Hydrologic Monitoring

Equipment and sampling needs vary among monitoring levels. At the lowest level, hydrology is usually the only aspect monitored. With an increase in maintenance and use, there is an increase in monitoring equipment and technology.

Infiltration rate is the change in stored water elevation with respect to time after a precipitation event. The rate is calculated as the slope of the receding limb of the water depth verse time (Ermilio 2005). Monitoring equipment includes:

  • Staff gage- a long rod with etchings used to denote depths Manually read at user defined intervals.
     When: Periodically throughout a storm event and continued until elevation equivalent to zero.
     Where: Placed at a location where water is presumed to pond.

  • Ultra-sonic level- a sensor housed in a water tight aluminum enclosure placed above a location of storage or depression. The sensor uses a sonar technology of bouncing echoes off the lowest point to determine water elevation. Depths are logged by an electronic signal sent to a data logger/flow meter and collected from data logger/flow meter at user defined intervals.
     When: Digitally recorded at user defined intervals.
     Where: Placed at a location where water is presumed to pond.

  • Pressure transducer- a sensor housed in a water tight aluminum enclosure which measures the change in pressure heads to determine the change it water elevation. Data is collected from a data logger/flow meter at user defined intervals.
     When:
    Digitally recorded at user defined intervals.
     Where: Placed at a location where water depth is presumed to pond. Most commonly placed in inlet, outlet or infiltration bed.

  • Area/Velocity bubbler- a pipe to measure the depth of water and Doppler Effect (the shift in frequency as it moves away with the flow) to determine velocity. Data is collected from a data logger/flow meter at user defined intervals.
     When: Digitally recorded at user defined intervals
     Where: Placed at location of continuous flow. Most commonly at an inlet or outlet.

Infiltration rates vary on a site by site basis due to soil properties. They are also affected by seasonal/temperature effects peaking in the summer and receding to their lowest values in the winter.

Overflow is the volume of water that exceeds the SCMs capacity and empties into a downstream water source. During storms, SCMs act to reduce peak flow by retaining the water in volume storage. When storage volume is exceeded, outflow can be measured by:

  • Area/Velocity Bubbler-a pipe to measure the depth of water and Doppler Effect (the shift in frequency as it moves away with the flow) to determine velocity. Data is collected from a data logger/flow meter at user defined intervals.
     When: Digitally recorded at user defined intervals
     Where: Placed at location of continuous flow. Most commonly at an inlet or outlet.

  • Pressure transducer with weir/orifice-

     - A pressure transducer is a sensor housed in a water tight aluminum enclosure which measures the change in pressure heads to determine the change it water elevation. Data is collected from a data logger/flow meter at user defined intervals.
       When:
    Digitally recorded at user defined intervals.
       Where: Placed at a location where water depth is presumed to pond. Most commonly placed in inlet, outlet or infiltration bed.
     
     - A weir/orifice is a structure used to measure the flow of water. Each structure has its own equation and in conjunction with a pressure transducer or area/velocity bubbler, the flow of water can be determined.
       When: Used in conjunction with pressure transducer when flow or volume calculations are required.
       Where:
    Commonly located at an inlet or outlet of a SCM.

Flow is important in determining the volume escaping and the volume being detained within the SCM. It must also be noted that some SCMs have a continuous baseflow, so therefore there is a minimum constant overflow monitored. There must also be an inflowing water source which can be monitored in a similar manner.

Moisture content determines the saturated conditions of the soil by measuring the volumetric water content. By measuring the moisture front as it passes, the infiltration rate can be determined. Automated equipment used for monitoring is:

  • Reflectometers-measure the passing moisture front as the soil moisture content changes due to infiltrating runoff. The Reflectometers measure volumetric water content which is transmitted via a data logger/ flow meter. Data is collected from a data logger/flow meter at user defined intervals.
     When:
    Digitally recorded at user defined intervals
     Where: Placed at various depths beneath infiltrating media. 

