Tag Archives: urban hydrology

the emerging science of aquatic system connectivity

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JAWRA has a nice literature review on this topic, particularly the science of how wetlands function hydrologically based on their placement in the watershed. I can see a lot of useful applications in both urban and rural hydrology. In my experience, and particularly on the urban end, there is very little hard science built into our legal and policy frameworks on this topic, for example in municipal drainage codes and stormwater management requirements.

the urban cool island

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Here’s an article on quantifying the urban cool island. Which, as you might expect, is the opposite of the urban heat island.

Quantifying the cool island effects of urban green spaces using remote sensing Data

Urban Heat Island (UHI) leads to increased energy consumption, aggravated pollution and threatened health of citizens. Urban green spaces mitigate UHI effects, however, it is still unclear how the green space characteristics and its surrounding environment affects the green space cool island (GCI). In this study, land surface temperature (LST) and land cover types within the outmost ring road of Shanghai, China were obtained from Landsat 8 data and high-resolution Google Earth data. The GCI effects were defined in three aspects: GCI range (GR), amplitude of temperature drop (TA) and temperature gradient (TG). Pearson correlation analysis was processed to get the relationship between the aspects and impact factors. The results indicated that the GCI principle could be explained by the thermal conduct theory. The efficient methods to decrease LST of green spaces include increasing green space area while staying below the threshold, adding complexity of green space shape, decreasing impervious surfaces and enlarging the area of water bodies. For the surrounding environment of the green spaces, increasing vegetation and water body fractions or decreasing impervious surfaces will help to strengthen GCI effects. The findings can help urban planners to understand GCI formation and design cool green spaces to mitigate UHI effects.

This is a subject where I’m out of my depth in terms of formal training, but certainly interested. There are at least two ways you can try to combat the urban heat island effect, which occurs when pavement and other man-made surfaces absorb heat during the day and release it slowly at night (and during the day). The first is to use light-colored materials to reflect sunlight back into space. Using white roof materials whenever practical seems like a no-brainer. Maybe we don’t want snow white paving materials everywhere at the ground level, because that could be displeasing and even painful to the eye, but certainly we could dispense with the asphalt. Even if asphalt didn’t absorb heat, it would still be a hideous, toxic, short-lived material. It’s better to use concrete or brick or stone or almost anything else – it may cost more up front but it will last longer and just generally make our urban areas better. Materials that are permeable to rain water are also available so let’s consider those where they make sense.

The other way is to maximize the use of soil and vegetated surfaces. Soil and vegetated surfaces also absorb heat, I think, but then dissipate much of it again through evaporation and transpiration. Then there is the simple process of tree canopy create shade at ground level (which I imagine satellite studies like the one above may have trouble picking up on). In very dry climates, this may not be practical because to state the obvious, you need water to have evaporation. In very, very wet climates, it might make sense to store rainwater and intentionally spray it on your paved surfaces to cool them down. This is assuming you want to get rid of the heat and water – if you are in a place where water is scarce and precious, you might not want to do that, and you might even want to think twice about having a lot of vegetated surface. Or maybe that is not the right place for large numbers of people to live. Unless you can create more or less a closed-loop water system, in which case it might be a good place, thinking in terms of ecological footprint and preparing for humanity’s possible future in space.

macroinvertebrates (aka worms and bugs) in rain gardens

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Even though the names imply they are living ecosystems, stormwater management engineers still have a tendency to think of rain gardens and bioretention basins as inert systems. It’s good to see the profession working with other disciplines and taking soil science more seriously these days. And where most are focused on physical, chemical, and plant-based processes, a few are looking more closely at the importance of animal activity.

Soil invertebrates in Australian rain gardens and their potential roles in storage and processing of nitrogen

Research on rain gardens generally focuses on hydrology, geochemistry, and vegetation. The role of soil invertebrates has largely been overlooked, despite their well-known impacts on soil nutrient storage, removal, and processing. Surveys of three rain gardens in Melbourne, Australia, revealed a soil invertebrate community structure that differed significantly among sites but was stable across sampling dates (July 2013 and April 2014). Megadrilacea (earthworms), Enchytraeidae (potworms), and Collembola (springtails) were abundant in all sites, and together accounted for a median of 80% of total soil invertebrate abundance. Earthworms were positively correlated to soil organic matter content, but the abundances of other taxonomic groups were not strongly related to organic matter content, plant cover, or root biomass across sites. While less than 5% of total soil N was estimated to be stored in the body tissues of these three taxa, and estimated N gas emissions from earthworms (N2O and N2) were low, ingestion and processing of soil was high (e.g., up to 417% of the upper 5 cm of soil ingested by earthworms annually in one site), suggesting that the contribution of these organisms to N cycling in rain gardens may be substantial. Thus, invertebrate communities represent an overlooked feature of rain garden design that can play an important role in the structure and function of these systems.

