50-year event. The
existing bridge over Threet Creek had zero freeboard. As a result of the
preliminary bridge hydraulics and discussions between the National Park
Service and Federal Highway Administration Design and Bridge staff it
was decided to raise the grade at Threet Creek to obtain additional
freeboard for the proposed bridge.

The freeboard of the bridge over Threet Creek for a 50-year flood would
be increased by raising the grade. The proposed bridge over Threet Creek
would provide 0.87 feet of additional vertical clearance for a
fifty-year flood. The grade raise would result in an increased backwater
of 0.43 feet for the 100-year flood.

It was decided not to raise the grade at Lindsey Creek because this
would create a hump in the road, which would limit the sight distance of
visitors driving the Natchez Trace Parkway and create safety concerns.
Instead it was decided to add a span to allow room for the main channel
so channel realignment would not be needed. The lengthening of the
Lindsey Creek bridge would reduce the constriction of the floodplain by
40 feet. Because of modern design requirements, the freeboard of the
bridge over Lindsey Creek for a 50-year-flood would be reduced. This
would cause no backwater increase for the 100-year flood.

Design considerations were sensitive to the historic importance of the
Natchez Trace Parkway. Altering the bridges drastically from their
existing state might cause an adverse affect on the Parkway.

## 

## CONCLUSION

The National Park Service concludes that there is no practical
alternative for replacement of the bridges over Lindsey Creek and Threet
Creek. The preferred alternative would substantially reduce potentially
hazardous conditions caused by continued bridge deterioration.
Mitigation and compliance with regulations and policies to prevent
impacts to water quality, floodplain values, and loss of property or
human life would be strictly adhered to during and after the
construction. Individual permits with other federal and cooperating
state and local agencies would be obtained prior to construction
activities. No long-term adverse impacts would occur from the Preferred
Alternative. Therefore, the National Park Service finds the Preferred
Alternative to be acceptable under Executive Order 11988 for the
protection of floodplains.