§ 10.4.3.4. USBR Type VI Impact Basin

The U.S. Bureau of Reclamation Type VI impact basin requires no tail water to function. The outflow hits the vertical hanging baffle and provides the necessary energy dissipation.

Figure 10.4.3.4.A - USBR Type VI Impact Basin

(Source FHWA, Hydraulic Design of Energy Dissipators for Culverts and Channels, 2006)

Figure 10.4.3.4.B - Design Curve for USBR Type VI Impact Basin
(Source FHWA, Hydraulic Design of Energy Dissipators for Culverts and Channels, 2006)

Table 10.4.3.4. - USBR Type VI Impact Basin Dimensions (ft)
(Source FHWA, Hydraulic Design of Energy Dissipators for Culverts and Channels, 2006)

W B	h 1	h 2	h 3	h 4	L	L 1	L 2
4	3.08	1.50	0.67	1.67	5.42	2.33	3.08
5	3.83	1.92	0.83	2.08	6.67	2.92	3.83
6	4.58	2.25	1.00	2.50	8.00	3.42	4.58
7	5.42	2.58	1.17	2.92	9.42	4.00	5.42
8	6.17	3.00	1.33	3.33	10.67	4.58	6.17
9	6.92	3.42	1.50	3.75	12.00	5.17	6.92
10	7.58	3.75	1.67	4.17	13.42	5.75	7.58
11	8.42	4.17	1.83	4.58	14.58	6.33	8.42
12	9.17	4.50	2.00	5.00	16.00	6.83	9.17
13	10.17	4.92	2.17	5.42	17.33	7.42	10.17
14	10.75	5.25	2.33	5.83	18.67	8.00	10.75
15	11.50	5.58	2.50	6.25	20.00	8.50	11.50
16	12.25	6.00	2.67	6.67	21.33	9.08	12.25
17	13.00	6.33	2.83	7.08	21.50	9.67	13.00
18	13.75	6.67	3.00	7.50	23.92	10.25	13.75
19	14.58	7.08	3.17	7.92	25.33	10.83	14.58
20	15.33	7.50	3.33	8.33	26.58	11.42	15.33

 

W B	W 1	W 2	t 1	t 2	t 3	t 4	t 5
4	0.33	1.08	0.50	0.50	0.50	0.50	0.25
5	0.42	1.42	0.50	0.50	0.50	0.50	0.25
6	0.50	1.67	0.50	0.50	0.50	0.50	0.25
7	0.50	1.92	0.50	0.50	0.50	0.50	0.25
8	0.58	2.17	0.50	0.58	0.58	0.50	0.25
9	0.67	2.50	0.58	0.58	0.67	0.58	0.25
10	0.75	2.75	0.67	0.67	0.75	0.67	0.25
11	0.83	3.00	0.67	0.75	0.75	0.67	0.33
12	0.92	3.00	0.67	0.83	0.83	0.75	0.33
13	1.00	3.00	0.67	0.92	0.83	0.83	0.33
14	1.08	3.00	0.67	1.00	0.92	0.92	0.42
15	1.17	3.00	0.67	1.00	1.00	1.00	0.42
16	1.25	3.00	0.75	1.00	1.00	1.00	0.50
17	1.33	3.00	0.75	1.08	1.00	1.00	0.50
18	1.33	3.00	0.75	1.08	1.08	1.08	0.58
19	1.42	3.00	0.83	1.17	1.08	1.08	0.58
20	1.50	3.00	0.83	1.17	1.17	1.17	0.67

 

The recommended design procedure for the USBR Type VI impact basin is as follows:

Step 1.  Determine the maximum discharge (Q (cfs)) and velocity (V _o (ft./s) and check against design limits. Calculate the flow area at the end of the approach pipe, A (ft ). Calculate equivalent depth, y _e = (A/2) ^1/2 (ft).

Step 2.  Calculate the Froude number (Fr) and the energy at the end of the pipe (H _o (ft)).

Step 3.  Determine H _o /W _B from Figure 10.4.3.4.B. Calculate the required width of basin (W _B (ft)).
W _B = H _o / (H _o / W _B )

Step 4.  Obtain the remaining dimensions of the USBR Type VI impact basin from Table 10.4.3.4 using W _B obtained from Step 3.

Figure 10.4.3.4.C - Energy Loss of USBR Type VI Impact Basin versus Hydraulic Jump
(Source FHWA, Hydraulic Design of Energy Dissipators for Culverts and Channels, 2006)

Step 5.  Determine exit velocity, V _B = V ₂ , by trial and error using an energy balance between the culvert exit and the basin exit. Determine if this velocity is acceptable and whether or not riprap protection is needed downstream. Use Figure 10.1.3.4.C to determine H _L /H _o percent by using the calculated Fr from Step 2.

H _B = Q/(W _B V _B ) + V _B ^2/ /(2g) = H _o (1- H _L /H _o )

This equation is a cubic equation yielding three (3) solutions, two (2) positive and one (1) negative. The negative solution is discarded. The two positive roots yield a subcritical and supercritical solution. Where low or no tail water exists, the supercritical solution is taken. Where sufficient tail water exists, the subcritical solution is taken.