Water sensitive urban design approach for flood prone area (Study case: Kauman Riverfront Area, Semarang)

Kauman Semarang Riverfront Area is the center of the trade and services area in Semarang, especially for traditional trade. This area is located between three ethnic villages and is adjacent to Kali Semarang, a river that has an important meaning and historical value for the city’s development. Kauman Area is located in Central Semarang District, Semarang City, and is within a 1 km radius of the Semarang Old Town, a tourist attraction in the cultural heritage field, which is known as the priority of socio-cultural development on the city and province scale. However, with this type of potential, Kauman Riverfront Area has a medium-high flooding threat. In 2021, the number of flood incidents was the highest within five years (88 incidents). This study aims to examine the WSUD approach for the study area that focuses on the urban water system to decrease flooding risk and its correlation with urban design elements. The chosen method for this research is a fragmental method that consists of four steps, which begin with collecting data until producing the WSUD component approach, which is likely to be implied. The results show that the WSUD components and water system for Kauman Riverfront Area to increase the stormwater infiltration and storage.


Introduction
The existence of the urban water cycle is being highly considered, both from the social, economic, and environmental aspects.Realizing this, many policies and regulations are made to respond the climate change issue, water quality, flood risk management, environment, public health, and the general concept of sustainability.According to Alan Hoban (2019), Water Sensitive Urban Design (WSUD) is one part of the green infrastructure concept approach that aims to approach urban planning and design related to water resources and environmental management and minimize the impact caused by the presence of water in the urban surface [1].
Water Sensitive Urban Design (WSUD) was a concept from Australia responding to the water problem and integrate water management into city development from the planning phase.[2] The term 'Water Sensitive Urban Design' was introduced formally by Whelans and Haperns Glick Maunsell on 1994.WSUD guidelines suggest that this concept has to decrease the TSS rate to 80%, TP rate to 60%, TN rate to 45%, and a whole pollutant to 90% compared to the stormwater runoff without treatment [3] The WSUD concept is summarized as the formulation of a development plan that combines several rainwater management objectives and involves a proactive process.This process involves urban design, landscape architecture, and rainwater management infrastructure.Kauman Riverfront Area is located in Central Semarang District, Semarang City and directly adjacent to the Semarang River, a river that has historical value for the city development.In addition, this area is also located between three ethnical settlements (Kampung Pecinan, Kampung Melayu and Kampung Kauman) and the Old City of Semarang, so this area is very probable to be developed with a heritage tourism focus.According to the Semarang Spatial Planning in 2011-2031, the Old City of Semarang 1318 (2024) 012010 IOP Publishing doi:10.1088/1755-1315/1318/1/012010 2 and the three ethnical settlements are proposed to become a heritage-based tourism centre, while the Kauman area itself is proposed to become a traditional trade centre and PKL centre.[4] However, with these various potentials, Kauman Riverfront Area has its own problems.The area has flooding/rob issue that can be an obstacle to the area development.According to Semarang City Regional Spatial Plan for 2011-2031, Central Semarang District is an area that is included to mediumhigh threat for flooding/rob.
The worst flood disaster that hit Semarang in 2021 was in February, where the impact of floods hit various sectors.In total, the incidents of flooding in February 2021 occurred in 11 sub-districts, 110 subdistricts with 27,346 families affected or 101,366 people, and 5 people died [5].Apart from the transportation sector, several buildings that have important value to the city were also affected, which are Johar Market and Kauman Mosque in Semarang.According to BPBD of Semarang, the flood height in Kauman area reach 30-50 cm.Based on the interview with the head of RW 5 and Semarang Public Works Department, this incident happened because Semarang City's water pump at that time was not functioning properly.The statement shows that Semarang has a huge dependence on water pumps in overcoming flood problems.In addition, the study area itself has fewer stormwater infiltration and storage areas, so the stormwater runoff does not have absorption media and storage.Therefore, it is necessary to arrange an area that pays attention to urban water management by considering the structure, geography, and geology of the area as a mitigation measure against the risk of flooding [6] This paper aims to determine the applicable WSUD components by considering the structure, geographical aspects, and geological aspects of the area.The approach will be correlated with urban design elements to create new opportunities in placing the WSUD concept, both components in open spaces and in building lots.The components of the WSUD that will be implemented focus on rainwater infiltration and water storage

