Soil and Water Conservation, Part-3

 

Soil and Water Conservation 

Part-3

Biological Methods for Erosion Control: Contour cultivation and Cropping Systems

Biological Methods for Erosion Control:

The methods used for controlling soil erosion through crops or vegetation and through agronomical practices are known as biological methods. The methods discussed herein are applicable both for controlling water and wind erosion.

The biological methods for controlling water erosion consist of:

(1) Contour cultivation,

(2) Cropping systems, and

1. Contour Cultivation:

Contour cultivation means carrying out agricultural operations like planting, tillage and inter-cultivation very nearly on the contour. Contour cultivation reduces the velocity of overland flow and retards soil erosion. In some cases, after the inter-cultivation operations a ridge and furrow system or the contour develops and offers greater resistance to surface runoff.Crops like maize, sorghum, pearl, millet which are normally grown in rows are ideally suited for contour cultivation. To layout the system in the field, guidelines are marked across the slope using a Dumpy level or even a hand level. All the subsequent agricultural operations are carried out making use of the guidelines.

Strip Cropping:

Strip cropping means growing different crops in alternate strips across the slope such that they serve as vegetative barriers to erosion. The alternate strips consist of close growing erosion resisting crops to erosion permitting crops like row crops.

To achieve the best results, strip cropping is to be done in combination with other farming practices, like good crop rotations, contour cultivation etc.

There are four types of strip cropping systems.

They are:

(1) Contour strip cropping,

(2) Field strip cropping,

(3) Buffer strip cropping, and

(4) Wind strip cropping.

Contour strip cropping means growing alternate strips of erosion permitting and erosion resisting crops along the contour. Depending upon the topography the widths of the strips will vary.

In field strip cropping the strips are laid across the slope in uniform width without taking into consideration the exact contours. This method is useful on regular slopes and with the soils of high infiltration rates.

In wind strip cropping the crop strips are laid out at right angles to the direction of the prevailing winds irrespective of the direction of the land slope. The objective here is to control wind erosion rather than water erosion.

In buffer strip cropping permanent strips of grasses are located either in badly eroded areas or in areas that do not fit into a regular rotation.

 

Grassed Waterways

Grassed waterways are natural or man made constructed channels established for the transport of concentrated flow at safe velocities from the catchment using adequate erosion resistant vegetation which cover the channels. These channels are used for the safe disposal of excess runoff from the crop lands to some safe outlet, namely rivers, reservoirs, streams etc. without causing soil erosion. Terraced and bunded crop lands, diversion channels, spillways, contour furrows, etc. from which excess runoff is to be disposed of, preferably use constructed grassed waterways for safe disposal of the runoff. The grassed waterways outlets are constructed prior to the construction of terraces, bunds etc. because grasses take time to get established on the channel bed. Generally, it is recommended that there should be a gap of one year so that the grasses can be established during the rainy season.

27.1 Purpose of Grassed Waterways

Grassed waterways are used as outlets to prevent rill and gully formation. The vegetative cover slows the water flow, minimizing channel surface erosion. When properly constructed, grassed waterways can safely transport large water flows to the down slope. These waterways can also be used as outlets for water released from contoured and terraced systems and from diverted channels. This best management practice can reduce sedimentation of nearby water bodies and pollutants in runoff. The vegetation improves the soil aeration and water quality (impacting the aquatic habitat) due to its nutrient removal (nitrogen, phosphorus, herbicides and pesticides) through plant uptake and sorption by soil. The waterways can also provide a wildlife habitat.

27.2 Design of Grassed Waterways

The designs of the grassed waterways are similar to the design of the irrigation channels and are designed based on their functional requirements. Generally, these waterways are designed for carrying the maximum runoff for a 10- year recurrence interval period. The rational formula is invariably used to determine the peak runoff rate. Waterways can be shorter in length or sometimes, can be even very long. For shorter lengths, the estimated flow at the waterways outlets forms the design criterion, and for longer lengths, a variable capacity waterway is designed to account for the changing drainage areas.

27.2.1 Size of Waterway

The size of the waterway depends upon the expected runoff. A 10 year recurrence interval is used to calculate the maximum expected runoff to the waterway. As the catchment area of the waterway increases towards the outlet, the expected runoff is calculated for different reaches of the waterway and used for design purposes. The waterway is to be given greater cross-sectional area towards the outlet as the amount of water gradually increases towards the outlet. The cross-sectional area is calculated using the following formula: 

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where,         a = cross-sectional area of the channel,

                   Q = expected maximum runoff, and

                   V = velocity of flow.

27.2.2 Shape of Water Way

The shape of the waterway depends upon the field conditions and type of the construction equipment used. The three common shapes adopted are trapezoidal, triangular, and parabolic shapes. In course of time due to flow of water and sediment depositions, the waterways assume an irregular shape nearing the parabolic shape. If the farm machinery has to cross the waterways, parabolic shape or trapezoidal shape with very flat side slopes are preferred. The geometric characteristics of different waterways are shown in Fig. 27.1 and Fig. 27.2 for trapezoidal and parabolic waterways respectively. 

Fig. 27.1. Trapezoidal Cross-section. (Source: Murty, 2009)

In the figure, d is the depth of water flow, b is bottom width, t is the top width of maximum water conveyance, T is top width after considering free board depth, (D - d) is the free board and slope (z) is c/d.

The design dimensions for trapezoidal and parabolic waterways are given in Tables 27.1 and 27.2 respectively.

