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HomeMy WebLinkAboutNHC Northern Watersheds Flood Study Results_03_2025 Date: March 31, 2025 To: New Hanover County Board of Commissioners Cc: Chris Coudriet, County Manager; Jessica Loeper, Assistant County Manager From: Ken Vafier, Planning Operations Supervisor; Rebekah Roth, Planning & Land Use Director Re: New Hanover County Northern Watersheds Flood Study After the adoption of the FY23 budget, which allocated funding for the New Hanover County Northern Watersheds Flood Study, staff initiated a flood mapping analysis with ESP Associates, Inc. to model and analyze areas of the county that are not shown as designated Special Flood Hazard Areas on the county’s current FEMA Flood Insurance Rate Maps (FIRMs) but are likely subject to a similar level of flood risk due to their proximity to local streams. Staff has received the final results and following your review, will be providing this information to affected property owners, interested stakeholders, and the public. In 2018, the county adopted most of its updated FEMA Flood Insurance Rate Maps (FIRMs), and subsequent updates of additional map panels were made effective in 2019 and 2025 once adjacent jurisdictions had completed the statutory due process to adopt these panels. However, certain areas within the county were not provided with detailed mapping studies showing flood risks, despite being physically located within the vicinity of or along the numerous waterways in the county. Areas of a community may not have a full detailed study performed in all locations due to cost, sparse development in a study area, or factors relating to a less likely risk of flood conditions. However, as development continues in the unincorporated areas of New Hanover County, especially in northern portions that are transitioning from agricultural and timberland to residential and commercial, these gaps in the flood mapping data should be analyzed since this information is required to inform a wide range of land use planning and policies for transportation, infrastructure design, water resources, environmental assessments, regulations/enforcement, and the formulation of comprehensive plans. With updated floodplain mapping data, stakeholders will have access to additional information necessary for decision making. As a result, county staff identified 12 study areas in the northern portion of the county in 2021 where additional study and mapping Example of study area where the effective FIRM detailed study terminates. of watersheds was necessary to provide adequate data to display potential flooding risks. Following the budget allocation to fund a study and a selection process, the county contracted with ESP Associates, Inc. to conduct a floodplain mapping study for these 12 locations within the county where the FIRMs do not have fully detailed mapping studies completed. This study has produced data and maps that will augment the existing FIRMs which have been produced in accordance with FEMA mapping standards. The results of this study will be used to provide additional data on flood risk in the study areas to improve awareness and resiliency in these areas. The study was performed using methodology that meets NCFMP and FEMA requirements so that consistent mapping results will be achieved. This generally consisted of field surveys of watercourse profiles, field measurement and assessment of conveyance features such as culverts and channels, analysis and modeling, production of preliminary results, and quality control to produce the final mapping products, which are attached to this memorandum. A final engineering report is a scoped deliverable which is anticipated to be provided by ESP prior to the study’s closure; such a document is common with a floodplain mapping study and includes a detailed narrative on the data collection, hydrologic and hydraulic analysis methodology, results and conclusions, and annotated appendices with data tables, calculations, cross section drawings, and associated files. Staff analysis of the Advisory Flood Hazard Area results show that approximately 910 acres of property are within these delineated areas. Within the mapped areas, there are approximately 1,135 parcels that have some extent of advisory flood hazard area delineation, and 765 structures within them. The data produced with this study will be utilized as a key piece of data as the Comprehensive Plan continues to be updated. Data will be shared with property owners, large landowners with potential development opportunities, internal and external agency representatives such as Emergency Management, Engineering, NCDOT, NHC Schools, and ILM International Airport so that future infrastructure or other initiatives may consider this data in future plans. It is intended to be used for advisory purposes so that property owners may take actions to decrease potential risk to flooding events, such as purchasing flood insurance or designing the development of sites in a manner to minimize the risk of flood damage. Staff intends to post information to the department website on this study along with the mapping results in early April. Property owners with Advisory Flood Hazard Areas on their property have been identified and will be notified of the results at this time. Additionally, staff plans to host a technical session for these property owners to learn more, and external stakeholders will be contacted to discuss the study and its findings this spring. An objective of the framework to provide the study results to interested parties is to allot at least 30 days before the official start of hurricane season, allowing property owners the opportunity to purchase flood insurance and have an active policy in place by June 1. The study and its results will be part of a larger body of technical information related to the Comprehensive Plan update and will be provided in advance of the joint meeting of the Board of Commissioners and Planning Board on May 1. As always, staff is available for additional discussion and presentation if needed. Attachments: - Flood Analysis Overview - Narratives and Final Mapping Delineations for Project Study Areas Attachments: Mapping Exhibits Figure 1: Overview of all study areas showing the extent of Advisory Flood Hazard Areas with Effective Special Flood Hazard Areas on FIRMs This Flood Analysis Overview was produced by NHC staff and depicts the entire study area results with the Advisory Flood Hazard Areas (labeled as 1%, 0.2%, and Floodway) as well as the current effective Special Flood Hazard Areas on the FIRMs (labeled 0.2% Annual Chance, AE, AE Floodway, and VE). The map depicts the terminus of each effective mapped SFHA and the extent of the Advisory Flood Hazard Areas. The AE Zone is synonymous with the 1% polygon, these are areas that have a 1% or greater annual chance of incurring a flood event to base flood conditions in any given year. The 0.2% polygons represent areas that have a 0.2% or greater annual chance of incurring this flood event. The AE Floodway and