Seventeenth Statewide Conference on Local Bridges Culvert Design An Overview of the NYS Highway Design Manual Chapter 8 Tuesday, October 25, 2011 Training Session: Culvert Design, Analysis - talk 2 Presented by: Peter Van Kampen, P.E. NYSDOT Peter Van Kampen, P.E. NYSDOT Design Services Bureau Outline Culvert Basics Culvert Basics Site Considerations Design Criteria Conventional Culvert Selection & Design Culvert Protection/Channel Linings Plans Culvert Failures Resources Questions
Culvert Chapter 8 HDM Types of Culverts - Shapes A culvert is usually a closed conduit (e.g., a pipe), but may be an open conduit (e.g., an arch), installed to convey runoff collected in roadside channels, and natural channels such as streams, underneath an embankment. Any single structure with a span greater than 20 ft (6.1 m) is a bridge. These structures require a different procedure for hydraulic analysis, and should be coordinated through the Regional Structures Group * Source - HDS 5 Culvert Materials Site Considerations Metal Steel (CSP) Structural plate, Aluminum, (CAP) Ductile iron (DIP) Concrete RCP (Class I Class V) Plastic Polyethylene SICPP Polyvinal chloride - PVC
What to Identify Stream Width. Size/Location of Up/Down Stream crossings. Stream Scour? Bed Material. Fish Bearing? Embankment Height. Photos to take View from roadway upstream and down stream. Inlet and outlet. Wingwalls and/or banks around inlet/outlet. Interior if possible. Any scour or erosion noted. Scaled reference of bed material. Design Criteria Or What do need to know before I select a culvert?
Design Size Criteria Constraints Design Storm Frequencies for Culverts and Channels Design Storm Frequency (Per HDM) Design Headwater (HDM & ACOE/DEC)* Outlet Velocity (Natural Velocity - Scour)* Aquatic Organisms (ACOE/DEC/Fish Wildlife) * Constraints defined/evaluated by designer HDM Chapter 8 page 8-21 Table 8.2 Roughness Coefficients n Conventional Culvert Design Reference HEC-22 Reference HDM, HEC-22, & HDS 5
Conventional Culvert Design Culvert Terminology Culvert defined as structure with a span less than or equal to 20 feet (6.1 m) Conventional culvert is a simple culvert placed on grade with standard headwall, end section inlet (improved inlets not covered here) Common shapes are box, circular, arch, and elliptical. Common materials are concrete, plastic, steel and aluminum. Hw Headwater EL Hd Headwater Elev. Inlet Invert Cut-off wall Roadway Embankment Outlet Invert H Head Loss Tailwater Elev. Tw Tailwater Cut-off wall Culvert Design Process 1. Calculate Q for design storm. 2. Select pipe material and shape for site. 3. Calculate tailwater depth 4. Calculate maximum allowable headwater 5. Size culvert 6. Check outlet velocity 7. Design outlet protection Calculate Q (flow) TR-20 / TR-55 HydroCAD Regression Equations StreamStats in NY http://water.usgs.gov/osw/streamstats/new _york.html Check your answers with historical sources if able.
Material Considerations Design life/service life (HDM Chapter 8.6.2.1) Stream conditions (Debris, Rocks, Ice movement) Chemical properties of project location (alkalinity/acidic nature of the surrounding soils, see HDM chapter 8.6.2.2) Height of fills Environmental considerations (fish passage, mussels) Economics Calculate Tailwater Depth Use calculated flow rate from hydrologic analysis and basic open channel flow calculation for downstream conditions and linings Consider affect of major rivers & streams downstream that could influence tailwater. Determine if downstream culvert/bridge size limitations exist Document non-conforming features. Maximum Allowable Headwater Determination (Hw) 2 ft. below lowest shoulder edge. No damage to upland properties. No increase to water surface elevation than that allowed by floodplain regs (generally 1 ft). Headwater to pipe ratio: Diameter or Rise Maximum H w /D Ratio < 5 ft. 1.5 5 ft. 1.0 Culvert Sizing Minimum Culvert Size Minimum culvert size is 2 ft. Smaller diameters acceptable for shallow depths and utility conflicts 1 ft. is acceptable for driveway pipes and field entrances. Meets allowable Headwater criteria.
