Hydrotopics

May 20, 2009

HYDRAULIC JUMP STILLING BASINS ON ROUGH BEDS: A STATE-OF-SCIENCE SUMMARY

Filed under: Uncategorized — Rajnikant Khatsuria @ 1:12 pm

INTRODUCTION

 Starting with the general equation of force-momentum balance (see Figure 1)

 

  hyd jump features

    Figure 1:   Parameters of hydraulic jump on a rectangular prismatic channel

 

  M2-M1=F1-F2-Fs                                                                                                   (1)

 Where, M =momentum and F =force as defined in figure1. Analytical solution of Eq (1) requires evaluation of the shear force function Fs, which may be either shear or form drag. This is usually not possible unless empirical values of relevant coefficients and velocity at that location are incorporated into the equation. In the most simplified case of hydraulic jump in a horizontal rectangular channel with smooth bed and sides, the frictional resistance offered by the bed and sides is taken equal to zero, resulting into the classical equation (2)

eq 1 jp

 

                                                                                (2)

 

 According to Eq (2), a conjugate depth Y2 corresponding to the pre-jump depth Y1 and Froude number F1 must be provided to form a satisfactory jump. Also, a basin length corresponding to the length of the hydraulic jump, of approximately 4-6 Y2 would be required. Thus, designing stilling basins on the basis of Eq (2) resulted in uneconomical structures entailing deep excavations and longer paved lengths. Therefore, emphasis was laid on evolving designs that required lesser tail water depths as compared to Y2 and shorter lengths of paved aprons. With a re-look at Eq (1), the term Fs then gained significance.

 ACCOUNTING FOR Fs

 Analytical solution of Eq (1) with inclusion of the term Fs was, however, not possible because the relevant coefficients Cf and CD in expressions

Fs= C ρ ( V²1/2)  Ar       and     Fs = CD ρ  (V²1 /2) Ap                              (3)

were not known for the various types of blocks and roughness. Here, Cf and CD are the coefficients of shear and form drag respectively, ρ is density of water, V1 is the approach velocity and Ar and Ap are the areas of roughness and of projection of blocks respectively. The research therefore relied heavily on experiments.

 FORM DRAG

 As the shear drag resulting from roughness of the bed and sides was not expected to make significant reduction in the conjugate depth Y2 and jump length, a force generated as a form drag, caused by placing obstacles like cubes and blocks in the flow, was relied upon. Thus, the earlier designs included blocks of various shapes and sizes, just downstream of the initial depth Y1. These designs included several rows of such blocks, but as further development took place in the form of stilling basins designated as USBR basins, low Froude number stilling basins and SAF basin, only one row of such blocks was introduced. Also, special designs were evolved for site-specific applications such as Lower Bhavani and Pit 6 dam stilling basin to minimize tail water requirement and apron length. Addition of chute blocks at the beginning of the jump (to corrugate the incoming jet and aid some dissipation) and end sill at the end of the jump (to lift the flow leaving the basin and protect the unpaved portion from direct action of current) also increased efficiency of performance.

 These designs resulted in smaller size structures as compared to the stilling basins without any appurtenances. The USBR type III basin, with chute blocks, one row of triangular shape baffle piers and end sill, required an apron length of about 2.75 Y2 and a tail water depth of about 0.83 Y2, against 6Y2 and Y2 respectively for a basin without any appurtenances. The SAF basin would permit even a length shorter than Y2 and a tail water depth of about 0.85Y2. The Bhavani stilling basin employing T-shape blocks and Pit 6 basin with special shape baffle piers also resulted in economical structures.

 The above designs, however, suffered from one serious drawback that these could not be used for entrance velocities in excess of about 15 m/sec. The appurtenances, especially the baffle piers are likely to be subjected to excessive drag and even cavitation. Many instances of damage to the structures employing these designs have been reported in literature, although some structures with entrance velocity up to 20 m/sec have performed without any problem.

 SHEAR DRAG

Limited application for entrance velocities higher than 15-20 m/sec together with risk of damage prompted designers to explore the alternative of shear drag to minimize the jump length and tail water requirements. Several ways to produce hydraulic jump on artificially roughened bed have been explored so far with varying degrees of success. Studies were performed with specific objectives starting from late seventies.

