During a rain event in 2015, the area of Winter Park Lane and Sherlock Drive experienced flooding that caused damage to one property and threatened another. Halff Associates, Inc., was under contract at the time to develop the Town's stormwater master drainage study, so staff asked them to analyze the causes of the flooding and provide alternatives for reducing the flooding in that area. The analysis showed that the intersection of the two streets is located in the lowest elevation portion of that area and is the collection point for 23 acres in that neighborhood. There are four 10' curb inlets, one 5' curb inlet, and a system of grate inlets in the immediate area, but during the heavy rain events that had occurred that summer, the increased amount of water was more than the storm drain system could handle. Because of the overflow of the system, the storm water flooded one of the houses causing $40,000 worth of damage and rose to the level of the garage of the other house.
The existing underground system downstream of this area consists of one 39" reinforced concrete pipe (RCP) with a capacity of approximately 110 cubic feet per second. Large storms, such as the one in May 2015 in Addison, cause the system to "surcharge" or overflow causing it to accumulate and pond onto the area around the inlets. The depth of the water can reach over 3 feet, causing damage to structures and creating dangerous driving for vehicles, especially those responding to emergencies in the area during a storm.
Based on Addison's Drainage Criteria Manual, this area should be designed for a 100-year, or 1% probability flood, with a minimal 6" flood ponding. Based on the storm water model results, the existing storm drain system along Sherlock Drive does not have capacity for the 1% probability storm, as required in Addison's design standards.
Several alternatives were evaluated to determine if they could reduce flooding at this location. Those alternatives were presented to Council on June 27, 2017 and Council authorized staff to pursue Alternative No. 2, which was to increase the size of the existing storm drain system in the area in order to handle the 100 year (1%) storm event.
The scope of the project will include: replacement and upsizing of the existing storm drain piping from the intersection of Sherlock Drive and Winter Park Lane to Belt Line Road, installation of several new inlets at the intersection of Sherlock Drive and Winter Park Lane, reconstruction of the intersection of Sherlock Drive and Winter Park Lane, and all associated site restoration.
Design for the project was completed in March and the project was placed on BidSync. In response to that solicitation, the Town received five bids. A breakdown of the bids is provided below:
| Bidder |
Cost |
| FNH |
$1,324,990 |
| Flow-Line |
$1,411,558 |
| Texas Standard |
$1,413,879.20 |
| DDM Construction |
$1,461,461 |
| Joe Funk Construction |
$1,656,908 |
| Camino |
$2,201,230 |
FNH has been determined to be the lowest bidder. Staff has checked their references and find them capable of performing this work. The project did come in over budget. After discussing this with several of the bidders, it appears that the volatility of the construction market, coupled with the potential liability of working adjacent to occupied residences, is what attributed to the higher construction costs. Cost estimates were developed by Halff Associates for this project using municipal projections from 2017. Those numbers were then updated using TxDOT bids from March of 2018 along with adding in a contingency. In that estimate, Halff Associates undervalued the cost of the 54" concrete pipe, which accounts for most of the cost overrun.
The total budget for this project was $1,200,000. Of this amount, $160,250 was spent on design, leaving a total of $1,039,750 for project construction. The low bid for the project is $1,324,990, leaving an overall budget shortfall of $285,240. The Stormwater Capital Improvement Program included $735,000 for Storm Water Assessment Basin Improvements in Fiscal Year (FY) 2018. The planned projects were the addition of inlets in the Oaks North area and along Montfort. The plan was to design and construct the Oaks North and Montfort improvements in FY2018. Given the cost overrun of the Sherlock project, staff proposes to move $500,000 in funds from the Oaks North and Montfort construction projects to cover the budget shortfall for Sherlock. The Oaks North and Montfort projects would still be designed in FY2018, but construction will be delayed until funding is available. |