CULVERTS:CULVERTS:

Design and Construction Design and Construction ConsiderationsConsiderations

Bridges and CulvertsBridges and Culverts

New York State DOT, by statute and following New York State DOT, by statute and following the FHWA, defines a bridge as any structure the FHWA, defines a bridge as any structure carrying a highway over an obstruction which carrying a highway over an obstruction which has an opening, measured along the centerline has an opening, measured along the centerline of the highway, in excess of 20of the highway, in excess of 20’’..

A similar structure having a centerline span of 20A similar structure having a centerline span of 20’’or less is defined as a culvert.or less is defined as a culvert.

This administrative definition does have significant This administrative definition does have significant implications.implications.

NYSDOT maintains a complete inventory of all NYSDOT maintains a complete inventory of all bridges in the state, whether State, County, or bridges in the state, whether State, County, or Locally owned. All bridges are rigorously Locally owned. All bridges are rigorously inspected and rated at intervals of no more than inspected and rated at intervals of no more than 2 years.2 years.

NYSDOT is working to develop an inventory of NYSDOT is working to develop an inventory of StateState--owned culverts. A few Counties and owned culverts. A few Counties and Towns inventory their culverts. The great Towns inventory their culverts. The great majority of culverts are not inventoried.majority of culverts are not inventoried.

No uniform standards exist for culvert inspection. No uniform standards exist for culvert inspection. NYSDOT inspects StateNYSDOT inspects State--owned large culverts owned large culverts (span > 5(span > 5’’), less rigorously than bridges, with a ), less rigorously than bridges, with a minimum frequency of once every 4 years.minimum frequency of once every 4 years.

Using these definitions, there are 12,663 Using these definitions, there are 12,663 ““bridgesbridges””over water in New York State.over water in New York State.

There are an estimated 50,000 large (>5There are an estimated 50,000 large (>5’’ span) span) culverts.culverts.

There are an estimated 1,000,000 small culverts There are an estimated 1,000,000 small culverts on the State, County and Local system on the State, County and Local system excluding private structures.excluding private structures.

The administrative definitions, though established The administrative definitions, though established by statute, are necessarily arbitrary and do not by statute, are necessarily arbitrary and do not reflect common usage.reflect common usage.

A perhaps better distinction might be a structural A perhaps better distinction might be a structural one, based on the method of support.one, based on the method of support.

A closed conduit stream crossing is self A closed conduit stream crossing is self supporting. It distributes the load of its own supporting. It distributes the load of its own weight, the fill above it, and the roadway and weight, the fill above it, and the roadway and traffic, over the bottom of the structure, which traffic, over the bottom of the structure, which is in contact with the soil.is in contact with the soil.

This type of support is simple and inexpensive, This type of support is simple and inexpensive, and with its large area of support is often a good and with its large area of support is often a good choice in soils having weak bearing capacity.choice in soils having weak bearing capacity.

An open bottomed structure, by contrast, must An open bottomed structure, by contrast, must support these loads on a much smaller area. It support these loads on a much smaller area. It requires foundations capable of dealing with the requires foundations capable of dealing with the more concentrated loads, either on stronger soils more concentrated loads, either on stronger soils (or rock) or on pile foundations, which add (or rock) or on pile foundations, which add significantly to the cost of the structure.significantly to the cost of the structure.

The smaller the structure, the greater the cost of The smaller the structure, the greater the cost of the foundations in proportion to the total cost the foundations in proportion to the total cost of the structure.of the structure.

PilesPiles

OverbankOverbank LowLow--flow flow channelchannel

DeckDeckSuperstructureSuperstructure

AbutmentAbutment PierPier Wing Wing wallwall

FootingFooting

It is the policy of New York State to use the 50It is the policy of New York State to use the 50--year recurrence event (Qyear recurrence event (Q5050 discharge) as the discharge) as the design flow and the 100design flow and the 100--year (Qyear (Q100100) as the check ) as the check flow for all highway crossings. Scour is evaluated flow for all highway crossings. Scour is evaluated for the Qfor the Q100100 event except for Interstate highwaysevent except for Interstate highways

For Interstates, scour is checked for the QFor Interstates, scour is checked for the Q500500..

