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Drainage swales are an environmentally sensitive means of providing roadside drainage that is promoted by the Florida Department of Environmental Protection (FDEP) through the issuance of swale exemptions to General Stormwater Permit requirements. Nearly all subdivisions constructed during the last 10 years have used swales to convey runoff from the subdivision roads to retention basins and outfalls.
In addition to conveying stormwater, the intention of a swale is to provide stormwater treatment by allowing some portion of the runoff water to infiltrate into the ground versus completely running off to an outfall. The infiltration process cleans the runoff water and provides a natural recharge to the ground water. The FDEP swale requirements allow the stormwater 72 hours to infiltrate into the ground.
Frequent problems with drainage swales are that they are filled in by residential landscaping efforts or the flow line to an outfall is blocked by a driveway without a culvert. Another frequent problem is the high water table found in many areas of Bay County. The water table will not allow the stormwater to infiltrate into the ground and the swales stay "wet" all the time.
Standing water can be caused by a number of factors. First and probably foremost the yard should be graded such that there are no depressions and the water is directed toward an outfall. That outfall may be the roadside drainage system of swales, ditches and pipes or backyard drainage swales or ditches that eventually lead to an outfall. Frequently the flow of backyard drainage systems is blocked by privacy fences or other yard improvements. While roadside drainage systems are generally located on county property (road right-of-ways) which is maintained by the County, backyard drainage systems are not generally dedicated to the County and maintenance is the responsibility of individual property owners or a homeowners association.
Given the relatively flat terrain of Bay County and the frequently found high water tables, stormwater can be expected to stand for several days prior to it’s being absorbed into the ground. This is especially true during the rainy season when the ground is already highly saturated and the storage capacity provided by wetlands is already full.
There may be a number of reasons for water standing in a ditch following a rainfall. First, the ditch may be blocked or obstructed so the stormwater runoff can not flow to an outfall point. Second, the ditch may be deeper than the flow line of the drainage structures (pipes and/or inlet boxes) and the remaining water will have to be absorbed into the ground. And finally, you may be located in an area with a high water table and the water in the ditch may be ground water which will not be absorbed. One means of checking this situation is to check the level of the water several days after the rainfall and determine whether the level is lowering over time. You can also check the level of the ground water by digging a hole on your property and determining the elevation of the water table of the adjoining land.
The Bay County Land Development Code regulates new developments within the county. Included in the provisions of the land development code are stormwater quality and quantity criteria. Stormwater quality refers to ensuring the quality of the stormwater runoff prior to the water exiting the development site. The land development code requires new developments to meet the criteria as established by the Florida Department of Environmental Protection and U.S Army Corp of Engineers. Specifically, the Bay County Land Development Code governs the quantity of stormwater generated by a development.
New developments are required to control their stormwater runoff through attenuation measures. Attenuation requirements include the stipulation that a development cannot release more stormwater than was released before the development occurred for the 25-year rainfall event. Attenuation is generally provided by retention/detention basins or other means of storage.
(As a side note: The 25-year rainfall event produces a quantity of precipitation that will fall over time based on a 100% probability that a rainfall event of this specific volume and duration will occur every 25 years. Because a 25-year rainfall event occurs this year does not mean there will not be another rainfall of this magnitude for another 25 years. There is a 4% probability that a 25-year event will occur in any given year and a 100% probability that at least one 25-year rainfall event will occur every 25 years.)
Stormwater can be a significant source of pollution if not treated properly. In Bay County the major pollution problem caused by stormwater is the sediments that runoff from dirt roads. These sediments not only affect the clarity of the water but also the deposits adversely affect seagrass beds and block navigation ways.
The County has undertaken an aggressive program of stabilizing dirt roads with open graded asphalt with limited County funds and through the Participating Paving Program (whereby the residents pay 50% of the road improvement costs). In addition to stabilization the County has built a number of sediment basins which detain the stormwater runoff long enough for the sediments to settle out of the water.
While there are no guarantees depending on the nature of the storm event, there are several indicators that can be checked to provide some reasonable assurances that your home will not be subject to routine flooding. First, the Federal Emergency Management Agency (FEMA) publishes flood control maps that depict the 100 year flood zone and for many areas provide the elevation of the 100 year flood. This elevation can be compared to the Finished Floor Elevation (FFE) of your new home.
Second, the FFE of your home should be one-foot or higher than the elevation of the adjacent road. Roads are generally built at a sufficient elevation to not "over-top" during normal rain events. Therefore if your home is higher than the road it is less likely to be subject to flooding. If your home is not higher than the road, there should be some means for conveying the stormwater runoff around your home to a lower area.
Finally, you may also get in touch with the County Stormwater Engineer who works in the Public Works Department and is aware of those areas that have previously been identified as flooding problem areas. Engineering's main phone number is 850-248-8301, an after-hours emergency number is 850-784-4000.