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Item 8G AGENDA ITEM # SG AUGUST 23, 2010 CITY OF ATLANTIC BEACH CITY COMMISSION MEETING STAFF REPORT AGENDA ITEM: Stormwater Runoff and Land Development Regulations On -Site Storage Requirements SUBMITTED BY: Rick Carper, P.E., Public Works Director, DATE: August 13, 2010 BACKGROUND: During the July 14 Commission Meeting, a resident with a new construction building permit with on -site storage required questioned the City's requirement as being over burdensome and costly to new home builders. On- site storage is required by Land Development Regulations Section 24 -66 (b). (Attachment A). The State has regulated stormwater discharge from larger sites for years using Environmental Resource Permits (ERP). The current thresholds for requiring a Water Management District permit are 4000 SF drivable impervious surface or 5000 SF of building or other impervious area not drivable, with the goal of limiting post development discharge (both quantity and quality) from a development site to Tess than or equal to pre - development discharge. Atlantic Beach extends these requirements to the single family lot level. History: In 2002, the City was beginning construction of the Core City Drainage Improvements to combat flooding issues. This $9 million dollar drainage and infrastructure improvement project was designed to minimize damage to trees in the right of way to maintain the canopy coverage that is so much a part of the Old Atlantic Beach ambience. To minimize these impacts, on many streets the drainage system was placed in the middle of the street and pipe sizes were limited to that necessary to carry the existing runoff volumes, meaning the system does not have capacity for increased runoff resulting from added impervious area. (Attachment B, City Manager letter from April 2006 provides additional details concerning stormwater collection system limitations). At the same time the Core City Project was proceeding, Engineers from CDM were finishing a Stormwater Master Plan Update (SMPU) for the city. As part of the SMPU, CDM was tasked to make recommendations for changes to the Land Development Regulations to ensure future development with increased impervious surface areas and resulting stormwater runoff did not result in a return to the flooding conditions the city was spending millions of dollars to correct. CDM's recommendations (Attachment C) included the requirement for on -site storage from every developed or redeveloped parcel. This requirement was added to the Land Development Regulations in March 2003. The City considered allowing developers to pay a stormwater impact fee rather than provide on -site storage, however this option was not adopted, partly because of a belief that the persons causing the problem should be responsible for the solution, but also because of the problem of quantifying a cost for public development of future facilities (as an example, the dramatic increase in land costs for the Hopkins Creek Retention Pond from when it was initially proposed in 1996 (-$400,000) and the land was finally purchased in 2006 ($860,000)). AGENDA ITEM # 8G AUGUST 23, 2010 Since the City started enforcing the on -site storage requirement, there have been periodic complaints from builders, developers and homeowners that control of storm water was a municipal function and should not be required at the lot level. City response has always been that it should be the responsibility of the developer / homebuilder to pay for impacts caused by increased stormwater runoff rather than City tax payers as a whole. Controlling the increase at the source also results in many small systems, with little environmental impact, rather than larger, public systems such as wet ponds with the attendant increase in public maintenance costs. Additionally, it would be Tess of a burden for the neighbors if the stormwater solutions were provided on site vs. in the ROW (no closed roads during construction, etc.) A comparison of costs for publicly financed stormwater retention systems to the cost for various systems used by private homeowners / builders to meet the requirements at the lot level shows that private storage is significantly Tess costly on a unit cost ($ /Cubic Foot) storage basis. Staff calculated this unit cost as $5.92/CF for Hopkins Creek using actual project costs vs. $4.96/CF for a proprietary in ground storage system already constructed in Atlantic Beach. Private storage may also be accomplished in many cases by surface swales or by perforated HDPE pipe in a gravel envelope at substantially lower unit costs than the proprietary systems. Because of rising land values, the lack of undeveloped land in Atlantic Beach, and limited areas in which to construct detention facilities, construction of under road vault systems with associated pumping stations may be the only method feasible for Atlantic Beach to construct a public stormwater facility. A unit cost estimate of at least $15 /CF is a more likely comparison point for future public vs. private costs. The resident voicing this latest concern over the on -site storage requirement provided a survey showing only an existing swimming pool with his original building permit application. Since approval of the building permit, the resident, with Staff assistance, has provided documentation of substantially more previously existing impervious area on this lot, such that the on -site