SPN7 Numerical investigations on the influence of hydraulic boundary conditions on the efficiency of sewer flushing Dr.-Ing. Joerg Schaffner

SPN7 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 Q dry Downstream water level Sewer slope Roughness

SPN7 Sewer flushing -Impoundage dry-weather runoff to designed storage level -Fast lifting of the flushing shield -Development of a turbulent flush wave downstream -Pipes mm in diameter -Cleaning distance up to several kilometers in length Reference: Chow, 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

SPN7 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 v t=0 = 0 m/s. -Downstream BC: Pressure boundary -Bottom shear stress: (Energy slope method)

SPN7 Results: Longitudinal slope -Bottom roughness: M = s/m 1/3 (constant) -Flushing volume: V = m³ (constant) -H stor = 0.31 m m -Adjustment of storage distance according to the slope in order to keep the flushing volume constant. -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. Variation of longitudinal slope I = ‰ I = 2.25 ‰

SPN7 Results: Longitudinal slope -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. Effective flushing distance - Location where:  <  crit = 3 N/m²

SPN7 Results: Bottom roughness Constant values: -I S = 1 ‰ -H stor = 0.55 m -V Flush = m³ Variation M = s/m 1/3 (very smooth concrete - medium sized gravel) -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/m 1/3 : wave running time t = 1446 s and  max = 2.29 N/m². -M = s/m 1/3 : wave running time t = 3538 s and  max = 4.21 N/m².

SPN7 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.

SPN7 Results: Constant downstream water level -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 h 0 = 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 h 0 = 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. I = 1 ‰ M = s/m 1/3 h = 0.55 m V = m³ h 0 = 0.15 m