Flow measurements provide needed data for estimating runoff and streamflow volumes and flow rates from a particular precipitation event or over an extended period of time. Methods exist that directly measure volumes over time or indirectly measure surrogate variables. These surrogates are then used to estimate flow rates. Streamflow is a generalterm used to represent volumes or rales of flow in a river, creek, stream, or channel. Discharge is used primarily for rate measurements and applied to Waters that flow from a specific watershed or pond area to a receiving water body. In this chapter, the more common methods for estimating flow rates are presented. Examples of their use in measuring streamflow in rivers and discharge fromstormwater control facilities (primarily from ponds) are presented. The actual sizing of reservoirs and stormwater ponds are dependent on discharge measurements.
This chapter also presents information on water quality measurement (including sediment movement), sampling techniques and development of rating curves (i.e., streamflow or discharge, versus elevation or stage; pollutant concentrations versus' streamflow or discharge; and stage versus other parameters such as velocity', drainage area, and pollutant loads).
METHODS OF MEASUREMENT
The accuracy with which one can estimate flow rales over time, (a hydrograph) for a stream location depends on the method used for estimation. This assumes an appropriate application of the procedures for measuring and interpreting the data. If theproper procedures are followed, the.accuracy of the estimates of discharge will be limited only by the range of the instrument and human error.
Some methods for discharge estimation are classified as shown in Table 7.1.
Stage is the water surface elevation recorded relative to some horizontal datum elevation, frequently mean sea level. Stage is a reflection of all the hydrologicprocesses and water transport characteristics of the watershed. Historical records of high water levels exist, and are either man-made or indirectly identified using the presence of water-related vegetation or slain marks on trees and slructures.
TABLE 7.1 Clcmificalion of Flow Measurement Melliodi
A. Stage (water surLacc elevation) ^
1. Visual observation
3. Pressurejeiisor ~* "3£fSal ^ f^CS^C'
4. Bectrical resistance
I). Discharge (nonstructural)
1. Current meter
4. Indirect via Manning's equation C. Discbarge (structural)
1. Dirvc't volume collcctioa
Stage records are valuable for the definition of high and low waler levels.Areal extent of Hooding, history of the rate of fluctuation, and watershed hydrologic characteristics can be documented. The record of stage is called the stage hydrograph, primarily because il can be translated into (low rate (discharge) units.
A stage recorder can be as simple as a ruler along a bridge or other structure. It can be read periodically but is usually automatically recorded. Theautomation is achieved using a water float, pressure sensor, or change in electrical resistance caused by water contact. The float and sensors are attached by wire or chain to a chart that ■ records all watei level changes. Typical guide pulley assemblies for a recording device with a counterweight are illustrated in Figure 7.1. The float and the counterweight should not encounter interference. Allsystems must be maintained and calibrated to ensure accurate results. Since rapid flow rates may move or destroy the float and sensors, protection from potentially damaging forces must be provided. In addition, waves or short-period water surface disturbances caused by boating or man-made events are not generally desired and certainly not reflective of nat.ural conditions. Thus, for stage-level...