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Automated Sewer and Drainage Flushing Systems in Cambridge, Massachusetts William C. Pisano, P.E.1; Owen C. O’Riordan, P.E.2; Frank J. Ayotte, P.E.3; James R. Barsanti, P.E.4; and Dennis L. Carr, P.E.5 Abstract: This paper summarizes the design of passive automatic flushing systems installed in the City of Cambridge’s storm and sanitary sewer system tributary to the Alewife Brook as part of a $75 million sewer separation program. Grit and debris deposition is severe in the existing combined sewers, storm drains, and sanitary trunk sewers due to the flat topography of the area. This condition is exacerbated by hydraulic constraints imposed on the system’s outlet by the Alewife Brook 共shallow stream兲 and downstream sanitary siphons 共again because of the Alewife Brook兲. The use of pumps to lift flows from sewers and drains to permit self-scouring velocities is prohibitively expensive. To overcome this problem, five automated flushing systems using quick opening 共hydraulic operated兲 gates discharging collected stormwater were constructed in conjunction with downstream collector grit pits covering a distance of 1,604 m for storm drain pipes ranging from 1.4 m circular to 1.2 m by 1.8 m rectangular. New 450 and 600 mm sanitary trunk sewers, 561 m long, will be flushed daily by two flushing systems using spent filtrate water from Cambridge’s water treatment plant recently constructed nearby. The flushing systems are sized to achieve wave velocity of 1 m/s at the end of the flushing segment. The flush vault volumes range from 11 to 40 m3 for the storm drain systems and 6 m3 for the sanitary system. Construction was completed in May 2002 and functional testing of the flushing systems is in progress. Partial test results are reported. DOI: 10.1061/共ASCE兲0733-9429共2003兲129:4共260兲 CE Database subject headings: Sewers; Drains; Sediment; Massachusetts; Automation; Flushing.

Introduction The deposition of sewage solids during dry weather in combined sewers has long been recognized as a major contributor to ‘‘firstflush’’ phenomena. Another manifestation of ‘‘first-flush,’’ in addition to the scouring of materials already deposited in the lines, is the first flush of loose solid particles on the urban ground surface that are transported into the sewerage system and not trapped by catch basins or inlets. These particulate materials may settle out in the system and be available for scour and resuspension during wet periods. Such materials also create first flush loading from storm drainage systems. Deposition of heavy solids is also a problem in separate sanitary systems and can also result in significant odor and corrosion problems. Sewer sediments create odor problems due to septic conditions in the sewer that result from the activity of microorganism and anaerobic conditions of the sediment layer. The process begins with the biological reduction of sulfate to sulfide by the anaerobic slime layer residing on pipe and sediment surfaces below the water in wastewater collection systems. The resulting 1

Vice President, MWH, 100 Boylston St., Boston, MA 02116. Deputy Commissioner of Public Works, Cambridge Dept. of Public Works, 147 Hampshire St., Cambridge, MA 02139. 3 Project Manager, MWH, 100 Boylston St., Boston, MA 02116. 4 Sr. Project Engineer, MWH, 100 Boylston St., Boston, MA 02116. 5 Sr. Project Engineer, MWH, 100 Boylston St., Boston, MA 02116. Note. Discussion open until September 1, 2003. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on October 30, 2001; approved on April 11, 2002. This paper is part of the Journal of Hydraulic Engineering, Vol. 129, No. 4, April 1, 2003. ©ASCE, ISSN 0733-9429/2003/4-260–266/$18.00. 2

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sulfide ion is transformed into hydrogen sulfide gas after picking up two hydrogen ions from wastewater. Once released to the sewer atmosphere, aerobic bacteria and fungi that reside on sewer walls and surfaces above the water line consume the hydrogen sulfide gas and secrete sulfuric acid. In severe instances, the pH of the pipe can reach 0.5 under these conditions, which causes severe damage to unprotected collection system surfaces and may eventually result in the failure of the sewer 共Cho and Mori 1995; Davis et al. 1998兲. Generally if sediments are left to accumulate in pipes, hydraulic restrictions can also result and blockages can eventually occur. In recent years, considerable attention has focused on appropriate maintenance strategies to maintain sewers free of sediment. Different strategies include: manual and physical cleaning methods, silt traps, and automatic ‘‘self-operating’’ flushing equipment.

