8.10 Collection System Flow Monitoring, Area-Velocity Metering, Weirs and Flumes & Capacity Assessment

Measuring flow in a pipe that is not full

WPI's Collection System Monitoring, Evaluation, and Adjustment content area is 14 percent of the Class I exam and opens with flow monitoring, and it separately lists monitor flow sensors and calibrate and adjust atmosphere testers. Collection flow monitoring is harder than pressure-pipe metering because a gravity sewer is a partially full open channel: to know the flow you must know both the depth and the velocity.

Area-velocity metering

The governing relation is continuity: Q = A x V.

• Area is computed from the measured depth and the known pipe geometry.
• Velocity is measured directly, most commonly by Doppler ultrasonic, which reflects sound off particles and bubbles in the flow and reads the frequency shift.
• Depth is measured by submerged pressure transducer, downward-looking ultrasonic, or radar.

Area-velocity meters work in surcharged and backwater conditions where a depth-only device would be badly wrong, which is why they dominate sewer system evaluation surveys.

Worked example. A 15-inch sewer is flowing at a depth of 6.0 inches with a measured velocity of 2.4 ft/s. From a circular-pipe table, a d/D ratio of 6/15 = 0.40 corresponds to a flow area of about 0.374 of the full-pipe area.

• Full pipe area = 0.785 x (15/12)² = 0.785 x 1.5625 = 1.227 sq ft
• Flow area = 0.374 x 1.227 = 0.459 sq ft
• Q = 0.459 sq ft x 2.4 ft/s = 1.10 cfs
• In gpm: 1.10 x 448.8 = 494 gpm, or about 0.71 MGD

Primary devices: weirs and flumes

Where a permanent, accurate measurement is needed, a primary device creates a known depth-to-flow relationship, and only depth has to be measured.

Two rules govern weirs and flumes and both are commonly tested. First, the device must discharge freely; if downstream water backs up and submerges the device, the depth-to-flow relationship is invalid and the reading is meaningless. Second, head is measured upstream of the crest or throat at the specified distance, not at the crest itself. For weirs, solids deposit in the pool behind the weir plate and change the approach conditions, so the pool is cleaned on a schedule — which is precisely why flumes are preferred in wastewater.

Capacity assessment

WPI lists performing adjustments on flow monitoring, force mains, gravity sewers, lift stations, manholes and cleanouts, and measuring and control systems. Behind those adjustments is a capacity question: is the pipe big enough for the flow it now receives?

Gravity capacity comes from Manning's equation:

Q = (1.486 / n) x A x R^(2/3) x S^(1/2)

where n is the roughness coefficient (about 0.013 for most sewer materials), A is flow area, R is hydraulic radius (area divided by wetted perimeter), and S is the slope in feet per foot. Two consequences an operator should carry:

• Capacity depends on the square root of slope. Doubling slope increases capacity only about 41 percent.
• A circular pipe carries its maximum flow at about 93 percent of full depth, not at exactly full, because the wetted perimeter grows faster than the area in the top of the pipe.

Design allowance for peak flow is what determines whether a system surcharges. A pipe sized for peak dry-weather flow will surcharge every time it rains if I&I is significant, which is why flow monitoring and capacity assessment are done together.

Interpreting flow data

• Diurnal pattern. Normal dry-weather flow shows the familiar morning and evening peaks. A flat, featureless hydrograph in a residential basin means the meter is reading something other than sewage — often groundwater infiltration dominating the signal.
• Rainfall response. Overlay flow on rainfall. A sharp, fast peak that recedes quickly is inflow — direct connections. A broad, delayed rise that persists for days is infiltration — groundwater entering through defects.
• Base flow shift. A seasonal rise in the dry-weather minimum tracks the groundwater table and quantifies infiltration.
• Surcharge. Depth exceeding the pipe crown means the reach is full and backing up; sustained surcharge is the direct precursor to an overflow or a basement backup.

Meter siting and maintenance

• Site the meter in a straight reach, away from drops, junctions, and bends that create turbulence and non-uniform velocity profiles.
• Avoid locations that silt in; sediment over the sensor destroys both depth and velocity readings.
• Confirm the pipe dimension and shape entered into the meter. An incorrect diameter propagates directly into every flow value.
• Field verify with an independent method — a bucket-and-stopwatch at a drop, a dye-dilution measurement, or a manual depth reading compared with the logged value.
• Clean the sensor on every visit; grease films change ultrasonic response.
• Calibrate atmosphere testers used during meter site entries, which WPI lists as its own task, and treat every manhole as a permit-required confined space.