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Town of Gander Department of Municipal Works and Services

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Town of Gander Department of Municipal Works and Services

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    1. Town of Gander Department of Municipal Works and Services Flood Investigation and Recommendations

    2. BACKGROUND The Town of Gander has two sewage treatment plants, and therefore two separate sewer systems. Magee Road Plant receives wastewater only and services all of the northeast area of Gander (areas of the town to the north of Memorial Drive). The Beaverwood Plant treats wastewater from all other areas of town, as well as residential storm water from weeping tile in the original townsite area. Generally, all properties north of Airport Boulevard drain to Beaverwood via gravity flow and all properties south of Airport Boulevard drain via gravity flow to Cobham Lift Station which then pumps to Beaverwood.

    3. BACKGROUND The lift station at Cobham reached full capacity and was overflowing during the storm event and we are therefore less concerned with this area’s affect on the flooding at Peterson and Edinburgh areas. The capacity of Beaverwood Plant is 21 million gallons per day (32.5 cfs). If there is a potential for exceeding the capacity of the plant, gates can be opened to provide a by-pass. On August 23, this by-pass was opened at approximately 9:30 p.m. and on August 30 the by-pass was opened at approximately 9 a.m.

    4. INVESTIGATION In response to the flooding incidents, the Town of Gander investigated the following areas: Peterson Drive, Edinburgh Avenue and Area Lower Yeager Street Bennett Drive Hornell Street Raynham Avenue Medcalf Street

    5. PETERSON / EDINBURGH & AREA This area of town is on a combined sewer system, meaning both sanitary sewer and weeping tile is connected to the same sewer main. This causes high peak flows in the sanitary sewer lines during storm events. Throughout both flooding events, Town crews discovered the main line to be at full capacity. Once the pipe reaches its full capacity, it causes a hydraulic pressure upstream of the narrowest (or flattest) point. All branch lines begin to be restricted, causing further pressures upstream of the branch intersections. Peterson Drive sewer system is situated at the downstream end of the main trunk sewer leading to Beaverwood. This line is at the lowest point in the south area of Gander and therefore most susceptible to back-up.

    6. PETERSON / EDINBURGH & AREA Edinburgh Avenue system is the 2nd lowest branch line and again very susceptible to back-up once the trunk line reaches capacity. Camera inspections were performed by Town staff on the sewer lines for Edinburgh Avenue and Peterson Drive. The Peterson line is a 12” diameter concrete asbestos line and appeared to be in good condition. The inspection revealed some gravel and small rocks in the bottom of the line approximately 50 feet east of Cooper Blvd. This debris was only 2”-3” high and not large enough to cause back-up of this sewer line. The lower Edinburgh system consists of 2 parallel lines, one 12” diameter and one 8” diameter. The inspection revealed tree roots in the 12” line at 17 Edinburgh Avenue. These roots would cause a minor restriction and infiltration but not be the only reason for the entire back-up.

    7. PETERSON / EDINBURGH & AREA Alfonso’s Diving conducted a camera inspection on the trunk sewer from Edinburgh Avenue to Beaverwood. There were no major deficiencies found in this section of the pipe. There was some concrete debris found in the bottom of the line but not enough to cause a back-up of the system. The final section of trunk main had a significant amount of sewer debris attached to the top of the pipe. An analysis of the system network and pipe sizes revealed that the main trunk line between the Peterson branch and Beaverwood is not large enough to carry all the incoming flow when all branch lines are full.

    8. PETERSON / EDINBURGH & AREA The Technical Services Division conducted a survey of roof gutter downspouts in the original town site and south east area of town. The major concern is the area which is gravity fed to Beaverwood, since it has an immediate effect on run-off into the sewer mains and trunk sewer. The area south of Airport Boulevard flows to Cobham, where it is pumped to the trunk sewer. The contribution of this flow to the trunk sewer can never be larger than the capacity of the two pumps at Cobham lift station.

    9. PETERSON / EDINBURGH & AREA

    10. PETERSON / EDINBURGH & AREA The Technical Services Division analyzed the sewer main network and performed a calculated recreation of the storm event with the best available data. The event was estimated to be a 1-in-25 year rainfall by comparing the rainfall records with the Rainfall Intensity Atlas for Atlantic Canada.

    11. PETERSON / EDINBURGH & AREA System Flows and Infiltration

    12. PETERSON / EDINBURGH & AREA Results of this analysis indicate that roof run-off contributed to over 60% of the capacity of the sewer main on Peterson Drive and 27% of the flows in the trunk main. In all cases, the reduction of direct roof run-off into the system would eliminate overloading of the branch lines and trunk line. Assuming all the branch lines leading to the trunk sewer were near or at full capacity, it is evident that the trunk sewer was overloaded. It appears that reduction of direct roof run-off may not eliminate overloading of the trunk sewer at Beaverwood in the event of a greater than 1-in-25 year storm frequency. Further reduction in system infiltration or an increase in the size of the trunk sewer line is required.

