How streamflow is measured and gaged and how floodplain maps are developed Raymond Slade, Jr., PH Certified Professional Hydrologist

What is a stream basin?

The U.S. Geological Survey currently collects streamflow data at over 7,000 sites San Francisquito Cr. nr. Stanford, CA Pacific Cr. nr Moran, WY The USGS currently operates over 7000 gaging stations.

Stage (gage height) can be sensed using PRESSURE TRANSDUCERS Pacific Cr. near Moran, WY Orifice Line Stage can also be sensed using pressure transducers. The pressure exerted by water at any point in the water column is a function of water depth. This pressure can be transmitted through an gas- or liquid-filled orifice line to a pressure transducer. The USGS usually uses orifice lines filled with nitrogen gas and pressure transducers that meet USGS specification to sense pressure. Such installations are used when stilling wells are difficult or impractical to install. Orifice Line

Most users of streamflow information need to know the discharge (ft3/s) of the stream Although stream stage is directly useful for some purposes, such as floodplain mapping, most users of streamflow information need to know the discharge of the stream. Discharge is the volume of flow passing a specific point in a given amount of time. Discharge Measurement Brian Loving, Salt Lake City

DISCHARGE IS USUALLY MEASURED USING THE VELOCITY-AREA METHOD (Area of water in cross section) x (Water velocity) = discharge square feet x feet per second = ft3/s Cross section area x Water Velocity The most practical method of measuring stream discharge is through the velocity-area method. Discharge is determined as the product of the area of the water and the water velocity..

Channel cross section is divided into numerous sub sections Measuring the average velocity of an entire cross section is impractical, so the USGS uses what’s called the mid-section velocity area method. Using this method the channel cross section is divided into a number of sub-sections. Most natural channels must be broken down into 25 or 30 sub-sections to adequately characterize their irregular geometry. The discharge of each sub-section is determined by measuring it’s area and water velocity. Discharge of each sub-section = Area x Average Water Velocity

Water velocity in each sub-section estimated using a current meter to measure water velocity at selected locations Average water velocity in each sub-section is estimated using current meters to measure water velocity at selected locations.. For shallow sections average velocity is estimated by measuring the velocity at 0.6 of the distance from the water surface to the streambed. When depths are large, the average velocity is best estimated by averaging velocities measured at 0.2 and 0.8 the distance from the water surface to the streambed.

Discharge measurements are used to develop rating curves A continuous record of discharge is determined by developing a relationship between stage, which is measured continuously, and discharge. This “rating” is developed using data from individual discharge measurements. Discharge Measurements

Discharge must be measured at all stages To properly develop ratings, discharge must be measured at all stages. Otherwise the relation between stage and discharge will be uncertain for some ranges in stage. USGS personnel visit gaging stations regularly, making discharge measurements when needed to define ratings.

Discharge measurements during high stages are particularly important Indirect measurement It is particularly important to make discharge measurements when stream stages are high. Such measurements define the upper ends of ratings. As such they form the basis for flood warning, flood forecasting and estimates of annual flow volumes. The means of making current-meter measurements are provided at most USGS gaging stations. Measurements during high stages are often made from bridges or cableways. Sometimes, it is impossible to make current-meter measurements during large floods. When direct current-meter measurements cannot be made, discharge is measured indirectly by surveying the high-water marks left by the flood and using hydraulic formulas to calculate discharge associated with the peak stage

Gage height data transmitted to USGS--discharge data added Then transmitted to public via Internet GOES DOMSAT Hydrologic Stations ( Internet ) Wallops, Virginia Command and Data Acquisition Center User Operations USGS streamflow data are generally telemetered using the GOES satellite system, as displayed here. Data from some sites are telemetered using FM radios or cellular phones Stage

Data dissemination-- Real Time (and historic data) Graphs and data Tables http://waterdata.usgs.gov Verde River near Scottsdale, AZ Real time information on stream stage and discharge are available as electronic data table and graphs from the world-wide web.

Example of gage height hydrograph

Example of discharge hydrograph

Uses for streamflow data Water resource appraisal and allocations - how much water is available and how is it being allocated? - For water supply plans As part of interstate agreements, compacts, and court decrees Engineering design - Reservoirs - Bridges, roads, culverts - Treatment plants Operations - Power production - Navigation

Uses for streamflow data (cont.) Identifying changes in streamflows due to changes in - Land use - Water use - Climate Flood planning and warning - Floodplain mapping - Flood forecasts Streamflow forecasting - water management Support of water quality sampling - Water quality conditions and trends - Contaminant transport Characterizing and evaluating instream conditions - Habitat assessments - Instream flow requirements - Recreation

How floodplains are mapped Hydrology and statistics Annual peak discharges for about 1,100 gaging stations exist in database for Texas. About 36,000 years of data.

Based on database of annual highest peak discharges, calculate annual exceedence probability for peaks at each gaging station

Annual peak discharges plotted on probability paper Exceedence probability in percent 50 10 4 2 1 Inverse of exceedence probability

Statistical characteristics—defining frequency distribution Use of L-moments: a sequence of statistics used to summarize the shape of a probability distribution--based on the mean, standard deviation, skewness and kurtosis of a database. Mean: Central tendency Standard deviation: Measure of variation Skew and kurtosis: Shape of distribution

2. Hydraulics (convert 1 % peak discharge to water surface elevations) The elevation of the water surface for a 100-year discharge at a point in a stream is a function of: Cross sectional area Cross sectional shape Channel roughness Channel slope Channel alignment (bends) Restrictions in channel (bridges, levees, etc.) The channel and floodplain are both integral parts of the natural conveyance of a stream. The floodplain carries flow in excess of the channel capacity. The greater the discharge, the greater the extent of inundation. …. Because of its devastating nature, flooding poses serious hazards to human populations in many parts of the world. “The Flood Disaster Protection Act of 1973” required the identification of all floodplain areas in the United States and the establishments of flood-risk zones within those areas.

FEMA mapped floodplain From Google Earth using FEMA NFHL kmz file KMZ is a file extension for a placemark file used by Google Earth. KMZ stands for Keyhole Markup language Zipped.

Has global warming increased the magnitude and frequency of floods? www.ncdc.noaa.gov

American Meteorological Society “The scientific literature and the Intergovernmental Panel on Climate Change (IPCC) have independently suggested that the intensity and frequency of extreme weather events will increase due to climate change.” American Meteorological Society Therefore, climate change may increase the frequency and severity of catastrophic floods, and the intensity and duration of severe droughts.

Early flood warning systems

Same site 5 minutes later… Example of flash flood Pedernales River at Pedernales Falls State Park Same site 5 minutes later…

http://water.weather.gov/afws/

Early Texas flood forecasting Ft Griffin—Roughest town in the west. Wyatt Earp, Doc Holiday, Pat Garrett, John Wesley Hardin July 1876 flood Clear Fork Brazos River Tonkawa Encampment Ft Griffin

Doppler radar

Doppler data Interpretations of radar return signals and algorithms converts signals to…

Doppler interpretations

National Weather Service Early Warning Flood System 1-kilometer, 5-minute estimated rainfall data (NEXRAD) input into rainfall-runoff models of streamflow basins to estimate timing and discharge for flood peak USGS real-time streamflow data used to calibrate and improve model during flood Warnings sent out by radio frequency to specific areas