5 Flood Control
6.4 Closed Conduit Network (Pipe Network)
All pipes should be designed using the Hydraulic Grade Line (HGL) method, as described in Section 4.6.
Appropriate energy losses should be accounted for in the design. Losses include:
Losses at junctions
Inlets and outlets
Obstruction and penetrations
Pipe branch losses, and
Transition losses
6.4.1 Minimum Size
The minimum size of pipe to be adopted shall be 910 mm in order to allow the passage of debris and minimize the risk of blockage.
6.4.2 Minimum Velocity
In order to encourage self-cleaning, and minimize sediment build up, pipes should be designed to ensure a minimum flow velocity of 0.8 m/s at pipe full.
6.4.3 Maximum Velocity
The maximum velocity to be adopted for piped drainage systems is 5 m/s.
6.4.4 Cover
Cover refers to the distance from the top of the pipe to the surface.
A minimum cover of 600 mm should typically be adopted.
For pipes under highways, or heavily trafficked areas, a cover of 900 mm should be adopted.
A cover depth of 450 mm may be adopted on private property or under open space that experiences only occasional traffic.
6.4.5 Alignment
Pipes should run straight between pits wherever possible. Where curves in the pipe are absolutely required, standard curved pipes from suppliers should be adopted.
Deflecting joints to achieve curvature is not recommended.
6.4.6 Capacity
The capacity of a pipe flowing full, but not under pressure, should be calculated using Manning’s equation, as discussed in Section 4.5.
It is generally recommended to avoid pipes flowing under pressure in drainage applications, although this may not always be possible.
6.4.7 Outlet Scour Control
Outlet scour control is discussed in Section 6.5.7.1.
6.4.7.1 Orientation of the Outlet Refer to Section 6.5.7.3.
6.4.8 Backflow Control Structures
Backflow control structures are discussed in Section 6.5.7.1.
6.5 Culverts
Culverts are a relatively short length of pipe or closed conduit used to convey stormwater through an embankment or road, connected at each end to an open channel.
6.5.1 Minimum Sizing
For culverts crossing under local roads, a minimum internal width and clear depth of 910 mm is required.
For culverts crossing under expressways, a minimum internal width and clear depth of 1 m is required.
6.5.2 Minimum Velocity
In order to encourage self-cleaning, and minimize sediment build up, culverts should be designed to ensure a minimum flow velocity of 0.8 m/s at pipe full.
6.5.3 Maximum Velocity
The maximum velocity to be adopted for culverts is 5 m/s.
6.5.4 Flow Conditions
Flow behavior through culverts varies depending on whether the inlet and outlet are submerged.
Computer design programs will automatically adjust the culvert flow conditions based on the upstream and downstream water levels.
Culvert flow calculations are discussed in Section 4.7
Further details on calculating culvert flow are provided in the Urban Drainage Manual (Federal Highways Administration, 2001).
6.5.5 Cover
The cover for a culvert depends on the concrete/ loading class. In general, a minimum cover of 600 mm should typically be adopted. A cover depth of 300 mm may be adopted on private property or under open space that experiences only occasional traffic.
6.5.6 Blockage
Blockage of a culvert is possible through debris as well as siltation of the culvert.
The effect of potential blockage should be considered in the design of the capacity of the culvert. While blockage of culverts tends to be associated with forested catchments, where wooded debris may mobilize during floods, urban catchments can also represent sources of debris through mobilization of man-made debris such as cars, garbage and other objects.
To date, there have been no studies of blockages of culverts within the Philippines, and in particular the likely blockages for different catchment types and land-uses.
In the absence of historical observations or studies, blockage factors as identified in Table 6-10 should be adopted in determining the discharge capacity.
When assessing blockage, blockage of the handrails should also be considered for overtopping flow.
Table 6-10 Blockage Factors to be Applied to Culverts
Culvert Size Blockage Factor *
Width < 5 m or Height < 3 m 20%
Width > 5 m and Height >3 m 10%
Handrails 50%
* Blockages are applied from the bottom of the culvert, upwards.
