Stormwater Management Performance of Green Roofs

Green roofs are gaining recognition in North America as effective tools for managing stormwater runoff in urban areas. A greater understanding of how green roofs perform with respect to fundamental stormwater management criteria, such as stormwater retention and peak flow attenuation is required. This study investigated the impact that changing climates have on the retention performance of three green roofs in London, Calgary, and Halifax. It was found that Calgary (67%) has significantly better retention performance then both London (48%) and Halifax (34%). However, London retained the greatest volume of stormwater (758 mm), followed by Halifax (517 mm) then Calgary (474 mm). Further monitoring of the hydrologic response for a fourth green roof in London Ontario was conducted to identify and measure the fundamental processes of peak attenuation on a green roof. It was determined that field capacity is a quantifiable point, after which peak attenuation performance significantly decreases. Before field capacity peak attenuation is governed by capillary storage (72%) and routing (7%). After field capacity, gravity storage provides peak attenuation (22%) and drainage routing plays a larger role (11%). A predictive model was developed using Richards equation to simulate the outflow hydrographs of a green roof. Model results show that there is no significant difference from observed data for the performance metrics (ie., water storage, drainage, and peak flow rate). For the first time in green roof literature the impact of climate on retention was assessed, the processes of peak attenuation were quantified, and an accurate predictive model was presented

Bioretention Cell Performance Under Shifting Precipitation Patterns Across the Contiguous United States

As climate change produces shifts in precipitation patterns, communities will need to understand how the performance of green stormwater infrastructure (GSI) may be impacted. Bioretention cells are one of the most commonly implemented forms of GSI for their ability to reduce peak discharge and filter pollutants and are a vulnerable component of stormwater infrastructure. Projections in future climate indicate that bioretention cells may be at risk of losing their existing function due to deviations in precipitation frequency and intensity. General circulation models (GCMs) downscaled to regional climate models (RCMs) can provide climate change projections at a high spatial resolution but often have a degree of bias introduced during the downscaling process. As such, an ensemble of 10 regional climate models and 17 locations across the contiguous United States were evaluated to provide the widest range of potential future outcomes. Bioretention cells were modeled using USEPA’s Storm Water Management Model (SWMM) to compare observed and future performances. Observed climate data from 1999 to 2013 were gathered from NOAA’s National Centers for Environmental Information data archive, and simulated future climate data from 2035 to 2049 were gathered from the North American Coordinated Regional Downscaling Experiment data archive. To reduce model bias, simulated future climate data was bias-corrected using the kernel density distribution mapping (KDDM) technique. Median annual rainfall and 99th percentile rainfall event depths were projected to increase across all 17 locations while median drying period was projected to decrease for 11 locations, indicating fewer events with higher magnitudes of rainfall for a majority of locations. Correspondingly, bioretention cell performance decreased across all 17 locations. Relative percent changes in infiltration loss decreased between 4.0-24.0% across all 17 locations while overflow increased between 0.4-19.6% for 15 locations. Results suggest that bioretention cells in the southern United States are at significant risk of losing their existing function while those in the Midwest and Northeast are at moderate risk. Bioretention cells in the western and northwestern United States performed the best under future climate scenarios but could still lose their existing function if unchanged. Most, if not all, bioretention cells across the contiguous United States will, therefore, require some degree of modification to maintain their existing function in the future. This study provides insight on future regional bioretention cell performance trends that can be used to add resiliency to stormwater infrastructure

CLIMATE CHANGE IMPACT ON URBAN STORMWATER SYSTEM AND USE OF GREEN INFRASTRUCTURE FOR ADAPTATION: AN INVESTIGATION ON TECHNOLOGY, POLICY, AND GOVERNANCE