Table 27.1. Design Dimensions for Trapezoidal Cross-section

 

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Fig. 27.2. Parabolic Cross-section. (Source: Murty, 2009)

Table 27.2. Design Dimensions for Parabolic Cross-Section

 

27.2.3 Channel Flow Velocity

The velocity of flow in a grassed waterway is dependent on the condition of the vegetation and the soil erodibility. It is recommended to have a uniform cover of vegetation over the channel surface to ensure channel stability and smooth flow. The velocity of flow through the grassed waterway depends upon the ability of the vegetation in the channel to resist erosion. Even though different types of grasses have different capabilities to resist erosion; an average of 1.0 m/sec to 2.5 m/sec are the average velocities used for design purposes. It may be noted that the average velocity of flow is higher than the actual velocity in contact with the bed of the channel. Velocity distribution in a grassed lined channel is shown in Fig. 27.3. Recommended velocities of flow based on the type of vegetation are shown in Table 27.3. The permissible velocities of flow on different types of soils are given in table 27.4. 

Fig. 27. 3. Velocity Distribution in Open Channel (Source: Murty, 2009) 

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Table 27.3. Recommend Velocities of Flow in a Vegetated Channel.

Type of vegetation cover

Flow velocity, (m/s)

Type Magnitude

Spare green cover Low velocity 1-1.15

Good quality cover Medium velocity 1.5-1.8

Excellent quality cover High velocity 1.8-2.5 

 Table 27.4. Permissible Velocity of Flow on Different Types of Soil. 

Type of soil

Permissible velocity, (m/s)

Clean water Colloidal water

Very fine sand 0.45 0.75

Sandy loam 0.55 0.75

Silty loam 0.60 0.90

Alluvial silt without colloids 0.60 1.00

Dense clay 0.75 1.00

Hard clay, colloidal 1.10 1.50

Very hard clay 1.80 1.80

Fine gravel 0.75 1.50

Medium and coarse gravel 1.20 1.80

Stones 1.50 1.80  

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27.2.4 Design of Cross-Section

The design of the cross-section is done using Equation 27.1 for finding the area required and Manning‟s formula is used for cross checking the velocity. A trial procedure is adopted. For required cross-sectional area, the dimensions of the channel section are assumed. Using hydraulic property of the assumed section, the average velocity of flow through the channel cross-section is calculated using the Manning‟s formula as below:

         

where, V = velocity of flow in m/s; S = energy slope in m/m; R =  hydraulic mean radius of the section in m and n = Manning‟s roughness coefficient.

The Manning‟s roughness coefficient is to be selected depending on the existing and proposed vegetation to be established in the bed of the channel. Velocity is not an independent parameter. It will depend on n which is already fixed according to vegetation, R which is a function of the channel geometry and slope S for uniform flow. Slope S has to be adjusted. If the existing land slope gives high velocity, alignment of the channel has to be changed to get the desired velocity.

Problem 27.1: Design a grassed waterway of parabolic shape to carry a flow of 2.6 m3/s down a slope of 3 percent. The waterway has a good stand of grass and a velocity of 1.75 m/s can be allowed. Assume the value of n in Manning‟s formula as 0.04.

Solution: Using, Q = AV for a velocity of 1.75 m/s, a cross-section of 2.6/1.75 = 1.485 m2 (~1.5 m2) is needed.

Assuming, t = 4 m, d = 60 cm. 

The velocity exceeds the permissible limit. Assuming a revised

t = 6 m and d = 0.4 m

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The velocity is within the permissible limit.

Q = 1.6 × 1.7 = 2.72 m3/s

The carrying capacity (Q) of the waterway is more than the required. Hence, the design of waterway is satisfactory. A suitable freeboard to the depth is to be provided in the final dimensions. 

27.2.5 Construction of the Waterways

It is advantageous to construct the waterways at least one season before the bunding. It will give time for the grasses to get established in the waterways. First, unnecessary vegetation like shrubs etc. are removed from the area is marked for the waterways. The area is then ploughed if necessary and smoothened. Establishment of the grass is done either by seeding or sodding technique. Maintenance of the waterways is important for their proper operation. Removal of weeds, filling of the patches with grass and proper cutting of the grass are of the common maintenance operations that should be followed for an efficient use of waterways.

27.3 Selection of Suitable Grasses

The soil and climate conditions are the primary factors in selection of vegetations to be established for construction of grassed waterways. The other factors to be considered for selection of suitable grasses are duration of establishment, volume and velocity of runoff, ease of establishment and time required to develop a good vegetative cover. Furthermore, the suitability of the vegetation for utilization as feed or hay, spreading of vegetation to the adjoining fields, cost and availability of seeds and redundancy to shallow flows in relation to the sedimentation are the important factors that should be considered for the selection of vegetation.

Generally, the rhizomatous grasses are preferred for the waterway, because they get spread very quickly and provide more protection to the channel than the brush grasses. Deep rooted legumes are seldom used for grassed waterways, because they have the tendency to loosen the soil and thus make the soil more erodible under the effect of fast flowing runoff water. Sometimes, a light seeding of small grain is also used to develop a quick cover before the grasses are fully established in the waterway.

27.4 Construction Procedure and Maintenance

Ordinary tools such as slip scraper can be easily used for construction of waterways. However, the use of grader blade or a bulldozer can be preferred, particularly when a considerable earth movement is needed. Since the channel is prone to erosion before

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vegetations are established, it is very essential to construct the waterway when the field is in meadow and the amount of runoff from the area is also very less. In addition, if the erosion hazard is very high, then runoff should also be essentially diverted from the waterway until a good grassed cover is developed in the waterway.