Floodway are the channels of each watercourse and immediate adjacent lands that should be unobstructed to effectively discharge waters from the base flood condition. The VE zones are areas that have a 1% or greater annual chance of incurring a base flood event but are expected to have additional hazards from wave action associated with high winds, surges or tides. The Advisory Flood Hazard Areas delineated from the study are inland and thus do not result in any additional VE zones. Island Creek Island Creek is a freshwater stream in the northeastern portion of the county that meanders through mix of forested and low-density rural areas, crossing into Pender County and eventually draining into the Northeast Cape Fear River just northeast of Holly Shelter Road. The mapped Special Flood Hazard Areas of Island Creek terminate just east of Crooked Pine Road, and the Advisory Flood Hazard Areas for this stream are delineated further west and south, ending on the northern side of I-140 at its interchange with the Military Cutoff Road Extension. Kings Grant Tributary The Kings Grants Tributary is a local freshwater stream that flows adjacent to developed residential areas generally south of Gordon Road in the northeastern portion of the county. The mapped Special Flood Hazard Areas begin on the north side of Gordon Road just east of White Road; these areas then meander generally westward north of the Kings Grant subdivision, crossing North College Road and drain into Smith Creek in an area north of Wilmington Christian Academy. The Advisory Flood Hazard Areas for this stream are delineated in three separate branches generally north and northeast following drainage conveyances in the residential areas of Meadowbrook and Farrington Farms west of Ogden Park. Kings Grant Tributary Kings Grant Tributary Murrayville Tributary The Murrayville Tributary is a local freshwater stream that flows generally eastward from an agricultural area west of the Northchase development and drains into Smith Creek on the east side of I-40. The mapped Special Flood Hazard Areas flow south of Northchase, Laney High School and Olsen Park before reaching the confluence with Smith Creek. The Advisory Flood Hazard Areas for this stream delineate more pocketed, isolated areas that generally follow drainage ditches in the forested and agricultural areas west of Northchase. Ness Creek, Ness Creek Tributary 1, Ness Creek Tributary 2 Ness Creek is a local freshwater stream located generally between Wrightsboro and Castle Hayne. The stream contains two tributaries that branch to the north of the main channel; upon reaching their confluence with the main channel, Ness Creek flows generally southwest to the Northeast Cape Fear River. The mapped Special Flood Hazard Areas on the west side of Castle Hayne Road generally flow through low density residential or undeveloped properties, while on the east side of Castle Hayne Road the mapped areas affect the residential developments of Wrightsboro Acres and Ivy Woods. The Advisory Flood Hazard Areas are delineated generally east of Castle Hayne Road along drainage conveyances, roadways, and in pocketed, isolated areas of low topography. Ness Creek Prince George Creek, Prince George Creek Tributary 3 Prince George Creek is a local freshwater stream that flows generally westward from the western Sidbury Road corridor area where it drains into the NE Cape Fear River; it also contains a tributary that follows the east side of I-40 south to its interchange with I-140. The mapped Special Flood Hazard Areas for the main branch terminate abruptly on the north side of Sidbury Road northwest of the new Sidbury Station development while the mapped Special Flood Hazard Areas of the tributary terminate on the northeast quadrant of the interchange. The Advisory Flood Hazard Areas of the main branch are delineated south of Sidbury Road to stormwater areas southwest of Sidbury Station. The Advisory Flood Hazard Areas of the tributary are delineated in agricultural areas on the northeast and southeast quadrants of the interchange as well as in an additional narrow tributary flowing southwest from the Sidbury Station vicinity. Prince George Creek Prince George Creek Pumpkin Creek Pumpkin Creek is a local freshwater stream that flows through undeveloped, forested areas with some sparse low density residential development. The stream flows generally northeast from the vicinity of the NHC Detention Center where it drains into Prince George’s Creek on the west side if I-40 southeast of Castle Hayne. The mapped Special Flood Hazard Areas terminate at Juvenile Center Drive, while the Advisory Flood Hazard Areas are delineated further west and south of this terminus; two tributaries are delineated further west into agricultural or wooded areas and a southern tributary is delineated further south, affecting areas in the far northern residential area of Northchase. Pumpkin Creek Spring Branch Spring Branch flows from an area on the far southwestern edge of Dutch Square/southern boundary of Kings Grant, generally westward in areas near the Martin Luther King Parkway before reaching its confluence with Smith Creek southeast of Wilmington International Airport. The mapped Special Flood Hazard Areas terminate in a small, undeveloped area between the transition from Kings Grant to Dutch Square. The Advisory Flood Hazard Areas are expanded slightly east and north of this terminus with some limited effects on residential and commercial properties. Wildcat Branch Wildcat Branch is a short local stream that flows from the Heritage Park area west of Wilmington International Airport westward to its confluence with the NE Cape Fear River. The mapped Special Flood Hazard Areas encompass much of the Heritage Park neighborhood, and the terminate on the west side of Blue Clay Road. The Advisory Flood Hazard Areas do not necessarily delineate further extent in the mapping in this area, rather slight alterations to the boundaries of each respective flood zone with some isolated pockets of residences “removed,” or showing a lower flood risk based on the modeling. Acorn Branch Acorn Branch is in an area generally between the N Kerr Industrial park and Wilmington International Airport which has a mix of commercial, low-density residential, and undeveloped, forested properties. The area was initially not an identified study area for this analysis as there is not an abrupt visual termination in the mapped Special Flood Hazard Areas on the FIRMs in this vicinity; rather, the area is largely free from being mapped. In response to multiple concerns regarding flooding and drainage presented to both the Planning Board and Board of Commissioners in a recent rezoning proposal, this area was added to the scope for this study. The delineated Advisory Flood Hazard Areas do show some risk in this area with a channel extending from Blue Clay Road on the west side of the industrial park southeast, traversing ILM property, and intersecting Smith Creek on the southeast side of the airport. Acorn Branch Acorn Branch Acorn Branch