Storage volume is the amount of water that can be captured and retained by the SCM. Storage volume for a given storm is represented by a contour map of the site or elevation-storage tables. If the storage space is filled with stone or gravel, the storage volume is represented by the void space. Equipment used to determine volume is:

  • Staff gage- a long rod with etchings used to denote depths. Manually read at user defined intervals.
     When: Periodically throughout a storm event and continued until elevation equivalent to zero.
     Where: Placed at a location where water is presumed to pond.

  • Ultra-sonic level- a sensor housed in a water tight aluminum enclosure placed above a location of storage or depression. The sensor uses a sonar technology of bouncing echoes off the lowest point to determine water elevation. Depths are logged by an electronic signal sent to a data logger/flow meter and collected from data logger/flow meter at user defined intervals.
     When: Digitally recorded at user defined intervals.
     Where: Placed at a location where water is presumed to pond.

  • Pressure transducer- a sensor housed in a water tight aluminum enclosure which measures the change in pressure heads to determine the change it water elevation. Data is collected from a data logger/flow meter at user defined intervals.
     When:
    Digitally recorded at user defined intervals.
     Where: Placed at a location where water depth is presumed to pond. Most commonly placed in inlet, outlet or infiltration bed.

  • Area/Velocity bubbler- a pipe to measure the depth of water and Doppler Effect (the shift in frequency as it moves away with the flow) to determine velocity. Data is collected from a data logger/flow meter at user defined intervals.
     When: Digitally recorded at user defined intervals
     Where: Placed at location of continuous flow. Most commonly at an inlet or outlet.

When maximum volume is reached, there is overflow from the site.

Total precipitation is the amount of rain that falls during a storm event.  Precipitation data is helpful in determining the volume of water entering the SCM. Rainfall is monitored by:

  • Analog raingage- graduated cylinder with markings delineating values of length. Manually read at user defined intervals. Emptied upon full capacity or completion of a storm event.
     When: Periodically throughout a storm event.
     Where: Placed on site, void of falling debris and other obstacles blocking contact with direct precipitation.

  • Tipping bucket raingage- cylindrical unit which must be placed on level. The cylinder collects and funnels precipitation towards a lever which tips for every 0.01 inch of rain. The tips are logged through an electric signal that is sent to a data logger/flow meter. Data collected from datalogger/flowmeter at user defined intervals.
     When: Digitally recorded at user defined interval.
     Where: Placed on site, void of falling debris and other obstacles blocking contact with direct precipitation.

If equipment is not present, one can download precipitation data for a storm event from a nearby raingage using NOAA-ATLAS 14 (National Weather Service 2007).

Water elevation is the depth of ponded water at a given location whether in a location of storage or stream channel. Water elevation is important to define the volume of water retained by the SCM and for infiltration calculations. Equipment used to monitor water surface elevation may include:

  • Staff gage- a long rod with etchings used to denote depths Manually read at user defined intervals.
     When: Periodically throughout a storm event and continued until elevation equivalent to zero.
     Where: Placed at a location where water is presumed to pond.

  • Ultra-sonic level- a sensor housed in a water tight aluminum enclosure placed above a location of storage or depression. The sensor uses a sonar technology of bouncing echoes off the lowest point to determine water elevation. Depths are logged by an electronic signal sent to a data logger/flow meter and collected from data logger/flow meter at user defined intervals.
     When: Digitally recorded at user defined intervals.
     Where: Placed at a location where water is presumed to pond.

  • Pressure transducer- a sensor housed in a water tight aluminum enclosure which measures the change in pressure heads to determine the change it water elevation. Data is collected from a data logger/flow meter at user defined intervals.
     When:
    Digitally recorded at user defined intervals.
     Where: Placed at a location where water depth is presumed to pond. Most commonly placed in inlet, outlet or infiltration bed.

  • Area/Velocity bubbler- a pipe to measure the depth of water and Doppler Effect (the shift in frequency as it moves away with the flow) to determine velocity. Data is collected from a data logger/flow meter at user defined intervals.
     When: Digitally recorded at user defined intervals
     Where: Placed at location of continuous flow. Most commonly at an inlet or outlet.

In conjunction with each other, the staff gage can be used as a calibrator for the level sensor, pressure transducer or area/velocity bubbler.