Glossary of Terms


# A B C D E F G H I J K L M N O P Q R S T U W

0.2 percent (500-year) flood – A 0.2 percent (500-year) flood is a flooding event that has a 0.2 percent (1 in 500) chance of occurring in any given year at any given location. Statistically, a 0.2 percent (500-year) flood has a 6 percent chance of occurring during a 30-year period of time, which is equal to the duration of many home mortgages. Contrary to what the term suggests, a "500-year flood" is not a flood that occurs only once every 500 years. A "500-year flood" can occur multiple times in a 500-year period of time.

0.2 percent (500-year) flood elevation – A 0.2 percent (500-year) flood elevation is the distance above sea level (or other datum) that the water level in a bayou, creek or other waterway reaches during a 0.2 percent (500-year) flood.

0.2 percent (500-year) floodplain – A 0.2 percent (500-year) floodplain is an area at risk for flooding from a bayou, creek or other waterway overflowing during a 0.2 percent (500-year) flood. Structures located in a 0.2 percent (500-year) floodplain have a minimum of a 0.2 percent chance of flooding in any given year. Statistically, structures located in a 0.2 percent (500-year) floodplain have a minimum of a 6 percent chance of flooding during a 30-year period of time, which is the duration of many home mortgages. And, the risk for flooding increases the closer a structure is to a bayou or a creek, assuming the structure is not elevated.

For more information about floodplains and FIRMs, visit FEMA’s website at www.fema.gov. For information about flooding and flood risks in Harris County, visit the Harris County Flood Control District’s Flood Education Mapping Tool at www.HarrisCountyFEMT.org.

1 percent (100-year) flood – A 1 percent (100-year) flood, also known as the Base Flood, is a flooding event that has a 1 percent (1 in 100) chance of occurring in any given year at any given location. Statistically, a 1 percent (100-year) flood has a 26 percent chance of occurring during a 30-year period of time, which is equal to the duration of many home mortgages. Contrary to what the term suggests, a "100-year flood" is not a flood that occurs only once every 100 years. A "100-year flood" can occur multiple times in a century.

1 percent (100-year) flood elevation – A 1 percent (100-year) flood elevation is the distance above sea level (or other datum) that the water level in a bayou, creek or other waterway reaches during a 1 percent (100-year) flood.

1 percent (100-year) floodplain - A 1 percent (100-year) floodplain, also known as a Special Flood Hazard Area on a Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM or floodplain map), is an area at risk for flooding from a bayou, creek or other waterway overflowing during a 1 percent (100-year) flood. Structures located in a 1 percent (100-year) floodplain have a minimum of a 1 percent chance of flooding in any given year. Statistically, structures located in a 1 percent (100-year) floodplain have a minimum of a 26 percent chance of flooding during a 30-year period of time, which is the duration of many home mortgages. And, the risk for flooding increases the closer a structure is to a bayou or a creek, assuming the structure is not elevated. A mapped 1 percent (100-year) floodplain is an area where land development is regulated by a city or a county.

For more information about floodplains and FIRMs, visit FEMA’s website at www.fema.gov. For information about flooding and flood risks in Harris County, visit the Harris County Flood Control District’s Flood Education Mapping Tool at www.HarrisCountyFEMT.org.

2 percent (50-year) flood – A 2 percent (50-year) flood is a flooding event that has a 2 percent (1 in 50) chance of occurring in any given year in any given location. Statistically, a 2 percent (50-year) flood has a 48 percent chance of occurring during a 30-year period of time, which is the duration of many home mortgages.

2 percent (50-year) flood elevation – A 2 percent (50-year) flood elevation is the distance above sea level (or other datum) that the water level in a bayou, creek or other waterway reaches during a 2 percent (50-year) flood.

2 percent (50-year) floodplain – A 2 percent (50-year) floodplain is an area at risk for flooding from a bayou, creek or other waterway overflowing during a 2 percent (50-year) flood. Structures located in a 2 percent (50-year) floodplain have a minimum of a 2 percent chance of flooding in any given year. Statistically, structures in a 2 percent (50-year) floodplain have a minimum of a 48 percent chance of flooding during a 30-year period of time, which is the duration of many home mortgages. And, the risk of flooding increases the closer a structure is to a bayou or a creek, assuming the structure is not elevated.

A 2 percent (50-year) floodplain lies within a mapped 1 percent (100-year) floodplain on the Federal Emergency Management Agency’s Flood Insurance Rate maps (FIRMs or floodplain maps). For more information about floodplains and FIRMs, visit FEMA’s website at www.fema.gov. For information about flooding and flood risks in Harris County, visit the Harris County Flood Control District’s Flood Education Mapping Tool at www.HarrisCountyFEMT.org.