 The roughened bed alternatives were: cubes of different shapes, strips, gravel laden beds and corrugations of various shapes as shown in Figure 2.

                   Blocks cubes etc                                                                                                           

                                       Gravels corrugations

                                       Figure 2:  Roughened bed alternatives

For the alternatives of cubes and strips, the length of coverage, longitudinal and lateral spacing of cubes, height of cubes and the intensity of coverage were the variables. For the gravel bed alternative, the size distribution of gravels was the important parameter. For the corrugated beds, different shapes such as sine wave, trapezoidal and triangular forms with various heights and wave lengths of corrugations have been studied.

 Of the various roughness elements, the cubes would offer a reduction in the range 10-20% in Y2/Y1 and 20-40% in jump length; strips would similarly offer a reduction of 10-15 % and 5-30% respectively. Gravel beds contribute towards reductions of 15-30 and 15-35% respectively. Corrugated beds resulted in 40% reduction in tail water depth requirement and 60% reduction in jump length. The above values correspond to the range of Froude numbers 5-9.

 A comparison reveals that while both the forms of Fs contribute nearly the same amount of reduction in the tail water depth, the shear drag offers less reduction in the jump length, accept corrugated beds, which offer significant reduction. This is largely due to the increased shear stress resulting from the interaction of the supercritical flow above the corrugations with eddies trapped in the cavities of corrugations.

 

  PRESENT STATUS

            Although, significant research has been conducted to evaluate the contributions of various roughness elements towards reduction in Y2/Y1 and jump length, there appears to be very little headway towards its practical implementation. There is no information about any stilling basin that has been constructed specifically with any of the roughness elements mentioned above. The main reason for this is that it is impractical to reproduce most of the roughness elements in nature, except small size cubes or blocks. Strips are difficult to construct and maintain. Gravel beds require binding with cementing material to hold the material intact against high velocity flow. Although, rubble concrete may be a better alternative, the upper surface resisting the flow may become smooth during course of time and loose effectiveness. Corrugations in the shape of sine wave, trapezoids or triangles are similarly difficult to construct in concrete and maintain. The apron floors of large stilling basins are cast in panels of suitable size with contraction joints on all the four sides of panels. Besides, the corrugations of each panel must match with those on all the adjoining panels. The concrete of the panel is also provided with appropriate reinforcement. This type of construction is difficult with corrugations at the top. An alternative of fixing steel plates with the desired corrugations on the base concrete of panels in place of casting the corrugations in concrete may be thought of, but then there will be a problem of seepage of flow beneath the plates and resulting dynamic uplift peeling of the plate itself. The only practical way of construction would be with small size cubes or blocks that can be cast along with the base concrete of the panel.  Another alternative may be to make rectangular corrugations as shown in Figure 3, instead of those involving curved boundaries like sine waves. This is basically large rectangular strips spaced at close interval, which would be easier to construct as on the base concrete of panels.

 

Rect corrugation details

              Figure 3:   Details of a typical floor panel with rectangular corrugations.

 

RESEARCH NEEDS

Evolving practicable configurations           

Although considerable research has been conducted on various types of roughness elements to produce hydraulic jump requiring lesser tail water depth and reduced length of jump, its direct application in practice has not made a head way as yet. Of all the alternatives, corrugated beds in the form of sine wave offer attractive results. However, there are difficulties in constructing stilling basin apron floors with corrugations as discussed above. A shape of corrugations made up of rectangular strips as shown in Figure 3 may be hydraulically efficient and easier to construct. Further research is therefore required to:

 Optimize configurations of alternative design as shown schematically in Figure 3. This should include height, width and spacing of strips and the length of the apron required to be covered. If a part covered length results only in a marginal loss of efficiency over that of the entire length of the apron covered, this may result in an economical design.

  • The hydraulic efficiency of such a design should be consistent with the structural safety in that this should be free from cavitation and excessive drag under the range of flow velocities anticipated.

 Construction practice

  •  Evolving appropriate technique of constructing apron floor in regard to determination of size of panels, stages of placement of concrete, construction of strips, contraction joints between the panels and water tight sealing between the panels.

