Analyzing Mitigation Strategies for Flood-Prone Philadelphia Community

Photo from Eastwick Friends and Neighbors Coalition

Hydrology is the study of the properties, distribution, and effects of water on the Earth’s surface, in the soil and underlying rocks, and in the atmosphere. The hydrologic cycle includes all of the ways in which water cycles from land to the atmosphere and back. Hydrologists study natural water-related events such as drought, rainfall, stormwater runoff, and floods, as well as how to predict and manage such events. On the application side, hydrology provides basic laws, equations, algorithms, procedures, and modeling of these events.

Hydraulics is the study of the mechanical behavior of water in physical systems. In engineering terms, hydraulics is the analysis of how surface and subsurface waters move from one point to the next, such as calculating the depth of flow in a pipe or open channel. Hydraulic analysis is used to evaluate flow in rivers, streams, stormwater management networks, sewers, and much more.

Combined hydrologic and hydraulic data, tools, and models are used for analyzing the impacts that waterflow – precipitation, stormwater, floods, and severe storms – will have on the existing infrastructure. This information is also used to make future land-use decisions and improvements that will work within the constraints of the hydrologic cycle and won’t exacerbate flooding or cause water quality impairment.

Simply put, hydrologic and hydraulic modeling is an essential component of any effective flood risk management plan.

Putting Hydrologic & Hydraulic Analysis to Work in Philadelphia

Eastwick, a low-lying urbanized neighborhood in Southwest Philadelphia, is located in the Schuylkill River Watershed and is almost completely surrounded by water: The Cobbs and Darby creeks to the west, the Delaware River and wetlands to the south, and the Schuylkill River and Mingo Creek to the east. The community is at continual risk of both riverine and coastal flooding, and faces an uncertain future due to sea level rise and riverine flooding exacerbated by climate change.

Princeton Hydro, along with project partners KeystoneConservation and University of Pennsylvania, conducted an analysis of Eastwick, the flood impacts created by the Lower Darby Creek, and the viability of several potential flood mitigation strategies.

Flood mitigation approaches can be structural and nonstructural. Structural mitigation techniques focus on reconstructing landscapes, including building floodwalls/seawalls and installing floodgates/levees. Nonstructural measures work to reduce damage by removing people and property out of risk areas, including zoning, elevating structures, and conducting property buyouts.

For Eastwick, studying stream dynamics is a key component to determining what type of flood mitigation strategies will yield the most success, as well as identifying the approaches that don’t work for this unique area.

Princeton Hydro Senior Ecologist Christiana Pollack CFM, GISP participated in a workshop for Eastwick residents held by CCRUN and the Lower Darby Creek team. The goal of the workshop was to get the community’s input on the accuracy of the predictive models.Princeton Hydro’s study focused on the key problem areas in Eastwick: the confluence of Darby Creek and Cobbs Creek; a constriction at Hook Road and 84th Street; and the Clearview Landfill, which is part of the Lower Darby Creek Superfund site. Additionally, the study sought to answer questions commonly asked by community members related to flooding conditions, with the main question being: What impact does the landfill have on area flooding?

The built-up landfill is actually much higher than the stream bed, which creates a major disconnection between the floodplain and the stream channel. If the landfill didn’t exist, would the community still be at risk? If we increased the floodplain into the landfill, would that reduce neighborhood flooding?

Princeton Hydro set out to answer these questions by developing riverine flooding models primarily using data from US Army Corps of Engineers (USACE), Federal Emergency Management Agency (FEMA), The National Oceanic and Atmospheric Administration (NOAA), and NOAA’s National Weather Service (NWS). FEMA looks at the impacts of 1% storms that are primarily caused by precipitation events as well as coastal storms and storm surge. NOAA looks at the impacts of hurricanes. And, NOAA’s NWS estimates sea, lake and overland storm surge heights from hurricanes.

This is an example of a 2D model showing where the water is originating, how the water flows through the neighborhood, moves to the lower elevations, and eventually sits.

This is an example of a 2D model showing where the water is originating, how the water flows through the neighborhood, moves to the lower elevations, and eventually sits.

The models used 2D animation to show how the water flows in various scenarios, putting long-held assumptions to the test.

The models looked at several different strategies, including the complete removal of the Clearview Landfill, which many people anticipated would be the silver bullet to the area’s flooding. The modeling revealed, however, that those long-held assumptions were invalid. Although the landfill removal completely alters the flood dynamics, the neighborhood would still flood even if the landfill weren’t there. Additionally, the modeling showed that the landfill is actually acting as a levee for a large portion of the Eastwick community.

This model was developed to illustrate how the removal of the landfill impacts waterflow through the Eastwick community.

This model was developed to illustrate how the removal of the landfill impacts waterflow through the Eastwick community.

