Efficiency of parks in mitigating urban heat island effect: An example from Addis Ababa

Exploring changes in land use land cover (LULC) to understand the urban heat island (UHI) effect is valuable for both communities and local governments in cities in developing countries, where urbanization and industrialization often take place rapidly but where coherent planning and control policies have not been applied. This work aims at determining and analyzing the relationship between LULC change and land surface temperature (LST) patterns in the context of urbanization. We first explore the relationship between LST and vegetation, man-made features, and cropland using normalized vegetation, and built-up indices within each LULC type. Afterwards, we assess the impacts of LULC change and urbanization in UHI using hot spot analysis (Getis-Ord Gi∗ statistics) and urban landscape analysis. Finally, we propose a model applying non-parametric regression to estimate future urban climate patterns using predicted land cover and land use change. Results from this work provide an effective methodology for UHI characterization, showing that (a) LST depends on a nonlinear way of LULC types; (b) hotspot analysis using Getis Ord Gi∗ statistics allows to analyze the LST pattern change through time; (c) UHI is influenced by both urban landscape and urban development type; (d) LST pattern forecast and UHI effect examination can be done by the proposed model using nonlinear regression and simulated LULC change scenarios. We chose an inner city area of Hanoi as a case-study, a small and flat plain area where LULC change is significant due to urbanization and industrialization. The methodology presented in this paper can be broadly applied in other cities which exhibit a similar dynamic growth. Our findings can represent an useful tool for policy makers and the community awareness by providing a scientific basis for sustainable urban planning and management.

Characterizing the relationship between land use land cover change and land surface temperature

The topic of rainfall thresholds for landslide occurrence was thoroughly investigated, producing abundance of case studies at different scales of analysis and several technical and scientific advances. We reviewed the most recent papers published in scientific journals, highlighting significant advances and critical issues. We collected and grouped all the information on rainfall thresholds into four categories: publication details, geographical distribution and uses, dataset features, threshold definition. In each category, we selected descriptive information to characterize each one of the 115 rainfall threshold published in the last 9 years. The main improvements that stood out from the review are the definition of standard procedures for the identification of rainfall events and for the objective definition of the thresholds. Numerous advances were achieved in the cataloguing of landslides too, which can be defined as one of the most important variables, together with rainfall data, for drawing reliable thresholds. Another focal point of the reviewed articles was the increased definition of thresholds with different exceedance probabilities to be employed for the definition of warning levels in landslide early warning systems. Nevertheless, drawbacks and criticisms can be identified in most part of the recent literature on rainfall thresholds. The main issues concern the validation process, which is seldom carried out, and the very frequent lack of explanations for the rain gauge selection procedure. The paper may be used as a guide to find adequate literature on the most used or the most advanced approaches followed in every step of the procedure for defining reliable rainfall thresholds. Therefore, it constitutes a guideline for future studies and applications, in particular in early warning systems. The paper also aims at addressing the gaps that need to be filled to further enhance the quality of the research products in this field. The contribution of this manuscript could be seen not only as a review of the state of the art, but also an effective method to disseminate the best practices among scientists and stakeholders involved in landslide hazard management.

A review of the recent literature on rainfall thresholds for landslide occurrence

A review of mitigating strategies to improve the thermal environment and thermal comfort in urban outdoor spaces

The number of hot days is increasing in many parts of the world because of the heat island phenomenon and global climate change. High air temperature greatly affects human thermal comfort and public health, particularly in urban areas. Therefore, the challenging task of, urban designers and urban planners in accommodating the increasing population is to make cities with the least level of vulnerability to future climate change. Interest in transferring urban climatic knowledge into urban planning practices, and developing mitigation strategies to adapt to climate change, has been increased in recent years. The use of vegetation and appropriate urban geometry are shown very promising in mitigating the adverse effects of heat island and providing a better pedestrian thermal comfort. This article reviews studies on pedestrian level urban greening and geometry in improving thermal comfort in cities. Such strategies can be applied at the preliminary stages of urban planning and thus directly affect the microclimate. The analyzed data include simulation and field measurement studies. The discussion of this research clearly reflects how urban design guidelines can be applied to enhance outdoor thermal comfort and minimize the heat island effect. This study is helpful in controlling the consequences of city design from the early design stage.

Review on the impact of urban geometry and pedestrian level greening on outdoor thermal comfort

The cooling effect of green spaces as a contribution to the mitigation of urban heat: A case study in Lisbon

Rainfall is the most important physical process responsible for the landslide triggering in Portugal. Results obtained worldwide have shown that control of rainfall on landslides differs substantially depending upon landslide depth and kinematics and the affected material. Therefore, the critical rainfall conditions for failure are not the same for different types of landslides, and may be strongly influenced by regional geologic and geomorphologic conditions. Rapid debris flows are typically triggered by very intense showers concentrated in just a few hours, and shallow translational soil slips are usually triggered by intense precipitation falling within the 1–15 days long range. On the contrary, activity of deep-seated landslides of rotational, translational and complex types is related to periods of nearly constant rainfall, lasting from several weeks to several months. The different rainfall intensity–duration conditions are associated with different hydrologic mechanisms for slope failure. The generation of surface run-off and high peak discharges in first-order mountain catchments is a critical triggering mechanism for debris flows. The intense rainfall allows the rapid growth of pore water pressure and the drop of capillarity forces that sustain the apparent cohesion of thin soils. As a consequence, shallow soil slips occur within the soil material or at the contact with the underlying less permeable bedrock. Long lasting rainfall episodes enable the steady rise of the groundwater table and the development of positive pore water pressures into the soil. Consequently, deep-seated failures occur in relation to the reduction of shear strength of affected materials. In this work, we present the state of the art concerning the proposition of empirical rainfall thresholds in Portugal for different types of landslides observed in different zones of the country: the Lisbon region, the Douro Valley and the NW Mountains, and the Povoacao Municipality in Sao Miguel Island (Azores). The empirical thresholds applied in Portugal are based on the identification of past landslide events and include (i) the computation of antecedent rainfall threshold defined by linear regression, (ii) the normalization of rainfall by the mean annual precipitation, (iii) the definition of lower limit and upper limit rainfall thresholds and (iv) the definition of combined rainfall thresholds, which integrates the rainfall event and the antecedent rainfall for different time periods.

