Common Terms

100-year flood

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.

Experts designate the size of a 100-year flood for specific areas -- the actual size will vary from one place to another. Other common flood designations include 50-year floods (2% chance, or 1 in 50) and 500-year floods (0.2% chance, or one in 500). In the Upper Brushy Creek Watershed, the 100-year flood is created by the runoff from 9.9 inches of precipitation within a 24-hour rainfall duration.

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.

Cafeteria Plan

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.

Dam

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.

Early Warning System (EWS)

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.

Emergency Action Plan (EAP)

A formal plan that is designed to identify and react to emergency conditions which could develop at a dam. The three objectives of an EAP are:

Identify emergency conditions posing a threat to a dam.

Enact response actions to prevent failure of the dam.

Prevent or reduce loss of life and property damage should a dam failure occur.

Hazard Classification of a Dam

Dams are classified on 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 indicate any structural or physical condition of the dam itself.

Hazard
Classification

Loss of Human Life

Economic Loss

Low Hazard

None expected

(no permanent structures for human habitation)

Minimal

(undeveloped to occasional structures or agricultural improvements)

Significant Hazard

Possible, but not expected

(small number of inhabitable structures)

Appreciable

(notable agricultural, industrial, or commercial development

High Hazard

Expected

(urban development of large number of inhabitable structures)

Excessive

(extensive public, industrial, commercial, or agricultural development)

Hydrologic Criteria for a Dam

State dam safety rules require dams to safely pass various flood events based on the size and hazard classification of the dam. The magnitude of the minimum flood event which is required to be safely passed is the dam’s hydrologic criteria.

Hazard Classification	Size Classification	Minimum Flood Event to Pass
Low Hazard	Small	25% PMF
	Intermediate	25% to 50% PMF
	Large	100% PMF
Significant Hazard	Small	25% to 50% PMF
	Intermediate	50% to 100% PMF
	Large	100% PMF
High Hazard	Small	100% PMF
	Intermediate	100% PMF
	Large	100% PMF

Probable Maximum Flood (PMF)

The flood magnitude that may be expected from the most critical combination of meteorologic and hydrologic conditions that are reasonably possible for a given watershed. In terms of probability, the PMF event is usually assigned a 0.01%, or one in 10,000, chance of occurring in any given year. In the Upper Brushy Creek Watershed, the PMF is created by the runoff from 44.7 inches of precipitation within a 24-hour rainfall duration.

Size Classification of a Dam

State dam safety rules classify the sizes of dams based on the height of the dam and the maximum reservoir storage capacity:

Size Classification	Maximum Reservoir Storage (acre-feet)	Height of Dam (feet)
Small	Less than 1000	Less than 40
Intermediate	Equal to or Greater than 1000 and less than 50,000	Equal to or Greater than 40 and less than 100
Large	Equal to or Greater than 50,000	Equal to or Greater than 100