In this article we will discuss about the section and classification of drain.
Drain Sections:
An ideal drain section should fulfil the following conditions:
1. It should develop self-cleaning velocity with minimum dry weather flow.
2. It should have sufficient free board at its top, even during maximum discharge.
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3. It should be easy in construction and maintenance.
4. It should be such that it can be easily cleaned.
5. It should be structurally safe and stable.
It should be constructed with non-corrosive materials and should have sufficient resistance to the erosion.
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In practice only four common sections of sewers are used, which have been successfully tested by experience.
There are:
(a) Semi-circular section,
(b) U-section,
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(c) F-section, and
(d) Rectangular section.
(a) Semi-Circular Section:
This type of drain is mostly used for small drains due to its easiness in laying, stability and cheapness. Half round glazed stone-ware pipe is used for its construction. If the discharge in this section decreases, the self-cleaning velocity will not develop and deposits will settle down in the bottom and will cause obstruction to the flow of sewage.
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Semi-circular drains are not suitable for large discharges, because in such cases they will occupy more space. If this drain is deep, there are always chances that the wheels of the vehicles may drop in it causing damage both to the vehicle and the drain. In sandy areas during high winds, these sections are filled with sand, causing clogging of drain.
(b) U-Section:
In semi-circular section if the sides are raised, it becomes U-section. It is used at such places where discharge is more and cannot be taken by a semi-circular drain. Half stone ware pipe is laid in the bottom and over it masonry work is done as shown in Fig. 6.2.
It has the same disadvantages as that of semi-circular section:
(c) V – Section:
This section gives self-cleansing velocity even for very small discharges because greater depth is available in such case. In the invert one-fourth to one-fourth stoneware pipe or concrete block is laid and the sides are constructed with brick or flag stones and are plastered. It is easy to clean this type of section. These sections may subtend 120°, 100° or 60° at the centre of stoneware pipe.
Fig. 6.3. shows three most common type F-section drains. Fig. 6.3 (a) shows the section used for taking small discharges. Fig. 6.3 (b) shows the section used for taking DWF with small quantity of storm water.
Oftenly in this section the sides above the stoneware pipe are made in soil to reduce the cost. Fig. 6.3 (c) shows another common section used or taking large discharges. Due to restriction of space, the sides are provided with more slopes. In this section stoneware pipe is not required in the invert.
(d) Rectangular Section:
This type of section is only for large discharges because for small discharges self-cleansing velocity will not develop causing settlement of suspended particles. This drain is constructed by laying plain cement concrete bed over which sides are constructed with stone or brick masonry and finally plastered inside. Due to difficulty in cleaning, this section is not commonly used.
Classification of Drains:
Depending upon their discharge-carrying, capacities and their cross-sections the surface drains are classified into numerous types. In Punjab state the cement concrete drains are classified as Type 1, Type II,… Type XII. Out of these twelve six types. Type I to Type VI are most commonly used.
Figs. 6.5 to 6.10 shows Type I to Type VI drain sections. The discharge carrying of these sections are given in table 6.1 on the basis of Crimp and Bruges formula. The discharges are calculated on the basis of self-cleaning velocity from 0.60 to 0.90 m/sec.
