Investigations on reservoir sewers

with an endoscope to detect the presence of an organic layer. [Oms et al., 2002] The eroded pollutant mass was calculated taking the difference in concentrations of samples during the dry-weather flow and the flushes into account. The available mass of the organic layer was calculated with the measured surface size of the endoscope observations.

The analysis of the flushing experiments showed that the concentration increased for the first and the second flush where 65 % of the eroded mass was removed. After the third flush the concentrations decreased to a stable value. Further long-term investigations showed that even with one year difference between two flushing investigations the eroded masses are of the same magnitude. It can therefore concluded that erodeable sediments reform to the same level and measurements showed that this process needs less then 8 days. Inspections after a flushing series revealed that the organic layer was removed to a large extend and larger particles were eroded by the flush waves. [Laplace et al., 2002]

The numerical model mentioned in Campisano et al. (2004) has been used for further investigations substituting the stainless steel plate with a Hydrass gate model. [Camp-isano et al., 2005] Again numerical results were compared with experimental results from a laboratory channel. The Hydrass gate as used by Bertrand-Krajewski et al. (2002) in Lyon was reduced to a 1:7 scale model to investigate the unsteady flow conditions caused by the gate. The Hydrass gate outflow relations (Bertrand-Krajewski et al., 2004) were implemented into the numerical model as internal conditions. The results showed a good agreement of the model, which is suitable to reproduce the unsteady flow of flushing operations and the steady outflow through the Hydrass gate.

The dissertation of Creaco (2005) gives a very good and detailed summary of all the investigations carried out at the University of Catania in Italy. [Campisano et al., 2002, 2004, 2005, 2006] [Bertrand-Krajewski et al., 2004, 2005a]

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Figure 7.14: Hydrograph of the water level and the velocity [Gathke und Borchering, 1996]

The velocity of the opening of the flush gate was also of importance for the propagation of the flush wave. The slow opening of the flush gate resulted in a lower (77 %) and slower (10 s) flush wave compared to the flush wave created by a fast opening. But in a distance of 450 m to the flush gate the difference of both flush waves is not visible any more. The opening time is important for the sewer range close to the gate but not for longer flushing distances.

The controlling parameters for the remobilisation of the deposits are based on a minimum flow velocity of 1 m/s and a bottom shear stress of 4 N/m². [Mittelstaedt, 1981] [Sander, 1989] These values are still exceed in a flushing distance of 1800m. (Figure 7.15)

Figure 7.15: Hydrograph of bottom shear stress [Gathke und Borchering, 1996]

A further statement was the massive reduction of the water level during the wave pro-gression. At the end of reservoir sewer the flush wave had a height of 30 cm (∼ 1/15 DN). The loss of energy of the flush wave was given with 80 % for the first quarter of the sewer length. The loss of energy caused by erosion and transport was not considered.

Numerical and practical investigations in Bochum on the flushing of combined sewers with storage capacity and overflow are given by Dettmar and Stauffer (2004). The field investigations were carried out in a large sewer with storage capacity and bottom-end overflow. The size of the channel ranged between pipe diameters of 2500 mm to 3400

mm and a length of about 400 m. The bed slopes of the sewer varied between 3.6 and 4.6 .

Figure 7.16: Overview of the investigated sewer [Dettmar & Stauffer, 2004]

An automatic flushing gate was installed in the upper part of the sewer to remove settled particles and bed-load material. The gate divided the sewer in a 300 m flushing section downstream and in a 100 m storage section upstream. In both parts of the sewer deposit are found. The throttle at the outlet of the sewer channel was a pipe with a diameter of 700 mm and a maximum discharge of 0.71 m³/s. Figure 7.16 shows an overview of the investigated sewer.

At the end of the sewer channel the concentration of flow at the throttle reduced the bottom shear stresses and limited the sediment transport rates. Therefore the flow rate of the flushing waves needed to be optimised at the outlet to minimise the risk of sedi-mentation of remobilised solids and of transportating the solids back into the sewer by reflection of the flushing waves. For an optimised flushing concept the authors chose a bottom shear stress of 5 N/m² to exceed the erosion shear stresses.

