November 1996

State Street Watershed Improvements Oldsmar, Florida

by

David W. Hamstra, P.E. Professional Engineering Consultants, Inc.

INTRODUCTION

In September of 1988, the City of Oldsmar (located between Tampa and Clearwater), was inundated during a four-day period with approximately 15 to 20- inches of precipitation. The unusually large amount of rainfall resulted in substantial overland flooding for 4 to 5 days following the storm event. The overland flooding originated north of the City limits and sheet flowed south, over State Road 580/584 (Tampa Road), where direct stormwater runoff eventually entered Mobbly Bay and ultimately Old Tampa Bay (Class II marine system). Several buildings within an industrial park experienced substantial flooding. In addition, State Road 580/584, a hurricane evacuation route, was overtopped with an average depth of flow of approximately 3 to 4-inches extending several hundred feet along the roadway.

During the Spring of 1989, the City of Oldsmar decided to address the flooding experienced during the September 1988 storm event by initiating a watershed study for the eastern portion of the City. During September 1989, PEC completed an analysis of the City's existing conveyance systems and drainage basins. Based on the recommendations of the PEC study and the observed conditions during the September 1988 storm event, the State Street watershed was identified as the priority drainage basin to implement flood control and conveyance improvements. During September 1990, PEC completed a detailed analysis of the 585-acre State Street Drainage Basin and recommended improvements to provide a much needed drainage conveyance and outfall system. Based on the findings and recommendations of the September 1990 Engineering Report, the City Engineer, Mr. Fred Schildhauer, P.E., prepared applications and the City received a $250,000 grant from the Pinellas-Ancolte River Basin Board to construct the flood control improvements.

Upon receipt of the grant, the City Council authorized (January 1991) engineering services to design and permit for drainage improvements within the State Street Basin in order to provide adequate flood control. Concurrently, the Florida Department of Transportation (FDOT-District Seven) began design for a six - eight lane improvement of State Road 580/584 from Tampa to northern Pinellas County, with priority given to improving the two-lane segments within the City of Oldsmar.

OBJECTIVES

The initial design consisted of enlarging existing ditches, constructing new ditches and culverts, and utilizing a portion of Mobbly Bay as a stormwater management facility. The stormwater facility would provide pre-treatment of stormwater runoff, since several roadways within the State Street Basin are unpaved. Therefore, the City's primary goal was to contain the sediment loading in a maintained facility ultimately reducing the impacts to Mobbly Bay.

During July of 1991, the proposed improvements were submitted to Pinellas County and FDEP. Pinellas County had previously accepted delegation from the Southwest Florida Water Management District to review and approve permit applications. Based on the extensive comments received from Pinellas County and FDEP, it became obvious that the proposed improvements within waters of the State would not be a permittable project. At this point, the City decided to re-evaluate the project in order to address the environmental concerns of both agencies and the concerns of the general public with respect to flood protection.

Since a design revision was required to resolve the permit problems, Mr. Fred Schildhauer, P.E. established additional objectives and coordinated with PEC to redesign the project during the Spring of 1992 in order to achieve such objectives, as follows:

Objective No. 1 - Flood Control
The original and number one priority of this project was still flood control. It became quite apparent during the September 1988 storm event that the existing drainage system was inadequate to convey peak runoff rates and prevent substantial flooding. The City's goal was to convey peak runoff during a 25-year, 24-hour storm event through a series of ditches, channels, and culverts.

Objective No. 2 - Paving of Dirt Roads
The second objective was to pave all existing dirt roads within the State Street Basin (approximately 18,300 feet or 3.5 miles). The reasons for paving the dirt roads are two-fold. First, to improve air quality through the reduction of dust entering the atmosphere. Second, to improve water quality of the receiving waterbody through the reduction of sediment loading. Typically, the regulatory agencies (FDEP and WMD's) will allow municipalities to pave existing dirt roads without permits as long as the roadway width is maintained, the section is not curbed and guttered, and the amount of traffic is not increased. However, there is one case that prevents agencies from granting a permit exemption. It involves any roadway that currently discharges to a Class II waterbody. In this case, a stormwater permit is required in order to demonstrate that adequate pollution abatement volume will be provided.

