Open channel spillway

Open channel spillways are dam spillways that utilize the principles of open-channel flow to convey impounded water in order to prevent dam failure.

Chute spillways carry supercritical flow through the steep slope of an open channel.

There are four main components of a chute spillway:[1] The elements of a spillway are the inlet, the vertical curve section (ogee curve), the steep-sloped channel and the outlet.

In order to avoid a hydraulic jump, the slope of the spillway must be steep enough for the flow to remain supercritical.

Proper spillways help with flood control, prevent erosion at the ends of terraces, outlets, and waterways, reduce runoff over drainage ditch banks and are simple to construct.

However, they can only be constructed at sites with natural drainage and moderate temperature variation and have a shorter life expectancy than other spillways.

Stepped spillways are used to dissipate energy along the chute of the channel.

Roller-compacted concrete (RCC) stepped spillways have become increasingly popular because of their use in rehabilitating aged flood control dams.

[2] Stepped spillways are useful for flood control, increasing dissolved oxygen (DO) levels downstream of a dam, aid wastewater treatment plants for air-water transfer of gases and for volatile organic compound (VOC) removal and reduces the spillway length or eliminates need for stilling basin.

[3] Side channel spillways are typically used to discharge floods perpendicular to the general direction of flow by placing the control weir parallel to the upper portion of the discharge channel.

[4] It offers low flow velocities upstream and minimizes erosion.

However, it can cause a sudden increase in reservoir level if the channel is submerged.

Different agencies have different methods and formulas for quantifying flows and conveyance capacities for chute spillways.

The Natural Resources Conservation Service (NRCS) produced handbooks on dam design.

NEH14 provides the following discharge-head relationship for straight inlets of chute spillways, which is given by the flow equation for a weir: where: If the flow rate per unit width is defined as q = Q/W, then the equation can be written as:[5] The coefficient, 3.1 varies for different entrance conditions.

The value of the coefficient is slightly higher if the conveyance channel has a greater width than the inlet.

The value 3.1 is based on the assumption that He and va are measured at a location that exhibits subcritical flow conditions.

NEH14 also provides the following relationship for side channel inlets: where: The United States Bureau of Reclamation (USBR) also uses the weir formula to quantify flow over a chute spillway.

The USBR flow equation is:[5][6] where: Example: For a spillway crest length/width of 25 ft, Q will vary with H as follows: For the NRCS computations, the mean velocity of approach was assumed to be zero.

For the USBR computations, it was assumed that linear interpolation could be used to obtain C from H. For a given depth at the spillway crest, the flows calculated using the USBR method are higher than those from the NRCS method because of the higher discharge coefficients.

Nappe flow regimes occur for small discharges and flat slopes.

The onset of skimming flow can be defined as: (dc)=1.057*h - 0.465*h2/l Where: For the nappe flow regime, a partially or fully developed hydraulic jump occurs as a result of the jets created between each step.

In the case of a dam spillway, this can be caused by turbulence or vortices in flowing water.

Cavitation occurs within the body of flow of a given distributed roughness.

In the case of chute spillways, cavitation occurs at velocities between 12 and 15 m/s.

Cavitation can be prevented by decreasing the flow velocity or by increasing the boundary pressure.

Plunge pools (also called stilling basins) and impact boxes are two examples of energy dissipators used on dams.

Many USBR dams use energy dissipating blocks for chute spillways (also called baffled aprons).

These blocks help induce a hydraulic jump to establish subcritical flow conditions on the downstream side of the dam.

Discharge coefficient and energy dissipation over stepped spillway under skimming flow regime.