Consideration of Alternatives for Replacing Vanadium Street Culvert

Purpose -
The purpose of this case it to evaluate options for replacing the existing 48" Corrugated Metal Pipe (CMP) culvert that conveys storm water runoff flowing through Anoka County Ditch #14 under Vanadium Street, north of Green Valley Road.  This culvert, which is approximately 40 years old, is failing due to severe corrosion and should be replaced as soon as practical to prevent it from collapsing, resulting in the subsequent collapse of a portion of Vanadium Street.  Figure 1 (attached) includes two pictures showing the existing corrosion.

Background -
The city completed a storm sewer maintenance project this fall along Vanadium Street, north of Green Valley Road.  Figure 2 (attached) shows the project location, and Figure 3 (attached) shows where the culvert crossing is.  This project consisted of extending and/or replacing various culverts and establishing new turf in the impacted areas within the roadside ditches along Vanadium Street.  The project was awarded to Dryden Excavating, who submitted a low quote of $8,630.  Final project costs came in at $10,850.  This storm sewer maintenance project was not identified in the City’s 5-year CIP.
 
During this maintenance project, the property owner at 17730 Vanadium Street NW notified staff that a sink hole was forming in their back yard, and that the slope in their back yard along Anoka County Ditch #14 has been eroding more and more over the last several years.  Upon reviewing their concerns, staff noted that the sink hole was forming around a culvert apron that collects runoff from the ditch along the west side of Vanadium Street and directs it into County Ditch #14. Staff also noticed that the erosion issues in the back yard were due in large part to the existence of a bend in the County Ditch, which appeared to be created by the 48" culvert under Vanadium Street being installed at an angle to the natural ditch line.  Upon checking the condition of the 48" CMP culvert under Vanadium Street, staff found it to be in poor condition with extensive corrosion, particularly along the spring-line of the pipe as shown in Figure 1.  When the culvert is replaced, staff plans to realign the culvert by moving the inlet end on the west side of Vanadium Street to the north about 10 feet which will straighten the ditch line out to minimize the potential for future erosion of the ditch bank. 

Since this culvert conveys runoff through County Ditch #14, staff contacted Anoka County to inquire what the County's requirements are for replacing such culverts.  Staff was directed to replace the culvert with a new culvert of the same material and size, and to install it at the same elevations to ensure the drainage capacity of the culvert, and therefore the ditch, remains unchanged.  Staff was also informed that if a different culvert material was to be proposed the city would need to first complete storm water modeling to determine the effects of the new culvert on flows in the ditch, both upstream and downstream.  Based on the County's response, and considering the time of the year, staff immediately requested quotes from three local contractors for replacing the culvert with a new 48" CMP culvert.  Quotes ranged from $33,655 to $50,184 and covered all costs for removing the old culvert, trenching the new culvert across the road one half at a time to allow one lane of traffic to remain open at all times, for establishing turf on all disturbed areas, and for repairing/paving Vanadium Street.

It is important to note that one lane of traffic must remain open at all times since there is no other access to the properties north of this culvert so everyone must use Vanadium Street and Green Valley Road for access.

None required.

Staff also evaluated the cost to install a new lining in the existing culvert, rather than replacing it, which would provide needed structural support.  However, the cost for materials alone would exceed $20,000, and when factoring in labor the costs to line the culvert would exceed the cost of the lowest quote received for installing a new CMP culvert, even with the needed road repairs.  As such, staff does not recommend this option be considered.

Staff also evaluated costs to replace the 48" CMP with 48" Reinforced Concrete Pipe (RCP) and 48" High Density Polyethylene (HDPE) Pipe.  The life expectancy of CMP has proven to be about 30 to 40 years, whereas the life-expectancy of RCP has proven to exceed 50 years.  The life expectancy of HDPE is estimated to be more than 50 years too, but since HDPE has not been in use this long this has yet to be proven.  However, based on the condition of HDPE pipe that has been installed for 20 years or so, the general consensus is that the life expectancy of HDPE pipe will be at least 50 years.

Below is a table outlining the estimated costs for replacing the 48" CMP culvert with 48" CMP, 48" HDPE, or 48" RCP pipe.  Even though the drainage capacity of HDPE and RCP pipes are greater than CMP pipe due to their smooth inside walls, staff does not recommend reducing the pipe diameter because the culvert immediately upstream is a 48" culvert and it is generally not an accepted practice to reduce pipe sizes downstream.  The project cost for each pipe type in the last column includes costs for the two flared end sections.  And if RCP pipe were to be used, there would be an additional project cost of about $5,000 for additional grading and traffic control requirements due to the shorter pipe lengths. 

 

Pipe Size & Material	Estimated Furnish & Install Cost per Lineal Foot	Estimated Furnish & Install Cost for Project (includes 2 FES's)
48" CMP	$185	$14,200
48" HDPE	$160	$12,500
48" RCP	$325	$26,200

   

Per the estimated costs above, HDPE pipe appears to be the most economical to install while CMP pipe is not far behind.  RCP is by far the most expensive.

The replacement of this 48” CMP culvert is not identified in the 5-year CIP, and no money was budgeted for this work in 2013, or in 2014.  This work is therefore proposed to be paid for through the Stormwater Fund.

Staff recommends replacing the existing culvert with 48" CMP pipe as soon as possible.

Since County Ditch #14 is currently dry and the residential neighborhood north of this culvert crossing has no other access, one lane of traffic must be maintained at all times during the work.  If this work is delayed until next spring/summer, the ditch will likely be conveying storm water runoff which would require the water to be pumped through a hose across Vanadium Street, thereby prohibiting or even preventing one lane of traffic from being maintained at all times.  Also, considering the small cost savings that could be realized by switching from CMP to HDPE pipe, this savings may not justify the delays associated with switching to HDPE pipe due to completing the required storm water modeling and the potential cost increase associated with ripping through frost if the delays are substantial and/or the weather turns cold quickly.   

Bituminous plants will be closing soon so the sooner this work can be completed the better the chance is that the street can be paved this fall.  If staff needs to complete storm water modeling due to switching to HDPE pipe, paving would most likely not be able to be completed this fall.  However, if a small section of Vanadium Street were to remain a gravel surface over the winter, Public Works does not envision this as being an issue for snow plowing.  In this event, the street would be paved in the spring in conjunction with another paving project in this area.

If Council approves the use of 48" CMP pipe, the contractor that provided the low quote of $33,655.16 has informed staff that they could still likely complete the work as soon as a contract is awarded.

Staff also recommends approving a new inspection program whereby staff will inspect all similar culverts on a regular basis to allow all future culvert replacement projects to be added to the CIP and budgeted for through annual culvert replacement projects, as needed.

Motion approving a regular culvert inspection program to be implemented by staff in 2014, the results of which will be added to the CIP and used to budget for annual culvert replacement projects, as needed.

-- AND --

Motion to replace the existing 48" CMP culvert with a new 48" CMP culvert.

--OR --

Motion to replace the existing 48" CMP culvert with a 48" HDPE culvert.

-- OR --

Motion to evaluate other alternatives as directed by Council.