10 percent (10-year) flood – A 10 percent (10-year) flood is a flooding event that has a 10 percent (10 in 100) chance of occurring in any given year in any given location. Statistically, a 10 percent (10-year) flood has a 95 percent chance of occurring during a 30-year period of time, which is the duration of many home mortgages.

10 percent (10-year) flood elevation – A 10 percent (10-year) flood elevation is the distance above sea level (or other datum) that the water level in a bayou, creek or other waterway reaches during a 10 percent (10-year) flood.

10 percent (10-year) floodplain – A 10 percent (10-year) floodplain is an area at risk for flooding from a bayou, creek or other waterway overflowing during a 10 percent (10-year) flood. Structures located in a 10 percent (10-year) floodplain have a minimum of a 10 percent chance of flooding in any given year. Statistically, structures in a 10 percent (10-year) floodplain have a minimum of a 95 percent chance of flooding during a 30-year period of time, which is the duration of many home mortgages. And, the risk of flooding increases the closer a structure is to a bayou or a creek, assuming the structure is not elevated.

A 10 percent (10-year) floodplain lies within a mapped 1 percent (100-year) floodplain on the Federal Emergency Management Agency’s Flood Insurance Rate maps (FIRMs or floodplain maps). For more information about floodplains and FIRMs, visit FEMA’s website at www.fema.gov. For information about flooding and flood risks in Harris County, visit the Harris County Flood Control District’s Flood Education Mapping Tool at www.HarrisCountyFEMT.org.

78 to 01 Adjustment – The 78 to 01 adjustment is a calculation that accounts for vertical changes in the Earth’s topography (typically as a result of land surface subsidence) from 1978 to 2001. The adjustment is used to compare elevations from a 1978 datum to elevations from a 2001 datum.

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A – “A” is the letter that the Harris County Flood Control District has designated to represent the Clear Creek watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “A100” refers to Clear Creek, the main stem in the Clear Creek watershed, and “A119” refers to Turkey Creek, a tributary of Clear Creek.

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B – “B” is the letter that the Harris County Flood Control District has designated to represent the Armand Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “B100” refers to Armand Bayou, the main stem in the Armand Bayou watershed, and “B112” refers to Willowspring Creek, a tributary of Armand Bayou.

Benchmark – A benchmark is a known elevation at a point on the ground typically marked by a brass disk embedded in a bridge or sidewalk. A benchmark is used to determine the elevation of the water levels in bayous and creeks as measured by the Flood Warning System equipment.

Bottom of Channel – The Bottom of Channel (BOC) is a measurement of elevation above sea level at a bayou’s lowest point and is generally measured from the downstream side of a bridge near a gage station site. To determine the depth of a bayou, subtract the Bottom of Channel elevation measurement from the Top of Bank elevation measurement.

Bubbler – A bubbler is one of four types of real-time, water-level measuring devices used at gage stations throughout Harris County. A bubbler system is submerged under water and uses a tube with an opening, called an orifice line, which runs from a transmitter at a gage station to the opening of the tube on the submerged part of the device to measure pressure and determine water levels. A fixed flow of air is passed through the tube, and the internal pressure of the air in the tube and the external pressure of the water outside the tube provide a water-level reading.

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C – “C” is the letter that the Harris County Flood Control District has designated to represent the Sims Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “C100” refers to Sims Bayou, the main stem in the Sims Bayou watershed, and “C106” refers to Berry Bayou, a tributary of Sims Bayou.

Channel – A channel is a waterway through which stormwater moves or is directed. It is a generic term used by the Harris County Flood Control District in reference to bayous, creeks, small tributaries and some ditches. A channel can vary in shape and size and can be either natural or man-made.

Cross Section – A cross section is a particular view of a bayou, creek or other waterway that appears cut in half for the purpose of showing its internal structure from top to bottom.

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D – “D” is the letter that the Harris County Flood Control District has designated to represent the Brays Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “D100” refers to Brays Bayou, the main stem in the Brays Bayou watershed, and “D109” refers to Harris Gully, a tributary of Brays Bayou.

Datum – A datum is a plane from which all ground surface elevations are referenced. All of the elevation data for Harris County is surveyed to the current datum, which is the North American Vertical Datum (NAVD) 1988, 2001 Adjustment.

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E – “E” is the letter that the Harris County Flood Control District has designated to represent the White Oak Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “E100” refers to White Oak Bayou, the main stem in the White Oak Bayou watershed, and “E121” refers to Vogel Creek, a tributary of White Oak Bayou.

