Cross-drainage systems: CulvertsGupta, Chapter 14, pp 722-731

Purpose Design objective Design parameters Design Procedure Regime Classification Examples

PURPOSE

Pass natural stream flows or runoff under roadways

Outlet 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 Plant

Crossing below dam at Douthat State Park, Virginia

Culvert Types

box culvert(typ. unsubmerged)

circular culvert(typ. submerged)

Section Views (Looking Downstream)

Design Objective

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

Design Q– Rational method– TR-55 method– USGS Regression

method, pp. 393-394.

drainagearea

road culvert

Design Parameters (Profile)

datum

DH

road deck

headwater/inlet

outlet/tailwater

outlet invert El.intlet invert El.

h4

L, So

Ao=area of culvert barrel; A3=area of section of flow at outletlW=typ. one stream width

1 2

3

4

V1

lw

Qz

Design Procedure

1. Estimate Qdesign for drainage area & design return period.2. Select culvert shape, material and trial size, D and calculate H/D.3. Design for desired culvert flow type (Table 14.5) & select discharge formula.

Unsubmerged flow conditions (e.g. box culverts supporting roadways)a) Calculate dc & dn and classify slope as Mild or Steep

b) Compare dn, dc and tailwater depth h4 and classify flow type: dn <dc; h4<dc : S2 profile, inlet control with dc as control depth, TYPE 1

c) dn >dc; h4 <dc : M2 profile, outlet control with dc as control depth, TYPE 2

d) dn >dc; h4 >dc : M2 profile or M1(h4 > dn), outlet control with h4 as control depth, TYPE 3

Submerged flow conditions (most culverts)a) If culvert submerged at inlet and outlet: outlet control, TYPE 4b) If culvert unsubmerged at outlet use Figs. 14.6 & 14.7

c) Compute 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.d) For concrete pipes, use Fig. 14.6e) For rough pipes, use Fig. 14.7.

f) Plot the point So, L/D.g) If the point plots to the right of the curve it is outlet control, TYPE 5h) If the point plots to the left of the curve it is inlet control, TYPE 6

4. Calculate Qtrial and compare with Qdesign. Iterate until they equal.

Regime Classification: Unsubmerged, 1-3

Table 14.5, p. 724. 34

22

,22.2 R

LVnh ba

Regime Classification: Submerged, 4-6

Table 14.5, p. 725.

Criteria for Types 5 & 6: Submerged (H/D > 1.2) Conditions for Smooth (Concrete) Culverts

Figure 14.6, p. 727.

r = radius of rounding

W = effective bevel

Criteria for Types 5 & 6: Submerged (H/D > 1.2) Conditions for Rough (Corrugated) Culverts

Figure 14.7, p. 728-729.

USDOT Federal Highway Administration Design References & Culvert Design Model

Hydraulic Design of Highway Culverts– http://www.fhwa.dot.gov/bridge/hds5SI.pdf

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

FHWA HY8 Culvert Analysis Computer Program, v6.1– http://www.fhwa.dot.gov/bridge/hyddescr.htm#hy_8_culvert_a

nalysis