Infrastructure and Stream Valleys
Most buildings and roads in Arlington were built prior to regulations requiring stormwater be slowed down or treated, so runoff from these areas flows uncontrolled to streams. Roughly 42 percent of Arlington’s land has been converted to impervious surfaces that don’t allow water to soak into the ground. This is one of the key watershed management challenges in Arlington that has a significant impact on stream health. This increased runoff to the streams has caused erosion as the stream channels enlarge to accommodate the higher storm flows.
Storm sewer and sanitary sewer systems in Arlington rely on gravity to move the flow through the pipes, so in many locations the pipes were installed in the stream valleys because they are at a lower elevation.
A County-wide stream assessment was completed from 2011-2012. The study assessed the physical stream conditions, the infrastructure conditions, and the restoration potential of the County streams. A prioritized list of stream restoration projects was created based on the assessment findings. These projects are included in the Stormwater Master Plan. The stream assessment will be updated in the next several years.
Natural Channel Design
For stream improvement projects, Arlington uses a technique called natural channel design to create a new stream channel that’s in balance with the runoff it receives from the surrounding land. The watershed characteristics, such as the amount of runoff generated during rain events and the stream’s slope, guide the stream design. The streams are sometimes raised up to reconnect with a floodplain area. During higher flows, the stream can flow onto the floodplain and the water will slow down and reduce its energy. In addition, step pool structures and meanders are often added to help manage the energy of the flow. Natural channel design is recognized as an official Best Management Practice by the US EPA and Virginia Department of Environmental Quality.
Improvement Projects Benefit Residents and the Environment
Stream and infrastructure improvement projects:
- Repair infrastructure such as recreational trails, sanitary sewer pipes, drinking water mains and storm drain pipes.
- Restore stability to the stream channel, allowing it to withstand storm flows without eroding
- Help the County meet its regulatory requirements to reduce nitrogen, phosphorus and sediment pollution.
- Improve the long term health of the stream valley forest. Stream erosion kills trees along the stream. Creating a more stable stream channel for the long term will help existing and newly planted trees thrive.
Adding Stormwater Management in the Watershed
Upstream stormwater management projects, also referred to as stormwater retrofits, and stream improvement projects are two tools for improving stream-related problems. Both practices serve an important purpose related to stream health, but they don’t address the same needs.
Stormwater retrofits are designed to treat the first inch of a rainfall event. The retrofits slow down and treat stormwater runoff, which will help water quality in our streams in the long term. Examples include rain gardens and permeable pavement. A County-wide 2012 study identified appropriate locations where retrofits could be installed.
Stream improvement projects address the physical degradation of the stream. Large amounts of stormwater runoff are funneled to our streams through the storm drain network during storm events. The fast infusion of stormwater into our stream erodes our stream channels and unearths the infrastructure that is buried within the stream corridor. To minimize the impact to our stream corridors and be as efficient as possible with County funds, drinking water, sanitary and storm pipe network repairs are incorporated into the construction plan and completed as a part of the project. The County has prioritized its stream projects based on erosion and infrastructure needs.