Rainfall thresholds for landslide activity in Portugal: a state of the art

. This work proposes a comprehensive method to assess rainfall thresholds for
landslide initiation using a centenary landslide database associated with a
single centenary daily rainfall data set. The method is applied to the Lisbon
region and includes the rainfall return period analysis that was used to
identify the critical rainfall combination (cumulated rainfall duration) related to each landslide event. The spatial
representativeness of the reference rain gauge is evaluated and the rainfall
thresholds are assessed and calibrated using the receiver operating
characteristic (ROC) metrics. Results show that landslide events located up to 10 km from the rain gauge
can be used to calculate the rainfall thresholds in the study area; however,
these thresholds may be used with acceptable confidence up to 50 km
from the rain gauge. The rainfall thresholds obtained using linear and
potential regression perform well in ROC metrics. However, the
intermediate thresholds based on the probability of landslide events
established in the zone between the lower-limit threshold and the upper-limit threshold are much more informative as they indicate the probability
of landslide event occurrence given rainfall exceeding the threshold. This
information can be easily included in landslide early warning systems,
especially when combined with the probability of rainfall above each
threshold.

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Regional rainfall thresholds for landslide occurrence using a centenary database

A geodatabase of landslides and other geomorphologic and hydrologic phenomena, triggered by earthquakes in Portugal, was built based on the analysis of historical records. As a result, 28 earthquake-triggered landslides were identified; the landslides were associated with 10 earthquakes that occurred between 382 and 1969. However, the landslide recognition and the definition of its precise location were difficult due to the time elapsed since the occurrence of most of the landslides and to the changes in the morphologic features of the affected slopes. Despite these limitations, it was possible to establish that most landslides were disrupted landslides that affected jointed rocks. The earthquake-triggered landslides are concentrated in the southern and central areas of Portugal, close to the main seismic sources, as opposed to the recurrent landslides induced by precipitation in the northern and central parts of the country. Other identified geomorphologic and hydrologic phenomena induced by earthquakes in mainland Portugal include: ground cracks (48 cases), liquefaction (75 cases) and water spring and fountains anomalies (262 cases). These secondary earthquake effects were reported for 5 earthquakes, but these may be considered representative only in two cases: the earthquakes of 1 November 1755 and of 23 April 1909. The distance between the site of the above-mentioned phenomena and the corresponding earthquake epicentre varies according to the earthquake magnitude and the type of phenomena. The 1755 earthquake (Mw 8.7) induced geomorphologic and hydrologic effects farther from the epicentre (maximum distance of 658 km) than the 1909 earthquake (Mw 6), whereas the effects associated with the latter were felt much closer to the earthquake epicentre (maximum distance of 172 km). The hydrologic effects brought about by the 1755 earthquake probably occurred throughout the country (distance of at least 658 km from epicentre) in contrast to ground cracks (526 km) and liquefaction (504 km).

Landslides and other geomorphologic and hydrologic effects induced by earthquakes in Portugal

Green urban areas are a potential touristic attraction, providing opportunitiesfor recreation and social interaction. In this study, two green areas in Lisbon wereselectedto investigate their influence on the thermal environment in and outside them andto analyse the usage of these parks in view of the perception of thermal comfort, the preferencesand personal characteristics of the visitors. Measurements of temperature and relativehumiditywere carried out for several successive weeks in summer and in winter, indifferentlocations around and inside the green areas. Temperature differences between thegreen spaces and the surrounding areas were greater in hotter conditions and reached up to7oC difference at certain times on particular days. This cooling effect provided by greenareas has a potential role in mitigating climate change impacts. In order to analyse theperception of thermal comfort and users’ preferences, face-to-face interviews were carriedout simultaneously with measurements of weather parameters. In summer, a positive correlationbetweenthe measuredvalues of temperature and relative humidity with people’sperception was observed. A significant association between individual’s characteristicsand type of usage of the parks was also revealed, particularly related to age and gender.Understanding the relationship between weather parameters, people’s perception and visitors’preferences is paramount to create green areas more attractive and beneficial for theurban environment.

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Teresa Vaz

Papers

The cooling effect of green spaces as a contribution to the mitigation of urban heat: A case study in Lisbon

Rainfall thresholds for landslide activity in Portugal: a state of the art

Regional rainfall thresholds for landslide occurrence using a centenary database

Landslides and other geomorphologic and hydrologic effects induced by earthquakes in Portugal

Perception of thermal comfort by users of urban green areas in Lisbon