urban vegetation design for heat

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When you see completely mangled English in a paper that has supposedly passed peer review, you have to wonder about the quality of the peer review. Nonetheless, I was interested in the results of this study that looked at trees, shrubs, and lawn to see which had the most effect on urban heat.

Numerical simulation of the impact of different vegetation species on the outdoor thermal environment

For air temperature at 1.5 m and thermal comfort and safety (PET and WBGT), the sequence is trees> lawn> shrubs, but for surface temperature, the sequence is lawn> shrubs> trees

I’m always interested in the idea of designing urban areas to maximize hydrologic function, ecological function, and human comfort simultaneously. There is so much that could be done, and so much closed-mindedness and poor communication among the various professions and disciplines that could be doing it.

I’ve always assumed trees are the gold standard, because you get both the evapotranspiration function and the shading function, whereas with lawn and shrubs you only get the former. Also, you only need a small area of soil to plant the tree (although often more than we allow in urban areas), and then its leaves can cover a large area of concrete or asphalt, which would otherwise be generating a lot of heat and polluted runoff. Also, grass provides very little ecological function (unless you let it grow taller and/or take a lenient approach to what some of your neighbors choose to define as “weeds”, which can be socially unacceptable), where trees and bushes provide ecological function. Bushes take up a lot of space, either in a sidewalk context or a small urban yard – paradoxically, once trees mature a little bit they take up less space because there is space under them. On the other hand, I’ve argued with purists that if people really want lawn in urban areas, it is a lot better than concrete in terms of hydrology, heat, and aesthetics. Although if you’re in a water-stressed area, that adds another factor to the hydrology equation that those of us in wetter areas have the luxury of not worrying about too much.

green roofs

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Here’s a green roof modeling study from Singapore. Green roofs reduce peak flows enough to help with flooding. They reduce the volume of runoff a little bit through increased evapotranspiration, which would have an effect on the water supply in Singapore where urban runoff is used as a water source.

Effect of Catchment-Scale Green Roof Deployment on Stormwater Generation and Reuse in a Tropical City

Low-impact development (LID) comprises a broad spectrum of stormwater management technologies for mitigating the impacts of urbanization on hydrological processes. Among these technologies, green roofs are one of the most adopted solutions, especially in densely populated metropolitan areas, where roofs take up a significant portion of the impervious surfaces and land areas are scarce. While the in situ hydrological performance of green roofs—i.e., reduction of runoff volume and peak discharge—is well addressed in literature, less is known about their impact on stormwater management and reuse activities at a catchment or city scale. This study developed an integrated urban water cycle model (IUWCM) to quantitatively assess the effect of uniform green roof deployment (i.e., 25, 50, and 100% conversion of traditional roofs) over the period 2009–2011 in the Marina Reservoir catchment, a 100-km2100-km2, highly urbanized area located in the heart of Singapore. The IUWCM consists of two components: (1) a physically based model for extensive green roofs integrated within a one-dimensional numerical hydrological-hydraulic catchment model linked with (2) an optimization-based model describing the operation of the downstream, stormwater-fed reservoir. The event-based hydrological performance of green roofs varied significantly throughout the simulation period with a median of about 5% and 12% for the catchment scale reduction of runoff volume and peak discharge (100% conversion of traditional roofs). The high variability and lower performance (with respect to temperate climates) are strongly related to the tropical weather and climatic conditions—e.g., antecedent dry weather period and maximum rainfall intensity. Average annual volume reductions were 0.6, 1.2, and 2.4% for the 25, 50, and 100% green roof scenarios, respectively. The reduction of the stormwater generated at the catchment level through green roof implementation had a positive impact on flood protection along Marina Reservoir shores and the energy costs encountered when operating the reservoir. Vice versa, the drinking water supply, which depends on the amount of available stormwater, decreased due to the evapotranspiration losses from green roofs. Better performance in terms of stormwater reuse could only be obtained by increasing the time of concentration of the catchment. This may be achieved through the combination of green roofs with other LID structures.

downscaling

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Here is a useful (to me, at least) Hydrology and Earth System Sciences open article on spatial and temporal downscaling of climate change model output.

