Sizing of Stormwater Control Measures for Resiliency - The Four Rs

In order for stormwater control measures (SCMs) to achieve the required removal they must be sized properly. In many cases sizing involves literally choosing the physical size of the SCM. Sizing an SCM is complicated and there is no standard process. This makes it difficult for stormwater managers to know if the size they have chosen is appropriate. The effects of climate change further increase the uncertainty. The goal of this presentation is to provide stormwater managers with an overview of existing sizing methods so that they can be sure the method they are using is state of the art. It will also present recent thinking on adapting the process to improve resiliency. Stormwater control measures (SCMs) have two basic mechanisms: gravity settling and filtration. These two mechanisms operate differently but they are both less effective at higher flow rates. This means that detailed sizing depends on the specific product or practice but the general sizing process is the same regardless of the SCM. Sizing SCMs is fundamentally a three-part problem: 1.Determine how much rainfall needs to be treated, particularly the maximum expected intensity 2.Determine how much flow will be generated by the expected rainfall 3.Determine how much removal can be expected for the given flows In most cases at least one of these parts, and perhaps all of them, are a black box. Assuming the boxes are functioning properly, which seems to be the case most of the time, this has not necessarily created a problem for stormwater managers in the past. Now that rainfall is known to be changing it is a problem because an unknown process cannot be modified to account for future rainfall. A system sized for today will be undersized in a few years, well before the end of its service life. This presentation will crack the lid on all three boxes and at least partially illuminate them. A detailed discussion could fill a university course but a basic understanding can be achieved in 20 minutes. Different ways to model each aspect will be discussed, along with the strengths and weaknesses of the different options. How models can be adapted to create resilient infrastructure will then be discussed. Section 1: Rainfall will address design storms, using existing rainfall data from the National Oceanic and Atmospheric Association (NOAA) Precipitation Frequency Data (PFD) Server and, finally, modelling rainfall data. Since rainfall data is typically shown as a frequency distribution (see Figure 1) fitting it requires complex mathematics. The model will be based on the generalized extreme value (GEV) family of curves. The presentation will not get into the detailed mathematics but it will describe the thought process. It will also present a basic goodness of fit test, the chi square test. Section 2: Runoff will address the rational method and the probabilistic rational method. The probabilistic rational method has the runoff coefficient, C, as a function of intensity. There is no consensus yet on what this function should be so a few options will be discussed. Section 3: Removal will discuss available removal data, particularly that published by the New Jersey Department of Environmental Protection (NJDEP). The Critical Particle Size Theory (CPST) will be introduced as a way to estimate performance of a system for different particle size distributions (PSDs) based on data from the NJDEP PSD. An example of how to use CPST will be shown. Section 4: Resilience will discuss ways to adapt sizing models to account for recognized changes in rainfall will look at building resilience through the use of climate change models and through different interpretations of NOAA rainfall data. Again there is no consensus on a correct approach, indeed there may be no 'correct' approach, so the focus will be on possible options and their implications.