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How to achieve flood prevention by On-site Stormwater Detention?

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.

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What is residual flooding?

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.

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How can porous pavement help to relieve flooding problem?

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.

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Should we cater for manhole loss in design?

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:

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What is the mechanism of cavitation in pipes and drains?

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.

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What are the effects of hydraulic jump?

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.

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What is the difference between on-line storage and off-line storage in the design of storage pond?

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.

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What is the significance of critical slope in hydraulic design of box culvert?

Critical slope is the minimum slope in which maximum discharge shall occur without requiring the box culvert to flow full. For box culverts with slope less than critical slope, at low headwater it tends to flow full and eventually requires a higher headwater depth to convey the same amount of water required for culverts with slopes greater than critical slope.

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What is the significance of tailwater level in culverts?

The headwater level and tailwater level of culverts are important parameters in hydraulic design. The headwater level cannot be set too large, otherwise flooding upstream may occur leading to the loss of life and properties. On the other hand, the tailwater level of culverts has to comply with the following requirements:

(i) For low tailwater levels at the outlet of culverts, the small depths of flow may cause significant erosion of downstream channels.

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What are the potential problems of channels to carry supercritical flow?

Supercritical flow involves shallow water flowing in high velocity. The shallow water depth results in higher velocity head when compared with subcritical flow. The fast flow of water causes erosion to channel linings and beddings. When the channel slope becomes flat, the flow can become subcritical causing the formation of hydraulic jump which further causes erosion to channel bed.

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