Information on extreme precipitation for future climate is needed to assess the changes in the frequency and intensity of flooding. The primary source of information in climate change impact studies is climate model projections. However, due to the coarse resolution and biases of these models, they cannot be directly used in hydrological models. Hence, statistical downscaling is necessary to address climate change impacts at the catchment scale.

This study compares eight statistical downscaling methods (SDMs) often used in climate change impact studies. Four methods are based on change factors (CFs), three are bias correction (BC) methods, and one is a perfect prognosis method. The eight methods are used to downscale precipitation output from 15 regional climate models (RCMs) from the ENSEMBLES project for 11 catchments in Europe. The overall results point to an increase in extreme precipitation in most catchments in both winter and summer. For individual catchments, the downscaled time series tend to agree on the direction of the change but differ in the magnitude. Differences between the SDMs vary between the catchments and depend on the season analysed. Similarly, general conclusions cannot be drawn regarding the differences between CFs and BC methods. The performance of the BC methods during the control period also depends on the catchment, but in most cases they represent an improvement compared to RCM outputs. Analysis of the variance in the ensemble of RCMs and SDMs indicates that at least 30% and up to approximately half of the total variance is derived from the SDMs. This study illustrates the large variability in the expected changes in extreme precipitation and highlights the need for considering an ensemble of both SDMs and climate models. Recommendations are provided for the selection of the most suitable SDMs to include in the analysis.

What is potentially useful to me is that they went to a one day time scale, and they defined an “extreme precipitation index” for storms expected to happen once a year or less on average. I am interested in how or whether these concepts can be applied to “typical” hydrologic conditions that happen at the more-than-once-a-year level. Drought and flooding are probably the two most concerning conditions impacted by climate change, but there are also questions being asked about water quality, and it is the “typical” conditions that most come into play.

green roofs

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Green roofs are still pretty expensive and not all that common, at least in North America. But here’s a study in Ecological Engineering where they turned out to work better than people thought in Hong Kong, a humid subtropical area.

Urbanization replaces permeable surfaces with relatively impervious ones to intensify mass and temporal response of stormwater runoff. Under heavy rainfalls, urban runoff could impose tremendous stress on the drainage systems, contributing to combined sewer overflow and flooding. Green roof offers an on-site source-reduction sustainable stormwater management measure that mimics pre-development hydrologic functions. It can retain and detain stormwater as well as delay and suppress peak discharge. However, previous studies were conducted mainly outside the tropics. Since green-roof hydrologic performance can be notably influenced by local meteorological conditions, dedicated investigation in the tropics are necessary. Moreover, substrate depth has long been regarded as an influential factor in green-roof stormwater retention, but recent findings have implicated that such relationship may be more complex. This study (1) evaluates green roof stormwater mitigation performance and potentials in humid-subtropical Hong Kong; and (2) investigates systematically the effect of substrate depth and addition of rockwool, a high water-retention growth medium, on quantitative performance. Using multiple 1.1-m2 raised green-roof platforms placed on an urban rooftop, the effect of four substrate-depth treatments on stormwater mitigation performance was examined over a 10-month study period. The results show that, while the retention under Hong Kong’s frequent and heavy rainfall regime seems to be less effective, remarkable peak reduction and peak delay were evidently expressed even when the green-roof systems have reached full moisture-storage capacity. No statistical significance was found between treatments, despite the slightly higher mean performance of the 80-mm soil substrate. Satisfactory peak performance of the 40-mm soil substrate implies that a thin substrate can provide effective peak mitigation, especially if building loads are of concern. Extensive green roof remains as a promising alternative mitigation strategy to urban stormwater management in Hong Kong with potential application to other tropical areas.

Part of me doesn’t like using an inorganic material like rockwool. But if somebody comes up with a simple, cheap material that we can practically just staple or spray on to roofs in urban areas, it could be a quick way to restore a lot of hydrologic function – retention, evaporation, peak flow reduction, and cooling – in urban areas. It could be a transitional step on our way to restoring both hydrologic and ecological functions together – ideally we would want to capture that water and use it to grow something of use to either people or wildlife or both. But we are far from ideal today, so I’m all for some smaller steps in the right direction.