Analysis of the Sycamore/Mud Creek Floodway
Chico, California
The Sycamore/Mud Creek Flood Diversion was completed by the Army Corps of Engineers in 1966 to divert flood flows of Big Chico Creek around the city of Chico. The project has successfully prevented any significant flooding from Big Chico Creek for 33 years. However, like any stream modification, it has caused some significant changes in the syatem.
The project consists of box culverts at the head of Big Chico and Lindo channels and an overflow weir to permit excess flow to go via a diversion channel to Sycamore Creek and Mud Creek, both of which were improved as floodways, excavated and levied to form trapezoidal channels.
The design flow for the project is 16,000 cfs at which point the flow division is supposed to be: Big Chico, 1,500 cfs; Lindo, 6,000 cfs; and Sycamore, 8,500 cfs. In practice, this design flow does not occur, because woody debris clogs the Big Chico and Lindo box culverts so that a substantially greater volume of water spills into the Sycamore diversion at lower flows than intended. The flow control structures also affect movement of bed-load materials. Cobbles and gravels drop out in the upstream stilling basin; some smaller materials are sluiced through the box culverts,particularly into the Chico Channel, but essentially no gravel or larger rock material crosses the Sycamore Weir. This flow of "gravel-hungry" water, far in excess of the natural flow in tiny Sycamore Creek, has substantial ability to rearrange the channel.
Stream channels are maintained by the high flows they receive. Because of reduction in high flows,the Big Chico Channel downstream of the box culvert has experienced plant encroachment until the channel has been reduced in size so it just carries its maximum flow, 1500 cfs. The Lindo Channel has been largely deprived of gravel so has scoured to bedrock in its upper reach and is experiencing entrenchment in downstream reaches.
The greatest effect, however, has been in little Sycamore Creek, which had its flow agumented by a factor of 10 while receiving no bedload materials from Big Chico Creek.The graph below shows a longitudinal profile of the Sycamore/Mud Floodway. The high-gradient reach between 5,000 and 12,000 feet (Sycamore Creek to Floral Avenue) is experiencing severe channel incision with headcuts extending back up the diversion channel and tributary channels. The floodway was excavated in cemented alluvial deposits. Very possibly the engineers felt that the bedrock nature of the channel matrix would provide sufficient armor to prevent erosion and, since little bedload material would be transported that way, deposition would not be a problem. Unfortunately, the exposed "bedrock" weathers very rapidly into easily eroded materials and, just 32 years after project completion, serious problems are arising.
When the eroded material reaches the low-gradient reach from 16,000 to 25,000 feet (upstream and downstream of Cohasset Highway) it deposits, producing a braided channel and marsh environment, with spikerush, hardstem bulrush, and willows. This marsh environment has been converted to permanent wetland by summer runoff from adjacent housing and now supports a fish community, bullfrogs, and a family of beaver. The permanent water is permitting the vegetation to thrive, substantially increasing channel roughness and decreasing flow capacity. The reduction in channel volume and increased roughness result in much higher flood levels in this reach; large deposits of flotsam may be seen very near the levy top and Cohasset Highway is in danger of being closed by flood flows.
Between 28,000 and 38,000 feet (Hicks's Lane to Bell Road), the channel again becomes erosional, displaying incision and exposure of bridge abutments, but further downstream (from Meridian Road to the confluence with Big Chico Creek) the backwater base level created by the Sacramento River at flood slows the flow, inducing deposition, and the flood channel is filling, creating a braided stream surrounded by marshy habitat. In this area, also, flotsam deposits may be seen within a few feet of the levy top.
Urban planners should not assume that the project still meets design flood protection, particularly in portions of Sycamore and Mud Creeks where deposition is occurring in the channel.
Niche Space Last Updated August 4, 1999