Monday, April 13, 2015

First Flush Design for Stormwater Case Study Introduces Localized, Continuous Modeling Approach to Estimating Design Parameters

OHM Advisors' Murat Ulasir, PhD and City of Novi’s Timothy Kuhns (City Water and Sewer Engineer) teamed to present, First Flush Design for Stormwater, at the 28th Annual Michigan Stormwater Floodplain Association Conference, February 25-27, 2015.

That’s where they unveiled a case study involving a unique approach to estimating first flush on a site for stormwater treatment infrastructure. 

Ready? Let’s dig in.

What is first flush?

First flush is the initial runoff from a rain. In stormwater management circles, the first flush is important because that’s when the pollutants are most concentrated.

Picture a parking lot. Cars and trucks sit on the pavement, dripping engine grease and other chemicals onto the paved surface. The chemicals mostly stay on that paved surface…until it rains. Then, the rainwater washes those chemicals, along with dirt and debris to the nearest low point and eventually to your community’s storm sewers. 

State stormwater regulations require that communities evaluate and treat the first flush to remove those highest concentrations of harmful substances before they reach rivers and streams.

Why the first flush is so important

“To reduce the impacts of runoff on urban streams, the EPA expanded the Clean Water Act in 1987 to require municipalities to obtain permits for discharges of stormwater runoff. As a result, many communities have adopted regulations requiring developers to install stormwater management practices that reduce the rate and/or volume and remove pollutants from runoff generated on their development sites.” (Center for Watershed Protection)
The first flush is typically used for estimating design criteria for infrastructure to improve water quality. This means that the capital investment required for water quality enhancements is directly related to the first flush rainfall volume induced runoff volume and peak flow rates.

How does the industry calculate first flush and why can it be inadequate?

Standards are predominantly based on rainfall depth statistics and a prescribed rainfall volume.

One of the common practices in stormwater management is to calculate the first flush using what’s called the Rational Method to estimate the peak discharge rates that are used to determine treatment requirements.

However, the Rational Method has drawbacks: it assumes that the rainfall is uniformly distributed across the drainage area and evenly distributed. It doesn’t take soil moisture into account, either.

These assumptions can impact a community’s compliance costs unnecessarily. Stormwater funds are scarce and communities want to stretch them as far as possible. Overestimating the first flush pollutants means designing and building stormwater management infrastructure that’s bigger than you need.

Localized rainfall, runoff and moisture measurements
Because runoff volume and peak flows vary continuously with rainfall volume, rainfall intensity, watershed runoff characteristics, and antecedent moisture conditions, it’s more accurate to measure the rainfall and flows locally. 
That’s the approach Kuhns and Ulasir presented at the Michigan Stormwater Floodplain Association Conference: using localized continuous model to establish design flows. 

First, they used a hydrologic model calibrated to local rainfall data and local flow data. There are several continuous simulation models available:

·       Basins (HSPF)
·       IHACRES
·       H2OMetrics
·       EPA SWMM

These models use continuous modeling routines that establish the non-linear relationship between rainfall, temperature, and runoff.


Friday, April 10, 2015

Planning for Climate Change Impacts on the Environment

Climate change. What impact will it have on infrastructure? And how can we plan for the next 50 to 100 years, today?

OHM Advisors' Charles Humpriss, PE, gave a presentation at the Michigan Water Environment Association’s Watershed Seminar, Tangible Approaches to Quantify Climate Change for Infrastructure Planning and Design.

In it, he talked about a recent update to the EPA’s National Stormwater calculator, a free online modeling tool that estimates the annual amount of rainwater and frequency of runoff from a specific site based on local soil conditions, land cover, and historic rainfall records.

The new update adds a Climate Adjustment tool that lets engineers and planners evaluate the impacts of future climate change projections on water infrastructure performance. Some of the scenarios include more frequent high intensity storms and changes in evaporation rates of seasonal precipitation.


                    Median Change

The Climate Adjustment Tool also includes the ability to apply scenario projections to the near term (2020-2049) and far term (2045-2074.) showing how the different far term scenario projections affect monthly rainfall levels and extreme storm events.

Sanitary sewer analysis

Hydraulic engineers from OHM Advisors used the Climate Adjustment Tool to help the City of Ann Arbor evaluate future capacity needs for its sanitary sewer system. 

As part of a sanitary sewer wet weather evaluation study, the project team evaluated several scenarios and ran models for each, to determine the impact on the sanitary infrastructure.

In using the tool to evaluate Ann Arbor’s sanitary sewer system, the team observed that the Warm/Wet scenario results in higher average rainfall, while the Hot/Dry scenario produces slightly larger extreme storms.

The hydraulics engineers performed a Frequency Analysis using Hydrologic Model Output and 60 years of historic rain data to analyze the frequency of storms with the potential to overwhelm the City’s wastewater treatment plant.  

Ultimately, the project team, which included an advisory committee of involved citizens, elected to use a 10% increase in future peak flow rates, to account for climate change, based on the results of the modeling.

A Technical Oversight and Advisory Group, which included nationally recognized authority on climate change impacts on landscape ecology, Dan Brown, of the University of Michigan, reviewed the project team’s recommendations:

“…TOAG Members indicated that the recommendation to increase the 25 year Design Event flow rate by 10% is reasonable since this is a mid-range value which falls near the center of the climate change forecast models showing “best case” and “worst case” future conditions…”

Charles adds a reminder for those using the tool, “It is important to point out that EPA Climate Adjustment Tool provides data specific to the geographical region of interest. It may be safe to assume that other communities in Southeast Michigan should increase their future design flow rates by 10% based on the Ann Arbor results, but keep in mind that the EPA Climate Adjustment Tool projections may be very different for communities outside of Southeast Michigan.”