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Side slopes

The Side slopes of the drains in general are recommended as given below Firm soil 1.0:1 (horizontal: vertical) Loam soil .1:5:1 Sandy soil 2:5:l

However, it is desirable to design the side slope of a channel from consideration of angle of repose of the soil.

Channel grade

The channel should be as uniform as possible. The grade should be as steep as possible provided the maximum allowable flow velocities are not exceeded. Design grades should be from 1- 0.3 % and should never be less than 0.05%.

Channel depth and width

The depth of channel should be sufficient to carry the design discharge. In general the depths of main and sub main should be kept between 2-3 m. The bed width depends on peak design discharge at different points.

Channel bottom width

The bottom width can be computed for a given discharge after the channel grade, grade depth and side slopes are selected. The bottom width for most efficient cross section (hydraulic radius is one half the depth) The minimum bottom width should be 1.2 m except in small lateral. The cross section is designed to meet the requirement of the capacity, velocity, and side slope, bottom width. The parabola has the smallest wetted perimeter and it is well suited for concrete channels. A trapezoidal section with minimum recommended bottom width of 0.60 m is recommended for earthen channels. The following factors may be considered for designing: A deeper ditch gives a higher velocity than a shallow one and also may provide future opportunity for pipe drainage it will remain effective for a longer period sediment bars may cause less obstruction, it requires less waterways than a shallow one. It may uncover unstable layers of soil. Shallow drain may be more practical to maintain by pasturing or by mowing flat side slopes, the depth should be. related to a good outlet condition ns, Design velocity should be selected so as to maintain the ditch cross s section with time. In channels that flow intermittently, some scouring may be desirable at high flows to counteract sediment deposition that occurs at low flows.

Berms and spoil banks

Berms are required to provide for work areas and facilitate spoil bank spreading, Prevent excavated materials falling back into the ditch. The berm width for drains should not be less than the depth of cutting. The minimum berm widths are given in the following Table 4. The berm width should be increased in unstable soil where it is feared that the drain will enlarge in the section. The spoil is spread until the height is reduced to an economical figure usually not more than lm.

Table 4. Minimum berm widths

Depth of drain, m

Minimum berm width, m

0.6-1.2

1.2

1.2-1.8

1.8

1.8-2.4

3.0

>2.4

4.5

Free board

The free board is additional depth above the design water level used to provide a safety factor for the design storm. A free board of 25% of designed depth is kept.

 
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