The construction of grassed waterways is carried out using the following steps.

Step-1: Shaping (Soil Digging)

The shaping of the waterway should be done as straight and even as possible. Any sudden fall or sharp turn must be eliminated, except in the area where the structure is planned to be installed in the waterway. In addition, the grade should also be shaped according to the designed plan. Also, the stones and stumps which are likely to interfere with the discharge rate must be removed.

Step-2: Grass Planting

After shaping the waterway channel, the planting of grasses is very important. Priorities should always be given to the local species of grasses. The short forming or rhizome grasses are more preferable as compared to the tall bunch type grasses.

In large waterways, the seeding is cheaper than the sodding. Therefore, the seeding should be preferred for grass development. It is also suggested that the seeded area should be mulched especially for production purposes. Immediately after grass planting, the waterways should not be allowed for runoff flow.

Step 3: Ballasting

Ballasting is done in those localities where rocks are readily available adjacent to the sites and waterway gradient is very steep. Ballasting is generally recommended for the waterways in the small farms. The stones to be used for this purpose should be at least of 15 to 20 cm diameter; and they should be placed firmly on the ground. From stability point of view, on very steep slopes, wire mesh should be used to encase the stones. In parabolic shaped waterways, partial ballasting should be done in the centre, leaving the sides with grass protection.

Step 4: Placing of Structure

Structures (drop) are essential if there is sudden fall in the waterway flow path. Because under this situation, there is a possibility of soil scouring due to falling of water flow from a higher elevation to a lower elevation. For eliminating this problem, the constructed structure must be sufficiently strong to handle the designed flows successfully. As a precautionary measure, care should be taken to see that the water must not flow from the below or around the structure but through the top of the structure. In addition, the structure should be constructed on firm soils with strong and deep foundation. The apron or stilling basin of drop structures should be sufficiently strong and able to absorb or dissipate the energy/impact of falling water. After construction, earth filling should be done around the structure and it should be properly consolidated to prevent further settlement. Proper sodding should also be provided at the junction of earth filling and the structure to prevent tunneling. 

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27.4.1 Maintenance

The grasses grown in waterway should always be kept short and flexible, so that they shingle as water flows over them, but do not lodge permanently. For this purpose, the grass should be mowed two to three times in a year. The mowed grasses must be removed from the waterway, so that they do not get accumulated at some spots in the waterway and also should not obstruct the flow. The deposition of mowed grasses in the section of the waterway reduces the flow capacity of the waterway and also diverts the direction of flowing water which can cause turbulence and thus damage of the channel. It is also possible to keep the grasses short by light pasturing, which should not be done in wet condition. When the grass is pastured, it is necessary to apply manure to discourage grazing. The waterway should not be used as a road for livestock. After the vegetative cover is established and runoff passes through them for a long time, a light application of fertilizer should be done because the flowing runoff removes the plant food from the soil of waterway.

Similarly, if waterways are to be crossed by tillage implements, they should be disengaged, plough should be lifted and disc straightened. Tillage operation should also be done following nearly the contour. The waterway and its sides should not be touched during tillage operation. It is also essential that if there is any damage of the waterway, it should be quickly repaired so that the damage may not enlarge due to rainfalls. Overall, it should always be remembered that the waterways are an integral part of watershed conservation or land treatment system. If they fail to handle the peak discharge due to lack of proper maintenance, then the prolong flow of runoff through them can develop gullies in the area. Briefly, the maintenance of waterways can be taken up using the following process.   

a)     The outlets should be safe and open so as not to impede the free flow.

b)    Grassed waterways should not be used as footpaths, animal tracks, or as grazing grounds.

c)     Frequent crossing of waterways by wheeled vehicles should not be allowed.

d)    Newly established waterways should be kept under strict watch.

e)     The large waterways should be kept under protection with fencing.

f)      Waterways must be inspected frequently during first two rainy seasons, after construction.

g)     If there is any break in the channel or structures, then they should be repaired immediately.

h)    The bushes or large plants grown in the waterway should be removed immediately as they may endanger the growth of grasses.

i)       The level of grass in waterway should be kept as low and uniform as possible to avoid turbulent flow.   

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Module 9: Water Harvesting

Lesson 28 Water Harvesting

28.1 Importance of Water Harvesting

Rainwater harvesting, in its broadest sense, is a technology used for collecting and storing rainwater for human use from rooftops, land surfaces or rock catchments using simple techniques such as jars and pots as well as engineered techniques. Rainwater harvesting has been practiced for more than 4,000 years, owing to the temporal and spatial variability of rainfall. It is an important water source in many areas with significant rainfall but lacking any kind of conventional, centralised supply system. It is also a good option in areas where good quality fresh surface water or ground water is lacking. Water harvesting enables efficient collection and storage of rainwater, makes it accessible and substitute for poor quality water. There are a number of ways by which water harvesting can benefit a community.

 Improvement in the quality of ground water,

 Rise in the water levels in wells and bore wells that are drying up,

 Mitigation of the effects of drought and attainment of drought proofing,

 An ideal solution in areas having inadequate water resources,

 Reduction in the soil erosion as the surface runoff is reduced,

 Decrease in the choking of storm water drains and flooding of roads and

 Saving of energy to lift ground water.

