Ignacio Creek Outfall Repairs
Novato, CA
Private Homeowners Assoc.
Summer 2009
Clearwater Hydrology of Berkeley, CA planned and designed this project to remedy erosion issues at three separate culvert outfall locations along Ignacio Creek. Hanford ARC contracted with the HOA to construct the repairs.
Site 1: A culvert outfall created scour and erosion of the toe of the slope immediately downstream of the outfall. This erosion had undermined the roots of a large oak tree. The outfall was compromised and the tree was senescing. Prior to our work, the HOA hired a tree service to remove the oak. We graded the bank to a 3:1 slope, placed approximately 30 TN of ½ ton rock at the toe of the slope and constructed vegetated soil lifts (VSL) above the rock and planted willow stakes around the structure.
Site 2: Similar to Site 1, this site included an outfall and a larger area of downstream scour. The outfall is located immediately downstream of a bridge on a slight outer bend. The design for this site combined stabilizing structures downstream of the culvert (VSL, rock toe, slope grading and erosion control fabric), upstream structures to redirect flow away from the outfall, as well as minor floodplain grading to increase the channel capacity and slow water velocities during bankfull and greater events. The upstream structures consisted of two ‘spur dikes’, or rock barbs protruding into the channel to direct the thalweg towards the opposite bank.
Site 3: This site involved the most intensive repairs due to multiple factors impacting the outfall. Incision in Ignacio Creek undermined the culvert support, causing the end section of the outfall to dislodge and drop into the creek. This condition then caused the bank to unravel, exposing the next section of pipe. In addition, sheet flow from the adjacent parking area discharged over-bank, further complicating the situation.
Clearwater designed a multi-functional drain system and bank structure to address all issues, including future incision in Ignacio Creek. The solution included extending the culvert to its previous length and rebuilding the bank using a stacked, vegetated cellular confinement system (CCS). The toe of the CCS is supported with ½ ton rock extending into and beneath the channel bed.
To address the surface drainage, we installed a cobble lined swale leading to a vegetated infiltration swale. The infiltration swale consisted of 6” perforated pipe, beneath loamy sand/soil blend topped with ½” minus topsoil mix and erosion control fabric. At the base of the swale is a drainage inlet to capture sheet flow that does not percolate. The DI and the perforated pipe tie into a solid drain pipe that discharges at the base of the CCS. Finally, we vegetated the swale and CCS with container plants and seed. |
