CULVERTS: Design and Construction Considerations

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1 CULVERTS: Design and Construction Considerations

2 Bridges and Culverts New York State DOT, by statute and following the FHWA, defines a bridge as any structure carrying a highway over an obstruction which has an opening, measured along the centerline of the highway, in excess of 20. A similar structure having a centerline span of 20 or less is defined as a culvert.

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

5 NYSDOT is working to develop an inventory of State-owned culverts. A few Counties and Towns inventory their culverts. The great majority of culverts are not inventoried. No uniform standards exist for culvert inspection. NYSDOT inspects State-owned large culverts (span > 5 ), 5 less rigorously than bridges, with a minimum frequency of once every 4 years.

6 Using these definitions, there are 12,663 bridges over water in New York State. There are an estimated 50,000 large (>5 span) culverts. There are an estimated 1,000,000 small culverts on the State, County and Local system excluding private structures.

7 The administrative definitions, though established by statute, are necessarily arbitrary and do not reflect common usage. A perhaps better distinction might be a structural one, based on the method of support.

8 A closed conduit stream crossing is self supporting. It distributes the load of its own weight, the fill above it, and the roadway and traffic, over the bottom of the structure, which is in contact with the soil. This type of support is simple and inexpensive, and with its large area of support is often a good choice in soils having weak bearing capacity.

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

11 Superstructure Deck Abutment Pier Wing wall Piles Overbank Low-flow channel Footing

12 It is the policy of New York State to use the 50- year recurrence event (Q 50 discharge) as the design flow and the 100-year (Q 100 ) as the check flow for all highway crossings. Scour is evaluated for the Q 100 event except for Interstate highways For Interstates, scour is checked for the Q 500.

13 Design policy for bridges and bridge-sized culverts is to design the proposed structure to provide a minimum of 2 2 of freeboard over the design flood whenever possible, and to pass the check flood with reduced freeboard. Hydraulically, bridges are designed as open- channel flow. Flow is purely gravity-driven, and the water surface is unconstrained, determined only by the hydraulics of the flow.

14 Culverts are generally designed and analyzed using simplified, empirical relationships. Material roughness, length and size of the barrel determine friction losses, and end geometries are assigned inlet and outlet losses. These and other minor losses make up the total energy or hydraulic head needed to push water through the culvert. Culvert hydraulics assume an elevated headwater to supply this push.

15 Culverts are said to be in either inlet or outlet control. In inlet control the barrel can carry more water than the inlet will allow in. It is hydraulically inefficient, and results in supercritical (shallow, high-velocity) flow and hydraulic jumps,, which block fish movement.

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

17 It can be difficult to determine whether a proposed culvert design will be in inlet or outlet control. A culvert may also change from one to the other over a range of discharges. For these reasons, culvert design usually involves analysis of the proposed design for both conditions, and assuming the worst case, whichever produces the greater headwater for a given design discharge.

18 Culvert-sized structures use the same design flows as bridges. Since culverts are designed to function in pressure, there is no freeboard target. To prevent overtopping, however, the headwater at design discharge is required to be a minimum of 2 2 below the shoulder elevation of the roadway, and headwater depth cannot exceed 1.5 times the culvert rise.

19 Issues relating to closed-cell cell culverts: Material limitations corrosion, abrasion, roughness, shape and size availability Structural limitations maximum span, cover depth, excavation needs Connectivity breaks physical barriers, hydraulic barriers, habitat interruptions

20 Open-bottomed Culverts The major types of open-bottomed structures used by NYSDOT are 3-sided 3 rigid frames (e.g. Con/Span, Hy-Span Span ) ) and arches, either concrete or metal (steel or aluminum). Some counties and towns also use sheet pile and slab culverts, or block abutment structures.

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23 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.

24 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.

25 Available sizes: Type Span Rise Min Max Min Max Con/Span Hy-Span PC Arches (Bebo, Kistner, etc.)

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

27 Arches, too, typically come in a limited range of rises for a given span. For example, a Bebo arch having a span of 29 to 30 must have a rise between 8.5 and If the roadway embankment requires a shorter vertical rise, a shorter span must be used or the embankment raised.

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

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

30 It is vital that designers use the appropriate specification number for 3-sided 3 structures (Sec. 562) to assure proper transport. During construction as well, care must be taken in handling the units. Open-bottomed units, until properly placed and backfilled, are inherently less stable than closed boxes. Swaying or setting them on end can result in damage.

31 Benefits of 3-sided Structures: Ecological health Recreation industry Economic output of NY freshwater fishing: $755M (American Sportfishing Association, 2006 data) Mitigation cost savings Less need for routine maintenance (debris and sediment removal) May ease permitting/regulatory concerns

32 Costs High initial cost Structure Foundation Transport / handling Labor

33 Construction Issues with 3-Sided 3 Structures Excavation often reduces this item Water management may allow simpler dewatering, diversion structures Site operability requires more room for equipment Schedule differences Takes longer than traditional box culverts Much faster (and more easily staged) than bridges

34 Cost Comparison: 3-sided 3 vs. 4-sided4 (span 24 ft x rise 7 ft, same location) Select structure fill: 19% cost reduction Concrete piles and pile driving equip: add l items Footing concrete: add l item Concrete for structures: 15% cost reduction Reinforced concrete span units: 124% cost increase Mobilization: 59% cost increase Total culvert cost increased by 59% or $227K 4-sided: $387K 3-sided: $614K

35 Stream Simulation Sediment size match range in natural channel at surface Channel cross-section section fix with larger stones Channel configuration mimic natural stream, allow room for self-adjustment

36 (Ken Bates, 2006) Stream Simulation Cross Section

37 Stream Simulation Open-bottom structures Bed is created within the assembled culvert

38 Photo Credit: Mendocino Co. DPW

39 Streambed Simulation Open-bottom structures Bed is created within the assembled culvert, or Simulated bed is created between the footings before precast units are placed

40 Photo Credit: US Forest Service

41 Streambed Simulation Open-bottom structures Bed is created within the assembled culvert, or Simulated bed is created between the footings before precast units are placed Embedded 4-sided structures Bed is created within the assembled culvert, or Bed material is allowed to naturally fill the embedded portion up to streambed grade

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

43 Contact Information George Long, PE (Structures-Hydraulics) Brandon Greco (Environmental Science Bur.)