eChapters

A canal of approximate discharge of less than 80 cumecs (3000 cusecs) discharge may be called a medium or low capacity canal).

3.1 Mehboob Section

A Mehboob Section is only a modification of best trapezoidal section by making its complete bottom circular as shown in figure 3.1. Also see figure 1.6 and para 1.7. That is why these canals are also known as canals of zero bed width, or trapezoidal canals with curved bottom, or cup shaped canals.

But these cannot be called as triangular canals with curves, as often spoken. In fact it is a truncated triangle circumscribing the semi circle. Trapezium is also a truncated triangle circum scribing a circle.

More accurately it can be said as a combination of semi circular and best trapezoidal section so as to take full advantage of their hydraulic efficiency and practicability.

This is such a beautiful section that the designer can never commit a mistake, even if he wants to commit a mistake. The dimensions are so geometrically fixed that no one can play with any dimension.

The value of this section was realised for the first time by Er S I Mehboob (1943) in his Punjab Irrigation Branch Paper No.260" Lining of Canals". There after this canal section became well known as Mehboob Section after his name. He also stated that the most economical section i.e. the maximum area for the minimum perimeter is when the centre of the arc is at the full supply line of the section. This is also evident in figure 1.6. This is also the condition for all best hydraulic sections. This section is also useful to a higher silt carrying capacity than a wide shallow one..

A. Rectangular Canals

5.1 Design of Best Rectangular Canals

After the Mehboob and Trapezoidal canals, next best section is rectangle. This section is also generally adopted for small discharges say upto 10 cumecs (350 cusecs). This section ranks at No. 4. Yet, this is the only section most commonly adopted for flumes of discharges upto140 cumecs (5000 cusecs). Typical example is Orsang acqueduct, 1 km. long for 1140 cumecs (40000 cusecs) in ractangular sections on Narmada Canal Project, Gujrat, India, built in the years from 1990 – 2000. Such sections are common in power house canals due to limitation of space and very sharp curves. It is free from the effect of change of side slopes as in the trapezoidal canals (see para 3.3) Still some small side slope (batter) is adopted in thick masonry / concrete lining. It adds to structural stability with economy in thickness of wall and as well as hydraulic efficiency. It may still be called as rectangular section.

Details of its hydraulic efficiency in comparison to semi circle, Mehboob and trapeziodal sections are discussed in chapter 1, para 1.8

The pipe network in a drip system consists of a mainline, sub mains, laterals, and distributors. The layout of the pipes depends on the physical factors like shape and size of the area, topography and any other obstacles in the field. In any field there may be the scope of different layout of the pipes. The most economic one should be selected. In general, contour line is followed in laying the laterals in sloping land, sub main length usually be shorten in uphill and equally spaced laterals on either side of a sub main in a flat land provides good layout.

Hydraulic Formulae /Head Losses in Pipes

There are numerous equations for solving the head loss in pipes. The most commonly used are the Darcy-Weisbach, Hazen-William and Scobey’s.

A canal of more than 80 cumecs (3000 cusecs) approximetly may be called as a high capacity canal.

It has already been pointed out in chapter 1, that for the best section of any shape, R is always equal to d/2 only, and such depth is the most optimum depth. But such channels are not always practical or most economical due to many other practical reasons, (see para 4.1 and 6.3) and many times shallower depth then this depth is adopted. Typical examples are lining of existing earthen channels, given in para 6.3.

All the best sections have fixed ratio of dimensions very near to the semi circle. For large discharges, radius of semi circle increases that, means depth of canal as well as velocity increases. But higher velocity is not permissible with every type of lining (see table 2.1 and para 2.3). Similarly depth also has limitation of construction, maintenance, stability of slopes, etc. see example–4.2.

Because of limiting velocities and depth, deeper canal section are not practically feasible/possible. Therefore for large discharges, say more than 100 cumecs, Mehboob or best trapizodal sections are just not possible/practical, and recourse has to be taken to trapezoidal section with increased bed width then depth. This section is hydraulically inefficient or uneconomical. Larger the B/D ratio, larger is the inefficiency and section is more costly. (For Mehboob Section, B:D ratio is zero and for best trapizodal section it is much less than 1, see para 3.1, table 3.1 para 3.5 and table 3.4).

Remote sensing has proved its credibility beyond doubt in natural resources survey and environmental monitoring owing to its synoptivity, multispectral photographic capability and repetitive coverage. While its synoptivity gives unbiased information on the regional panorama of the earth surface, its multispectral possibility aids precise interpretation of various objects, as different terrestrial objects exhibit various spectral responses in different bands of the electromagnetic spectrum. Amongst various terrestrial objects, water has a characteristic spectral response of its own, i.e. it permits the blue / red part of EMR (electromagnetic radiation, 0.4-0.8 µm) to penetrate inside. On the contrary, it totally absorbs the infrared part of EMR (0.8-1.5 µm). As such blue/red part of EMR penetrates the water bodies, the impurities and suspended sediments are seen very clearly in various shades of gray as and when the water bodies are shallow. Many workers, therefore, have used blue / red band of the satellite data in such water-quality studies^l-3. Hardy⁴ has observed that the infrared light in the spectral regions of 0.6-1.4 µm has the potential to map the turbidity in the ocean water, whereas Kritakos et al ⁵ have used the rationed data of visible and near infrared bands (0.4-1.1 µm) and obtained better results for water quality monitoring. Keeping such facts in mind, the blue/red band (band 2) of the IRS 1A satellite data was used to configurate the bathymetry along Madras coast in Bay of Bengal (Figure 1 a).