The flushing gate used for the investigations was based on a new concept with an auto-matic operation using a pneuauto-matic drive. The parts were of stainless steel and synthetic materials. The field investigations included recordings of the water level during the pro-gression of the flush wave and measurements of the sediment height and location. The composition of the sediments was also analysed. The measurements of the flush wave at four locations along the sewer showed a mean velocity of the flush wave of 3.6 m/s.

For the numerical investigations the program Fluvius-1D was used which is a one-dimensional hydrodynamic model. The model was developed for calculating non-steady and discontinuous discharges in near-natural channels and is bases on the Saint-Venant equations, which were discretised conservatively by the method of Gudonov. The method of Gudonov characterises the transition between two areas of continuous flow by the Riemann’s problem for non-linear equations of conservation. In this case the Riemann’s problem is solved with the method of Roe. Parameters for the calibration of the nu-merical model using the measurements of the real sewer were the discharges and the roughness for several sections along the channel. The flush waves behaved highly un-steady and discontinuously and were described as dam-break waves. The calibration of the modelled flush waves at two locations of the sewer channel are shown in figure 7.17.

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Figure 7.17: Measured and calculated water levels over time [Dettmar & Stauffer, 2004]

The results of the numerical calculations lead to the fact that the critical shear stress of 5 N/m² was exceeded by the calculated mean shear stresses at the bottom for at least 240 s for the lowest dam height of H = 0.80 m. Taller dam heights caused even higher shear stresses up to 140 N/m² for H = 1.40 m at the gate. Therefore the process of erosion was started and the deposits were transported downstream. As expected the cleaning success at that point was greater with a bigger wave than those created with a smaller dam height.

When choosing the dam height at the flushing gate the backwater effect at the end of the sewer channel, at the throttle, had to be taken into account. Only dam heights below H = 0.9 m showed a free discharge at the throttle without any backwater effects and low shear stresses. For larger heights the shear stresses droped as soon as the flow rate exceeded the given limit by the throttle. That might cause re-sedimentation at lower parts of the sewer. The degree of re-sedimentation probably depends on the time the impact of the throttle is activated.

On basis of the simulation results the authors were applying for a two phase cleaning strategy composed of basic and preventive cleaning. The basic cleaning should remove all existing deposits and two different strategies are possible. First numerous small waves (dam height: H < 0.9 m) could remobilise settled solids without a blockage. Therefore up to 20 waves a day would be necessary. Assisting the small waves single big waves can be flushed to loosen consolidated deposit or to transport gross solids towards the outlet.

The second choice could be an operation where the dam height at the gate is adjusted smoothly as the height of deposit decreases and the capacity of the outlet rises.

After the basic cleaning the operation for the preventive cleaning would be started.

Flushing waves for preventive cleaning are dimensioned with only 1 or 2 waves per day and a dam height at the gate between 1.2 and 1.3 m. According to the authors, with this two-part strategy the sewer the sewer channel could be cleaned permanently without causing overflow events by exceeding the maximum runoff at the throttle. [Dettmar &

Stauffer, 2004, 2005a] A detailed report on this investigation can be found in Dettmar and Stauffer (2006).

A comparison of the one-dimensional numerical model Fluvius and the tree-dimensional

model SSIIM was carried out by Stauffer et al. (2006). Here he compared the 3-D results of Schaffner (2003b) with result he achieved by re-modelling the sewer channel originally investigated by Schaffner.

As a result of all the investigations presented in this chapter it can be stated that the flushing volume and the frequency of flushing are the key parameters for a potential successful cleaning of a sewer channel. The initial storage height should not be not less than 50 cm to create a typical dam-break wave. But compared to the flushing volume a large initial storage height does not necessarily lead to long flushing distances. It is more the flushing volume which needs to be large enough for the chosen or demanded sewer stretch to be cleaned. The chosen frequency of flushing should be related to the existing deposits in a sewer channel. Their characteristics can vary to a large extend and should be taken into account. High levels of sediments in old sewers will need a high frequency of powerful flush waves to be cleaned in the long run. New sewers or already cleaned sewers will need a smaller frequency of flush waves to be cleaned of new and mobile sediments.

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