CDM -- Camp Dresser & McKee Inc.

Stormwater Management Element



TO: Bill Spikowski

FROM: Mike Heyl, QEP

DATE: December 10, 1996

SUBJECT: Stormwater Management Element



One of the problems facing coastal communities is how to provide adequate drainage to minimize flooding. Coastal flooding can arise from two different sources. One source of flooding in Fort Myers Beach is unrelated to rainfall and stormwater, and occurs when the water in the Gulf of Mexico and Estero Bay rise to unusual heights due to strong on-shore winds. Often, this type of flooding occurs without rainfall.

In contrast to flooding caused by water flowing onto the island, flooding caused by stormwater (the second source) is the result of a conveyance system which is inadequate to get water off of the island and into the Gulf or Bay. Usually barrier islands have several intrinsically good drainage features because of their narrow profile and the short drainage pathways coupled with highly pervious sandy soils. However, the overall drainage process can be stymied because of low relief and slope, with the simple result that there is no place for the water to flow. It is also aggravated by increased development reducing the natural drainage functions.

Disregarding water quality concerns for the moment, typical solutions to stormwater flooding attempt to move larger volumes of stormwater runoff away from roads and building at a faster rate, or to store the runoff until a later time when the system can accept flow without flooding. For existing development, this is accomplished by increasing the size of drainage pipes, eliminating obstructions, and cleaning or enlarging ditches.

However, it should be noted that these same improved stormwater conveyances will also allow rising water in from the Gulf at a faster rate. At the community level, the only effective technical remedy to rising flood water is to dike the island and outfit the outfalls with flapper valves - an impractical solution for an island of this size. There are however, community activities which can remedy some of the damage. For example, adding dunes to the Gulf side with walk-overs will provide a form of energy dissipation for onshore waves. Rising water would still encroach from the Bay side and flooding will occur, but wave damage would be reduced.

In some respects, stormwater quality issues stand in stark contrast to the causes and solutions to stormwater flooding. Most flood control efforts are focused on the prevention of stormwater flooding up to and including flooding associated with abnormal events, such as extreme rainfall that occurs only once every 5, 10 or 25 years. Because of the infrequent nature of these events, these storms are of little consequence in stormwater quality. The water quality concern is about pollution carried in numerous small storms. Generally, the west coast of Florida experiences about 100 storm events annually. Of these, more than 90 percent produce less than one inch of rainfall. Stormwater treatment technology therefore is geared to treat the runoff from up to a one-inch rainfall, thus providing treatment for 90 percent of the events.

Whereas part of the solution to flooding is to move stormwater as quickly as possible to the Gulf or Bay, several forms of stormwater treatment rely on slowing the movement of water to allow solids and metals to settle out, or storing it in depressions and allowing it to soak into the ground. For example, grassed swales provide good treatment for the small storm events where the depth of water in the swale is small and is slowed by the vegetative cover. During bigger events, the swale fills and the effectiveness of the vegetation in slowing the water becomes progressively less.

The term Best Management Practices (BMP) is used to describe techniques for stormwater management. Structural BMPs are physical devices intended to control the quantity and/or quality of stormwater. A stormwater pond is one example of a structural control. Other BMPs are categorized as source controls, which are designed to control the problem at the source and minimize the need for structural controls. For example, reducing the amount of impervious area results in less runoff. This results in more room in the drainage system for the remaining runoff and results in less water that needs to be treated. Source controls are often the only alternative for built-out communities with little room to install structural controls.

The susceptibility of a community to flooding or water quality problems due to stormwater can be measured by assessing the level of service (LOS) available. For flooding issues, a LOS can be expressed in terms of the degree of roadway flooding and/or the extent of first floor flooding for a given hypothetical storm event. For example, for some communities, a 'C' level of roadway service is defined as no more than six inches of water on evacuation routes during the largest one-day rain event expected every 25-years. A 25-year recurring storm means a storm has 1/25 of a chance of occurring during a given year. The current Lee County Comprehensive Plan stormwater management LOS is that designated evacuation routes shall not be flooded for more than 24 hours by rainfall from a "25-year, 3 day" storm, and . . . new development (except widening of existing roads) shall hold excess stormwater to match the predevelopment discharge rates for a "25-year, 3-day storm." (Note that the definition applies only to flooding which results from rainfall and not to flooding from rising water.)

LOS definitions vary considerably by community. In 1993, a task force consisting of FDEP and representatives from each of the water management districts jointly published a recommended set of criteria (Report to Plan Oversight Committee Stormwater Level of Service Conventions Committee) for flooding LOS. These recommendations defined level 'C' as standard flood protection, which means evacuation routes and arterial roadways must be passable during a 100-year flood event, and collector roadways must be passable during a 25-year event.

The same task force also developed standards for water quality. Compared to a flooding LOS, the concept of a water quality LOS is new in the State of Florida. The water quality ranking system promotes land use controls, followed by structural treatment measures, and penalizes untreated discharge from urban areas. A water quality LOS is not required in the Comprehensive Plan, but should the Town wish to include one, this approach can be considered.