In the present study, the IRS lA band 2 data (0.52-0.59 µm) of March 1991 was used and subjected to edge enhancement studies for the coastal zone of Madras (Figure 1 a). Such edge enhancement normally sharpens the edges of two contrasting objects or sharpens the edges of objects of contrasting relief. In such edge enhanced data, few

A detention reservoir temporarily stores storm runoff and releases it gradually at a controlled rate, thereby reducing the peak discharge reaching downstream channels and communities. Unlike a conventional conservation reservoir, its principal function is short-term flood moderation rather than permanent water storage. Its performance depends on basin capacity, inlet and outlet arrangements, catchment response, design storm, allowable downstream discharge, and available drawdown time. This chapter explains the concept, purposes, and functioning of detention reservoirs and distinguishes them from ordinary storage reservoirs. It covers their classifications, site requirements, storage estimation, outlet design, routing principles, benefits, limitations, maintenance, and application in rural watersheds, urban drainage systems, and integrated flood-control programmes.

Boron occurs as anion in soils and isrequired by plants in very small quantity. Water soluble B makes the estimate of its availability to plants. Total boron in soils varies from 20 to 200 mg kg^-1 and available (water soluble) boron in soils ranges from 0.03 to 12 mg kg^-1 respectively. The threshold value ranging from 0.1 to 0.5 mg kg^-1 (water soluble B) depends upon the soil type, crops, and other factors, below which the response to applied boron may be expected. Its availability is affected by soil pH as under:

Soluble chloride (Cl) concentration in the extract is determined by silver nitrate titration. This method quantifies the concentration of Cl (meq/L) in the saturation paste extract. Chloride may be determined using anion selective electrode (potentiometric). Plant tolerance to Cl can be related to its concentration in the soil saturation paste extract. The method detection limit is approximately 0.1 meq/L, dependent on the method of analysis, and is generally reproducible within ± 10 %.

Apparatus Required

Burette, pipette, conical flask and mechanical shaker etc

Diethylene tetramine penta acetic acid (DTPA) being a chelating agent is used in the determination of available Fe, Mn, Cu and Zn. When the soil is shaken with a solution of DTPA, it combines with metal ions in the solution and form soluble complexes of Fe++, Mn++, Cu++ and Zn++. The CaCl2 and triethanolamine (TEA) solution slightly raise and buffer the soil pH and mitigates the effect of Ca++ and Mg++.

[Alkaline Permanganate / Subbiah and Asija (1956) Method]

Introduction

Nitrogen (N) is found in the arable horizon of the soil, mostly in organic material. In the soil solution, organic N is gradually transformed into ammoniacal (NH₄ ⁺), nitrite (NO₂ ^–) and nitrate (NO₃ ^–) nitrogen by microbial processes. Organic-N is, in itself, of very little use to plants, as it can not be absorbed as such. It is, therefore, necessary to estimate the different forms of mineralized or available N. The NO₃ ^–_N and NO₂ ^–-N together, hardly, exceed 1 % of the total N in normal soil.

The available N in soil refers to a fraction of the total N which is converted into forms accessible to the plants. This constitutes, on an average, only 0.5-2.5 % (rarely 5 %) of the total N in a soil at any given time. Nitrogen is, generally, absorbed by plants as NO₃ ^–, under oxidized environment (upland condition), and as NH₄ ⁺, under reduced condition. NO₂ ^– is sometimes, detectable, but the amount is, generally, very small vis-a-vis NH₄ ⁺ or NO₃ ^– and, hence does not warrant its determination. The sum total of NH₄ ⁺ and NO₃ ^–- N is smaller than the total N, which becomes available to plants and the difference being attributed to the mineralization of the organic N, during the crop growth cycle.

The Kjeldahl method is mainly divided into three main steps. This method needs to be carried out in proper sequence. 

The major part of soil nitrogen exists in complex combination in the organic compounds. It becomes available to plants after breakdown to simple form followed by mineralization. The chemical methods for determination of soil nitrogen availability involve either direct or indirect of total nitrogen or measurement of a fraction of easily hydrosable nitrogen by employing solution of dilute acid or alkali. Distillation of alkaline potassium permanganate solution has often been adopted for estimating the ox disable and reactive form of soil nitrogen. The available nitrogen in soil is determined by alkaline permanganate method as per procedure suggested by Subbiah and Asija (1956).