This is the case for all dirt roads within the State Street Drainage Basin, since they currently discharge to Mobbly Bay. Therefore, the proposed redesign of the stormwater facility had to provide sufficient treatment volume to off-set the proposed paving. Based on meetings with the regulatory agencies, it was agreed to provide pollution abatement volume for one-half inch of runoff over the proposed impervious area (approximately 15.10 acres).

Objective No. 3 - Water Reuse
The third objective set forth by the City was to redesign the stormwater management facility and integrate it as a component of the City's current and proposed water reuse program. The City planned to construct ground storage tanks adjacent to the existing wastewater treatment plant (WWTP) located near the outfall of the State Street watershed.

While the treated wastewater effluent would be stored as reuse water in the proposed ground storage tanks, stormwater could then be stored in the abandoned effluent ponds. Under normal conditions, effluent will be distributed throughout the City for irrigation. However, during periods of peak demand, the stormwater facility would provide water to augment the demand via an intake pipe located below the control water elevation. If required, the City would also have the capability to re- direct the stormwater runoff through the WWTP for further treatment prior to distribution.

Objective No. 4 - Water Quality
Another objective of this project was to provide "some" level of pollution abatement volume for the State Street Basin in order to reduce the pollutant loading currently entering Mobbly Bay. Historically, the runoff from the State Street Drainage Basin discharged directly, without treatment, to Mobbly Bay. The current land use consists primarily of residential, commercial, and industrial, and with sediment loading caused in part by the dirt roads contributing various pollutants to Mobbly Bay. The water quality objective was also consistent with the goals of the Tampa Bay Surface Water Improvement and Management (SWIM) Program with respect to reducing pollutant loads.

Objective No. 5 - NPDES
During February of 1991, Pinellas County received official notice from the United States Environmental Protection Agency (EPA), Region IV, that they would be required to submit a National Pollutant Discharge Elimination System (NPDES) Municipal

Separate Storm Sewer System (MS4) permit application as a "medium municipal separate storm sewer system" that is required to file a Part 1 and 2 permit application. Pinellas County was also designated as the "Lead Applicant" for the area-wide permit, and all other incorporated municipalities, including the City of Oldsmar, as "co-applicants."

The 1987 Clean Water Act amendments required EPA to develop regulations under the NPDES Permit Program to address stormwater discharges. A major portion of these regulations (40 CFR, Part 122) deal with the reduction of pollutant loads from "major outfalls." The existing outfall ditch that drains a significant portion of the State Street Basin is classified as a "major outfall." Therefore, the final objective of the project was to reduce pollutant loads to the Maximum Extent Practicable (MEP) in order to meet the intent of the Federal Storm Water Regulations.

PROPOSED IMPROVEMENTS

In order to address the multiple project objectives, Mr. Schildhauer suggested converting the existing effluent holding ponds to an 18-acre regional "wet" detention stormwater facility. As a result of converting the holding ponds to provide stormwater management, the City was: (a) able to address pollution abatement volume associated with paving of dirt roads; (b) forced to construct the ground storage tanks and expand the water reuse program; (c) provide water quality treatment associated with 585-acres of urban runoff; and (d) satisfy the Federal storm water regulations by reducing pollutant loads to Mobbly Bay.

The total cost of construction, including the 8,000 linear feet of channel and culvert improvements for flood control, was approximately $1,100,000. In order to off-set the wetland impacts associated with severing the basin from waters of the State, the City created a 21-acre wetland/upland preserve. The permitting involved the removal of Brazilian pepper in order to encourage the growth of mangrove systems. As previously noted, the City received a $250,000 grant from the Pinellas-Ancolte River Basin Board in addition to another $250,000 grant received prior to construction. The remainder of the construction costs were paid from the City's stormwater utility fund. The City's stormwater utility fund was established in 1991 through the efforts of the City Engineer, Fred Schildhauer, P.E., in order to fund the many needed stormwater improvements throughout the City.

SUMMARY

In summary, it would have been much easier to permit the State Street project if it involved conveyance improvements only, or if the project involved constructing typical ponds and swales associated with the paving of dirt roads. However, the City felt these alternatives would be short-sighted and decided to address all five objectives as one project. This involved the additional costs and time to permit the conversion of the abandoned effluent holding ponds to a regional "wet" detention stormwater management facility.