"April showers bring May flowers". Those same rain showers can contribute to other types of blooms; algal blooms. Plants need sunlight, nutrients, water and a favorable environment in which to grow. Algae have similar requirements. A shallow basin allows intense sunlight to shine to the bottom and allows the water to heat up quickly. Longer spring days mean more sunlight available to the algae. Warming temperatures also contribute to the growth of the algae. Nutrients that are bound to sediments can wash into the basin from the surrounding drainage area and help drive algal blooms.
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Why does algae in a basin matter? Algae is a part of any heathy aquatic ecosystem so at normal levels they are not a problem. When one species of algae blooms, it can lead to maintenance issues and other problems. Let’s look at algae blooms and their impact on the function of the stormwater basin. Dense algal scums interfere and obstruct parts of the basin’s outlet structure, causing it to lose storage capacity or overfill. Algae that die off will sink to the bottom, decay and will eventually fill the basin with muck. This can lead to costly maintenance. 

Beyond the maintenance implications of an algae bloom, there are health and aesthetic implications. As an algal bloom dies off and decays dissolved oxygen in the water is depleted. This can lead to a fish kill. While not all algae blooms would cause a health impact, there is one class of “algae”, the cyanobacteria, that can release harmful toxins when they bloom. The toxins they release can harm pets that wade or drink the water. There can also be respiratory impacts from severe blooms.  Water quality also includes aesthetics. A homeowner who just paid $500K to move into a nice community is less likely to appreciate a basin with a thick algal scum. This can hurt real estate values in the area.

The best way to reduce the impact of a harmful algae bloom is to work to prevent blooms. This starts in the design phase. Designing a wet extended retention basin to a depth of 6-8 feet and with sufficient bank grading will reduce plant growth and algae blooms in the basin. A dry basin should be constructed so it doesn’t hold water for more than 72 hours. During the construction phase, limit the amount of sediment that flows into the basin. This sediment will have bound up nutrients and will provide the food leading to future algae blooms. Remove built up sediment in the basin, the presence of a forebay should make this maintenance easier. Stabilize the basin walls and surrounding ground with seed and straw to help lock soil and nutrients in place, keeping them out of the basin.
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Post-construction measures like keeping a vegetated buffer around the basin will help keep the sides of the basin from caving in. The taller vegetation will also discourage geese from hanging out around the pond and adding more nutrients. The vegetation buffer will act as a filter strip. Native sedges, blue flag iris and rushes are a few plants recommended as native buffers. Woody vegetation and nuisance vegetation should be discouraged. Keep grass clippings out of the basin. Bottom-up aeration can help support a healthy ecosystem and help regulate temperature. Also encourage residents to be responsible if they are applying fertilizer to their lawns. Adopting the four Rs of nutrient management for your lawn can be useful.

​Considering that fertilizer will lead to more costs, mowing, and maintenance, some may decide to forego it.  With proper design and maintenance, the stormwater basin will remain functional and be more enjoyable to have in the community.

​For more information, contact our office at 513.695.1337

​Sediment and erosion control is always a challenge on construction sites when soil is exposed to the wind and rain. As rain falls on the exposed soil, it loosens it up and begins to wash away as fine-grained sediment. Sediment is the top pollutant impacting Ohio’s streams. It clouds the water column, choking out aquatic life, and changes stream morphology by settling out in areas of slow flow. Alluvial fans are a great large-scale visualization of this (Fig. 1). As the water slows down, it drops the sediment out which raises the ground level. Water will seek a new path of least resistance. Hopefully not towards someone’s house! Sediment can change stream morphology too. Deep pools within a stream provide cool-water habitat for fish and these can be filled in as sediment is deposited. What once was a great swimming hole is now too shallow. One way we can reduce sediment discharge to Ohio’s lakes and streams is by installing sediment settling basins on construction sites. These are required by the Construction General Permit as the stormwater must be treated before being discharged from the site.

​A sediment basin is a temporary settling pond that filters runoff and releases it at a controlled rate. They can be used for sites where the drainage area is 100 acres or less. Any project that increases the impervious surface is required to have a permanent, post-construction stormwater basin, so it makes sense to install the sediment basin on the low point of a site, where it will be converted to its post-construction design at the end of the project. They should be installed as a first step to the mass grading process, so they can be functional and ready to accept sediment-laden runoff once the earth is disturbed. Since sediment basins trap sediment, the sediment will need to be removed before project completion. This ensures that the basin has the proper water quality storage capacity when it gets converted to the post-construction stormwater design at the end of the project. All these considerations should guide the design process.

​Ohio EPA’s Rainwater and Land Development Manual (https://epa.ohio.gov/divisions-and-offices/surface-water/guides-manuals/rainwater-and-land-development) is the guiding document for stormwater practices used during development. Chapter 6 covers sediment basin design. Figure 3 illustrates the various components of the design criteria, which includes pool design, embankment design, dewatering design, and spillway design. A floating skimmer device (Fig. 4) is required throughout the duration of the project to drain the water from the top of the water column, where the sediment is less concentrated. So, install the skimmer and let it simmer!

The Ohio EPA has created a sediment basin compliance tool, which we require to be completed for all projects. This tool is helpful to determine the sizing of your basin by inputting numbers relating to total disturbed area and total drainage area draining to the basin. The dewatering zone shall be a minimum of 1800 cubic feet per acre of drainage while the sediment storage zone shall be 1000 cubic yards per disturbed acre that drains to the basin. After inputting the drainage area and disturbed drainage area, it will calculate the required sediment and dewatering zone volumes as well as the orifice size and skimmer size. There is also a Water Quality Volume tool very similar to this tool that guides the post-construction stormwater design. Our office is happy to provide these tools to you upon request.
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Stabilization is not just important for upland controls, but the basin itself should be smooth graded and stabilized as a first step to the mass grading process. That way the basin can be in place before any upslope disturbance occurs to accept sediment-laden stormwater. Towards the end of the project, when the entire site is stabilized, the sediment basin needs to be converted to its post-construction stormwater design. This usually entails removing the skimmer device and installing an orifice plate to achieve the proper water quality orifice size for post-construction treatment of stormwater. As-Builts need to be completed and submitted to the Engineer of Record for the municipality.

​Sediment basins should not solely be relied upon as the only sediment and erosion control measures on a site. Ohio EPA has found that they are only about 50-80% effective at removing sediment. The finer silt particles remain suspended in the water column and are too small to filter out. That’s why a whole toolbox of SECM’s should be used on a site.  The best management practice (BMP) is to prevent erosion and sediment transport in the first place by stabilizing areas at final grade or areas that lie dormant for longer than 14 days. Ohio EPA has found that erosion control BMPs like stabilization are 90-98% effective. So, it takes multiple tools to properly handle stormwater runoff on a site. Sediment basins are a crucial part of the stormwater solution by satisfying both water quantity and water quality requirements. And don’t forget, install the skimmer and let it simmer!

For assistance, please call our office at 513.695.1337

​Article Written by Justin Bedocs

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. 
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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.
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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