In this article we will discuss about:- 1. Objectives of Grassed Waterways 2. Waterway Location 3. Selection of Suitable Grasses 4. Layout 5. Construction 6. Maintenance 7. Effectiveness 8. Advantages and Disadvantages.
Objectives of Grassed Waterways:
Different objectives of grassed waterways are mentioned as under:
i. To remove the excess runoff from the terraced field, bunded field or from a conservation-treated areas.
ii. To carry the rainwater down the slope to a safe place.
iii. To reduce the soil erosion caused by high velocity runoff.
Waterway Location for Grassed Waterways:
The grassed waterways should be located at that point, where the water can drain into the waterway from all the sides of watershed. The places which require a minimum earthwork are also preferred for location of waterway. In addition, where the soils are deep, fertile, moisture is favourable for good grass growth and top soil surface is in depression form, the waterways can also be constructed. However, if suitable natural depressions are not available, then waterway should preferably be constructed along the field boundary.
The natural depressions are normally being good sites for waterways construction, but they need to be proper shaping and protection. However, if no such places are available in the area, then waterways can be best located close to the field boundary to minimize interferences in field operation. The waterway can also be located in the middle of the field, provided that there is a good outlet or protected areas, down-stream side. The climatic condition of the area plays very important role in deciding the necessity of grassed waterways.
Based on the climatic conditions of area the need of grassed waterways can be assessed as per following points:
i. In humid regions, where rainfall intensity is high and yield of excess runoff is very frequent.
ii. In semi-arid or arid regions, where infiltration rate is low, and yield of concentrated runoff is there.
iii. The area treated with terracing system; and drainage of rainwater is essential from there.
iv. In uncultivated lands to provide a safe connection of runoff water to the structures like diversions, cutoff ditches etc.
Selection of Suitable Grasses for Grassed Waterways:
The soil and climatic conditions are the primary factors in selection of vegetations to-be established for construction of grassed waterways. The other factors which are considered for selection of suitable grasses, are the duration of establishment, quantity and velocity of runoff, ease of establishment and time required to develop a good vegetative cover.
Furthermore, the suitability to the farmers for utilization of the vegetation as seed or hay, spreading of vegetation to adjoining fields, cost and availability of seeds and retardance to shallow flows in relation to sedimentation, are the important factors, should be considered for selection of vegetations to grow in the channel section.
Generally, the rhizomatous grasses are preferred for establishing in the waterway, because they get spread very quickly and also provide more protection to the channel than the bunch grasses. Deep rooted legumes are seldom used for grassed waterways, because they have tendency to lose the soil, and thus to 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.
Layout of Grassed Waterways:
The layout of grassed waterway is prepared under following considerations:
i. The elevation of waterway should be slightly lower than the drainage or terrace outlets.
ii. All the outlets must be connected to the waterway, which should be as straight and uniform as possible.
iii. Small structures or basins should be laid at regular intervals in long grassed waterways; or on the sites where change in direction or slope is unavoidable.
iv. If possible, a well-protected area at the lower end of waterway should be located for safe disposal of runoff.
Construction of Grassed Waterways:
Ordinary farm tools such as slip scraper can be easily used for construction of waterways. However, the use of grader blade or a bulldozer can also be preferred, particularly when a considerable earth movement is needed.
Since, the channel is subject to erosion before vegetations being established, so it is very essential to construct the waterway when the field is in meadow and the amount of expected runoff generated 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 grass cover is developed in the waterway.
The construction of grassed waterways is carried out under following steps:
1. Shaping (Soil Digging):
The shaping of waterway should be done as evenly as possible. If there is any sudden fall or sharp turn, then they must be eliminated, except in that area where structure is planned to install in the waterway. In addition, the grade should also be shaped according to the design plan. Also, the stones and stumps, which are likely to interfere the discharge rate, must be removed.
2. Grass Planting:
After shaping the waterway channel, the planting of grasses is very important. Priority should always be given to the local species grasses. The sod-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, seeding can be preferred for grass development. It is also suggested that, especially for protection purposes, the seeded area should be mulched. Immediately, after grass planting, the waterway should not be allowed for runoff flow.
Ballasting is done in that localities, where rocks are readily available adjacent to the sites, and waterway gradient is very steep. Ballasting is generally recommended for the waterways on small farms. The stones to be used for this purpose should be of at least 15 to 20cm diameter; and they should be placed firmly on the ground. For stability point of view, on very steep slopes the wire mesh should also be used. In parabolic shape waterways, the partial ballasting should be done in the centre, leaving the sides with grass protection.