Culvert Sizing Determine Headwater Elevation Headwater is controlled by Inlet or Outlet control of the culvert. Inlet and outlet control influenced by several factors Design must be checked for inlet & outlet conditions to determine controlling feature. Culvert Sizing Inlet vs Outlet Control Inlet Control - Culvert is in inlet control when culvert is capable of delivering more flow than inlet will allow. Typically in supercritical flow. Outlet Control Culvert is in outlet control when the barrel losses are greater than the inlet loss. Typically in sub-critical flow. Culvert Sizing Inlet vs Outlet Control Culvert Sizing Inlet Control Factors Headwater depth determines pressure flow In general, we utilize calculations to determine this depth given other criteria Inlet Area determined from the culvert size selected. Inlet Shape Determined from culvert chosen
Culvert Sizing Inlet Control Factors Inlet edge configuration - The major factor in culvert performance Thin Edge Projecting Ke = 0.9 (poor efficiency) Thick walled inlet acts similar to square edge headwall & typical end sections (typical installation) Ke = 0.5 Grooved end projecting/ box with bevel (highest efficiency) Ke = 0.2 Bell end of projecting pipe upstream changes Ke from 0.7 to 0.2 See Table 223 of HDS #5 for additonal Ke factor values Find Ke value for given end treatments. 0.9 0.5 square 0.2 beveled Culvert Sizing Inlet Control Factors Beveling the inlet headwall is a cheap and effective way to improve hydraulic flow through the culvert 0.5 or 0.2 0.7
Beveled Headwall Sizing Culverts HDS-5 http://www.fhwa.dot.gov/engineering/hydraulics/ library_arc.cfm?pub_number=7&id=13 HY-8 http://www.fhwa.dot.gov/engineering/hydraulics/ software/hy8/ Culvert Sizing Outlet Control Factors Inlet or Outlet Control? All inlet control factors apply. Headwater will be controlled by barrel properties. Outlet Barrel shape, length, area, slope and roughness influence flow. Tailwater elevation determined by open channel flow calculations for downstream conditions. Inlet
Flow Types (HDM Figure 8-5) Inlet Control Flow Types (HDM Figure 8-6) Outlet Control Outlet protection for stream and culvert Check Velocity Design outlet protection (stone fill, energy dissipater) Outlet Velocity Calculations Inlet Control Determine normal depth and velocity of culvert barrel [Manning s Equation V = (R.67 S 0.5 )/n ] Culvert outlet velocity is assumed to be the same as the culvert barrel. Outlet Control Determine area of flow from the following (V=Q/A): TW < Dc Use Dc D>TW<Dc Use TW TW > D Use D
Outlet Velocity Calculations Outlet Velocity Calculations Visual URBAN program https://mctrans.ce.ufl.edu/store/shopcart1.asp Or use output from HY-8 Outlet Protection & Linings Cut Off Walls for End Sections Standard Sheet 603-04 Prevents Scour Prevents Erosion
Bank and Channel Linings - Geotechnical Procedure GDP-10 Dumped stone Inlet Apron Geotechnical Procedure GDP-10 June 1995 SECOND EDITION GDP-10 Provides guidance on the selection of protective linings for stream banks and channels Apron sized by diameter of culvert Design to minimum stone blanket thickness or greater. STATE OF NEW YORK DEPARTMENT OF TRANSPORTATION Concrete and asphalt linings are not recommended. Extend height to calculated headwater depth. Culvert Outlet Protection Dumped Stone Outlet Protection Design
Culvert Outlet Protection GDP-10 Channel Lining Design Charts Culvert Outlet & Lining Protection Multiple Culvert Installation Standard Sheets 203-5 & 204-1 DEC NYS Standards and Specifications for Erosion and Sediment Control. (AKA The Blue Book ) also contains information for outlet and channel protection. Standard Installation Std. Sheet 203-5 Installation with CLSM Std. Sheet 204-1 See Section 5, Pg. 5B.21