The world is urbanizing at an unprecedented rate, and cities are dominantly and increasingly becoming hubs for agglomerations of human population and economic activities, as well as major sources of environmental problems. Accordingly, humanity′s pursuit of global sustainability is becoming increasingly reliant on urban sustainability. Unfortunately, the traditional approaches of urbanization and urban stormwater management are inappropriate from the sustainability standpoint. By removing vegetation and topsoil and creating impervious structures, urbanization destroys natural biodiversity and hydrological processes. As a result, urban societies are disconnected from nature and deprived of ecosystem services including flood control, fresh air, clean water, and natural beauty. Due to disrupted hydrology, an urban landscape transforms most rainwater into stormwater runoff which is conveyed off the site through a system of curb-gutter-pipe, called gray infrastructure. While gray infrastructure efficiently mitigates the problem of flash floods in urban areas, it results in multiple other adverse environmental consequences such as loss of freshwater from urban landscapes, transfer of pollutants to receiving waters, and an increased potential of downstream flooding. Green infrastructure (GI) is regarded as a sound alternative that manages stormwater by revitalizing the natural processes of soil, water, and vegetation, and restoring ecosystem structures and functions. Thus, the approach re–establishes the lost socio–ecological connectivity and regenerates ecosystem services. However, despite being inevitably important for urban sustainability, and despite being the object of unrelenting expert advocacy for more than two decades, the approach is yet to become a mainstream practice. To widely implement GI, cities need to address two critical challenges. First, urban stormwater managers and decision makers should be ensured that the approach can adequately and reliably manage stormwater. In the time when flooding problems are rising due to climate change, this concern has become more prominent. Second, if there exist any other barriers, they should be replaced with strategies that help expedite the use of GI. This multidisciplinary research dealt with these two challenges. The study consisted of two major parts. In the first part, a computer model was developed for a combined sewer system of St. Louis, a city in the US state of Missouri, using U.S. EPA SWMM. Simulations for historical (1971-2000) and future (2041-2070) 50-yr 3-hr rainfall scenarios were then run on the model with and without GI. The simulation results showed a significant impact of increased precipitation on the system, which was considerably reduced after adding select GI measures to the modeled system. The following 4 types of GI were used: bio–retention cell, permeable pavement, green roof, and rain barrel. In the second part, a survey of relevant policies and governance mechanisms of eleven U.S. cities was conducted to identify potential barriers to GI and determine strategies to address them. The study also included the assessment of relevant city, state, and federal policies and governance structures. A total of 29 barriers were identified, which were grouped into 5 categories. Most of the identified barriers stem from cognitive barriers and socio–institutional arrangements. A total of 33 policies, also grouped into 5 groups, were determined to address the barriers. The investigation on governance revealed that current governance is highly technocratic and centralized, and hence has less opportunity for public involvement. Therefore, it is inherently inappropriate for GI, which requires extensive public involvement. This dissertation proposes a two–tier governance model suitable for implementing GI

Kentucky Water Resources Research Institute Annual Technical Report FY 2016

The 2016 Annual Technical Report for Kentucky consolidates reporting requirements for the Section 104(b) base grant award into a single document that includes: 1) a synopsis of each student research enhancement project that was conducted during the period, 2) citations for related publications, reports, and presentations, 3) a description of information transfer activities, 4) a summary of student support during the period, and 5) notable awards and achievements. No funds were requested for general program administration activities. However, travel funds were provided to support the participation of the director and associate director in the annual meeting of the National Institutes for Water Resources in Washington, DC from February 27 - March 1, 2017

Understanding sewer infiltration and inflow using impulse response functions derived from physics-based models

Infiltration and inflow (I&I) are extraneous flow in a sanitary sewer system that originate from surface water and ground water. I&I can overload the sewer system and wastewater treatment plants, and cause sanitary sewer overflows (SSOs) or basement flooding. This flow can account for as much as ten times dry weather flow (DWF) but the estimation of the volume and peak of I&I involves a great deal of uncertainty. Temporal and spatial variability of the I&I processes make it difficult to understand the phenomena. Depending on the time scale of different I&I processes, some watershed properties may only affect specific I&I sources. For example, the configuration of sewer network and the geology of the watershed may affect fast and slow I&I processes differently. In this study, the physical process of three major I&I sources: roof downspout, sump pump, and leaky lateral, are investigated at the residential lot scale using physics-based models. The typical flow response of each I&I source is calculated and these flow responses, called Impulse Response Functions (IRFs), are evaluated. I&I estimation using the three IRFs, calibrated using a genetic algorithm (GA), was performed on a catchment in the Chicago area at the sewershed scale. Results are compared with one of the most widely used I&I estimation methods, the RTK method. The IRF method shows more stable solutions as the model is based on physical processes. The RTK method better predicts the monitoring data, however it is suspected that this is mainly because the RTK method is an empirical curve fitting method. Uncertainty related to rainfall induced infiltration (RII) is further investigated on six different input parameters: Antecedent moisture condition (AMC), pedotransfer functions (PTFs), soil hydraulic conductivities, initial conditions (IC), sewer pipe depths, and rainfall characteristics. The uncertainty analysis indicates that the model result is most sensitive to the soil hydraulic conductivity, which defines the maximum infiltration rate. Rainfall characteristics, including duration and hyetograph shape turn out to be the least influential factors affecting the infiltration response. Results from this study help understand the sewer I&I process in a complex urban system. In particular, using a small scale, detailed distributed model enables examination of the sensitivity of the I&I process to the different factors contributing to uncertainty. While the modeling results are site specific to Hickory Hills, IL, this study can provide insights to researchers and engineers about characteristic behaviors of different I&I sources and the uncertainty factors that affect sewer infiltration response including AMC

Extreme Floods and Droughts under Future Climate Scenarios

Hydroclimatic extremes, such as floods and droughts, affect aspects of our lives and the environment including energy, hydropower, agriculture, transportation, urban life, and human health and safety. Climate studies indicate that the risk of increased flooding and/or more severe droughts will be higher in the future than today, causing increased fatalities, environmental degradation, and economic losses. Using a suite of innovative approaches this book quantifies the changes in projected hydroclimatic extremes and illustrates their impacts in several locations in North America, Asia, and Europe