28.2 Types of Water Harvesting

Rainwater Harvesting: Rainwater harvesting is defined as the method for inducing,    collecting, storing and conserving local surface runoff for agriculture in arid and semi-arid regions. Three types of water harvesting are covered by rainwater harvesting.

 Water collected from roof tops, courtyards and similar compacted or treated surfaces is used for domestic purpose or garden crops.

 Micro-catchment water harvesting is a method of collecting surface runoff from a small catchment area and storing it in the root zone of an adjacent infiltration basin. The basin is planted with a tree, a bush or with annual crops.

 Macro-catchment water harvesting, also called harvesting from external catchments is the case where runoff from hill-slope catchments is conveyed to the cropping area located at foothill on flat terrain.

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Flood Water Harvesting: Flood water harvesting can be defined as the collection and storage of creek flow for irrigation use. Flood water harvesting, also known as „large catchment water harvesting‟ or „Spate Irrigation‟, may be classified into following two forms:

 In case of „flood water harvesting within stream bed‟, the water flow is dammed and as a result, inundates the valley bottom of the flood plain. The water is forced to infiltrate and the wetted area can be used for agriculture or pasture improvement.

 In case of „flood water diversion‟, the wadi water is forced to leave its natural course and conveyed to nearby cropping fields.

Groundwater Harvesting: Groundwater harvesting is a rather new term and employed to cover traditional as well as unconventional ways of ground water extraction. Qanat systems, underground dams and special types of wells are a few examples of the groundwater harvesting techniques. Groundwater dams like „Subsurface Dams‟ and „Sand Storage Dams‟ are other fine examples of groundwater harvesting. They obstruct the flow of ephemeral streams in a river bed; the water is stored in the sediment below ground surface and can be used for aquifer recharge.

28.3 Water Harvesting Technique

This includes runoff harvesting, flood water harvesting and groundwater harvesting.

28.3.1 Runoff Harvesting

Runoff harvesting for short and long term is done by constructing structures as given below.

28.3.1.1 Short Term Runoff Harvesting Techniques

Contour Bunds: This method involves the construction of bunds on the contour of the catchment area (Fig. 28.1). These bunds hold the flowing surface runoff in the area located between two adjacent bunds. The height of contour bund generally ranges from 0.30 to 1.0 m and length from 10 to a few 100 meters. The side slope of the bund should be as per the requirement. The height of the bund determines the storage capacity of its upstream area. 

Fig. 28.1. Contour Bunds. (Source: Barron and Salas, 2009)

Semicircular Hoop: This type of structure consists of an earthen impartment constructed in the shape of a semicircle (Fig. 28.2). The tips of the semicircular hoop are furnished on the contour. The water contributed from the area is collected within the hoop to a maximum depth equal to the height of the embankment. Excess water is discharged from the point around the tips to the next lower hoop. The rows of semicircular hoops are arranged in a

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staggered form so that the over flowing water from the upper row can be easily interrupted by the lower row. The height of hoop is kept from 0.1 to 0.5 m and radius varies from 5 to 30 m. Such type of structure is mostly used for irrigation of grasses, fodder, shrubs, trees etc. 

Fig. 28.2. Layout of Semi-Circular Hoop. (Source: Barron and Salas 2009)

Trapezoidal Bunds: Such bunds also consist of an earthen embankment, constructed in the shape of trapezoids. The tips of the bund wings are placed on the contour. The runoff water yielded from the watershed is collected into the covered area. The excess water overflows around the tips. In this system of water harvesting the rows of bunds are also arranged in staggered form to intercept the overflow of water from the adjacent upstream areas. The layout of the trapezoidal bunds is the same as the semicircular hoops, but they unusually cover a larger area (Fig. 28.3). Trapezoidal bund technique is suitable for the areas where the rainfall intensity is too high and causes large surface flow to damage the contour bunds. This technique of water harvesting is widely used for irrigating crops, grasses, shrubs, trees etc. 

Fig. 28.3. Layout of Trapezoidal Bund. (Source: Barron and Salas, 2009)

Graded Bunds: Graded bunds also referred as off contour bunds. They consist of earthen or stone embankments and are constructed on a land with a slope range of 0.5 to 2%. The design and construction of graded bunds are different from the contour bunds. They are used as an option where rainfall intensity and soils are such that the runoff water discharged

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from the field can be easily intercepted. The excess intercepted or harvested water is diverted to the next field though a channel ranges. The height of the graded bund ranges from 0.3 to 0.6 m. The downstream bunds consist of wings to intercept the overflowing water from the upstream bunds. Due to this, the configuration of the graded bund looks like an open ended trapezoidal bund. That is why sometimes it is also known as modified trapezoidal bund. This type of bunds for water harvesting is generally used for irrigating the crops.

Rock Catchment: The rock catchments are the exposed rock surfaces, used for collecting the runoff water in a part as depressed area. The water harvesting under this method can be explained as: when rainfall occurs on the exposed rock surface, runoff takes place very rapidly because there is very little loss. The runoff so formed is drained towards the lowest point called storage tank and the harvested water is stored there. The area of rock catchment may vary from a 100 m2 to few 1000 m2; accordingly the dimensions of the storage tank should also be designed. The water collected in the tank can be used for domestic use or irrigation purposes.

