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
Design Objective Design culvert (D,S o ) to pass flood of given return period (10 or 100- year event). Design Q – Rational method – TR-55 method – USGS Regression method, pp drainage area roadculvert
Design Parameters (Profile) datum D H road deck headwater/inlet outlet/tailwater outlet invert El. intlet invert El. h4h4 L, S o A o =area of culvert barrel; A 3 =area of section of flow at outlet l W =typ. one stream width V1V1 lwlw Q z
Design Procedure 1. Estimate Q design 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 d c & d n and classify slope as Mild or Steep b) Compare d n, d c and tailwater depth h 4 and classify flow type: d n dc; h 4 dc; h 4 >d c : M2 profile or M1(h 4 > d n ), 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 4 b) If culvert unsubmerged at outlet use Figs & 14.7 c) Compute ratios L/D, r/D or w/D, So (and 29n 2 H/R o 4/3 for rough pipes), where r is the radius of rounding and w is the effective bevel. d) For concrete pipes, use Fig e) For rough pipes, use Fig f) Plot the point S o, L/D. g) If the point plots to the right of the curve it is outlet control, TYPE 5 h) If the point plots to the left of the curve it is inlet control, TYPE 6 4. Calculate Q trial and compare with Q design. Iterate until they equal.
Regime Classification: Unsubmerged, 1-3 Table 14.5, p. 724.
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 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
USDOT Federal Highway Administration Design References & Culvert Design Model Hydraulic Design of Highway Culverts – Hydraulic Charts for the Selection of Highway Culverts – FHWA HY8 Culvert Analysis Computer Program, v6.1 – nalysis nalysis