Ultimately, the research and modeling helped conclude that for the specific scenarios we studied, altering stream dynamics – a non-structural measure – is not a viable flood mitigation strategy.

The USACE is currently undergoing a study in collaboration with the Philadelphia Water Department to test the feasibility of a levee system (a structural control measure), which would protect the Eastwick community by diverting the flood water. Funding for the study is expected to be approved in the coming year.

Take a Deeper Look at Eastwick Flood Mitigation Efforts

There are many studies highlighting flood mitigation strategies, environmental justice, and climate change vulnerability in Eastwick. Princeton Hydro Senior Project Manager and Senior Ecologist, Christiana Pollack CFM, GISP, presented on the flooding in Eastwick at the Consortium for Climate Risk in the Urban Northeast Seminar held at Drexel University. The seminar also featured presentations from Michael Nairn of the University of Pennsylvania Urban Studies Department, Ashley DiCaro of Interface Studios, and Dr. Philip Orton of Stevens Institute of Technology.

You can watch the full seminar here:

For more information about Princeton Hydro’s flood management services, go here: http://bit.ly/PHfloodplain.

Part Two: Reducing Flood Risk in Moodna Creek Watershed

Photo of Moodna Creek taken from the Forge Hill Road bridge, New Windsor Post Hurricane Irene (Courtesy of Daniel Case via Wikimedia Commons)

This two-part blog series showcases our work in the Moodna Creek Watershed in order to explore common methodologies used to estimate flood risk, develop a flood management strategy, and reduce flooding.

Welcome to Part Two: Flood Risk Reduction and Stormwater Management in the Moodna Creek Watershed

As we laid out in Part One of this blog series, the Moodna Creek Watershed, which covers 180 square miles of eastern Orange County, New York, has seen population growth in recent years and has experienced significant flooding from extreme weather events like Hurricane Irene, Tropical Storm Lee, and Hurricane Sandy. Reports indicate that the Moodna Creek Watershed’s flood risk will likely increase as time passes.

Understanding the existing and anticipated conditions for flooding within a watershed is a critical step to reducing risk. Our analysis revealed that flood risk in the Lower Moodna is predominantly driven by high-velocity flows that cause erosion, scouring, and damage to in-stream structures. The second cause of risk is back-flooding due to naturally formed and man-made constrictions within the channel. Other factors that have influenced flood risk within the watershed, include development within the floodplain and poor stormwater management.

Now, let’s take a closer look at a few of the strategies that we recommended for the Lower Moodna Watershed to address these issues and reduce current and future flood risk:

Stormwater Management

Damage to Butternut Drive caused when Moodna Creek flooded after Hurricane Irene (Courtesy of Daniel Case via Wikimedia Commons)

Stormwater is the runoff or excess water caused by precipitation such as rainwater or snowmelt. In urban areas, it flows over sewer gates which often drain into a lake or river. In natural landscapes, plants absorb and utilize stormwater, with the excess draining into local waterways.  In developed areas, like the Moodna Creek watershed, challenges arise from high volumes of uncontrolled stormwater runoff. The result is more water in streams and rivers in a shorter amount of time, producing higher peak flows and contributing to flooding issues.

Pollutant loading is also a major issue with uncontrolled stormwater runoff. Population growth and development are major contributors to the amount of pollutants in runoff as well as the volume and rate of runoff. Together, they can cause changes in hydrology and water quality that result in habitat loss, increased flooding, decreased aquatic biological diversity, and increased sedimentation and erosion.

To reduce flood hazards within the watershed, stormwater management is a primary focus and critical first step of the Moodna Creek Watershed Management Plan. The recommended stormwater improvement strategies include:

  • Minimizing the amount of impervious area within the watershed for new development, and replacing existing impervious surfaces with planter boxes, rain gardens and porous pavement.
  • Utilizing low-impact design measures like bioretention basins and constructed-wetland systems that mimic the role of natural wetlands by temporarily detaining and filtering stormwater.
  • Ensuring the long-term protection and viability of the watershed’s natural wetlands.

The project team recommended that stormwater management be required for all projects and that building regulations ensure development does not change the quantity, quality, or timing of run-off from any parcel within the watershed. Recommendations also stressed the importance of stormwater management ordinances focusing on future flood risk as well as addressing the existing flooding issues.

Floodplain Storage

Floodplains are the low-lying areas of land where floodwater periodically spreads when a river or stream overtops its banks. The floodplain provides a valuable function by storing floodwaters, buffering the effect of peak runoff, lessening erosion, and capturing nutrient-laden sediment.

Communities, like the Moodna Creek watershed, can reduce flooding by rehabilitating water conveyance channels to slow down the flow, increasing floodplain storage in order to intercept rainwater closer to where it falls, and creating floodplain benches to store flood water conveyed in the channel.  Increasing floodplain storage can be an approach that mimics and enhances the natural functions of the system.

