1. Two contraction joints and one expansion joints are usually adopted for drainage channels. Why? (C1)
In the life cycle of a concrete structure (not prestressed concrete), it will generally undergo the following process of contraction and expansion:
Contraction: (a) Early thermal movement
(b) Seasonal contraction owing to drop in temperature (c) Shrinkage
Expansion: (a) Seasonal expansion owing to drop in temperature
The order of magnitude for items (a) to (c) is more or less the same. Hence, qualitatively speaking, for a given length of concrete structure, the number of contraction joints should be more than the number of expansion joints and they are roughly in the order of 3:1 to 2:1 based on the number of expansion and contraction process above. Of course, the actual spacing and number of contraction joints and expansion joints should be determined case by case.
2. Should joints in box culverts and channels be completely watertight? (C1)
The joints for box culverts and channels should be capable of accommodating movements arising from temperature and moisture changes. However, the joints are not necessarily designed as watertight except the following conditions [15]:
(i) There is a high possibility of occurrence of high water table in the vicinity of box culverts/channels. The high groundwater level and rainwater seepage through embankment may cause water passing through the joints and washing in soils. Consequently, the loss of soils may lead to the failure of the structures.
(ii) If the box culvert/channels are designed in such a way that water flow through joints from the structures causes the washing out of bedding materials, the requirement of watertightness of joint has to be fulfilled.
(iii) In cold countries, road salt is sometimes applied on roads above box culvert or at crossings of channels to prevent freezing and thawing. The leaching of road salts into the joints may cause corrosion of joint
reinforcement.
3. Should the outer side of drainage channel at a bend be elevated to cater for superelevation?
Flow around a bend results in a rise of water surface on the outside of the bend and it is natural to consider that extra height of channel wall on the outside of bend to prevent overflow of water.
However, for supercritical flow in channel, owing to the effect of superelevation extra height of channel wall should be provided on both sides of the bend. This is because supercritical flow around a bend would make water level go up alternatively on the outside and inside of the bend owing to cross waves. This cross wave pattern may continue for some distance downstream. As such, both sides of bend shall be lengthened to cater for this effect.
4. Should the same freeboard be maintained along a channel? (C2) The freeboard is defined as the vertical distance from water surface to the top of channel bank. The selection of freeboard is dependent on the consequence should overflow out of channel bank occurs. Other than that, consideration should also be given to prevent waves, superelevation and fluctuations in water surface from overflowing the channel banks.
Generally, a 300mm freeboard is generally considered acceptable. For steep channels, it is preferably to have the height of freeboard equal to the flow depth to account for high variations in swift flow induced by waves, surges and splashes.
5. In designing of access ramps for drainage channels, why should the direction of access ramps be sloping down towards downstream?
(C3)
In the design of access ramps, the direction is normally specified to be sloping down towards downstream so as to avoid the occurrence of over-shooting of flowing water for supercritical flow in case of aligning the ramps in the reverse direction of channel flow.
Note: Access ramps refer to ramps used for maintenance vehicles during routine maintenance of channels.
6. What is the purpose of using riprap in drainage channels? (RC1) Riprap is an erosion-resistant ground cover made up of large, angular and loose stones (rock, concrete or other material) with geotextile or granular layer underneath. Riprap is commonly used in drainage channel to provide a stable lining to resist erosion by channel water. It is also used in channels where infiltration is intended but the velocity of flow is too large for vegetation.
A layer of geotextile is normally provided under riprap to perform separation from underlying soils. This prevents the migration of fined-grained soils from sub-grade into riprap and results in settlement and loss of ground.
7. Should riprap be constructed by dumping or by hand-placing?
(RC1)
Riprap by dumping involves the dumping of graded stone by dragline or crane in such a way that segregation would not take place. Dumped riprap is a layer of loose stone so that individual stones independently adjust to shift in or out of the riprap. The dumped riprap is very flexible and would not be damaged or weakened by minor movement of the bank caused by settlement. Moreover, local damage or soil loss can be readily repaired by placement of more rock.
Riprap by hand-placing involves laying of stones by hand and by following a pattern with the voids between the large stones filled with smaller stones and the finished surface is kept even. The interlocking riprap produces a tidy appearance and decreases flow turbulence. Also, owing to the interlocking nature of riprap it allows the formation of riprap on steeper bank slopes. The thickness of riprap can usually be reduced when compared with dumped riprap. However, it requires much labour for installation of riprap. Another drawback is that the interlocking of individual rocks produces a less flexible revetment so that a small movement in the base material of the bank could cause failure of large portions of the revetment.
8. Should angular or rounded stones be used in riprap channel?
(RC1)
Rock used for riprap should be blocky and angular, with sharp edges and flat faces. Angular stones proved to be effective to withstand external
forces. Rounded stones have a high tendency to roll and inadequately protect the channel bed and bank. The ratio of length to thickness of angular stones should be less than 2.
If rounded stones have to be used, they should not be placed not steep embankments. Moreover, the size of rounded stones shall be increased (say 25%) with the corresponding increase in thickness of riprap layer.
9. Why is stoplog seldom used in drainage channels? (C4)
Stoplog consists of several sets of horizontal beams/logs stacked vertically.