 

 ( Neither this article nor any part of it may be reproduced in any form without the permission of the author in writing)             

 

 

 

                                                                                         

 

April 17, 2009

GATE SLOTS FOR HIGH HEAD CONDUITS: PHASES OF DEVELOPMENT

Filed under: Uncategorized — Rajnikant Khatsuria @ 4:04 am

  The most practical application of slots is in high head outlet works. A slot configuration, in general, is shown in figure 1.If the downstream boundary of the slot is sharply cornered, flow separation at the corner creates a negative pressure zone. The pressures in the slot can also be negative if a clockwise vortex is caused in the slot. This is particularly so in the case with gates having skin plate and seal on the upstream side. The implications of these are quite obvious; the pressures can be as low as to cause cavitation damage to slot boundaries and negative pressure zone inside the slot can induce a downward force on the bottom of the partly open gate, viz. downpull.

  flow-around-slot8

  Improvements in slot configurations were aimed at modifying downstream corner so as to eliminate flow separation. Rounding the corners of the slot together with recessing the downstream corner and some part of the conduit wall in continuation of the slot or providing tapered transition there, was effective in reducing magnitude of negative pressures and intensity of slot vortex. Other modification consisted of providing a 5:1 quarter ellipse in continuation of the downstream corner. The improved configurations are shown in figure 2.

improvement-slot-configure4

While the above improvements were adequate for the outlets operating in relatively silt free waters, special modifications were required for the outlets handling sediment laden flows. Slot vortices can cause silt to undergo a circulatory motion in the slot area thereby inducing abrasion damage to slot boundaries. Deposition of silt into the slot can also take place and hamper smooth operation of the gate. Modifications were directed to accomplish slot configurations that would deflect sediment flow away from the slots. This indicated a need to attach some sort of a device that can deflect the flow towards centre of the outlet so that the area in the vicinity of slots remains relatively calm. Two designs were evolved; a configuration involving a slot flow deflector at the upstream corner of the slot, without projecting into the flow, and a simpler arrangement of a deflector projecting into the flow. These are shown in figures 3 and 4.

 

slot-with-deflectors3

offset-projecting-into-the-flow2

The flow conditions in the vicinity of the slots were improved to such an extent that downpull on the gates was reduced to nearly 60% of that without the modifications. However, there still remains a possibility of slight negative pressures on the slot boundaries and some abrasion damage. Therefore, such boundaries are generally provided with steel lining.

(Copyright: Neither this article nor any part may be reproduced or copied without the permission of the author in writing)

 

January 12, 2009

REMODELING OF EXISTING SPILLWAYS (PART 3)

Filed under: Uncategorized — Rajnikant Khatsuria @ 6:30 am

Remodeling for ensuring structural safety

 

A necessity for remodeling to ensure structural safety may arise because

 

  • the dam is pretty old and turns out to be structurally unsafe according to present practice
  • increased inflow/outflow discharge requires a wider section of the spillway
  • additional loads are introduced following a major earthquake

 

In the above cases, hydrologic and hydraulic safety considerations are also involved together with structural safety that generally requires strengthening of the dam cross section with buttresses and other similar measures. However, for spillway, widening of the cross section is the only alternative available. In addition to ensuring a good bond between the old and the new construction, such a modification would have serious implications on the hydraulics of the spillway and energy dissipating arrangements. Figure 1 depicts a possible remodeling of an existing spillway by way of flattening of the rear slope for widening of the section, which would indicate such implications.

 

   flattening-of-profile4

 

 

  

 Two case studies are discussed here to illustrate the aspects involved. The first case deals with an old spillway subjected to increased design inflow and concerns about the structural stability due to the increased height of the dam. The second case deals with a spillway requiring wider section due to additional load to be accounted for the increased seismicity.

 

Loch Raven Dam, USA

 

The Loch Raven dam, in Maryland, USA is a 96 year old structure with a 30 m high dam and an 88 m long spillway. It was originally designed for an inflow flood of 1189 cub met/sec (42000 cfs) and an outflow flood of 637 cub met/sec (22500 cfs) with a depth of overflow of 2.44 m (8 ft). The safety considerations resulted in a revised inflow flood equivalent to PMF of 6796 cub met/sec (240 000 cfs) and corresponding outflow of 4870 cub met/sec (172 000 cfs) with a depth of overflow of 8.84 m (29 ft). To accommodate this, the height of the dam was increased with the spillway crest elevation remaining unchanged. The spillway section was widened and the stilling basin was converted to a submerged slotted bucket, noting that excessive scour may result for the higher discharges. For further details, refer Redeveloping Loch Raven