One of the major causes of flooding along the Lower Moodna was the channel’s inability to maintain and hold high volumes of water caused by rain events. During a significant rain event, the Lower Moodna channel tends to swell, and water spills over its banks and into the community causing flooding. One way to resolve this issue is by changing the grading and increasing the size and depth of the floodplain in certain areas to safely store and infiltrate floodwater. The project team identified several additional opportunities to increase floodplain storage throughout the watershed.

One of the primary areas of opportunity was the Storm King Golf Club project site (above). The team analyzed the topography of the golf course to see if directing flow onto the greens would alter the extent and reach of the floodplain thus reducing the potential for flooding along the roadways and properties in the adjacent neighborhoods. Based on LiDAR data, it was estimated that the alteration of 27 acres could increase floodplain storage by 130.5 acre-feet, which is equivalent to approximately 42.5 million gallons per event.

Land Preservation & Critical Environmental Area Designation

For areas where land preservation is not a financially viable option, but the land is undeveloped, prone to flooding, and offers ecological value that would be impacted by development, the project team recommended a potential Critical Environmental Area (CEA) designation. A CEA designation does not protect land in perpetuity from development, but would trigger environmental reviews for proposed development under the NY State Quality Environmental Review Act. And, the designation provides an additional layer of scrutiny on projects to ensure they will not exacerbate flooding within the watershed or result in an unintentional increase in risk to existing properties and infrastructure.

Conserved riparian areas also generate a range of ecosystem services, in addition to the hazard mitigation benefits they provide. Protected forests, wetlands, and grasslands along rivers and streams can improve water quality, provide habitat to many species, and offer a wide range of recreational opportunities. Given the co-benefits that protected lands provide, there is growing interest in floodplain conservation as a flood damage reduction strategy.


These are just a few of the flood risk reduction strategies we recommended for the Lower Moodna Creek watershed. For a more in-depth look at the proposed flood mitigation strategies and techniques, download a free copy of our Moodna Creek Watershed and Flood Mitigation Assessment presentation.

Revisit part-one of this blog series, which explores some of the concepts and methods used to estimate flood risk for existing conditions in the year 2050 and develop a flood management strategy.

Two-Part Blog Series: Flood Assessment, Mitigation & Management

For more information about Princeton Hydro’s flood management services, go here: http://bit.ly/PHfloodplain

Two-Part Blog Series: Flood Assessment, Mitigation & Management

In this two part blog series, we showcase our work in the Moodna Creek Watershed in order to explore some of the concepts and methods used to estimate flood risk for existing conditions and the year 2050 and develop a flood management strategy (Part One), and traditional engineering and natural systems solutions used to manage and reduce flood risk (Part Two).

Part One: Flood Assessment & Mitigation Analysis in the Moodna Creek Watershed

The greater Moodna Creek watershed covers 180 square miles of eastern Orange County, NY. The watershed includes 22 municipalities and hundreds of streams before joining the Hudson River. This region has seen tremendous growth in recent years with the expansion of regional transit networks and critical infrastructure.

The Moodna Creek watershed can be split into two sub-basins — the Upper Moodna Creek and the Lower Moodna Creek. In the span of 15 months, Hurricane Irene, Tropical Storm Lee, and Hurricane Sandy each have caused significant flooding throughout the Moodna Creek watershed, damaging public facilities, roadways, and private properties. Both sub-basin communities have noted a concern about increased flood risk as more development occurs.

As global temperatures rise, climate models are predicting more intense rainfall events. And, the flood risk for communities along waterways — like the Moodna Creek watershed — will likely increase as time passes. In order to understand existing and future risk from flood events in this flood-prone area, a flood risk management strategy needed to be developed. The strategy uses a cost-benefit analysis to review the feasibility of each measure and the overall impact in reducing flood risks.

With funds provided from a 2016 grant program sponsored by the New England Interstate Waters Pollution Control Commission (NEIWPCC) and the New York State Department of Environmental Conservation’s (NYCDEC) Hudson River Estuary Program (HEP), Princeton Hydro along with a variety of project partners completed a flood assessment and flood mitigation analysis specific to the Lower Moodna Creek watershed.

Let’s take a closer look at our work with the Lower Moodna Creek watershed, and explore some of the methods used to estimate flood risk and develop a flood management strategy:

Lower Moodna Creek Watershed Flood Assessment & Analysis

The primary Lower Moodna Creek project goals were to assess flood vulnerabilities and propose flood mitigation solutions that consider both traditional engineering strategies and natural systems solution approaches (land preservation, wetland/forest restoration, green infrastructure and green water management). The project team focused on ways to use the natural environment to reduce risk.  Instead of strictly focusing on just Moonda Creek, the team took a holistic approach which included all areas that drain into the river too. These analyses were incorporated into a Flood Assessment Master Plan and Flood Mitigation Plan, which will serve as a road map to reducing flooding issues within the watershed.

