LINED OR NON-ERODIBLE DESIGN CHANNEL STUDY

By :Ledib Aprilansi 145060401111017

Faisal Farid Habibi 145060401111020Dwandika Vicky N. 145060401111022Dea Rozan Aqil P. 145060401111024

Alif Rinaldy 145060401111025Roid Ghozi 145060401111029Titih Pawestri 145060401111031

Introduction

A critical topic in the area of open-channel hydraulics is the design of channels capable of transporting water between two points in a safe, cost-effective manner. Although economics, safety, and esthetics must always be considered. In addition, this discussion will be limited to the design of channels for uniform flow, and only three types of channels will be considered : (1) lined or nonerodible ; (2) unlined, earthen, or erodible ; and (3) grass lined. In examining the design procedures for three types of channels, there are some basic concepts which are common to all three, and these commonalities will be discussed first.

The proportions of the best hydraulic section of specified geometric shape can be easily derived. It should be noted that from the point of view of applications, the best hydraulic section is not necessarily the most economic section. In practice the following factors must be considered :

1. The best hydraulic section minimize the area required to convey a specified flow ; however, the area which must be excavated to achieve the flow area required by the best hydraulics section may be significantly larger if the over burden which must be removed is considered.

2. It may not be possible to contruct a stable best hydraulic section in the available natural material. If the channel must be lined, the cost of the linning may be comparable with the cost of excavation.

3. The cost of excavation depends not only on the amount of material which must be removed, but also on the ease of access to the site and the cost of disposing of the material removed.

4. The slope of the channel in may cases must also be considered a variable since it is not necessarily completely defined by topographic considerations..

In most design problems, the longitudinal slope of the channel is determined by topography, the head required to carry the design flow, and the purpose of the channel. For example, in a hydroelectric power canal, a high head at the point of delivery is desirable, and a minimum longitudinal channel slope should be used.Table 1.2 Suitable side slope for channels built in various types of materials (Chow, 1959)

Material Side SlopeRock Nearly verticalMuck and peat soils  ¼ : 1Stiff clay or earth with concrete lining  ½ : 1 to 1:1Earth with stone lining or earth for large channels  1 : 1Firm clay or earth for small ditches  1,5 : 1Loose, sand earth  2 : 1Sandy loam or porous clay  3 : 1

In deep cuts, side slope are often steeper above the water surface than they are below the surface. In small drainage ditches, the side slope are steeper than they would be in irrigation canal excavated in the same material. In many cases, side slopes are determined by the economics of construction. With regard to this subject, the following general comments are appropriate :

1. In many unlined earthen canals on federal irrigation projects, side slopes are usually 1,5 : 1 however, side slopes as steep as 1 : 1 have been used when the channel runs through cohesive materials.

2. In lined canals, the side slopes are generally steeper than in an unlined canal. If concrete is the lining material, side slope greater the 1 : 1 usually require the use of forms, and with side slopes greater than 0,75 : 1 the lining must be designed to withstand earth pressures. Some types of linning require side slopes as flat as those used for unlined channels.

3. Side slopes through cuts in rock can be vertical if this is desirable.

DESIGN OF LINED CHANNELS

Lined channels are built for five primary reasons :

1. To permit the transmission of water at high velocities through areas of deep or difficult excavation in a cost-effective fashion

2. To permit the transmission of water at high velocity at a reduced construction cost

3. To decrease canal seepage, thus conserving water and reducing the waterlogging of lands adjacent to the canal

4. To reduce the annual costs of operation and maintenance

5. To ensure the stability of the channel section

The design of lined channels from the viewpoint of hydraulic engineering is a rather elementary process which generally consists of proportioning an assumed channel cross section. Some typical cross sections of lined channels used on irrigation projects in United States are summarized in Table 1.3. In This table, it is assumed that the design flow QD, the longitudinal slope of the channel S, the type of channel cross section, e.g., trapezoidal, and the lining material have all been selected prior to the initiation of the channel design process.

These channels are lined with materials that do not erode easily, e.g. concrete, stone pitching, steel, wood, glass, plastic, etc.

The choice of material depends on availability and cost of respective materials. The advantage of nonerodible channels is that lower roughness values allow higher velocities to be maintained in a specific channel resulting in the building of a smaller, cheaper structure. Costs must be minimized when designing non-erodible channels. Two aspects need to be taken into consideration, namely the quantity of lining material and excavation required.

Trapezoidal channels are usually used where flows are > 8 m3/s, with side slopes of 1:1,5 generally being used.Rectangular channels should only be used where space is limited and where small quantities of water are to be transported. In such cases rectangular channels have the advantage of being more stable than trapezoidal channels, therefore also requiring less maintenance. With large channels the cost of a rectangular channel may be up to three times more than the equivalent trapezoidal channel.

The dry board of a channel is chosen such, that the distance is sufficient to prevent overtopping due to waves or variations in water level. There is no generally accepted rule for determining dry board, as wave action and variations in water level are caused by uncontrollable factors. A dry board variation of 5% – 30% of the normal flow depth is generally accepted.

Picture 1.1 Types Of Open Channel, Non-Erodible Channel

THE EXAMPLE OF LINED OR NON-ERODIBLE DESIGN CHANNEL

Picture 1.2 Geotextile Lined to Solve California’s Drought Problems

Picture 1.3 Rehabs of Irrigation System

Example 1The normal flow depth in a trapezoidal concrete channel is 2 m. The base width is 5 m with side slopes 1:2. The channel slope is 0,001 and Manning's n = 0,015. Determine the flow rate and average flow velocity.

Solution :

W = b + 2zy

= 5 + ( 2 x 2 x 2 )

= 13 m

A = ( b + zy ) y = ( 5 + 2 x 2 ) x 2 = 18 m2

P = 22 2122512 xzyb = 13,94 m2

21

32

1 ASPA

nQ = 2

132

)001,0(1894,13

18015,01 xxx = 45 m3

smAQv /5,2

1845

Exsample 2Determine flow depth and average flow velocities for a concrete channel with slope 1:2 500 changing to 1:3 000. Assume Manning's n-value = 0,017. The channel is rectangular with a base width of 3 m and must be able to handle a flow rate of 2 m3/s.

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