Runoff Retention Research Articles

Analysis of the effect of green roof substrate amended with biochar on water quality and quantity of rainfall runoff.

Green roofs are becoming a popular ecological alternative in urban areas worldwide. In this study, we constructed two modular green roofs (commercial substrate green roof and biochar substrate green roof) and analyzed the effects that the green roof substrate amended with biochar on the runoff retention capacity, water quality, pollutants releasing characteristic, and pollution load by simulating rainfall experiment (rainfall levels 10~80mm). Results showed that the mean retention ratio was no significant differences between the commercial substrate (72.54%) and the biochar substrate (72.08%). Both the two kinds of substrates showed that the concentrations of total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), and iron (Fe) decreased gradually with the extension of rainfall time. Electrical conductivity (EC) and pH, as well as mean concentrations of TN, COD, TP, total suspended solids (TSS), and Fe, showed no differences between the green roof runoff of two kind of substrate. However, the neutralizing capacity of biochar substrate for the pH of green roof runoff was stronger than the commercial substrate, and the mean concentration of the TN and COD in the commercial substrate (16.14mg/L and 171.79mg/L, respectively) was about two times higher than the biochar substrate (9.85mg/L and 97.31mg/L, respectively). Similarly, the pollution load of TN and COD in the commercial substrate was significantly higher than that in the biochar substrate. Therefore, the biochar substrate could effectively reduce the pollution load of TN and COD in the runoff of green roof. Consequently, we suggest that the biochar could be applied to green roof substrates in order to reduce the impact of city non-point pollution on receiving water bodies.

The future orientation of Austria’s flood policies: from flood control to anticipatory flood risk management

This paper analyses the future orientation of flood risk management in Austria. Framed by a systematic review of risk reduction measures, we assess the extent to which Austrian policies are characteristic of an anticipatory, forward-looking flood management approach to cope with expected future stresses on the flood risk, such as climate change and land development. The analysis shows that risk reduction measures build on status quo assessments and do not explicitly consider future changes in the flood hazard or vulnerability. However, new design standards for extreme events, the proliferation of large-scale flood retention, tightened land development and building restrictions, and novel planning instruments indicate that Austrian policies are increasingly forward-looking to more effectively mitigate future increases in the flood risk. This temporal reorientation is a consequence of a wider policy shift from flood control to integrated flood risk management. Faced with the increasing spatial interplay of risk reduction measures (e.g. securing land for flood runoff and flood retention), our analysis shows that inter-sectoral cooperation between water management and spatial planning can support long-term flood management decisions and maintain the ability to adapt to changing future conditions.

Regional differences in the soil and water conservation efficiency of conservation tillage in China

The benefits of conservation tillage on water erosion control have been extensively tested in China, while few studies have involved their regional differences. We synthesized 81 recent publications about the water erosion control by conservation tillage in China, to explore the regional difference of conservation tillage and identify the optimal tillage methods of each region. The benefits of conservation tillage for runoff retention and sediment reduction are different in different regions due to the special environmental condition of each region. Conservation tillage demonstrated the lowest efficiency in the Northwest China Loess Plateau region and highest in the North China mountainous region, while no significant differences were found among other 4 regions. In the Northeast China black soil region, micro-basins tillage (MBT) is the optimal tillage method due to its greatest benefits in reducing runoff and sediment. In the North China mountainous region, the values of regional efficiency evaluation index (REEI) of runoff and sediment were both larger than 1 only by contour tillage with hedgerow (CTH), thus CTH will be the optimal tillage method in this region. In the Northwest China Loess Plateau region, some engineering measures should be the priority due to the limited benefits of conservation tillage. In the South China red soil region, no tillage with mulch (NTM) presented the greatest benefits on water erosion control and largest values of runoff and sediment REEI, thus CTH is the priority in this region. In the Southwest China purple soil region, though the values of runoff and sediment REEI by collecting soil to form ridges with no tillage (CSNT) are less than those by MBT, CSNT is the most suitable tillage method in this region. The small sample size leads to the unreliable results in the Southwest China karst region, thus more researches are needed in this region.

