Flooding is the most common, and costliest, natural hazard facing the United States. Flooding has many causes, including heavy rain, snow melting too fast, and dams or levees breaking. Damaging flooding may happen with only a few inches of water, or it may cover a house to the rooftop. Floods can occur within minutes or over a long period, and may last days, weeks, or longer. Floods are the most common and widespread of all weather-related natural disasters.
Flash floods are the most dangerous kind of floods, because they combine the destructive power of a flood with incredible speed. Flash floods occur when heavy rainfall exceeds the ability of the ground to absorb it. They also occur when water fills normally dry creeks or streams or enough water accumulates for streams to overtop their banks, causing rapid rises of water in a short amount of time. They can happen within minutes of the causative rainfall, limiting the time available to warn and protect the public. 
Where do floods occur?
A flood can happen anywhere but some areas that may be more vulnerable to floods than others include:
- Dense urban areas
- Areas along or near rivers
- Canyons and wildfire burn areas
Property owners and communities can determine their current and future risk based on using FloodFactor, a tool developed by peer-reviewed research from the world’s leading flood modelers. Additionally, Pennsylvania’s Department of Community and Economic Development produces maps that allow you to assess whether your property requires flood insurance, your flood insurance rate and grant programs your property may qualify for.
How do local governments manage flood risks?
A municipality may take one of two basic approaches in enacting its floodplain management regulations: updating the regulations within an existing code (typically the zoning ordinance) or adopting a single-purpose ("stand-alone") ordinance. Montgomery County Planning Commission maintains a model floodplain ordinance and aid localities tailor the model ordinances or update an existing floodplain ordinances.