In this article we will discuss about:- 1. Introduction to Diversion Drains 2. Location and Spacing of Diversion Drains 3. Design Steps 4. Layout and Construction 5. Care and Maintenance 6. Uses.
- Introduction to Diversion Drains
- Location and Spacing of Diversion Drains
- Design Steps for Diversion Drains
- Layout and Construction of Diversion Drains
- Care and Maintenance of Diversion Drains
- Uses of Diversion Drains
1. Introduction to Diversion Drains:
These are the individually designed channel, constructed across the slope for intercepting the surface flow (runoff) and diverting to a safe point. Its use is urgently required, when a specified part of catchment is required to keep under protection using soil conservation measures, and this area is also situated below an unprotected area. If the area kept under soil conservation measures is not protected, then the runoff coming from the unprotected or upper area will cause damage to this area. In this situation, the use of diversion is very essential.
The diversion drain is constructed by excavating the soil from the land across the slope, above unprotected area to intercept and conduct runoff safely to a suitable point. The diversion drain can also be constructed at the boundary of the area, proposed for protection.
If there is difference between the elevation of land along the boundary of the sub-catchment and the bed grade to be used along the proposed alignment of the diversion drain, then it can be overcome by providing bed stabilizers, which will permit the designed bed grade for diversion drain.
2. Location and Spacing of Diversion Drains:
The diversion drains are used to cut the runoff away from gully head; from the area kept under treatment of erosion control etc. Apart from this, they also serve as spillway to take out the water from the earth dam, and to protect critically planted areas from washing.
The location of diversion drains should be fixed at the head of area, contributing runoff. Diversion drains are mainly constructed for handling smaller flows. The flows more than 500 l/s are normally avoided. The drains should be located on stable slopes to prevent sloughing into the gully, above over-fall. The diversion drains should be constructed above the gully head at a distance ones or twice the depth of gully.
The drains should be wide and shallow with a regulated grade to cause low runoff velocity. The capacity should be sufficient enough to carry all the water originated from contributing drainage areas during 5 to 10 years return period.
Spacing of these drains is decided on the basis of soil types, location, width of strips and crop rotation used, particularly where strip cropping is followed to reduce the length of slope and soil erosion.
For gully control work, the diversion of water is done by locating the drains above the tip of the gully over-fall, so that stable slope can still exist after sloughing of the bank. And where diversions are to be used to protect the cropped land from overflow, the drains should generally be placed just above the cropped land.
The diversions are designed based on the flow velocity of such a range, which will be safe for the vegetation present in the section of the drain. Normally, the following velocities (Table 15.7) are considered as safe velocity.
3. Design Steps for Diversion Drains:
The design of diversion drain is accomplished under following steps:
Determine the alignment of diversion drain on topographic map of the area. The drain should be aligned on non-erosive grade.
Determine the catchment area of the outlet of the drain and at other points, where it is desired to change the grade or cross-section of drain.
Compute the quantity of runoff anticipated from the catchment area for diversion. For computation 10-years return period is taken into consideration. For small water disposal using diversion drains, the runoff which such drains are expected to hold and carry, is calculated by using the rational formula, given as under –
Q = CIA/36
Calculate the cross-sectional area of the drain to carry the computed peak runoff rate in step (3). For this purpose, the following equation can be used –
Q = A.V
or A = Q/V … (15.15)
In this equation, the value of flow velocity is unknown, which can be taken from Table 15.7 based on the condition of vegetation at full developed stage. The permissible flow velocity is also based on the soil conditions.
Determine the non-erosive grade of the diversion drain, using Manning’s formula, i.e.
In above equation except R all variables are known. The value of ‘R’ is computed based on the final condition of the drain, permissible flow velocity and retardance condition.
Find the depth of diversion drain. For parabolic section, it is approximated by the following equation –
d = (3/2) R … (15.17)
Calculate the top width (T). For shallow parabolic channels the top width can be approximated as under –
T =1.5 (A/d) … (15.18)
In which, T is the top width, A is the cross-sectional area and ’d’ is the maximum depth of flow in diversion drain.
The above design is based on the assumption that, the excavated channel section is sufficient to carry the peak rate of runoff; and the earth materials deposited at down-stream of the channel is the free board.
4. Layout and Construction of Diversion Drains:
The diversion drains are constructed before the erosion control measures are placed in the area, and when contributing watershed is covered by the grass to check the silting problem of the diversion. For the places, where watershed is not under vegetation or vegetative cover, then the provision of non-silting grade should be essentially provided to the diversion.
The layout of diversion drain is done, using the same procedure as followed in terracing system.
However, the following points should always be taken into consideration:
1. They should be aligned on non-erosive grade.
2. A uniform grade can be used throughout the length of diversion or it can also be reduced on upstream side depending on the length of drain. If the grade is changed, then the size of drain should also be kept accordingly at the point where grade has been changed.
