1.Background
Wilmington is within the North Carolina Coastal Plain physiographic province, which extends from the Piedmont eastward to the North Carolina coast. Six regional aquifers are present in the region surrounding Wilmington, including the Surficial Aquifer, the Castle Hayne Aquifer, the Peedee Aquifer, the Black Creek Aquifer, and the Upper and Lower Cape Fear aquifers. The aquifers are water-yielding formations that are more permeable than the finer-grained formations (confining units) that are typically above and/or beneath these coastal aquifers.
2. Problem
Although the aquifers are more permeable, Wilmington is stilling facing flood problem. Precipitation in Wilmington occurs year-round, but it has a seasonal precipitation characteristic. April is the driest month, with less than 3 inches (76 mm) of rain in 2019, and July to September are the wettest months, with over 7 inches (180 mm) of rain each in 2019. In an average year, the July to September period delivers about 40% of annual rainfall. Due to the proximity of warm Atlantic Ocean waters and prevailing tropical-system tracks, the Wilmington area is subject to hurricane or tropical storm activity, mostly from July to early October. Such tropical systems can bring high winds and very heavy rains, which caused its seasonal precipitation.
In September 2018, Wilmington sustained a direct hit from Hurricane Florence. More than 450 people required rescue across Wilmington. By the morning of September 16, Wilmington had recorded more rain from Florence than any other single weather event in the city’s history. The city of Wilmington became entirely isolated, as all roads to the city flooded and were deemed impassable. The Cape Fear River crested at 61.4 ft (18.7 m)—about 35 ft (11 m) above flood stage early on September 19. The magnitude of flooding greatly exceeded the levels observed due to last big Hurricane: Hurricane Matthew in 2016.
In Wilmington, Sweeney Water Treatment Plant treats and distributes water within the City of Wilmington. Surface water drawn from the Cape Fear River is processed through Sweeney’s facility that incorporates the water treatment technologies including the use of ozone and UV for disinfection purposes. Sweeney Water Treatment Plant has the capacity to treat up to 35 million gallons of water per day already during dry season.
The wastewater treatment system carries sewage and wastewater from homes and businesses, to the sewage treatment plants. Wastewater is treated and cleaned during dry season before it is discharged into the Cape Fear River.
Wilmington is facing a severe problem with its sewer system during hurricanes season because has a combined sewer system. Combined sewers are pipes that catch both sewage and storm water and route it to a waste water treatment plant. The biggest problem with combined systems are that during rainfall events like hurricane, the water flow often exceeds the systems’ capacity. When the systems’ capacity is exceeded, the water is discharged directly into local waters without treatment. This polluted water carries with it bacteria, chemicals, and other pollutants that harm our rivers, streams and public health. We believe water infrastructure investments are essential for the future.
3. Solutions
3.1 City’s proposed solution
To solve the problem, Wilmington came up with the Storm water capital projects.
Storm water capital projects are constructed when the existing storm water drainage system is inadequate and results in flooded streets and property. Storm water capital projects also strive to improve water quality whenever possible. The blue area in the map shows the area that have completed the improvement. Storm water utility fees provide the funding and staff resources for planning, design and construction of capital improvement projects. Another strategy is the New Hanover County Hurricane Florence Long-Term Recovery Plan. First, it planned to remove debris and sedimentation from key streams and storm water drainage conveyances where flooding damage occurred. Second, it aimed toadd or increase outflow in watershed areas that experienced flooding that affected homeowners and businesses. Also, the New Hanover County Hurricane Florence Long-Term Recovery Plan want to elevate some portions of the highway in the county, like I40 and I95.
Sweeney Water Treatment Plant treats and distributes water within the City of Wilmington, parts of Ogden, Monkey Junction, King’s Grant and the Flemington/421 Corridor. Surface water drawn from the Cape Fear River is processed through Sweeney’s facility that incorporates the water treatment technologies including the use of ozone and UV for disinfection purposes. Sweeney Water Treatment Plant has the capacity to treat up to 35 million gallons of water per day. The wastewater treatment system carries sewage and wastewater from homes and businesses, to the sewage treatment plants. Wastewater is treated and cleaned before it is discharged into the Cape Fear River. The wastewater treatment plants are located along the Northeast Cape Fear River and Smith Creek.
