EEC's Blog

There is Nothing Retro about the Latest Basin Retrofits 

By Brian Storz 

Development can take a toll on the natural environment by creating a major issue: storm water runoff.  

An occasional flood is good for overall stream health and biodiversity, but regular torrents of stormwater coming from surfaces like roads, roofs and parking lots damage our local creeks.  

“When we have all this additional runoff from impervious surfaces, it’s sending water into our creeks at a higher volume and faster flows,” said Nicole Clements, watershed coordinator for the Banklick Watershed Council. “When you subject our creeks to this additional volume and velocity, it quickly erodes our streams and dumps sediment into our water.” 
The problem with stormwater runoff 

An authority on northern Kentucky stormwater basin issues, Clements said one of Kentucky’s greatest challenges is the delivery of stormwater into creeks.  

Stormwater runoff has a tremendous amount of energy when it hits a nearby creek, which can result in stream bank erosion, siltation and reduced water clarity, disruption of the stream bed, loss of animals living in the stream and an overall degradation and instability of the entire creek habitat.  

“We see more and more runoff coming in at a velocity that the creeks just can’t handle,” Clements said. “Streams erode, banks fail and sediment fills our waters. It’s not just about flood control anymore. We have to start designing for erosion control.” 

Caption: A degraded tributary of the Salt River, showing severe incising. Photo credit: Perry Thomas 

Stormwater runoff is the main contributor to nonpoint source pollution, pollution without a distinct, identifiable source.  Stormwater runoff pushes fertilizers, fecal bacteria, sediment, pesticides, and herbicides into our creeks, which affects water quality.  

The repeated pattern of stormwater flashing into creeks not only negatively impacts the stream habitat and biological systems, but also hits human communities in the pocketbook by causing erosive damage to public and private property and to roads, bridges, sewers and water lines.  

To tackle this issue, the first step starts with science.  

Caption: Public infrastructure damage due to creek erosion. Photo credit: SD1 

The science of determining a stable stream flow 

The answer to understanding how to reduce stormwater efficiently and quickly without damaging local habitats and biological systems in our local creeks comes down to understanding Qcritical.  

Qcritical is a method to estimate the threshold above which the stormwater discharge begins to pick up and move the streambed material, which can negatively impact the physical and biological properties of the creek.  

According to Bob Hawley of Sustainable Streams, LLC, Qcritical is the link between stormwater management, stream erosion, water quality and even biology.  

”Optimizing stormwater controls to reduce discharges below Qcritical will not only help to reduce erosion in local streams, but also reduce sediment loads and improve habitat stability for the aquatic animals that inhabit the streambed,” Hawley said.   

The Qcritical must be calculated for each creek because creek geomorphology (wide-shallow, thin-deep, etc.) and streambed bottom type (sand, gravel, cobble, or boulder) impacts the calculation. 

Conventional Stormwater Detention Basin 

Conventional stormwater detention basins are depressions dug into the landscape meant to collect runoff from impermeable surfaces before it reaches a nearby creek, and after which, slowly release the runoff into said nearby creek.  

Caption: Stormwater detention basin illustration courtesy of


Conventional stormwater detention ponds are supposed to hold the stormwater long enough to allow particles and pollutants to settle out and release the runoff at a rate to prevent flooding and creek erosion, but, they often maintain drainage levels above the Qcritical threshold for longer periods, allowing flows strong enough to erode the downstream channel.  

They also maintain drainage at levels above the Qcritical for a much longer time, which means the possible negative impacts are occurring for a longer period of time as well.  

These basins were a first attempt at protecting our local creeks from runoff quantity and quality issues, but in order to reduce the negative physical and biological impacts occurring in our creeks, stormwater detention basins need to be integrated with the Qcritical model.  
The future is retro 

If our goal is to stay below the Qcritical threshold during the duration of a stormwater runoff event, the existing, conventional stormwater detention basins need retrofits that match their newly calculated Qcritical values. 
Retrofits are a simple reduction in the size of the outlet control structure, which restricts outflow of the stormwater to at or below the Qcritical. Not all are suitable for retrofitting.  

Caption: These images show two of the different options for modifications of the outlet control structure. The left image is a metal restrictor plate that attaches to the existing concrete outflow and the amount of water discharged through it should be below Qcritical. Photo credit: Matt Wooten 
Image on the right is an Outflow “Detain H2O” version of stormwater basin retrofit. This pipe structure is attached to an existing concrete outflow and the amount of water discharged through it should be below Qcritical. Photo credit: Bob Hawley. 
The retrofits can work just as intended, by means of outflow reduction reducing the outflow down to below the Qcritical and maintaining it there during the duration of the stormwater runoff event.  

Caption: A retrofitted (Detain H2O) stormwater detention basin. Photo credit: Bob Hawley.   

What about stormwater quality? 

To improve water quality even more, we can take further steps to retrofit our stormwater basins for additional nonpoint source pollution uptake.  

An example of a stormwater basin that can improve stormwater quality above and beyond the Qcritical/basin retrofit, is a bioretention basin. 

Caption: A bioretention basin, like above, can have permeable substrates on the bottom, allowing for infiltration, and native plants that soak up a tremendous amount of storm water and pollution while the stormwater is in the basin (Wang et al. 2022).  
While bioretention basins are substantially more costly, when combined with Qcritical/basin retrofit, our conventional stormwater basin can be quickly transformed into a super tool for effectively managing both the quantity and quality of stormwater entering our local creeks.  

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