Managing Flood Risk

The first step in managing flood risk is to understand what type of exposure the communities face. The Moodna Creek project modeled flooding within the watershed during normal rain events, extreme rain events, and future rain events with two primary goals in mind:

Visual assessment being conducted in flood-prone areas of Moodna Creek Watershed.

  • Assess the facilities, infrastructure, and urban development that are at risk from flooding along the Moodna Creek and its tributaries within the study area.
  • Develop a series of hydrologic and hydraulic models to assess the extent of potential flooding from the 10-year (10%), 100-year (1%),  and 500-year (0.2%) storm recurrence intervals within the study area. The modeling includes flows for these storm events under existing conditions and also hypothetical scenarios with predicted increases in precipitation and population growth.

 

The project team used these models and data to propose and evaluate a series of design measures that help reduce and mitigate existing and anticipated flood risk within the study area. Where possible, the proposed solutions prioritized approaches that protect and/or mirror natural flood protection mechanisms within the watershed such as floodplain re-connection and wetland establishment. In addition to flood protection, the project components also provide water quality protection, aesthetics and recreation, pollutant reduction, and wildlife habitat creation.

Land Use and Zoning

Zoning is a powerful tool that determines a region’s exposure to hazards and risk. Zoning determines which uses are permitted, or encouraged, to be built in moderate and high-risk areas. It also prevents certain uses, such as critical facilities, from being built in those areas. Zoning is also a determinant of a region’s character and identity.

In the Lower Moodna Creek watershed, a large majority (82%) of land is zoned for residential use. However, in the flood-prone areas, there is a higher ratio of areas zoned for non-residential uses (commercial, industrial) than in areas that are zoned for potential future development. Specifically, within the 10-year storm recurrence floodplain, 30% of the land is zoned for industrial use. This is likely because several facilities, such as wastewater treatment plants and mills, require access to the river and were strategically developed to be within immediate proximity of waterfront access. The Lower Moodna zoning analysis demonstrated a general preference within watershed to limit residential use of flood-prone areas. 

Land Preservation

Preserving land allows for natural stormwater management, as well as limits the exposure of development, and minimizes sources of erosion within the watershed. Preserved land also maintains the hydrologic and ecologic function of the land by allowing rainwater to be absorbed or retained where it falls and thus minimizing run-off. If the land within the floodplain is preserved, it will never be developed, and therefore the risk — a calculation of rate exposure and the value of the potential damage — is eliminated.  Therefore, land preservation, both within the floodplains and in upland areas, is the best way to minimize flood damage.

Conserved riparian areas also generate a range of ecosystem services, in addition to the hazard mitigation benefits they provide. Protected forests, grasslands, and wetlands along rivers and streams can improve water quality, provide habitat to many species, and offer a wide range of recreational opportunities. Given the co-benefits that protected lands provide, there is growing interest in floodplain conservation as a flood damage reduction strategy.

Within the mapped Lower Moodna floodplains, our assessment determined that there appears to be a slight priority for preserving land most at-risk for flooding. This is likely a consequence of prioritizing land that is closest to riparian areas and preserving wetland areas, which are the most likely to experience flooding. Within the floodplains for the 10-year storm, approximately 22.7% is preserved. For the 100-year storm, approximately 21.2% of the land is preserved. Within the 500-year storm, this number drops slightly to 20.3%. These numbers are so close in part because the difference between the 10-year, 100-year, and 500-year floodplains are small in many areas of the watershed.

Hydrology and Hydraulics

Hydrology is the scientific study of the waters of the earth, with a particular focus on how rainfall and evaporation affect the flow of water in streams and storm drains. Hydraulics is the engineering analysis of the flow of water in channels, pipelines, and other hydraulic structures. Hydrology and hydraulics analyses are a key part of flood management.

As part of this flood assessment, Princeton Hydro created a series of hydrologic and hydraulic (H&H) models to assess the extent of potential flooding from the 10-year, 100-year, and 500-year storm recurrence intervals within the Lower Moodna. The modeling, which included flows for these storm events under existing conditions and future conditions based on predicted increases in precipitation and population growth, makes it easier to assess what new areas are most impacted in the future.

These are just a few of the assessments we conducted to analyze the ways in which flooding within the watershed may be affected by changes in land use, precipitation, and mitigation efforts. The flood models we developed informed our recommendations and proposed flood mitigation solutions for reducing and mitigating existing and anticipated flood risk.

Check out Part Two of this blog series in which we explore flood risk-reduction strategies that include both traditional engineering and natural systems solutions:

Part Two: Reducing Flood Risk in Moodna Creek Watershed

For more information about Princeton Hydro’s flood management services, go here: http://bit.ly/PHfloodplain.