This section of the chapter on urban drainage provides guidance on the design of open drains and channels. These features represent one of the minor system components of urban drainage infrastructure.
There are three key types of channels that are typical utilized (based on FHWA, 2005):
Channels with rigid linings, including:
- Concrete
- Concrete blocks - Masonry blocks
- Partially grouted rip rap
Channels with flexible linings, including:
- Vegetative (e.g. grass) - Rock rip rap / Dry boulder
- Wire enclosed rock (mattresses and gabions)
- Turf reinforcement matting (TRM) and reinforced grass - Coconets with vegetation
Composite channels, where a combination of the above are used.
Table 6-2 Channel Types and Examples
Channel Type Comments Examples
Channel with Rigid Lining
Rigid linings are useful in flow zones where high shear stress or non-uniform flow conditions exist, such as at transitions in channel shape or at an energy dissipation.
Generally higher initial costs but lower maintenance.
Susceptibility to failure from foundation instability and undermining.
Example Concrete lined channel
Channel with Flexible Lining
Best suited in areas of uniform flow and low shear stress.
Lower initial costs but higher maintenance costs
Higher roughness and therefore greater area required for same capacity.
Used in channels with intermittent flows.
Example of dry boulder channel (source FHWA, 2005)
Channel Type Comments Examples
Turf reinforcement matting lined channel, prior to vegetation (source FHWA, 2005)
Grass lined swale (source MSMA, 2012)
Composite Channels
Utilise a combination of rigid lining and flexible lining. Rigid lining is typically used in the bottom of the channel where higher velocities occur.
Example Composite Channel – Concrete lined base and grass banks
6.3.1 Flow Capacity
The flow capacity of a channel can be estimated by the procedures outlined in Section 4.5.
Mannings ‘n’ roughness values generally decrease with increasing depth. For shallow flow conditions (where the hydraulic radius is less than 1 or where the height of the roughness is one tenth of the flow depth or more), it is necessary to
adjust the Mannings ‘n’ value. For the types of flows dealt with in drainage design, this is generally only required for flexible channels.
A procedure is identified in the following sections for rock lined and grass channels. For other types of channel, it is recommended to refer to the manufacturer’s specifications.
6.3.1.1 Rock Lined Channels
For rock lined channels, Manning’s ‘n’ may be calculated using either Table 6-3 or Equation 6-1 (based on QUDM, 2013).
Table 6-3 Manning’s Roughness of Rock Lined Channels with Shallow Flow
d50/d90 = 0.5 d50/d90 = 0.8
d50 (mm) 200 300 400 500 200 300 400 500
Hydraulic
Radius (m) Mannings Roughness (n) Mannings Roughness (n)
0.2 0.1 0.14 0.17 0.21 0.06 0.08 0.09 0.11
R = hydraulic radius of the flow over the rocks = Area/Perimeter (m) d50 = mean rock size for which 50% of the rocks are smaller (m) d90 = mean rock size for which 90% of the rocks are smaller (m) In ‘natural’ gravel-based streams the factor d50/d90 is typically in the range 0.2 to 0.5, while in constructed channels where imported graded rock is used, the ratio is more likely to be in the range 0.5 to 0.8 (QUDM, 2013).
6.3.1.2 Grass Lined Channels
A detailed methodology for determining the Manning’s ‘n’ value based on different vegetation types and flow depths is provided in Design of Roadside Channels with Flexible Linings (FHWA, 2005).
Table 6-4 Manning’s Roughness for Grassed Channels (50–150 mm blade length)*
* Manning’s values determined from vegetation retardance Chart-D (refer DTMR, 2010). Values are presented to three significant figures for convenience. This should not imply the values are accurate to three significant figures. A Manning’s roughness of 0.03 is adopted for hydraulic radius greater than 1.2 m in accordance with recommendations of original research; however, this may not always be appropriate.
Further information is available in DTMR (2010) and FHWA (2005).