Minimization of blockages can be achieved through implementation of features such as debris deflector walls (as shown in Figure 6-5).
Figure 6-5 Debris Deflector Walls
Source: QUDM, 2013
6.5.7 Inlet and Outlet Structures
Inlet and outlet structures are provided to direct the flow between the open channel and the culvert. Typical structures are shown in Figure 6-6.
Figure 6-6 Typical Inlet Structures
Source: DID, 2012
6.5.7.1 Backflow Control Structures
Outlet flow controls include structures such as tidal flaps, flood gates and duck billed valves. These structures control the backflow of water from the receiving water body into either the culvert or pipe. They may be incorporated for a variety of reasons, including:
To prevent tidal backflow into a culvert or pipe network
To prevent floodwaters from a river or creek from backwatering through a pipe network or culvert, particularly under a levee or dike
To provide water quality controls between two areas
These structures introduce additional head losses. Reference should be made to the appropriate manufacturer guidelines.
Maintenance of these structures is also critical for their performance.
6.5.7.2 Outlet Scour Control
Outlet scour control may be required at outlets to reduce flow velocities prior to discharging to watercourses in order to reduce the risk of erosion. Outlet protection may be required where:
The outlet velocity exceeds the scour velocity of the bed or bank material
The outlet channel and banks are actively eroding
There is a bend in the channel a short distance downstream
Protection requirements may range from a riprap apron to stilling basins and concrete structures.
In all cases, a concrete cut-off wall is required at the end of the culvert to prevent undermining.
Rock pad outlets or dry boulder outlets are commonly adopted for culvert outlets (refer to Figure 6-7). These should generally be considered where outlet velocities are less than 5 m/s and the Froude number of the flow is less than 1.7.
Figure 6-7 Dry Boulder (Riprap) Outlet
Source: QUDM, 2013
Figure 6-8 and Figure 6-9 provide guidance on the selection of mean rock size (d50) and the length of the dissipater (L). Note that these design graphs assume a specific gravity of 2.6. Refer to standard specification for riprap in Section 5.5.6.
The minimum recommended width of the rock pad is defined as:
Immediately downstream of the outlet: the width of the outlet apron, or the width of the outlet plus 0.6 m (if there is no apron).
At the downstream end of the rock pad: the above width plus 0.4 times the length of the rock pad (L) as shown in Figure 6-10.
If the width of the outlet channel is less than the recommended width of the rock protection, then rock protection should extend up the banks to either the height of the pipe’s obvert or to the design tailwater level.
Note that this type of protection is only applicable for slopes of less than 10%.
For information on designing alternative dissipation structures, refer to Hydraulic Design of Energy Dissipaters for Culverts and Channels (FHWA, 2006).
Figure 6-8 Sizing of Dry Boulder Outlet Structures for Single Pipe or Box Culverts
Source: QUDM, 2013
Figure 6-9 Sizing of Dry Boulder Outlet Structures for Multiple Pipe or Box Culverts
Source: QUDM, 2013
Figure 6-10 Typical Rock Pad Outlet Configuration
Source: QUDM, 2013
6.5.7.3 Orientation of Outlet
Where practical, storm water outlets should be recessed into the banks of any watercourse that is likely to experience bank erosion, channel expansion, or channel migration. Typically the minimum desirable setback (Figure 6-11) is the greater of (based on QUDM, 2013):
3 times the bank height from the toe of the bank
10 times the equivalent pipe diameter (single cell) or 13 times the equivalent diameter of the largest cell (multiple outlets) measured from where the outlet jet would strike an erodible bank.
