Equilibrium Time of Scour Near Water Intakes on River Floodplains
2016
Oskars Lauva

Defending
02.11.2016. 16:00, Rīga, Ķīpsalas iela 6 B, 250. auditorija

Supervisor
Boriss Gjunsburgs

Reviewers
Voldemārs Barkāns, Algirdas Radzevičius, Jana Pařílková

The study of equilibrium time of scour calculation methods at steady-flow and clear-water conditions revealed that the most common parameters used are: approach flow depth; approach flow velocity; critical flow velocity; structure size parameters (abutment length and width, or pier diameter); median size of the sand; and sand density; however, several parameters are left out or avoided, such as: contraction rate of the flow; local flow velocity at the structure; bed stratification; flood duration; flood sequence; flood probability; and flood frequency. The differential equation of the bed sediment movement in clear-water scour conditions was used and a new method for equilibrium time of scour calculation at water intakes with and without flow separation at the structure in clear-water scour conditions was elaborated. Data from previously conducted and published experiments were used for equilibrium time of scour development calculations and computer modeling. The proposed threshold criteria for equilibrium time of scour known from the literature are only depending on the size of the hydraulic structure, and not on hydraulic parameters of the flow. Ratio of the recalculated critical flow velocity to the local one at the head of the water intake was proposed as the hydraulic threshold criterion in equilibrium time of scour calculation. Different values of the new hydraulic threshold criterion were presumed to find the best fit between computer modeled and calculated equilibrium time of scour values. As a result, the proposed equilibrium time of scour calculation threshold criterion for water intakes that showed the best agreement is equal to βV0t/Vlt = 0.985. Using the new hydraulic threshold criterion values hequil, Aequil, Dequil, xequil, Nequil and finally time to equilibrium scour tequil can be calculated. An electronic time to equilibrium scour calculation model was created. Previously conducted flow-altering method against scour at abutments experiment results show that using flow-altering method against scour at abutments, results in equilibrium depth of scour and time of scour reduction. Since the flow-altering method mostly affects the wall, scour depth reduction at the wall was almost three times more effective, than at the abutments nose. Recently carried out live-bed pier scour experiment results revealed that as the ratio of approach flow velocity to critical flow velocity increases, equilibrium time of scour is reached faster with decreasing scour depth value. The duration of water intake laboratory tests of 7 hours was prolonged by using computer program “RoBo”, until the scour depth development reached the equilibrium stage. Calculated water intake test data revealed that with an increase in flow contraction rate and with an increase in approach flow Froude number, equilibrium time of scour increases as well. To verify the developed equilibrium time of scour evaluation method, calculated time of scour values were compared to computer modelled ones; a percent relative error was determined for each calculated and computer modeled time to equilibrium scour test values; using the determined percent relative errors a line of agreement was drawn for each set of water intake experiment data, the results showed good agreement. The developed method for equilibrium time of scour calculation at water intakes with flow separation at the structure was compared with other equilibrium time of scour evaluation methods at abutments available in the literature, and it showed that other calculation methods give great over and under-predicted equilibrium time of scour values, when the approach flow velocity is much lower than the critical flow velocity, ignoring the factor of flow contraction rate. Theoretical analysis of the developed equilibrium time of scour calculation method was made and it showed that equilibrium time of scour depends on: flow contraction rate; kinetic parameter of flow in contraction in open-flow conditions; kinetic parameter of the open flow; ratio of the Froude number to the river slope; dimensionless sand grain size; ratio of the recalculated critical flow velocity to the local flow velocity; relative flow depth; and relative scour depth. Graphical hydraulic and riverbed parameter dependence analysis of the proposed equilibrium time of scour calculation results was made and it showed that equilibrium time of scour depends on: flow contraction rate; relative depth of scour; Froude number; and relative velocity of the flow. This calculation method can be applied to river water intakes with and without flow separation at the structure at steady-flow and clear-water conditions, as well as, other water engineering and hydraulic structures like bridge abutments, piers, guide banks, dams and roads located on river floodplain area. Equilibrium time of scour values are used worldwide in equilibrium scour depth calculation methods, where it is essential to use the most precise predictors of time. A reliable time of scour prediction method can give an advantage to engineers, to know when the equilibrium depth of scour has been achieved, to understand the stability of a water engineering structure. Promotional work consists of: introduction, 5 chapters, conclusion, 3 appendixes, 91 references, 25 figures, 17 tables, and together 90 pages.


Keywords
Līdzsvara izskalojuma laiks, saspiesta plūsma

Lauva, Oskars. Equilibrium Time of Scour Near Water Intakes on River Floodplains. PhD Thesis. Rīga: [RTU], 2016. 90 p.

Publication language
English (en)
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