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 or may not be "engineered." 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 the levee face and other factors. Non-engineered levees are basically long piles of earth pushed up along a river. Engineered levees have a much higher success rate than non-engineered ones.
Levee failures are usually caused by a flow greater than their design flow; poor maintenance; and/or 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:
- "Structural" and "nonstructural" measures.
- Whether they are most suitable for protecting: a) individual structures or b) areas containing multiple structures and communities.
- Whether their purpose is to: a) modify the flood; b) reduce susceptibility to flooding; and/or c) reduce the impact of flooding.
Multiple measures are usually needed to provide protection to an area.
Most of the known floodplains in the U.S. were mapped by the Flood Insurance Administration, under the Federal Emergency Management Agency. These identified areas account for about 60% of flood insurance claims. The remaining 40% 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?
Steps that homeowners and business operators can take to protect themselves against imminent flooding depend on a variety of factors:
- The time available between a warning and the onset of flooding. Longer warning times, up to about 8 hours, enable greater reduction of damages. More than 8 hours warning time produces only a slightly greater reduction of damages in most cases.
- Availability of accurate information on the eventual height of flood waters. This may make the difference between raising furniture a foot or two, moving it the second floor, or moving everything out of the house to a safe area.
- Ability, strength, know-how, or availability of appropriate assistance.
- Preparations that have been made in advance to facilitate evacuation and protection of property.
The general approach in most cases should be to:
- Decide whether to simply raise contents, move them upstairs, or remove them from the site of flooding.
- If moving contents out, arrange a secure storage place and necessary transportation and helpers.
- Owners should prioritize the value of their property:
- Items that are essential to living, such as medical equipment/medicine
- Items that cannot be replaced such as heirlooms or photographs
- Important business/tax papers
- High value small items, such as art, furs, jewelry, and electronics
- Removing carpets (to ease cleanup as well as salvage them)
- Furniture/drapes/beds and bedding
- Remove or leave open all drawers, cabinet doors and room doors (they swell and stick shut).
- Turn off gas and electricity to make building safe against fire and explosion.
- Turn off water to help prevent loss of pressure in the system.
- Remove furnace and gas burners to prevent clogging by sediment.
For all immovable items:
- Remove motors, if applicable.
- Unplug or disconnect any gas or electric lines.
- If possible, wrap items in plastic to prevent water/sediment infiltration.
- Remove motors from equipment that can't be moved (i.e., furnaces).
- Unplug and wrap in plastic any appliances that can't be moved to keep out water/sediment and make cleanup easier.
- Disconnect gas lines to dryers and refrigerators to prevent breakage if the appliance begins to float.
- Tie to a tree or other fixed object any unanchored propane or other kinds of tanks. If tank can be opened safely, fill it with water to prevent flotation.
- If additional time is available and electrical power is disconnected, remove wall receptacles to prevent siltation. This will enable faster restoration of power after floodwaters recede.
- Evacuate or raise the supplies and equipment that will be needed for cleanup such as mops, hoses, gloves, boots, detergents, lubricants, etc.
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:
- Elevating the structure.
- Flood proofing the structure.
- Surrounding the structure with a small wall or levee.
- Facilitating future evacuations.
- Buying flood insurance.
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:
- Buying properties and turning the most vulnerable part of the floodplain into a green way, park, forest preserve or other use not subject to much damage.
- Tightening zoning ordinances to limit the kinds of development permitted in flood-prone areas.
- Developing or improving arrangements for warning of imminent flooding.
- Developing or improving flood preparedness plans.
- Working with state and federal agencies to provide or improve structural protection for the area.
- Implementing a full range of non-structural measures.
- Providing technical and/or financial assistance to property owners in flood proofing or otherwise protecting their property against flooding.
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.