**5.2 Does the residence time distribution get affected with the change of wetland depth and flow rate?**

According to [35], "A distribution of times that parcels of water spend in a constructed wetland is known as a residence time distribution". The residence time distribution (RTD) is a tool that has been used widely by engineers to measure wetland's characteristic that could affect its treatment capabilities. Retention or residence time can be calculated as retention time = (area *x* depth)/flow per day where flow is the average inflow and outflow of the wetland noting that the velocity profile in subsurface flow wetlands can influence the distribution of the hydraulic residence time. This is due to the velocity profiles that are influenced by the surface friction that potentially slows the water [36]. The RTD can be modeled on a computer and using tracer tests. A tracer test involves a dyed chemical tracer that gets dissolved into the wetland which can easily be detected to measure RTD [35]. Holland et al., [36] has conducted a study on RTD characteristics and how wetland depth and flow rate can influence it. The study was performed by using a series of dye tracers in a small-scale constructed wetland with a constant and controlled flow rates and water levels. Different water levels and flow rates are then used throughout the study to determine its effects on RTD.

Twelve experiments were conducted with different sets of water levels and flow rates, over a period of 13 weeks by adding 15 l of dye to the inlet of the constructed wetland. The average depth of water is 166 and 398 mm for low and high-water levels respectively. The results of this study indicated that there was no significant difference for the RTD values between the high and low flow rates (**Table 1**). There was however a significant difference for the RTD between the high and low water depth, the low water level result in distribution with one clear peak value whereas the higher water level result in a continuous probability with two different peaks. According to [36], the value differences are reflected in **Table 1** for the water levels are significant. The mean RTD spread, o2 e, values changed significantly when the water level changed from low to high. However, the change for the RTD centroid of the first moment, *λ*t, shows not much of a change in the values.

This study concluded that the residence time distribution changed significantly with the change of water levels but not so much with different flow rates. Moreover, a little change in the volume can influence the RTD characteristic in a constructed wetland because the volume is a function of depth. Depth or volume - of water effects the hydraulic efficiency of a constructed wetland and should be considered during the designing of wetlands [36]. Poor designing or sizing of wetland can affect its treatment performance. From this study, it can be stated that with the change of depth and volume in wetlands, the treatment process will be affected therefore the water quality can be assumed to be poor.

#### *Flood Risk in a Climate Change Context – Exploring Current and Emerging Drivers*


**Table 1.**

*Summarized results for flow rates and water level (using high and low flows) [36].*