Elevation – Elevation is the vertical distance of the ground above a datum, such as sea level. Elevations in Harris County range from 0 feet in the southeast near Galveston Bay to more than 300 feet in the northwest corner of the county near the City of Waller.

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F – “F” is the letter that the Harris County Flood Control District has designated to represent the Galveston Bay watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. Numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. Numbers within the 200-range represent diversion channels, which are constructed channels attached to existing channels that divert excess stormwater around an area to reduce flooding risks and damages. For example, “F101” refers to a tributary in the Galveston Bay watershed, and “F216” refers to Little Cedar Bayou, a diversion channel in the Galveston Bay watershed.

Flood Warning System – The Harris County Flood Control District’s Flood Warning System measures rainfall amounts and monitors water levels in bayous and creeks on a real-time basis to inform officials and the public of dangerous weather conditions. The system relies on 133 gage stations strategically placed throughout Harris County bayous and their tributaries. The stations contain sensors that transmit valuable data during times of heavy rainfall and during tropical storms and hurricanes. Some gages also measure air temperature, barometric pressure, humidity, road temperature and wind speed and direction.

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G – “G” is the letter that the Harris County Flood Control District has designated to represent the San Jacinto River watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “G100” refers to the Ship Channel, the main stem in the San Jacinto River watershed, and “G103” refers to the San Jacinto River, a tributary of the Ship Channel.

Gage – A gage is an instrument used to collect technical measurements. The Flood Warning System’s gages measure rainfall amounts and water levels in bayous and creeks. Some gages also measure air temperature, barometric pressure, humidity, road temperature and wind speed and direction. While “gauge” can serve as an alternate spelling, the Harris County Flood Control District uses the “gage” spelling to be consistent with that commonly used in technical reports and by the United States Geological Survey (USGS). The Harris County Flood Control District and the USGS house data collection devices in the same shelter and use the same water-level measuring devices at approximately 46 gage station sites throughout Harris County.

Gage Station Site – Gage station sites are strategic locations throughout Harris County where rainfall and water-level data are collected. Usually located on a bridge over a bayou, creek or other waterway, the stations contain sensors that transmit valuable data during times of heavy rainfall and during tropical storms and hurricanes. The Harris County Flood Control District’s Flood Warning System relies on 133 gage stations that contain sensors to measure rainfall amounts and water levels in bayous and creeks. Some gage stations also contain sensors to measure air temperature, barometric pressure, humidity, road temperature and wind speed and direction. However, the majority of gage stations in Harris County measure rainfall and water levels only. A gage station can use one of the following four real-time water-level measuring devices: bubbler, pressure transducer, radar or stilling well.

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H – “H” is the letter that the Harris County Flood Control District has designated to represent the Hunting Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “H100” refers to Hunting Bayou, the main stem in the Hunting Bayou watershed.

High Water Indicator – A high water indicator is a water-level measuring device constructed of a 0.75-to 1-inch pipe with holes in its bottom and a wooden stick and cork shavings inside. It usually is attached to the downstream side of a bridge. During high water conditions, water will enter the holes at the bottom of the pipe and cause the cork shavings inside to float. As the cork shavings rise, they attach to the wooden stick and leave a mark as the water recedes. After a flooding event, Harris County Flood Control District field crews open the pipe and measure the distance from the top of the wooden stick to the top of the mark left by the cork shavings. Based on this measurement and the surveyed top of the pipe, field crews can determine how high water rose at that gage station site during a particular rain event. High water indicators differ from other types of water-level measuring devices because they do not provide real-time water-level readings. The Flood Control District uses high water indicators to verify that electronic, real-time water-level measuring devices are providing accurate data.

Historical Storms – The Harris County Flood Control District considers a historical storm to be one that occurred in the past and produced exceptional flooding and structural damages. Flooding events are listed by date and include the name of the tropical system, if applicable, on the gage station site information page under the “Stream Elevation” tab.

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I – “I” is the letter that the Harris County Flood Control District has designated to represent the Vince Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “I100” refers to Vince Bayou, the main stem in the Vince Bayou watershed.

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J – “J” is the letter that the Harris County Flood Control District has designated to represent the Spring Creek watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “J100” refers to Spring Creek, the main stem in the Spring Creek watershed.

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K – “K” is the letter that the Harris County Flood Control District has designated to represent the Cypress Creek watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. Numbers within the 600-range represent levees. For example, “K100” refers to Cypress Creek, the main stem in the Cypress Creek watershed; “K166” refers to Mound Creek, a tributary of Cypress Creek; and “K600” refers to a levee at Cypress Creek near the Inverness Forest neighborhood.

