Design opportunities for flash flood reduction by improving the quality of the living environment
by A. Cruijsen
The overarching aim of this research project was to establish a sustainable urban water management design for Hoboken City to decrease vulnerability to extreme precipitation and improve urban quality. By carrying out a functional analysis of the area and the technical analysis of the water system, recommendations were made for soft and natural spatial solutions (i.e. blue-green measures). The base of the design is the existing urban environment, which is characterised by low area elevations, dense urban development, and high impermeability. The gravity based sewer system drains storm water into the Hudson River through combined sewer overflow valves. During high tide, the overflow valves close, preventing the storm water to overflow, whereby the surplus water remaining in the sewer pipes overflows in low-lying areas. Natural processes were used to recreate a naturally oriented water cycle. The main goals of the urban design were to reduce storm water floods, reduce the number of combined sewer overflows into the Hudson River, and improving the urban living environment. Urban quality was indicated by air quality, public green- and recreational space, and the reduction of urban heat island effects. The research question as a guide for the thesis reads: ‘What system of blue- green adaptation measures is most beneficial for Hoboken in terms of flood reduction and improving the urban quality?’ Various research methods are used to answer this question. To gain knowledge regarding urban water management and nature based flood adaptation measures, a literature review is conduced. tudies towards flooding and flood reduction in Hoboken were used as input for the water assignment. To design sets of blue-green measures, a functional analysis of the area is done using the six-layer approach for urban areas. With the approach, design opportunities and constraints for different layers are identified. The design strategies of appropriate blue-green measures for Hoboken are tested in a hydrology-hydraulic water management model.
Hoboken suffers both flash flooding overwhelming the sewer system and storm surge. Low elevations and high groundwater levels influence infiltration capacity and efficiency of the drainage system. Combined sewer outlets with valves to the Hudson River carry excess water directly into the river during storms. When heavy rainfall coincides with high tide, excess water backs up in the sewer, causing flood nuisance in particular the low-lying areas. In terms of the required storage and drainage capacity, the water assignment for Hoboken is calculated. This technical assignment provides a rough measure of the required delay and storage capacity in urban areas for given rain events that exceed the existing storage and pumping capacity. With a basic hydrological model, the storage is determined on the basis of 100 years of precipitation data. The water assignment isn’t normative. It does not take detailed area characteristics, elevation profile and sewer layout into account and therefore cannot determine the locations where flooding occurs.
With statistics afterwards, the storage volumes for 1, 2, 10 and 50 years design storms is determined. The table below shows the water assignment for these storm events. The volumes of water in the water assignment show the pressure on the current drainage system. During a T1 storm event, the required storage capacity is already twice the available sewer storage of 8.3 MG. For a T10 the excessive volume is 40.9 MG, corresponding to 62 Olympic swimming pools spread over the city. Based on the current storage capacity of 0.5 inch/day, SDF curves show that for a T10 storm, a pumping capacity of more than 15 inch per day would be needed. Due to interconnections, the calculated volumes per sub basin may differ from the actual volumes.
Future regulations may require the North Hudson Sewerage Authority (NHSA), to reduce the flooding frequency on average once every 4 years or a T4 return period (HobokenNJ, 2013). The preferred T10 drainage capacity (5.0 inch in 24h) as set out in the US urban drainage design manual even corresponds with a required T10 storage capacity. A number of blue-green adaptation measures have been selected for Hoboken based on site suitability. The six-layer approach, which integrates urban planning and subsoil characteristics, gives an overview of the opportunities and constraints for blue and green measures in the area. Well-fitting measures appeared to be subsurface detention storage in the higher elevated areas, storm water flow-through planters, (storm water infiltration) trees and permeable pavement in infrastructure. In public space, parks, water squares, green squares, detention below sports fields, open water and urban farming would fit well. The buildings in Hoboken found to be suitable for rainwater harvest tanks, blue- and green roofs, green facades and urban farming on rooftops. To maximize blue-green benefits, a citywide network of waterways, green areas, green streets and common gardens is proposed. Five design strategies were developed with combinations of blue-green measures, applied to different layers in the urban system. The first (1) design strategy improves the current situation with the application of subsurface adaptation measures. With a functional analysis, storage possibilities beneath parking lots and sports fields are determined. Two deep storage basins will are proposed. The remaining sports fields and parking lots will be equipped with shallow storage facilities. The second (2) design strategy improves the current situation with the application of infrastructural adaptation measures. This includes surface measures like permeable pavement, storm water infiltration planters and trees along major roads, and green in the streetscape. The third (3) design strategy applies adaptation measures on public space. Undeveloped public areas suitable to rain gardens, storm water trees and flow-through planters, water squares, detention below sport fields, parks, retention ponds (in new/existing green space), urban agriculture, bio retention swales, rainwater harvest cistern and seasonal storage are here for identified. For the fourth (4) design scenario, adaptation measures were applied on private space and buildings. Buildings suitable to place green roofs in the area are analysed. The fifth (5) strategy in the end, combines the proposed measures into an urban blue-green metabolism. Additional green is applied to the public space. Hollow roads discharge storm water towards open water bodies and green in the streetscape connects green area and open water in the city centre to green belt.