 

Koyna Dam, India

 

The 103 m high Koyna dam in Maharashtra state, India was designed with conventional method as a rubble concrete gravity dam. Seismic forces considered were nominal corresponding to horizontal acceleration of 0.05 g constant over the height of the dam. No vertical component of earthquake was considered. After the earthquake of 6.5 magnitude, which occurred on 11 December, 1967, damaged portions of NOF dam were strengthened with concrete backing and buttresses. There was no damage to spillway section. Another earthquake of 6.3 magnitude occurred in the region in September 1993. Though, there was no damage this time, the authorities decided to adopt strengthening measures based on the analysis considering seismic forces corresponding to a Maximum Credible Earthquake of 6.8 magnitude. While the NOF portions already strengthened were found to be safe, the spillway section needed strengthening. The Pseudo static analysis carried out or the load combination (Res. At FRL+EQ+extreme uplift) indicated the necessity of increasing the rear slope of the spillway section from 1V:0.725H to 1V:1.1H. The flattening o the slope resulted in an encroachment into the stilling basin, requiring extension of the latter in the downstream direction. The final section evolved as a result of hydraulic and structural analyses is shown in Figure 2.

 

     koyna2

 

   

 

  

Reference:

Bhave,A.P. and Joshi, S.G. (2006)- Evaluation of Existing Dams & Action Plan for Restoration Case study: Koyna Dam- Proc. A national Level Short Term Training Programme on Evaluation of Existing Dams and Action Plan for Restoraion, conducted by Sinhgad College of Engineering, Pune, India. Feb 2006.

 

 

 

 

 

 

 

 

 

           

           

October 29, 2008

REMODELING OF EXISTING SPILLWAYS (Part 2)

Filed under: Uncategorized — Rajnikant Khatsuria @ 7:10 pm

 

(For similar topics refer https://hydrotopics.wordpress.com)

 

R.M.Khatsuria

(email: rmkhatsuria@rediffmail.com

 

 

 (Want to refer Part 1?)

Remodeling for the purpose of increasing the storage of the reservoir involves raising of the reservoir level above the normal operating level and this would result in additional submergence of land in the upstream. In the case of ungated spillways, the normal operating level is at the crest level and there always is a temporary submergence caused by the overflow depth during the passage of floods. Thus, the full reservoir level (FRL) can be raised up to the maximum depth of overflow corresponding to the allowable limit of the upstream submergence. However, with the gated spillways, the FRL is required to be raised, which brings additional area under the submergence, beyond that corresponding to the original FRL. Thus, the remodeling is to be accomplished under the above mentioned pre-requisites.

When the FRL is raised in order to increase the storage in the reservoir, it also reduces the spillway outflow discharge because of the additional flood absorption rendered by the increased space in the reservoir. Thus, if additional submergence on upstream is permissible, an alternative of installing larger gates on the original crest level may be advantageous as compared to the alternative of lowering the crest and installing large gates.

UNGATED SPILLWAYS

Flash boards

Flash boards on ungated crests have been used traditionally to retain additional storage. These are suitable for small spillways and to impound small water depths. The boards are so designed that they fall down as soon as the water level reaches the top of the boards. Their disadvantage is that once they fall down, the additional storage created by them is lost and they have to be put in to the condition again. An improved design has been developed by Francois Lemperiere, with which the boards can be designed to retain water depth up to a predetermined level, can be allowed to be overtopped up to a designated discharge and would fall down for higher discharge. Thereafter, the boards have to be replaced. This design has been patented under US Patent 5061118.

Installing crest gates

Sometimes ungated spillways have piers on the crest for supporting a road bridge. In such cases, the temporary storage created by the depth of overflow during floods can be retained permanently by installing crest gates, yet retaining the Maximum Water Level (MWL) the same. It is advantageous to install inflatable rubber weirs on the crests to achieve ease and flexibility in the operation of gates.

Labyrinth weir

A labyrinth weir on the crest of the spillway can convert the temporary storage (corresponding to the depth of overflow) into permanent storage in such a manner that the existing MWL remains the same. The original depth of overflow is divided into the height of the labyrinth weir and the depth of overflow over the labyrinth weir to pass the design discharge. This is shown schematically in figure 1.