Design policy for bridges and bridgeDesign policy for bridges and bridge--sized culverts sized culverts is to design the proposed structure to provide a is to design the proposed structure to provide a minimum of 2minimum of 2’’ of of freeboardfreeboard over the design over the design flood whenever possible, and to pass the check flood whenever possible, and to pass the check flood with reduced freeboard.flood with reduced freeboard.

Hydraulically, bridges are designed as openHydraulically, bridges are designed as open--channel flow. Flow is purely gravitychannel flow. Flow is purely gravity--driven, and driven, and the water surface is unconstrained, determined the water surface is unconstrained, determined only by the hydraulics of the flow.only by the hydraulics of the flow.

Culverts are generally designed and analyzed using Culverts are generally designed and analyzed using simplified, empirical relationships. simplified, empirical relationships.

Material roughness, length and size of the barrel Material roughness, length and size of the barrel determine friction losses, and end geometries are determine friction losses, and end geometries are assigned inlet and outlet losses. These and other assigned inlet and outlet losses. These and other minor losses make up the total energy or minor losses make up the total energy or hydraulic head needed to push water through hydraulic head needed to push water through the culvert. the culvert. Culvert hydraulics assume an Culvert hydraulics assume an elevated elevated headwaterheadwater to supply this push.to supply this push.

Culverts are said to be in either Culverts are said to be in either inletinlet or or outletoutletcontrolcontrol. .

In inlet control the barrel can carry more water In inlet control the barrel can carry more water than the inlet will allow in. It is hydraulically than the inlet will allow in. It is hydraulically inefficient, and results in inefficient, and results in supercriticalsupercritical (shallow, (shallow, highhigh--velocity) velocity) flowflow and and hydraulic jumpshydraulic jumps, which , which block fish movement.block fish movement.

Outlet controlOutlet control occurs when the inlet does not limit occurs when the inlet does not limit the flow through the culvert. Rather, it is the flow through the culvert. Rather, it is controlled by the slope and the characteristics of controlled by the slope and the characteristics of the barrel. Flow in a culvert in outlet control will the barrel. Flow in a culvert in outlet control will be subcritical, presenting less of a barrier to fish be subcritical, presenting less of a barrier to fish passage.passage.

In either case, either the inlet, the outlet or both In either case, either the inlet, the outlet or both may be submerged. may be submerged.

It can be difficult to determine whether a It can be difficult to determine whether a proposed culvert design will be in inlet or outlet proposed culvert design will be in inlet or outlet control. A culvert may also change from one to control. A culvert may also change from one to the other over a range of discharges. the other over a range of discharges.

For these reasons, culvert design usually involves For these reasons, culvert design usually involves analysis of the proposed design for both analysis of the proposed design for both conditions, and assuming the conditions, and assuming the ““worst case, worst case, ””whichever produces the greater headwater for a whichever produces the greater headwater for a given design discharge.given design discharge.

CulvertCulvert--sized structures use the same design flows sized structures use the same design flows as bridges. as bridges.

Since culverts are designed to function in pressure, Since culverts are designed to function in pressure, there is no freeboard target. To prevent there is no freeboard target. To prevent overtopping, however, the headwater at design overtopping, however, the headwater at design discharge is required to be a minimum of 2discharge is required to be a minimum of 2’’below the shoulder elevation of the roadway, below the shoulder elevation of the roadway, and headwater depth cannot exceed 1.5 times and headwater depth cannot exceed 1.5 times the culvert rise.the culvert rise.

