Floods Are Becoming More Frequent and Severe. Here's a New Approach to Suit Challenging Sites.

Surface-mounted systems can provide a practical solution to the growing problem of flooding due to sea level rise and heavier rainfalls from increasingly severe storms.

Floods Are Becoming More Frequent and Severe. Here's a New Approach to Suit Challenging Sites.

An artist’s rendering shows a typical surface-mounted floodwall of the kind that can be used to protect wastewater treatment facilities.

Critical infrastructure is not always well equipped to resist rising floodwaters and coastal inundation and erosion, which can cause millions of dollars in damage and extensive downtime to bring essential equipment back online.

However, there are new solutions that can suit the most challenging settings. Many flood-control products on the market can protect a wide range of equipment and structures. These include smaller temporary systems designed to protect doors and windows to a height of about 3 feet. They rely on the building to provide structural support.

These systems are not practical for large facilities that require perimeter protection. Larger free-standing perimeter systems rely on extensive excavation with piles buried deep in the ground for added strength. That can be a problem where excavation is limited due to below-grade structures or buried infrastructure like pipelines or electrical conduits.

When a floodwall system cannot be embedded in the soil, additional features are required to keep water from seeping under the floodwall. This type of barrier must be engineered to resist the lateral forces from moving floodwater without sliding or overturning. The design of a surface-mounted perimeter floodwall for a wastewater facility that is no longer adequately protected by existing flood-control measures presents special challenges.

Engineering challenge  

In 2019, a retired City of Houston engineer working with a consulting firm sought bids for design of a floodwall system for a wastewater facility next to a bayou. The city provided as-built drawings, information about the threat levels (flood height) and where the transitions were to be on the campus. The city also granted permission to walk the site and film drone footage.

The resulting design, still under the city’s consideration, is a surface-mounted retrofit system to accommodate extensive underground infrastructure that could not be relocated, and to provide 2,000 linear feet of flood protection with just one foot of excavation.

The system includes a floodwall that provides 5 to 10 feet of flood protection, a special floodgate to close off the roadway while preserving access, and a pumping system to address flooding inside the perimeter walls from heavy rainfall.

The design calls for the relatively low-permeability soil under the wall to be compacted and the hydraulic gradient increased by providing an asphalt underlay extending beyond the limits of the wall footing, while ensuring a good seal along the bottom of the concrete. These structural elements ensure adequate support of the floodwall.

A successful surface-mounted floodwall needs a footing sized to ensure that the bearing pressure at the toe is sufficiently below the bearing capacity of the founding soil, and that there is no uplift at the heel.

Construction elements

Concrete reinforced with rebar is a practical choice for wall materials because it is relatively inexpensive compared to steel, and the mass helps anchor the system to the ground. This type of system can be constructed from precast sections or by casting in place.

Precast sections are generally limited in size by the capacity of the trucks used to haul them to the site and the crane needed to move them into place. Larger walls are cast in place with reusable forms. Precast sections are sealed between sections as they are installed. The cast-in-place method is constructed using a continuous pour with no seams to fill. Many sets of forms can be used simultaneously to speed up construction.

The waterproof floodgate designed for the access road is to be composed of precast concrete sections that match up to the roadway level and the adjacent walls. A base slab would be embedded in the ground flush to the full width of the roadway and would extend to the side where the gate resides when open. An inverted-angle guide rail is to be attached to the base to enable the gate to easily roll open and shut.

Abutment blocks at both ends of the base slab would secure the gate system to the walls and provide overturning resistance. A vertical wall panel would extend from the end abutment to another smaller abutment at the edge of the roadway. This panel would seal the area where the gate is stored when not in use. Two towers fabricated of steel would guide the gate and provide vertical stability. The gate design uses flexible seals at the base and both ends to prevent water intrusion around the gate when deployed.

There are many ways to power such a gate, from simple systems such as hand winches to complex control systems with automatic operation. The gate can be operated by an electric gear motor and a sprocket-and-chain system at the top of the guide towers, with the drive sprocket at the motor providing the force to move the gate.

Drainage grates and shallow sumps would capture rainfall to prevent flooding inside the walls. The drainage sumps would connect via underground pipework to sump pumps near the perimeter wall that discharge the water directly into the flood zone.

A float switch would sense when the sump is full and turn on the pump using a motor starter. Another float switch near the bottom would sense when the sump is nearly empty and turn the pump off, leaving enough water to prevent cavitation. A small control box at the pump location would house the motor starter and the interface to the switches. Power would be provided by the facility, but the design allows for a standby generator that can be shared by the gate system, if required.

Meeting needs

The Federal Emergency Management Agency has issued the FEMA 55 Coastal Construction Manual, which standardizes the load cases for hydrostatic, hydrodynamic and debris impact. The ASCE 7 Design Manual for Buildings and Other Structures also applies. Hydrostatic and hydrodynamic loads are calculated using empirical equations provided by these references. These equations are based on extensive research and post-event surveys, providing a rational approach to floodwall design.  

There is an obvious need for flood protection at wastewater treatment facilities due to increasing amounts of flooding and the devastating consequences to surrounding communities. Flood protection comes in many forms, but it must prevent overtopping by floodwaters, remain stable during the flood event, and protect against under-seepage.

Modular, retractable and surface-mounted floodwall systems offer flexibility and the ability to protect facilities where other flood control measures may not be feasible.

About the author

Rick Adler (radler@rsaprotect.com) is president of RSA Protective Technologies in Claremont, California, a company specializing in development of protective structures for counterterrorism and flooding.   



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