Principle

A known weight of the soil is mixed with alkaline potassium permanganate (KMnO₄) solution and distilled. The organic matter present in soil is oxidized by the nascent oxygen, liberated by potassium permanganate, in the presence of sodium hydroxide and the released ammonia is condensed and absorbed in known volume of a boric acid with mix indicator to form ammonium borate, the excess of which is titrated with a standard sulphuric acid.

In Bray’s method a mixture of ammonium fluoride and hydrochloric acid (NH4F + HCl) act as the extracting agent. It works in two ways, firstly, the hydrogen ions from HCl have a solubilising effect, especially on Ca-P, which makes phosphate get extracted. Secondly, the fluoride ions (from ammonium fluoride) complex with the Al3+ and Fe3+ ions, thereby releasing the Al and Fe bound phosphorous into the extract.

Olsen P were found to bear a quadratic relationship, with Olsen’s extractant underestimating the content in phytoavailable P of soils with high Olsen P contents relatively to soils with low contents. The ‘‘change point’’ at which phytoavailable P began to increase rapidly per unit change in Olsen P was 53 mg Olsen P kg–1 soil. For the acid soils, a significant quadratic relationship was found between the amount of P desorbed to water and Olsen P at the three soil:solution ratios studied.

The term available phosphorus (P) refers to the inorganic form, occurring in soil solution, which is almost exclusively ‘orthophosphate’. The available P is considered to be a fairly good indicator/measure of the P supplying capacity of a soil. A knowledge of available P content of soils is important for determining its critical limit , based on soil-test-crop response calibration study.

Principle

In soil, phosphorus exists in the form of various types of orthophosphates. A very small fraction of these is available to plants at a given time. Available phosphorus content of soil consists mainly of Ca-P, Al-P and Fe-P. In the neutral and alkaline soils particularly, Ca-P is the dominant fraction. Organic –P fraction is also considerable amount, but is usually not include in the determination of available phosphorus. A large number of extractions, buffer solutions, acids and chelating agents have been suggested for available phosphorus from time to time. However, no single extractant appears to be suitable for all types of soils. Two types of extraction methods are more popularly adopted. Under acidic soil conditions, Bray No.1, which involves soil extraction with a solution consisting of 0.003 N NH₄F and 0.025 N HCl is widely followed. The fluoride complexes Al and Fe in soil, thus releasing some bound p besides the easily acid soluble P (largely Ca-P). This extraction is suitable for soils containing less than 2% calcite or dolomite because in calcareous soils, carbonates quickly neutralize the acid, resulting in less extraction of P.

Potassium is extracted from the soil with the help of suitable extractant CH3COONH4 by shacking followed by filtration or centrifugation & is determined in the extract using flame photometer. The analysis photometer is based on the measurement of the intensity of characteristic lime emission given by the element to be determined .

The total potassium content (K) of a soil varies from 0.05 to 2.5%. The total K is distributed in mineral form (90-98%), fixed non-exchangeable (1-10%) and exchangeable plus water soluble K(1-2). Exchangeable K represents an average of 2-15% of the sum of exchangeable bases and 1-3% of the total capacity of cationic exchange.

Principle

It is based on the principle of the equilibrium of the soils with an exchangeable cation mode of the solution of neutral ammonium acetate, in a given soil:solution ratio. During the equilibrium, ammonium ions exchange with the exchangeable K ions of soil.The K content in the equilibrium solution is estimated with a flame photometer.

Soil is shaken with 0.15 % CaCl2 solution. Chloride ions displace adsorbed sulphate during extraction. Calcium ions suppress the extraction of soil organic matter and hence eliminate the contamination caused by extractable organic S.

Soil is shaken with 0.15 % CaCl2 solution. Chloride ions displace adsorbed sulphate during extraction. Calcium ions suppress the extraction of soil organic matter and hence eliminate the contamination caused by extractable organic S.

Sulphur (S) occurs in different forms in soil viz., sulphites, sulphates , sulphides and in organic compounds. Plants absorb S in the form of sulphate(SO₄ ^2-) . The organic forms of S compounds become assimilable, especially following microbiological transformation into SO₄ ^2-. The mineralization of organic sulphur in a soil depends primarily upon the N:S ratio and any SO₄ ^2- formed may be fixed against extraction, particularly if much Fe and Ba is present or if soil is very acidic.

Principle

Beside some amount in the soil solution, available sulphur in mineral soils occurs mainly as adsorbed SO₄ ^2- ions. Both CaCl₂ and phosphate solutions (as monocalcium phosphate) are generally used for replacement of the adsorbed SO₄ ^2- ions. Use of Ca salts have a distinct advantage over those of Na or Na, as Ca prevents deflocculating in heavy textured soils and leads to easy filtration. SO₄ ^2- in the extract can be estimated turbid metrically using a spectrophotometer. Sulphur in the extract can also be measured by colorimetric method using barium chromate but the turbimertic method given below is mainly used in the soil testing laboratories.