With the stormwater management paradigm shifting from “get it off site as quickly as possible” to “retain stormwater on site so it can either be recycled or infiltrated,” regulators have been quick to incorporate low impact development (LID) or green infrastructure (GI) policies. The main goal of LID and GI designs is to maintain pre-development hydrology, which is critically important because the previous “get it out of here” stormwater management policies have caused flooding and erosion and left tens of thousands of miles of waterways across the United States impacted.
Maintaining pre-development hydrology can be accomplished through detention, retention, infiltration, and reuse of stormwater, all of which will help to restore impacted waterways and reduce downstream flooding and erosion. No matter what type of stormwater management strategy is employed, infiltration can play a key role in any LID or GI design. Infiltration is typically broken down into two main types within the stormwater infiltration category, shallow infiltration systems and deep infiltration systems.
There are a number of factors that can affect the performance an infiltration system and none is more important than soil permeability, which must be evaluated carefully. Just as the types of infiltration systems are broken down into shallow and deep, so are soil conditions. Shallow soils tend to be consolidated and poorly drained and deep soils are most often made up of well drained alluvium, sand, and gravel. In other words, going deeper will often turn the most average of LID designs into blue ribbon winners.
Most shallow infiltration systems, which are comprised of modular components, made of concrete, plastic, or metal (shown below right), are completed anywhere from 8 feet to 12 feet below finished grade. At this depth, they are left in the upper, poorly drained soils, which means infiltration does not occur. Therefore, going deeper may be the answer. Moving to a deep infiltration system, such as a drywell (shown below left), which can be installed at depths up to 120’ into well drained material, is often the only choice for infiltration. Engineered drywells are perfectly suited for any LID or GI design that needs a reliable, maintainable system that is highly efficient at both pre-treating and infiltrating stormwater.
When considering infiltration as part of an LID or GI design, it is important to classify onsite soils as early as possible in the process in order to decide whether to go shallow or deep. Since engineered drywells are superior in function and often more cost effective than shallow systems, classifying deeper soils during the geotechnical investigation is often money well spent.