January 3, 2006 > Water, water everywhere
Water, water everywhere
Flood control in Alameda County
by Todd Griffin
We are all aware of the tragic events in New Orleans last fall. Could it happen here? We are unlikely to see a hurricane, but floods, and related events such as landslides, have occurred periodically in the Tri-City region, from the Mission and pioneer eras up to as recently as the 1950s and 60s. Many of the 20th century floods were significant enough to inundate roads, shut down schools and businesses, and interrupt utility services.
But since the 1960s, flooding has been less frequent and less serious. This is largely due to the actions of the Alameda County Flood Control and Water Conservation District (ACFCD), part of the Alameda County Public Works Agency. The Flood Control District was created in 1949. Over ensuing years, various East Bay cities and unincorporated areas joined the district, and the flood control infrastructure we enjoy today was begun in earnest in the 60s and 70s. The district is funded largely from property taxes and the benefit assessment tax, based on flood risk assessment of a property.
Any area that floods periodically in its natural state, with no flood control intervention, is considered a flood plain, and many low-lying East Bay areas are in this category. FEMA, the Federal Emergency Management Agency, identifies 100-year flood plains, and classifies these areas as Special Flood Hazard Areas. A 100-year flood plain is defined as an area in which the risk of a flood is at least 1 percent each year; i.e., the area is expected to flood at least once in a 100-year period. Residents of these areas are required to maintain flood insurance.
However, the danger from flood damage is not limited to residents in low-lying areas. TCV spoke with Tom Hinderlie, Maintenance Engineer for the Flood Control District, who emphasized the risk of landslide to those living in the hills. "When you have continual rain, day after day, with no sun to evaporate what has fallen, the hills absorb it to the point where they can't hold any more. Then, when you have a significant [rain] event, the hillsides start coming down. That's a significant danger for properties and homes that are built up in the hills."
This occurred as recently as 1998, when extended periods of rain caused a hillside to liquefy, creating a landslide above the Eastbay Business Park. Sediment flowed into drainage channels, causing localized flooding in some areas before the channels could be cleared.
Flooding could also result from the breaching of levees along the bay, most likely caused by a major earthquake. The resultant flooding would not be catastrophic in the sense of the New Orleans disaster, as very little East Bay land is below sea level. But a levee breach could allow tidal flow into developed bayside areas and drainage channels. This could not only result in property damage, but more importantly, this "reverse flow" would introduce salinity into the freshwater along the bay.
To ensure the risk of serious flooding remains small, the Flood Control District continually maintains hundreds of miles of channels, reservoirs, creeks, and pipelines, keeping them free of silt and other debris. For instance, waterways adjacent to Coyote Hills Regional Park are part of this system. In addition to natural materials, thousands of cubic yards of man-made items such as shopping carts, mattresses, and other trash are removed from the flood control system annually.
The district also maintains erosion control structures to reduce the risk of soil loss on hillsides, as well as a network of 22 pump stations, to ensure the flow of water through low-lying areas to the bay. These pump stations are also critical during periods of high tide, when the water level in the bay may be significantly higher than in flood control channels.
Projects are underway each year to improve the flood control infrastructure operated by the district. It is, however, sometimes a lengthy and difficult process to obtain approval for these projects, as they require an extensive environmental permitting process via state and local agencies. District projects increase capacity, restore aged construction, upgrade existing resources, and add new capability to the infrastructure. The goal is not only to maintain the current system, but to continue to reduce the risk of flooding.
As urban development has expanded, the additional roofs, asphalt, and concrete reduce the acreage available to percolate rainwater, thus increasing runoff, posing ever-greater challenges to the flood control system. Fortunately, building codes now require developments to provide the capability to retain some of the additional runoff they create until it can be safely released. Hinderlie explained, "Each storm has an intensity curve that creates a peak storm flow through the watershed, through the channels, and out to the bay. That peak is what can cause flooding. So by retaining a portion of the runoff, it keeps that peak at the same level."
The district routinely reviews plans for development and provides comments to the cities regarding the impact of planned development on flood control. Hinderlie acknowledged this partnership between the district and the cities that has contributed to the success of flood control efforts, saying, "The cities in the Tri-City area have been very cooperative with the Flood Control District, working with us to keep the potential for flooding as low as possible."
Thanks in part to the efforts of the Flood Control District, local residents are unlikely to experience a flood event similar to the horror in New Orleans. This once primarily agrarian region subject to floods is now able to support a dense urban population with a greatly reduced risk of serious flooding.
The district also yields benefits to our pocketbooks. Due to improvements in flood control that future projects will provide, ACFCD will eventually solicit FEMA to re-classify some Special Flood Hazard Areas. If successful, this would eliminate the mandatory flood insurance requirement for residents of those areas. But some initial investment will be required. As Hinderlie stated, "With this aggressive plan to get properties out of the flood plain, we'll need to go to the public to increase the benefit assessment rates to pay for those improvements, which could be very cost effective for the public in the long run," both in improved flood protection and potentially reduced insurance bills.
For more information, visit www.acgov.org/pwa or call (510) 670-5480.