§ 6.4.2. Concrete-Lined Channels

Concrete-lined channels may be needed in channel reaches where the velocities are excessive (See Section 6.4.1.A) or where the channel characteristics require such use.

A.

Velocity. In concrete-lined channels the probability of achieving supercritical flow is greatly increased. The designer must take care to insure against the possibility of unanticipated hydraulic jumps forming in the channel in considering the 25 and 100 year storms. Flow with a Froude number equal to one (1) is unstable and should be avoided. If supercritical flow does occur, then freeboard and superelevation must be determined. In addition, all channels carrying supercritical flow shall be continuously lined with reinforced concrete.

B.

Roughness Coefficient. Table 6-2 provides the Manning's Coefficients for concrete-lined channels.

C.

Freeboard. Adequate channel freeboard shall be provided for the 100 year storm in reaches where supercritical flow occurs by Equation 6-3 or using the energy grade line, whichever is less.

H _FB = 2.0 + 0.025V (d) ^1/3 (Eq. 6-3)

Where,

H _FB = Freeboard height, ft

V = Velocity, ft/sec

d = Depth of flow, ft

Freeboard shall be in addition to superelevation, standing waves and/or other water surface disturbances. Concrete sideslopes shall be extended to provide freeboard. Freeboard shall not be obtained by the construction of levees.

D.

Superelevation. Superelevation of the water surface shall be determined at all horizontal curves which deviate more than 45 degrees off the projected centerline. An approximation of the superelevation at a channel bend can be obtained from the following equation:

h = V T _w /gr _c (Eq. 6-4)

Where,

h = Superelevation, ft

V = Flow velocity, ft/sec

T _w = Top width of channel, ft

R _c = Centerline radius of curvature, ft

g = Acceleration due to gravity, ft/sec

The freeboard shall be measured above the superelevation water surface.

E.

Side Slopes. Since concrete lined channels do not require slope maintenance, the side slopes may be as steep as vertical with appropriate structural methods applied.

F.

Slope. The flow line slope of the channel shall be no less than one-half (0.5) percent and must also be sufficient to produce a velocity for the two (2) year storm flow of at least two (2) feet per second. Compliance with this requirement must take into account the variation in channel flow due to distributed inflows to the channel.