The goal in any water quality monitoring program is to collect samples that encompass the spatial and temporal variability of the site conditions (Harmel et al. 2003). When designing bioretention systems for the purpose of monitoring, the overall research questions, bioretention drainage configuration, final reportable units (e.g., concentration or mass), hydraulic conductivity, specific yield of the bioretention soil media, local rules and regulations, budgetary and logistical constraints, and proximity to underground utilities are important considerations (Law et al. 2008).
There are many different methods to sample stormwater, with time-based and flow-based sampling being the two most commonly used (Harmel et al. 2003). Time- based sampling is most appropriate for research in small watersheds, where land cover is fairly homogeneous (Harmel et al. 2003; Sansalone and Cristina 2004). Such conditions will produce a hydrograph that can be sampled with equally spaced samples over its rising limb, peak, and falling limb in an ideal storm (Alias et al. 2014; Harmel et al. 2003). Alternatively, flow-based sampling allows for samples to be taken after a specified volume of water has passed (Law et al. 2008) and is more robust to changing
precipitation intensities over time, and when site conditions are likely to alter flow rates, such as those which contain irregular surfaces, diverse land use, or when drainage areas are larger in size (Harmel et al. 2003).
In this research, discrete, time-based samples were collected at multiple
locations throughout the runoff hydrograph, from small, paved, road sub-watersheds. The timing of inflow samples was based on estimates of peak inflow discharge rates for the
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eight watersheds, which were determined using the time of concentration, rainfall intensity duration frequency (IDF), curves and the rational method, which are described in the following sections. These values were then used to determine the length of time required to take representative samples at multiple intervals throughout an idealized hydrograph. The sub-watersheds in this study were modeled as homogeneous paved road surfaces, using a runoff coefficient for paved asphalt.
3.3.2.1. Time of Concentration
The Time of Concentration (Tc) estimates how long it will take a drop of water
to travel from the most hydrologically remote part of the watershed, to the monitoring location, using the runoff coefficient, total distance, and slope as the main variables, as shown in Equation 3 (Kang et al. 2008; King et al. 2005). The distance from the farthest corner of the largest watershed to the monitoring device, in this research was
approximately 104 ft (31.7 m). A runoff coefficient of 0.95 for impervious asphalt (Allen Burton and Pitt 2002) and a slope value of 0.01 ft/ft were used to approximate the time of concentration. The time of concentrations from the smallest to largest watersheds ranged from 4.73 minutes to 8.27 minutes. The Tc value was then used to determine the
approximate rainfall intensity, using a rainfall IDF curve, and the rational method.
(3)
Where,
Tc is the time of concentration (min)
G is equal to 1.8 (FAA method, constant)
C is the runoff coefficient using the rational method (dimensionless) L is the longest distance from the fixed location within the watershed (ft) S is the slope of the watershed (ft ft-1or m m-1)
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3.3.2.2. Estimating Peak Discharge with Intensity Duration Frequency (IDF) Curves
Rainfall IDF curves depict the relationship between precipitation intensity and duration, given a selected frequency of return for a specific climatic region (Claytor and Schueler 1996; Davis and Cornwell 1998). In this research, a rainfall IDF curve for Chittenden County, Vermont was used (Figure 7), with a 1-year recurrence interval, from 5 minutes to 120 minutes (Northeast Regional Climate Center Precipitation Data). The rainfall intensities, which corresponded with the time of concentrations from each sub- watershed, ranged from approximately 3.32 in hr-1 (2.34 x 10-5 m s-1) to 2.57 in hr-1 (1.81 x 10-5 m s-1). The rainfall intensity for each watershed was used to estimate peak
discharge with the rational method, as shown in Equation 4.
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(4)
Where,
Q is the peak discharge (ft3 s-1 or m3 s-1) Cf is the runoff coefficient (dimensionless)
Ci is the rainfall intensity (ft s-1 or m s-1)
A is the drainage area (ft2 or m2)
The rational method is most appropriate for small watersheds, which are highly impervious (Natural Resources Conservation Service 1986). The assumptions of the rational method are as follows: (a) peak flow rate is a direct function of the drainage area and average rainfall intensity during the time of concentration, (b) rainfall is uniformly distributed over the paved road sub-watersheds, (c) rainfall intensity remains constant during the time of concentration, and (d) the runoff coefficient is constant and consistent throughout the sub-watersheds (Natural Resources Conservation Service 1986).
The peak flow rate occurs when the total watershed area is contributing runoff (Davis and Cornwell 1998). The peak flow rate values were used to estimate the total length of time needed to sample a specific rainfall depth (Equation 5).
(5)
The rainfall depth selected was 0.90 inches (0.0229 m), which is a common water quality volume to be treated with stormwater best management practices (Vermont Agency of Natural Resources 2002b).
3.3.2.3. Monitoring Duration for the Inflow Hydrograph
The time for the peak flow rate to reach the monitoring equipment in the eight sub-watersheds on this research site were between approximately 17 and 21 minutes. A
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multiplier of two was applied to the time, in order to account for the falling limb of the hydrograph (Table 30, Appendix). A larger multiplier may be warranted if the
assumptions used to determine the peak flow rate cannot be fully met.
The Teledyne ISCO 6700 series automated samplers can hold a maximum of twenty four 1-L bottles. To encompass the inflow hydrograph, the inflow samples from each cell were taken every two minutes for 48 minutes (n = 24), when inflow flow rates were consistently above the minimum sampling threshold of 0.21 ft (6.50 cm). If the inflow flow rate dropped below the minimum threshold, sampling stopped, and resumed if levels rose again, until all 24 bottles were filled. An example inflow hydrograph from the site is shown in Figure 8.
Figure 8. Example inflow hydrograph, showing samples (n=24) taken from watershed 6, 7/3/14.