The mitigation performance of every design strategy regarding the water assignment is tested on the basis of a Storm Water Management Model (SWMM). The five design scenarios were added to the basic model by Low Impact Development controls (i.e. green roofs, permeable pavement, bio swales). To quantify the contribution of the design strategies to the water assignment, a number of criteria and parameters are identified. The criteria for design performance are (a) flood volume reduction during heavy rainfall events, (b) CSO reduction, (c) vulnerability reduction of critical public buildings and infrastructure (f.e. Hoboken terminal, hospitals, wastewater treatment plant, electricity distribution locations), and (d) improvement the urban quality of living. Four parameters to evaluate the to evaluate the contribution to flood mitigation and overflow reduction are (1) the flood volume in the drainage area (internal outflow), (2) the runoff per sub-basin, (3) the total storm water storage per sub-basin (excluding sewer storage), (4) critical facilities threatened by flash floods.
All strategies showed contribution to flood volume reduction and creating urban quality, but not all strategies were as effective. Strategy 1 can store about 10% of the total flood volume. It has the least contribution to urban quality, since only subsurface storage, and no vegetated measures were used. Strategy 2 on the other hand, has a very broad effect on the green experience of the city. It includes the greening of three major roads. Strategy 3 uses undeveloped space to create a green network throughout the city. It has the least effect on total flood reduction of all strategies, but uses space that would otherwise lie fallow. In strategy 4, green roofs are applied on all suitable buildings throughout the city. This had a positive effect on both flood reduction, reduction of the urban heat island effect, and air quality. When the roofs are large enough, they can even function as a roof garden, roof restaurant or private kitchen garden. Design strategy 5, the city-wide metabolism and people design strategy, combines all design strategies into a citywide system of blue-green measures. Modelling showed that Hoboken benefits most from this city-wide design strategy (5) in terms of quantitative flood reduction. This design strategy also has the most additional green through a citywide network of vegetated measures.
The proposed spatial typology of the city-wide metabolism and people design strategy is elaborated with a number of illustrations. Green roofs, permeable pavement, bio-retention gardens, storage basins, an urban farm, urban wetlands and water squares are included in the adaptation design. Through the urban grid system, vegetated areas are connected to create a pleasant experience throughout the city. The riverbeds in the east and the areas at the bottom of the cliff are surrounding the city with a green belt. The green belt along the cliffs serves for both storage and infiltration of water. The three major roads with trees and plants create green veins through the heart of the city. In every part between the major roads parks, urban farms or wetlands can be found. A solid footpath or bicycle lane can be applied along the city borders to create an uninterrupted route. This citywide system of flood prevention measures provides a pleasant, physical appearance.
For future work, a number of suggestions can be presented towards the reliability and effectiveness of this study. To fill up data gaps in the current model, a follow up study regarding the costs and support of the proposed design is meaningful. The suggested design strategy 5 is the most extensive urban design is the most extensive one and therefor also the most costly. Historical research on blue-green measures resulted in implementation costs indications. These results need to be revised for the proposed design and with up to date financial taxes. Support for the design is needed when it comes to realisation. Inhabitants, businesses, landowners, the NJ transit and the governance are important stakeholders. Based on the outcome of this research, the improvement of the storm water management model is of biggest interest.
Student: Anna Cruijsen
Committee: Prof. dr. ir. N. C. van de Giesen (TU Delft), Frans van de Ven (TU Delft), F.L. Hooimeijer, N.C. Dolman