Measuring Flow

A major aspect in this analysis was measuring the flow rate of the inflows and outflows to the bioretention basin. This was necessary to determine the contaminant loadings and allowed for comparisons of the amount of pollutants at different flows. Multiple methods of measuring flow were utilized, as described in the following subsections. Section 4.1 describes the effectiveness of each method.

3.5.3.1 Bucket Method

One method is to time the filling of a bucket of a known volume. This will give an accurate measurement of the volume per unit time, as seen in Equation 3.

𝑄 = 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑏𝑢𝑐𝑘𝑒𝑡 𝑇𝑖𝑚𝑒 𝑡𝑜 𝑓𝑖𝑙𝑙 𝑐𝑜𝑚𝑝𝑙𝑒𝑡𝑒𝑙𝑦 = 𝑔𝑎𝑙𝑙𝑜𝑛 𝑠𝑒𝑐𝑜𝑛𝑑𝑠 ∗ 0.13368 𝑓𝑡3 𝑔𝑎𝑙𝑙𝑜𝑛 = 𝑐𝑓𝑠 Equation 3: Bucket Method Flow Rate Calculation

One challenge in pursuing this method is fitting a bucket under the inflow and outflow pipes. The inflow of Gate 27 basin has a wide apron sitting immediately under the 24” inlet pipe. This apron spreads the water out and after about 3 feet drops it into a small pooling area in the forebay. The discharge from the Gate 27 basin has many rocks which needed to be moved to place a bucket under the 4” pipe. At the River Street basin, the 36” inflow pipe has many rocks that needed to be moved as well. The 6” outlet pipe of this basin only has one rock

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that caused an issue in catching the water. In both basins’ inflows, a sheet of plastic and/or aluminum assisted in funneling the water into the collection bucket.

3.5.3.2 Weir

Another method which can fairly accurately describe the volumetric flow is with the use of a weir. The main function of a weir is to increase the water level, or the water head. This

obstruction is well documented in open channel hydraulics and equations have been derived to calculate volumetric flow based on the height or head of the upstream water (Engineering

ToolBox).Flow can be described by this equation for v-notch weirs: 𝑄 =_{15 𝐶}8 𝑑(2𝑔) 1 2tan𝜃 2 ℎ 5 2

Equation 4: Flow Over a V-Notch Weir Where:

Cd = Empirically derived discharge constant g = Acceleration due to gravity

θ = V-notch angle

h = Water head above the v-notch

A weir can be purchased or fabricated. A commercial one, though expensive, would prove to be the easiest, though a variety of sufficient ones could be made. One potential design was to create a box with a notched out weir on one side. The water would flow into the box and exit through the weir. This would create a controlled weir that could be placed below the inflow or outflow pipe. This method was utilized for the outflow at the Gate 27 basin. The height of the water immediately upstream of the weir would need to be measured to appropriately calculate the flow rate. This measurement was accomplished by hand with a tape measure when a spot measurement of the flow was required. The fabricated weir that was placed on the outflow of Gate 27 had a 47.56° angle and 8.75 cm height to the notch from the bottom. Details of the Gate 27 weir are located in Appendix F: Weir.

3.5.3.3 Manning’s Equation

A rough approximation of flow rate can also be made through the use of Manning’s

equation. Manning’s equation is used to find the velocity of a stream, and that coupled with the rough cross sectional area of the channel that the water flows through allows for the

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𝑣 =𝑘_{𝑛 𝑅}2/3_𝑆1₂

Equation 5: Manning’s Equation Where:

v = Cross sectional average velocity k = Conversion factor

n = Manning coefficient R = Hydraulic radius

S = Slope of the water surface

By using this method, a number of educated assumptions were made, which largely

affected the hydraulic radius. With such large-diameter inflow pipes and relatively small flow of water, it was difficult to calculate the cross sectional area and wetted perimeter necessary for Manning’s equation. Multiple iterations were required following a rough approximation and along with measuring the slope, a velocity of the water was calculated. To find flow (Q), the velocity is multiplied by the cross sectional area as in this equation:

𝑄 = 𝐴 𝑣

Equation 6: Flow Through a Channel

This is a good approximation for calculating the volumetric flow in a pipe, and served as a comparative value for other methods of measurement.

3.5.3.4 Depth Probe

A depth probe was used at both inflows and outflows for different storms. The probe that was used was the In-Situ Level TROLL 500 Instrument. It is a vented, or gauged, unit. These have a “vent tube in the cable [that] applies atmospheric pressure to the back of the strain gauge. The basic unit for vented measurements is PSIG (pounds per square inch ‘gauge’), measured with respect to atmospheric pressure. Vented sensors thus exclude the atmospheric or barometric pressure component” (In-Situ Inc., 2010). The following tables list the effective ranges and efficiencies of the probe.

Table 9 : Level TROLL® 500 Depth Probe Efficiencies (In-Situ Inc., 2010)

Depth Probe Component Temperature Range -20-80o_C Pressure/Level Sensor at: 15oC 0-50oC -20-0, 50-80oC Full scale ± 0.05% ± 0.1% ± 0.25%

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Table 10: Range and Usable Depth (In-Situ Inc., 2010)

Range Usable Depth

PSIG kPa Meters Feet

5 34.5 0-3.5 0-11.5 15 103.4 0-11 0-35 30 206.8 0-21 0-69 100 689.5 0-70 0-231 300 2068 0-210 0-692 500 3447 0-351 0-1153

Once these data were tabulated, Manning’s equation was used to calculate the flow in the pipe. The variables in the equation were modified to apply to flow through a pipe; the wetted perimeter is the arc length of the water, etc.

3.5.3.5 ISCO Meter

ISCO meters are area velocity meters that were provided by the DCR. The meters accurately display the flow in cubic feet per second for the individual pipe programmed into the unit. As stated in the ISCO manual, the area velocity meter requires three measurements: water level, water velocity, and channel dimensions. The internal sensor records the level and velocity by a differential pressure transducer that measures pressures transferred by a stainless steel diaphragm exposed to the streams flow. The recorded velocity is multiplied by the area, which is calculated from a combination of the water level and the pipe’s characteristics. The water level is determined from the pressure on the sensor and the pipe’s characteristics are programmed into the unit beforehand.