 

Fuse gates

Fuse gates can increase both spillway capacity and storage. If only the spillway capacity is to be increased, the crest of the fuse gate is set near the original crest level, thus increasing the depth of overflow substantially just before the fuse gate tilting. For increasing the storage only, the crest of the fuse gate is set higher than the original crest level. At the Terminus dam (Lake Kaweah), USA, world’s largest fuse gates, 6.4m (21 ft) in height have been installed, to increase the storage from 183,300 acre-ft to 225,300 acre-ft, about 25%.

The standard fuse gates are usually not reusable because of the damage due to falling over a considerable height. Recently, a design of what is termed as Recoverable fuse gate has been evolved, which is basically an improved version of the classical flash board.

Concrete fuse plugs

Hydrocoop, France have developed simple arrangement employing concrete fuse plugs that serve the purpose of creating additional storage or increased spilling capacity in the same manner as the fuse gates described above. The functioning of the plugs is depicted in figure 2. There are two types of the plugs; those that tilt before overtopping and those tilting after they overtop. Plugs that tilt before overtop have their heights about double the length whereas the plugs allowing considerable overflow before tilting have their heights about 3 to 10 times the length. Figure 2 shows general arrangement of fuse plug for increasing storage capacity.

 

Piano Keys Weir

The Piano Keys weir, popularly known as PK weir can be considered as an improved version of labyrinth weir. Its development is due to the commendable efforts of F. Lempérière under the famous Hydrocoop, France. PK weirs may be favored when the unit discharge to be handled is more than 20 m2/s, and where the construction of labyrinth weirs would be expensive. Another advantage of the PK weir is that it can be conveniently installed on a truncated spillway sill or on a non overflow dam unlike the labyrinth weir, which occupy larger space. Its functioning can be explained with reference to figure 3.

It is found that PK weir installation can affect about 20% increase in the storage capacity or the outflow capacity can be almost doubled with the same reservoir level.

 

(Copyright: Neither this article nor any part of it may be reproduced or copied in any form without permission in writing from the author)

 

 

 

 

 

 

 

 

 

 

October 23, 2008

REMODELING OF EXISTING SPILLWAYS (Part 1)

Filed under: Uncategorized — Rajnikant Khatsuria @ 6:37 pm

 

(For similar topics refer https://hydrotopics.wordpress.com)

 

R.M.Khatsuria

(email: rmkhatsuria@rediffmail.com)

 

Though, all the aspects concerning hydrology, geology, topography and operational needs are taken into account while designing a spillway, eventually some spillways of  existing dams require remodeling. This may be due to a variety of reasons. The most prominent are:

·         Safety of the dam- This may be hydrologic safety or structural safety

·         Need for increasing the storage capacity of the reservoir

·         Environmental consideration, i.e. reducing fish mortality during overflow 

·         Replacement of aging components such as gates 

The most compelling case is concerning the hydrologic safety of the dam. In the last decade, existing dams were specifically surveyed in various countries to assess their safety. One of the most comprehensive surveys was conducted by the Corps of Engineers of the US Army (USACE), which included more than 80,000 dams. It was revealed that about 36% of the dams were  unsafe due to various reasons. However, the most alarming finding was that about 80% of these  were unsafe because of inadequate spillway.

Significant advances in the science of hydrology during the past three decades have taken place.  Serious shortcomings in the spillway inflow floods of many dams that were designed prior to the beginning of the nineteenth century have been revealed. Actual floods that passed down the spillways often equalled or exceeded the design values, just when these dams were believed to be operating as anticipated. History is replete with the cases of inadequate spillway capacities and failures due to overtopping of dams. The example of the spillway of Machhu II dam, Gujarat, India would serve to amply illustrate this aspect. The inflow flood for this spillway, at the design stage was presumably arrived at with calculations on a simple formula involving only the catchment area and nothing else. Accordingly, for a catchment area of 1928 sq.km, the design flood was estimated as 5663m3/s. After seven years of operation of the spillway, the catastrophic floods of August 1979 resulted in an actual discharge of 16307 m3/s passing down the spillway. The earthen dam failed due to the overtopping of about two meters, while the masonry spillway remained intact. The hydrology of the project was revised and the modified inflow flood was 20925 m3/s. Since the existing spillway was not capable of handling this discharge, an additional spillway was added to take care of the increased flood.