storage requirement has been reduced from 1830 cubic feet to 540 cubic feet. He has submitted a plan that provides this volume using surface swales and in- ground storage with no requirement for pipes or other structures. If the original on -site storage requirement had remained, using a cost of $440,000 for the lot (Property Appraiser's website) and a reported construction value of $390,000, the reported cost of $7000 for materials, assuming a 50% cost for design and installation, would have amounted to -1.25% of the total cost for the home. BUDGET: No budget impact; for Commission information only. RECOMMENDATION: None. For Commission review and discussion, pending further direction to Staff. ATTACHMENTS: A. COAB Land Development Regulations Section 24 -66 (b) B. City Manager's Letter - Capacity of City of Atlantic Beach Storm Water System and Related Storm Water Regulations C. SMPU 2002 Appendix D, Stormwater Criteria for Development and Redevelopment REVIEWED BY CITY MANAGER: 4741.. AGENDA ITEM # 8G AUGUST 23, 2010 Attachment A Sec. 24 -66. Stormwater, Drainage, Storage and Treatment Requirements. (b) On -Site Storage. The Applicant shall be required to provide on -site storage, such that there is no increase in the rate or volume of flow to off-site, from every developed or redeveloped Parcel, and for any addition or modification that increases the impervious surface area on a developed lot by 10% or 400 SF, whichever is smaller and provide documentations and calculations to demonstrate compliance. Development Projects previously permitted by the St. Johns River Water Management District (SJRWMD), which have an in- compliance retention or detention system that collects and controls run -off, are exempt, however a copy of the Engineer's Certification of As -Built Construction to the SJRWMD must be submitted to the City before issuing building permits for individual lot construction may begin. The requirement for on -site storage may be waived by the Director of Public Works if storage is determined to be unnecessary or unattainable. If on -site storage is required, an As -Built survey, signed and sealed by a licensed Florida surveyor, documenting proper construction and required volume of the storage system, must be submitted to the Director of Public Works prior to permit closeout or issuance of a Certificate of Occupancy. For an under- ground system, a notarized letter from the General Contractor, along with red -lined plans and construction photographs, will be sufficient to document proper construction. Volume calculations for Lots that require on -site storage should be based on the difference in run -off volume generated by the new impervious area ( "delta volume ") and would be calculated by: V= CAR/12, where V = volume of storage in cubic feet, A = area of the lot in square feet, R = 25 year and 24 hour rainfall depth (9.3 inches) over the lot area, and C = run -off coefficient, which is 0.6 for the 50% maximum imperviousness, 0.4 for 25% imperviousness, and 0.2 for 0% imperviousness. This delta volume (post V minus pre -V in cubic feet) must be stored at least 1 foot above the wet season water table and below the overflow point to off -site (in many cases this may be the adjacent road elevation). As an option, and as approved by the Director of Public Works, the owner of the parcel to be developed or redeveloped may implement, at the applicant's cost, off -site storage and necessary conveyance to control existing flood stages off -site. AGENDA ITEM # 8G AUGUST 23, 2010 ATTACHMENT A APRIL 10, 2006 COMMISSION MTG Capacity of City of Atlantic Beach Storm Water System and Related Storm Water Regulations April 5, 2006 There is no excess capacity in the City of Atlantic Beach storm water system. In recent years, engineers working for Atlantic Beach have designed storm water improvements to prevent street flooding during most major storms and flooding of homes during a 25 year storm. This means a rainfall event that will occur on average once every 25 years. In Atlantic Beach, that would equal 9.3 inches of rain over a 24 hour period. It has been many years since a storm of that magnitude has been seen here. The city recently spent over $9,000,000 in the old Atlantic Beach area making storm water and other utility improvements. The final plan for these improvements was identified only after two previous designs were rejected by the public because of their environmental impacts. Those plans would have included converting major portions of two parks into retention ponds, the construction of large storm water lines and pumping systems and the removal of many street side trees. Atlantic Beach is basically flat and getting storm water to move rapidly out of residential neighborhoods is difficult. Every additional square foot of impervious surface that is added in old Atlantic Beach will increase the level of flooding in surrounding homes. For this reason, in 2001 the Mayor and Commission adopted a 50% limit on the amount of impervious surface on any given lot and in 2003 adopted regulations requiring the on -site retention of additional storm water runoff resulting from development. In spite of these regulations, the total impervious surface in Atlantic Beach will grow in the future because most of the present lots have less than 50% impervious area, and they can legally expand under current regulations. The purpose of the current regulations is to slow