Ground Catchment: In this method, a large area of ground is used as catchment for runoff yield. The runoff is diverted into a storage tank where it is stored. The ground is cleared from vegetation and compacted very well. The channels are as well compacted to reduce the seepage or percolation loss and sometimes they are also covered with gravel. Ground catchments are also called roaded catchments. This process is also called runoff inducement. Ground catchments have also been traditionally used since last 4000 years in the Negev (a desert in southern Israel) where annul crops and some drought tolerant species like pistachio dependent on such harvested water are grown.

28.3.1.2 Long Term Runoff Harvesting Techniques

The long term runoff harvesting is done for building a large water storage for the purpose of irrigation, fish farming, electricity generation etc. It is done by constructing reservoirs and big ponds in the area. The design criteria of these constructions are given below.

 Watershed should contribute a sufficient amount of runoff.

 There should be suitable collection site, where water can be safely stored.

 Appropriate techniques should be used for minimizing various types of water losses such as seepage and evaporation during storage and its subsequent use in the watershed.

 There should also be some suitable methods for efficient utilization of the harvested water for maximizing crop yield per unit volume of available water.

The most common long term runoff harvesting structures are:

 Dugout Ponds

 Embankment Type Reservoirs

Dugout Ponds: The dugout ponds are constructed by excavating the soil from the ground surface. These ponds may be fed by ground water or surface runoff or by both. Construction of these ponds is limited to those areas which have land slope less than 4% and where water table lies within 1.5-2 meters depth from the ground surface (Fig. 28.4). Dugout ponds

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involve more construction cost, therefore these are generally recommended when embankment type ponds are not economically feasible. The dugout ponds can also be recommended where maximum utilization of the harvested runoff water is possible for increasing the production of some important crops. This type of ponds require brick lining with cement plastering to ensure maximum storage by reducing the seepage loss. 

Fig. 28.4. Illustration of Dugout Pond. (Source: Barron and Salas, 2009)

Embankment Type Reservoir: These types of reservoirs are constructed by forming a dam or embankment on the valley or depression of the catchment area.  The runoff water is collected into this reservoir and is used as per requirement. The storage capacity of the reservoir is determined on the basis of water requirement for various demands and available surface runoff from the catchment.  In a situation when heavy uses of water are expected, then the storage capacity of the reservoir must be kept sufficient so that it can fulfill the demand for more than one year.

Embankment type reservoirs are again classified as given below according to the purpose for which they are meant. 

Irrigation Dam: The irrigation dams are mainly meant to store the surface water for irrigating the crops. The capacity is decided based on the amount of input water available and output water desired. These dams have the provisions of gated pipe spillway for taking out the water from the reservoir. Spillway is located at the bottom of the dam leaving some minimum dead storage below it.

Silt Detention Dam: The basic purpose of silt detention dam is to detain the silt load coming along with the runoff water from the catchment area and simultaneously to harvest water. The silt laden water is stored in the depressed part of the catchment where the silt deposition takes place and comparatively silt free water is diverted for use. Such dams are located at the lower reaches of the catchment where water enters the valley and finally released into the streams. In this type of dam, provision of outlet is made for taking out the water for irrigation purposes. For better result a series of such dams can be constructed along the slope of the catchment.

High Level Pond: Such dams are located at the head of the valley to form the shape of a water tank or pond. The stored water in the pond is used to irrigate the area lying downstream. Usually, for better result a series of ponds can be constructed in such a way that the command area of the tank located upstream forms the catchment area for the

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downstream tank.  Thus all but the uppermost tanks are facilitated with the collection of runoff and excess irrigation water from the adjacent higher catchment area.

Farm Pond: Farm ponds are constructed for multi-purpose objectives, such as for irrigation, live-stock, water supply to the cattle feed, fish production etc. The pond should have adequate capacity to meet all the requirements. The location of farm pond should be such that all requirements are easily and conveniently met.

Water Harvesting Pond: The farm ponds can be considered as water harvesting ponds. They may be dugout or embankment type. Their capacity depends upon the size of catchment area. Runoff yield from the catchment is diverted into these ponds, where it is properly stored. Measures against seepage and evaporation losses from these ponds should also be.

Percolation Dam: These dams are generally constructed at the valley head, without the provision of checking the percolation loss. Thus, a large portion of the runoff is stored in the soil. The growing crops on downstream side of the dam, receive the percolated water for their growth.

28.3.2 Flood Water Harvesting

To harvest flood water, wide valleys are reshaped and formed into a series of broad level terraces and the flood water is allowed to enter into them. The flood water is spread on these terraces where some amount of it is absorbed by the soil which is used later on by the crops grown in the area. Therefore, it is often referred to as "Water Spreading" and sometimes "Spate Irrigation". The main characteristics of water spreading are:

 Turbulent channel flow is harvested either (a) by diversion or (b) by spreading within the channel bed/valley floor.

 Runoff is stored in soil profile.

 It has usually a long catchment (may be several km)

 The ratio between catchment to cultivated area lies above 10:1.

 It has provision for overflow of excess water.

The typical examples of flood water harvesting through water spreading are given     below.

Permeable Rock Dams (for Crops)

These are long low rock dams across valleys slowing and spreading floodwater as well as healing gullies (Fig. 28.5). These are suitable for a situation where gently sloping valleys are likely to transform into gullies and better water spreading is required.