Study Area
The delineation of the study area was determined by overlaying the flood map of Semarang City, the boundary of ethnical villages, and a little part of Kauman as a landmark for the area, so the study area has 13.36 hectares that consists of 48% as a trade and services, 34% as residential, 14% as green open space, and 4% as a worship center.The study area is adjacent to Kelurahan Purwodinatan in the east, Kampung Pecinan in the south, Kampung Kauman in the west, and Semarang Old City in the north.Within the study area, there is a settlement that is classified as a slum [7] and two cultural heritage buildings.From 13.36 hectares, 61% of the land status in the case study area is unregistered, and the rest is registered [8].The climate in the study area is a tropical climate with an AM (tropical monsoon) climate type.The rainy season has a period of 6 months (October-March) although conditions often change, the highest precipitation is in the period December-March.The peak of high precipitation is in February with a rainfall level of 393 mm, the lowest level of precipitation is in June with an amount of 22 mm, and potentially dry weather.The average level of precipitation in the study area is 208.5 mm.[9].
The study area is located in the district of Central Semarang with alluvial soil type [10].Alluvial soil has a high mineral content so that it has a fairly good water absorption capacity.Its permeability in passing small particles is also relatively low [11].

Methodology
The chosen method for this research is fragmental method.This method is another form of rational comprehensive method (synoptic method) but in a shorter form [12].This study used four steps, which begin with collecting primary and secondary data, analysing data using GIS (Geographic Information System) and literature review, formulating the concept for WSUD component implementation in study area, and structuring the design guidelines.

Natural Stormwater Runoff Treatment
Semarang City has an average precipitation rate of 208.5 mm/month which is categorized as moderate rainfall with the highest level of 393 mm/month in February which is categorized as high rainfall [13].However, with this kind of climate, a special treatment for stormwater runoff is needed efficiently to reduce the risk of flooding in the area.Basically, natural stormwater runoff treatment is sufficient to let the water naturally absorbed into the ground and then evaporated as opposed to flowing directly into the sanitation system [14] so the WSUD concept that should be applied to the area is one that can maximize stormwater infiltration and storage.These goals will be packaged in an integrated urban water system as seen in figure 2. To apply the integrated urban water system concept, the topography of the area is very important to predict the direction of water flow.The study area is located in the Central Semarang district which is classified in the slope I area with a slope level of 0-2% [10].The land in the study area has a slope that tends to be flat.According to an analysis conducted with Google Earth, the design area is 3-6-meters above sea level.With the help of GIS analysis, the detailed topography of the area tends to decline to the northwest and east of the area as seen in Figure 3.The nature of water is to flow from a high place to a low place [15].In Figure 3, the lowest level in the area is showed in green so the infrastructure that can support the rainwater infiltration will be maximized in areas that are located higher up and the provision of rainwater storage will be placed at a lower level.
Infrastructure that can maximize rainwater infiltration includes permeable paving, bioswale, rain gardens, green roofs as well as maximizing the natural green open space in the area [1].Meanwhile, the area for providing rainwater storage can be implemented with various media, such as detention ponds or rainwater harvesting tank.

Urban Design Measure
Urban design is one of so many WSUD approaches because urban design affects the placement of circulation, the division of land, and the appearance of the building.Urban design also has an impact on land cover design which has an effect on water infiltration rates [1].To determine the correlation of the WSUD approach to urban design elements in the area, analysing the existing conditions is important.

Land Use
Based on the results of the survey and analysis using GIS, the area consists of 48% of trade and services, 34% of settlements, 14% of green open spaces and 4% of worship centre.As we can see from the composition of existing land use, the study area consists of various functions with the largest percentage is trade and services, in line with the direction of developing the area into a traditional trade centre and PKL center [4].The figure ground map shows that the majority area is a built-up area and is quite dense.This affects the land use that supports the rainwater infiltration area in the form of green open space is quite minimal, whereas green open space in an area has to be 30% that consists of 20% public green open space and 10% private green open space [16] UU no. 26 tahun 2007.Therefore, with these backgrounds, functional mixed-use development will be proposed to decrease the rate of land consumption, mobilization by motorized vehicles will be reduced, opportunities for green area and permeable surface preservation will be greater [17] and air pollution will be reduced [18].

Circulation
Overall, the study area is served by secondary collector roads (Pemuda Road) and secondary local roads (KH Agus Salim Road and Wahid Hasyim Road).There are traffic jams in the area caused by on-street parking and a lot of activities as seen in Figure 5.There are five types of roads within the study area which are described as follows in Table 1.As seen on the table, not all roads are facilitated by pedestrian way and greenbelts, where greenbelts are also an opportunity to place water sensitive urban design components [1].

Building Elevation
The appearance of the building is a component that can be intervened with the concept of water sensitive urban design, such as by implementing green walls and green roofs [1].Green roof is one of the stormwater runoff management efforts by binding rainwater, then releasing it to the ground around the building and also re-evaporating it into the air [19].Meanwhile, the green wall is one of the green infrastructures to reduce heat in the building and minimize the urban heat island [20].In the study area, it appears that the building has not responded to this.Details of the characteristics of the buildings in the study area can be seen in Table 2.
• Space between building is still available, • Open space between lot is still available as a void.

