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Hydraulic Design of Energy Dissipators for Culverts and Channels
Hydraulic Engineering Circular Number 14, Third Edition

Chapter 12: Stilling Wells

The design of the US Army Corps of Engineers' stilling well energy dissipator is based on model tests conducted by the US Army Corps of Engineers (USACE, 1963; Grace and Pickering, 1971). It is illustrated in Figure 12.1. The stilling well can be used in channels with moderate to high concentrations of sand or silt and where debris is not a serious problem. The stilling well should not be used in areas where large floating or rolling debris is expected unless suitable debris-control structures are used. The highway uses of stilling wells are at the outfalls of storm drains, median, and pipe down drains where little debris is expected.

Figure 12.1. US Army Corps of Engineers' Stilling Well (USACE, 1963)

Profile view of stilling well defining D sub w as the well diameter, h sub 1 as the distance from the invert of the approach culvert down to the bottom of the well; and h sub 2 as the distance from the invert of the approach culvert up to the top of the well.

The design of the stilling well is initiated after the size and discharge of the incoming pipe are determined. Figure 12.2 is used to select the stilling well diameter, DW. The model tests indicated that satisfactory performance can be maintained for KuQ/D5/2 ratios as large as 10, with stilling well diameters from 1 to 5 times that of the incoming conduits. (Ku is a unit conversion constant equal to 1.811 in SI and 1.0 in CU.) These ratios were used to define the curves shown in Figure 12.2.

The optimum depth of stilling well below the invert of the incoming pipe is determined by entering Figure 12.3 with the slope of the incoming pipe and using the stilling well diameter, DW, previously obtained from Figure 12.2. The height of the stilling well above the invert is fixed at twice the diameter of the incoming pipe, 2D. This dimension results in satisfactory operation and is practical from a cost standpoint; however, if increased, greater efficiency will result.

Tailwater also increases the efficiency of the stilling well. Whenever possible, it should be located in a sump or depressed area.

Riprap or other types of channel protection should be provided around the stilling well outlet and for a distance of at least 3DW downstream.

The outlet may also be covered with a screen or grate for safety. However, the screen or grate should have a clear opening area of at least 75 percent of the total stilling well area and be capable of passing small floating debris such as cans and bottles.

Figure 12.2 (SI). Stilling Well Diameter, DW (USACE, 1963)

Series of curves relating culvert diameter, D (y-axis) to discharge, Q (x-axis). Separate curves are provided for well diameter, D sub w, equal to D, 2D, 3D, and 5D. (SI units).

Figure 12.2 (CU). Stilling Well Diameter, DW (USACE, 1963)

Series of curves relating culvert diameter, D (y-axis) to discharge, Q (x-axis). Separate curves are provided for well diameter, D sub w, equal to D, 2D, 3D, and 5D. (CU units).

Figure 12.3. Depth of Stilling Well Below Invert (USACE, 1963)

Ratio of depth of well below invert (h sub 1) to diameter of stilling well (D sub w) (y-axis) as a function of culvert slope (x-axis).

The design procedure is summarized as follows:

Step 1. Select approach pipe diameter, D, and discharge, Q.

Step 2. Obtain well diameter, DW from Figure 12.2.

Step 3. Calculate the culvert slope. The depth of the well below the culvert invert, h1 is determined from Figure 12.3.

Step 4. The depth of the well above the culvert invert, h2, is equal to 2D as a minimum, but may be greater if the site permits.

Step 5. The total height of the well, hW = h1 + h2.

Design Example: U.S. Army Corps of Engineers' Stilling Well (SI)

Determine the stilling well dimensions. Given:

  • D = 600 mm CMP
  • S = 0.5 V/H
  • Q = 0.424 m3/s

Solution

Step 1. Select approach pipe diameter and discharge.

D = 0.600 m

Q = 0.424 m3/s

Step 2. Obtain well diameter (DW) from Figure 12.2

DW =1.5 D = 1.5 (0.600 m) = 0.90 m

Step 3. Calculate the culvert slope. The depth of the well below the culvert invert is determined from Figure 12.3.

Slope = 0.5

h1 = 0.38 m

Step 4. The depth of the well above the culvert invert is equal to 2D as a minimum, but may be greater if the site permits.

h2 = 2(D) = 2(0.600) = 1.20 m

Step 5. The total height of the well.

hW = h1 + h2 = 0.38 + 1.20 = 1.58 m

Design Example: U.S. Army Corps of Engineers' Stilling Well (CU)

Determine the stilling well dimensions. Given:

  • D = 24 in CMP
  • S = 0.5 V/H
  • Q = 15 ft3/s

Solution

Step 1. Select approach pipe diameter and discharge.

D = 2 ft

Q = 15 ft3/s

Step 2. Obtain well diameter from Figure 12.2

DW =1.5 D = 1.5 (2 ft) = 3 ft

Step 3. Calculate the culvert slope. The depth of the well below the culvert invert is determined from Figure 12.3.

Slope = 0.5

h1/DW = 0.42

h1 = 0.42(3) = 1.26 ft, use h1 = 1.3 ft

Step 4. The depth of the well above the culvert invert is equal to 2D as a minimum, but may be greater if the site permits.

h2 = 2(D) = 2(2 ft) = 4 ft

Step 5. The total height of the well.

hW = h1 + h2 = 1.3 + 4 = 5.3 ft

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Cynthia Nurmi
Resource Center (Atlanta)
404-562-3908
cynthia.nurmi@dot.gov

Updated: 04/07/2011
 

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