Preparing for the Flood: Rightsizing Stormwater Design in Unpredictable Times

Building too much or too little stormwater infrastructure can be expensive. Involving civil engineers early in the design process can save money, increase efficiency and protect facilities from storm damage.

By Kathereen M. Shinkai, PE, F.ASCE

Principal, Director of Civil Engineering

Climate change and extreme weather events are taking a toll around the country. Rainfall levels are rewriting records. Places that once seemed immune to flooding are no longer so.

Every project must wrestle with creating stormwater management systems appropriate for the development’s goals, budget and local conditions. A blanket strategy of designing every site to withstand a 100-year storm is not the answer. Planning for storms with a 1% chance of occurring in any given year can be expensive and, frankly, futile. Not only would infrastructure and transit connections remain at risk, but once neighboring lands flood, the project will still face dangers from the rising water.

As a civil engineer, my job is to help keep projects safe from risks through practical design strategies. This includes ensuring water drains away from the building and overflow locations are integrated into the design. But stormwater design can take many forms. Focusing on extreme conditions can lead to unnecessary expense. At the same time, failure to take the risk of flooding seriously and spending too little can lead to costly damage in the future. Understanding the potential risks appropriately is essential to rightsizing the storm management system to conditions on the ground.

In an integrated design process, civil engineers are involved from the beginning planning discussions, allowing us to introduce stormwater management considerations early and align the system with a project’s needs and aspirations. A focus on researching options and merging stormwater management with other design decisions can save money and lead to a better overall project.

The process of developing appropriate and resilient storm infrastructure requires designer and owner collaboration.

SITE REVIEW

An integrated design team can identify floodplain areas, coordinate utility tie-ins and develop a beautiful and sensible site plan from the project’s onset. A client once proudly shared with me their recently completed campus master plan. On paper, it looked stunning and meticulously conceived. But after a quick review, I had to tell them that it was not feasible: The buildings were arranged without consideration for the site’s 80-foot elevation change. Massive retaining walls would be needed to support the land and the buildings. Had the master planning team included a civil engineer, the layout would have accounted for the site topography.

GRADING

Regardless of the terrain, the project team must direct how and where your site is going to drain. That may require shaping the land to facilitate drainage, in order to avoid pools of standing water in areas like sidewalks, parking lots and playgrounds.

Civil engineers will endeavor to balance your site’s earthwork to minimize the amount of outside fill hauled in, which adds expense and can introduce contaminants. If possible, we will use excavated earth to build embankments and achieve the desired topography.

WATER QUALITY

Projects typically must treat stormwater runoff to a local standard before discharging it into the natural environment or into public infrastructure. However, there are options. A project on an urban infill or industrial site can use a proprietary or underground water storage and treatment system. If 8% to 10% of the total disturbed site area is open, we can design bioretention basins to filter runoff and recharge underground aquifers naturally. Increasingly, clients are interested in natural retention methods, which often integrate native plantings, to enhance their landscaping and offer talking points and educational opportunities.

BUDGET

Like the building’s structure and HVAC equipment, stormwater management falls under the category of investments the everyday user may never notice. Factors affecting its cost include the choice of water collection and treatment method and the type of pollutants in the runoff. Bioretention ponds are generally the least expensive option. Next up are proprietary systems, which are manufactured, often underground, stormwater control measures designed to treat stormwater runoff or provide water-quality control. If an underground detention or retention system is required, the cost will rise to cover the deeper excavation and additional infrastructure, such as pumps, to send the discharge into the existing storm drain system. For sites with very limited space, drywells have a compact footprint, but their 30- to 50-foot depth will require boring and deep excavation.

MAINTENANCE

Like any home or vehicle purchase, a new capital project can lose value and years of operating life if it is not maintained properly. Owners also have a social and possibly legal responsibility to make sure that the water leaving their site boundaries does no harm down the line. Many owners fail to plan and budget for their site’s long-term maintenance. A successful water-collection and treatment system requires the regular removal of trapped trash and the occasional changing of treatment media, depending on your system type. Early planning can reduce future maintenance costs and ensure a system’s longevity.

An integrated design approach will minimize unpleasant surprises throughout a project’s lifecycle. The earlier we can jump into a project, the greater effect we will have on the design. By presenting stormwater options early in the planning process, we can work as a team to develop strategies that work for the project’s site, budget and larger performance goals.