FlowFlowDiagonalDuckbillLabyrinthGlobal Journal of Environmental Resea - PDF document

FlowFlowDiagonalDuckbillLabyrinthGlobal Journal of Environmental Research 2 (3): 122-125, 2008ISSN 1990-925X© IDOSI Publications, 2008Corresponding Author: Dr. A.D. Ghare, Department of Civil Engineering. V N I T, Nagpur, India122Hydraulic and Environmental Aspects of Long Crested WeirsA.D. Ghare, P.J. Wadhai, N.J. Mistry and P.D. Porey Flow Labyrinth ScouringINTRODUCTIONdesign flow with smaller head. This means that largeWeir is deliberate obstruction constructed inasmaller changes in head and small changes in flow intowatercourse. Provision of weir increases water surfacethe lateral or farm turnouts upstream of weir. Large crested 3 2 QCLHLL0.2H=−Global J. Environ. Res., 2 (3): 122-125, 2008123Hydraulic Aspects: The discharge over a long crestedWaterStress Indicators (WSI) as found in literatureweir can be calculated by are as follows:(1)whereC is the coefficient for the type of weir crest,Smakhtinet al. [4] modified this indicator byLe is the effective weir crest length and H is the total headaccounting water use for maintaining ecology andover the weir crest. The effective crest length of weir isenvironment as defined by(2) requirement for aquatic Eco system)(4)Obtaining correct crest coefficient C is difficult asFor examining environmental issues, it is necessarythis coefficient changes with head on the weir. Generally,to consider both short term impacts which are likely to befora well aerated sharp crested weir C is about 1.85. For aencountered during the progress of the works and in neardiagonal or duckbill weir this value is in the range of 1.4 tofuture, as well as the longer term impacts that would1.5. For a labyrinth weir, this coefficient depends on (H/P)featurein the distant future. Because of raising of waterratio and also the labyrinth angle [1]. P denotes thelevels, weirs offer the opportunity of creation of wetlandheight of weir. For all the long crested weirs the capacityand conservation habitat as well as enhance rivers andof flow magnification diminishes and it approaches to thetheir surrounding areas. The provision of weir negates thecapacity of a perpendicular linear weir as (H/P) increasespossibility of the river drying out also it increases theand exceeds over 0.8 or so.aeration of river water as it cascades over the weir crest.EnvironmentalAspects: Water is perhaps the mostcompared to a linear weir. This can lead to a rich anddominant constituent of the environment asitdiverse environment for both aquatic and terrestrialsupports life on the globe. The action planspecies. Thus weirs have a significant impact on theenvisaged in National Environmental Policy [2]amenity value of rivers, creating opportunities forincludes the following:enhanced use of water. However, at the same time, the Promote integrated approaches to management ofdetrimental to nature conservation. This barrier preventsriverbasins by the concerned river authorities,the migration of fish upstream and downstream, thusconsidering upstream and downstream inflows andlimiting their access to suitable spawning sites as wellwithdrawalsby season, pollution loads and naturalas reducing the overall biological value of fishery.recreational capacities, to ensure maintenance ofHence it is important that all the possible environmentaladequate flows and adherence to water qualityimpacts are considered before a decision on weirstandards throughout their course in all seasons.provision is taken. Consider and mitigate the adverse impacts on riverflora and fauna and the resulting changes in theWeir Aeration: DO is generally used as an indicator ofresourcebase for livelihoods, of multipurpose riverthe quality of water used by humans or serving asavalley projects, power plants and industries.habitat for aquatic flora and fauna. Respiration by Consider mandating the installation of water savingaquatic life serves to reduce DO. Weir aeration occurs inclosets and taps in the building byelaws of urbanwatercourses and in water treatment plants. Labyrinthcentres.notches can be introduced for increased turbulence for Set up a legally enforceable regulatory mechanismmixing the chemicals and also to aerate the water. Higherforidentified valuable wetlands to prevent theirthe head higher will be the aeration and longer cresteddegradation and enhance their conservation.weirs offer this advantage. When free nappe plunges intoDevelop a national inventory of such wetlands.downstream tail water, air entrainment and turbulenceAs per Alcamo et al. [3] WSI = Withdrawal / Mean Annual Runoff (MAR)(3)WSI =Withdrawal/(MAR-Environmental waterThe aeration is even more in case of labyrinth weir asfact that the weir creates a barrier in the river can be cos0.1661.955 2 Q0.0033qh1.193 Global J. Environ. Res., 2 (3): 122-125, 2008124contribute to oxygen exchange. Baylaret al. [5] concludedunderside of a turbine, providing surface water pumpsthat weir shape was the important factor influencing thewhich push warm, oxygen rich surface water downward oraeration efficiency and contended that the weir geometryintroducing oxygen injection systems can be adopted.defines nappe shapes that are unique to each weir andHowever, more cost effective alternative can be providingtheoxygen transfer seems to strongly depend on thelong crested aeration weirs at a short distancenappe shapes.downstream from dams.Forlow flows associated with aeration the preferreddesign is a labyrinth weir, which increases the river’s DOFish Migration: Fish migration in water courses may becontent by creating a number of waterfalls. As water flowsrestricted if a structure fails to make proper provision forover the weir, it splashes into the plunge pool below,their passage. The primary factors affecting the passagecreating number of small bubbles and high turbulence.of fish through a structure are the flow velocities in andSystematic experimental study on triangular planformdownstream of the critical section, the available width andlabyrinth weir was carried out to measure the airdepth of flow at the structure and the drop in water levelentrainment rate by Emiroglu et al. [6]. For horizontal sills,over the structure as also the swimming capabilities andfollowing correlation equation was proposedleap height of migratory species. The most common typechannel partitioned into a number of pools. For the long(5)crested weir like labyrinth, one of the legs may be in thewhereQ = air entrainment rate in cumecsVegetation Response: The assessment of theq = unit discharge in m/senvironmental consequences of any flow regimeh = drop height in metersrequires an assessment of the impact of the regime on theè = weir included anglefloodplain environment. One of the important aspects ofItwas also observed that the values of airmay provide the habitat for many migratory water-birdentrainment rate of the linear weir and the weirs withspecies and other floodplain biota. Perennials are thehorizontal sills were generally lower than the other typeplants which are longer-lived ( one to hundreds of years).of weirs.Hence perennials have many flowering and seedingThermal Stratification: During hot weather, deepseeds germinate and seedlings become well establishedreservoir water stratifies i.e. separates into two layers.(known as regeneration stage) can be relatively long andOne warm surface that is relatively rich in DO and anotherlast from several months to a few years. The adult stagerelatively cold bottom layer. The oxygen in the lower layer( maintenance) is much longer. The influencing variablesis gradually consumed by organic material which isfor regeneration vegetation stage are seasonality,brought to the reservoir when it rains or is dischargedduration of inundation and depth of inundation. Similarly,fromsewage treatment plants, industries or any otherfor maintenance stage the influencing variables aresource and settles to the bottom and decays. Theseasonality, drying time between inundations andstratifiedlayers of water do not mix because ofduration of inundation. As the long crested weirs affecttemperature difference and hence the oxygen in the lowerthewater levels and in turn the inundation during floods,layer does no get replaced. Towards the end of thethe effect is to be ascertained by modeling floodplainsummer, the oxygen supplies near the bottom may beinteractions with reference to vegetation response.entirely depleted. Hydro power plants typically drawwater from the deeper levels and create low oxygenDownstream Scouring: For mobile bed rivers, the bedconditions downstream of the dam. This results inis constantly subjected to special and temporalproblems for aquatic life which depends on oxygen forchanges which modify the habitat characteristics.theirexistence. In order to improve this situation variousOneof the structural measures to maintain or restoremeasures such as provision of hub baffles on thea river’s habitat diversity is provision of weir. It isof fish ways have been in the form of sloping or steppedform of fish way.the floodplain environment is floodplain vegetation whichcycles during their lifetime. For perennials, the stage when Global J. Environ. Res., 2 (3): 122-125, 2008125establishedthat the extent of scour depends onwater aeration rate and it can also be used to counter thecharacteristics of flow, bed material composition andeffects of thermal stratification of reservoirs when usedgeometry of structure. For a linear weir, the maximumfor hydropower. Long crested weir geometry alsoscour depth is expressed asinfluences aquatic life, fish migration, vegetation response ds = f(Q, y, y, P, b, B, , D)(6)must be accounted for before providing such weirs onwhereQ = discharge, REFERENCESy = depth of flow over the crest, y = tailwater depth, 1.Tullis, B.P., J.C. Young and M.A. Chandler,P = height of sill, 2007. Head-Discharge Relationships forb= width of sill, Submerged Labyrinth Weirs. J. Hydraul. Eng.,B = width of channelASCE, 133(3): 248-253. and D= relative density and characteristic sediment of2.National Environmental Policy Document, 2004.the bed material, respectivelyMinistry of Environment and Forests, GovernmentLaboratory studies on W type labyrinth weir were3.Alcamo, J., P. Doll, T. Henrichs, F. Kaspar, B. Lehner,carried out by Bhuiyanet al. [7] and a typical scouringT. Rosch and S. Siebert, 2002. Water GAP 2: A Modelpattern was observed, which indicated the presence offor Global Assessment of Freshwater Resources.deeper scour holes on downstream side. Hydrological Sciences Journal. (Water Policy IssuesItis a matter of field investigations coupled withof India).laboratory investigations to arrive at a generalized model4.Smakhtin, Vladimir, Revenga, Carmen, Doll Petra andforassessment of scour depth for labyrinth weirs. ThisTharme, Rebecca, 2003. Putting the Waterwill be of help while designing downstream apron orRequirements of Freshwater Ecosystems into theenergy dissipating system for the weir.Global Picture of Water Resources Assessment.CONCLUSIONSMarch 8th, 2003. (Water Policy Issues of India)Long crested weirs such as duckbills and labyrinthsperformance of weirs. J. Water S A, 26(4): 521-526.have inherent advantage of flow magnification in the6.Emiroglu, M.E. and A. Baylar, 2005. Influence ofavailable water course width. This facilitates the waterIncluded Angle and Sill slope on Air Entrainment oflevel management leading to increased water storageTriangular Planform Labyrinth Weirs. J. Hydraul.capacity without impounding the more area for the sameEng., ASCE, 131(3): 184-189.operatinglevel. For design of such weirs, environmental7.Bhuiyan, F., R.D. Hey and P.R. Wormleaton, 2007.aspects in addition to the hydraulic aspects have to beHydraulic Evaluation of W-weir for Riverconsidered.Long crested weirs are found to have higherRestoration. J. Hydraul. Eng., ASCE, 133(6): 596- 609.in floodplain and downstream scouring. All these aspectsnatural water courses.of India, New Delhi {www.envfornic.in/nep}Draft paper- 3rd World Water Forum, Kyoto, Japan,5.Baylar, A. and T. Bagatur, 2000. Aeration