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.

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. 

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

​Sediment basins are designed to trap sediment from a construction site. If not maintained properly the basin can become filled with sediment and algae. While keeping sediment and algae out of the basin may seem like an impossible task, there are steps that can lessen the buildup and costly maintenance associated with dipping out or treating the basin. The following article covers sediment basin function, maintenance, and considerations to reduce algae.
Sediment Basin Basics
A sediment basin is a settling pond. By slowing down the flow of sediment-laden runoff, solids can settle out and sink to the bottom of the basin. The sediment basin is constructed as a first step, prior to land disturbing activities. The settling basin is usually converted to a wet retention basin or a dry detention basin at the end of the construction project. During construction, the basin functions for sediment control while after construction, the basin functions for stormwater flood control and water quality improvement practice.
How it Works 
As construction site runoff flows into the basin its flow rate is slowed on reaching the basin. Heavier sediment particles sink to the bottom while clean water remains in the top dewatering zone. As the water makes its way to the outlet a skimmer pulls water from near the surface and directs it through the principal spillway. During extreme rain events water would build up in the basin then flow through the riser and the principal spillway.

Maintenance for Proper Function and Control of Algae
For a basin to function, accumulated sediment must be removed from the sediment storage zone once it exceeds 50 percent of the minimum required sediment storage design capacity and prior to the conversion to the post-construction practice. Excess sediment in the basin decreases the depth of the dewatering zone, brings in other potential pollutants, such as nutrients. Sediment buildup also increases the maintenance required to keep the basin functional.  
The shallow water also allows additional sunlight to make its way to the bottom of the basin further warming the water. Shallow water promotes the growth of vegetation, algae, and mosquitos. Algae can increase maintenance needs by creating additional solids that settle to the basin bottom and fill the sediment storage zone with muck. Some blue green algae can also produce toxins making the water unsafe.
​
Prevent Early Fill-up of the Sediment Storage Zone
The following steps can help minimize maintenance needs while keeping the basin functional.

1. Stabilize the inner bank of the basin as early as possible utilizing grass seed and straw matting.
2. Remove sediment from inflow into the basin by utilizing stabilized grass swales, silt fence, or filter sock and other best management practices on the construction site.
3. Excavate the construction site in phases so that less of the dirt is exposed and subject to erosion.
4. Keep the disturbed area of the site as small as possible.
5. Stabilize the construction site as soon as possible.
6. Have a pond management company treat algae blooms before they become severe.
7. Dip sediment once it exceeds (50%) of the minimum required sediment storage and prior to conversion to post-construction.

Prevention of excess sediment in the basin is often more cost effective than bringing in heavy equipment to dip out the basin and then hauling off the sediment. These steps can also reduce resident and local homeowner association complaints down the road.
 
Additional information can be found in the Rainwater and Land Development Manual, Chapter 6

​Questions? Contact our office at 513.695.1337