Cross-drainage systems: CulvertsGupta, Chapter 14, pp 722-731PurposeDesign objectiveDesign parametersDesign ProcedureRegime ClassificationExamples

PURPOSEPass natural stream flows or runoff under roadwaysOutlet for detention basins

Box culverts will protect streams crossed by the new road from Route 50. Photo courtesy of George Golden, Smithsonian Office of Physical PlantCrossing below dam at Douthat State Park, Virginia

Culvert Types

box culvert(typ. unsubmerged)circular culvert(typ. submerged)

Section Views (Looking Downstream)

Design ObjectiveDesign culvert (D,So) to pass flood of given return period (10 or 100-year event).Design Q

Rational methodTR-55 methodUSGS Regression method, pp. 393-394.drainagearearoadculvert

Design Parameters (Profile)datumDHroad deckheadwater/inletoutlet/tailwateroutlet invert El.intlet invert El.h4L, SoAo=area of culvert barrel; A3=area of section of flow at outletlW=typ. one stream width1234

V1

lwQz

Design ProcedureEstimate Qdesign for drainage area & design return period.Select culvert shape, material and trial size, D and calculate H/D.Design for desired culvert flow type (Table 14.5) & select discharge formula.Unsubmerged flow conditions (e.g. box culverts supporting roadways)Calculate dc & dn and classify slope as Mild or SteepCompare dn, dc and tailwater depth h4 and classify flow type: dn dc : M2 profile or M1(h4 > dn), outlet control with h4 as control depth, TYPE 3Submerged flow conditions (most culverts)If culvert submerged at inlet and outlet: outlet control, TYPE 4If culvert unsubmerged at outlet use Figs. 14.6 & 14.7Compute ratios L/D, r/D or w/D, So (and 29n2H/Ro4/3 for rough pipes), where r is the radius of rounding and w is the effective bevel.For concrete pipes, use Fig. 14.6For rough pipes, use Fig. 14.7.Plot the point So, L/D.If the point plots to the right of the curve it is outlet control, TYPE 5If the point plots to the left of the curve it is inlet control, TYPE 6Calculate Qtrial and compare with Qdesign. Iterate until they equal.

Regime Classification: Unsubmerged, 1-3Table 14.5, p. 724.

Regime Classification: Submerged, 4-6Table 14.5, p. 725.

Criteria for Types 5 & 6: Submerged (H/D > 1.2) Conditions for Smooth (Concrete) CulvertsFigure 14.6, p. 727.r = radius of roundingW = effective bevel

Criteria for Types 5 & 6: Submerged (H/D > 1.2) Conditions for Rough (Corrugated) CulvertsFigure 14.7, p. 728-729.

USDOT Federal Highway Administration Design References & Culvert Design ModelHydraulic Design of Highway Culvertshttp://www.fhwa.dot.gov/bridge/hds5SI.pdf

Hydraulic Charts for the Selection of Highway Culvertshttp://www.fhwa.dot.gov/bridge/hec05.pdf

FHWA HY8 Culvert Analysis Computer Program, v6.1http://www.fhwa.dot.gov/bridge/hyddescr.htm#hy_8_culvert_analysis