Alain Mailhot

Design Criteria of Urban Drainage Infrastructures under Climate Change

Actual projections provided by climate models suggest that the probability of occurrence of intense rainfall will increase in a future climate due to increasing concentrations of greenhouse gases. Considering that the design of urban drainage systems is based on statistical analysis of past events, an increase in the intensity and frequency of extreme rainfall events will most probably result in more frequent flooding. The design criteria must therefore be revised to take into consideration possible changes induced by climate change. A procedure is proposed to revise the design criteria of urban drainage infrastructures. This procedure integrates information about (1) climate projections for extreme rainfall over the region under consideration; (2) expected level of performance (or acceptable level of risk); and (3) expected lifetime of the infrastructure/system. The resulting design criterion ensures that the service level remains above the selected “acceptable” level over a predefined portion of the inf...

A Bayesian perspective on input uncertainty in model calibration: Application to hydrological model “abc”

[1] The impact of input errors in the calibration of watershed models is a recurrent theme in the water science literature. It is now acknowledged that hydrological models are sensitive to errors in the measures of precipitation and that those errors bias the model parameters estimated via the standard least squares (SLS) approach. This paper presents a Bayesian uncertainty framework allowing one to account for input, output, and structural (model) uncertainties in the calibration of a model. Using this framework, we study the impact of input uncertainty on the parameters of the hydrological model ‘‘abc.’’ Mostly of academic interest, the ‘‘abc’’ model has a response linear to its input, allowing the closed form integration of nuisance variables under proper assumptions. Using those analytical solutions to compute the posterior density of the model parameters, some interesting observations can be made about their sensitivity to input errors. We provide an explanation for the bias identified in the SLS approach and show that in the input error context the prior on the input ‘‘true’’ value has a significant influence on the parameters’ posterior density. Overall, the parameters obtained from the Bayesian method are more accurate, and the uncertainty over them is more realistic than with SLS. This method, however, is specific to linear models, while most hydrological models display strong nonlinearities. Further research is thus needed to demonstrate the applicability of the uncertainty framework to commonly used hydrological models.

Calibration of hydrological model GR2M using Bayesian uncertainty analysis

[1] An outstanding issue of hydrological modeling is the adequate treatment of uncertainties in model calibration and prediction. The current paradigm is that the major sources of uncertainties, namely input, output and model uncertainty should be accounted for directly, instead of assuming they can be safely lumped into the output uncertainties. In this paper, Bayesian analysis is used to calibrate the conceptual hydrologic monthly model GR2M taking into account input, output, structural and initial state uncertainty through error models and priors. Calibration is performed under different error assumptions to study the influence of the initial state uncertainty, the consequences of large input errors, the impact of error assumptions on calibrated parameter posterior distributions and the definition of error models. It is shown how such an analysis can be used to separate, a posteriori, the different sources of errors, and in particular, to identify structural errors from data errors.

Modeling the evolution of the structural state of water pipe networks with brief recorded pipe break histories: Methodology and application

Water managers are concerned about the poor structural state of their water pipe networks and are confronted with the lack of tools to analyze their data in order to evaluate both the present and future state of their networks. Researchers have developed statistical models with the annual number of pipe breaks as an indicator of the structural state of their networks. Unfortunately, most statistical modeling strategies do not perform well when used in municipalities with short recorded pipe break histories. Pipe breaks must have been recorded since the installation of the first pipes, and this is rarely the case in most municipalities. This paper presents a methodology to estimate calibration parameters of statistical models in municipalities with short recorded pipe break histories. The application of this methodology to a municipality with a century-old water pipe network and a recorded pipe break history of 21 years is also presented.

GIBSI — An integrated modelling system prototype for river basin management

This paper introduces GIBSI, an integrated modelling system prototype designed to assist decision makers in their assessment of various river basin management scenarios in terms of standard water physical and chemical parameters and standards for various uses of the water. GIBSI runs on a personal computer and provides a user-friendly framework to examine the impacts of agricultural, industrial, and municipal management scenarios on water quality and yield. A database (including spatial and attribute data) and physically-based hydrological, soil erosion, agricultural-chemical transport and water quality models comprise the basic components of the system. A geographical information system and a relational database management system are also included for data management and system maintenance. This paper illustrates potential uses of GIBSI by presenting two sample applications applied to a 6680 km2 complex river basin (63.2% forest, 17.2% agricultural land, 15.3% bush, 3.1% urban development and 1.2% surface water; population: 180 000) located in Québec, Canada: (i) a timber harvest scenario and (ii) a municipal clean water program scenario. Simulation results of the timber harvest scenario showed how clear-cut activities could lead to earlier and larger spring runoff than in the investigated reference state. Results of the municipal clean water scenario revealed that substantial reduction in coliform counts and total phosphorus could be made by constructing and operating wastewater treatment plants. These applications provide a framework of how GIBSI can be used as a decision support system and ultimately as a means of incorporating sound science into political decisions involving river basin management.

Relationship between Surface Temperature and Extreme Rainfalls: A Multi-Time-Scale and Event-Based Analysis*

AbstractRecent studies have examined the relationship between the intensity of extreme rainfall and temperature. Two main reasons justify this interest. First, the moisture-holding capacity of the atmosphere is governed by the Clausius–Clapeyron (CC) equation. Second, the temperature dependence of extreme-intensity rainfalls should follow a similar relationship assuming relative humidity remains constant and extreme rainfalls are driven by the actual water content of the atmosphere. The relationship between extreme rainfall intensity and air temperature (Pextr–Ta) was assessed by analyzing maximum daily rainfall intensities for durations ranging from 5 min to 12 h for more than 100 meteorological stations across Canada. Different factors that could influence this relationship have been analyzed. It appears that the duration and the climatic region have a strong influence on this relationship. For short durations, the Pextr–Ta relationship is close to the CC scaling for coastal regions while a super-CC sca...