Physical Cleaning Methods Manual methods of sediment removal have commonly been employed and these methods usually involve the movement of the sediment to a location for subsequent removal by mechanical or suction equipment. A number of such methods are discussed below. The most unique example of a manual technique is that employed within the Brussels, Belgium sewer system where a wagon incorporating a flushing vane is physically moved along the sewer. The vane disturbs the sediment, which is subsequently transported with the sewer flow. This successful method evolved due to the ‘‘cunette shape’’ of the sewer inverts. Common sewer cleaning techniques include the use of rodding, balling, flushing, poly pigging, and bucket machines. These methods are used to clear blockages once they have formed, but also as preventative maintenance tools to minimize future problems. With the excep-

tion of flushing, these methods are generally used in a ‘‘reactive’’ mode to prevent or clear up hydraulic restrictions. Power rodding includes an engine and drive unit, steel rods, and a variety of cleaning and driving units. The power equipment applies torque to the rod as it is pushed through the line, rotating the cleaning device attached to the lead end. Power rodders can be used for routine preventative maintenance, cutting roots, and breaking up grease deposits. Power rodders are efficient for pipes up to 300 mm. Balling is a hydraulic cleaning method in which the pressure of a water head creates high velocity water flow around an inflated rubber cleaning ball. The ball has an outside spiral thread and swivel connection that causes it to spin, resulting in a scrubbing action of the water along the pipe. Balls can remove settled grit and grease buildup inside the line. This technique is useful for sewers up to 600 mm. Poly pigs, kites, and bags are used in a similar manner as balls. The rigid rims of bags and kites cause the scouring action. Water pressure moves these devices against the tension of restraining lines. The shape of the devices creates a forward jet of water. The poly pig is used for large sanitary sewers and is not restrained by a line, but moves through the pipe segment with water pressure buildup behind it. Jetting is a hydraulic cleaning method that directs high velocities of water against the pipe walls at various angles. The basic jetting machine equipment is usually mounted on a truck or trailer and consists of water supply tank of at least 4,000 L, a high pressure water pump, an auxiliary engine, a powered drum reel holding at least 150 m of 25 mm hose on a reel having speed and direction controls and a variety of nozzles. Jetting is efficient for routine cleaning of small diameter, low flow sewers. It effectively removes grease buildup and debris. The power bucket machine is a mechanical cleaning device effective in partially removing large deposits of silt, sand, gravel, and grit. These machines are used mainly to remove debris from a break or an accumulation that cannot be cleared by hydraulic methods. Silt traps have successfully been used to collect sewer sediments at strategic locations within the system with the traps being periodically emptied as part of a planned maintenance program. The design and operational performance of two experimental rectangular 共plan兲 shaped silt traps in sewer systems in France was reported by Bertrand-Krajewski et al. 共1996兲. Similar but full scale traps have been used in Marseille, France since the beginning of 1990 共Chebbo et al. 1996兲 where they have been developed and first implemented 共Laplace et al. 1993兲. Information on design procedures and methodology for silt traps is scarce. The most recent synthesis of knowledge about French traps has been published by Laplace et al. 共1998兲.

Sewer Flushing Flushing of sewers either by manual or by automated means is generally used to reduce hydraulic restriction problems and infrequently as a pollution prevention approach. The concept of sewer flushing is to induce an unsteady waveform by either rapidly adding external water or creating a ‘‘dam break’’ effect by the quick opening of a restraining gate. This aim is to resuspend and transport deposited pollutants to the sewage treatment facility during dry weather and/or to displace solids deposited in the upper reaches of large collection systems closer to the system outlet. During wet weather events these accumulated loads may then be more quickly displaced to the treatment headworks before overflows occur or be more efficiently captured by wet weather first flush storage facilities.

Manual methods usually involve discharge from a fire hydrant or quick opening valve from a tank truck to introduce a heavy flow of water into the line at a manhole. Flushing readily removes organic deposits, saturated water logged floatable solids, fine sand, and grit, but is not very effective for removing heavy debris.