A laboratory study to determine the use of polluted river sediment as a substrate for extensive green roofs

In this study, laboratory-scale green (e.g. living) roof platforms were established to assess the potential use of polluted river sediment in their substrate mixture. The mean runoff retention of the green roof platforms, which contained peat and/or river sediment, after 11 artificial rainfall events was >72%, significantly higher than traditional roofs. However, green roof platforms that had been filled with peat soil showed chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) leaching. Green roofs that had used river sediment showed good leaching control for COD, TN and TP. The cumulative leaching masses from the green roofs contained 30% (COD), 42% (TN) and 47% (TP) as much as the total leaching mass from traditional roofs, and the Cu, Zn, Cd and Pb leaching risk from green roofs when river sediments are used as part of a substrate mixture was relatively low. Despite some nutrient leaching in the initial phase of runoff from the green roofs, river sediment has the potential to be used as a substrate for extensive green roofs.

The influence of structural factors on stormwater runoff retention of extensive green roofs: new evidence from scale-based models and real experiments

Green roofs, in contrast to conventional roofs, retain significant stormwater, and are considerably appealing for urban regions to help mitigate flooding events and addressing serious environmental impacts of excessive or uncontrolled runoff. In designing a green roof, a careful selection of substrate material/depths, vegetation or slopes, or its optimal combination, is imperative but this issue remains underexplored by practicing hydrologists. Scale-based models of extensive green roofs were purposely designed and simulated rainfall experiments were conducted to comprehensively quantify runoff retention performance of extensive green roofs, to precisely determine the contribution of structural factors to stormwater retention. The results indicated that green roofs can effectively retain stormwater and delay the start of the runoff. To aver with this finding, influential contribution of four structural factors are ordered as follows: substrate material > substrate depth > slope > vegetation. This finding denotes that substrate material and its depth are major contributors to a green roof’s stormwater retention capability. Statistical tests conducted on runoff retention showed the substrate materials and vegetation types had statistically significant effects, no statistically significant difference in runoff retention abilities among the three substrate depth treatments and the slope gradients considered. Given the same conditions, the study ascertains that runoff retention of low intensity events is likely to be relatively higher, while that of the heavy events is lower than its counterparts. The antecedent moisture contents of the substrate have a significant negative influence on the runoff retention and the time to the runoff.

Street tree stormwater control measures can reduce runoff but may not benefit established trees

Directing stormwater runoff to irrigate urban trees has the potential to simultaneously: (i) reduce the volume of runoff generated by impervious surfaces and (ii) reduce tree drought stress and increase growth. Many papers promote this concept, but few have quantified both potential benefits. In this study, we quantified both the runoff retention performance of infiltration trenches retrofitted alongside established street trees and tree drought stress and growth. We compared retention for different soil types, tree phenological types and inlet designs over a period of 18 months. Retention was low on average (18.3%) but was highly variable among infiltration trenches (5.2–43.7%), driven by variation in inlet capacity due to blockages, as well as the flow velocity of runoff leading to the inlet. With appropriately designed inlets, street tree stormwater control measures have significant potential to retain runoff. We observed no clear reduction in tree drought stress or increase in tree growth. This suggests that the established trees in our study already had access to adequate soil water within their rooting volumes, supplemented by nearby irrigated private gardens, or leaky water infrastructure. We suggest that i) mature trees surrounded by greater impervious surface area, ii) newly planted trees, or iii) trees growing in hotter, drier climates, may respond more positively to similar passive irrigation systems. The evolving practice of passive irrigation of street trees with stormwater requires innovative design to maximise runoff capture efficiency under typical sediment and debris loads.

Factors Affecting Runoff Retention Performance of Extensive Green Roofs

The runoff retention effectiveness of 10 extensive green roof (EGR) modules (100 mm substrate planted Sedum lineare Thunb.) were analyzed in Beijing for 22 rainfall events (2.4–46.4 mm) from 1 July to 30 September 2017. Differences between minimum inter-event dry periods, module scales, substrate hydraulic conductivity and depths, drainage layer types and rainfall characteristics were examined to study their correlation to the retention performance of EGRs. In general, EGRs with lower substrate hydraulic conductivity, deeper substrate and lower rainfall depth had higher runoff retention performance. By comparsion, no siginificant correlation was found between rainfall duration, prior dry period, average rainfall intensity, drainage layer type and EGR runoff retention rate. Analyses of variance (ANOVA) and Tukey tests supported these results. Low or moderate rainfall (<15 mm) may or may not have an effect, but heavy rainfall (>25 mm) definitely affects the EGR retention performance of the next rainfall event.

