(A) General. The design of drainage channels can involve highly complex hydraulic analysis techniques and considerations. The existence of transitions, culverts, channel curves, changes in flow regime, etc., can produce hydraulic conditions significantly different from those determined from steady, uniform-flow analyses. This chapter provides certain considerations that are commonly encountered in open channel analysis and design. However the design engineer is referred to the following publications for detailed presentation of these and other procedures and considerations associated with the design of open channel facilities:
Hydraulic Design of Flood Control Channel (1970), U.S. Army Corps of Engineers.
Design Manual - Hydraulic (1982), Los Angeles County Flood Control District.
Design of Small Canal Structures (1978), U.S. Department of the Interior, Bureau of Reclamation.
(B) Channel geometry.
(1) Open drainage channels shall be designed using trapezoidal, rectangular, or compound cross sections, unless prior written approval of an alternate design is provided from the Public Works Engineer. Side slopes for constructed earthen or dumped riprap channels shall be no steeper than 3:1, unless an approved soil analysis demonstrates that steeper side slopes are stable. Side slopes for lined channels may be steeper, depending upon the structural stability of the lining and the underlying soils. Reinforced concrete linings may have vertical side slopes, provided that the design is adequate to prevent failure from hydrostatic or earth pressures. Shotcrete may be placed on side slopes as steep as 1:1, provided this slope is not significantly steeper than the natural angle of repose of the soil. Soil cement lining may be placed on 1:1 side slopes, provided it is of sufficient thickness to be structurally stable. The minimum thickness of soil cement on a 1:1 side slope should be four feet, measured normal to its face. Where soil cement is used as slope paving, with a maximum thickness of one foot, the maximum allowable side slope should be 3:1.
(2) In the case where a channel outside of public right-of-way will be accepted for maintenance by the town, a minimum bottom width of 10 feet is required, and an access road, along at least one side of the channel bank, having a minimum width of 12 feet. Exceptions to the minimum requirements for public drainageways may be permitted if deemed appropriate by the Public Works Engineer. Privately maintained channels have no minimum bottom width, except as dictated by hydraulic considerations.
(C) Flow regime. Flow regime in an open channel can be either subcritical (tranquil) or supercritical (rapid). The Froude number for subcritical flow is less than 1.0 and the Froude number for supercritical flow is greater than 1.0. Critical flow is defined as having a Froude number of 1.0. Flow that is in the proximity of critical depth is generally unstable and excessive wave action or undulations of the water surface may occur. For this reason channel designs should avoid flow regimes that have Froude numbers in the range of 0.86 to 1.13.
(D) Freeboard.
(1) Freeboard is the additional depth required in a channel beyond the depth calculated for conveyance of the design discharge. The purpose of freeboard is to protect against hydraulic disturbances such as waves, unforeseen obstructions to flow, debris and inherent inaccuracies in assumptions and analyses techniques. Following are the minimum freeboard requirements for open channels, with a minimum freeboard of one foot for design depths of flow of three feet or more:
Subcritical Flow (i.e. Froude number < 0.86): Minimum Freeboard = 1.0 feet
Supercritical Flow (i.e. Froude number > 0.86): Minimum freeboard calculated from Equation 6.3. If the calculated minimum freeboard is less than one foot and the flow depth is three feet or greater the minimum freeboard shall be one foot.
FB = 1/6(y + v2/2g) (Eqn. 6.3)
Where:
FB = Freeboard, in feet
y = Maximum depth of flow, in feet
v = Average velocity of flow, in feet per second
g = Gravitational constant = 32.2 ft./sec
(2) The freeboard requirements described above are for uniform channel reaches where no unusual flow disturbances are anticipated. Additional freeboard is required at channel bends and junctions, where backwater effects or superelevation may occur, or where hydraulic jumps may occur. The engineer should consult the references provided at the beginning of this chapter for computing hydraulic conditions at such locations. The lining of protected channels shall extend to a height necessary to include the freeboard requirement, unless approval to the contrary is granted in writing from the Public Works Engineer.
(E) Hydraulic jump.
(1) A hydraulic jump occurs when flow changes from supercritical flow to subcritical flow. Hydraulic jumps can occur:
(a) When the slope of the channel changes from steep to mild;
(b) At sudden expansions or contractions in the channel section;
(c) At culverts or bridges in steep channels;
(d) At the downstream side of dip crossings;
(e) At channel junctions; and,
(f) Sharp bends.
(2) Hydraulic jumps are useful in dissipating energy, and consequently they are often purposely forced to occur at drainageway outlet structures in order to minimize hydraulic forces and erosion. However, because of the large amount of energy dissipated in hydraulic jumps, it is not advisable to allow them to occur except under controlled circumstances. Therefore, if during the design of a channel, a hydraulic jump is expected to occur, computations shall be made to determine the height, length and other characteristics of the jump. In addition, steps shall be taken to either eliminate the jump or contain it, in order to prevent damage to the channel or surrounding property.
(3) Procedures for analyzing the hydraulic jump are well documented in the references cited at the beginning of this chapter as well as in numerous other easily available hydraulic texts and manuals.
(F) Curved channels.
(1) Flow in a curved channel will create centrifugal forces that will cause a rise in the water surface along the outside of a bend. At the same time, a corresponding depression will be created in the water surface along the inside of the bend. In addition, spiral secondary currents tend to form within the bends. These currents can cause scour to occur along the outside of a bend, and deposition along the inside of a bend. Cross-channel waves that propagate downstream will also form, if the flow in the channel is supercritical.
(2) Although curves are inevitable in the design of most open channels, they should be minimized in order to avoid the special problems associated with their design. The design of channel bends must include considerations for superelevation, limiting curvature, bend scour, and special design curves.
(G) Transitions.
(1) Transition sections designed to collect and/or discharge flow between the natural floodplain and constructed channels can be located at either the upstream or downstream ends of constructed channels. They can also be located along a segment or segments of a constructed channel itself. In either case, it is necessary to design the flow transition to minimize the disturbance of flow. In the case where flow in a constructed channel is being transitioned back to the natural floodplain, sufficient distance must be allowed for the flow to adequately expand to the original width of the natural floodplain.
(2) Procedures for analyzing curved channels and transitions are well documented in the references cited at the beginning of this chapter as well as in other easily available hydraulic texts and manuals.
(Res. 1637, passed 2-28-02)