Detailed Changes in New Jersey and Other States Offer Guidance for Successful Adaptation
Using a design storm rainfall is one of the primary ways runoff from storm events is managed in planning regulations and modeled in stormwater management design. In order to properly utilize the design storm, both rainfall amount and distribution in space and time are used to determine a design flood or design peak discharge[1] . The storm return frequency, which is also known as the exceedance probability of a design storm, is based on historical storm data for a defined area; these typically range from a 1-year to a 1000-year period[2]. Hydrologists, engineers, researchers and regulators are finding more and more that the rainfall depths associated with each of the return frequencies are routinely being surpassed. This means that the 100-year storm can occur more often than every 100 years, exceeding the expected 1% chance of a 100-year storm occurring in a given year.
A main goal of stormwater designers and regulators is to protect the health, safety and welfare of citizens and property. If infrastructure is under-designed, this goal often cannot be fully met. As a result, planning for resiliency is also being emphasized as an important consideration. One method for addressing the storm frequency issue that is gaining support at the regulatory level is revising the rainfall depths calculated for each of the return periods considering recent observed storm events and developing rainfall trends. Justifications for this method are based in recent rainfall data sets and field observations of infrastructure. Changes to regulations with this approach will affect predicted peak discharges and flood elevations, thereby affecting stormwater management and infrastructure design.
Related Services
Basis for Updating Design Storm Rainfall Depth Regulations
The motivations for updating parameters in stormwater regulations vary amongst agencies, states and municipalities. Regulators across many jurisdictions are proposing revisions based on:
- Storm events that have recently occurred in and around their jurisdiction,
- Anticipated impacts of climate change on storms events informed by climate modeling, or
- A blend of both recent and anticipated storm events.
Howard County, Maryland, for example, updated their Stormwater Design Manual in response to two high-intensity, flash rain events that flooded Ellicott City in both July of 2016 and May of 2018. A 3.55-hour, 6.6-inch storm event modeled after these two recently occurred storms was added to certain Howard County watersheds for quantity management[3]. Some counties in California, including San Mateo County near San Francisco, are making changes centered on projections of climate models considering multiple representative concentration pathways (RCPs) for anticipated global temperature changes (i.e., RCP4.5 as a moderate greenhouse gas emissions scenario and RCP8.5 as a high greenhouse gas emissions scenario). In its 2020 technical memorandum titled Climate Adaptation Risk Analysis for the San Mateo Countywide Sustainable Streets Master Plan, the methodology presented leads to an increase in stormwater runoff ranging from 15% for the 2-year storm to 50% for 100-year storm[4].
New Jersey as a Regulatory Case Study
New Jersey has been compelled by both damage from recent high-intensity storm events and by climate research to propose state-wide changes to design storm rainfall depths. The latest update proposed by the New Jersey Department of the Environmental Protection (NJDEP) draws inspiration from 2021 tropical storm Ida as well as from current climate science to generate county specific rainfall adjustment factors. These adjustment factors will update the National Oceanic and Atmospheric Administration’s (NOAA) Atlas 14 Point Precipitation Frequency Estimates currently used. NOAA Atlas 14 data, which is employed by many municipalities and agencies nationwide, is considered to be outdated and obsolete by the NJDEP and is also viewed as unable to appropriately inform New Jersey stormwater design to protect the public from flooding hazards[5]. While New Jersey’s newly proposed rule is specific to the geography of the state, it is illustrative to review how these factors were developed and what analyses were involved.
The NJDEP update builds on a Cornell University study of New Jersey rainfall[6]. The report found that increases in extreme precipitation were observed at over 75 percent of the weather stations in New Jersey over the period from 1999 to 2019. The study was tied to the locations of 55 weather stations; however, the state was interested in generating adjustment factors by county to make the changes more practical. In order for the NJDEP to determine increased rainfall values they utilized Thiessen polygons to delineate the area surrounding each weather station, similar to how infiltration test results for infiltration stormwater management practices can be applied to a certain area based on soil characteristics. These delineated areas were used to develop rainfall adjustment factors for each of the 21 counties is New Jersey.