Tree pits to help mitigate runoff in dense urban areas

Tree pits are attractive stormwater control measures (SCMs) for implementation in dense urban areas because of their small footprint, their potentially low cost and the co-benefits they may bring through improved street tree growth. While they provide street trees with passive irrigation, it remains to be determined if tree pits may achieve meaningful reductions in stormwater runoff. We undertook a streetscape experiment to quantify runoff retention of tree pits in a heavy clay soil with low-rates of exfiltration. We calibrated and validated a water balance model using the field experiment data to identify tree pit characteristics driving runoff retention performance. We then applied the model to different implementation scenarios to ascertain how useful these tree pits may be at reducing runoff to return a more natural flow regime in dense urban areas. The main drivers of runoff retention were identified as exfiltration rates from the tree pits and the connected impervious catchment size. Our results show that it is possible, even in dense urban streetscapes with low conductivity (heavy-clay) soils, to achieve a 90% reduction in annual runoff and to reduce days of runoff to just 15 days per year. However, to achieve this, tree pits need to be sized between 2.5% and 8% of the impervious catchment area, depending on pit exfiltration rates. In practice, achieving these tree pit to catchment area ratios for a dense urban streetscape will require consideration in the planning stages of development works and is expected to be most feasible through implementation of tree pits alongside a suite of other complementary SCMs.

Stormwater volume reduction and water quality improvement by bioretention: Potentials and challenges for water security in a subtropical catchment

Climate change scenarios tend to intensify extreme rainfall events and drought in Brazil threatening urban water security. Low Impact Development (LID) practices are decentralized alternatives for flood mitigation and prevention. Recently, their potential has increasingly been studied in terms of stormwater harvesting. However, there is still a lack of knowledge about their potentialities in subtropical climate regions. Therefore, this study evaluated the behavior of a bioretention cell in a Brazilian city, during the dry period, which is critical in terms of pollutant accumulation and water availability. In addition to the runoff reduction and pollutant removal efficiency, this paper analyzed the potential for water reuse in terms of the stored volume and water quality guidelines. The results obtained show an average runoff retention efficiency of 70%. Considering only the water availability aspects, the potential stored runoff could be reused for non-potable purposes, reducing the water demand in the catchment by at least half during the dry season. On the other hand, the bioretention presented two different conditions for pollutant removal: Condition A – the concentration values are within the recommended limits for water reuse. The parameters found in this condition were NO3, NO2, Zn, Mn, Cu, Cr; Condition B – the pollutant concentrations are above the guideline limits for water reuse and cannot be directly used for different purposes. The parameters found in this condition were Fe, Pb, Ni, Cd and color. Considering water reuse, an additional treatment is required for parameters in this second condition. Further studies should evaluate the design aspects that can allow collection of LIDs effluent, additional treatment if necessary, and reuse in the catchment.

Tree water-use strategies to improve stormwater retention performance of biofiltration systems

Biofiltration systems are highly valued in urban landscapes as they remove pollutants from stormwater runoff whilst contributing to a reduction in runoff volumes. Integrating trees in biofilters may improve their runoff retention performance, as trees have greater transpiration than commonly used sedge or herb species. High transpiration rates will rapidly deplete retained water, creating storage capacity prior to the next runoff event. However, a tree with high transpiration rates in a biofilter system will likely be frequently exposed to drought stress. Selecting appropriate tree species therefore requires an understanding of how different trees use water and how they respond to substrate drying. We selected 20 tree species and quantified evapotranspiration (ET) and drought stress (leaf water potential; Ψ) in relation to substrate water content. To compare species, we developed metrics which describe: (i) maximum rates of ET under well-watered conditions, (ii) the sensitivity of ET and (iii) the response of Ψ to declining substrate water content. Using these three metrics, we classified species into three groups: risky, balanced or conservative. Risky and balanced species showed high maximum ET, whereas conservative species always had low ET. As substrates dried, the balanced species down-regulated ET to delay the onset of drought stress; whereas risky species did not. Therefore, balanced species with high ET are more likely to improve the retention performance of biofiltration systems without introducing significant drought risk. This classification of tree water use strategies can be easily integrated into water balance models and improve tree species selection for biofiltration systems.