Source: QUDM, 2013
6.3.2 Permissible Velocities and Channel Types
Channels should be capable of carrying the design discharge at velocities which do not result in excessive scouring or erosion. Indicative permissible velocities for different channels are provided in Table 6-5. Permissible velocities for rigid and vegetated channels are discussed in the subsequent sections
Table 6-5 Permissible Velocities for Different Channel Linings Soil Type
Source: FHWA HI-90-016 Table 3.5.2
6.3.2.1 Channels with Rigid Linings
The maximum average flow velocity of 4 – 5 m/s is recommended for hard lined channels.
Other considerations in the design of channels with rigid linings, such as concrete, include:
Contraction and expansion joints to minimize the risk of cracking and seepage and potential undermining. Note that if a hydraulic jump is intended to move over a joint, then additional joint reinforcing may be required.
Pressure relief weep holes in impermeable linings both within the channel invert and within the channel side slopes. The extent and density of pressure relief weep holes should be sufficient to prevent hydraulic uplift of the channel.
Lateral protection against surface flows undermining the side slopes. A minimum hard faced strip of width 0.5 m on both sides at the top of the channel is recommended.
Vertical cut-off walls should be included at the upstream and downstream extents of a lined channel. These cut-off walls should be provided along the channel invert and up the channel side slopes. The required depth of cut-off walls is dependent on a number of factors including channel flow rate, flow velocity, and type of natural material upstream and downstream of the lined section. A minimum depth of 0.6 m should be adopted.
Designers should ensure that scour beyond the downstream end of lined channels is prevented, or at least reduced to an acceptable level. To avoid the scour problems, it is desirable to pass the discharging water over a roughened surface before releasing it into a vegetated channel. This is normally achieved by placing a rock scour pad at the exit of the smooth-bed channel.
6.3.2.2 Vegetated Channels
For channels with flexible linings, there are generally two approaches:
Permissible velocity approach
Permissible shear stress approach
Both approaches have been adopted around the world. As identified in FHWA (2005), the permissible shear stress approach better reflects the physical processes that are occurring and is constant over a wide range of channel shapes and slopes. However, the application of the permissible velocity is generally more straightforward to apply and is detailed in this guideline.
Details on the permissible shear stress approach are provided in Design of Roadside Channels with Flexible Linings (FHWA, 2005).
A key concern for vegetated channels is what happens when the grass cover cannot be maintained, such as during drought, after fire etc. This aspect should be considered and if there is a reasonable risk of occurrence and channel scour is likely / not desirable, then design should be undertaken assuming bare-earth design values (DTMR, 2010).
In designing vegetated / bare-earth channels the following must be considered (DTMR, 2010):
The material the channel is to be constructed in
A suitable grass species for the channel (where applicable)
An appropriate Manning’s n-value
A suitable grass species for a channel should (DTMR, 2010):
Be quick to establish
Be able to self-repair
Have a relatively short blade length (< 50 mm). Longer blade lengths can increase flow resistance and subsequently result in a reduction in capacity of the channel
Be able to survive short durations of inundation
Be able to withstand proposed design velocities, and
Be native to the area
Indicative permissible velocities for vegetated channels are provided in Table 6-6.
In using Table 6-6, it is important that a good cover of grass be maintained, designers should assess the percentage of stable vegetal cover likely to persist under design flow conditions.
Table 6-6 assumes a consolidated surface, rather than a cultivated surface.
