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In the manufacturing process of vitrified clay pipe, the clay material particles are fused into an inert and chemically stable compound. It is capable of carrying a wide range of commercial, industrial and domestic sewage. In fact, it is chemically resistant against sulfuric acid induced by hydrogen sulfide. Moreover, it is reported to be unaffected by the presence of solvents.
The main function of PVC lining is to protect concrete surface against hydrogen sulfide attack. Moreover, it also guards against attack by a wide range of acids and alkalis. Hydrogen sulfide generated in sewerage by bacterial action under anaerobic (absence of oxygen) conditions is converted to sulphuric acid by aerobic bacteria growing on wet sewer walls. This acid reacts with the lime in the concrete, causing breakdown of the concrete pipes. The addition of lining could also improve the flow capacity.
The market price of vitrified clay pipes is generally less than that of precast concrete pipes with PVC lining. Therefore, for small size of pipes (pipe diameter less than 600mm) it is more economical to use vitrified clay pipes.
(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.
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.
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).
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.
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.
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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.
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.