Investigating the Plant Species and Rainfall Factors on Stormwater Retention Performance of Extensive Green Roofs in Malaysia

Information on the influences of native plant species and rainfall characteristics on the stormwater retention performance of extensive green roofs in Malaysia is still scarce. More local data is still required because rainfall and runoff generation processes in tropical environment are very different from the temperate regions. This study is aimed to investigate the stormwater retention performance of native plant species in extensive green roofs with respect to different rainfall characteristics in Malaysia. Two test beds consist of pro-mixing potting soil were vegetated each with different native plant species (Axonopus Compressus (cow grass), Portulaca Grandiflora (sedum) and one test bed with no vegetation (bare ground) was prepared as a control. A total of 22 significant storm events were collected over a 3-month period from March 2016 to May 2016. The rainfall depths for monitored storm events were ranged from 4.5 mm to 63.2 mm and rainfall intensities ranged from 0.5mm/hr to 58.5 mm/hr. The results showed that sedum exhibited higher mean runoff retention percentage than grass which is 75.8% and 70.9%, respectively. Bare soil was the least effective for reducing water runoff with retention percentage of 62.6%. Overall, sedum and grass species provided 90.6% and 88.2% of cumulative rainfall retention in this study. The rainfall depth and intensity are correlated negatively with stormwater retention performance of green roofs. Meanwhile, longer dry weather period are likely to increase the water retention capacity of green roof.

Assessing regulating ecosystem services provided by the Ege University Rectorship Garden

Urban ecosystem services are generated in a diverse set of natural and managed urban green areas, including parks, urban forests, cemeteries, vegetated corridors, vacant lots, gardens, yards, and campus areas. Private gardens are generally undervalued for the ecosystem services they provide along with the other urban green areas.This paper aims to calculate three regulating ecosystem services; runoff retention, carbon storage and sequestration generated by the Ege University Rectorship Garden, which is one of the few former Levantine gardens remaining in the highly urbanized Bornova district in İzmir. The carbon storage and sequestration capacity of the trees in the area was calculated based on allometric equations. Runoff retention was computed by using the SCS-CN method. Findings show that pervious surfaces cover approximately two-thirds of the garden with 1203 trees. The estimated carbon storage of both the above and below-ground parts of the trees in the garden is 648.25 t. The total annual carbon sequestration rate is estimated to be 7.87 t year−1 (0.10 kg m−2). The potential storm water runoff value was predicted to be approximately 7,018.9 m3. This indicates that the garden has a high value of runoff retention and substantial capacity carbon storage and sequestration.It can be concluded that private gardens and associated ecosystem services in urban landscapes can play an important role in enhancing the quality of life in cities. Therefore, an integral approach is needed where all types of green areas are planned and managed in a systematic way, so that they can provide maximum services.

Evaluating the Capability of Grass Swale for the Rainfall Runoff Reduction from an Urban Parking Lot, Seoul, Korea.

This field study elaborates the role of grass swale in the management of stormwater in an urban parking lot. Grass swale was constructed by using different vegetations and local soil media in the parking lot of Mapu-gu Seoul, Korea. In this study, rainfall runoff was first retained in soil and the vegetation layers of the grass swale, and then infiltrated rainwater was collected with the help of underground perforated pipe, and passed to an underground storage trench. In this way, grass swale detained a large amount of rainwater for a longer period of time and delayed peak discharge. In this field study, various real storm events were monitored and the research results were analyzed to evaluate the performance of grass swale for managing rainfall runoff in an urban area. From the analysis of field experiments, grass swale showed the significant rainfall runoff retention in different rain events. Grass swale markedly reduced total rainfall runoff volume and peak flow during the small storm events of intensity about 30 mm/h. From the analysis, on average rainfall runoff retention from the grass swale was found around 40 to 75% during the various small rain events. From the results, we can say that grass swale is a stormwater mitigation practice which can help avoid flash flooding problems in urban areas.

Green Roof for Stormwater Management in a Highly Urbanized Area: The Case of Seoul, Korea

Urbanization changes natural pervious surfaces to hard, impervious surfaces such as roads, buildings and roofs. These modifications significantly affect the natural hydrologic cycle by increasing stormwater runoff rates and volume. Under these circumstances, green roofs offer multiple benefits including on-site stormwater management that mimics the natural hydrologic conditions in an urban area. It can retain a large amount of rainwater for a longer time and delay the peak discharge. However, there is very limited research that has been carried out on the retrofitted green roof for stormwater management for South Korean conditions. This study has investigated the performance of retrofitted green roofs for stormwater management in a highly urbanized area of Seoul, the capital city of Korea. In this study, various storm events were monitored and the research results were analyzed to check the performance of the green roof with controlling the runoff in urban areas. Results also allowed us to conclude that the retention mainly depends on the intensity and duration of the rain events. From the analysis, average runoff retention on the green roof was 10% to 60% in different rain events. The application of an extensive green roof provides promising results for stormwater management in the highly urbanized area of Seoul.

