**6. Modeling of existing wetlands**

#### **6.1 Accuracy of modeling**

The modeling of stormwater pollutants in runoff and the modeling of Stormwater Quality Treatment (SWQT) assets has developed significantly over the years. The first modeling primarily consisted of relatively simple mathematical equations which were considered somewhat crude, now software programs model many of the complex interactions which occur through the stormwater runoff and treatment process, via user-friendly interfaces [37]. Within Australia, the most widely used SWQT modeling program is MUSIC [38]. MUSIC is a stochastic model which utilizes probability to help determine the pollutants in stormwater runoff and the performance of SWQT assets [39]. Consistent with the rest of Australia, in the Melbourne region MUSIC is also the SWQT modeling software of choice. To approve new SWQT assets and to assess whether a new development is meeting best practices, Melbourne Water and Councils throughout Melbourne require a MUSIC model [40].

However, even though MUSIC models are a requirement by Councils and the major water authority within the Melbourne region, some research suggests that MUSIC models are not completely accurate and may over-treat or under-treat depend on the situation [38, 41]. In their paper Modeling stormwater treatment systems using MUSIC: Accuracy [41] undertook a series of comparisons between existing SWQT assets (located in Australia, Sweden, and New Zealand} and MUSIC models which had been created, with modified parameters to represent the actual conditions (e.g., inflows concentrations) and existing assets. They found that depending on the type of treatment asset, the accuracy of the MUSIC model may vary; in some cases, the MUSIC models overestimated treatment whereas in other cases the modeling underestimated the treatment. It should be noted that the study did not investigate MUSIC's ability to accurately model wetlands, a topic which requires further examination, however, it does call into question MUSIC's ability to create accurate models and may provide a portion of the answer to the research question.

The uncertainty of the accuracy of MUSIC SWQT modeling could be due to setting up and calibrating the model. Several studies have found that some of the parameters in MUSIC, e.g., soil storage and field capacity, are crucial for obtaining accurate results and require calibrating based on local data [38, 39]. When creating MUSIC models, errors may occur in the modeling when the user uses the default MUSIC parameters, accidently inputs the incorrect parameters, is unaware of the

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

correct parameters to input, or intentionally inputs the wrong values. To mitigate these potential human errors and improve the quality of MUSIC modeling, Melbourne Water produced MUSIC guidelines, which state the parameters to be used when creating a MUSIC model and general information about SWQT modeling elements. Whilst Melbourne Water's MUSIC Guidelines provide some recommended parameters, [38, 42] recommend further research into assessing the parameters required in MUSIC for catchments with "similar land use, climatic characteristics and hydrological behavior".

### **6.2 Condition of the wetland**

The condition of the wetland could contribute to why modeling results may differ from wetlands. Wetland conditions that may influence the performance of the wetland include bad construction, outlet blockages, modification of the terrain by animals, etc. [6, 37]. Models must make some assumptions and they generally assume that assets will function as in intended. However, this is not always the case as there are many operational factors that affect the functioning of a wetland.

### **6.3 Timing of water sampling**

To assess whether a comparison between the modeling and real-life conditions should occur, the first aspect that has to be determined is if the wetland is fully developed. Kadlec and Wallace [37] state that it can take more than 2 years for the wetland to develop fully. This is the duration required for the bio-system to mature, which requires amongst many other things the build-up of a layer of plant detritus over the base of the wetland so that congregations of periphyton and bacteria can form. The congregations of periphyton and bacteria are essential as these organisms' form part of the nutrient removal process [37]. Thus, if sampling is undertaken before the wetland is fully developed, it may not be reflective of the future potential of the wetland as the bio-system has not matured and is not working to full capacity.

Another aspect that is of importance to the timing of the sampling, is the change of seasons. As plants are seasonal and sprout and perish on an annual basis, the natural biological process dictates that there will be fluctuations in concentrations of nutrients due to the cycling, uptake and release of nutrients, by the plants [37]. Spring generally produces higher uptake of nutrients as the plants are growing and absorb more nutrients in this period whereas in autumn plants are generally dyeing-off and their decaying litter releases nutrients in the waterways [37]. As a result, depending on the timing of the sampling, the same fully developed wetland may produce significant results. However, [12] found that in their two-year study covering all seasons, there was no increase in nitrogen concentrations over the autumn and winter period, which raised the question that there may be other nitrogen removing mechanisms at play. Although, to mitigate this potential error, it is suggested that long-term sampling occurs so that a baseline performance can be determined which takes into account seasonal fluctuations [43].

### **6.4 Maintenance**

One aspect which plays a crucial role in the condition of the wetland is maintenance. Regular maintenance is vital to the performance of a wetland as it facilitates the correct functioning of the wetland [37]. One maintenance task which enables the

proper functioning of a wetland, is the cleaning-out of the sediment pond [16]. If the sediment pond fills up beyond its designed depth, the sediment pond may not have the required depth for sediment to settle and therefore the sediment will remain suspended in the water [37]. This suspended sediment may flow into the macrophyte zone and settle, or it may remain suspended and resist treatment. This has various implications to the effectiveness of the wetland, one being that if sediment settles on the base of the macrophyte zone and builds up, the water velocities in the macrophyte zone may increase due to the smaller flow area, which may cause erosion or impact on detention times [37]. Additionally, the makeup of plants in the macrophyte zone may change due to the lack of habitat, e.g., there may be no deep marsh plants, due to sediment changing the makeup of this zone to the shallow marsh.

Another way in which maintenance can impact the performance of a wetland is through the outlet structure. The outlet structure may become blocked due to litter or the natural decay of plant species. This blockage may result in the water level to rise for extended periods of time which can kill off a number of the plant species that inhabit the wetland [37]. Similarly, without regular maintenance of the wetlands, certain plants such as Typha may grow rampant and effect the hydraulic efficiency of the wetland. In doing so it may cause the water level to rise and kill off several plants [37]. To prevent this, it is suggested that scheduled maintenance of the wetland is to occur.

### **6.5 Frequency of water quality sampling and testing**

Another area that may add to the differences between modeling and existing wetlands is in the sampling and testing. There are many aspects that need to be controlled to obtain accurate results. An important factor which must be taken into consideration to obtain an accurate result is frequency of the sampling. In one study it was found that "to sample TSS adequately within a storm event, at least 12 flow-weighted samples were required, and that polluto graphs of seven storm events needed to be sampled within a year to estimate mean annual loads at a reasonable level of accuracy" [43]. Building on this research [43] recommend that to have an error of less than 10% for sediment sampling, sampling must occur every three days or less for TSS–TN and TP will be different.

### **6.6 Samples' contamination**

Differences between the modeling and actual results may result from the contamination of samples. This could occur in a multitude of ways hence the samplers must be vigilant and follow the protocols when proceeding to take samples. The following are some examples of ways that contamination may occur [44]:


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