Numerical modelling of urban stormwater management with grassed road divider as bioretention system

Bioretention system is one of the best management practices for rainwater runoff redirecting and storing before discharge into existing stormwater system. On the other hand, road divider is designed for dividing the traffic flow for road safety. However, it may be a blockage for surface runoff on road and possibly created ponding during heavy rainfall event. This scenario could become a hazard for motorised vehicles. In this study, a grassed road divider in Broga Road, Semenyih, Malaysia, is modelled as bioretention system by EPA's Storm Water Management Model (SWMM) to investigate the performance of its application. A case of grassed road divider without bioretention cell was also modelled for comparison. A series of simulations were carried out for the ARI of 2, 5, and 10 years to further study the performance of grassed road divider as a bioretention system. Four different types of soil including sand, loamy sand, loam, and sandy loam are selected as filler soil in the bioretention cell. Results from the model simulations showed that the performances of grassed road divider as a bioretention system can reduce the surface runoff into the stormwater system up to 49.9% and 56.77% for different ARIs. The effect of this implication is more significant on the reduction as the ARI increased. Results also showed that the impact of soil types is insignificant. The findings show that a bioretention system in a grassed road divider may supplement conventional urban road drainage and provide an effective stormwater management.


Introduction
Malaysia is a developing country with a territory of approximately 320000 km square. Majority of the population is living in the country's capital which is Kuala Lumpur. Evidently, urbanization has boosted economic activity and aided in the expansion of the global population. Simultaneously, urban development disrupted the ecological system by deforestation, paving over dirt, and polluting atmosphere and water source. Effective management techniques that are commonly used include wetlands, detention basins, gross pollutant barriers, and bioretention. The centre business district (CBD) of Kuala Lumpur has limited lands for the development of gigantic civil structures for stormwater management such as detention ponds and wetland.
A grassed road divider can be commonly observed in Kuala Lumpur. The fact of limited space in Kuala Lumpur inspires an idea about applicability of road divider as a stormwater storage facility [1]. There is a lack of understanding in Malaysia about the use of grassed road dividers as a mechanism of natural system to retain stormwater [2]. By redirecting stormwater from the pavement further into grassed road divider, the divider becomes a valuable component on the road, providing a green space in the urban road drainage mechanism. As a result, an effort is made here to explore the appropriateness of grassed road dividers as bio-retention systems in the equatorial region's heavy rains [2].
Usage of bio-retention system method on existing locations aims to establish more appealing perspective on urban landscape which are typically applied to small places. Bioretention systems are typically positioned throughout building, alongside highway and road drainage riverbeds, next to parking lots, and then within landscaping in impervious or highdensity environment [3].
The objectives of this study are: 1) To simulate the numerical model of grassed road divider as bio-retention system.
2) To study the effectiveness of grassed road divider as bio-retention system.

Study area
Broga Road has been selected as the field of study. After a strong storm event, the road pavement around the Broga road complex is regularly flooded, it is inconvenience for the road user. The grassed road divider along the Broga road is modelled as normal road divier and bio-retention system road divider.

Model setup
The methodology used directly targeted storm water runoff management through the use of green infrastructure, which further contributes towards the achievement of sustainable development goals. Using EPA SWMM 5.1, model was simulated to investigate the influence of green infrastructure on storm water runoff control. Example of model setup is showed in Figure 2, where the shaded regions are the designated sub-catchment of the study.

Soil type of bio-retention road divider
Appropriate soils can increase the infiltration rates of surface runoff discharged to the stream, regulate infiltration rates for pollutant removal, and promote plant growth and long-term sustainability [2]. For bio-retention systems, sandy loam, loamy sand, or loam texture is suggested. Depending on water flow calculations performed in a research, soils with infiltration rates greater than 0.5 in/hr were favoured for bio-retention systems. Minimum infiltration values ranged between 0.52 in/hr to 2.41 in/hr for loamy sand, sandy loam, and loam soils. Loamy soils also including silt loams and sandy clay loams with penetration and infiltration rates equivalent to or less than 0.27 in/hr were not ideal. For the system to work optimally, the soil must contain 15 percent organic matter and not more than 25 percent clay [4]. Table 1 displays the hydraulic conductivity, suction head, porosity, field capacity, and wilting point of sand, loamy sand, sandy loam, and loam that may be used in simulation.

Design rainfall
The design rainfall in the model is chosen according to Urban Stormwater management Manial for Malaysia 2 nd edition (MSMA 2 nd edition) [5]. The design storm duration 1 hr and with 3 different sets of ARI which are 2 years, 5 years and 10 years. The design rainfall intensity is calculated according to the empirical equation (Equation 1) from MSMA 2 nd edition as the input of the study model and listed in Table 2. The study area is Semenyih which is near to Kajang, thus, the selected station for IDF constant is Sector JPS Kajang (Station ID: 2917001).

Bio-retention road divider specification
According to Malaysian Urban Stormwater Management Manual guideline, the length to width ratio for constructing a bio-retention scheme is 2:1 as shown in Table 3. As a result, a single unit of bio-retention system of 3 m x 6 m in scale was constructed. Since the selected road strip had two-way lanes, the width was considered to be half for one lane and half for the opposite lane. Conversely, the scale used for SWMM modelling was 1.5m x 6m. Table 3. Physical specification and geometry of bio-retention system [5].

Result
According to Table 4, with the presence of bio-retention cell, the runoffs reduced with a percentage range of 49.9% to 56.77% from the original scenario which is without the presence of bio-retention cell. The 2 year ARI reduced the least percentage in runoffs whereas the 10 year ARI reduced the highest percentage in runoffs which mean 10 year ARI is the most accurate outcome. Despite so, all the estimate reductions were less than a percentage of 10%. Although the soil types were different, but they were having a same bio-retention performance when subjected to 1 hour of increasing rain intensities. For ARIs that is higher than 10 year, the percentage reduce will be higher and eventually become 100% which does not make sense, so the optimum ARIs was not more than 10 year to obtain an accurate result. Additional conclusion of the study showed that in the equatorial areas, roadside drainage was required for rainfall distribution. In the face of heavy rains and runoff, eliminating a roadside drain was a poor decision. Additionally, by including a bio-retention system, portions of the flow as well as dust/soil elements from the road surface may be retained. To save costs on construction and operation, the design of the roadside drainage may be simplified.

Conclusion
From this research paper, we can determine the performance of the bio-retention cell presence in grassed road divider to store water effectively. Other than storing water, bio-retention cell also helps to infiltrate the stones and others particle from road surface to maintain a healthy and sustainable environment of the urban area. Although the soil types are different from each other but the performance and the outcomes from each are similar with the others which are water retention/detention. Using SWMM 5.1, research investigated bio-retention systems by constructing stormwater transportation systems. For road drainage situations with and without bio-retention mechanisms, two models were constructed. Existing site constraints at the site might be replicated using a case study. For discussion, a practical bio-retention mechanism was tested to 1 hour of tropical rainfall of 2, 5 and 10 year ARIs.