3.
Source: Private Document, 2022 Kanjengan Market • Medium to high building intensity.• There are permanent and nonpermanent building.• Space between building is still available.
• Space between building is still available.• The remaining lot is hard pavement and green belt in the edge of site.(Source: Analysis, 2022)

Implementation of WSUD Approach to Urban Design Elements
Based on data from field surveys and conducting literature and regulatory studies, the study area can be intervened by implementing WSUD components and will be correlated to urban design elements, especially in aspects of land use, building appearance, and circulation.The synthesis of the urban design element principles and the elaboration of the implementation of the WSUD concept can be seen in Table 3. Providing bioswale in pedestrian protector that integrated with urban drainage system.
(Source: Analysis, 2022) With the urban design element principles approach, an urban framework and site plan can be formulated that regulates area zoning, spatial planning, and area design focusing on expanding green areas, so the site-specific WSUD elements can be placed to maximize infiltration and rainwater storage in the water cycle.This is likely to be achieved by encouraging vertical development and implementing mixed-use development [21].Cultural heritage buildings and the Kanjengan market are maintained to support the direction of the RTBL Kali Semarang as a traditional trading center.The implementation of infrastructure for maximizing rainwater absorption can be applied to land cover in the form of permeable paving and can be traversed by vehicles or used for activities.Permeable paving has a hard surfacing that allows stormwater to percolate to an underlying reservoir base where rainfall is either infiltrated to ground or removed by a subsurface drain [1].Other rainwater infiltration media that will be implemented in the landed green space are bioswale and rain garden, while in the building lot is green roof.Bioswale is a simple vegetated water channel that carries water runoff and can maximize sedimentation and water absorption to reduce the flow velocity [1].Meanwhile, the green roof in the area is used as an efficient stormwater runoff catchment medium to bind water up to 80% of the standard roof [19].The water plaza in the area is used as a detention pond to temporarily hold water and then releasing it to the nearest water body during high precipitation.When the precipitation rate is low, the water plaza is used as an amphitheatre.The placement of this water plaza is at the lowest ground level in the area as we can see in Figure 9.If each of these components are integrated, then the water cycle in the area will be as follows: (1) stormwater runoff will be absorbed by the green roof and then distributed to the rain water tank through the downpipe diverter in the building.The water stored in the rainwater tank can be used for daily needs.
(2) Stormwater runoff will be absorbed naturally in green open spaces and maximized its absorption with the help of green infrastructure in the form of bioswales and rain gardens.The water absorbed by the bioswale will be distributed into the city drainage channel to the nearest water body and the water plaza as a detention pond.In the process of distributing water to water bodies and water plazas, water infiltration is also maximized by providing infiltration wells at several points to avoid overflows that are likely to occur in drainage channels.Meanwhile, the water absorbed in the rain garden will be absorbed directly to the ground.The stormwater runoff system diagram can be seen in Figure 11.

Conclusion
Kauman Riverfront Area has the potential to be developed into a trade and service center as well as a cultural heritage-based tourism node.However, this potential can be hampered because the area along the Kauman River is included in an area with a medium-threat of flooding/rob.The current flood management relies heavily on the Semarang water pump, which operates every time in high precipitation rate to empty the water body first.This shows that water pumps are not the only solution and require other mitigation measures, such as urban water management that focuses on maximizing rainwater infiltration and storage.The application of urban water management in the study area begins with a geographical analysis of the area, especially the topographical and geological aspects.Urban water management is implemented by elaborating urban elements and the WSUD concept, which are interventions on land use, building design, and circulation.The stormwater management system in the area focuses on rainwater infiltration through natural green areas, accelerated by green infrastructure.Water that is not completely absorbed is directed to the lowest level and is received by the detention pond in the form of a water plaza.The suggestions for further research are to be able to elaborate on aspects of heritage tourism and traditional trade nodes such as the RDTR directive on the WSUD component more specifically.

Figure 1 .
Figure 1.The Location of Study Area Between Important Issues and Sites (Source: Analysis, 2022)

Figure
Figure 5. Circulation Analysis (Source: Analysis, 2022) Less of green space area.

Figure 9 .
Figure 9.The Placement of Stormwater Infiltration and Storage Infrastructure (Source: Analysis, 2022)

Table 1 .
Circulation Analysis within the Area

Table 2 .
Building Characteristic within the Study Area Source: Google Earth, 2022 Trade and Center • High building intensity • No spaces between buildings • Active and passive green open space is unavailable.2.

Table 3 .
Correlation between Urban Design Element Principles and WSUD Approaches