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Fig. 28.5. Permeable Rock Dams. (Source: Barron and Salas, 2009)

Water Spreading Bunds (for Crops and Rangeland): In this method, runoff water is diverted to the area covered by graded bund by constructing diversion structures such as diversion drains. They lead to the basin through channels, where crops are irrigated by flooding. Earthen bunds are set at a gradient, with a "dogleg" shape and helps in spreading diverted floodwater (Fig. 28.6). These are constructed in arid areas where water is diverted from watercourse onto crop or fodder block.

 

Fig. 28.6. Floodwater farming systems: (a) spreading within channel bed; (b) diversion system. (Source: Barron and Salas, 2009)

Flood Control Reservoir: The reservoirs constructed at suitable sites for controlling the flood are known as flood control reservoirs. They are well equipped with self-operating mechanical outlets for letting out the harvested water into the stream or canal below the reservoir as per requirement.

28.3.3 Groundwater Harvesting

Qanat System: A qanat consists of a long tunnel or conduit leading from a well dug at a reliable source of groundwater (the mother well). Often, the mother well is dug at the base of a hill or in the foothills of a mountain range. The tunnel leading from the mother well slopes gradually downward to communities in the valley below.  Access shafts are dug intermittently along the horizontal conduit to allow for construction and maintenance of the qanat (Fig. 28.7). The Qanat system was used widely across Persia and the Middle East for

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many reasons. First, the system requires no energy, relies on the force of gravity alone. Second, the system can carry water across long distances through subterranean chambers avoiding leakage, evaporation, or pollution. And lastly, the discharge is fixed by nature, producing only the amount of water that is distributed naturally from a spring or mountain, ensuring that the water table is not depleted. More importantly, it allows access to a reliable and plentiful source of water to those living in otherwise marginal landscapes (Fig. 28.8). 

Fig. 28.7. Cross Section Showing Qanats. (Source: Barron and Salas, 2009) 

Fig. 28.8. Ariel view of Qanats. (Source: www.visualphotos.com)

28.4 Runoff vs. Flood Water Harvesting

 Water harvesting techniques which harvest runoff from roofs or ground surfaces fall under the term rainwater harvesting while all systems which collect discharges from watercourses are grouped under the term flood water harvesting.

 Runoff harvesting increases water availability for on-site vegetation while flood waters harvesting provide a valuable source of water to local and downstream water users and play an important role in replenishing floodplains, rivers, wetlands and groundwater.

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 Runoff harvesting reduces water flow velocity, as well as erosion rate and controls siltation problem while in flood water harvesting, floodwater enters into the fields through the inundation canals, carrying not only rich silt but also fish which can swim through the canals into the lakes and tanks to feed on the larva of mosquitoes.                           

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Module 10: Water Quality and Pollution

Lesson 29 Water Pollution

Water pollution, whether in groundwater or surface water, is contamination or alteration of the physical, chemical or biological property of the water that causes the water to be harmful, detrimental or injurious to the public health, safety or welfare; or to the plant, animal or aquatic life dependent on the water or that impairs any designated beneficial use of the water.

29.1 Types of Water Pollution

Different types of water pollution can be listed as below.

1. Surface water pollution

2. Groundwater pollution

3. Microbial pollution

4. Oxygen depletion pollution

5. Nutrient pollution

6. Suspended matter pollution

7. Chemical pollution

1) Surface Water Pollution: Surface water pollution is the most visible form of pollution and can be seen floating on the water surface in lakes, streams, and oceans. Trash from human consumption, such as water bottles, plastics and other waste products, is most often evident on water surfaces. It also originates from oil spills and gasoline waste, which floats on the surface and affects the water and its inhabitants.

2) Groundwater Pollution: Groundwater pollution is becoming more and more relevant because it affects our drinking water obtained from the aquifers. Groundwater pollution is usually caused by highly toxic chemicals and pesticides that leak through the ground to contaminate the wells and aquifers below the surface.

3) Microbial Pollution: Microbiological pollution is the natural form of water pollution that is caused by microorganisms in uncured water. Most of these organisms are harmless but some bacteria, viruses, and protozoa can cause serious diseases such as cholera, typhoid, etc. This is a significant problem in third world or developing countries where many people have no clean drinking water and/or facilities to purify the water.

4) Oxygen Depletion Pollution: Microorganisms that thrive in water feed on biodegradable substances. When there is an influx of biodegradable material from sources such as waste or erosion from farming, the numbers of these microorganisms increase and utilize the usable oxygen. When the oxygen level is depleted, beneficial aerobic microorganisms die and anaerobic microorganisms thrive. Some of these organisms produce damaging toxins like sulfide and ammonia.

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5) Nutrient Pollution: Nutrients are usually found in wastewater and fertilizers. Excess concentration of nutrients in water bodies can cause increased vegetation in the water bodies such as algae and weeds, using up the oxygen in the water and affecting the surrounding marine life and other organisms in the water.

6) Suspended Matter Pollution: It occurs when pollutants enter the water and do not mix with the water molecules. These suspended particles form fine silt on the waterbed, harming the marine life by taking away the nutrients, restricting oxygen diffusion into the water body and disturbing their habitat.

7) Chemical Pollution: From industrial plants and farms, chemical runoff flows into the nearby rivers and water sources. Metals and solvents flow out of factories into the water, polluting the water and affecting wildlife. Pesticides from farms also endanger the aquatic life. These dangerous pesticides and toxins can get transferred through infected fish and affects human health. Petroleum is also a type of chemical pollutant that dramatically affects the aquatic life.