Evaluating rain gardens as a method to reduce the impact of sewer overflows in sources of drinking water

The implications of climate change and changing precipitation patterns need to be investigated to evaluate mitigation measures for source water protection. Potential solutions need first to be evaluated under present climate conditions to determine their utility as climate change adaptation strategies. An urban drainage network receiving both stormwater and wastewater was studied to evaluate potential solutions to reduce the impact of combined sewer overflows (CSOs) in a drinking water source. A detailed hydraulic model was applied to the drainage basin to model the implementation of best management practices at a drainage basin scale. The model was calibrated and validated with field data of CSO flows for seven events from a survey conducted in 2009 and 2010. Rain gardens were evaluated for their reduction of volumes of water entering the drainage network and of CSOs. Scenarios with different levels of implementation were considered and evaluated. Of the total impervious area within the basin directly connected to the sewer system, a maximum of 21% could be alternately directed towards rain gardens. The runoff reductions for the entire catchment ranged from 12.7% to 19.4% depending on the event considered. The maximum discharged volume reduction ranged from 13% to 62% and the maximum peak flow rate reduction ranged from 7% to 56%. Of concern is that in-sewer sediment resuspension is an important process to consider with regard to the efficacy of best management practices aimed at reducing extreme loads and concentrations. Rain gardens were less effective for large events, which are of greater importance for drinking water sources. These practices could increase peak instantaneous loads as a result of greater in-sewer resuspension during large events. Multiple interventions would be required to achieve the objectives of reducing the number, total volumes and peak contaminant loads of overflows upstream of drinking water intakes.

Information technologies in a wider perspective: integrating management functions across the urban–rural interface

Abstract Recent developments in hydroinformatics, namely watershed modeling tools, show potentials in using these information technologies to implement integrated watershed management and, thus, offer to stakeholders comprehensive decision support systems. This paper presents an overview of hydroinformatics as a means to predict impacts of various point and nonpoint (i.e., diffuse) discharges on the water quality and yield of a watershed, and discusses the practical and technical issues related to their role and use within the context of integrated watershed management. Sample applications of a watershed modeling tool, namely GIBSI, are reported to illustrate some of the capabilities of conducting studies such as: (i) establishing environmental load allocations or total maximum daily loads (i.e., the U.S. framework for managing watersheds) and (ii) evaluating pollutant trading assessment studies at the urban–rural interface. Finally, the paper discusses some of the key practical issues that arise at the interface of integrated urban water management (IUWM) and integrated water resources management (IWRM). From a watershed management point of view, this discussion suggests that IWRM should be used to evaluate the overall efficiency of various urban control strategies as well as to compare the benefits of IUWM with some other alternative solutions (e.g., best management practices).

Planning Unidirectional Flushing Operations as a Response to Drinking Water Distribution System Contamination

This paper addresses the issue of contaminated water evacuation as part of the response process in cases of intentional or accidental contamination of drinking water distribution systems (DWDSs). Following a previous study which addressed contaminant isolation, an original flushing procedure (FP) is introduced. Through a heuristic set of rules, the FP defines unidirectional flushing (UDF) strategies in a safe, structured, and efficient manner. In addition to the rather theoretical considerations related to hydraulics modeling, the FP also takes into account the operational issues of UDF implementation. Compared to other similar studies, the latter aspect establishes the main contribution of this paper. The FP has been applied to two real-world networks and application of the heuristic rules is demonstrated with the aid of a detailed example. General results suggest possible improvements to existing network configurations and to the design of projected networks to better support contamination response plan...

Trend analysis of winter rainfall over southern Québec and New Brunswick (Canada)

Abstract Winter rainfall is a non‐negligible issue for urban drainage in Canada as it can generate significant flooding, especially when it occurs at the same time as high air temperature and in the presence of an appreciable snow cover. According to climate change scenarios, it is expected that the occurrence of these events will increase in a future climate. The purpose of this paper is to perform a trend analysis on six indices related to winter rainfall (January–February) at 60 weather stations located in southern Québec and New Brunswick (Canada) in order to detect possible trends in the frequency or intensity of winter rainfall events during the twentieth century. Datasets were provided by Environment Canada and come from the Canadian Daily Rehabilitated Precipitation Database. The bootstrap‐based Mann‐Kendall test is used to detect possible non‐stationarities in the dataset, while Sens slope estimator is used to quantify the magnitude of the slope. Results show that 19 stations out of 60 present a significant trend (18 of them being positive) at a 5% level for winter (January–February) total rainfall. In most cases where a trend was detected for winter rainfall there was also an increase in the number of days with rainfall (42% of the stations). These results suggest that globally, for the region under study, rainfall during January and February was more likely to occur, often resulting in a significant increase in the total rainfall during these months. Increasing trends in maximum daily rainfall during January and February were also observed for 9 stations (15% of the stations). The spatial distribution of stations where significant trends were detected is consistent with the hypothesis that trends in winter rainfall are more likely to be observed for stations located in the southern part of the region under study.