From the first HEC-RAS model of no dredging, we established that there is a need for dredging of the channel. The flow rate needed to overflow the defined bank of the channel was only 350 cfs which is very likely to happen every year according to our flood frequency analysis. Flow rates greater than 350 cfs will start to overflow the main channel of the Yahara River.

​

Dredging 2 feet of sediment along a 50-foot width of the channel was modeled in the second HEC-RAS model. This is the model that fits the plans of Dane County Sediment project. From this dredging, the capacity of the river channel increased to be 550 cfs, and reduced the water surface for the 10-year storm by 1.3 feet. This offers a significant improvement in flow volumes before overtaking the banks of the channel. The flood frequency analysis shows that the maximum allowable flow rate has a probability of happening about every 2 years. 

​

Finally, the third model shows that dredging the entire width of the channel increases the capacity of the channel to 800 cfs. This flow rate has the probability of happening every 11 years. This model shows that the water surface for the 10-year storm is only reduced by an additional .3 feet as compared to the 50 foot dredged model. 

 

More dredging leads to a higher channel capacity, however the benefit of an additional .3 feet of water surface reduction for the 10-year storm is not worth the cost of dredging the entire channel. For this project, we think that the cities plan is a good approach since their main goal is to decrease the amount of time it takes for 2 inches of rain to flow through the chain of lakes by half. The current plan to remove 2 feet of sediment from a 50 foot width increases the capacity of the channel by around 200 cfs and will help storm events flow through this section of the Yahara Lakes system.

​