Sampling Interval for Flow Hydrograph

The sampling interval or period is the time interval between successive samples and applies to all hydrometric data collection, including Acoustic Doppler Velocity Meters (ADVM).? Acoustic Doppler Velocity Meters are in-situ acoustic Doppler instruments, grouped under either up-looking (bed mounted) or side looking (bank mounted) instruments.

Before we further discuss sampling interval requirements, it’s important that we define the different timing parameters that ADVM’s instruments utilizes during the measurement process.? The timing parameters can be grouped under, Ping Rate, Averaging Period, and Sampling Interval.

• Ping Rate: ?Frequency at which water velocity is sampled, with a higher ping rate reducing the variance in measurement accuracy.? The decreased variance means a shorter averaging period can be used to achieve certain instrument uncertainty.? It’s important to understand that reduced variance in measurement accuracy only applies to the instrument generated uncertainty.? Modern ADVM instruments make use of either a fixed or auto adaptive ping rate, which is not user configurable.

The ping rate does not account for real world variations in water velocity at a monitoring site and an averaging period long enough should be selected to average variations.        

• Averaging Period:? Averaging period is the time the instrument averages data for each sample reported.? Long enough averaging period should be selected to average small and large scale (total water depth) turbulence and large natural fluctuations. ?In most flow conditions natural variation is more critical than the instrument uncertainty when selecting averaging period.?

In most applications the maximum averaging period is selected within the constraints of sampling interval (equal or shorter), power supply (longer period, higher power consumption) and data output type (allow time for data transmission, SDI12).? Longer averaging period does improve the measurement uncertainty.        

• Sampling Interval: ?Sampling period is the time interval a sample is reported or transmitted via SDI12, etc. ?The sampling period should be based on the response of the stream and or slope of hydrograph (rising limb) to accurately define the flow hydrograph.

The selection of appropriate sampling interval for a flow monitoring site can be achieved by one or combination of the following approaches,

• Catchment Category: ?Fixed time interval (Urban 2-5min, Mountainous 5-10min, Bottom / Flat Slope 15min, Flood Plain 15 min).
• Event based Logging:? Sampling triggered with change in water level (Urban 5mm, Mountainous 10mm, Bottom / Flat Slope 15mm, Flood Plain 15mm)
• Site Calibration:? Data collected at one-minute interval over at least 2-3 consecutive flow hydrographs.? The one-minute raw data are re-sampled at higher interval (e.g.5, 10 or 15 minutes).? Volume calculations are performed based on the one-minute raw data and re-sampled data.? The difference in volume calculations between the two data sets should be less than 5% over the entire period selected.

It’s important to understand that the Averaging Period and Sampling Interval selection can have impact on the overall measurement uncertainty.? Careful consideration should be given in the selection of appropriate timing parameters for a flow monitoring site as these are dependent on the site and hydraulic conditions.