Figure 6-11 Typical Orientation and Set-Back of Outlet
Source: QUDM, 2013
Outlets that discharge into a ‘narrow’ receiving channel should be angled 45 to 60 degrees to the main channel flow. A receiving channel is considered ‘narrow’ if:
The channel width at the bed is less than 5 times the equivalent pipe diameter, or
The distance from the outlet to the opposite bank (along the direction of the outlet jet) is less than 10 times the equivalent pipe diameter, and
The inflow is more than 10% of the receiving channel flow
Stormwater outlets that discharge in an upstream direction need to be avoided wherever practical (QUDM, 2013).
6.6 Inlet Manholes
6.6.1 Inlet Manhole Location
Inlet pits should be located:
Such that the capacity of the reach between inlet pits is not exceeded. This will require an iterative process of pit location. An initial spacing can be determine based on individual pit catchment areas and pit inlet capacities. For a worked example, refer to FHWA (2001), Example 4-15.
In all low points/depressions in order to prevent the unwanted collection of stormwater.
Upstream of bridges/crossings to prevent stormwater flowing onto the bridge/crossing.
In locations were overland flow may present a hazard to pedestrians or vehicles.
Where they do not interfere with pedestrian or vehicular access (for example, driveways).
6.6.2 Inflow Capacity
The capacity of an inlet is dependent on the depth of water over the inlet. Under shallow flow conditions the inflow behaves as for a sharp crested weir. As the depth increases, the inlet becomes submerged, and the inflow behaves as for an orifice.
Equations for determining the inflow capacity under weir flow conditions and orifice flow conditions are provided in Section 6.6.2.1 and Section 6.6.2.2, respectively.
Alternatively, the inflow capacity can be estimated from the inlet rating curves shown in Figure 6-12 and Figure 6-13.
Note that these curves are applicable only to pits located in low points and depressions. For grated pits on grade (such as roadside drains) refer to Volume 4:
Highway Design.
Figure 6-12 Grated Pit (in depression) Inflow Rating Curves
Source: FHWA, 2001
Figure 6-13 Side Opening Pit (in kerb or gutter) Inflow Rating Curves
Source: FHWA, 2001
6.6.2.1 Weir Flow
Weir flow behavior is illustrated in Figure 6-14. Inflow under weir flow conditions can be derived based on the simplified version of the weir formula, as identified below:
Equation 6-6
𝑄𝑄𝑔𝑔= 𝐵𝐵𝐵𝐵 × 1.66. 𝐿𝐿. ℎ3 2⁄ where:
Qg = inflow
BF = blockage factor 1.66 = weir coefficient
L = perimeter of the grate, disregarding any sides against vertical edges (such as kerbs or walls)
h = height of the energy level above the weir crest (Equal to the water level at low velocities)
Figure 6-14 Inlet Weir Flow Behavior
Source: QUDM, 2013
6.6.2.2 Orifice Flow
Orifice flow occurs under two conditions. Free flow, where a free surface remains within the inlet and atmospheric pressure is within the chamber, and fully drowned, where the pit is filled with water and the pressure within the pit is governed by the head and flow conditions. These flow conditions are illustrated in Figure 6-14.
The flow under both conditions should be assessed and the less capacity adopted in design.
Orifice flow is given by the orifice equation:
Equation 6-7
𝑄𝑄𝑔𝑔 = 𝐵𝐵𝐵𝐵 × 𝐶𝐶𝑜𝑜. 𝐴𝐴𝑔𝑔. (2𝑔𝑔. ℎ)1 2⁄ where:
Qg = inflow
BF = blockage factor (refer Section 6.6.3) Co = orifice coefficient = 0.67 Ag = clear opening area
h = average depth of water over grate g = acceleration due to gravity (9.8m/s)
Figure 6-15 Inlet Orifice Flow Behavior
Atmospheric Non-Atmospheric
Source: QUDM, 2013
6.6.3 Blockage
In determining the inflow capacity of inlets, an appropriate blockage rate should be adopted.
For inlets located on-grade, a blockage of 20% should be adopted.
For inlets located in depressions and low points, a blockage of 50% should be adopted.