Hydraulic Modeling and Restoration Design
Restoration design of floodplains typically must incorporate hydraulic modeling of the project site to predict how the vegetation will impact flood flows. O’Connor Lakes is a River Partners’ restoration project that illustrates how hydraulic modeling influenced the restoration planting to produce a flood neutral design.
O’Connor Lakes story
O’Connor Lakes (OCL) is a unit of the Feather River Ecological Reserve, owned and managed by the California Department of Fish & Game. Prior to acquisition by F&G the land supported orchards of walnuts and peaches. OCL is located at a relatively narrow place between the flood control levees (about 4,000 feet wide), with a consequent area of high velocity (10 fps) flow at the north upstream edge of the property. The project site covered all of the floodplain between the levees. The two-dimensional hydraulic model of the Feather River for the entire reach was developed subsequent to the large floods of 1997 (which was at or slightly above the design flow) and 1998. These floods allowed engineers to measure flood water elevations at known flows, thereby validating the output predictions from the hydraulic model.
The primary goal of the O’Connor Lakes Project is that riparian restoration designs must be flood-neutral. That is after the plants have grown for 10 (or more) years they must not alter the pre-project river elevation nor its velocity.
The hydraulic model for the Feather River identified the water depth and velocity of flow within the 200 foot-square cells across the entire site. From this model-output, planting locations for trees and shrubs were identified. The iterative procedure to develop the planting design involved testing different planting designs (plant densities and proportions of trees and shrubs) that the engineer could assign a roughness value.
Pre-Project velocity contours (feet per second) at O’Connor Lakes on the Feather River. Note how flow velocity changes at different points in the floodway.
Then, the model was run with the proposed planting design to determine any changes in water depth and velocity compared to the pre-project vegetation. For example, field four occurs at the east edge of the project with the river channel along its north and east edges. If this field were planted with a complete set of trees and shrubs at the typical densities (200+ per acre), then the model predicted that the velocity against the east levee would increase by 0.2 feet per second. This did not meet flood neutrality; a different design had to be modeled. A second planting design for field four consisted mostly of flexible-stemmed shrubs species (rose and blackberry) planted in hedgerows parallel to flow in the channel with 100 feet between hedgerows. This design had a lower n-value in the model equations and resulted in no change in the flow velocity against the east levee.
Final planting design at O’Connor Lakes based upon results from hydraulic model.
Velocity contours for the As-Built restoration at O’Connor Lakes
The Feather River at OCL has been depositing sand on the site for a long time – the site has one to two feet of recently deposited sand over silty loam soil. This build-up of sand is reducing channel capacity. The land downstream of OCL is a dense cottonwood willow forest that greatly slows the velocity of flow as it move through it. A 100 feet corridor had been dozed through the forest many years ago in order to facilitate the movement of sand during high flows. After reviewing the hydraulic model results the engineers decided that widening the corridor to 400 feet would benefit the transport of the sand across the site and down the river. In January 2006 a few months after planting and widening of the corridor, a flood moved across the OCL (ca. 80,000 cfs). The land surface was under 2 to 5 feet of water moving fast enough to mobilize the sand. After the flood receded, monitoring of OCL revealed that the sand had moved down the corridor through the forest as planned. The dense forest on either side of the corridor forced the flow into the open corridor where the flow maintained its velocity to transport the sand to its downstream end.
In summary, the plant design at OCL met the flood-neutral criterion according to the hydraulic model in that water elevation and velocity did not change against either the east or west levees after the planting was installed. In addition, opening a corridor through the dense vegetation south of OCL resulted in the sediment transport process moving significant amounts of sand through the project and downstream.