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Numerical investigations on the influence of hydraulic boundary conditions on the efficiency of sewer flushing Dr.-Ing. Joerg Schaffner. www.steinhardt.de. Downstream water level. Roughness. Sewer slope. Introduction. Recent investigations:

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  1. Numerical investigations on the influence of hydraulicboundary conditions on the efficiency of sewer flushingDr.-Ing. Joerg Schaffner www.steinhardt.de

  2. Downstream water level Roughness Sewer slope Introduction • Recent investigations: • - Focused on behaviour of flush waves on initially dry sewer/tank bottom • Simplified assumption does not match reality • Present investigation: • - Analysis of the influence of hydraulic boundary conditions on bottom shear stresses : • Longitudinal sewer slope and the bottom roughness • Initial downstream water levels caused by lateral inflows or Qdry

  3. Reference: Chow, 1959 Sewer flushing • Impoundage dry-weather runoff to designed storage level • Fast lifting of the flushing shield • Development of a turbulent flush wave downstream • Pipes 600 - 3500 mm in diameter • Cleaning distance up to several kilometers in length • Flush wave acts hydraulically like a dam-break wave • Historical analytic equations are not suitable for sewer channels • Numerical modelling (1-D) is a good tool for fast and realistic results Oldest formulation: Ritter (1892) dam-break wave

  4. Numerical Modelling • 1 – D Numerical model EDWA • Developed by Technical University of Darmstadt / Germany with special regard to the calculation of flush waves • Full Saint – Venant equations - Finite Volume Method • Godunov-Upwind scheme with approximated HLL – Riemann solver • Basic geometry, numerical grid and initial conditions • Circular sewer 1600 mm diameter ( L = 2200 m) • Location of the flushing shield according to investigations • Grid distance in flow direction: ∆ x = 0.5 m • Upstream BC was a free standing water body with vt=0 = 0 m/s. • Downstream BC: Pressure boundary • Bottom shear stress: (Energy slope method)

  5. Results: Longitudinal slope Variation of longitudinal slope I = 0.25 - 2.25 ‰ • Bottom roughness:M = 0.013 s/m1/3 (constant) • Flushing volume:V = 139.6 m³ (constant) • Hstor = 0.31 m - 0.77 m • Adjustment of storage distance according to the slope in order to keep the flushing volume constant. I = 2.25 ‰ • High bottom shear stresses at the beginning with 46 N/m². • Then fast declination of the values. • At the end of the sewer channel crit = 3 N/m² still exceeded.

  6. Results: Longitudinal slope Effective flushing distance - Location where:  < crit = 3 N/m² • Linear rise of the effective flushing distances depending on the slope. • Difference from 101 m (I = 0.25 ‰) to 2992 m (I = 2.25 ‰). • Increase of 2992 % • Major influence of longitudinal slope on cleaning efficiency of flush waves. • Fortunately: Slope of sewer channel is usually well known and reliable value.

  7. Results: Bottom roughness Variation M = 0.01 - 0.025 s/m1/3 (very smooth concrete - medium sized gravel) • Constant values: • IS = 1 ‰ • Hstor = 0.55 m • VFlush = 139.6 m³ • Distribution of the shear stresses at the end of the sewer channel • Shear stresses increase with a higher M-value while the flow velocity drops. • M = 0.01 s/m1/3: wave running time t = 1446 s and max = 2.29 N/m². • M = 0.025 s/m1/3: wave running time t = 3538 s and max= 4.21 N/m².

  8. Results: Bottom roughness • High influence of bottom roughness on: • Wave flow velocity • Water level development • Bottom shear stresses • On the necessary flushing volume (design volume). • Correct choice of the bottom roughness very difficult for the planning engineer when modelling a flush wave. • Bottom roughness is usually unknown new and existing sewer channels. • Existing sewer channels: - Measurement of sediments heights and characteristics. • New projects: • - No prior knowledge how and which sediments will develop. • - Trust in calibrated models based on sediment and wave measurements.

  9. Results: Constant downstream water level I = 1 ‰ M = 0.013 s/m1/3 h = 0.55 m V = 139.6 m³ h0 = 0.15 m • Downstream water levels: Remaining dry-weather runoff a/o lateral inflows. • Deceleration of flush wave and reduction in cleaning efficiency. • Variation of downstream water levels between h0 = 0.01 – 0.2 m. •  drops fast due to flow resistance of DWL. •  < crit = 3 N/m² after 191 m running distance. • Reduction of effective flushing distance of 75 % by h0 = 0.10 m. • Strong effect of downstream water levels on the efficiency of the flush wave. • DWL very important when modeling flush waves for a practical applications.

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