Stormwater Improvement for St. Armands Key

Transcription

1 Stormwater Improvement for St. Armands Key Michael A. Hanson, P.E. Senior Engineer Boyle Engineering Corporation 5971 Cattleridge Boulevard, Suite 200 Sarasota, Florida Rajaraman Jayakrishnan, Ph.D. Water Resources Specialist Boyle Engineering Corporation 5971 Cattleridge Boulevard, Suite 200 Sarasota, Florida Southeastern Conference on Stormwater Management Florida Stormwater Association June 19-21, 2002 Orlando, Florida ABSTRACT St. Armands Key, a barrier island located in western Sarasota County in Florida, has had a history of chronic flooding. It is relatively low and bowl-shaped and has experienced flooding caused by both excessively high tides and rainfall events. The existing stormwater drainage system on the Key includes one pump station and gravity drainage pipes, which are not adequate to meet the level of service requirements (No street flooding for 3-year, 1-hour rainfall event and no structural flooding for 25-year, 24-hour event at a design tide elevation of 2.5 feet). A hydraulic model of the existing drainage system was developed using Advanced Interconnected Channel and Pipe Routing (AdICPR) model. Hydrologic and hydraulic simulation using 3-year, 1-hour design rainfall and 2.5 feet design tide indicated severe street flooding of depths up to 16 inches from the gutter line. This flooding is most prevalent in the northern half of the Key. Simulation using 25-year, 24-hour rainfall and 2.5 feet design tide indicated twelve structures whose finished floor elevations fall below the predicted flood stages. Different alternatives were evaluated to complement the existing drainage system that would enable it meet the level of service requirements. Restoration of an existing stormwater pump station, installation of four 1

2 additional pump stations strategically located throughout the Key, and enlargement of some existing storm sewer pipes proved to be the most cost-effective and feasible option that would address the level of service criteria. Hydraulic modeling using the improved stormwater drainage system demonstrated that it would effectively reduce flooding problems on St. Armands Key within the established tidal limits. INTRODUCTION Flooding is one of the major natural disasters in the United States costing billions of dollars to the national economy every year. Florida and other states bordering the Gulf of Mexico and eastern seaboard are especially vulnerable to severe flooding caused by hurricanes and tropical storms from the Atlantic Ocean during the hurricane season of June to October every year. The severe flooding event of June 2001 in Houston, Texas caused by the Tropical Storm Allison is one recent example, which caused more than 4 billion dollars of damage and declaration of Major Federal Disaster for 30 counties in East Texas and several parts of Louisiana. Such flood events and related damages underscore the importance of improved stormwater control systems. Ever increasing population growth, development activities, and environmental concerns place tremendous pressure on existing flood control systems. Retrofitting of existing stormwater systems is often the solution to solve increased flooding due to urbanization and development. This paper focuses on one such retrofit project to improve the stormwater system on St. Armands Key in Sarasota County in Florida, hydraulic modeling of existing and proposed alternative solutions, and the unique challenges met by this project. STUDY AREA St. Armands Key is located in west Sarasota County in Florida, south of Longboat Key and east of Lido Key, in the Gulf of Mexico. Figure 1 provides a general location map of the Key. This Key has had a history of chronic flooding from major storm events as well as more frequent and less intense seasonal rainfall patterns. The Key is relatively low and bowl-shaped and has experienced flooding caused by excessively high tides alone and in connection with rainfall events. The flooding problems have caused structural damage and frequently create disruptions to the businesses and impede vehicular access along this important evacuation route. 2

3 The St. Armands Key drainage basin encompasses an area of approximately 105 acres. This drainage area consists mainly of residential and commercial areas, with minimal open/green space. The roads in the area vary in elevation from 2 to 4 feet NGVD (National Geodetic Vertical Datum 1929), with the lowest floor elevation of numerous structures ranging from 3 to 4 feet NGVD. Figure 1 Location Map of St. Armands Key EXISTING STORMWATER SYSTEM The existing stormwater drainage system on St. Armands contains seven discharge locations to tidally influenced waters. One such location in the northeast quadrant of St. Armands also contains a stormwater pumping station. Most of the storm sewers range from 12 to 15 in diameter. Many are silted in and in need of repair due to frequent exposure to tidal salt water. There is currently no stormwater treatment prior to discharge from the Key. 3

