October 2008

MIDTOWN FLOOD IMPACT STUDY

CITY OF COLUMBUS, MUSCOGEE COUNTY, GEORGIAGEORGIA

LINDSEY CREEK STUDY AREA

• From railroad bridge on south end to Interstate 185 on north end approximately 2 5 miles long185 on north end, approximately 2.5 miles long

• Located approximately 2 miles south of the local airport and 2 5 miles east of downtownlocal airport and 2.5 miles east of downtown Columbus

• 9 3 square mile basin9.3 square mile basin• Flood plain affects commercial and residential

areas along corridora eas a o g co do• Consists of both natural open channel and

concrete channel

End Project

B i P j tBegin Project

FLOOD PLAIN HISTORY

• Published FEMA Flood Insurance Study (FIS) for this stream dates back to October 1980for this stream dates back to October 1980

• Current FEMA Flood Insurance Rate Maps (FIRMs) revised September 2007 based on(FIRMs) revised September 2007, based on 1980’s data

• Only data available from FEMA was theOnly data available from FEMA was the Floodway determination for the 100-year event

• No significant flooding along corridoro s g ca ood g a o g co do• FIRM Panels did not overlay with GIS data

accuratelyy

INITIAL STUDY OBJECTIVES

• Analyze northern end from Boxwood Boulevard to Macon RoadBoulevard to Macon Road

• Determine if flooding limits could be decreased• Could channel be modified to reduce Flood• Could channel be modified to reduce Flood

Plain and Floodway• Could modifications be permitted• Could modifications be permitted

environmentally• Increase amount of land for redevelopment• Increase amount of land for redevelopment

• FEMA Channel alignment does notalignment does not match background image

• Very wide Flood Plain and Floodway

• Concrete Channel with no historic flooding

EXPANDED STUDY LIMTS

• City of Columbus shows interest to expand study limitsstudy limits

• Determine if flooding limits could be decreased for a longer stretch of the corridorfor a longer stretch of the corridor

• Work with Midtown, Inc.• Permit through the City to not incur FEMA fees• Permit through the City to not incur FEMA fees• Reduce amount of properties paying flood

insuranceinsurance

Steps North of Morehouse Street

D t fDownstream face at Decatur Street

Upstream of pRailroad Bridge

Upstream of Melrose DriveMelrose Drive

Upstream Face ofUpstream Face of Melrose Drive

Downstream of Melrose Drive

Footbridge

Bridge at “K”

Steps South of “K”

U t fUpstream of Boxwood Blvd

Bridge atBridge at Boxwood Blvd

Downstream of Boxwood Blvd

U d R dUnnamed Road at “N”

Unnamed RoadUnnamed Road at “N”

Midtown Loop Bridge

Upstream Macon Road

Bridge

Upstream natural channel of Macon Road Bridge

Bridge

Downstream Macon Road

Bridge

Downstream of Interstate 185

Section “R”

THE PROCESS

• Subcontracted with local surveyors (Moon, Meeks Mason and Vinson) to perform surveyMeeks, Mason and Vinson) to perform survey of published and unpublished sections from model

• Additional surveyed cross sections taken at steps in channelp

• Develop Duplicate Effective Floodplain and Floodway models

• Develop Corrective Effective Floodplain and Floodway models

SOME HURDLES

• Original model performed in HEC-2• Converting the model into HEC RAS• Converting the model into HEC-RAS• Naming convention of cross sections

V ti l D t f NGVD29 t NAVD88• Vertical Datum from NGVD29 to NAVD88• Even with vertical correction used by FEMA

( 0 090 feet) did not match survey data(-0.090 feet) did not match survey data.• Surveyed deviation from -2.0 to 2.9 feet

from FEMA channel elevationfrom FEMA channel elevation

DUPLICATE EFFECTIVE280

Lindsey Creek Flood Plan: Duplicate Effective 1/12/2009

Legend

WS 100 yr

WS 10 yr

Lindsey Creek Creek

270

y

Ground

250

260

on (f

t)

240

250

Ele

vati

230

0 2000 4000 6000 8000 10000 12000 14000 16000220

Main Channel Di stance (ft)

EXISTING280

Lindsey Creek Flood Plan: Exis ting 2/3/2009

Legend

WS 100 yr

WS 10 yr

Lindsey Creek Creek

270Ground

250

260

atio

n (ft

)

240

Ele

va

230

0 2000 4000 6000 8000 10000 12000 14000220

Main Channel Di stance (ft)

EXISTING vs. DUPLICATE280

Lindsey Creek Flood Plan: 1) Exis ting 2/3/2009 2) Dup Eff 1/12/2009

Legend

WS 100 yr - Existing

WS 100 D Eff

Lindsey Creek Creek

270

WS 100 yr - Dup Eff

Ground

260

on (f

t)

240

250

Ele

vatio

230

0 2000 4000 6000 8000 10000 12000 14000 16000220

Main Channel Di stance (ft)

WHY THE DIFFERENCES

• Original model never modeled steps in channelchannel

• Manning “n” value of concrete channel reduced from 0 03 to 0 015reduced from 0.03 to 0.015

• Overbanks remained essentially the same with a manning “n” of 0.06t a a g o 0 06

• Modeled rectangular channel under bridges and not trapezoid as HEC-2p

DID WE ACCOMPLISH THE GOAL

• Not totally…• Upstream was actually worse off• Upstream was actually worse off• Downstream had higher walls, wider

channel flows remained in channelchannel, flows remained in channel• Floodway improved for downstream also

OWHAT NOW….

PARTIAL REVISION

• Submit to FEMA the downstream portion for map revisionmap revision

• Stream gauge the upstream to get more accurate dataaccurate data

• Suggest channel modifications to reduce width and elevation of floodplain and dt a d e e at o o oodp a a dfloodway

• How? Wider natural channel to aid in environmental permitting. More consistent slope in channel to reduce hydraulic jumps…

QUESTIONS?QUESTIONS?

CONTACT INFORMATION:

Michael Bywaletz, PEGresham Smith and Partners

Michael_Bywaletz@GSPnet.com