By Daniel Johnson
Many concerns have been expressed that the South Boulder Creek Flood Mitigation Project will degrade the groundwater regime on the north side of U.S. 36, downstream of the embankment dam and concrete wall spillway. Based on the geotechnical and hydrological investigations and design reports prepared by the city’s design engineer, this issue has been thoroughly studied by gathering years of real-time groundwater data over the entire region, performing complex groundwater modeling, evaluating alternatives and developing designs that permit the continuation of natural groundwater flows.
The engineer’s geotechnical report provides groundwater data from 42 monitoring wells that were installed for groundwater monitoring on both the north and south sides of U.S. 36. Data presented extends over a three-year period, providing an excellent indication of existing groundwater conditions. This data shows that U.S. 36 has little impact on groundwater flow down the South Boulder Creek valley — this result is an excellent comparison to predict the flood mitigation project’s potential impact on groundwater.
Two types of foundation designs are presented in the project’s Preliminary Design Report, and one will be selected as the project goes to the next stage of design. One type considers an underseepage (groundwater) barrier consisting of a low permeability wall through the existing soil (alluvium) and extending into bedrock with a groundwater bypass system. The other type has no underseepage barrier and allows the free flow of groundwater.
The groundwater bypass system has infiltration galleries (buried water conduction channels) upstream of the dam, conduits through the barrier, and dissipation galleries downstream of the dam, all to permit groundwater to flow under the dam. Therefore, both types of foundation design will have minimal impact on natural groundwater flow. Both options are presented in the Preliminary Design report and, in the opinion of this writer, the option with no underseepage barrier is preferable, simply because it could significantly reduce project complexity and cost.
Extensive modeling has been performed to evaluate the impact on groundwater by underseepage barriers with bypass features. The engineer has analyzed the impact using models based on real-world data taken from field studies and sophisticated computer simulations of groundwater flow. The results of these models show that the impact on groundwater depths in the region north of U.S. 36 will be less than three inches.
The groundwater bypass system being considered has been used on prior projects — one of which I was the project manager for design and construction. That project was a flood retention dam of 110 feet in height across an ephemeral (intermittent) stream near the White Mountains in Arizona. The canyon has rich riparian habitat, partially because of the alluvial flow (groundwater). The dam would block this alluvial flow, so a groundwater bypass system was designed and installed. The system consists of one upstream intake gallery, a conduit through the dam base, and a discharge gallery. This one conduit system is similar to the multi-conduit system proposed for the flood mitigation Project. According to communication with the owner’s representative, the system functioned well — as the groundwater behind the dam mounded the conduit passed the flow and enabled the downstream riparian habitat to flourish. The project was put in service in 2006 and was still functioning until it was no longer monitored in 2015. However, aerial photos from 2019 show continued benefit, even though no longer monitored.
The successful performance of the groundwater bypass in the ephemeral stream in Arizona shows that a similar system should function well for the South Boulder Creek Flood Mitigation Project. As a community, we need to vote NO on the upcoming ballot referendum regarding the Annexation Agreement. The next flood will surprise us, as the September 2013 flood did.
Daniel Johnson is a water resources engineer with 50 years of worldwide experience and a past WRAB member. He lives in Boulder.