Two sets of adjustment factors were developed, both intended to be applied to the NOAA Atlas 14 data. One set is for current precipitation and the other set is for future precipitation. The rule proposed that, “both current and projected precipitation totals be utilized for each of the two-, 10- and 100-year for both the pre- and post-construction conditions.” The table below summarizes how the proposed adjustment factors affect the rainfall in the five most populous New Jersey counties; reported rainfalls are in inches.
Current and Future Precipitation Adjustment Factors
Developed for New Jersey’s Five Largest Counties by Population
Seeing the adjustment factors applied to a rainfall depth helps to demonstrate how different the updated values can be, particularly for the future precipitation adjustments.
| County (County Seat) |
NOAA Atlas 14 (100-year, 24-hour) |
Current Precipitation Adjustment Factor | NOAA Atlas 14 (Current Precipitation Adjusted) |
Future Precipitation Adjustment Factor | NOAA Atlas 14 (Future Precipitation Adjusted) |
|
Bergen (Hackensack) |
8.25 | 1.06 | 8.75 | 1.37 | 11.30 |
| Middlesex
(New Brunswick) |
8.55 | 1.03 | 8.81 | 1.33 | 11.33 |
| Essex
(Newark) |
8.41 | 1.06 | 8.91 | 1.33 | 11.19 |
| Hudson
(Jersey City) |
8.75 | 1.09 | 9.54 | 1.23 | 10.76 |
| Monmouth
(Freehold) |
8.58 | 1.02 | 8.75 | 1.26 | 10.81 |
Next Steps: TRC Can Help
Municipalities, agencies, and designers that are currently using NOAA Atlas 14 data or, in some cases, even older rainfall data, can refer to these rainfall updates happening nationally to initiate conversations about how each jurisdiction can adapt to the observed changes in rainfall for the benefit of its citizens and investments in infrastructure. TRC is closely tracking regulator updates around rainfall precipitation values and revisions to stormwater management rules and guidance documents. TRC offers stormwater design services that can assist with adjusting to developing regulations and with planning the future. Contact our experts to learn more.
Related Materials
American Meteorological Society Glossary of Meteorology
American Meteorological Society Glossary of Meteorology Published March 27, 2024
History of the Application of Design Storm Frequency and Intensity
Published May 18, 2017
Howard County Department of Public Works Revisions to Howard County Design Manual
Published February 19, 2020
Climate Adaptation Risk Analysis for the San Mateo Countywide Sustainable Streets Master Plan
Published October 14, 2020
NJ Department of Environmental Protection Watershed and Land Management Stormwater Management Rules
Published December 5, 2022
Changes in Hourly and Daily Extreme Rainfall Amounts in NJ since the Publication of NOAA Atlas 14 Volume
Published October 1, 2021
Adapt to
Change
Partner With TRC’s Tested Practitioners
Sharing Our Perspectives
Our practitioners share their insights and perspectives on the trends and challenges shaping the market.
Webinar Replay
March 26, 2024
TRC’s panel of practitioners discuss the Hydrogen Hub (H2Hub) funding provisions of the Bipartisan Infrastructure Bill (BIL).
U.S. EPA’s EJScreen Evolves as the Agency Advances Environmental Justice
August 21, 2023
EJScreen is currently at the forefront of federal efforts to identify potential disproportionate environmental burdens and communities with potential environmental justice (EJ) concerns.
U.S. EPA and Army Corps of Engineers to Develop Revised Rule to Define “Waters of the United States”
August 8, 2023
The US EPA and Army Corps of Engineering are redefining “Waters of the United States”.
Regulators Update Design Storm Rainfall Depths in Response to Climate Science Projections and Recent Storm Data
August 3, 2023
Regulators are responding to anticipated increases in extreme rainfall events by updating design storm rainfall depth regulations.
