Memorandum

City of Lawrence

Public Works

 

TO:

David L. Corliss, City Manager

FROM:

Matt Bond, P.E., Stormwater Engineer

CC:

Charles Soules, P.E., Director of Public Works

David Cronin, P.E., City Engineer

DATE:

April 24, 2015

RE:

Maple Street Pump Station, 6th & Maple Street - Project No. PW1133

 

Background

In 2005, a comprehensive drainage study (North Lawrence Drainage Study, NLDS) was completed for North Lawrence.  The NLDS took a more detailed and comprehensive look at localized flooding issues in North Lawrence when compared to the original 1996 Stormwater Master Plan.  It provided recommended improvements to the existing drainage system, one of which was the Maple Street Pump Station located within "System 6" (map). The capacity of the existing pump station was identified as being too small to adequately drain the area causing localized flooding of ditches, streets, and yards of area residents. 

 

In November 2008, the community supported a 0.3% sales tax for infrastructure improvements.  The Maple Street Pump Station project was identified as one of the projects that would be completed with the sales tax at an estimated cost of $4.5 million for both engineering design and construction.

 

The engineering design team of Bartlett & West Engineers, Inc. and CDM Smith was chosen for engineering design services.  During and after the development of the engineering drawings several public meetings were conducted. These meetings consisted of a general public information meeting at the Train Depot, two presentations to the North Lawrence Neighborhood Association as well as meetings with individual property owners.  The engineering drawings for the project were also reviewed and approved by the U.S. Army Corps of Engineers.

 

Project Description

The existing pump station is located near the northwest corner of 6th & Maple Streets.  It was previously a sanitary sewer lift station and was retrofitted to help remove stormwater.  However, due to its original purposing it is greatly undersized to handle the volume of stormwater that it currently experiences. The existing pump station is limited by its pumping capacity of 2700 gallons per minute (6 cfs) as well as its 9” diameter outfall pipe.  The area designated as “System 6”, in the NLDS, contains roughly 192 acres (see Attachment A)  and experiences localized flooding during significant rainfall events.  The majority of the “System 6” watershed is north of the Union Pacific Railroad Tracks and is approximately 10 feet below the tracks profile grade.  The property to construct both the pump station and the wet well were previously acquired (see Attachment B).  Once stormwater has been collected at the pump station location it will then be pumped via force main underneath the railroad tracks and into a junction box on the northeast corner of the intersection of 6th & Locust Streets.  From there it will drain by gravity to the Kansas River.  A new outfall and discharge pipe will be constructed at the levee.

 

All of the temporary construction easements have been acquired for this project.  In addition all of the private utilities have been relocated so that there are no conflicts with the proposed storm sewer alignment.  

 

Project Status

On March 3, 2015, bids were received for the Maple Street Pump Station Project No. PW1133, City Bid No. B1503 from the two (2) bidders listed in the table below.  The base bid included the pump station at 6th & Maple Street and the downstream system under the railroad and through the levee to the Kansas River.  In addition to the base bids, contractors also provided an alternate bid for the additional storm sewer work upstream from the pump station.  The alternate bid included extending the storm sewer north of the pump station along the west side of 6th Street and then east along the south side of Lincoln Street to Lyons Park. 

 

CONTRACTOR

BADE BID TOTAL

ALTERNATE BID TOTAL

GRAND TOTAL

Engineer’s Estimate

$4,128,379.00

$1,047,267.00

$5,175,646.00

BRB Contractors

$5,947,275.00

$1,589,338.00

$7,536,613.00

Nowak Construction, Inc.

$6,525,544.00

$1,503,376.00

$8,028,920.00

 

The lowest bid significantly exceeded the engineer’s estimate.  City staff recommended that the bids be rejected and the City Commission formally rejected the bids at its March 10th City Commission Meeting.  As a result of the higher than anticipated bid prices city staff, Bartlett & West Engineers, Inc., and the bidding contractors have met to discuss potential cost savings which include the following:

 

Ø  Modifying the hydrologic criteria to reduce the overall design flow which in turn directly affects the total pumping capacity of the pump station and size of the storm sewer piping.

