evaluation of stream culvert crossings for function and...
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Evaluation of Stream CulvertCrossings for Function and Aquatic
Life PassageErin Shanaberger, City of Charlotte
Program HistoryMissions of Charlotte Storm Water Services:
Maintain storm waterinfrastructure
Reduce floodingImprove surface waterquality
Charlotte
• Charlotte has thousandsof headwater streams
• Charlotte is urban– Flashy flows >> eroded
streams• We have to fix
infrastructure in a waythat will not furtherdegrade our waterways
RequirementsEngineering Clean Water Act; Sections 401/404
• City of Charlotte hasdesign standards– 10, 25, 50, 100-yr
events– Standard varies by
location• Needs to be sustainable
– Our infrastructureneeds to last!
• “Safe passage of fish andaquatic organisms”– Buried culverts– Slope
• No alteration of upstream anddownstream dimension,pattern, profile
• Dimensions and gradient ofculvert must pass averagehistorical low flow withoutadversely altering flow velocity
• Minimize destabilization andheadcuts
Potential CompromisesPrevious suggestions to meet all requirements:
Study Initiation
• Complianceinspectionshighlighteddeficiencies in olderprojects– Repairs = time + $
• Do suggested“alternatives” work?– maintenance
challenges
Goals and Potential Outcomes
• Many design alternatives can be used to achieve aquatic life passage.We want to:– Eliminate configurations that do not work– ID alternatives that consistently function correctly
Evaluate completedprojects and assess howthey are functioning*
• Buried• Sills• Baffles• Multiple barrels
Find a solution thateveryone “agrees” with
• Meet environmentalgoals
• Maintain infrastructurein a cost-effectivemanner
Save time & $
• Fewer complianceissues
• Fewer repairs/re-designs
Methods
Key Variables
• Culvert slope• Presence of sills or baffles• Presence of grade controls
up/downstream• Fill depth• Width difference• Aquatic life obstructions• Stream match percentage
Level of Function
• Level of function for each culvert was defined in relation topermitting requirements
q Ideal: No obstructions to aquatic life; no signs of degradation; properfill (or bottomless); and 75 to 100% matching stream within culvert
q Deficient: Obstructions to aquatic life, signs of degradation, lack of fillat inlet and outlet (or for bottomless exposed footer); and 0%matching stream within culvert
q Functioning: allows aquatic life passage, but lacking one or severalconditions to categorize it as ideal or deficient
Results
Observations:• Arches have highest % ideal• Majority of boxes are
functioning• Circular are deficient
Culvert Shape is significant
Deficient Functioning IdealMax 5.77 2.38 2.11Q3 2.235 1.0375 1.4Mean 1.73 0.7686667 1.050588Median 1.38 0.51 0.89Q1 0.73 0.5 0.6Min 0.5 0.12 0.3
Slope plays a roleObservations:• Culverts designed at greater than ~2%
have little chance of functioningproperly
• Culverts with slopes off ~ 1.4% or lesshave the best chance of functioningproperly
Slope and Fill Depth
So, are sills the solution?
Observations:• Little difference between average
fill depth with and without sills• Small sample size• Filled culverts with higher slopes
tend to have more obstructions toaquatic life passage– When material is scoured at
inlets, sills serve as barriers toaquatic life
Influence of multiple barrels
Box Culverts Circular Culverts
Observations:• Multiple barrel box
culverts tend tofunction better
• Multiple barrelcircular culverts notsuperior to a singlebarrel
Overall Conclusions
• Bottomless arches are most “successful”• Culverts with lower slopes tend to function better
than culverts with higher slopes (>~1.4%)• Multiple barrel box culverts> Single barrel boxes• Multiple barrel circular culverts > Single barrel
circular
Next Steps• Analyze data collected in 2019• Collaboration
– NCDOT statewide study– Agency advisory team (USACE,
NCDWR, NCWRC, USFWS, NCDCM)• Identify new solutions
[email protected](704) 562-2691