Reservoir Model Background
Cherry Creek Reservoir is a flood control reservoir that has had periodic blue-green algae blooms and high chlorophyll α concentrations.
To increase oxygen at the bottom of the Reservoir and reduce the amount of internal nutrient loading, a destratification system was installed in the Reservoir in 2008. The system was also intended to help control cyanobacteria in the Reservoir by disrupting their buoyancy. However the destratification system has not achieved these goals. Additionally, there has not been enough evidence to show how the destratification system may influence cyanobacteria blooms.
Due to continued water quality concerns, a water quality model of the Reservoir was developed to:
- Better understand the causes of chlorophyll α standard exceedances and cyanobacteria blooms;
- Determine the impacts of the destratification system;
- Provide a tool to help predict the effects of future management strategies.
CCBWQA chose a two-dimensional hydrodynamic and water quality model of the Cherry Creek Reservoir that simulated in-reservoir water quality for 2003-2013. In 2019, the Reservoir Model was updated with data through 2017. To investigate the findings of Model 2 (Increased Destratification System Mixing) a Bubble Plume Model was coupled with the Reservoir model to mechanistically simulate the effects of the destratification system on mixing in Cherry Creek Reservoir. In addition, a Destratification System Study was initiated and will be completed in 2020.
How would the Reservoir respond to the best currently anticipated reduction of nutrients (nitrate, ammonia, and orthophosphate) through watershed controls?
The Reservoir would respond with chlorophyll a concentrations at or below the 18 ug/L summer average standard over the 11-year simulated years (2003-2013). Lower summer cyanobacteria peak concentrations are also predicted.
How much increased vertical mixing is needed for the destratification system to meet the original bottom DO design target of 5 mg/L?
Model results indicate that a destratification system with three times the vertical mixing effect of the current destratification system would result in at least 5 mg/L of DO at the bottom of the Reservoir at nearly all times from 2003 through 2013. Average summer chlorophyll awould decrease in all years, though the value would be above the 18 ug/L standard value in one or the eleven years.
Additional Investigation: Bubble Plume Model
How would the Reservoir respond to a combination of the best anticipated watershed controls and destratification mixing that achieves 5mg/L DO at the bottom?
As would be expected, Run 3 results predict that simulated average summer chlorophyll a concentrations would be lower than those of both Run 1 and Run 2, resulting in summer averages at least 4.8 ug/L below the 18 ug/L standard in all years. Peak summer cyanobacteria concentrations would also be sharply reduced.
What percent reduction in inflow orthophosphate concentration is needed to meet the 18 ug/L Chlorophyll a standard value for all simulated years?
Modeling indicated that a 75% reduction in inflow orthophosphate concentrations from Cherry Creek was needed to meet the chlorophyll a standard value in all eleven simulated years.
Does the model indicate an adverse effect of increased cyanobacteria in response to a disproportionate reduction of inflow nitrogen relative to orthophosphate in this nitrogen-limited system?
Based on the model results, further reductions to inflow nitrogen relative to orthophosphate are not expected to have a large effect on summer cyanobacteria, but could increase spring blooms of the nitrogen‐fixing cyanobacteria, Anabaena flos‐aquae.