To obtain necessary flow measurements along with storm water samples, two devices are required: one for flow metering and one for flow metering with an interconnection to insure synoptic collection of sample and flow data. Common characteristics of adequate devices are summarized below:
• Sample transport velocity of 3.0 fps or more to prevent sedimentation
• Minimum of 24 discrete sample bottles or ability to composite samples in one container
• Constant sample size over different sampling lines for rising and falling streams
• Air purging of sampling intake line before and after sample collection
• No solids deposition in sample train
• Chemically inert surfaces in contact with sample
In general, the intake should point upstream, extended slightly upstream from any obstacles in the flow, and should not excessively obstruct flow to avoid clogging or damage. Locations are recommended along the pipe periphery at about one third of the average water depth above the bottom. The intake should be placed at a cross-section where the flow is highly turbulent and well mixed. At such locations, a single intake, instead of multiple intakes, may be acceptable.
Sample withdrawal is accomplished by a pump controlled by timers or flow meters. The best devices for urban pollution studies fall into the following categories of pumping methods: positive displacement, peristaltic, and
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