CITY OF COLUMBUS, MUSCOGEE COUNTY, …€¢ From railroad bridge on south end to Interstate 185 on...
Transcript of CITY OF COLUMBUS, MUSCOGEE COUNTY, …€¢ From railroad bridge on south end to Interstate 185 on...
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