4 PROJECT OBJECTIVES Based largely upon community efforts, the Metropolitan Planning Organization (MPO), Florida Department of Transportation (FDOT), the City of Sarasota, and Sarasota County are working together to address the flooding. Because the MPO is composed of members of each of these groups, a St. Armands Drainage Subcommittee was also formed to help oversee the efforts. St. Armands Key homeowners and business representatives are also on this Drainage Subcommittee. Specific level of service (LOS) criteria developed by technical staff was established to identify flooding problems and evaluate alternatives, as follows: No structure flooding for a 25-year, 24-hour rainfall event (8 inches) at a design tide elevation of 2.5 feet, and No roadway flooding for a 3-year, 1-hour rainfall event (2.55 inches) at the same design tide elevation. It should also be noted that a key factor in determining the LOS was the perimeter elevation of the Key and the expected storm surge from extreme events. Many seawalls surrounding the Key have a top elevation of 3.0. During hurricane events, the Sarasota Bay may be subject to storm surge elevations as high as 11.0 feet. Under these conditions, residents will be evacuated from the Keys. Therefore, the focus of these improvements is mainly on those events that can be reasonable controlled and may facilitate the evacuation under more extreme events. HYDROLOGIC/HYDRAULIC MODEL Topographic survey was performed for St. Armands Key by combining both field survey and aerial photogrammetry to develop one-foot contour interval elevation map. An inventory of the storm sewer conveyance system including respective pipe sizes, inverts, and grate elevations was also prepared using the drainage atlas map of the Key. Further, the finished floor elevation survey was conducted for 80 structures within the Key. These data were utilized to delineate the drainage areas for the storm sewer system, develop a hydrologic/hydraulic model of the stormwater system on St. Armands Key, assess the existing conditions and LOS deficiencies, and determine if the proposed improvements satisfied the LOS criteria. 4

5 A hydrologic/hydraulic model of the stormwater system was developed using AdICPR model (Singhofen, 1990; Singhofen, 1994). The existing stormwater pump station at the northeast quadrant for the Key is dated to the early 1970s and no as-built information is available. Though the pump station is still functioning, its discharge capacity is suspected to be severely limited. Hence it was assumed that this pump station was inoperable for existing conditions modeling. All pipe systems modeled were assumed to be free from sediment and groundwater infiltration for the purposes of this study. A review of records from the Federal Emergency Management Agency (FEMA) concerning flood damage claims on St. Armands Key revealed 12 individual structures that were associated with flood losses. This information was compared to the results of the existing conditions modeling for purposes of providing a general validation of the model. Existing conditions model indicated eleven structures as being flooded by the 25-year 24-hour design storm, out of which 7 structures could be confirmed as being contained in the FEMA flood loss records and 2 additional structures in the FEMA records are within 0.1 feet of flooding (Boyle Engineering Corporation, 1999). The results of the existing conditions modeling also indicated street flooding of depths up to 16 from the gutter line for the 3-year, 1-hour design storm. This flooding was most prevalent on the northern half of St. Armands Key. Based on the simulated flood elevations at model nodes for the existing conditions and contours and spot elevations throughout the Key, a 25-year floodplain map was developed. EVALUATION OF ALTERNATIVES Preliminary model runs were conducted to establish the sensitivity of the system to various potential solutions including pipe size, pump size, check valves and tidal levels. Two alternatives were evaluated by hydraulic modeling to alleviate existing flooding problems on St. Armands Key to meet the LOS criteria; each alternative relied on one particular methodology to solve the flooding problem (Boyle Engineering Corporation, 1999). Alternative 1 considered restoring the existing stormwater pump station and installing up to four additional stormwater pump stations, strategically located throughout the Key. This alternative also involved enlarging 5

6 some storm sewer pipes generally away from the commercial district located around the center of the Key. This alternative was determined to be effective in addressing the LOS criteria. Alternative 2 envisioned restoring the existing stormwater pump station and constructing an underground storage vault and pump station in the center of the Key. Additionally a smaller pump station was included at each (north and south) end of the Key. This alternative also required the installation of new storm sewer pipes to feed water into this central facility. Although this alternative was found to be effective in addressing the LOS criteria, it would result in significant disruptions to traffic and businesses in the commercial district. Both alternatives included the use of rubber check valves on all outfalls to eliminate tidal waters from backing into the storm sewers. A third alternative was also considered and it involved gravity drainage option by enlarging the existing storm sewer system as well as restoring the existing stormwater pump station. But this alternative was deleted from further consideration since many of the roads are below the design tide level of 2.5 and could therefore not be addressed no matter how much the storm pipes are enlarged. Following the hydraulic modeling of the two alternatives, the pros and cons of each alternative along with preliminary cost estimates were evaluated. Significant efforts were made to estimate the potential costs of utility conflicts for these alternatives. Based on discussions with local contractors it was assumed that a 50% increase in the per linear feet cost of pipe installation would address utility issues in this older urban section of the city. Approximate cost of Alternative 1 was estimated to be $4.1 million, while implementation of Alternative 2 would cost about $5.7 million (Boyle Engineering Corporation, 1999). Both alternatives satisfied the LOS criteria discussed in the previous section. Both alternatives were compared based on costs, utility conflicts, and potential disruptions to residential and commercial traffic. Based upon this evaluation, and in compliance with the specified LOS criteria set forth, Alternative 1 was recommended for implementation. This alternative provided the most cost-effective resolution of the flooding problem on St. Armands Key. Although disruption of traffic would occur, construction could be phased and this alternative has the advantage of being focused away from the commercial district of the Key. 6