For narrow openings, the logs span between support slots at the ends of the openings. For wide openings, intermediate removable support posts may be required. They are prevalent in the past because of its low establishment cost, simple erection and easy operation.
However, there are also several drawbacks of this closure system. Since it requires a long lead time to mobilize workers and equipment for installation, it needs an accurate long-range weather forecast to allow for the long lead time. The situation is even worse for wide stoplogs which requires special lifting equipment for installation of intermediate support posts. When compared with time required to close sluice gates, the installation time of stoplogs is much longer. Moreover, a storage building has to be provided to prevent material loss by theft and damage by vandalism.
Fig. Stoplog system
10. Why is gabion apron necessary for gabion retaining wall to retain river embankment?
Gabion aprons are provided at gabion walls to protect its toe from scouring due to river flow. The scouring would eventually lead to undermining of the gabion structure and affects its structural stability. The length of gabion apron should be long enough such that it reaches beyond the limit where scouring may form. A layer of getextile filter is normally placed at the base of gabion structures to prevent leaching out of foundation soils.
11. How to cater for energy dissipation at drainage outlets? (C5)
Flow velocity at outlets is usually high. Without proper control of this energy, the subsequent bank erosion may result in failure of the banks. Therefore, some energy dissipating structures are designed to cope with this problem.
Impact energy dissipaters may be provided at outlets by making use of impact walls to dissipate energy. Alternatively, the flows at outlet are dispersed artificially to achieve a significant loss of energy. However, the problem of cavitation may occur in this type of energy dissipating structures.
Fig. A typical drainage outlet.
12. In selection of dams in drainage channels, what are the advantages of using rubber dams instead of steel-gate dams? (C6) The advantages of rubber dams are as follows:
(i) Since rubber is flexible in nature it is capable of performing deflation even in the presence of dirt and sedimentation on the downstream side. However, for rigid steel-gate dams, it may not be possible to open when there is excessive sediment.
(ii) Since the foundation of rubber dams is comparatively lighter than that
of steel-gate dams, it saves both construction cost and time.
(iii) Rubber dams can be designed with longer spans without piers while steel-gate dams require intermediate piers for long spans.
13. How do we compare air-filled and water-filled rubber dams? (C6) Most of the existing rubber dams are of air-filled types. Water-filled rubber dams are not preferred for the following reasons:
(i) By giving the same sheet length and dam height, the tensile stress for water-filled dams is higher than that of air-filled rubber dams.
(ii) A significant size of water pond is normally provided for water-filled water dams for filling the rubber dams during the rising operation of dams.
14. Under what conditions should engineers consider using stilling basins? (C7)
Stilling basins are usually introduced to convert supercritical flow to subcritcal flow before it reaches downstream. A typical stilling basin consists of a short length of channels located at the source of supercritical flow (e.g. end of spillway). Certain features are introduced to the basins like baffles and sills to provide resistance to the flow. As such, a hydraulic jump will form in the basin without having conducting significant amount of excavation for the stilling basin if baffles are installed [31].
15. When a drainage system (i.e. u-channels with catchpits) is connected to a main drainage channel, a segment of short pipe is used. What is the reason of such arrangement?
There are three scenarios of such connection arrangement: (a) a new drainage system is connected to an existing drainage channel (b) an existing drainage system is connected to a new drainage channel (c) a new drainage system is connected to a new drainage channel.
For all scenarios, what engineers consider is the total amount of differential settlement or lateral movement to be encountered between the drainage system and main drainage channel. For scenario (b) and (c), it is very likely that substantial differential settlement will occur and this will cause damage to the connecting concrete pipes. Therefore a segment of short pipes are designed so that they serve to provide flexibility to the pipes in case of uneven settlement occurring between drainage system and main drainage
channels.
Fig. Short pipe
16. Nowadays, most flap valves are made of HDPE. What are the advantages of using HDPE when compared with cast iron? (C8)
(i) It has no reaction with sewage and seawater and does not suffer from the corrosion problem associated with cast iron.
(ii) No protective coating is required and it is almost maintenance-free.
(iii) HDPE flap valves require very low opening pressure in operation (like 5mm water level difference). For cast iron flap valves, due to its own heavy self-weight, the required opening pressure of cast iron flap valves is higher than that of HDPE flap valves. This criterion is essential for dry weather flow conditions.
However, the pressure resistance of HDPE flap valves is not as good as cast iron flap valves. For instance, a typical 450mm wide HDPE flap valve can only withstand about 5m water column.
17. What is the difference between on-seating and off-seating head in penstock? (C9)
A penstock is commonly used to control the flow and water level and for isolation of fluid. It mainly consists of a sliding door which is controlled by mechanical spindle moving through a hole in a frame built onto a structure.
Penstock is the term used in UK while sluice gate is more commonly adopted outside UK. In the design of penstock, it is important to identify if it would take on-seating head or off-seating head.
On-seating head refers to the water pressure forcing the penstock into the wall while off-seating head refers to the water pressure forcing the penstock out of the wall as shown below.
Fig. On-seating (left) and off-seating head (right)