Presenter/Contact
Presenter
Melissa Reynolds, Assistant City Engineer
Contact Info
(830) 221-4020 - mreynolds@nbtexas.org
Subject Header
SUBJECT:
Title
Approval to amend Chapter 3 Design Rainfall in the Drainage and Erosion Control Design Manual.
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BACKGROUND / RATIONALE:
City staff is proposing amendments to Chapter 3 Design Rainfall in the Drainage and Erosion Control Design Manual to include new design rainfall information. Rainfall intensities are one of the factors used in calculating the peak flow or design discharge used for sizing detention ponds. The proposed changes to the rainfall intensity data used in the detention design are integral in sizing flood control measures to recognize local rainfall conditions.
The Subdivision Platting Ordinance requires the City Engineer to submit criteria for drainage facilities to the Planning Commission for recommendation to City Council and the Drainage Code allows for the use of information not presented in the Drainage and Erosion Control Design Manual. City staff is presenting the amendments for approval as they represent the most current information.
On September 27, 2018, the National Oceanic and Atmospheric Administration (NOAA) released an analysis finding significantly higher rainfall frequency values in parts of Texas, redefining the amount of rainfall it takes to qualify as a 100-year event. The study, published as NOAA Atlas 14, Volume 11 Precipitation-Frequency Atlas of the United States, Texas, found increased values in parts of Texas, that would result in changes to the rainfall amounts that define 100-year events, which are those that on average occur every 100 years or have a one percent chance of happening in any given year.
In New Braunfels 100-year rainfall amounts for 24 hours increased to 13.1 inches from 12.3 inches.
The updated values supersede those currently available for Texas from the 1960s and 1970s. The new values are more accurate than estimates developed 40 to 50 years ago due to decades of additional rainfall data, an increase in the amount of available data, both in the number of stations and their record lengths, and improved methods used in the analysis. NOAA’s Atlas 14 is a peer-reviewed publication that serves as the official government source of precipitation frequency values for the U.S.
NOAA Atlas 14 rainfall values are used for infrastructure design and planning activities under federal, state and local regulations. They also help delineate flood risks, manage development in floodplains for FEMA’s National Flood Insurance Program and are used to monitor precipitation observations and forecasts that can indicate flooding threats by NOAA’s National Weather Service.
The revision was released in an Engineering Bulletin to the Development Community on October 16, 2020 effective October 27, 2020 for all new floodplain permits and City capital improvement projects submitted for new application after the effective date are required to adhere to Atlas 14 rainfall data. These changes are required for all floodplain models and designs within the floodplain as required by the Federal Emergency Management Agency (FEMA) as the data is represented as the most current and best available hydrology information. This item was presented to the Watershed Advisory Committee on October 29, 2020 and to Planning Commission on November 4, 2020 receiving a unanimous recommendation of support to City Council.
Staff is proposing the new design rainfall data be required for all new development projects effective January 1, 2021. The new design rainfall data is not required for a project with an approved or filed master plan, final plat, letter of certification, or permit prior to the effective date except for projects including the regulatory floodplain or public works projects located on public right-of-way or easements. If an approved master plan requires a major revision as specified in the City of New Braunfels Code of Ordinances, the development shall comply with the most current Drainage and Erosion Control Design Manual requirements.
ADDRESSES A NEED/ISSUE IN A CITY PLAN OR COUNCIL PRIORITY:
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City Plan/Council Priority: Envision New Braunfels Comprehensive Plan |
Action 5.15: Ensure that developers adequately address drainage in their projects and developments. |
FISCAL IMPACT:
N/A
Recommendation
COMMITTEE RECOMMENDATION:
This item was presented to the Watershed Advisory Committee on October 29, 2020 and to Planning Commission on November 4, 2020. A unanimous recommendation of support was received at each meeting.
STAFF RECOMMENDATION:
Staff recommends approval of the proposed amendments to Chapter 3 Design Rainfall in the Drainage and Erosion Control Design Manual.