Building dams and creating retention ponds are part of a 123 page Flood Mitigation Study
As Eastern North Carolina recovers from the aftermath of Hurricane Florence, which brought a crest of 25.8 feet to the Neuse River at Kinston and major flood status reported in several counties, it’s hard to forget that similar devastation occurred only two years ago with Hurricane Matthew. Hurricane Matthew made landfall Oct. 8, 2016 and eight days later the Neuse River at Kinston reached a record 28.31 feet.
Hurricane Matthew flooding prompted the Neuse River Basin Flood Analysis and Mitigation Strategies Study, a report published by North Carolina Emergency Management (NCEM) in partnership with North Carolina Department of Transportation (NCDOT) and River Basin Advisory Committees. The study published on May 1.
This report is an attempt to summarize some of the strategies mentioned. To download the full report, click on the button below.
The report highlights 12 different flood mitigation strategies developed and analyzed by NCEM:
New detention structures
Retrofit of existing detention structures
New embankment structures
Existing levee repair/enhancement
Roadway elevation/clear spanning
Large scale wet flood-proofing
Land use strategies
River corridor greenspace
Each strategy has varying implementation time frames, benefits and costs to various communities in the Neuse River Basin, which can be found on pages 4-6 in the report.
The strategies most relevant to these communities that were studied at length by NCEM include building new detention structures, channel modification and land use strategies.
New Detention Structures
Eight scenarios were investigated combining various new sites for both both wet and dry detention sites throughout the Neuse River Basin.
Wet sites hold a permanent conservation pool but still provide flood storage. There is the opportunity for recreational activities and water supply from the conservation pool, but there is the potential for water quality issues and disruption of connectivity of the waterway.
Dry sites only hold water during a flood event and allow for more flood storage with less impact on streams and wetlands than wet detention.
Both options have long implementation timeframes, with a dry reservoir taking 7-15 years and a wet reservoir taking 15-30 years or more.
Of the 10 sites investigated for potential new detention centers, Wilson’s Mills featured in several scenarios as a location for a dry dam, configured to continually release water during the flood event but still retain some water, reducing the flood peak.
The proposed Wilson’s Mills site would be located on the Neuse mainstem to the northeast of the town of Wilson’s Mills, roughly 30 miles from Goldsboro. Scenarios 1, 3, 4 and 8 all involve Wilson’s Mills, and the proposed costs, losses avoided, and benefit-cost ratio can be found in Table 7.1.
The proposed strategy of channel modification would involve dredging the Neuse River in Kinston to increase the efficiency of water flow in an effort to reduce water surface elevations during a flood event.
The implementation timeframe for this strategy would be seven to 10 years, owing to permitting, legislative and environmental concerns.
Flood damage reduction in Lenoir County as a result of dredging would amount to 61 percent reduction for a 100-year recurrence flood event and 82 percent reduction in Kinston. The direct benefit-cost ratio for a 30-year time horizon would be 1.10, estimating costs of dredging, disposal and maintenance.
Other considerations of the dredging scenario include increased velocities from the dredging resulting in increased erosion, decreased bank stability and disruption of biodiversity in the river, making detailed studies necessary to pursue such a strategy.
The elevation/acquisition/relocation strategy involves reducing the amount of damage done by floods instead of attempting to reduce the flood event itself and has one of the shortest timeframes for implementation at 3-5 years.
Elevation involves physically raising a building so that the floor is above the base flood elevation, acquisition involves purchasing and demolishing the building and relocation involves the relocating the structure to a property outside of the floodplains.
Elevation, acquisition and relocation efforts are still ongoing in the wake of Hurricane Matthew and as such, those were not considered in the analysis of the scenarios.
Mitigating all structures identified with a finished floor elevation less than the base flood elevation was calculated to have a benefit-cost ratio of 0.54 on a 30-year time horizon and would include 1,562 treated structures.
Various benefit-cost ratios were calculated to determine the effect of prioritizing the most at-risk structures and the effect in different communities, which can be viewed in the full report.
NCEM concluded that elevation/acquisition/relocation is the most effective flood mitigation strategy based on the implementation timeframe, scalability of funding allocation, ability to target most vulnerable structures and communities and the benefit-cost ratio.
U.S. Rep. Walter Jones agrees with this conclusion, according to Allison Tucker, his communications director.
“There is definitely a strong need and role for state and federal cooperation for flood mitigation strategies,” Tucker said. “Most notably, we believe the best opportunity to do so according to benefit/cost ratio is through elevation/acquisition/relocation strategies through FEMA’s Hazard Mitigation Grant Program.”
Land Use Strategies
Though analysis of upstream development did not show statistically significant contribution to flooding on the mainstem of the Neuse River, it is important to understand how increased development contributes to flash flooding and future damages.
In Wake and Johnston Counties, there have been 19 percent and 16 percent increases in population, respectively, which is also where the most developed areas in the Neuse River Basin are located.
Nearly 16 percent of land cover in the Neuse Basin was developed as of 2011, up from 13.8 percent in 2001; 3.4 percentage points of that cover is impervious cover, which is any type of human-made surface that doesn’t absorb rainfall. The runoff from these surfaces can create secondary problems by overwhelming the capacities of bodies of water.
Land use strategies such as smart growth planning and eliminating new development in flood prone areas can help prevent flash flooding and future damages.
While many residential zones in the Neuse Basin keep impervious cover below the 10-percent threshold established by the Center for Watershed Protection, large-lot zoning that requires houses be spaced far apart increases impervious surfaces through extensive creation of roads and utility infrastructure.
Strategies to eliminate some of these issues include natural buffer areas of grass and plants that can absorb the water and green design that utilizes green roofs, permeable surfaces and natural habitat creation.
The successful implementation of any mitigation strategy depends on a variety of factors, including cooperation between federal, state and local governments.
Currently, the Senate is continuing the effort from Hurricane Matthew to get supplemental funding for flood mitigation strategies across eastern North Carolina, including the Neuse River Basin, according to a source from Sen. Richard Burr’s office.
According to Adam Webb, press secretary for Sen. Thom Tillis, “Sen. Tillis believes bringing together experts and officials on every level of government to examine ways to prevent these damages from reoccurring is an important next step in the coming months, and he will continue to talk with federal and state officials about preventative measures to protect these communities moving forward.”