Swale

Description

Swales have been used for stormwater conveyance and infiltration for over 50 years. In the last 25–35 years, annual removal credit has been granted based on research and field measurements conducted in the State of Florida.

Annual removal can be quantified:

  • As a retention BMP
  • For reducing annual stormwater pollutant mass load

As a retention system, the Seasonal High Water Table (SHWT) must be at least 2 feet below the bottom of the swale to earn annual removal credit using the BMPFast removal equations.

Concentration reduction is accounted for by the residence time in the swale (Wanielista, M.P. and Yousef, Y.A. (1993). Stormwater Management. John Wiley & Sons, New York, pp. 244–251).

In general, concentration reduction occurs when the longitudinal slope is less than 1% (1 foot drop in 100 feet).

Statutory and Design Definition

Additional design details defining a swale are found in:

  • Chapter 403.803, Florida Statutes
  • Applicant’s Handbook, Vol 1, Appendix O (June 28, 2024)

A Swale is defined as a manmade trench which:

  • Has a top width to depth ratio equal to or greater than 6:1
    OR side slopes equal to or flatter than 3 horizontal to 1 vertical
  • Contains contiguous areas of standing or flowing water only following a rainfall event
  • Is planted with or has stabilized vegetation suitable for soil stabilization, stormwater treatment, and nutrient uptake
  • Is designed considering soil erosion, soil percolation, slope, slope length, and drainage area to prevent erosion and reduce pollutant concentration of discharge

Swale Categories

Swales designed for stormwater treatment can be classified into three categories:

  1. Swales with swale blocks or raised driveway culverts
  2. Swales without swale blocks or raised driveway culverts
  3. Swales incorporated into landscaping (elongated rain garden)

Typically, swales are online retention systems, and their treatment effectiveness is directly related to the amount of annual stormwater volume infiltrated.

Annual removal follows the retention curves, and it is assumed the swale empties the design volume within 72 hours.

Input Data

Data required to define the characteristics of a swale include:

  1. Geometric data of the swale:
    • Bottom width (if any)
    • Side slopes
    • Length of swale
    • Width at flood control
  2. Limiting infiltration rate (inches/hour)
    • Usually obtained from field tests
    • Includes a factor of safety
  3. Manning’s overland flow coefficient (N)
  4. Bottom slope (ft/ft)
  5. Swale block dimensions (if used)

The catchment area generating runoff must equal the catchment specified in the watershed characteristics worksheet for post-development condition.

Example input:

  • Catchment area = 1.2 acres

Discovery and Optimization (Plot Feature)

Alternative designs can be evaluated using the Plot feature of BMPFast.

The plot function shows:

  • Annual removal of runoff expressed as a percent of total annual runoff
  • As a function of rainfall depth

For a specific location and design, the example plot is shown below.

By adjusting:

  • Geometric dimensions
  • Swale size
  • Replacing infiltration soils with media
  • Other design parameters

The resulting change in annual capture can be determined.