The Diversion Weir and its Types. Alignment. As stated earlier, a diversion weir or an anycut or an intake weir is a raised pucca structure with or without shutters and laid across the river width. It is, essentially, of a height say up to 9 metres or so. The height of shutters over the weir crest seldom exceeds 1.2 metres or so. The entire length of the weir is divided into a number of bays by means of divide piers so as to avoid cross-flow in floods. As far as possible, the weir should be aligned at right angle to the direction of the main river current. This ensures lesser length of the weir, better discharging capacity and lesser cost. This right-angled alignment is better and, therefore, common, especially, when the river bed is silty or sandy. Sometimes, the weir may be aligned at an oblique angle to the direction of the river current, and thereby, obtaining more safe and better foundations. In such a case, the weir will be of greater length, will have less discharging power and will be costlier. Moreover, due to non-axial flow, cross-currents may be developed, which may undermine the weir foundation. An oblique alignment may sometimes become necessary, when the river bed consists of gravel and shingle, which could otherwise enter the head regulator of the main canal and get deposited into the head reach of the main canal.
Types of Tank Weir and Their Cross-sections.
Tank escape weirs are similar to river-weirs (i.e. diversion weirs or anicuts), and hence, they may be classified into the following three general types:
Type A: Masonry weirs with a vertical drop;
Type B: Rockfill weirs with a sloping apron; and
Type C: Masonry weirs with a sloping masonary apron (glacis).
These three types of river weirs have already been discussed in article.
Besides these three important types of river weirs, which are used as tank weirs also, the fourth type of a tank weir which is a combination of type A and type C, may also be used. In such a D type weir, a number of vertical steps are made (as in the case of a stepped fall) instead of providing a horizontal or sloping downstream apron. Such weirs of type d are called weirs with stepped aprons. The A and D type of weirs are most widely adopted, although every type of weir has been used in Sourth Indian tanks.
The A type of weir i.e. weir with a masonary pucca horizontal floor, may sometimes by provided with depressed floors, so as to provide sufficient water-cushion for absorbing the impact of high water falls. Typical sections of the two sub-types of A type, and D type of weirs, and the conditions under which each is adopted are shown in figs. (a) (b), and respectively.
Figure: (a) A typical masonry tank weir with a vertical drop, with a horizontal under pressed floor, suitable for low drops of 0.6 to 0.9 metre or for even higher drops when the soil is a rock.
Figure: (b) A typical masonary tank weir with a vertical drop with depressed floor, suitable for high drop (say 2.5 m upward) or in soft soils but only slightly pervious.
The general principles and considerations which guide the design of tank weir sections, are the same as in the case of river weirs founded on rocks or loose pervious or impervious soils But in case of tanks, the conditions regarding the nature of foundation soil and the working conditions are more varied than in the case of river weirs.
Say for example, the river weirs or anicuts are generally founded on soft sandy soils, whereas in tank weirs, almost every class of soils is occasionally to be built on, except this rarely met foundation sand.
Similarly, the tail water conditions in anicuts differ from those in tank weirs. This is because, the tail water backing up during floods and affording some protection to the talus and to the soil at the downstream toe of the apron, are absent in the majority of tank weirs; and, therefore, the tank weirs built on soft oil would, other things being equal, be more exposed to damage by scour at the downstream toe of the apron and retrogression of levels than river weirs.
Figure A typical stepped-apron tank-weir suitable for sites on soft soil when the drop exceeds 0.9 metre.
On the other hand, the depth of water passing over the crests of tank weirs is generally far less than that over anicuts, and in the case of very many tanks, it is only, on an average, for a few days each year, that any flow at all passes over the weir, and while the duration of continuous heavy flow is frequently limited to a maximum of two to three days. Tank weirs are, thus, usually worked, far less heavily and far less continuously than anicuts, and this fact may be duly considered while designing them.
Empirical Formulas for Determining Width of Floors of Tank Weirs. The width of the horizontal floors of type A and D weirs, from the foot of the drop wall to the downstream edge of the floor should never be less than 2 (D + H) where D is the height of the drop wall, and H is the maximum water head over the wall. In important works, this width may be increased to 3 (D + H). The rough stone apron forming a talus below the last curtain wall may be of varying widths depending on the nature of the soil and the velocity and annual probable quantity and intensity of runoff ; it would generally vary from 2.5 (D + H) according to varying conditions.