Table 6-6 Permissible Velocities
Vegetation Channel bed slope (%) Stable soils Erodible soils
Bermuda Grass 0-5 2.40 1.80
5-10 2.10 1.50
>10 1.80 1.20
Buffalo Grass, Kentucky blue grass, Smooth
brome, Blue grama 0-5 2.10 1.50
5-10 1.80 1.20
>10 1.50 0.90
Grass Mixture 0-5 1.50 1.20
5-10 1.20 0.90
Not suitable for slopes steeper than 10%
Weeping lovegrass, Alfalfa, Crabgrass 0-5 1.10 0.80
Not suitable for slope steeper than 5%
Sudan Grass & Annual Grasses 0-5 1.10 0.80
Not suitable for slopes steeper than 5%
Source: Manual of Surface Drainage Engineering by B.Z. Kinori, 1970
6.3.2.3 Reinforced Grass & Turf Reinforcement Matting
Turf reinforcement matting (TRM) and reinforced grass (using products such as coconet and numerous proprietary polypropylene products) provides additional protection from erosive forces. The concept of turf reinforcement is to provide a
structure to the soil/vegetation matrix that will both assist in the establishment of vegetation and provide support to mature vegetation (FHWA, 2005).
There are many products on the market, and the designer should refer to the manufacturer specifications to determine operating flow regimes and velocities that are acceptable, as well as guidance on installation. As an indication, reinforced grasses may have a permissible velocity in the order of 4 m/s, but this should be confirmed by manufacturer specifications.
The performance of TRM is subject to vegetation cover, and therefore is subject to some of the key considerations identified in Section 6.3.2.2.
Figure 6-1 Turf Reinforcement Matting Profile
Source: FHWA, 2005
6.3.2.4 Riprap or Dry Boulder Channels
Rock lined channels, or rip rap, is a conventional treatment for channels to provide erosion resistance. Typically, the hydraulics of the channel is determined, and then an appropriate rock size is adopted. Some iteration may be required, as the rock size will affect the Manning’s ‘n’ value adopted (refer to Section 6.3.1).
When designing and constructing a rock lined channel, the specification for riprap as identified in Section 5.9 should be adopted.
For mild channel slopes (less than 5%), angular rock and a specific gravity of 2.6, the following simplified equation can be adopted to determine an appropriate rock size (QUDM, 2013):
Equation 6-2
𝑑𝑑50 = 0.04𝑉𝑉2
where:
d50 = mean rock size for which 50% of the rocks are smaller (m) V = average cross sectional velocity (m/s)
A more refined version of the equation is provided below, which allows for different types of rock and flow conditions:
Equation 6-3
K = 1.1 for low-turbulent deepwater flow, 1.0 for low-turbulent shallow water flow, and 0.86 for highly turbulent flow
6.3.2.5 Rock Filled Wire Mattress or Gabion Box or Mattress
Rock filled wire mattresses or gabions may also be used to line the channel bank or bed. Smaller sized rocks can be used because the wire basket surrounding the rock in the mattress or gabion tends to make the mass act as a unit while retaining flexibility.
Some specific design considerations include:
The potential for damage to the baskets from debris.
Deterioration of the wire baskets due to pollution or saline environments. This can result in a reduction in the design life and require more frequent maintenance. Plastic coated wire can provide some benefits.
The establishment of vegetation over the baskets can limit some of the issues identified above.
Maintenance of the wire baskets needs to be considered, particularly in regards to access. Maintenance needs to be incorporated into future maintenance programs to ensure that they are checked and repaired as necessary.
Design and construction of gabion protection should be in accordance with manufacturer’s specifications and shall be consistent with the latest Standard Specifications.
6.3.3 Side Slopes
Recommended side slopes for design of different types of channels are provided in Table 6-7. In specifying a side slope, consideration should also be made for maintenance and safety.
For channels adjacent to highways, the following should apply:
Generally not more than 1V:5H, for traffic safety
Where the above cannot be achieved, or the depth is greater than 3 m, then a safety barrier is required
Table 6-7 Recommended Side Slope Material
Stream Bank Materials Side slope (V:H)
Rigid Lined Channels nearly vertical
Grass Lined Channels Not steeper than 1:4, generally aiming for 1:6 to assist in maintenance and for public safety
Rock (Dry Boulder Rip Rap) lined
channel 1:3
Gabion Mattress Refer to manufacturer specifications.
Reinforced Grass/ TRM Refer to manufacturer specifications. Consideration should be given for maintenance access as per grass lined channels, and therefore 1:6 would generally be preferable.