4. Placing of Structure:
Structures (drop) are essential in that condition, when there is sudden fall in the waterway flow path. Because at this condition there is possibility of soil scouring due to falling of water flow from a higher elevation to lower elevation point. For checking this type of possibilities the used structure must be sufficient strong to handle the designed flows, successfully. As precautionary measure the water must flow from the top of the structure; not from the below or around the structure.
In addition, the structures should be constructed at firm soils, and have strong and deep foundation, as well. The apron or stilling basin of drop structures should be sufficiently strong and capable to absorb or dissipate the energy/impact of falling water. After construction, the earth filling should be done around the structure; and it should be firmly pressed to prevent the settlement. The sodding should also be done at the juncture of earth filling and the structure to prevent tunneling.
The cost of grassed waterway construction depends on various factors such as the equipment and labor used for construction, costs of grading, seed and fertilizer cost, the cost of establishing grasses in the waterway etc. The costs expenditure from the process of site surrey to the care and maintenance of waterway, are the input costs. And the potential returns or revenue generated from waterways in terms of area protection against runoff, production of hay etc. are taken as the benefit achieved.
Maintenance of Grassed Waterways:
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 grasses should be moved two or three times in a year. The mowed grasses must be removed from the waterway, so that it cannot be collected at some spots in the waterway, and also could not form obstruction to flow.
The deposition of mowed grasses in the section of waterway reduces the flow capacity of the waterway and also deflects the direction of flowing water which can cause turbulence and thus cutting of the channel may be there. It is also possible to keep the grasses short by light pasturing, which should not be done in wet weather.
When the grass is pastured, it is necessary to apply manure to discourage grazing. The waterways should not be used as road for livestock. After the vegetative cover is established and runoff is passing through them from long time, a light application of fertilizer should be done, because the flowing runoff removes the plant’s food from the soil of waterway.
Similarly, if waterways are to be crossed by tillage implements, they should be disengaged, ploughs should be lifted and disc straightened. Tillage operations should also be done nearly on the contour, never up and down the sides of the waterway. It is also essential that if there is any damage in the waterway, they should be quickly repaired so that the damage may not become too large due to rain events.
Overall, it should always be remembered that the waterways are the integral part of watershed conservation or land treatment system; if they fail to handle the requisite discharge due to lack of proper maintenance, then the prolong flow of runoff through them can develop the gullies in the area, which is not tolerable at any cost.
In brief, the maintenance of waterways is done as per below:
i. The outlets should be safe and open, so as not to impede their free flow.
ii. Grassed waterways should not be used as footpaths, animal tracks, or as grazing grounds.
iii. Frequent crossing of waterways by wheeled vehicles should not be allowed.
iv. Newly established waterways should be kept under strict watch.
v. The large waterways should be kept under protection with fencing.
vi. Waterways must be inspected frequently during first two rainy seasons, after construction.
vii. If there is any breaks in the channel or structures, then they should be repaired immediately.
viii. The bushes or large plants grown in the waterway should be removed immediately, because they could endanger the growth of grasses.
ix. The level of grass in waterway should be kept as low and uniform as possible to avoid turbulent flow.
The effectiveness of waterway gets change as per below:
i. The soil characteristics; the land slope/topography; the vegetations; the area for establishment and the construction and maintenance are the important factors affecting the effectiveness of grassed waterways.
ii. A wide grassed waterway with well-established vegetation is being more effective to trap the sediments from the runoff; and thus reducing the level of sediments in the flow because of greater surface contact area and greater contact time with the runoff.
iii. The shape of waterway greatly affects the flow velocity and its erosive force, so proper maintenance must be followed for keeping the shape, proper.
iv. The time of waterway construction also affects its effectiveness. The construction should be done especially when there sufficient time is available to attain good vegetative growth before commencement of monsoon season.
v. The vegetation species also produce effect on waterway effectiveness. Therefore, the selection of vegetation should be done to that one, which provide a good cover, quickly, and also form a deep- rooted sod. As note, if the growth is not good, and seeding is required to do, then it should be done across the flow direction, as it further causes reduction in the flow velocity.
vi. The waterway should be inspected after occurrence of each large runoff events. The bare or eroded spots should be repaired or reseeded, immediately.
Advantages and Disadvantages of Grassed Waterways:
The main advantages of grassed waterway are mentioned below:
1. It carries large flows; and makes it suitable for large watersheds.
2. Farm machineries can easily cross them.
3. After establishment of grasses, its maintenance becomes less.
Apart from above advantages, there are few disadvantages, too; they are:
1. Use of farm machineries around waterway is difficult.
2. It restricts the installation of tile drainage outlet.
3. At few locations the establishment of vegetations is being very difficult.