Automated Flushing Equipment In recent years, three types of commercial automated flushing equipment have emerged in France and Germany. The most commonly used system is the flush gate system that has been recently used in North America. Pisano et al. 共1998兲 provide an in-depth review of flushing gate and pertinent flushing technologies for CSO tanks and sewerage and drainage conveyance systems. Hydrass This system, developed and patented in France 关B. Sikora, ‘‘Vanne cyclique autocurante a decantation,’’ French Patent No. 2643971 共1989兲兴, is comprised of a balanced hinged gate with the same shape as the cross section as the sewer. At low flows the self weight of the gate holds the gate in the vertical position and the sewer flow builds up behind the gate. The depth of flow continues to build up behind the gate until the force created by the retained water becomes sufficient to tilt the gate. As the gate pivots about the hinge to a near horizontal position, the sewer flow is released and this creates a flush wave which travels downstream and subsequently cleans deposited sediment from the invert of the sewer. The gate then returns to the vertical position and the cyclic process is repeated, thus maintaining the sewer free of sediment. Gates are positioned in series at intervals dictated by the nature, magnitude, and location of the sedimentation problem. Chebbo et al. 共1996兲 reported the effective operation of the HYDRASS system. Hydroself In Germany, over 13,000 CSO tanks have been constructed with over 500 being in-line pipe storage tanks 1.8 –2.1 m in diameter with lengths of 125–180 m. Many different methods for cleaning these pipe storage tanks were tried over the years. A popular method used to clean pipe storage facilities as well as conveyance systems is the HYDROSELF system The HYDROSELF system has been used to clean settled debris in sewers, interceptors, tunnels, and detention tanks in Germany and Switzerland. In Europe, there are over 280 installations in operation since 1986. Approximately 37% of these projects are designed to flush sewers, interceptors, and tunnels ranging from 250 to 4,300 mm in diameter and flushing lengths up to 340 m for large diameter pipes and up to 1,000 m for small diameter pipes. The balance of these facilities are used in CSO tanks. This system consists of a hydraulically operated flap gate, a flush water storage area created by the erection of a concrete wall section, a float or pump to supply hydraulic pressure, and valves controlled by either a float system or an electronic control panel. The water level in the sewer can be used to activate the release and/or closure of the gate using a permanently sealed float controlled hydraulic system. Without external system control, the flushing system control is designed to operate automatically whenever the in-system water level reaches a predetermined level, thereby releasing the gate and causing a ‘‘dambreak’’ flushing wave to occur. Activation by remote process control is also possible, noting when both the flushing volume chamber has reached a predetermined level and the downstream discharge level is favorable. JOURNAL OF HYDRAULIC ENGINEERING © ASCE / APRIL 2003 / 261

Fig. 1. Flushing gate installation, Whitten, Germany

For large diameter sewers greater than 2 m, the flushing system may be installed in the sewer pipe itself. The required storage volume for the flush water is created by erecting two walls in the sewer pipe to form a flush water storage area inbetween the two walls. For the area to remain free of debris, a floor slope of 10–20% must be provided in the storage area. The requirements for the storage area slope will determine, in most instances, the maximum flushing length possible for a single flush gate. In order to increase the maximum flushing length it is also possible to build additional flush water storage area by creating a rectangular chamber in-line or adjacent to the sewer line itself. The largest pipe storage project using a single flushing chamber is in Whitten 共near Dortmund兲, Germany 共Pisano et al. 1997兲 共depicted in Fig. 1兲 and has been operational since 1994. The project entails flushing a rehabilitated brick storage channel with a single flushing chamber having a diameter of 2 m 共with a 300 mm dry weather channel兲 770 m long with a slope equal to 0.22%. The upstream storage flushing chamber holds about 35 m3 . Normally at least two chambers would have been neces262 / JOURNAL OF HYDRAULIC ENGINEERING © ASCE / APRIL 2003

sary to flush a pipe of this length with this slope. Since an external water supply was readily available it was decided to use a single chamber having the flexibility to flush several times during a single operation. The facility went into successful operation in October 1996. BIOGEST Vacuum Flushing System A variation of the HYDROSELF is the BIOGEST system comprised of a concrete storage vault and a vacuum pump system to create a cleaning wave to resuspend sediment in the sewer invert. The system consists of a flush water storage area, diaphragm valve, vacuum pump, level switches, and a control panel for automatic operation of the system. The water level in the sewer is used to activate the vacuum pump. The vacuum pump evacuates the air volume from the flush chamber and as the air is evacuated the water is drawn in from the sewer and rises in the chamber. The vacuum pumps shuts off when a predetermined level in the flushing vault is reached. A second level sensor detects the water level in the sewer and activates the flush wave. The flush wave is

Fig. 2. Fresh Pond Parkway sewer separation project

initiated by opening the diaphragm valve above the flush chamber and subsequently releasing the vacuum and vault contents.