Issues relating to closedIssues relating to closed--cell culverts:cell culverts:

Material limitations Material limitations –– corrosion, abrasion, corrosion, abrasion, roughness, shape and size availabilityroughness, shape and size availability

Structural limitations Structural limitations –– maximum span, cover maximum span, cover depth, excavation needsdepth, excavation needs

Connectivity breaks Connectivity breaks –– physical barriers, physical barriers, hydraulic barriers, habitat interruptionshydraulic barriers, habitat interruptions

“Open-bottomed Culverts”

The major types of openThe major types of open--bottomed structures used bottomed structures used by NYSDOT are 3by NYSDOT are 3--sided rigid frames (e.g. sided rigid frames (e.g. Con/SpanCon/Span®®, , HyHy--SpanSpan®®) and arches, either ) and arches, either concrete or metal (steel or aluminum).concrete or metal (steel or aluminum).

Some counties and towns also use sheet pile and Some counties and towns also use sheet pile and slab culverts, or block abutment structures.slab culverts, or block abutment structures.

The important thing to remember about open-bottom “culverts” is that structurally, they are bridges.

They require the same type of separate foundation, capable of supporting concentrated loads and resisting scour, as any other bridge.

For State-owned installations, this means they must be founded on competent rock, or on a pile foundation designed to accommodate predicted scour, unless they span the entire 100-year floodplain.

Available sizes:

Type Span RiseMin Max Min Max

Con/Span 12’ 48’ 3’ 13’Hy-Span 6’ 40’ 2’ 10’PC Arches (Bebo,

Kistner, etc.) 11’ 60’ 3.5’ 22’

An additional constraint on size is the span/rise An additional constraint on size is the span/rise ratio. For 3ratio. For 3--sided frames, NYSDOT prefers to sided frames, NYSDOT prefers to limit spans to no more than 4 times the rise. limit spans to no more than 4 times the rise. Greater ratios increase top slab moments and Greater ratios increase top slab moments and lateral foundation loads to the point where they lateral foundation loads to the point where they are impractical or uneconomical to construct.are impractical or uneconomical to construct.

Thus, an increase in desired span frequently results Thus, an increase in desired span frequently results in an increased rise as well.in an increased rise as well.

Arches, too, typically come in a limited range of Arches, too, typically come in a limited range of rises for a given span.rises for a given span.

For example, a For example, a BeboBebo® arch having a span of 29 to 30’ must have a rise between 8.5’ and 13.5’. If the roadway embankment requires a shorter vertical rise, a shorter span must be used or the embankment raised.

Existing intersections, driveways, etc. may Existing intersections, driveways, etc. may constrain raises in roadway profile. In addition, constrain raises in roadway profile. In addition, since the highway embankment must have at since the highway embankment must have at least a 1V:2H slope, each 1least a 1V:2H slope, each 1’’ rise in profile means rise in profile means a 4a 4’’ widening of the roadwidening of the road’’s footprint, which can s footprint, which can lead to ROW problems, and encroachment of lead to ROW problems, and encroachment of fill onto surrounding areas. And it requires a 4fill onto surrounding areas. And it requires a 4’’lengthening of the culvert.lengthening of the culvert.

The additional highway work also adds greatly to The additional highway work also adds greatly to project expense.project expense.

Depth of cover may also be a concern. Arches will Depth of cover may also be a concern. Arches will support much deeper cover than will simple support much deeper cover than will simple frames. Some structures, notably metal shapes frames. Some structures, notably metal shapes that lack the structural strength of concrete, may that lack the structural strength of concrete, may require at least a certain minimum cover depth, require at least a certain minimum cover depth, to distribute loads and prevent crushing.to distribute loads and prevent crushing.

It is vital that designers use the appropriate It is vital that designers use the appropriate specification number for 3specification number for 3--sided structures (Sec. sided structures (Sec. 562) to assure proper transport. During 562) to assure proper transport. During construction as well, care must be taken in construction as well, care must be taken in handling the units.handling the units.

OpenOpen--bottomed units, until properly placed bottomed units, until properly placed and backfilled, are inherently less stable and backfilled, are inherently less stable than closed boxes. Swaying or setting them than closed boxes. Swaying or setting them on end can result in damage.on end can result in damage.

