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Flood Types
Flooding typically occurs in our area when large amounts of rain falls over a short period of time from a single, heavy storm, tropical system, or hurricane. After these storms, we rely on a man-made flood control system to drain excess water from the low, flat lands. Existing surface waters levels (i.e. ocean or sound), impact the ability of the connecting drainage systems to receive or store new rainfall. If the surface water conditions are elevated, this impacts the drainage system's ability to transport additional water. If the underground water table is high, water cannot soak into the already saturated ground. We have observed these conditions occurring simultaneously, which has led to rainfall in streets, swales, yards and low-lying areas.
A majority of the existing development in Nags Head is concentrated in the lower-lying areas close to sea level. This is different from inland areas where watersheds have a natural fall line that directs run-off into creeks, streams, and rivers, which then carry excess water downstream. In our area, run-off will collect and stand wherever the ground is saturated, where run-off cannot access natural or man-made drainages, or along impervious surfaces such as roadways and parking lots where absorption is restricted.
Emergency Floodwater Dewatering Plan
Under emergency flood conditions, when public health and safety are endangered, approval may be issued by the North Carolina Department of Environmental Quality (NCDEQ) to allow the direct pumping of floodwaters to the Atlantic Ocean or Roanoke Sound. A comprehensive emergency floodwater dewatering plan has been developed by the Town and filed with NCDEQ Division of Water Resources (DWR) in order to expedite emergency approval. The emergency plan outlines the criteria by which the plan may implemented as authorized by NCDEQ-DWR.
Coastal flooding is caused by high tides coinciding with hurricanes, tropical storms, nor’easters or other low-pressure storm systems which raise sea and tidal water levels, overwhelming coastal defenses. This may be made worse by gale force winds blowing the raised body of water onto the coast. Flooding may be in the form of overwash, where floodwaters erode and overtop coastal dunes or estuarine shorelines. Coastal flooding may affect not only property along oceanfront and estuarine shorelines, but also property inland from the coastline, due to floodwater being forced up connecting ditches, canals and outfalls by raised sea levels. Land areas that are at high risk for coastal flooding are defined by Special Flood Hazard Areas or flood zones.
Surface water flooding in times of heavy rain in prolonged, exceptionally heavy downpours, which are becoming more frequent, the ground may become saturated and the drains and sewers which carry away surface water may not be able to cope, leading to surface water flooding. Although this is more likely in low-lying areas in Special Flood Hazard Areas, it can happen to many other properties which are not specifically designated as being at risk of flooding.
Surface water flooding may be triggered or made worse in densely developed areas where the ground consists of mostly hard surfaces such as concrete or asphalt, as the rainwater flows straight off, rather than soaking into the ground. Surface flooding can also occur when ditches or underground storm systems are unable to carry away floodwaters. Adjacent low-lying properties and land are then liable to be flooded. Surface water flooding can cause widespread and extensive damage because of the sheer volume of water and may be longer-lasting and more difficult to drain away.
Rising groundwater levels resulting from heavier and frequent rainfall can present problems. Groundwater flooding generally occurs during long and intense rainfall when infiltration into the ground raises the level of the water table until it exceeds ground levels. It is most common in low-lying areas or in areas with a naturally high water table.
The water table can rise quickly with every 1 inch of rain resulting in the water table rising approximately 5 inches, causing groundwater to surface in low-lying areas. Several small events can greatly impact our groundwater levels, in addition to the daily tidal changes that can also influence groundwater levels.
Significant ponding can also occur during rainfall due to limited drainage features and flat topography. In some areas where stormwater drainage systems exist, the system may be disjointed or does not effectively convey runoff due to filled swales and ditches or clogged pipes. Homes in low-lying areas constructed below street level can experience an increase in the frequency of flooding. Areas susceptible to groundwater flooding may not be located within Special Flood Hazard Areas or flood zones.