7 UNIQUE CHALLENGES The recommended alternative for stormwater system improvement on St. Armands Key will have 5 forced-discharge pipes draining into the Sarasota Bay. Hence environmental issues were reviewed including point source and existing/proposed outfall locations to evaluate the potential impacts from scour and sedimentation due to the proposed improvements. The condition of the bay bottom around the current outfalls and proposed outfalls is disturbed, with no sea grass beds within 25 feet. These findings are further supported by the habitat assessment of bay bottom conducted in 1993 by the Mote Marine Laboratory for the Sarasota Bay National Estuary Program (Culter and Leverone, 1993). This report indicated that the entire bay area surrounding St. Armands Key has disturbed bay bottom. The proposed outfall discharge pipes would drain to such areas of the bay that already have disturbed bottom habitats. Therefore, the outfalls and pressure discharge of stormwater will have negligible adverse impacts to sea grass beds. Further, discharge locations are largely dictated by available public property access to the tidal waters. Within the available areas, outfalls will be located so that they discharge to the most effective tidal mixing zones. This will allow stormwater to dissipate and mix quickly, resulting in limited nutrient impacts on the bay and wildlife. Riprap aprons will need to be provided at the end of forced-discharge outfalls to minimize the impact of stormwater discharge. The discharge at the outfalls is anticipated to be submerged or partially submerged discharge. Water quality was another environmental concern for this project. Currently the island has virtually no water quality treatment facilities, such as retention ponds or grass swales. Boyle Engineering Corporation (1999) identified several limitations to providing reductions to the pollutant loading to Sarasota Bay from St. Armands Key. Being located on tidally influenced waters, combined with the low elevations of the Key, result in groundwater elevations being close to natural surface elevations. These high groundwater elevations would require significant increases in the size of wet or dry treatment facilities. Due to the built-out nature of St. Armands Key, no sizable parcels of land exist for the creation of such facilities. In addition, the public involvement with this project revealed a desire to minimize open water in public areas. Furthermore, three of the five proposed sites for pump stations are located in the medians along the major roads on the Key. These medians along with the St. Armands Circle located at the center of the Key are listed in the National Register of Historic Places as a Historic District. 7

8 Since no other public land is available on the Key to locate the pump stations, medians are the only choice for three pump stations and the Historic District designation requires development of a strategy to minimize the impacts to median appearance (Boyle Engineering Corporation, 2001). Considering that all public areas are within the existing medians and the Circle, the traditional method for treatment of stormwater is effectively eliminated. Possible options for water quality treatment were evaluated and the recommended option was the use of smart boxes in the form of Stormceptor inlets, Vortechnics stormwater treatment systems or equivalent system. These structures are not considered as replacements for water quality treatment ponds, and require a significant maintenance commitment. However, with an appropriate maintenance schedule, as recommended by manufacturers, up to 80% reduction in pollutant loading for Total Suspended Solids (TSS) may be anticipated depending on the operation conditions. A detailed evaluation of the reduction potential will be performed during the final design and permitting phases of the project. Baffle walls will also be provided in the pump station wet wells to enhance sediment trapping. Other low cost options for pollutant load reduction such as grate inlet skimmer boxes (Kimley-Horn and Associates, 2000a and 2000b) are available, but their pollutant load reduction potential is significantly less than the more sophisticated systems suggested above. STATUS OF THE PROJECT Based on the recommendations from the stormwater study of St. Armands Key (Boyle Engineering Corporation, 1999), preliminary design of the recommended alternative is complete. This involved review of the existing utility locations, proposed and existing outfalls, preliminary geotechnical investigations, preliminary structural and electrical design of the pump stations, architectural options, and analysis of discharge pipe sizing and routing options (Boyle Engineering Corporation, 2001). A permit application was submitted to the State Division of Historical Resources incorporating the strategies to minimize the impact to the appearance of the medians due to the pump stations. This was approved in April Final design of the proposed improvements by Boyle Engineering Corporation is pending Board approval from the Sarasota County and is anticipated to be complete by summer of Competitive bid for construction will be out by Fall 2003 and the construction will be completed in two phases during

9 REFERENCES Boyle Engineering Corporation, St. Armands Key Stormwater Study. Submitted to the Sarasota County, FL. Boyle Engineering Corporation, St. Armands Key Stormwater Improvement Project Preliminary Design Report. Submitted to the Sarasota County, FL. Culter, J. K. and J. R. Leverone, Bay Bottom Habitat Assessment. Final Report. Submitted to Sarasota Bay National Estuary Program. Mote Marine Laboratory, Sarasota, FL. Kimley-Horn and Associates, 2000a. Burnt Store Isles Catch Basin Filter Study Technical Memorandum. Submitted to the City of Punta Gorda, FL. Kimley-Horn and Associates, 2000b. Burnt Store Isles Catch Basin Filter Study. Permit Modification. Submitted to the City of Punta Gorda, FL. Singhofen, P. J., Modeling Tailwater Dependent Stormwater Systems Using adicpr. Symposium Proceedings on Urban Hydrology. M. E. Jennings (ed.). American Water Resources Association, Bethesda, MD. Singhofen, P. J., Comparison of adicpr (Version 2.0) and SWMM-EXTRAN (Version 4.3). Paper presented at Conference on Practical Applications of the Storm Water Management Model (SWMM). ASCE Florida Section (East Central Branch). Orlando, FL. April 15. 9