Evaluation of a Residential Green Roof System for Stormwater Runoff Retention

Evaluation of a Residential Green Roof System for Stormwater Runoff Retention

The effect of green roof configurations on runoff retention performance

The effect of green roof configurations on runoff retention performance

Evaluation of turfgrid porous pavements for urban stormwater management in Xiamen Island, Southeast China

This paper has investigated the runoff retention and pollution removal effectiveness of a frequently used parking area paved with porous turfgrid in Xiamen Island, southeast China. The results have shown that a turfgrid porous paving area of 72 m2 could delay the start time of the runoff generated on it by up to 30 min when rainfall intensity is less than 3.88 mm/h. Although the retention of total suspended solids is only 13.86%, the grass-paved surface can effectively reduce and remove pollutants from the runoff, with the removal of ammonia and NO3 −-N being significant (at 39 and 76.55%, respectively). In addition, high reductions (>60%) in concentration were found for Zn, Cu, and Pb. However, it was found that pollutant concentrations in the infiltration water samples analysed are higher than in the runoff, which suggests the possible occurrence of pollution accumulation as an emerging pollution source. Furthermore, in 2015 no grass was observed in the pores of the pavement, which suggests that a poor maintenance strategy is in place; with this resulting in the development of a potentially high and concentrated source for pollutants.

Sensitivity, stability and future evolution of the world's northernmost ice cap, Hans Tausen Iskappe (Greenland)

Abstract. In this study the dynamics and sensitivity of Hans Tausen Iskappe (western Peary Land, Greenland) to climatic forcing is investigated with a coupled ice flow–mass balance model. The surface mass balance (SMB) is calculated from a precipitation field obtained from the Regional Atmospheric Climate Model (RACMO2.3), while runoff is calculated from a positive-degree-day runoff–retention model. For the ice flow a 3-D higher-order thermomechanical model is used, which is run at a 250 m resolution. A higher-order solution is needed to accurately represent the ice flow in the outlet glaciers. Under 1961–1990 climatic conditions a steady-state ice cap is obtained that is overall similar in geometry to the present-day ice cap. Ice thickness, temperature and flow velocity in the interior agree well with observations. For the outlet glaciers a reasonable agreement with temperature and ice thickness measurements can be obtained with an additional heat source related to infiltrating meltwater. The simulations indicate that the SMB–elevation feedback has a major effect on the ice cap response time and stability. This causes the southern part of the ice cap to be extremely sensitive to a change in climatic conditions and leads to thresholds in the ice cap evolution. Under constant 2005–2014 climatic conditions the entire southern part of the ice cap cannot be sustained, and the ice cap loses about 80 % of its present-day volume. The projected loss of surrounding permanent sea ice and resultant precipitation increase may attenuate the future mass loss but will be insufficient to preserve the present-day ice cap for most scenarios. In a warmer and wetter climate the ice margin will retreat, while the interior is projected to thicken, leading to a steeper ice cap, in line with the present-day observed trends. For intermediate- (+4 °C) and high- warming scenarios (+8 °C) the ice cap is projected to disappear around AD 2400 and 2200 respectively, almost independent of the projected precipitation regime and the simulated present-day geometry.

An Evaluation of the Urban Storm Water Runoff Pollutant Removal Efficiency with a Focus on the Retention Time

In urban areas, during heavy rain events various pollutants are flushed off impervious surfaces with storm water runoff and through the storm water drainage system directly discharged to surface water bodies. Commonly, storm water runoff sedimentation tanks are used to remove suspended solids and total petroleum hydrocarbons from the first portions of storm water runoff. Unfortunately, the operation of especially storm water sedimentation tanks is still quite complicated and not very effective. The regulation of outflow rate can be one of the ways to increase the effectiveness of pollutants removal using sedimentation tank technology. The research was performed at the storm water runoff treatment unit based on the sedimentation process. The aim of the study was to investigate the removal of suspended solids (SS) and total petroleum hydrocarbons (TPH) at the storm water runoff sedimentation tank with a focus on the retention time. Initially, the distribution of concentration of SS and TPH in various places of sedimentation tank after the rain event was investigated. The obtained results showed that the concentrations of SS and TPH are very similar all over the tank after a rain event. Therefore, it is more reasonable to take one composite sample instead several grab samples during the research of sedimentation process efficiency. According to the obtained SS and TPH removal efficiency results it was determined that optimal storm water runoff retention time in the researched sedimentation tank is 48 hour, when the inflow and outflow of sedimentation tank are closed.

Effects of substrate types on runoff retention performance within the extensive green roofs

Effects of substrate types on runoff retention performance within the extensive green roofs