While addition of a spillway resolved the problem at Machhu II dam,it is clear that such a solution would not be possible for every spillway that is found to be inadequate. Conditions existing at a particular site would call for a solution unique to that site. In many cases, modifications to the existing spillway may be the only alternative available. Besides, requirements arising out of the need for additional storage, structural strengthening of the spillway or some appurtenances, etc. would call for a modification. This is generally known as remodeling. 

As mentioned above, the scope and the extent of remodeling would be governed by the local conditions at the site. It would be the designer’s imagination and experience that would shape the remodeling to cater to the needs. Case studies of the projects underwent the remodeling serve as the guide lines. These are discussed to illustrate the approaches and methodologies.

CATERING TO THE HIGHER INFLOW FLOOD

If an additional spillway to handle the increased flood is not feasible, other alternative may be to extend the spillway laterally to replace a portion of non overflow or earth dam. This may be possible only if there is additional space available. 

Several other alternatives are available depending upon whether the spillway is gated or ungated.

Ungated spillways

·     Increasing the depth of overflow is an easier means if the resulting increase in the Maximum Water Level (MWL) is acceptable. Sometimes, increased area under the reservoir submergence may not be allowed and under this circumstance another solution is to be found out. Also, the ratio of the actual increased depth of overflow to the design head should not be larger than say 1.3 or so. This is to ensure that the sub-atmospheric pressures developed for the increased head are within acceptable magnitude to avoid cavitation. The encroachment in the available free board for the non- overflow dam has to be made good by increasing the dam height. If the requirement is small, usually a parapet is added to recoup the margin lost.

 

·    If the increased area under the submergence or encroachment in the free board is not acceptable or if raising the height of the non- overflow or earth dam is not feasible, the only means available is to lower the crest level of the spillway so that the actual depth of overflow is more than the design head and MWL is retained the same. This is shown schematically in figure 1.

 

 

Figure 1: Lowering the crest lavel

  

·    If the increased discharge is too large to be handled by the available increase in the depth of overflow, the alternative of installing a labyrinth          weir  on the crest will be suitable. A fuse gate can also be installed instead of a labyrinth weir. These alternatives are shown in figures 2 and 3.

 

 

·    The Piano Keys weir, popularly known as PK weir can be considered as an improved version of labyrinth weir. Its development is due to the commendable efforts of F. Lempérière under the famous Hydrocoop, France. PK weirs may be favored when the unit discharge to be handled is more than 20 m2/s, and where the construction of labyrinth weirs would be expensive. Another advantage of the PK weir is that it can be conveniently installed on a truncated spillway sill or on a non overflow dam unlike the labyrinth weir, which occupy larger space. Its functioning can be explained with reference to figure 4. It is found that PK weir installation can affect about 20% increase in the storage capacity or the outflow capacity can be almost doubled with the same reservoir level. Installing PK weirs at the Goulours dam in France indicated that a discharge capacity of 69 m2/s could be obtained on a 11.5 m long PK weir with a head of 1 m. A conventional spillway would have required a head of 2 m on a 25 m length.

 

Figure 4: Piano Key (PK) Weir- Schematic

 

     It must, however, be noted that higher discharge is passed down the spillway through the same width as before, resulting in the increased discharge intensity. Its implications on the performance of the energy dissipator should be studied for the condition of higher discharge. This is particularly so for the hydraulic jump stilling basin or roller buckets, which if subjected to discharges higher than the design discharge, could result in a sweep out condition leading to excessive erosion. If the increase in discharge is small, a slight deficiency could be tolerated as passing of the higher discharge would be rare and the damage would not be of the magnitude to endangering the safety of the dam.

 

Gated Spillways

 

      Increasing the depth of overflow along with replacing the existing gates with a larger size gate and increasing the height of the non overflow or earth dam (if the available free board is not adequate) is the easiest means for passing the increased discharge. This is, however, subject to the conditions that additional submergence is acceptable, the crest piers can accommodate larger gates and the non overflow or earth dam can be raised as necessary.  In case, the additional submergence cannot be accepted or the non overflow or earth dam cannot be raised as required, the only alternative will be lowering the crest level of the spillway and installing larger gate so that the MWL remains unchanged. This is indicated in figure 5.

