Project to protect the lower reach of Pittman Wash
There is a portion of Pittman Wash that is in critical need of protection from flood events. This section is about 1,800 feet known as the “lower reach” located between the Union Pacific Railroad trestle and Arroyo Grande Boulevard. This section is much narrower and steeper than upstream portions of the wash, and therefore experiences a greater level of destruction during flood events. In the last few years, some areas of this section have been eroded by as much as three feet.
The project to protect the lower reach of Pittman Wash consists of the installation of a concrete channel along the bottom that is approximately 45 feet wide and up to 10 feet tall. The height was determined to accommodate a 100-year flood event in which the stormwater flow can reach up to seven feet tall.
The upstream portion of Pittman Wash adjacent to Silver Springs Recreation Center, also known as the Project GREEN area, will not be impacted by this project.
The concrete channel will occupy only the bottom of the lower slopes of the wash, and will result in flatter slope areas adjacent to the channel that are more amenable for recreational use. The natural habitat will be preserved in the upper sections of the wash near the trails that are maintained by the City’s Parks and Recreation Department. While it is not practical to maintain the existing habitat at the bottom of the active wash, there will be a flatter area created above the concrete lining that can be used for re-vegetation.
Alternatives that were evaluated for flood protection
Several alternatives for protecting the lower reach of Pittman Wash were examined and evaluated for their effectiveness at protecting the wash during a storm event. These alternatives are summarized below:
Use of riprap
“Riprap” is large rocks or concrete rubble that is commonly used to stabilize stream and river banks, and even coastlines. Riprap is currently used throughout the Pittman Wash, but the higher velocity of stormwater that occurs in this section washes the rocks downstream every year. The use of larger, heavier riprap that is less likely to be washed downstream was considered, but this material would require a larger footprint within the wash and cutting into the sides of the wash to such an extent that the environment would be permanently disrupted.
Construction of drop structures
Drop structures are manmade spillways that “drop” water from a higher elevation to a lower elevation, and in the process slow the destructive velocity of the water flow. Drop structures work well in wider, flatter waterways, but this portion of Pittman Wash is too narrow and steep for this alternative. It would require extensive re-grading to flatten the longitudinal slopes between drop structures and to widen the channel, which is not feasible given the current depth of the channel relative to the adjacent houses. This grading would also result in significant, permanent disturbance to the environment.
Lining the channel with concrete
This alternative consists of the lining of the bottom of the wash with a strip of concrete that is approximately 45 feet wide and concrete walls that range in height up to 10 feet. The concrete lining would occupy a rectangular footprint at the bottom of the wash and will result in flatter slope areas adjacent to the channel, which can be used for re-vegetation, trails, and natural habitat.
Use of vegetation
Vegetation is a natural way to control erosion and stabilize slopes. Once vegetation is established, the plant root system helps hold soil in place. This alternative is not feasible in this section of Pittman Wash where the potential velocity of water from a 2-year or 5-year, much less a 100-year flood event, is too much for vegetation to withstand.
The engineering analysis for this project concluded that due to the erosive nature of the soils, the high velocities of the design flood flows, right-of-way limitations, maintenance access considerations, and desire to create usable areas for recreation and minimize disturbance to the wash, the concrete-lined channel is the least environmentally-damaging alternative and fulfills the project purpose. Other benefits of the concrete lining include stabilizing the channel banks, preventing erosion, protecting sewer infrastructure, and reducing maintenance costs.