Blog post by stormwater design expert Hannah Lancaster.
CDR’s proprietary design methodology for stormwater management systems and expertise with the complex permitting process saves clients time and money both during and after construction.
Stormwater is defined as rainwater runoff that moves over land to a waterway, eventually flowing to the ocean. Management of stormwater is an important and usually costly consideration in the construction of any land development project more than 5,000 square feet. Tracking, collecting, and conveying stormwater onsite, determining the treatment method, and providing a design and Water Quality Management Plan (WQMP) that the agency will approve does not have one right answer. There are many different approaches and treatment systems that can be approved by the agency. Creating a design and picking the most cost-effective approach is a skill. Every site is unique, a cookie cutter approach for all sites does not work. CDR has the expertise, experience, and methodology to analyze site conditions and select the most cost-effective stormwater treatment system which saves clients time and money.
When working in stormwater management, the first question to answer after, “is this actually required?” is, “what’s going on underneath the surface?” The answer lies in the soils report which reveals the level of percolation that exists at the site and groundwater level. As you may know, in the state of California, stormwater must be addressed and treated to get almost anything built. The answer to percolation and groundwater will direct you to a hierarchy of treatment methods: above and/or underground retention/percolation, biofiltration with retention or partial retention, proprietary media-filtration, and dry well, see Table 5-1 below for an example.
Stormwater Treatment Options
Stormwater treatment options largely depend on the project site’s soil. If there is an acceptable percolation rate (i.e. how quickly water absorbs into the ground), an at-grade or underground retention system can be put in place. Because land value is a premium in southern California, typically a design effort tries to utilize landscape setbacks and planter islands for surface infiltration and/or retention to fulfill stormwater treatment and mitigation requirements before implementing an underground retention system. Trash and smaller debris will be removed in pre-treatment, and then the water will percolate into the soil where it will be treated naturally and recharge the groundwater table. For projects with limited landscaping and soils with higher percolation rates, we prefer to use large diameter perforated pipe. This option is the least expensive while also requiring the smallest excavation volume and gravel backfill. These can be deeper, but are ultimately cheaper! At a specific percolation rate, a deeper system with a smaller footprint may be more cost-effective than constructing a shallower system with a bigger footprint. 
When on-site infiltration is low due to clayey soil, for example, a shallow tank or plastic arch system is used for stormwater mitigation. Both systems require less excavation and gravel backfill and are easy to assemble at the construction site. (In this case, the generally-preferred perforated pipes are inefficient and more expensive because the system would require a smaller diameter pipe with a larger quantity of gravel backfill to avoid vector control issues.) For some project sites, typically in urban settings, where the buildings have zero setback, a dry well system may be used. However, this requires an expensive percolation test to be completed around 50-100 feet below surface and many developers don’t want to go this route unless they have to as a last resort.
If on-site infiltration is entirely infeasible due to soils with zero percolation or a high groundwater table, a standard biofiltration system must be used. Standard biofiltration basins are landscaped planters with layers of sand, gravel, and engineered soil media to biofilter on-site stormwater flows. However, if the project site lacks sufficient landscaped area, a proprietary biofiltration or flow-thru treatment device may be used. These are pre-constructed systems with a smaller footprint and the aforementioned filter layers, but come at a higher cost. Natural infiltration and biofiltration systems are preferred over proprietary treatment systems because they are cheaper and require less long-term maintenance – they simply need to be cleared of trash and maintain plant growth.
Cost Savings and Timelines
CDR obtains preliminary costs from three manufacturers during conceptual design and stores all cost data on previous projects in dollars per cubic foot of stormwater stored. Relying on their past project experience and data and obtaining real time costs when creating designs, CDR provides stormwater management designs that are extremely cost effective. This could mean working within the existing landscaping, directing water to a swale, and/or utilizing sheet flow before collecting and conveying to a large diameter corrugated metal pipe (CMP). Every additional storm drain pipe length, inlet, or inlet with a filter adds to the total cost of a project.
Typically, stormwater reports are submitted to the permitting agency along with precise grading plans. To stay on schedule, due diligence is vital, and CDR coordinates with the various involved parties to facilitate a quick and accurate flow of information, ensuring a better outcome in the permitting process.
CDR is diligent about staying current with what is available in stormwater system products, revisiting cost, availability, and product quality each time a stormwater management plan is created. Knowing what’s new, what’s working, and getting quotes and lead times on new products for comparison guarantees that the most affordable and available option is being selected before it’s included in the final plans, even if it’s been a long-time trusted choice.
CDR’s in-house expertise with both stormwater design and the permitting process can reap significant savings in both the initial cost and ongoing maintenance.
To discuss a project, contact CDR today.