Ø  Using submersible pumps in the design will raise the elevation of the wet well within in the pump station.

Ø  Raising the wet well of the pump station directly affects upstream flow line elevations of the storm sewer system which in turn lowers installation costs.

Ø  Reusing excavated soil as backfill for storm sewer installation.

Ø  Adding polypropylene pipe as an alternate to concrete pipe for the storm sewer.

Ø  Reconstructing the entire width of 6th Street as an asphalt road instead of replacing half of the road in concrete.

 

The Stormwater Management Criteria was developed concurrently with the 1996 Stormwater Master Plan and was adopted by Ordinance 6778.  The criteria established the return design frequency at a 10-year return period for peak design flows.

 

The North Lawrence Drainage Study calls for a design flow of 302 cfs for the new Maple Street Pump Station.  It should be noted that the NLDS accounted for substantial redevelopment within the watershed which increased development densities and subsequently increased rainfall runoff coefficients.  This design flow was re-analyzed by CDM Smith using ASCE runoff coefficients based on the rational method.  This resulted in a design flow of 195 cfs and this is the design flow that was used when the project was initially bid. 

 

Subsequently, city staff and Bartlett & West have agreed that due to the sandy soil types and the open and relatively flat road side ditches justify the use of an ASCE runoff coefficient within the lower range of values.  In addition higher times of concentration were also utilized to reflect the flat topography.  These design modifications still maintain a 10 year return period design frequency (10% annual chance the storm event takes place) which is consistent with the City's Stormwater Management Criteria and establishes a design flow of 100 cfs.

 

A smaller design flow translates to smaller pumps.  This smaller design flow allows submersible pumps to be utilized in the design.  This in turn results in a higher wet well elevation for the pump station.  The smaller design flow also allows the use of a single 48” diameter force main as opposed to the two that were shown in the original plan set that was bid.  A higher wet well elevation will reduce, trenching, shoring and dewatering cost associated with constructing the pump station.  A higher wet well elevation also results in reduced buoyancy forces and smaller side wall pressures which in turn reduces the thickness of the substructure's floors and walls.  Therefore, the pump station cost will be reduced by using a smaller design flow.  Another benefit of specifying submersible pumps is that it increases the number of suppliers and manufactures available to provide quotes for the bid.  Concrete storm sewer pipe was used exclusively in the original bid.  Including an alternate pipe material (polypropylene) in addition to the concrete pipe should help to lower costs and provide additional manufacturer/supplier competition. 

 

Finally, the plans locate the new storm sewer on the west side of 6th Street from Locust Street south through the levee to the Kansas River.  Originally it was planned to remove and replace half of the concrete street.  Through discussions with the contractors, they felt that the entire street would have to be rebuilt due to construction activities and included the cost of rebuilding the entire concrete street in their respective bids.  The new plans will indicate the entire street be reconstructed using asphalt thus eliminating contractor concerns for repairing the portion of the street that was to remain in place.  Sixth street is a local road serving neighborhood traffic. 

 

Staff Recommendation

It is staff’s recommendation to proceed with maintaining a 10 year design storm frequency with revisions to the hydrologic criteria, using submersible pumps and including two pipe material alternates (concrete and polypropylene) for an estimated base bid of $4.32 million.

 

This would require substantial redesign with a cost of $75,944 for Bartlett & West Engineers, Inc.  The redesign would take approximately 8 weeks.  The project could be rebid in the summer and begin early fall.

 

Also included in the new plans will be an alternate bid package.  This package will include all of the work necessary to install the new storm sewer north of the new pump station to Lyons Park.  If the cost of the base bid plus the alternate bid exceed the budget then this alternate bid package would have to be completed at a future date possibly with City forces.

 

Schedule

May 5, 2015 - Presentation to City Commission

May 6, 2015 - Providing City Commission approval of scope of services direct Bartlett & West to     begin with revising plan set (approximately 8 weeks needed for plan revision)

July 2015 - Revision and review of plans

Fall 2015 - Construction of project

 

Action

Authorize staff to proceed with the 10 year design storm frequency of 100 cfs and approve the engineering scope of services fee for plans and specifications from Bartlett & West Engineers, Inc. in the amount of $75,944.