3. The diversion drains should be located at the boundary of the area to be protected.
4. If there is difference in elevation between the land along the boundary of the sub catchment as existing in the field and the bed grade to be provided along the proposed alignment of the diversion drain, then this can be removed by providing bed stabilizers, which will permit the designed grade to be given. The stabilizers may be in the form of masonry drops, which are installed at the point where actual slope of land exceeds the designed grade of the diversion drain.
5. The diversion drain should be linked to the existing or stabilized outlet to deliver the runoff in proper way without having erosion problem.
The construction of diversion drain is performed, similar to the terraces. The soil is excavated from the site, and is deposited on the lower side of the drain, leaving a berm of about 30 cm, and is sectioned in trapezoidal shape with side slope not steeper than 1:1. This spoil bank serves as freeboard for the diversion drain. Due to this fact, additional free board is not provided to the diversion drain.
After end of construction work, the final checking of finished grade and ridge height is very important to determine the adequacy of completed job. For this purpose, level shots should be taken on the channel bed and ridge. The level readings should be recorded in the field book, as it will serve as a permanent record for future need. Finally, on the basis of recorded data the grade etc. are verified.
5. Care and Maintenance of Diversion Drains:
The working performance of this system depends very much on its care and maintenance. It has similar importance as its design and construction.
Normally, maintenance is done under following heads:
1. Growing of vegetation in the channel and at upstream slope.
2. Mowing of the channel vegetation at proper time.
3. Removal of silt from the section, if any.
4. Repair of rodents hole and breakage made in the body of diversion drain.
The growing of vegetation in the channel’s section and on the upper back slope has important role in success of diversion drains. The vegetation should be established in proper way, at proper time, otherwise, the design is not being fit for grassed condition. The vegetation has multiphereous effect on drain and soil erosion both.
The mowing of vegetations grown in the drain’s section is also essential, otherwise, bushy growth of vegetation may create hindrance in flow path of water. The mowing work should be done at least ones in a year. This work should be accomplished at the stage, when vegetations can be easily cut by the cutting tools. If woody growth gets too large, then mowing becomes very difficult, as a result the channel’s section is completely covered by dense vegetations and causing the drain to become ineffective to dispose the runoff.
The deposition of silt in the channels’ section is a common happening; and if care is not undertaken, then drain becomes in the level of surrounding land (i.e. the diversion drain is completely silted). Taking of action in this regard, is very essential. Normally, the small silt deposits can be removed using spade, or scraper. But if the siltation has been taken place throughout the length in more thickness, then channel should be ploughed to lose the silt deposition and then scraping should be done.
Due to existing of taller size vegetations in the section of diversion for longer time, the rodents, particularly ground hogs make several holes in the drain’s section and also in the ridge formed around the diversion, as a result the diversion as a whole becomes very weak to carry the runoff water.
Normally, in this situation the probability of breaching gets increase. In this direction, it is very essential to make proper inspection. The ridge should be examined at the time of mowing grasses and if any break or hole is found, that should be repaired. By the way, if attention is not paid to repair them, then they are so enlarged that they cannot be easily repaired.
There have been devised several methods for estimating the peak runoff rate from smaller areas, which do not have measured runoff data. They are mainly the rational formula, Curve number, Cooks method etc. All these methods require meteorological informations which are not easily obtainable.
Hudson (1981 a) reported another method for computing the value of peak runoff with the minimum available data, is described herewith under following steps:
Collect the data related to runoff generating characteristics of the catchment. They are basically the land use, soil types, drainage and percent land slope.
Find the score for each above mentioned watershed characteristics, from the Table 15.8.
Convert the score of each characteristics into area-weighted form.
Find the sum of area-weighted score of all the characteristics. (C.C.)
From Table 15.9, determine the peak runoff rate (m3/s) for the obtained CC and given area of the watershed (ha). The value of peak runoff obtained so, is valid for 10-years return period.
This value can also be modified for different rainfall intensities, catchment shapes and return periods by multiplying with their respective conversion factors. The conversion factors are given in Table 15.10.
6. Uses of Diversion Drains:
The diversion drains are used for following purposes:
1. To reduce the length of slope.
2. To divert the runoff away from the protected area.
3. To divert water away from the action of gully over-falls.
4. To protect bottom land from overflow.
5. To intercept the coming water from the top terraced area, into un-terraced area.
Apart from above, the following points should be considered for proper use of diversion drains.
1. The catchment area of diversion drain should be covered by grasses or kept under cultivation to check the soil erosion or sediment yield. Otherwise, the section of diversion drain gets filled by the eroded soil (sediment).
2. They should not be constructed wider than the width of three strips in strip cropped field.
3. They should not be used as permanent control measure for the fields requiring terracing practice.