Key Map – A Key Map is a detailed map, generally bound in book form, used by the general public, the Harris County Flood Control District and other government agencies for quick reference to specific locations. All Key maps share the same page numbers and lettered sections. References to Key Map locations on this website, such as “Key Map 451C” and “Key Map 368B,” are provided to help you locate a specific gage station site

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L – “L” is the letter that the Harris County Flood Control District has designated to represent the Little Cypress Creek watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “L100” refers to Little Cypress Creek, the main stem in the Little Cypress Creek watershed.

Land Surface Subsidence – Land surface subsidence is the lowering of the land surface elevation from changes that take place underground. In Harris County, land surface subsidence is typically caused by pumping water from aquifers made up of many layers of clay. As water is drawn from these aquifers, the layers of clay compact over time.

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M – “M” is the letter that the Harris County Flood Control District has designated to represent the Willow Creek watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “M100” refers to Willow Creek, the main stem in the Willow Creek watershed.

Measuring Plate – A measuring plate is an iron plate attached to the downstream side of a bridge, usually near the center of a bayou, creek or other waterway, that has been surveyed to determine its elevation. Harris County Flood Control District field crews measure the distance from the measuring plate to the top of the water and subtract that distance from the elevation of the measuring plate to determine the elevation of the water level in the bayou at that location. This measurement is compared to the reading from the water-level measuring device in that waterway to test the accuracy of the device’s sensor.

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N – “N” is the letter that the Harris County Flood Control District has designated to represent the Carpenters Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “N100” refers to Carpenters Bayou, the main stem in the Carpenters Bayou watershed.

North American Vertical Datum 1988, 2001 Adjustment – The North American Vertical Datum (NAVD) 1988, 2001 Adjustment is a plane from which points on the Earth’s surface are referenced.  In 2007, all of the Flood Warning System equipment was updated from the previous datum, the North Geodetic Vertical Datum 1929, 1978 Adjustment, to the current datum, the NAVD 1988, 2001 Adjustment.

North Geodetic Vertical Datum 1929, 1978 Adjustment – The North Geodetic Vertical Datum (NGVD) 1929, 1978 Adjustment is a plane from which points on the Earth’s surface are referenced. In 2007, all of the Flood Warning System equipment was updated from the previous datum, the NGVD 1929, 1978 Adjustment, to the current datum, the North American Vertical Datum 1988, 2001 Adjustment.

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O – “O” is the letter that the Harris County Flood Control District has designated to represent the Spring Gully and Goose Creek watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “O100” refers to Goose Creek, the main stem in the Spring Gully and Goose Creek watershed.

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P – “P” is the letter that the Harris County Flood Control District has designated to represent the Greens and Halls bayous watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “P100” refers to Greens Bayou, the main stem in the Greens Bayou watershed, and “P130” refers to Garners Bayou, a tributary of Greens Bayou.

Pressure Transducer – A pressure transducer is one of four types of real-time, water-level measuring devices used at gage stations throughout Harris County. A pressure transducer is submerged under water and uses a tube, called an orifice line, which runs from a transmitter at a gage station to the water-level measuring device. The water pressure above the transducer causes the pressure within the orifice line to change, and the internal pressure of the air in the orifice line provides a water-level reading.

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Q – “Q” is the letter that the Harris County Flood Control District has designated to represent the Cedar Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “Q100” refers to Cedar Bayou, the main stem in the Cedar Bayou watershed.

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R – “R” is the letter that the Harris County Flood Control District has designated to represent the Jackson Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “R100” refers to Jackson Bayou, the main stem in the Jackson Bayou watershed, and “R102” refers to Gum Gully, a tributary of Jackson Bayou.

Radar – A radar is one of four types of real-time water-level measuring devices used at gage stations throughout Harris County. A radar is typically installed on a swing arm on the side of or under a bridge and is placed above the water level of a bayou or creek. The radar transmits short wave pulses to the water’s surface and a separate receiver antenna receives the reflected waves. The radar uses the waves to measure the distance between the sensor at a gage station and the top of the water to determine the water levels in the bayou or creek.

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S – “S” is the letter that the Harris County Flood Control District has designated to represent the Luce Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. For example, “S100” refers to Luce Bayou, the main stem in the Luce Bayou watershed.

