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

QUESTIONS?ERIN.SHANABERGER@CHARLOTTENC.GOV(704) 562-2691