Repurposing Flood Control Infrastructure to Leverage Stormwater Resources

Combining the drought and evidence of a “Godzilla” El Niño event likely this winter, stormwater is increasingly valued as an asset worth integrating into California’s overall water management strategy. Re-thinking how to leverage stormwater resources opens up opportunities for repurposing infrastructure designed and built to dispose of flood waters.

More than one in five Californians—7 million—and nearly $600 billion in assets (crops, structures, and public infrastructure) are exposed to the risk of flooding in any given year. Over the past 60 years, California has experienced more than 30 major flood events, resulting in more than 300 lives lost, more than 750 injuries, and billions of dollars in disaster claims.

Improving Existing Stormwater Infrastructure

Dudek engineers who specialize in stormwater facilities said most of the state’s flood infrastructure has yet to be rehabilitated and falls into three categories: aging, undersized, and absent.

Much of today’s infrastructure was planned and constructed when ‘flood control’ was designed to convey ‘problem’ water as quickly as possible to the ocean. With the ongoing drought calling into question the practice of discharging all stormwater to the ocean, repurposing infrastructure may offer opportunities to better convey and conserve the expected deluge of stormwater.

However, repurposing a drainage structure to take better advantage of stormwater resources can introduce complicating factors. Flood control facilities are typically confined to areas only sufficient to support narrow, linear conveyance infrastructure, so converting existing structures into a greenbelt park or recharge basin requires additional considerations, such as hazard mitigation and securing more land and funding.

Dudek engineers reiterated that evaluating potential repurposing of existing flood control infrastructure involves four aspects: planning, prioritization, permitting, and funding.

Using Real-Time Data to Plan Projects

Older infrastructure was designed using one-dimensional “steady state” hydraulic models. These models assume magnitude and direction of flow are constant through time and physical domain, which provides conservative (less accurate) results, and does not provide a realistic view of water behavior.

Two-dimensional “dynamic” models allow flow to be evaluated as changing in both magnitude and direction over time. Using two-dimensional modeling shows that it makes more sense to design infrastructure to accommodate the majority of the flow that is lower than the peak-flow rate, and it allows for surface flooding in planned locations during the peak.

The newest dynamic hydraulic models generate more real-time data and results, which drive better decision making. If these more accurate models are used during concept-level planning, it may show that facilities and infrastructure do not need to be as large as once thought, meaning flood control infrastructure could be developed with a smaller environmental impact.

Additional Considerations for Repurposing Infrastructure

Dudek recommends that flood control managers evaluate their current capital improvement program list and the age of existing infrastructure to determine which facilities may be appropriate for repurposing. For example, if a decrepit channel system is scheduled for major rehabilitation, there may be an opportunity to revisit the plans to see if repurposing alternatives exist. Additional considerations include:

  • Partnering Opportunities – Consider outreach to local municipalities and developers. The repurposing of facilities can open the door to new and improved uses for public and recreational facilities. Partnering may also provide additional funding resources.
  • Stormwater as a Resource − Local water agencies are keenly aware of the resource potential that stormwater provides. Review potential nearby recycled water storage or groundwater recharge facilities that may benefit from the influx of additional water resources. If no local resources exist, review geotechnical conditions within or immediately adjacent to the drainage structure. Do geotechnical conditions provide for in-stream recharge opportunities?
  • Permit Regulations and Ramifications – Impacts must be evaluated because, for example, transitioning from a concrete channel to a meandering soft-bottom facility may create jurisdictional waters. Maintenance impacts should be evaluated and permitted to ensure long-term success of the project.
  • Linked Modeling – Consider employing a linked one-dimensional/two-dimensional modeling approach. Less conservative modeling techniques are more accurate and, thus, identify more realistic conduit sizes. For subsurface storm drain facilities, this can result in construction cost savings of approximately 20% to 30%.

Funding Available Through 2016

The state has set aside nearly $1.5 billion ($1.1 billion of Prop. 1E and $395 million of Prop. 1) in flood control funding. Remaining Prop. 1E funding must be appropriated by July 1, 2016 or be forfeited, so Governor Brown’s proposed budget allocates all remaining Prop. 1E funds to the Department of Water Resources for program categories that prioritize flood management projects. The Governor is also seeking legislation that will allow the appropriation of those funds “early in the legislative session” and prior to the Budget Act.

For more information on stormwater management, flood control engineering, and water resource system design contact us!