Flood Facts - The Basics of Flooding
This page provides a few basic facts and suggestions concerning flooding and an opportunity to pose questions to flood experts.
Basics of Flooding and Flood Protection
A "floodplain" is the lowland adjacent to a river, lake or ocean. Floodplains are designated by the frequency of the flood that is large enough to cover them. For example, the 10-year floodplain will be covered by the 10-year flood and the 100-year floodplain by the 100-year flood.
Flood frequencies, such as the "100-year flood," are determined by plotting a graph of the size of all known floods for an area and determining how often floods of a particular size occur. Another way of expressing the flood frequency is the chance of occurrence in a given year, which is the percentage of the probability of flooding each year. For example, the 100-year flood has a 1% chance of occurring in any given year.
Dams, levees, channels and other protective works are designed to provide protection against some specific level of flooding. The "level of protection" is selected based on cost, desire of the community, potential damage, environmental impact, and other factors. Engineers can design and construct levees, dams and other measures providing a very high level of protection. Communities tend to choose lower levels of protection because of the initial financial cost rather than overall costs and benefits.
The National Flood Insurance Program has established a de facto minimum standard of protection against the 100-year flood. This is a relatively low level of protection. For example, there is a 26% chance that a levee or channel designed to contain the 100-year flood will be at that design capacity at least once over a 30 year period. All residents and businesses in areas vulnerable to flooding should have flood insurance.
Levees may be either "engineered" or not. Engineered levees are those in which professional consideration has been given to the underlying soil conditions, the kind of earth used in building the levee, proper compaction of the levee materials, armoring of the levee face if needed and other factors. Non-engineered levees are basically long piles of earth pushed up along a river. Engineered levees have a far lower rate of failure than non-engineered levees.
Levee failures are usually due to either: a) a flow greater than their design flow; b) poor maintenance; and/or c) erosion or undercutting of the levee by high flow.
There is a wide range of measures that can be used to protect against flooding. They may be grouped in various ways, such as:
Multiple measures are usually needed to provide protection to an area.
Most of the known floodplains in the U.S. have been mapped by the Flood Insurance Administration, one of the parts of the Federal Emergency Management Agency. These identified areas account for about 60% of flood insurance claims. The remaining 40% of the claims occur in areas not previously recognized as being vulnerable to flooding, and are generally not located near a river or other water body.
The National Weather Service is responsible for warning the public of the possibility of flooding. Flood predictions generally are made at the regional "River Forecast Center". There are several different warning messages that may be issued, based upon the conditions and/or probability of flooding.
What Can Homeowners Do To Protect Themselves Against An Imminent Flood?
The steps that homeowners and business operators can take to protect themselves against imminent flooding vary depending on the following factors:
The general approach in most cases should be to:
For all immovable items:
How Can Homeowners Protect Themselves From Future Floods?
Home and business owners can protect themselves from future floods or reduce the effect of future floods by various means including:
What Should Communities Think About As Soon As The Emergency Is Over?
Preferably before a flood, (and certainly following a significant flood), communities should give immediate consideration to how future flood damages might be prevented or reduced. Some of the means for doing that are listed below. Community officials should consider putting a moratorium on building permits until there has been a chance to give adequate consideration to the following:
If you have any questions concerning floods, flood damage, damage prevention techniques, or governmental programs related to flooding that are not answered here, e-mail the Comal County Flood Plain Administrator. A reply will be sent as soon as possible. Please limit questions to general matters that do not require knowledge about a specific area or an ongoing flood to answer. Easy questions that can be handled by the administrator will be answered quickly. In the case of more complex questions requiring detailed knowledge of a program to answer, please realize that state and federal floodplain management agencies and their personnel are very busy during, and usually for several weeks following, a flood. Some time may be required for response.