Structures for Flow Diversion, Investigation Planning and Layout (Part - 3) - Civil Engineering (CE) PDF Download

Planning and layout 

A barrage, by definition, is a weir structure fitted with gates to regulate the water level in the pool behind in order to divert water through a canal meant for irrigation, power generation, flow augmentation to another river, etc.  By following the general guidelines mentioned in section 4.14, the location and alignment of the barrage axis and that of the canal headworks may be decided but the other details, like the width of the barrage and headworks, levels of weir crests, lengths of weir floors, river training works, pond level etc. have to be finalized based on the hydraulic conditions and geologic characteristics of the river bed and banks of the site.  This section is devoted to these planning and layout concepts of a barrage project consisting of the main structure and its appurtenant works.  

The planning part decides the various parameters necessary for designing the structures. Further, planning is also necessary for chalking out a construction program. The major planning aspects are as follows:

Design flood

The diversion structure has to be designed in such a way that it may be able to pass a high flood of sufficient magnitude (called the design flood) safely.  It is assumed that when the design flood passes the structure all the gates of the structure are fully open and it acts like a weir across the river with only the obstruction of the piers between the abutments.  The abutments are the end walls at two extremes of the structure and the length in between the two is termed as the waterway. Naturally, a high design flood would necessitate a longer waterway.  In general, a design flood of 1 in 50 years frequency  has been recommended for design of all items except free board for which a minimum of 500 year frequency flood or the Standard Project Flood has been recommended as per Bureau of Indian Standard Code IS: 6966 (Part1) - 1989 “Hydraulic Design of Barrages and Weirs – guidelines”, some of the barrages built in the past have considered very high design floods, as may be seen from the data given below: 

Though a high design flood may ensure safety of the structure against large floods, there is a consequent adverse affect related to sediment deposition in the pool.  This results from the fact that since a design flood is expected to pass once in that many years, with a full gate opening, the intervening years having lesser magnitude of floods would see the gates of the barrage being operated to raise the pond level at the desired elevation.  Naturally, this would result in a slower velocity in the pool and a consequent deposition of suspended sediments.  If sediment deposition continues for many consecutive years, they tend to form large mounds, called shoals, within the pool, not far upstream from the barrage bays.  This phenomena, which has been noticed in many of the large barrages of India, like Farakka, Mahanadi, etc.,  can cause not only reduction in the pool volume but more importantly, may cause obstruction to the free flow of the river that is approaching the barrage.  This results in what is called the washing of bays, with the flow through the bays directly downstream of the shoals being reduced and the excess flow passed through the other bays.  As a result, it causes inclination of the approaching flow to the barrage which may cause other undesired phenomena.  It has been observed that barrages with large shoal formations just upstream have flow inclinations to the extent of 600 or more to a normal through the barrage axis. 

Afflux 
If the flood in the river is less than the design flood, then some of the gates would be fully opened but the remaining opened to such an extent which would permit the maintaining of the pond level.  However, when a design flood or a higher discharge through the barrage structure, all the gates have to be opened. Nevertheless, the structure would cause a rise in the water level on the upstream compared to level in the downstream at the time of passage of a high flood (equal to or more than the design flood) with all the gates open.  This rise in water level on the upstream is called afflux.  The amount of afflux will determine the top levels of the guide bunds and marginal bunds, piers, flank walls etc.  Naturally a smaller waterway would result in larger afflux and vice versa.  Hence, reduction in water way may cause in lowering the cost of the barrage structure but may result in higher afflux and a resulting larger height of bunds and piers.  

Figure 7. Afflux explained, (a) River at normal flow:Depth almost same as on the downstream of a ponded flow as in (b). (c) River at flood level: Downstream depth almost the same as that of the original river ; upstream depth higher than downstream depth, the difference being called Afflux as in (d).

Free Board 

Once the permissible afflux is decided, the necessary water way can be accordingly worked out and the upstream water level estimated for the design flood.  Over the gauge-discharge curve on the downstream side and estimated on the upstream, sufficient Free Board has to be provided so that there is no overtopping of the components like abutments, piers, flank walls, guide bunds, afflux bunds etc.  The Free Board to be provided depends on the importance of the structure generally, 1.5 to 2 m Free Board above the affluxed water level on the upstream and above the high flood level on the downstream is provided.  A freeboard is provided over an affluxed water level due to a flood with 1 in 500 year frequency.   

Figure 8. Free board has to be kept above the respective high flood levels, on the upstream considering afflux and on the downstream without afflux

Pond Level 

Pond level is the level of water, immediately upstream of the barrage, which is required to facilitate withdrawal of water into the canal with its full supply.  The pond level has to be carefully planned so that the required water can be drawn without difficulty.  By adding the energy losses through the head regulator to the Full Supply Level of the canal at its starting point just downstream of the canal head-works, the pond level is evaluated. 

Figure 9. Section through a canal head regulator. Pond level should be equal to the canal full supply level added to the head loss through the head works while delivering the canal flow

The provision of a high pond level with an elevation almost equal to the high flood level or above has to be planned very carefully since such a provision is likely to induce shoal formation on the upstream. This has happened in the Durgapur Barrage on river Damodar.

Waterway

As discussed earlier, waterway, or the clear opening of a barrage to allow flood flow to pass has a bearing on the afflux.  Hence, a maximum limit placed on the afflux also limits the minimum waterway. Many a times, the Lacey’s stable perimeter for the highest flood discharge is taken as the basis of calculating the waterway. However, it should be remembered that Lacey’s formula is based on studies of canals in the alluvial regime and may not be quite correct for large rivers, and also for rivers in boulder or clayey reaches.  Nevertheless, application of the Lacey’s waterway would require the following calculations as given in Bureau of Indian standard Code IS: 6966-1989 “Guidelines for hydraulic design of barrages and weirs: Part 1 Alluvial Reaches”. 

P = 4.83 Q^1/2              (1) 

Where Q is the design flood discharge in m3/s for the 50 year frequency flood.  In the case of rivers in bouldery reaches, the width available at the site is limited by the firm banks. For meandering rivers in alluvial reaches, a factor is usually multiplied with the perimeter obtained by Lacey’s formula, which is called the looseness factor, as given below 

Silt factor f is calculated by knowing the average particle size d₅₀ is in mm of the soil from the following relationship  

f =1.76 (d₅₀)^1/2               (2)

By limiting the waterway, and consequently increasing the velocity and discharge per unit width, the shoal formation in the pond upstream of the barrage can possibly be minimized.  However, it has an adverse effect also since increase in the intensity of discharge, requires longer solid apron and deeper sheet piles due to higher expected scour depths.  Nevertheless, the performance of many barrages has led to the general observation that high looseness factor, more than about 1.0, results in shoal formation in the upstream pool.  Hence many recent barrages have been designed with a low looseness factor, nearing 0.5.  However, there is a need for a systematic study to evolve a scientific analysis for evaluating the waterway.  

A restricted waterway for a barrage is obtained by the use of guide bunds, approach and afflux embankments in Figure 10. 

FIGURE 10 . Typical layout of river training works for restricting waterway of barrage