29.2 Sources of Water Pollution

Based on the sources, water pollution is broadly divided into two groups (Fig. 29.1):

1. Point Sources Pollution

2. Non-Point Sources Pollution

1) Point Sources Pollution: Contamination that enters a waterway from a single, identifiable source, traced to a specific source is considered as point source pollution of water. Point source pollution comes directly from a known source like an industrial or sewage outfall pipe. Point sources are typically associated with manufacturing processes. Point source contamination includes leaking chemical tanks, effluents coming from a waste treatment of industrial plant, manure spill from a hog confinement lagoon, discharge from a sewage treatment plant, factory, city storm drain, industrial storm water, discharge from construction sites, leakage of oil tankers in the sea, septic tank systems, storage lagoons for polluted waste, municipal landfills, underground storage tanks containing pollutants such as gasoline, public and industrial wastewater treatment plants etc.

2) Non-point Sources Pollution: Contamination that does not originate from a single discrete source is called non-point source pollution. It is the cumulative effect of small amounts of contaminants gathered from wide spread area. They can‟t be tracked to a single point or source. They come from many miscellaneous or diffuse sources rather than from an identifiable, specific point. It includes soil erosion, chemical runoff, animal waste pollution, leaching out of fertilizers from agricultural lands and nutrient runoff in storm water from agricultural field and forest. It also includes contaminated storm water washed off from parking lots, roads and highways, also called urban runoff. Other significant sources of non- point source pollution include litter; disposal of wastes in catch basins; hazardous waste improperly stored or discarded; improperly operating septic systems; erosion from construction sites, farms or home sites; acid deposition including acid rain and fog; pollution from roadways and road salting activities; leaking sewer lines; storm-water runoff from city and suburban streets (oil gasoline, dog faeces, litter); pesticides and fertilizers from croplands; and salt on roads for snow and ice control.

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Fig. 29.1. Point and Non-Point Source Pollution. (Source : Calhoun, 2009)

29.3 Effects of Water Pollution

Physio-Chemical Effects: A large number of pollutants can impart colour, tastes and odors to the receiving waters thus, making them unaesthetic and even unfit for domestic consumption. The changes in oxygen, temperature and pH affect the chemistry of waters resulting in the formation of unwanted products. The addition of organic matter results in depletion of oxygen. The direct addition of nutrients through various sources enhances the algal and other biological growths which when die and decompose, further deplete the oxygen. The decomposition of excessive organic matter when undergoes in absence of oxygen results in odorous and unaesthetic conditions due to accumulation of several obnoxious gases like ammonia, hydrogen sulphide and methane etc.

Biological Effects: The addition of pollutants leads to the shift in flora and fauna due to homoeostatic (self-regulating) factors operating in the aquatic systems. Most freshwater algae are highly sensitive to pollutants and their elimination modifies the prey-predatory relationships by breaking down the food chains. This results in change of the whole plant and animal communities. The diversity of organisms increases due to encouragement of the growth of only a few tolerant forms in the polluted conditions.

Toxic Effects: These are caused by pollutants such as heavy metals, biocides, cyanide and other organic and inorganic compounds having detrimental effects on organisms. These substances have usually very low permissible limits in waters and their presence beyond these limits can render the water unfit for aquatic biota and even for human use.

Pathogenic Effects: In addition to the chemical substances, polluted water has several pathogenic, nonpathogenic microorganisms and viruses. The clostridium, perfingersans, staftoculus, ficaliris cause various types of food poisoning. Apart from this, many waterborne diseases like cholera, typhoid, paratyphoid, colitis and infective hepatitis (jaundice) are spread by consumption of sewage contaminated waters.

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There could be big list of pollutants and their specific effects. A few of the effects of specific pollutants present in water are summarized in Table 29.1.

Table 29.1. Effects of Specific Pollutants Present in Water

Pollutants Effects

Zinc (Zn)

It is an important cell component in several metalloenzymes. Heavy doses of Zinc salt (165 mg/l) for consecutive 26 days cause vomiting, renal damage, cramps, etc.

Copper (Cu)

Excess of Cu in human body (more than 470 mg) is toxic, may cause hypertension, sporadic fever, uremia and coma. Copper also produces pathological changes in brain tissue.

Barium (Ba)

Excess of Ba (more than 100 mg) in human body may cause excessive salivation, colic, vomiting, diarrhoea, tremors, paralysis of muscles or nervous system, damage to heart and blood vessels.

Iron (Fe)

It is a component of blood cells and liveral metalloenzymes. However, more than 10 mg per kg of body weight causes rapid respiration and pulse rates, congestion of blood vessels, hypertension and drowsiness. It increases hazard of pathogenic organisms, as many of them require Fe for their growth.

Cadmium (Cd)

50 mg may cause vomiting, diarrhoea, abdominal pains, loss of consciousness. It takes 5–10 years for chronic Cd intoxication. During first phase, discolouration of teeth, loss of sense of smell and mouth dryness occurs. Afterwards it may cause decrease of red blood cells, impairment of bone marrow, lumber pains, disturbance in calcium metabolism, softening of bones, fractures, skeletal deformations, damage of kidney, hypertension, tumor formation, heart disease, impaired reproductive function, genetic mutation, etc.

Mercury (Hg)

Excess mercury in human body (more than 100 mg) may cause headache, abdominal pain, diarrhoea, destruction of haemoglobin, tremors, very bad effects on cerebral functions and central nervous system, paralysis, damage of renal tissues, hyper coagulability of blood, mimamata disease, inactivates functional proteins and even causes death. It may cause impairment of vision and muscles and even coma. It disturbs reproductive and endocrine system. It also causes insomnia, memory loss, gum inflammation, loosening of teeth, loss of appetite, etc.

