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Cement will mix with more water than is required to eventually combine during hydration of cement paste. As such, some voids will be left behind after the hydration process which affects the strength and durability of concrete. With the presence of air voids in concrete, it is vulnerable to penetration and attack by aggressive chemicals.
For gravity pipes, air tests or water tests are carried out after completion of laying and jointing of the pipes. These tests are conducted to check the watertightness of joints and to ensure the pipelines are free from damage where leakage may occur.
Air test has the advantage that the test itself is simple and faster to be carried out. It does not require the disposal of significant quantities of water used in the test which is a mandatory requirement for water test. However, in case leakage exists in the constructed segment of gravity pipelines, the position of leakage can hardly be located in air test. Moreover, the rate of water leakage cannot be determined from air tests. In addition, air test is readily affected by atmospheric condition because air has a relatively high coefficient of thermal expansion. The test is also influenced by the moisture condition of the test pipelines because it affects the passage of air through the pipelines.
Froude number is used when gravitation forces is predominant in the channel flow. Reynolds number is adopted when viscous forces are predominant in the channel flow. It is almost impossible to make Froude number and Reynolds number identical in model and prototype.
The concept of On-site Stormwater Detention involves the temporary storage of stormwater with limited runoff from the site. It essentially modifies the runoff behaviour of the site so as to prevent flooding in area downstream. The allowable rate of discharge from the given area is Permissible Site Discharge while the minimum storage area for water detention is called Site Storage Requirement.
It is not uncommon that local flooding still occurs despite that drainage improvement measures have already been implemented in its vicinity. In some low-lying areas the ground level is lower than the water level in the nearby main drainage channel.
Common structural approaches to combat flooding problem includes replacement/upgrading of drains, temporary flood storage with pumping facilities, cross catchment diversion and infiltration. The concept of infiltration involves the reduction of surface runoff so that the amount of overland flow is reduced.
Manholes are provided in locations where there are changes in size, direction and gradient of gravity pipelines. In normal practice for straight pipelines manholes have to be installed at a certain spacing to facilitate the maintenance of pipes. With the introduction of manholes, there are various reasons which account for the manhole loss:
Cavitation refers to the formation of air bubbles in fluid in low-pressure condition which is lower than the saturation pressure. It is a potentially damaging condition in which the fluid in pipes or sewers is at high velocities. By Bernoulli’s Equation, at high flow velocities, the pressure head of fluid is reduced accordingly.
The use of hydraulic jump in hydraulic engineering is not uncommon and the creation of such jumps has several purposes:
(i) Its main aim is to perform as an energy-dissipating device to reduce the excess energy of water flows.
(ii) The jump generates significant disturbances in the form of eddies and reverse flow rollers to facilitate mixing of chemicals.
The design of storage pond is commonly divided into on-line storage and off-line storage. The on-line storage concept involves the inclusion of storage facilities in series with the pipelines so that overflow at the storage facilities is allowed. One simple application of on-line storage is to enhance a large size of drainage pipes.
