§ 7.4.3. Example 7-1

The following example problem utilizes computation Table 7-2 for a culvert rating curve calculation.

Given:

Culvert size	= 48 inches RCP
Length L	= 110 feet
n value	= 0.012
Inlet elevation	= 720.0 feet
Outlet elevation	= 718.8 feet
Slope S 0	= 0.010
Entrance condition (square edge), K e	= 0.50
Maximum elevation for embankment	= 732.0 ft.

 

Find: Culvert rating curve

Table 7-2 is used to make the computations for the culvert design.

Step 1. List the elevations for headwater depths in Column 1. Then put headwater depth and ratio of headwater depth to culvert height (or pipe diameter) in Column 2 and Column 3.

Step 2. Based on the inlet control conditions, the ratio of HW/D is used to find the flows (Q) which are put in Column 4. In this example, the FHWA nomograph for a circular pipe under inlet control conditions is utilized.

Step 3. For outlet control conditions, the flow rate Q in Column 4 is used to determine the headloss (H) in Column 5. In this example, the FHWA nomograph for a circular RCP under outlet control conditions is utilized.

Step 4. If the tailwater rating curve is available, the tailwater (TW) depth can be entered in Column 6. If the tailwater rating curve is not available, an estimate of the tailwater can be used.

Step 5. If the tailwater depth is less than the diameter of the culvert, Column 7 and Column 8 should be calculated. If TW is larger than D, the TW value is entered in Column 9 for ho.

Step 6. The critical depth (dc) is found from the FHWA nomographs for critical depth, and then used to compile Column 8.

Step 7. The headwater depth(HW) now can be computed from the following equation:

HW = H +ho - Lso

Where,

HW = vertical distance from flow line at the entrance to the pool surface, feet

H = headloss, feet (use appropriate nomograph)

ho = vertical distance from flow line at the outlet to the hydraulic grade line, feet

So = slope of barrel, ft/ft

L = culvert length, feet

Step 8. Compare the two (2) headwater depth values from Column 2 and Column 10. The controlling headwater depth and type of control are recorded in Column 11 and Column 12, respectively. The calculated elevation is written in Column 14.

Step 9. The rating curve for the culvert can be plotted from the values in Column 4 and Column 13.

To size a culvert crossing, the same table can be used, with some variation in the basic data. First a design Q value is selected and the maximum allowable headwater is determined. An inlet type i.e., headwall) is selected and the invert elevations and culvert slope are estimated based upon site constraints. A culvert type is then selected and first rated for inlet control then outlet control. If the controlling headwater exceeds the maximum allowable headwater, the input data is modified and the procedure repeated until the desired results are achieved.