3.2 Our proposed solution
A storm water system with open channels for the discharge of rainwater exists in most urbanized areas. Wilmington’s sewer system also includes a lot of channels.
Here is an example
The sewer system in the neighborhood is highly impacted and impaired due to excessive storm water flows from upstream development. The drainage system consists of street inlets, pipe networks, small ditches/swales and larger man-made and natural drainage channels located throughout the system. The upstream section of the drainage basin contains the majority of the street collection systems draining to the main outfall run. The main run consists primarily of large underground storm sewers (interspersed with open channels) in the area upstream of College Acres Drive. In the basin area, downstream of College Acres Drive, the main run consists primarily of a natural open channel with one road culvert crossing. The city is planning to Install large box culverts (pipes) to convey storm water under College Acres Drive and Mallard St. and install large culverts to capture and convey floodwaters from the S. College Road/New Centre Drive commercial areas. However, we believe that use stream restoration to increase flood storage capacity is also necessary for neighborhood that rely largely on natural open channel.
4. Strategies
4.1 Blue Infrastructure
- Cross-vanes — To reduce erosion to the stream banks, stones are placed across the stream in the shape of a C or V to guide the water into the center of the channel and away from the channel banks.
- Step pools — The stream bed is stepped down in elevation. Typically, rocks are placed in a series of rows that mimic stairs, creating small pools of water between each “stair”. This technique is used to stabilize a stream with a steep slope while the pools dissipate the energy from high flows and provide habitat for aquatic life.
- Planting — Along the stream, plants are planted to control erosion of the stream bank by keeping the soil in place with their roots. The stems of woody plants also help reduce the energy from stream flows along the banks, providing additional bank protection and ultimately provide shading for temperature control and a source of wood and biomass input to the stream.
- Woody debris — Logs and other woody material on the side of the stream bank or extending into the stream channel have several benefits. Depending on where the woody debris is placed, it can reduce erosion by acting as a buffer to a stream bank. Additionally, woody debris of various sizes, from small branches to large logs, can provide habitat for a variety of aquatic life.
4.2 Green Infrastructure
Besides the solutions for the blue infrastructure, we consider some green infrastructure improvements in Wilmington as Philadelphia does. The Philadelphia Green City, Clean Waters program aims to use green storm water infrastructure (GSI) to reduce the amount of polluted storm water overflow into natural water systems in around Philadelphia by 85% by 2035. It is beneficial to increase community buy-in to green storm water infrastructure and help bridge the information gap to promote buy-in and ownership in the neighborhood. Community programming around GSI technology can be paired with community workdays to convert some vacant lots into rain gardens. In addition, public and institutional buildings can adopt GSI technologies like green roofs and rain gardens in schools, libraries and community centers. It needs government support and a lot of funding for maintenance.
5. Conclusion
To conclude, stream restoration strategies could increase flood storage capacity based on the natural open channels, and the green infrastructure strategies could help absorb water and help reduce the strain on local sewer overflows. So, we imagine Drainage Improvement Project could mitigate the flooding issues caused by the climate change and create a green image in Wilmington.
Reference:
https://www.cfpua.org/257/Water-Treatment
https://www.wilmingtonnc.gov/departments/public-services/stormwater/education-outreach/get-educated
https://www.newsobserver.com/news/business/article235963052.html
https://www.cfpua.org/CivicAlerts.aspx?AID=1206
https://www.cfpua.org/CivicAlerts.aspx?AID=1203
https://www.wilmingtonnc.gov/departments/public-services/stormwater/projects
https://www.cfpua.org/CivicAlerts.aspx?AID=1167
https://nhcgov.maps.arcgis.com/apps/webappviewer/index.html?id=b32500df56894a158c34595a05763e13
https://extension.umn.edu/landscape-design/rain-gardens
https://www.wilmingtonnc.gov/departments/major-construction-projects/clear-run-branch