    13. PETERSON / EDINBURGH & AREA In an effort to reduce the potential to overload sewer systems, all direct connections from roof gutter downspouts and french drains should be priority. In addition to this, the backwash discharge from the Water Treatment Plant is directed to the sanitary sewer. This discharge is estimated to be 7.5% of the capacity of the trunk line and therefore should be removed from the sanitary system and directed to the storm sewer. Further study into the wastewater flows in the combined sewer system is recommended before major upgrades are planned. This analysis is an estimation of flows between specific times. Seasonal flow readings should be taken in order to determine the actual peak flows received by the wastewater collection system.

    14. YEAGER STREET During both flooding events, the main drainage ditch along the east side of Magee Road reached capacity and the water elevation reached the top of the 36” diameter culvert which diverts water to Cobb’s Pond. Further investigation on August 30 revealed that two storm sewer systems overflowed, directing water to the Magee Road ditch. This flooding appears to have caused back-up in the storm sewer system upstream at Airport Boulevard and Yeager Street. This may have caused a storm water back-up into a basement on Yeager Street. This assumption is not conclusive since elevation readings of the hydraulic head at the time of the storm appear to be below the footing height of the dwelling on Yeager Street.

    15. YEAGER STREET It is recommended that the 18” concrete culvert crossing Magee Road at the Rotary Park be replaced with a 36” HDPE culvert. This will accommodate overflow from the T’Railway storm sewer as well as the future Yeager Street development. During capital works upgrades in 2011, consideration should be given to increasing the capacity of the storm sewer system crossing Magee Road at the T’Railway. It is also recommended that the future Yeager Street development storm sewer not be connected to the existing system. A new outfall should be incorporated to direct storm drainage away from the existing development and into Cobb’s Pond through the culvert near the Rotary Park.

    16. BENNETT DRIVE Town crews inspected the storm sewer on Bennett Drive during the rain event and discovered that the manholes in front of 60 and 62 Bennett were full. The sanitary sewer was only half full at this time. An analysis of the storm system network and pipe sizes revealed that the main line between 54 and 62 Bennett Drive is not large enough to carry the incoming flow if all branch lines are full.

    17. BENNETT DRIVE Alfonso’s Diving attempted to perform a camera inspection on this section of storm sewer but the bottom was rusted out of the corrugated pipe, making it impassable with the camera equipment. A resident on Bennett reported water back-up in the basement through the floor drains and the foundation wall. Back-up of the floor drains would indicate that they may be tied into the storm sewer rather than the sanitary sewer. A further inspection of the service lines is warranted before a recommendation can be made.

    18. HORNELL STREET The catch basin grate behind 8 Hornell Street was blocked with debris from the drainage ditch between Hornell Street and Wright Crescent. There was no damage to any structures from this incident. The culvert crossing Cooper Boulevard located north of the Raynham Avenue intersection was overloaded and caused water to back up in the main drainage ditch between Hornell Street and Cooper Boulevard.

    19. HORNELL STREET The run-off from this subdivision is very fast since there is little vegetation cover at this time. The speed and volume of run-off in this area should decrease significantly once the development is complete. The drainage ditch between Scott Place and Hornell Street shall be grubbed and cleaned to reduce the potential for debris. The grate openings are to be increased. The ditch in the area immediately downstream of the culvert crossing at Cooper Boulevard should be cleaned to provide additional overflow for the storm ditch.

    20. RAYNHAM AVENUE There were reports of isolated storm sewer back-ups on Raynham Avenue immediately after the rainfall incident. Alfonso’s Diving performed a camera inspection in this section of storm sewer and found no obstructions or deficiencies in the pipe. The Town performed a camera inspection on the sanitary sewer main in this area. The inspection revealed that there were no blockages in the line. There was a minor sag in the sewer main but not significant enough to cause back-up into basements. Follow-up inspections are recommended to monitor the affect of the sag in the sewer main.

    21. MEDCALF STREET Three homes on Medcalf Street reported basement flooding after one or both storms. The Town of Gander performed a camera inspection on the sanitary sewer main along the section servicing these homes. The inspection revealed tree roots in the pipe joints in this section of sewer main. The roots may cause a minor obstruction but would also be a point of entry for groundwater infiltration. These two factors may have contributed to water back-up. It is likely the tree roots are affecting the service lines of the homes in the area which would further explain the isolated flooding. The tree roots found in the sewer main appear to be from a large maple tree. This tree should be removed. Further study of the service lines to the affected homes is required.

    22. METHODOLOGY The investigation was conducted using standard practices in response to isolated flooding reports in specific areas of town. The calculations were performed using standard engineering principles for sewer design with rainfall intensities and peak factors obtained from the Rainfall Intensity Atlas for Atlantic Canada. Background infiltration is estimated from average flows monitored from two existing sewers in the Town of Gander as well as theoretical values. The conclusions presented herein are based solely upon the professional experience and qualifications of staff and time limitations of individuals involved.

    23. For more information, contact: Town of Gander Department of Municipal Works and Services Town Hall, 100 Elizabeth Drive Gander, NL, A1V 1G7 Tel: 709-651-5915 Fax: 709-256-5809 www.gandercanada.com/complaints

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