Fresh Pond Parkway Sewer Separation Project: Storm Drain and Sanitary Sewer Flushing Systems Over the last 20 years, the City of Cambridge has aggressively separated old combined and over 共sanitary兲 and under 共storm兲 sewerage systems throughout the city to enhance drainage service and to improve the water quality in the Alewife Brook and the Charles River. Presently, the City is in the construction phase of separating a 100 ha catchment north and west of Harvard Square within a highly urbanized and heavily traveled area. Grit deposition within both existing sewerage and storm drainage systems is a major problem because of general flatness of the area, presence of several shallow streams that the sewerage 共storm and sanitary兲 systems must cross then streams under as siphons, and the hydraulic level of the receiving water body that frequently backwaters into the storm systems. The existing and

recently constructed storm drains on Fresh Pond Parkway have invert slopes of approximately 0.0003–0.0005 ft/ft. Deposition of any residual stormwater solids not captured by the surface best management practices that discharge into these conduits would be severe. Since no chemical salting during winter conditions can be tolerated in the low, flat Fresh Pond Reservation watershed, heavy winter sanding only exacerbates potential deposition problems To overcome this problem, automated flushing systems using quick opening 共hydraulic operated兲 flushing gates to discharge collected stormwater will flush grit and debris to downstream enlarged manholes with open grit pits. Grit pits are not provided on the sanitary systems being flushed. The storm drain and sanitary sewer systems to be flushed are within the Fresh Pond Parkway near the Cambridge Water Treatment Plant 共CWTP兲, continue east to Concord Circle, and then northeast to the Fresh Pond Circle. Both systems then proceed out of the area. See Fig. 2 for the general locus plan showing locations for the two sanitary sewer and two storm drain flushing vaults. The piping systems consist of approximately 560 m of JOURNAL OF HYDRAULIC ENGINEERING © ASCE / APRIL 2003 / 263

Table 1. Design Information Summary, Fresh Pond Parkway Flush-

ing Program Downstream flushed pipe diameter 共m兲

Flushing length 共m兲

Flush volume 共1,000 L兲

0.91–1.37 1.06 1.37 1.22 by 1.83 共box兲 1.83

376 212 213 345 458

17.4 11.3 12.5 22.7 39.7

— 0.46 0.60

1,604 194 367

103.6 5.7 6.8

—

561

12.5

Site Drain Drain Drain Drain Drain

Vault Vault Vault Vault Vault

No. No. No. No. No.

1 2 3 4 5

Total drain vaults Sanitary Vault No. 1 Sanitary Vault No. 2 Totals

sanitary trunk sewers, ranging from 460 to 600 mm, and approximately 1604 m of existing storm drains with pipe sizes ranging from 900 mm to 1.2 m by 1.8 m. Pertinent design information for the flushing systems are provided in Table 1. The flushing volumes for the storm drain vaults noted in Table 1 were developed as follows. Information regarding pipe size, roughness, shape, slope, distance between location of flushing vault, and the downstream receiving grit pit were provided to the HYDROSELF equipment vendors, Grande, Novac and Associates, Inc. Proprietary flushing volume sizing rules have been developed in Germany based on a combination of physical modeling, mathematical modeling, and empirical visual observations of prototype pipe flushing installations using rapid opening flush gate and other conventional more slowly opening valve schemes. The salient feature of the flushing gate technology is the ability of the gate to be nearly instantaneously unlatched, to fully open, and to a create flush wave with rapid initial velocities. The flush wave is designed to have a depth of approximately 50–75 mm and a velocity range between 0.9 and 1.2 m/s at the end of the pipe segment being flushed. Based on this information, the vendor provided the recommended flushing volumes. These volumes were then adjusted upward by 15–20% by the designers where feasible, to account for uncertainty, expected high amount of sand used during winter operations on Fresh Pond Parkway, and the extreme space limitations imposed by other utilities within the Fresh Pond Parkway. It is noteworthy that 14 other utilities share the same four lane corridor. Flushing volumes for the sanitary sewers were also similarly upsized. Justification for providing flushing systems for the new 600 mm sanitary trunk sewer system are provided in Table 2. Average peak dry weather and peak infiltration flow velocities throughout most of the year excluding inflow periods will not approach 1 m/s as a limit. Peak daily velocity and shear stress conditions for the upstream 450 mm sanitary trunk sewer are less than the estimates provided for the downstream 600 mm sanitary sewer noted in Table 2. The criteria adopted for the project assumed that taken Table 2. Fresh Pond Parkway Sanitary Sewer System, Design Flow

and Velocity Evaluation 600 mm Sanitary Trunk Sewer Measured flows 共11 months兲 Peak daily dry weather flow Average yearly dry weather flow Average summer dry weather flow