EPA Issues Clarification of Free Product Removal Requirements
June 20, 2023
EPA recently clarified requirements for LNAPL recovery and remediation.
New Executive Order 14096 Broadens Environmental Justice Initiatives
May 9, 2023
Executive Order 14096, Revitalizing Our Nation’s Commitment to Environmental Justice for All, seeks to deepen the Biden administration’s “whole-of-government” approach to environmental justice (EJ) by fully integrating the consideration of unserved and overburdened communities and populations into all aspects of federal agency planning and delivery of services.
TRC Assists in Securing $11M in funding for small Communities
April 14, 2023
On May 18, 2023, EPA published the proposed Hazardous and Solid Waste Management System: Disposal of Coal Combustion Residuals from Electric Utilities; Legacy CCR Surface Impoundments (proposed rule) in the Federal Register to amend subpart D of part 257 of Title 40 [Docket ID No. EPA-HQ-OLEM-2020-0107; FRL–7814-02-OLEM]. TRC initially summarized key aspects of EPA’s proposed rule in Summary of Proposed Coal Combustion Residuals Legacy CCR Surface Impoundments Rule, June 15, 2023. Drawing from our expertise and extensive experience supporting utilities as they navigate the intricacies of the CCR rule, TRC has submitted public comments related to EPA’s proposed compliance timeline and schedule.
Proactive Enforcement is Key in the EPA FY2022-2026 Strategy
October 19, 2022
A core element of the EPA FY2022-2026 Strategic Plan focuses on environmental compliance.
Preparing for EPA Inspections in Environmental Justice Communities
October 4, 2022
The EPA Office of Enforcement and Compliance Assurance Have Expanded Goals to Strengthen Enforcement and Protections Within EJ Communities
New National Emerging Contaminants Research Initiative
September 12, 2022
The Executive Office of the President of the United States announced a National Emerging Contaminant Research Initiative
Locana Awarded Wildlife Habitat Analysis Task Order For The Bureau Of Land Management
April 27, 2022
Locana, a leading geographic data and technology company, has won a 10-year National Geospatial Data and Technology Support Services contract for the U.S. Bureau of Land Management (BLM).
TRC Acquires New Jersey’s Clean Energy Program Contract and Assumes Program Administrator Role
January 17, 2021
NEW BRUNSWICK, NJ and LOWELL, MA. – Jan. 17, 2017 – TRC Companies Inc., a recognized leader in engineering, environmental consulting and construction-management services, today announced it has acquired the contract to serve as Program Administrator of New Jersey’s Clean Energy Program™ (NJCEP), which has provided more than $300 million annually in support to homeowners, businesses…
Ecological Risk of PFAS from AFFF-Impacted Sites
June 30, 2020
The facts on evaluating exposure to wildlife
Security Stew: How to Follow the Federal Regulatory Recipe for Safe Chemical Storage
September 19, 2017
Three different federal agencies regulate the storage of chemicals at facilities in the United States – and each for different reasons. Learn about the various rules from the alphabet soup of agencies (DHS, EPA and OSHA) involved and how to keep your business compliant.
TRC and partners win $1 million grant for engineering innovative New York microgrid
April 20, 2017
TRC is proud to support Huntington, NY bolster power reliability and climate-change resiliency with a sophisticated new “community microgrid’’ combining solar energy, a fuel cell, biogas and traditional natural gas to deliver electricity and heat to local customers and institutions.
EPA to Include CERCLA Sites and RCRA Facilities in Site Remediation NESHAP
June 23, 2016
EPA has published a proposed rule that would extend the requirements of the Site Remediation National Emission Standards for Hazardous Air Pollutants (NESHAP) regulations to previously exempt soil and groundwater remediation activities under CERCLA and RCRA.
Manchester Moves Legacy Sites
April 9, 2014
The City of Manchester, NH continues to transform blighted areas of the City into community-based redevelopment projects supported by EPA’s Brownfields Program to improve the environment and public health and expand opportunities for neighborhood and economic development.