 

 

Some examples of increasing the discharge capacity  with remodeling involving lowering the crest level /installation of crest gates: 

 

 Riolunato dam in Italy : Lowering the crest level and increasing its hydraulic efficiency effected a 55% increase in spillway outflow

 

 Peublo Viejo dam spillway in Guatemala : Loweing the crest level and installing gates effected 12% increase in spillway outflow 

 

 Nacimiento dam in USA : Loweing the crest level and installing rubber weir on crest ensured 42% increase in spillway outflow

 

 

(Copyright : Neither this article nor any part may be reproduced/copied without permission in writing from the author)

 

September 4, 2008

FLOW OF WATER- SINGLE AND ACCOMPANIED!

Filed under: Uncategorized — Rajnikant Khatsuria @ 10:06 pm

 

(For similar topics refer https://hydrotopics.wordpress.com)

R.M.Khatsuria

(email: rmkhatsuria@rediffmail.com)

 

Although, one differentiates between a fluid and a rigid body, there are many similarities in the fluid dynamics and the dynamics of rigid body. Both are covered under the general theory of continuum mechanics. Several problems concerning flow of water have been successfully treated with the principles of rigid body dynamics. As an example, the equations of the trajectory of a high velocity jet of water issuing from a nozzle have been derived on the basis of dynamics of rigid body, assuming that the flow of water consists of a series of water particles travelling in succession, at a given velocity and inclination. It could also account for the effects of external force such as resistance by air. We know that the flow of water is more susceptible to this force than a rigid body, and the analysis is applicable here also.  However, beyond this limit, the treatment differs. The flow of water is subjected to an internal force also, conspicuously absent in a rigid body. This is the internal turbulence in the flow that works to exert a retarding effect. If the velocity is beyond certain limit, the flow also entrains air from the surroundings and then it is known as two-phase flow. Entrainment of air in the flow of water may result in bulking and disintegration of the water mass. These phenomena have, of course been analyzed- only a little theoretically and mostly empirically. Thus, the flow of water alone is amenable to the analysis, much the way as in the dynamics of rigid body, but the flow of mixture of water and air needs different treatment.

Another example of the two-phase flow is the mixture of water and sediment. If water flows along a bed consisting of loose material like sand, gravel or clay particles, this material can also be picked up by the flow and carried along with it. While, both the flow of clear water and rigid particles can be analyzed individually, a sediment laden flow requires different approach and here again, much is empirical.

So what are the implications of such accompanied flows, in addition to their not being amenable to analysis through simple mechanics? Entrainment of air in flow of water dampens part of turbulence and results in reduction of drag at the boundary of the flow.  Air entrainment results in bulking of water mass so the depth increases in comparison to clear water depth. This would necessitate higher side walls to contain the flow. Yet, air entrainment is not always a problem; it is a remedy also! It is known that a small quantity of air entrained in flow (some 8% or so) close to the flow boundary significantly reduces cavitation damage. The surfaces of spillways, outlets and tunnels subjected to flows of high velocity-of the order of 25m/s or so-are protected against cavitation damage by artificially entraining them with required quantity of air. This technique of forced aeration has been successfully applied to a large number of spillways, outlets and tunnels, to reduce cavitation risk, during the last two decades. The devices that force air in the bottom layers of flow are known as aerators.

While the air entrainment in the bottom layers of high velocity flows on spillway surfaces is beneficial in preventing cavitation damage, flows with higher air contents in bottom layers in stilling basins and river downstream are detrimental to aquatic biota. There is some conflict between the two objectives discussed above. More discussion on how these are tackled will be discussed later.

Sediment laden flows on surfaces of hydraulic structures are always a problem. Flows containing coarse sediment in bottom layers with high velocities have damaged surfaces of spillways, outlets and tunnels by abrasion. Even when special arrangements are made to evacuate sediment load from the flow, presence of finer material in suspension have damaged blades of hydro turbines by abrasion. Devices called settling tanks are employed to evacuate as much of the sediment as possible before the flow is admitted to power plant. If the flow containing coarse sediment is to pass down a spillway, the best course would be to provide a protective coating on the spillway surface. In some cases, a large part of sediment can be flushed out by a special operation.

          (Copyright: Neither this article nor any part may be reproduced, copied or transmitted in any form without permission in writing from the author)

 

 

         (In the next issue:  Remodeling of existing spillways)

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