Much of western Alameda County lies in a floodplain. A floodplain is an area along a creek or river that is subject to flooding during major storms when water overflows the banks.
The District’s primary goal is to manage stormwater so that flooding is infrequent and minimal. Managing stormwater flow can take many forms. For example, making the channels bigger enables them to convey more stormwater. Removing or enlarging obstructions or bottlenecks, for example, narrow culverts, is another way to let more stormwater flow. The District also manages levees, floodwalls, and stormwater detention basins to protect the property of Alameda County citizens.
Typical Flood Control Infrastructure
A system of built channels and natural creeks collect and convey stormwater runoff from paved and unpaved surfaces throughout Alameda County—from the East Bay hills to the flatland, and finally to San Francisco Bay.
Built channels may take many forms, from earthen ditches to concrete-lined conduits to miles of underground pipe. Channels and creeks are often called “Lines” by the District. District staff maintains the channels and creeks, so they carry stormwater unimpeded by sediment, vegetation, or trash.
Earthen channels are monitored to spot erosion problems that require bank repair or possible channel widening. Improvements to earthen channels also enable the District to replace nonnative vegetation with California native plants, grasses, and trees.
Desilting silt (dirt and sediment) can be carried downstream where it builds up along the channel bottom, thereby restricting stormwater flow. Dredging sediment—known as desilting—is sometimes necessary, but is very expensive.
Creek Restoration Projects create healthier and more attractive environments and wildlife habitat while improving stormwater capacity and water quality. Some creek restoration sites are accessible by the public to enjoy.
Concrete-lined channels are found in many spots throughout western Alameda County. Building channels with concrete dates back to the 1960s, when many of the District channels were originally constructed.
Culverts are concrete or steel conduits that totally enclose flowing water. They are frequently used underneath roads, railways, or embankments.
Levees are earthen banks or constructed walls that keep stormwater within the creek. Levees must be managed and monitored for stability and capacity so that stormwater cannot seep underneath or rise high enough to overtop levee banks. As climate change causes rising sea levels and tides, levees will play an important part in protecting low-lying shoreline in Alameda County.
Levees are the focus of an extensive certification program that the District is doing with the Army Corp of Engineers to comply with new Federal Emergency Management Agency (FEMA) standards. Earthen levee banks are routinely monitored for erosion and large vegetation (i.e., trees), which can weaken the stability.
Several access roads atop District levees double as hiking trails for the public.
Stormwater Detention Basins
As new communities and business parks are built in Alameda County, stormwater must be routed as it flows off paved surfaces. Site developers are required by law to control the rate of stormwater flow so that flooding does not occur in downstream channels. Detention basins are one way to slow the water.
Detention basins may be various sizes. Shallow “dry” basins are seasonal and dry up in summer. Deeper “wet” basins are ponds that hold water year-round. Some are used as sources of irrigation water for nearby landscaping.
Bioswales and Rain Gardens
Bioswales are slightly graded drainage areas planted with vegetation or filled with compost that redirect and slow stormwater flow. As stormwater moves through the bioswale, contaminants may adhere to the vegetation to improve the water quality before the stormwater enters a channel or the ground.
Rain gardens are usually depressions in the ground that are planted with native vegetation. Rain gardens filter contaminants as the stormwater drains through plants, which improves the water quality before the stormwater seeps into the ground.
The District operates and maintains 22 pump stations for optimum flood control. Most of the pump stations are located in low-lying areas near the bay. Stormwater reaches pump stations by way of creeks, pipes, and channels, and then is pumped to an elevation high enough to allow the stormwater to then flow by gravity into San Francisco Bay. That makes pump stations a critical part of the District’s system to reduce flood hazards.
All pumps function with state-of-the-art technology called Supervisory Control and Data Acquisition (SCADA), which allows District staff to monitor station operations from remote locations ,so they can maximize the performance of each pump station.