Sensor – A sensor is a data collection device that transmits data collected at gage station sites at certain intervals, either as weather conditions and water levels change or at pre-determined points in time.  When it begins to rain, data-collecting sensors at each gage station transmit rainfall amounts via radio frequency every time 0.04 inches of rain is measured by the sensor. Sensors that transmit bayou/stream levels report every 0.10-foot rise in water levels. The sensors transmit data to two primary repeaters located in the Huffman and Clodine areas. The repeaters then relay the data to primary and back-up base stations located at Houston TranStar and at the Harris County Appraisal District. The data is monitored daily by Harris County Flood Control District staff to ensure the gages are properly functioning and transmitting accurate data. Gage station sensors also can measure: air temperature, barometric pressure, humidity, rainfall, road temperature, water level and wind speed and direction.

Stream Elevation – Stream elevation is a measurement above sea level (or other datum) of the water’s surface in a bayou or creek.  For example, if the bottom of a waterway is surveyed at an elevation of 40 feet above sea level, and the depth of the water is 30 feet, the stream elevation would be 70 feet above sea level.

Stream Elevation Sensor Number – The stream elevation sensor number is an identification number used to differentiate gage station sites from each other. Each gage station site is assigned a number that refers to the entire gage station site and its equipment.

Stilling Well – A stilling well is one of four types of real-time, water-level measuring devices used at gage stations throughout Harris County. A stilling well is constructed of a corrugated, or ridged, pipe and mounted on a bridge. Four holes are located at the bottom of the pipe to allow water to enter as water levels in a bayou or creek rise. Inside the pipe, a float is attached to a wire that runs to a shaft encoder, which is a wheel that turns as the float rises. As the wheel of the shaft encoder turns, it records the height of the water.

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T – “T” is the letter that the Harris County Flood Control District has designated to represent the Barker Reservoir watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “T100” refers to Upper Buffalo Bayou, the main stem in the Barker Reservoir watershed, and “T101” refers to Mason Creek, a tributary of Upper Buffalo Bayou.

Tip of Orifice – The Tip of Orifice (TOO) is a measurement that refers to the distance between the lowest tip of a water-level measuring device and the bottom of a waterway. In many cases, the tip of a measuring device does not sit on the bottom of a waterway, and water must rise to reach the measuring device before it will record water-level readings. In some rural creeks, the water must be 4-to-6 feet deep before it reaches the water-level measuring device.

Tributary – A tributary is a channel through which water moves or is directed and ultimately flows into a larger channel, such as a bayou.

Top of Bank – The Top of Bank (TOB) is a measurement of elevation that refers to the height of the banks of a bayou or creek. Elevation measurements are taken every 50 feet upstream and downstream of a bridge for a distance of 200 feet. The lowest elevation is called the “Top of Bank,” as that would be the first location where water could overflow during a heavy rain event. To determine the depth of a particular waterway, subtract the Bottom of Channel measurement from the Top of Bank measurement.

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U – “U” is the letter that the Harris County Flood Control District has designated to represent the Addicks Reservoir watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “U100” refers to Langham Creek, the main stem in the Addicks Reservoir watershed, and “U102” refers to Bear Creek, a tributary of Langham Creek.

USGS– USGS is the abbreviation for the United States Geological Survey, a scientific federal agency. USGS scientists study the landscape of the United States, including its natural resources and natural hazards. The Harris County Flood Control District and the USGS house data collection devices in the same shelter and use the same water-level measuring devices at approximately 46 gage station sites throughout Harris County.

On the Flood Warning System website, gage station sites shared with the USGS include an external link to the USGS website and the USGS water-level reading for the same location. Note that the Flood Control District and USGS readings may be different because some USGS sensors base water-level readings on stream level, or depth, and not on stream elevation, or the water’s actual elevation above sea level. Some USGS sensors also may be based on a different datum than Flood Control District sensors.

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W – “W” is the letter that the Harris County Flood Control District has designated to represent the Buffalo Bayou watershed. When identifying bayous and tributaries, the Flood Control District uses a combination of letters and numbers to help easily identify the location of a waterway. Letters refer to a particular watershed. The number “100” represents the primary waterway in a watershed. Other numbers within the 100-range represent tributaries, which are often smaller off-shoots of bayous and creeks. For example, “W100” refers to Buffalo Bayou, the main stem in the Buffalo Bayou watershed, and “W140” refers to Spring Branch, a tributary of Buffalo Bayou.

Watershed – A watershed is a geographical region that drains to a common bayou, creek or other waterway. There are 22 major watersheds in Harris County.

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The Flood Warning System interactive map and website are for general information and educational purposes only. Their purpose is to provide information collected at gage stations, which monitor rainfall and water levels of streams and bayous as well as other weather information in the Harris County region. Use and interpretation of this data and information is the responsibility of the user. Neither the Harris County Flood Control District nor Harris County represents the data as 100 percent accurate. You should follow directions provided by emergency management officials when making decisions during flooding or other weather-related events.