Monday, January 19, 2009

Bruce Shackleford explains relationship of gravity-flow sewer mains and trail construction in Fayetteville, Arkansas

Please click on image to ENLARGE view of riparian zone of the Cato Springs/West arm of the Town Branch of the West Fork of the White River where a teenage boy is looking down at the ice-covered stream on January 17, 2009. The view is downstream to the east from about 100 feet southeast of Greathouse Park. The clearing of the wide strip of floodplain to the left was done for placement of a gravity-flow sanitary sewer leading to the Noland Wastewater-treatment plant on the White River leading to Beaver Lake.

Regarding our discussion of trails being constructed within stream riparian zones, the minutes quoted me as saying:

"Bruce commented that in other cities, trails are often constructed along sewer line corridors."

This is not exactly what I said. The "in other cities" should be deleted. When someone mentioned that vegetation was being wiped out along streams to build trails, I was trying to clarify that the City of Fayetteville has built many trails along streams on top of sewer lines that run along streams. Therefore, I would like to clarify the matter.

During the current Fayetteville Wastewater System Improvements Project (WSIP), the city has constructed over 38 miles of new sewer lines. Roughly half of the Fayetteville sewer service is located within the Illinois River watershed, within the Arkansas River Basin, and the other half is within the Beaver Reservoir watershed within the White River Basin.

Where possible, the engineering design firms design gravity sewers so that the wastewater can flow to a given point via gravity. Where gravity sewers are not feasible, a force main must be constructed. A force main is a pressurized line that requires a pump station. A pump station can cost several million dollars to build, and there are significant long term expenditures for energy to pump the wastewater. Consequently, designers try to design gravity sewers whenever it is feasible. For the sewer system to serve some locations, it would require a trench over 50 feet deep to construct a gravity sewer. In such cases, construction difficulties are not feasible. This necessitate the construction of lines that are not sloped to allow gravity flow. These are referred to as "force mains" where wastewater has to be pumped.

Prior to the WSIP, wastewater from both drainage areas was conveyed to the Paul Noland Wastewater Treatment Plant (WWTP) on the White River side. The treated effluent is discharged proportionately from Outfall 001 on the eastern side of the City, into the White River, and on the western side of the city, into Mud Creek, which flows into Clear Creek, a tributary of the Illinois River. Thirteen pumps/lift stations are required to transport the sewage over the ridge from the Illinois River watershed to the Noland WWTP, and then pump treated wastewater back over the ridge for discharge into Mud Creek.
Objectives of the WSIP are to improve the existing sewer collection system, upgrade the existing Paul Noland WWTP, and construct a new (Westside) WWTP, in order to implement corrective actions to eliminate/reduce the odor and overflow problems associated with the existing treatment plant and collection system, and to provide wastewater treatment to areas currently outside the treatment area while reducing the total loading to the existing Noland WWTP, which was approaching design capacity. In doing so, the WSIP allowed for the elimination of eight pump stations, and for wastewater to be discharged into the watershed from which it is generated.

During the Design Phase, I worked with the engineering firms to evaluate alignment alternatives for each sewer line in order to route around ecologically sensitive areas where possible. The selected alignment also had to consider land ownership. Sewer lines that are designed to run right through the middle of someone's property will guarantee a law suit against the city most of the time because it splits the property and reduces the amount of usable land, therefore reducing the value of the land. The law suits cost the taxpayers more money and delays the project, so they are avoided where possible.

For gravity sewers to work and not require an extremely deep ditch for the sewer line, they must be built next to streams. This also usually puts it in the floodplain where the landowner cannot build anything anyway. Construction of the gravity sewers along streams often involves removal of riparian vegetation. Furthermore, I know of no city that allows trees to grow back on top of sewer lines because it can damage the lines and create access difficulties when repairs may be needed in future years.

The WSIP coordinated with City Trails so that trails could be built on top of the sewer lines where clearing had already occurred and no trees would be allowed to grow anyway, thus preventing the clearing of new ground for new trail construction.

There is opportunity to replace some of the trees that were removed during construction of the sewer lines. Typically, the City had a 100 ft. wide temporary construction easement, and a 25-50 ft wide permanent sewer easement. This varies with location. Some of the new sewer lines were 48 inch diameter lines. It requires a wide path to have access with large excavators to install lines of this size. The city allows trees to be put back within the bounds of the temporary construction easement, but not within the more narrow permanent sewer easement.

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