Safety & Protection Culvert End Sections Safety Culvert End Safety Grates NYSDOT end sections not hydraulically efficient Ke=0.7 STRUCTURAL LIMITATIONS Product Limitations Precast Concrete Span Rise* Structures Bottomless (3-sided) Min Max Min Max Conspan(arch) 12ft 48ft 3ft 13ft Hyspan (flat top) 6 ft 40 ft 2 ft 10 ft PC Arches (Bebo, 11 ft 60 ft 3.5 ft 22 ft Kistner, etc.) Box culvert (4-sided) 2 ft 20 ft 2 ft 10 ft STRUCTURAL LIMITATIONS 3 SIDED STRUCTURES Need scour protection Need piles driven to rock Piles increase construction time $$ Typically more expensive $$ *Minimum rises are limited by NYSDOT practice that designs have span-to-rise ratios of 4 or less. A larger ratio (greater span in relation to rise) increases moments in the top slab and foundation loads to the point where they are impractical/uneconomical to build. Maximum rise is limited by shipping constraints for Conspan, Hyspan and Box Culverts. Listed above are some of the more common manufacturers of precast 3 sided structures.
STRUCTURAL LIMITATIONS Span to Rise Ratio Longer span may increase rise Raise the highway profile Widen toe of slope $$ Increase culvert length $$ May also impact a greater amount of stream bed and any adjacent wetlands STRUCTURAL LIMITATIONS Fill Height Arched and round culverts handle large fills better As cover nears 10 ft and span approaches 20 ft, top slabs of box structures becomes excessively thick and impractical. Arch types should be used for fills over 10 ft. For other material fill limitations see HDM chapter 8, Appendix A PLAN SET Typical Sections (Culvert & Highway) Plan View Highway Profile (Existing & Proposed) Stream Profile Hydraulic Data Culvert and Wing Wall Elevations Staging Plan (if necessary) Guiderail Plans / Details
Culvert Failures Seepage (Piping) Undermining Buoyancy Overtopping Blockage Materials Culvert Failures Seepage Caused from flow of water on exterior of pipe. Removes material, eventually leading to failure. Common on multiple pipe installations due to poor compaction between pipes. Prevention: Cut-off walls at inlet Proper compaction of material along barrel Proper materials used for backfill (not crushed stone) Anti-seep collars Culvert Failures Undermining - Caused by scour at the outlet, removing material upstream. Prevention: Use cut-off wall at outlet Proper outlet protection
Culvert Failures Buoyancy Caused by high water tables, with air being trapped in barrel of culvert. Typically causes lifting of culvert resulting in seepage. Common with CSP and Plastic pipes. Prevention: Proper barrel material selection. Design of culvert to prevent air entrapment in barrel Culvert Failures Overtopping Caused by water going over feature, eroding material from both upstream and downstream. Can be caused by inlet blockage. Prevention: Proper design of culvert for storm event. Review of watershed for debris potential. Culvert Failures Blockage Primarily debris accumulation on inlet, constricting flow and causing overtopping to occur. Can also be caused by end sections not being anchored. Culvert Failures Materials Caused by soil conditions reacting with pipe materials. Resulting in seepage and undermining. Prevention: Cut-off walls on end sections Review of watershed for debris potential Use of debris deflectors Prevention: Proper selection of culvert materials for project site.
Resources Highway Design Manual, Chapter 8 Open Channel Flow, FHWA Hydraulic Design Series No. 3 (HDS-3) Open Channel Flow, FHWA Hydraulic Engineering Circular 22 (HEC 22) Culvert Design, FHWA Hydraulic Design Series No. 5 (HDS-5) Culvert Design, NHI Course 135056 Questions? Peter Van Kampen MO Design Albany, NY e-mail: pvankampen@dot.state.ny.us