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Stormwater Master Plan and Infrastructure Improvement Strategy

Sta 26 & Bayonne St

Project Summary:

Seamon Whiteside and Associates (SW+’s) has been contracted to develop a resilient and comprehensive drainage infrastructure improvement strategy. The intent of this study will be to complete a holistic investigation of the entire island (including the SCIIP and HMGP project areas) to provide an understanding of existing flood conditions, develop solutions to mitigate existing flood conditions, and provide conceptual designs to springboard project implementation once funding becomes available. Based on understanding the Town’s needs, the following sections outline SW+’s approach to providing a cost-effective and scientific, yet constructable solution.

Sullivan’s Island is a low-lying barrier island that is mostly residential in nature but serves as a recreational haven for tourists and neighboring communities. This unique coastal community is full of history and has done a great job at preserving an outstanding quality of life. However, extreme flood events and aging drainage infrastructure are beginning to create challenges in maintaining and achieving long-term coastal resiliency. As a result, this project is critical for holistically investigating drainage deficiencies and developing a sound comprehensive strategy to address flooding experienced today while also preparing for tomorrow’s changing coastal environment.

Pending and Potential Stormwater Improvement Project Map
Click on the image below for an interactive map of pending and potential stormwater improvement projects. For the non-interactive PDF version please click here.

Pending and Potential Stormwater Improvement Projects

Existing Conditions:

A considerable portion of the island has existing drainage infrastructure, much of which was installed by the South Carolina Department of Transportation SCDOT). Rather than discounting these systems with a full-blown replacement, which can be costly and burdensome on the community, there may be cost-effective maintenance and/or retrofit solutions that should be explored. Such solutions, as well as new drainage systems in areas without any drainage systems, can be explored with town staff and community members to identify the best and most viable long-term solutions for the community. This is the crowning goal of this project; finding the best and most cost-effective solutions that will enable Sullivan’s Island to continue thriving for hundreds of years to come.

A primary challenge faced by the Town’s staff is addressing regular complaints related to nuisance stormwater-related flooding within conveyance systems owned and maintained by the South Carolina Department of Transportation (SCDOT). The Island’s drainage system is comprised of a mix of open ditches and closed pipe drainage systems owned by SCDOT with all administrative functions related to NPDES 2 (DHEC permitting) overseen by the Charleston County Stormwater Management Program. Heavy rainfall events, extreme high tides, sea level rise, high water table, and coastal storms, combined with inadequately sized and poorly maintained infrastructure, creates flooding conditions throughout the Island. The impacts of this flooding include:

  • impassable streets;
  • flooding from the right-of-way onto adjacent lots;
  • flood damage to non-elevated structures; and
  • inflow into the Town’s sanitary sewer collection system.

Project Phases:

The Town’s engineering consultant, Seamon Whiteside and Associates (SW+’s), will develop the stormwater master plan in phases by systemically inspecting and cataloging existing drainage infrastructure, leveraging innovative technologies, and welcoming resident feedback while developing a hydrologic and hydraulic model which can be used to identify drainage system deficiencies and coastal food risk.

Upon completion, a report detailing the process and results of the stormwater master plan will be delivered to the Town along with any supporting data. This final report (or plan) will serve as a foundation for the Town to move forward with pursuit of project funding, engineering design, permitting, and construction. The project will generally follow the phases noted below:

Phase 1 – Data Gathering:

  1. Data collection of reports, studies, plans, land use, and other resources necessary to understand the existing conditions.
  2. Field inventory of the existing drainage system including identification of contributing factors (drainage system data currently held by SCDOT, US Department of Agriculture, Charleston County Stormwater Management Program and the Sullivan’s Island Water and Sewer Department will be made available for use).
  3. Inventory of pipe sizes and conditions documented by way of GIS and Television as needed.
  4. Inventory of all marsh stormwater outfalls with elevations.
  5. Inventory all random changes in elevation from southern collection point to outfalls island-wide.
  6. Geographic Information Systems mapping: Ensure all GIS data, shapefiles, and raster data are collected and provided to Town Staff as a Geodatabase via ArcMap version 10.8.1 (or more current version).

Phase 2 – Analysis:

  1. Development of existing hydrologic and hydraulic models, including but not limited to:
    • Rainfall – runoff;
    • Overland and pipe network flow;
    • Coastal flooding; and
    • Extreme tidal flooding.
       
  2. Development of future conditions hydrologic and hydraulic models, which account for:
    • Future land cover;
    • Future hydraulic setting; and
    • Changes in rainfall patterns combined with sea level rise.
       
  3. Problem area identification
     
  4. Development of design criteria targets.

Phase 3 – Alternative Analysis and Findings:

  1. Create an alternatives analysis for problem area remedy.
     
  2. Create Summary Report of findings, including but not limited to:
    • GIS Mapping;
    • Summary of the existing drainage system;
    • List of priority projects (see Part 4, below)
    • Documentation of methods; and
    • Technical data and related information.
       
  3. Sensitivity Analysis to study the impact of higher flood frequency events or sea level rise.

Phase 4 – Project Recommendations:

  1. Prepare cost estimates for recommended improvements.
  2. Create a ranking system to prioritize projects.
  3. Develop “what if” scenarios.
  4. Presentation of Recommendations:
    • Meeting 1: Facilitate a kick-off meeting to discuss the current situation and establish goals for the project
    • Meeting 2: Provide a mid-project update to Town Council. Explain concepts, strategies and progress report
    • Meeting 3: Final presentation of Master Plan with accompanying GIS deliverables.                                                                 

Project Documents