Flow 共L/s兲

Velocity 共m/s兲

Shear 共N/m2 )

79 37 28

0.73 0.58 0.56

1.8 1.3 1.1

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individually and/or jointly, minimum velocity and minimum average shear stress levels of 1 m/s and 2 N/m2 be realized on a regular basis for the sanitary systems. In addition to the low discharge velocities, the domestic waste tributary to the Fresh Pond Parkway sanitary system is unusual for two reasons. First, the waste contains high quantities of fats, oils, and grease 共FOG兲 discharged into the sewers from the numerous restaurants in the catchment. While a rigorous FOG program is in place, complete control is not possible. Grease buildups have been a significant problem and are expected to continue. Second, the new CWTP will dispose 共by permit兲 filtration backwash process waste on a daily basis into the sanitary sewer system. High levels of silt within a congealed matrix of coagulants and other flocculent aids will be disposed into the sewer system on a daily basis. Since the new sewers will be fairly flat in the area, significant deposition problems exacerbated by the combination of FOG and CWTP process wastes are expected. The design basis for the self cleansing of the storm drain system assumed that the peak flow velocities for the 3-month storm should exceed 1 m/s. The US Environmental Protection Agency SWMM EXTRAN model was used to simulate system flows for the trunk sewers for the regional 3-month storm having a peak hourly intensity equal to 10 mm/h with a total rainfall depth of 50 mm. The results indicated that peak velocities for the new storm drain system consisting of existing drains, rehabilitated combined sewers, or new drains 共box culvert兲 designed to handle the 10year storm having a peak intensity of 58 mm/h did not exceed 0.5 m/s. Flow velocities for lesser, more frequent storms will be even smaller and more problematic with respect to solids deposition. Automated flushing systems with downstream grit collection manholes were therefore provided. Fig. 3 shows in closer detail the proposed new sewerage and drainage system piping at the intersection of Fresh Pond Parkway and Lakeview Avenue. Sanitary Vault No. 2 and Drain Vault No. 1 are also depicted in Fig. 3. The pumped process 共backwash兲 filtrate flow from the new CWTP is pumped daily into Sanitary Vault No. 1. This vault is filled and overflow continues 194 m down to Sanitary Vault No. 2. This scheme is used in lieu of an external water source to flush the sanitary trunk sewers. Both vaults will be flushed at least once daily. Controls at both vaults are programmed to flush in sequence once full. During a rainfall event, stormwater from the incoming storm drain to Drain Vault No. 2 as shown in Fig. 3 fills the sump adjacent to the flush chamber. Submersible pumps then pump stormwater from the sump into the flush chamber. A level sensor within the flushing volume chamber relays water level data to the PLC in the control panel which terminates pump operation when the chamber reaches a predetermined fill elevation. A level sensor in the downstream storm drain notes when the water level in the downstream drain is sufficiently low to initiate the flushing operation. Activation of the hydraulic power pack then causes the flush gate to unlatch, creating the flush wave. Once the system has been activated it is possible to repeat the process during a multipeaked storm event. A generic 24 h time clock function adds an additional level of operational flexibility. For example, it is possible to interrogate the system 24 h after the first flush to unlatch any partially filled flush volumes. This procedure is the same for all other drain vaults. An adjustable bottom acting gate on the side of the entrance to the pump wet well controls the depth of storm flow entry. This feature can be used to ensure that the vault is not filled with base flows and allows bed load sediment to flow into the sump during storm events. The four receiving grit pits have been sized to pro-

Fig. 3. Sanitary and drain vault installation on Fresh Pond Parkway

vide maximum capture volume given the extraordinary spatial site constraints along the parkway. Average capture volume per pit is about 3 m3 . Inspection of the grit pits is programmed on a biannual basis with cleanout annually. The initial phase of functional testing of the flushing systems commenced in May 2002 and will continue during the summer of 2002. Preliminary testing results are noted in Table 3. The test procedure involves noting the elapsed time between flush onset and sighting of the flush wave at intermediate and terminus flushing segment locations. The flush vaults were filled during dry weather with water from fire hydrants. Average velocities from two separate trials are reported in Table 3. With the exception of results for Drain Vaults Nos. 4 and 5, the flush velocities at the end of the flush segments exceeded design expectations. Additional pipe cleaning and invert repair work for drains downstream of the Drain Vaults Nos. 4 and 5, and the last 30 m of the sanitary trunk sewer downstream of Sanitary Vault No. 2 will be completed prior to final functional testing.