the increase of storm water into the city's system and thereby push the need for additional and more invasive storm water solutions as far into the future as possible. Other areas of Atlantic Beach face the same problems. Some of those, like the area around Aquatic Drive and Hopkins Creek, experience considerable flooding during major rainfall events and will certainly experience house flooding during a 25 year storm. Planning -is underway for improvements in those areas. In other neighborhoods such as Oceanwalk, little or no flooding has been apparent even in the worst of storms. However, as the lots in Atlantic Beach are redeveloped with larger homes, bigger driveways, new swimming pools and other impervious surfaces, more storm water will be put into the drainage systems which inevitably will cause future flooding and the need for additional capital projects to bring that flooding under control. Just because most of the city drainage system works now doesn't mean that we can be careless in the design of new development. For additional information on the status of the city's storm water system, consult the Atlantic Beach Storm Water Master Plan dated February 1995 and the Master Plan Update dated August, 2002. Both are available in City Hall. AGENDA ITEM # 8G AUGUST 23, 2010 Appendix D Stormwater Criteria for Development and Redevelopment Background The City of Atlantic Beach has committed significant capital funds to reduce flooding in the Core City. This Storm Water Master Plan Update (SWMPU) has identified a series of projects with additional significant capital costs to reduce /control flooding in other parts of the City. The City is characterized by a series of remnant depressional areas from the coastal dune system, which along with development patterns has created a series of closed and quasi -dosed depressions or "bathtubs" where flooding occurs. As such, these areas are sensitive to the volume of runoff discharged to them. All of the analyses (Core City, SWMP, and SWMPU) have been based on the existing land use and storage conditions in the City. Therefore, redevelopment and new development could increase the volume and rate of runoff discharged to offsite through new impervious areas and /or the filling of onsite depressional (natural) storage. As an example of this, CDM used the Hopkins Creek stormwater model to simulate the effects of increased impervious area if the upstream areas were to develop to the allowable 50 percent level, and the results show that the flood stages in houses and buildings would be increased. The St Johns River Water Management District (SJRWMD) regulates the peak discharge from development greater than 40 acres or 12 acres of impervious surface. CDM recommends that the City extend these criteria to smaller projects (individual lots) to provide and preserve the City's desired levels of service for flood protection. Reconunendations Therefore, CDM recommends that the City consider the following criteria for implementation: • The City already requires a maximum of 50 % impervious area per parcel. CDM recommends that this apply to parcel subdivision for all platted lots (e.g., split a lot into two lots). • CDM recommends that the City require the applicant to provide onsite storage such that there is no increase in the rate or volume of flow to offsite from every developed or redeveloped parcel. This includes documentation and calculations to demonstrate compliance. Subdivisions and developments that have been permitted by the SJRWMD and have an in- compliance retention / detention system that collects and controls runoff would be exempted (e.g., Ocean Walk, Fleet Landing, Tiffany By The Sea, Paradise Cove). 1 CDM D-9 c020378.A.rwy, bessIO saoo_ uve twa.noffna amc Appendix D StormwaterCritena - a • Or, the City should require from the developer or landowner a fee -in- lieu -of onsite storage. This fee would be based on the fair share of capital and annual costs for the City to construct and maintain the piping and storage facilities downstream in � order to cause no adverse impact offsite (e.g., no increase in flood stages). This fee would include the cost of land acquisition, which could include existing building acquisition. ■ The volume calculations for lots that require onsite storage should be based on the difference in runoff volume generated by the new impervious area ( "delta volume ") and would be calculated by V= C*A *R/12; where, V = volume of storage in cubic feet, A = Area of the lot in square feet, R = 25 year 24 hour rainfall depth (9.3 inches) over the lot area, and C = runoff coefficient, which is 0.6 for the 50% maximum imperviousness, 0.4 for 25 % imperviousness, and 0.2 for 0 % imperviousness. This delta volume (in cubic feet) must be stored at least 1 foot above the wet season water table and below the overflow point to offsite (in many cases this may be the adjacent road elevation. • The City should require no ty equir net loss of 100 -year floodplain storage for areas where a floodplain elevation has been defined by either the Federal Emergency Management Agency (FEMA) on flood Insurance Rate Maps (FIRMS), the 1995 SWMP, the Core City project, or the 2002 SWMPU (e.g., Hopkins Creek). Therefore, site grading must create storage onsite to make up for filling of volume onsite. This storage is in addition to the storage required for the increase in impervious area. COM D -2 o: ene-urere e«o uoa.w.wowvuw di.