**5. Are existing wetlands capable of treating flows larger than what they are designed for?**

The most crucial part of designing wetlands is the sizing of wetlands. It is recommended that the capacity of a wetland should be at least 3% of its catchment size or can take 1 in 3 months flow to remove pollutants within the guidelines for stormwater quality treatment [17]. Since there is limited research on wetlands receiving flows larger than their design intent, this section will review the following fundamental questions:

### **5.1 How do depth, duration, and frequency of flooding influence wetlands plants?**

The duration and frequency of overland flow (water regime) can be a major influence on the development of wetland plants. In Australia, wet to dry seasonal changes cause different water levels each season which can potentially affect plant growth and responses [33]. Wetlands plants are utilized to prevent erosion, capture fine particles and to trap pollutants from runoff [3, 17].

An experiment was done to determine whether the depth, duration and frequency of flooding influence the development of wetlands plants. The experiment was done with different combinations of depth, duration, and frequency of flooding with different types of wetlands plants collected from (a) wetlands that are rarely dry (near permanent) and (b) poorly drained wetlands (intermittent) in New South Wales [33]. All 17 treatments were placed outdoor in uncovered tanks to allow the plants to be exposed to rainfall and some tanks were filled with water depending on the treatment. According to [33], the experiment was conducted for more than 16 weeks using different combinations to maximize results as plants are well established after this period of time. The results show that the depth of flooding does have a significant impact on the biomass and the different species of wetland plants for both wetland types (near permanent and intermittent) and the results of the durations of inundated plants varied between plants from the two wetlands.

The results indicated that the inundated plants from the near-permanent wetlands show no difference between different durations of flooding (4, 8 and 12 weeks) whereas plants from the intermittent wetlands show there was a decline in some

#### *Understanding the Role of Constructed Wetlands in Stormwater Management DOI: http://dx.doi.org/10.5772/intechopen.102912*

species of plants. As for the flooding frequency, no significant impact on plants establishments for all 17 treatments for plants that are from the near-permanent wetland but for the plants from the intermittent wetlands some decline of plants species are reported. The highest biomass and the vast plant species can be seen from the treatment tank that were never flooded [33]. According to [34] some wetland plants species are sensitive to the change of water level and could cause a distinct loss in its species which are caused by oxygen depletion and the ability to go through the photosynthesis process when it is fully submerged. Therefore, depending on the wetland type (near permanent and intermittent) wetlands plants that are frequently flooded can have an adverse effect on the treatment process.