Benefits of 3-sided Structures:

Ecological healthEcological healthRecreation industryRecreation industry

Economic output of NY freshwater fishing: $755MEconomic output of NY freshwater fishing: $755M(American (American SportfishingSportfishing Association, 2006 data)Association, 2006 data)

Mitigation cost savingsMitigation cost savingsLess need for routine maintenance (debris and Less need for routine maintenance (debris and sediment removal) sediment removal) May ease permitting/regulatory concernsMay ease permitting/regulatory concerns

CostsCosts

High initial costHigh initial costStructureStructureFoundationFoundationTransport / handlingTransport / handlingLaborLabor

Construction Issues with 3Construction Issues with 3--Sided Sided StructuresStructures

Excavation Excavation –– often reduces this itemoften reduces this itemWater management Water management –– may allow simpler may allow simpler

dewatering, diversion structuresdewatering, diversion structuresSite operability Site operability –– requires more room for requires more room for

equipmentequipmentSchedule differencesSchedule differences

Takes longer than traditional box culvertsTakes longer than traditional box culvertsMuch faster (and more easily staged) than bridgesMuch faster (and more easily staged) than bridges

Cost Comparison: 3Cost Comparison: 3--sided vs. 4sided vs. 4--sidedsided(span 24 ft x rise 7 ft, same location)(span 24 ft x rise 7 ft, same location)

Select structure fill: 19% cost reductionSelect structure fill: 19% cost reductionConcrete piles and pile driving equip: Concrete piles and pile driving equip: addadd’’ll itemsitemsFooting concrete: Footing concrete: addadd’’ll itemitemConcrete for structures: 15% cost reductionConcrete for structures: 15% cost reductionReinforced concrete span units: 124% cost increaseReinforced concrete span units: 124% cost increaseMobilization: 59% cost increaseMobilization: 59% cost increase

Total culvert costTotal culvert cost increased by 59% or increased by 59% or $227K$227K

44--sided: sided: $387K$387K →→ 33--sided: sided: $614K$614K

Stream Simulation

Sediment size Sediment size –– match range in natural match range in natural channel at surfacechannel at surface

Channel crossChannel cross--section section –– fix with larger fix with larger stonesstones

Channel configuration Channel configuration –– mimic natural mimic natural stream, allow room for selfstream, allow room for self--adjustmentadjustment

Stream Simulation Cross Section

(Ken Bates, 2006)

Stream Simulation

OpenOpen--bottom structuresbottom structuresBed is created within the assembled culvertBed is created within the assembled culvert

Photo Credit: Mendocino Co. DPW

Streambed SimulationStreambed SimulationOpenOpen--bottom structuresbottom structures

Bed is created within the assembled culvert,Bed is created within the assembled culvert, or…

Simulated bed is created between the footings before precast units are placed

Photo Credit: US Forest Service

Streambed SimulationStreambed SimulationOpenOpen--bottom structuresbottom structures

Bed is created within the assembled culvert, orBed is created within the assembled culvert, or……Simulated bed is created between the footings before Simulated bed is created between the footings before precast units are placedprecast units are placed

Embedded 4-sided structuresBed is created within the assembled culvert, or…

Bed material is allowed to naturally fill the embedded portion up to streambed grade

Summary

ClosedClosed--cell culverts are generally economical, cell culverts are generally economical, easily designed and installed, but require that easily designed and installed, but require that proper care be used to assure they do not proper care be used to assure they do not present unacceptable barriers to fish and present unacceptable barriers to fish and wildlife.wildlife.OpenOpen--bottomed structures avoid many of the bottomed structures avoid many of the environmental pitfalls, but are likely to be more environmental pitfalls, but are likely to be more expensive, more limited in applicability, and expensive, more limited in applicability, and require greater care in handling.require greater care in handling.

Contact InformationContact Information

George Long, PE (StructuresGeorge Long, PE (Structures--Hydraulics)Hydraulics)glong@dot.state.ny.usglong@dot.state.ny.us518518--457457--97309730

Brandon Greco (Environmental Science Bur.)Brandon Greco (Environmental Science Bur.)bgreco@dot.state.ny.usbgreco@dot.state.ny.us518518--485485--77537753