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Isaias: Stormwater Management Matters

Property flooded by stormwater

A few weeks back, in “When It Rains It Pours”, we took a look at some basic information on floodplains. But since then, we experienced Tropical Storm Isaias. Let’s take a deeper look at this topic and what civil engineers are doing today to prevent these outcomes.

Historical Flooding

To start, we have to look at the last major flooding events. Though we may feel like we are seeing more flooding, it is less severe. Pennsylvania has only experienced five major flooding events before this one:

The Great Johnstown Flood in 1889 | The St. Patrick’s Day Floods in 1936 | Hurricanes Connie and Diane in 1955 | Hurricane Agnes Flooding in 1972 | The Snowmelt Floods in 1996

Hurricane Agnes would then be the most recent major flooding event. Agnes dumped up to 15” of rain, but in our area most people saw around seven inches. There was widespread flooding, but the rain occurred over a four-day period. This was a 100-yr storm per NOAA Atlas 14, Volume 2, Version 3. Next would-be Hurricanes Connie and Diane in 1955. Now we are stretching everybody’s memories, so let’s take a look back at it. This was a double header of hurricanes, only separated by one week. Hurricane Connie was the same scale as Agnes. But then came Diane, who dumped between seven and nine inches of rain in a six-hour period. This event was greater than a 1,000-yr (6.06” in 6-hr) storm.

So where does Tropical Storm Isaias compare to these events? It was eight inches of rain and lasted about six hours. Thus, Isaias was actually greater than a 1,000-yr storm and on par with Diane.


The Advent of Regulations

I know a lot of you will be saying, that is well and good but “I saw flooding like I never have before.” Yes, that may be true, but do you remember Diane and Agnes? For those of us that do not, history tells us that the flood damage from those storms was greater. It also caused massive flooding in areas that did not flood during Isaias. So why is that?

In 1978, the Commonwealth of Pennsylvania passed the Storm Water Management Act (No. 167) . This was the first major stormwater focused legislation in the state. As the Act states in Section 2 (Storm Water Management Act §§167, 1978):

The General Assembly finds that:

1. Inadequate management of accelerated runoff of storm water resulting from development throughout a watershed increases flood flows and velocities, contributes to erosion and sedimentation, overtaxes the carrying capacity of streams and storm sewers, greatly increases the cost of public facilities to carry and control stormwater, undermines floodplain management and flood control efforts in downstream communities, reduces ground-water recharge, and threatens public health and safety.

2. A comprehensive program of stormwater management, including reasonable regulation of development and activities causing accelerated runoff, is fundamental to the public health, safety and welfare and the protection of the people of the Commonwealth, their resources and the environment.


Stormwater Management Today

In the years since the passing of Act 167, we have seen a developing set of regulations. Until the late 1990s, stormwater designs focused on conveying the flow of stormwater. Then came the need to detain the flow of larger events. Then engineers started designing detention basins, with low flow channels. Finally, came the government regulations to meet rate, volume and water quality standards.

Today, local stormwater ordinances vary to some degree. These ordinances set requirements for flow rate leaving the property. Additionally, they set the standards for sizing structures, stream crossings and pipes. When dealing with projects along or near a state road, PennDOT regulations can apply too. For projects that disturb greater than one acre, there is the NPDES Permit. This is a Federal Program under the Clean Streams Law, delegated to Pennsylvania. The Permit ensures projects meet certain rate, volume and water quality requirements. So how does this relate to Isaias?

The NPDES program ensures a project does not increase the rate or volume of water leaving a property. These program requirements are for storms up to the 100-year event. Some local ordinances require a project to reduce the discharge rates even further. The goal is to decrease the speed and volume of water leaving a property, thereby reducing or preventing downstream flooding. Additionally, are the ordinances and regulations that set standards for sizing stormwater structures. The sizing calculations use rainfall intensities, coverage type, and area. The intention of these regulations is to protect health and safety. This is why roadway crossings must pass at least the 100-year storm.


So, What Happened?

First, these regulations have not always existed. This means older projects did not have to meet today’s standards. Additionally, older structures and crossings may be undersized.
Today, engineers are making up for the designs of the past. The regulations force engineers to over compensate for the past. So, the designs are reducing rates and volumes of water leaving a site. This is why during Isaias, many saw those massive basins that never had water in them before, filling up. The filling of the basins indicates that they worked. Otherwise, that water would have added to the flooding too.

Even then, this storm dropped so much rain in a short period of time, that systems could not keep up. This storm was over a 1,000-year event. That means it had a 0.001% chance of happening in a given year. This is something no one could design for or expect. But the implementation of flood mitigation projects, like Blue Marsh Dam, and stormwater management regulations have reduced the damage they cause. As time goes on, more detention basins, infiltration basins, rain gardens and other practices will be installed. These stormwater management controls are what will prevent the floods of the past from happening again.


Who is ACA?

Started in 1992 and located in Chester County, PA ACA has grown to become a full-service civil engineering firm. Today, we merge professional services with practical knowledge for residential and commercial projects. No matter the scale, from installing a fence, to building a structure or developing land, you need permits. Because the approval process includes many permits and agencies, it can be a headache. Working with ACA’s full-service team saves you time, money, and headaches.  Every step of the way, we are here to support you and educate you about the process. Here are some of the basic services we provide:

Construction Management | Civil Engineering | Environmental Permitting | Septic System Testing and Design | Land Surveying | Wetland Delineations and Mitigation

Please feel free to browse our website or if working on a project or need help, contact us at (610) 469-3830. 

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Flooding in Pennsylvania. (2020, August 9). Retrieved from National Weather Service (National Oceanic and Atmospheric Administration):
Storm Water Management Act §§167. (1978). Commonwealth of Pennsylvania.

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