Conclusions Deposition of solids within flat drainage and sewerage conveyance pipes can result in problematic hydraulic restrictions, poten-

Table 3. Functional Testing Results, May 2002, Downstream Dis-

tances from Flushing Vaults and Wave Velocity Location 1

Storm Vault No. 1 Storm Vault No. 2 Storm Vault No. 3 Storm Vault No. 4 Storm Vault No. 5 Sanitary Vault No. 1 Sanitary Vault No. 2

Location 2

Distance from vault 共m兲

Wave velocity 共m/s兲

Distance from vault 共m兲

Wave velocity 共m/s兲

183 212 128 159a 282 194 233

1.72 1.50 2.84 1.76 1.60 1.75 2.43

376 N/Ab 213 345 458c N/Ab 367d

0.84 N/Ab 2.24 0.52 N/Ab N/Ab 1.42

a

Partial obstruction within pipe segment. Not available. c Vertical pipe sag and deposits within pipe segment. d Partial restriction near end of pipe segment. b

JOURNAL OF HYDRAULIC ENGINEERING © ASCE / APRIL 2003 / 265

tial odor and corrosion conditions, and the initial flush of pollutants and solids to receiving waters. This paper reviews methods and equipment for cleansing and flushing deposited sediments in pipe inverts. Details of a recently completed sewer separation project in Cambridge, Mass. are presented describing the design and implementation of seven automated pipe flushing systems which use quick opening 共hydraulic operated兲 gates discharging stored waters from off-line vaults ranging in size from 6 to 40 m3 . These systems are used to resuspend and transport deposited solids in 1,600 m of large flat storm drains and 560 m of new sanitary sewage trunk sewers. Preliminary functional testing indicates that the passive flushing systems create flush waves having velocities in excess of 1 m/s at the terminus of the flushing segments.

References Bertrand-Krajewski, J., Madiec, H., Moine, O., Heneau, T., Tougne, P., and Schaal, C. 共1996兲. ‘‘Assessment of experimental bed load sediment traps to replace usual grit chambers in sewer systems.’’ Proc., 7th International Conf. Urban Storm Drainage, Hannover, Germany, Vol. 2, 737–742.

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Chebbo, G., Laplace, D., Bachoc, A., Sanchez, Y., and Le Guennec, B. 共1996兲. ‘‘Technical solutions envisaged in managing solids in combined sewer networks.’’ Water Sci. Technol., 33共9兲, 237–234. Cho, K., and Mori, T. 共1995兲. ‘‘A newly isolated fungus participates in the corrosion of concrete sewer pipes.’’ Water Sci. Technol., 31共7兲, 263–271. Davis, J., Nica, D., Shields, K., and Roberts, D. 共Patent for the Hydrass gate兲. 共1998兲. ‘‘Analysis of concrete from corrode sewer pipe.’’ Int. Bio-deterioration Biodegradation, 42, 75– 84. Laplace, D., Bachoc, A., and Sanchez, Y. 共1993兲. ‘‘Solutions techniques pour gerer les depots en collecteurs visitables 共Technical solutions to manage sewer solids in man entry sewers兲.’’ PSM, 共10兲, 519–523 共in French兲. Laplace, D., Bertrand-Krajewski, J., Chebbo, G., and Felouzis, L. 共1998兲. ‘‘Les pieges a charriage: de la theorie a la pratique 共Bed load traps: from theory to practice兲.’’ Proc. NOVATECH 98, Lyon, France, Vol. 2, 329–336 共in French兲. Pisano, W., Barsanti, J., Joyce, J., and Sorensen, H. 共1998兲. ‘‘Sewer and tank sediment flushing: case studies.’’ Rep. No. EPA/600/R-98/157, U.S. Environmental Protection Agency, Cincinnati. Pisano, W., Novac, G., and Grande, N. 共1997兲. ‘‘Automated sewer flushing large diameter sewers.’’ Proc., Collection Systems Rehabilitation and O&M Conf.: Solving Today’s Problems and Meeting Tomorrow’s Needs, Water Environment Federation, Alexandria, Va., 9–20.