Lead (Pb) More than 400 mg of lead in human body can cause brain damage, vomiting, loss of appetite, convulsions, uncoordinated body movements, helplessly

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amazed state, and coma. It is retained in liver, kidney, brain, muscle, soft tissues, and bones. It leads to high rate of miscarriages, affects skin, and respiratory system, damages kidney, liver and brain cells. It also disturbs endocrine system, causes anaemia, and long term exposure may cause even death.

Arsenic (As)

It is poisonous to fishes, animals and humans. More than 25 mg of arsenic in human body causes vomiting, diarrhoea, nausea, irritation of nose and throat, abdominal pain, skin eruptions inflammations and even death. It binds globulin of blood haemoglobin in erythrocytes. It may cause cancer of skin, lungs and liver, chromosomal aberration and damage, gangrene, loss of hearing, injury to nerve tissue, liver and kidney damage. Minor symptoms of As poisoning, are weight loss, hair loss, nausea, depression, fatigue, white lines across toe nails and finger nails.

Vanadium (V) It is very toxic, may cause paralysis.

Silver (Ag)

It causes pathological change in kidney, liver and may even damage kidney. and may cause Argyria (discolouration of skin). It affects mucous membranes and eyes. In high doses, it may be fatal to humans.

Radioactive materials/

metals/

substances

These generally cause „Gene‟ mutation, ionization of body fluids, chromosomnal mutations and cancers. It destroys body cell tissue, and adversely affects reproductive system. If the mother is exposed to radiation during pregnancy, it causes severe mental retardation and leukaemia in infants. Radioactive metals like heavy metals are nephrotoxic and damage kidneys.

Fluoride

Excess fluoride intake in body results in progressive crippling scourge (sponging)/fluorosis of bones, and teeth. It may cause metabolic alternations in soft tissues and their functional mechanism.

Selenium (Se)

Signs of Se poisoning (more than 4 mg) are fever, nervousness, vomiting and low blood pressure. It causes damage to liver, kidney and spleen, loss of nails and hair and blindness to animals. It affects enzyme systems and interferes with sulphur metabolism. It can cause growth inhibition, skin discolouration, bad teeth, psychological problem, and gastro intestinal problems, but trace amount of Se is protective against poisoning by Hg, Cd, and Ag.

Chromium (Cr)

Any chromium compound is toxic but hexavalent Cr greater than 70 mg is very toxic. It causes cancer, anuria, nephritis, gastrointestinal ulceration, and perforation in partition of nose. It penetrates cell membrane and badly affects the central nervous system. It causes respiratory trouble and lung

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tumors when inhaled. It may cause complications during pregnancy. and has an adverse effect on aquatic life. Trace amount of CrIII is essential for normal glucose, protein and fat metabolism and hence it is an essential trace element in diet.

Manganese (Mn)

Mn is essential for mammals but in concentration greater than 100 ppm, is toxic and causes growth retardation, fever, sexual impotence, muscles fatigue, and eye blindness.

Cobalt (Co) High dose (27 mg or above) can cause paralysis, diarrhoea, low blood pressure, lung irritation and bone defects.

Nickel (Ni) More than 30 mg may cause changes in muscle, brain, lungs, liver and kidneys and can also cause cancer, tremor, paralysis and even death.

Boron (B)

Boron in traces is essential for plant growth. In higher concentration it is harmful to crops and affects metabolic activities of plants. It also affects central nervous system.

Alkalinity and Acidity

Permissible range of pH value if violated may cause health problems to human and animals and loss of productivity in agriculture.

Phosphate and nitrates

Phosphates and nitrates are soil nutrients and not toxic in low concentration. They deplete oxygen by promoting excess algae production in water and - giving bad odour and taste of water which are detrimental to aquatic life. They are toxic for human and animal life if concentration is beyond the permissible limits. Nitrates also cause cyanosis or blue body disease.

Chlorine (Cl) It destroys plant and aquatic life and is a biocide.

Sulphide It gives bad odour, toxic to many aquatic organisms and animals.

Salinity Salinity is very harmful for soils as it destroys agricultural land.

Oil/Grease/

Oil Sludge

Petroleum products in general are very harmful for soils, aquatic life, animal, human and plant life. They are very toxic. Agricultural land may suffer accumulation of oily waste affecting aeration and fertility. Many constituents of oily sludge are even carcinogenic and potent immunotoxicants.

Surfactants They are toxic and harmful for aquatic life, animals and humans. They

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and detergents

inhibit self-purification of water.

Phenols They are toxic and impart objectionable odour. They generally subdue plant growth. Some phenols (nitrophenyl etc) are carcinogens.

Cyanides Cyanide poses a serious health hazard. Apart from acute toxicity and chronic toxicity, it leads to development of iodine deficiency disorders.

Pesticides/

Insecticides

They are highly poisonous for humans and animals. Also they lower seed germination, play a role in the development of Parkinson‟s disease, destruction of nerve cells in certain regions of brain resulting in loss of dopamine which is used by nerve cells to communicate with brain. Some of these are physical poisons, some are protoplasmic poisons causing liver damage, some are respiratory poisons and some are nerve poisons.

Aluminium (Al)

It is especially toxic for brain and sometimes may lead to Alzheimer‟s disease in humans.