Glossary of Terms
A flood event that has a 1%, or one in 100, chance of occurring in any given year. The term refers to the flood's size, not how often it occurs. Several 100-year floods can occur within the same year or within a few short years. A 100-year flood does not mean a flood of that size will occur once every 100 years.
Ad Valorem Tax
Tax based on a certain percentage of a property's appraised value. In May of 2002, voters of Williamson County approved an ad valorem tax equal to 2 cents per $100 property valuation to fund the District's dam safety program. For example, a home with an appraised value of $100,000 would have a tax bill of $20 each year.
A menu of non-structural mitigating options for existing high hazard dams to provide flexibility when the dam cannot safely pass the PMF but will safely pass at least 50% of the PMF. Instead of having to structurally modify a high hazard dam to pass the PMF, the dam owner may choose from a list of options that are appropriate for the particular situation at the dam while still providing substantial protection to the downstream public. Examples of cafeteria plan options include: public information programs, early warning systems, emergency action plans, instrumentation and monitoring programs, operation and maintenance programs, and regular dam safety inspections.
State dam safety rules define a dam as any barrier, including one for flood detention, designed to impound liquid volumes and which has a height greater than six feet. This does not include highway or roadway embankments, low water crossings, levees, or closed dikes.
Dam Emergency Action Plan (EAP)
An Emergency Action Plan (EAP) is a formal document that identifies potential emergency conditions at a dam and specifies pre-planned actions to minimize the potential loss of life and damage to property. An EAP contains actions the owner can take to identify and moderate potential emergencies at the dam. The EAP includes procedures and information to assist the dam owners in issuing early warnings to the responsible downstream emergency management agencies. The EAP contains inundation maps to show emergency management authorities the critical areas for action in case of an emergency (such as road closures).
Early Warning System (EWS) or Flood Monitoring System (FMS)
An instrumentation system located at the dams to automatically measure reservoir levels. The reservoir data is transmitted (usually by radio) to a base station where information is processed to determine the rate of rise in the reservoir. The rate of rise is then used to predict the expected time when emergency conditions could develop.
Dams are classified based on down-stream hazard, in accordance with state dam safety rules, based on the potential loss of human life and/or property damage within either existing or potential developments in the area downstream of the dam in the event of failure or malfunction of the dam or appurtenant facilities. Hazard classification does not reflect any condition of the dam itself.
High Hazard Dam
Hazard classification does not reflect any condition of the dam itself. Texas dam safety criteria state that for a dam to be classified as high hazard, loss of life or excess economic loss (i.e., urban development exists or large numbers of inhabitable structures are at risk) is expected. The District has designated all of its dams as high hazard classification because of existing and anticipated urban development downstream of its dams.
The easement (land controlled by the District) above the dams that stores flood water during a rain event. While houses and businesses are restricted from these areas, greenbelt-type use is allowed, such as parks or golf courses. An example is Dam No. 7, which has Avery Ranch Golf Course on one side and a City of Cedar Park park on the other.
Probable Maximum Flood (PMF)
The flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions. Texas dam safety criteria require high hazard dams to be able to safely store and discharge the PMF floodwaters without causing the dam to fail. The PMF for the District's basin is 44 inches of rain falling within 24 hours.