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10/29/2024

Settling Time

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As cooler weather moves in, I hope you find time to kick back with a warm cup of your beverage of choice and this article considering Settling Time. For this article we will talk about different factors which impact how long it takes for particulates to settle out of stormwater once in the retention basin.

Stormwater retention basins are features that help to do the following:
*  Hold back water to help mitigate downstream flooding.
*  Settle out particulates to improve downstream water quality.
*  Slow down the rate of stormwater flow to help control erosion.
​*  Lower the temperature of the stormwater before it can flow downstream.​

As storm water flows over construction sites, parking lots and roads it picks up dirt and other debris that make up the suspended solids. The suspended solids can be sand, clay, silt and other particulates. The sediment load will vary based on the amount and duration of the rain event. Once the stormwater reaches the basin its flowrate drops off. In a quiescent pool the solids carried by the stormwater are given time to settle. If there is still flow through the pool or a second rain event occurs particulates may not have time to completely settle. These flow conditions would result in dynamic settling or possibly resuspension of sediment. Some particles are so small they will not settle. Others will have a settling time that is based on their particle size, shape, density and the water temperature. Solid settling in a stormwater basin can be estimated by Stokes’ Law.

Where:
V = settling velocity of the solid
g = acceleration of gravity (constant)
p1= mass density of the solid
p = mass density of water (constant)
d = diameter of the solid (assuming spherical shape)
µ= kinematic viscosity of water (varies only with temperature).

From the above equation we can see that particulates of higher mass density (p1) will settle out a bit more quickly than particles with lower mass density. (p1) We can also see that larger diameter (d2) particulates will settle out much faster than smaller particles. As diameter increases the settling velocity increases exponentially. Suspended solids vary in size with a particle of coarse sand being about 1,000x larger than a particle of fine clay. Given the larger diameter of the sand particles, we would expect them to settle much more rapidly than clay. Lastly temperature comes into play. Water becomes more viscous (µ) as temperature decreases. This higher viscosity leads to slower settling times.
Picture
https://www.kindergartenworksheetsandgames.com/beach-summer-density-tower-science-experiment-for-kids/
​This children’s science experiment provides a good visual overview for settling and density. You can see that sand settled to the bottom with less dense water above and the least dense oil floating on top
Other factors come into play when considering settling time. Though the Stokes equation assumes that particles are round, few particles would be perfectly spherical. The irregular shaped particles would likely settle more slowly. Concentration of particles also plays a role. Like snowflakes falling from the sky, sediment particles can collide forming larger floc particles. These floc particles will settle more rapidly. In fact, flocculant is sometimes used to bond to suspended sediment or chemicals to sink them to the bottom and reduce the amount of polluted water leaving the basin. Chemical properties will also play a role. An extreme being road salt. The salt mineral is dense and of large diameter but will dissolve into the water. 
Picture
The dissolved road salt will not settle out of the stormwater. Particles with a greater attraction to water will settle out more slowly or will remain in solution.
A well designed and maintained basin will slow water and hold it long enough to allow sediment to drop out of solution. Oversizing a basin is one way to ensure plenty of settling time. Depth of the basin also comes into play by providing a large enough sediment storage zone. Maximizing the distance between the inflow and outflow helps maximize detention time. The challenge is designing a basin that will provide time for solids to settle, store sediment, minimize the valuable real estate it will take up and provide for easy maintenance down the road. 
​
For more information contact our office at 513.695.1337

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    Urban team BLOG

    Every month, the Warren Co SWCD Urban Team dives deep into the world of land development as it relates to stormwater pollution prevention. The blog covers topics like erosion & sediment control best management practices (BMPs), state and local regulations, retention/detention basins, and the conservation of our natural resources. Stay up to date with Development Digest by signing up for WCSWCD Urban eNews!

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  • Home
    • About
    • Contact Us
    • Board Supervisors
    • Staff
    • NRCS
    • Employment Opportunities
    • Volunteer Opportunities
  • Upcoming Programs/Events
    • Ag Tour
    • Future Conservationist Day Camp
    • Introduction to Kayaking
    • Lebanon Oktoberfest
    • Stream Clean Ups
    • Stream Encounters
    • Summer Contests
    • Urban Chat Series
    • Warren Water Striders
    • Women for the Land
    • Partner Events
  • Services
    • EQUIPMENT RENTAL
    • For Residents >
      • Drainage, Erosion & Pond Assistance
      • Soil Testing
      • Water Testing
      • Stormwater Basin Inspections
      • Maps & Historic Aerial Images
      • Backyard Conservation >
        • Ohio Native Plants
        • Pollinator Pathways
        • Rain Gardens
        • Rain Barrels
        • Composting
    • For Agriculture Producers >
      • Nutrient Management
      • Land Preservation
      • Cover Crops
      • Cover Crop Incentive Program
      • GIS/Mapping
      • Caesar Creek Collaborative >
        • Meet the Collaborative
        • Our Work
    • 2024 H2Ohio Program
    • For Businesses >
      • Stormwater Basin Inspections
    • For Municipalities >
      • Illicit Discharge Detection Program
      • Operation Rain Garden Grants
  • PERMITS
    • Earth Disturbing in Warren County >
      • Earth Disturbing Permit Application
    • Floodplain Management >
      • Floodplain Determination Request Form
      • Flood Applications, Forms & Documents
  • Education
    • Programs >
      • Classroom Programs
      • Youth Programs
      • Community Outreach
    • Virtual Classroom
    • Urban Conservation Learning Lab
    • Dagmar the Dragonfly
    • Turtle Education Ambassador
    • Program Loan Portal
    • Scholarship Opportunities
    • HS Envirothon
    • Educator Workshops
  • Blogs
    • Conservation Connection
    • Development Digest
    • Education Connection