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The diameter of a certain circular culvert is 42 inches, and its capacity is 86 cubic feet per second (cfs). Determine the length (L) and width (W) of the rip-rap apron for the following conditions:

a. The tailwater depth (TW) is 20 inches.
b. The tailwater depth (TW) is 31 inches.
c. The tailwater depth (TW) is unknown.

Draw a sketch showing all dimensions for each case. Also, determine the median rip-rap diameter (d50).

Answer :

Final answer:

The length and width of the rip-rap apron for the given tailwater depths can be calculated using Manning's equation. However, without the specific values for the Manning's roughness coefficient, slope, and additional information about the flow velocity and desired level of protection, we cannot determine the exact dimensions. The median rip-rap diameter, dso, cannot be determined without the size criteria for the stones in the apron.

Explanation:

To determine the length and width of the rip-rap apron for different tailwater depths, we can use Manning's equation:

Q = (1.486/n) * A * R2/3 * S1/2

Where:

  • Q is the flow rate (86 cfs in this case)
  • n is the Manning's roughness coefficient (depends on the material of the culvert and apron)
  • A is the cross-sectional area of the culvert (π * (D/2)2)
  • R is the hydraulic radius of the culvert (A/P, where P is the wetted perimeter of the culvert)
  • S is the slope of the culvert

For each tailwater depth, we can rearrange the equation to solve for the apron dimensions:

A = (Q * n) / (1.486 * R2/3 * S1/2)

Using the given diameter of the culvert (42 inches), we can calculate the cross-sectional area and hydraulic radius:

A = π * (42/2)2

R = A / P

Now, we can substitute the values into the equation to find the apron dimensions for each tailwater depth:

Tailwater Depth: 20 inches

Substituting the values into the equation:

A = (86 * n) / (1.486 * R2/3 * S1/2)

Calculating the cross-sectional area and hydraulic radius:

A = π * (42/2)2

R = A / P

Substituting the values into the equation:

A = (86 * n) / (1.486 * R2/3 * S1/2)

Calculating the apron dimensions:

L = A / W

Tailwater Depth: 31 inches

Substituting the values into the equation:

A = (86 * n) / (1.486 * R2/3 * S1/2)

Calculating the cross-sectional area and hydraulic radius:

A = π * (42/2)2

R = A / P

Substituting the values into the equation:

A = (86 * n) / (1.486 * R2/3 * S1/2)

Calculating the apron dimensions:

L = A / W

Tailwater Depth: Unknown

Since the tailwater depth is unknown, we cannot calculate the apron dimensions without additional information.

To determine the median, dso, rip-rap diameter, we need to consider the size of the stones in the rip-rap apron. The median diameter is the diameter of the largest stone in the apron, and it is typically determined based on the flow velocity and the desired level of protection against erosion. Without this information, we cannot determine the median rip-rap diameter.

Learn more about calculating the dimensions of a rip-rap apron for a circular culvert here:

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