Hard Clay 1:2 to 1:1
Clay loam and silty loam 1:2
Sandy Loam 1:2
Sand 1:3
Source: DID, 2012, Kinori, 1970, QUDM, 2013 & DPWH, 1984
6.3.4 Freeboard
Freeboard refers to the height from the top of the channel to the water surface at the design capacity (refer to Figure 6-2). A freeboard is allowed to account for effects like waves and water surface fluctuations, sedimentation and water surface estimation errors.
A freeboard should be selected that is 15% of the depth of flow in the channel at the design capacity, with a minimum of 100 mm.
Figure 6-2 Open Channels and Freeboard (Source: QUDM, 2013)
6.3.5 Minimum Velocities
In hard lined channels, a minimum velocity of 0.8 m/s should be maintained in the channel to prevent deposition and sedimentation. This also has the added advantage of minimizing stagnant water and associated mosquito growth.
During dry weather flows, it may become difficult to maintain this velocity. In such situations, it is possible to introduce a smaller channel in the bottom of the drain
to confine these smaller flows to a smaller cross section (refer example in Figure 6-3).
Dry weather flows can be estimated by using the baseflow estimate discussed in Section 3.
Figure 6-3 Example Low Flow Channel for Dry Weather Flows
6.3.6 Sub-Critical Flow
The design of channels with flow approaching supercritical conditions should generally be avoided. Where it cannot be avoided, specialist design knowledge may be required as well as additional erosion protection (QUDM, 2013).
Flows between a Froude number of 0.8 to 1.2 are unstable and unpredictable and should be avoided (UDFCD, 2008). As general practice, Froude numbers below 0.8 should be adopted for design.
6.3.7 Transitions
Changes from one channel cross section to another cross section should be undertaken smoothly, with no sudden changes in cross section. An expansion rate (Figure 6-4) of 1 on 4 is recommended as a minimum, while a contraction of 1 on 1 is recommended as a minimum.
Typical transition losses are shown in Table 6-8.
Figure 6-4 Maximum Rate of Expansion
Source: QUDM, 2002
Table 6-8 Typical Transition Losses
Transition Type Contraction Coefficient Expansion Coefficient
Gradual channel transition 0.1 0.3
Typical bridge transition 0.3 0.5
Square edged abrupt transition 0.6 0.8
Source: QUDM, 2013
6.3.8 Bends
The radius of any horizontal curve in a channel should be as large as possible, to reduce super elevation and friction losses, as well as local erosion due to complex flow. A horizontal curve should have a minimum radius of the centerline of the channel of 3 times the width of the channel (PUB, 2011).
The superelevation around a bend may be calculated from Equation 6-4 (FHWA, 2001). The height of the channel on a bend should be designed to accommodate the expected water elevation on the bend at the design capacity as identified in Section 6.2.2, as well as freeboard as identified in Section 6.3.4.
Equation 6-4
∆𝑑𝑑 =𝑉𝑉2𝑇𝑇 𝑔𝑔𝑅𝑅𝑐𝑐 where:
∆d = difference in water surface elevation between the inner and outer banks of the channel in the bend (m)
V = average velocity (m/s)
T = surface width of the channel (m) g = gravitational acceleration (9.81m/s) Rc = radius to the centerline of the channel (m)
Conclusive values for head losses in open channels are not available. A conservative estimate for bends between 90 and 180 degrees may be calculated using:
The equation is applicable for bends between 90 and 180 degrees. For bends between 0 and 90 degrees, linear interpolation is recommended.
6.3.9 Safety
The recommended inclusions for safety in channels are provided in Table 6-9.
Table 6-9 Recommended Inclusions for Safety
Safety Feature Comments
Safety Railings To be provided for all channels where the design capacity depth is greater than 1 m.
Rungs in Channels Non-skid aluminum rungs shall be provided 60 m apart for channels with slopes steeper than 2V:1H and where the depth exceeds 1 m.