The process of sampling both the DD and X4 with auto samplers has been outlined in section 3.10.1. There were a number of factors which complicated this monitoring process (particularly in respect of the DD), and the subsequent analysis of the collected data. Normally surface water drains (SWD) only contain discharge during rainfall, meaning they are empty during dry weather, making it easy to differentiate periods of discharge induced by storms. However between the original investigation and the construction of the DD, it was found that the selected SWD had a constant base discharge passing through it. This fluctuated between approximately 5 to 8 l/s, and is thought to be caused either by a small brook which has been diverted into the drain, or through a water main leak. This flow was observed to be clear, with little or no TSS material visible in samples taken, indicating that the latter of these explanations was more plausible. This meant that differentiating between base flow flux, and the start of storm events was sometimes difficult, as the hydrograph was altered by the addition of this base flow.
It also meant triggering the sampling equipment during storm events was more difficult. For example, a flow sensor was used to trigger the sampling programme, by setting a threshold value in respect to either level or velocity. Due to the fluctuation in the base flow, this needed to be set above the typical highest values it produced (with respect to level or velocity); otherwise the samplers would be triggered by the base flow and not as the result of a storm event. As a consequence of this, the beginning of some events were missed, because, if the base flow was low the samplers would not commence collecting samples, until the storm had raised discharge enough to pass the trigger level.
The frequency at which samples were collected was generally set at 5 minute intervals for TSS and 10 minutes intervals for metals. In one event a shorter sample interval (2 minute)
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was used to determine if there was a significant change in sample concentration, when they were taken at shorter frequencies. However there was no appreciable additional fluctuation in pollutant concentration during this shorter interval, so for all following events, 5 minute intervals were used. Thus samples were considered to be representative of the 5 minute period, during which they were collected.
An important part of analysis has been to accurately compare the volume of pollutants entering the system against that exiting it, but due to their significant cost it was not possible to purchase two separate flow sensors to allow the monitoring of discharge both upstream and downstream of the DD unit. The significant internal volume of the DD (approximately 15 m3) caused a ‘lag’ effect on the discharge as it passed through the system. For example, when discharge increased as a result of a storm event, the change passes through the upstream sample chamber, being recorded by the flow sensor; it would then take a period of time for it to pass through the connecting pipe into and through the DD chamber until it was observed in the downstream sample chamber.
Figure 31 – Demonstration of lag time on sample concentration
As the samplers were set to take simultaneous samples, it was necessary due to lag for the downstream sample results to be offset in terms of the time they were collected, when comparing them with the log of the discharge, collected in the upstream sample chamber. The lag time also varied as discharge fluctuated, being larger at low discharges and smaller at
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high discharges. If two flow meters had been available, the calculation of the volume of pollutants upstream and downstream would have been completed using a log of discharge from each manhole. However as only one flow meter was available, the effect of the lag time between sample chambers has to be considered. This effect is summarised in Figure 31.
Initially it was proposed to take flow weighted composite samples, however due to the problems encountered with triggering the flow meter discussed above this was not possible. As only one flow meter was available the only method available to trigger the downstream sampler was to mirror the upstream sample program to the downstream sampler. If flow weighted sampling has been utilised this would have resulted in the downstream sampler forming composite samples based on flow data at a different location as the time lag effect of transition through the DD chamber meant that the downstream flow rate was different to the upstream. While the method adopted may be considered less representative, with the potential for greater error, it was considered that time weighted sampling was more suitable to deal with the constraint posed by the absence of a downstream flow meter. As the number of samples collected for each event was at least 12 the relatively high density of samples can be considered to reduce the level of error significantly.
The manufacturer (Hydro International) has monitored a similar device under laboratory conditions to ascertain the lag time of discharge passing through the DD chamber. This work determined that the lag time of water between the upstream and downstream of a DD system was equal to 0.61% of the mean residence time2 of water within the system. Lag was then, defined as:- Lag Time (2) 𝐿𝑇 = 𝑁𝑀𝑅𝑇 𝑋 𝑣 𝑄 Where: LT = lag time
NMRT = Normalised Mean Residence Time v = Volume
Q = Discharge
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Application of Equation 2, allowed the water samples collected downstream to be offset backwards based on the discharge at the time a sample was taken and compared against the actual representative discharge from flow sensor located in the upstream sample chamber. This allowed a more accurate comparison with the equivalent upstream discharge.
Level and velocity were logged at 2 minute intervals by the flow sensor, but it was often found during low flow periods that the sensor struggled to detect velocity accurately and this is partly why water level was selected as a more accurate trigger mechanism. This also meant that for some intervals, the sensor recorded a value of zero for the rate of velocity and so, to allow the calculation of discharge in these periods, a second equation has been used to provide missing velocityreadings.
To calculate this equation, velocity readings were taken from the whole of the monitoring period. Readings for each increase in level were then divided and sorted, with missing velocity values removed, and the average of the remaining values for each level interval plotted on a common scatter plot. A polynomial line was applied to the plot, and the equation governing this line was used to calculate missing velocity values in the analysis and is shown Equation 3. The plot is also depicted in Figure 32.
Velocity (3)
V = 1911.4 × L2− 325.84 × L + 14.387
Where: V = Velocity L = Level
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Figure 32 - Average Velocity against Level
y = 1911.4x2- 325.84x + 14.387 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0.085 0.09 0.095 0.1 0.105 0.11 Average Velocity Poly. (Average Velocity)
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