The stormwater management policies in the Fort Myers Beach comprehensive plan will be influenced by a variety of federal, state, and regional regulations. For our immediate purposes, the most direct involvement is through Chapter 163.3177(6)(c) of the Florida Statutes and Rule 9J-5.011 of the Florida Administrative Code. These require that the local comprehensive plan have an element establishing broad and long-term policy guidance for implementing stormwater management throughout the Town. Specific management techniques are not contained in these regulations; but through the formal review process, state and regional agencies will ensure that the policies are coordinated with surface water management policy contained in a variety of other plans. Attachment A contains a complete summary of other federal, state, regional and local objectives for management of stormwater and its potential impact on the Town of Fort Myers Beach, including the impending implementation of the National Pollutant Discharge Elimination System (NPDES) process.



While there appears to be very little water quality data collected within the Town's corporate boundaries, the regional evaluations for Charlotte Harbor (including Matlacha Pass) provided by Florida Department of Environmental Protection (FDEP) are applicable. In FDEP's 1994 biennial report to EPA, FDEP stated that "The predominant pollution problems are associated with development: bacteria from accelerated urban runoff through canals[,] and sediments from construction . . . ." The water quality in urban canals tends to be poor for a variety of reasons. First, urbanization introduces higher pollutant loads from stormwater runoff. Lawn care adds nutrients, pesticides, herbicides and fungicides to the land, some of which will be broadcast directly into the canals during application, or indirectly carried as stormwater runoff. Stormwater runoff also washes off roadway pollution into the canal systems. Roads collect oil, anti-freeze, brake fluids, petroleum products, brake and tire dust, and combustion products. These residues contain high levels of toxic metals and organic compounds, many of which are attached to solids. In the absence of a stormwater treatment facility for settling and removal, these solids and attached pollutants are washed directly into the Gulf and canal systems. In other cases where the drainage is routed through unvegetated areas such as beaches, high rates of runoff will cause erosion which compounds the problem.

Other impacts from urbanization include direct and indirect discharges of wastes, both domestic and industrial. Septic tanks drainfields contribute pollutants through groundwater seepage into the canals. Local contractors have reported that many discharges still remain from Estero Island homes and businesses despite central sewer service. Because many of these canals are dead-end, circulation is poor and pollutants tend to accumulate in the water column and in the sediments, adversely affecting the flora and fauna with the canal system. Fish kills, increased tissue levels of toxic compounds in fish and shellfish, reduced productivity and diversity all result from degraded water quality. While there are regulations against causing pollution through direct, or indirect discharges, there are no federal, state, or regional requirements to sample the ambient waters for pollution except when such monitoring is included as a permit condition. Sampling and monitoring of existing conditions must generally be initiated at the local level. In the future, however some monitoring will be required of the Town by the stormwater NPDES permit.

The major impediment to better flood control on Estero Island is the lack of available land for structural improvements in the older, northern third of the island where Estero Boulevard frequently floods. Improving flood control in this portion of the island will require solutions for both coastal flooding due to rising water and for better control of stormwater runoff. For many areas, drainage simply flows overland to the beach, bay or nearest canal. The existing drainage system is largely undocumented, and some facilities are partially buried or otherwise poorly maintained. In the absence of increased maintenance, the performance of the remaining structures will diminish, or cease due to siltation. The best opportunity for drainage improvement may consist of identifying and maintaining the existing system, coupled with land-use controls for redevelopment. For improvements to the stormwater quality, source controls should be emphasized and structural controls incorporated wherever possible during retrofits.

Conditions improve to the south, where drainage facilities are more abundant and better maintained. Properly maintained, these facilities have a life expectancy of 20-50 years. The commercial and multi-family residential developments constructed after the mid 1980s were built to meet the SFWMD requirement that the rate of runoff after development be no greater than before development (for the highest 3-day rainfall total expected every 25 years). Thus, in cases where the development occurred over undisturbed lands, the rate of runoff is equal to the natural rate of runoff.



Coastal Flooding - Options to reduce the frequency and severity of road and structural flooding resulting from rising water are limited, and best addressed during redevelopment. Technical options include installation of flapper-valves on discharge pipe outfalls located above high tide, raising roadways and structures, berming, and flood proofing structures. While berming is effective at keeping the rising water out, some mechanism (usually pumps) would be required to remove water from within the bermed enclave during large rain events, and raising of roadways often trades dry evacuation routes for flooded structures. Not withstanding evacuation issues and in lieu of a prevention strategy, the most cost-effective strategy is to design/build or redevelop in a manner that will minimize the damage of coastal flooding.

Stormwater Flooding - The performance of the neglected existing drainage facilities could be improved by routine maintenance. Pipes and outfalls should be located, and cleaned. Swales on private property provide small on-site storage and reduce the amount of stormwater that must flow through the conveyance system. Swales also provide water quality treatment and recharge of the surficial aquifer as additional benefits. In the north, it is likely that many pipes are undersized due to the need to drain increased impervious area which has been added over time. The extent of improvement which that be obtained can be determined with mapping and master planning the drainage of the north end of the island, as suggested in the recommendations section of this memorandum. Minimizing impervious area remains a good strategy for both quantity and quality concerns, but some of the popular structural solutions to flooding have not withstood the test of time well. For example, after five years of use, 75% of a porous pavement tested failed to percolate properly, and 50-100 % of the infiltration facilities (without pre-treatment) failed after five years. Another strategy, raising roadways, may improve the roadway flooding LOS, but potentially at the cost of additional off-road flooding or nearby buildings. Despite these limitations, strategies which can effectively minimize impervious area and maximize infiltration will reduce the flooding potential and water quality problems.

Stormwater Quality - There are several options available to improve the quality of stormwater runoff: