Hydraulic optimisation of a postulated sediment flushing system in low-level dam outlets

Van der Spuy, Liam (2020-04)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Reservoir sedimentation is an ongoing critical concern worldwide with a recorded global average of 33% of reservoir storage capacity already been lost therefrom, which is expected to exceed 50% by 2050 (i.e. not considering new dam projects). Drawdown or pressure flushing at hydropower intakes are common mitigation measures for removing locally deposited sediment from reservoirs. This is periodically necessary to especially keep hydropower intakes free of coarse non-cohesive sediment, as sand fractions will typically damage turbines in hydropower conduits. The aim of this study was to provide design guidelines for a postulated configuration for a sediment flushing system of a low-level outlet at a dam for control of reservoir sedimentation to protect the hydropower intakes. The postulated design, which comprised of a hybrid low-level outlet conduit and intake structure, was optimised by means of numerical and physical modelling for effectively flushing sediments (i.e. sand, gravel and boulders when the settled sediment delta has reached the dam) during different flood and water level scenarios. The intake structure (a semi-circular low-weir and ogee-type spillway with wing-walls) was designed and positioned in the vicinity of the low-level intake inside of the typical sediment scour cone that forms during pressure flushing. The main aim of the spillway was to produce desirable supercritical flow conditions upstream of the conduit intake, as well as through it. This was necessary to help optimise the flushing of sediments during free-surface flow conditions as well as to prevent deposition of coarse sediment near the outlet gates, thereby preventing closure. The submerged weir was, however, also designed for pressure flushing where the aim was: (1) local sediment removal; and (2) preventing main reservoir sediments from depositing near the outlet gate and hindering gate closure. For this study, 3D CFD modelling in ANSYS Fluent v19.1 was used to hydraulically compare four different proposed flushing system designs, from which the best design option could be chosen and further refined. Flow vectors (representing the degree of streamlined flow) and flow depths and velocities (representing the degree of supercritical flow) along the model during free-flow flushing conditions were considered for comparisons. A 1:40 scale physical model of the postulated flushing system design, which evolved from the findings of the numerical model simulations, was built for further testing and refinement. The main aim of the physical model was to test the robustness, reliability and actual flushing capability of the design. The following prototype conditions were considered during hydraulic testing of the physical model: (1) two different upstream sediment bed configurations for both pressure and free-flow flushing (2) four different sediment sizes (𝑑𝑒𝑓𝑓 = 3.6, 468, 659 and 1532 mm) for both pressure and free-flow flushing; and (3) three different upstream water levels (𝐻𝑢 = 24.16, 29.08 and 34 m), above the conduit inlet invert level, for pressure flushing. Similar testing was also performed in the absence of the intake structure in order to derive comparative results to determine the degree of influence of the postulated structure. It was found from physical model tests on the numerically optimised design that the postulated intake structure: (1) can fully flush the intake area of the considered sediment groups regarding the different upstream sediment bed configurations during free-flow flushing; (2) does not appear to affect the pressure flushing extent upstream of the intake; and (3) is crucial during free-flow flushing, where the weir is required to hold back main reservoir sediments from moving towards, or depositing near, the outlet gate operation area.

AFRIKAANSE OPSOMMING: Damtoeslikking is wêreldwyd 'n kritieke probleem aangesien ongeveer 33% van die totale reservoirkapasiteit reeds verlore is en wat na verwagting teen 2050 tot 50% sal styg (d.w.s. sonder om nuwe damprojekte in ag te neem). In die gevalle waar damtoeslikking ‘n impak het op die inlate van byvoorbeeld hidrokragstasies, word die uitspoel van sediment in die lokale omgewing van die inlate van tyd tot tyd soos nodig gedurende die bedryf daarvan toegepas om die inlate vry te hou van growwe, nie-kohesiewe sediment wat turbines kan beskadig. Die doel van hierdie studie was derhalwe om riglyne te ontwikkel vir die ontwerp van 'n gepostuleerde konfigurasie vir 'n laevlak dam-uitlaat vir die uitspoel van sediment (op ‘n vlak na aan die rivier bedding en onder die inlaat) van ‘n hidrokragstasie om die inlaat na laasgenoemde vry te hou van sediment wat turbines kan beskadig. Die gepostuleerde ontwerp is geoptimeer deur numeriese en fisiese modellering vir die effektiewe spoel van sediment (nl. sand, gruis en rotse wanneer die gevestigde sediment-delta die dam bereik het) tydens verskillende vloed- en watervlak-scenario's. Die sediment uitspoel struktuur ('n lae kruin semi-sirkelvormige keerwal met ogee oorloop-profiel en vleuelmure) is ontwerp en geposisioneer op die rivier bedding aan die stroomop kant van ‘n laevlak dam uitlaat in die sedimentuitskuur-keël wat natuurlik tydens drukvloei-spoeling vorm. Die hoofdoel van die keerwal was om in geval van beide drukvloei- en vryvlakvloei-stromingstoestande effektiewe superkritiese vloeitoestande te bewerkstellig in die gebied stroomaf van die keerwal om sedimentasie (wat die werking van sluise kan belemmer) te verhoed. Die hidrouliese aspekte (vloeisnelhede, vloeilyne en waterdieptes) van die gepostuleerde spoelstelsel is ge-optimeer vir vryvlak-stromingstoestande met behulp van numeriese modellering (ANSYS Fluent v19.1 3D CFD-model). Vier parameters is verstel in hierdie numeriese model studie waaruit die mees effetiewe konfigurasie gekies is om verder verfyn te word ten opsigte van sediment vervoer in ‘n 1:40 Froude-skaal fisiese laboratorium model. Verdere doelwitte van die fisiese model was om die robuustheid, betroubaarheid en werklike spoelvermoë van die ontwerp te toets. Die volgende afgeskaalde prototipe-opstellings is getoets in die fisiese model: (1) twee verskillende stroomop sedimentbedkonfigurasies vir druk- en as vryvlakvloei-spoeling (2) vier verskillende sedimentgroottes (𝑑𝑒𝑓𝑓 = 3.6, 468, 659 en 1532 mm) vir drukvloei- en vryvlakvloei-spoeling; en (3) drie verskillende stroomop watervlakke (𝐻𝑢 = 24.16, 29.08 en 34 m), bo die drumpel van die onderuitlaat, vir die drukvloei-spoeling. Soortgelyke toetse is ook uitgevoer in die afwesigheid van die keerwal vleuel-mure gedeelte van die spoelstelsel ten einde vergelykende resultate te verkry om die invloed van laasgenoemde te bepaal. Fisiese modeltoetse op die numeries geoptimeerde ontwerp het bevind dat die gepostuleerde spoelstelsel struktuur: (1) bewerkstellig volledige spoeling van die getoetste sedimentgroepe vir die verskillende stroomopstellings van die sediment tydens beide drukvloei- en vryvlakvloei-spoeling; (2) is krities tydens vryvlakvloei-stroming, waar uitskuring nodig is om reservoirsediment te weerhou om naby die uitlaatsluis te deponeer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107752
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