189 



(low. Although previous corps project 

 designs had designated a low-flow 

 channel in lower Wildcat Creek, they 

 did not include natural channel geome- 

 try or vegetation or grading plans that 

 would help define stable, low-flow 

 channels. Typically, the corps' low- 

 flow channels superimposed on open, 

 wide-bottom, trapezoidal channels are 

 unstable, braided, and choked with 

 bulrushes. 



The Consensus Plan is designed so 

 that sediment deposition will occur 

 where it is least harmful — on the flood- 

 plain and in the bay. By trapping as 

 much sediment in the upstream flood- 

 plains as possible, filling of the down- 

 stream marsh with sediment should be 

 prevented. The Consensus Plan assures 

 that the low-flow channels will scour 

 and transport as much sediment as pos- 

 sible to San Pablo Bay. To further pro- 

 tect the maish from sedimentation, the 

 plan also calls for widening the slough 

 channels through the marsh so that sus- 

 pended sediments can be conveyed by 

 the channels without overtopping into 

 the marsh and for excavating sediment 

 to increase the brackish marsh area and 

 restore the marsh's tidal action. 



Technical Issues 



The most contentious technical is- 

 sues faced by the design team included 

 making reasonable estimates of the 

 sediment loads carried by the creeks, 

 assessing the ability of the corps' pro- 

 posed sediment basin to colle:t sedi- 

 ment, judging the safely of concrete 

 box culverts, and assigning roughness 

 values to proposed revegetation areas. 

 The coalition's experts argued that the 

 natural creek channels were aggrading 

 with high sediment loads and predicted 

 that the even wider, trapezoidal chan- 

 nels proposed by the corps would fur- 

 ther increase sedimentation. The nar- 

 row, low-flow channels of the Modi- 

 fied Plan, therefore, were better de- 

 signed to transport sediment in suspen- 

 sion at higher velocities. Phil Williams 

 and Luna Leopold also questioned the 

 ability of the corps' proposed sediment 

 basin to perform as a sediment trap. 

 Later the corps' own specialists at the 

 Environment, Vol. 31, No. 10 



Waterways Experiment Station in Vicks- 

 burg, Mississippi, independently raised 

 the same concern. Therefore, the de- 

 sign team decided to locate the basin 



further upstream, and they adopted the 

 floodplains, wetland transition zone, 

 and higher velocity, low-flow channels 

 of the Modified Plan to keep the sedi- 



FIGURE 3. Cross-sections of creek channels designed for the 

 Selected, Modified, and Consensus plans for the flood-control 

 project on Wildcat and San Pablo Creeks in North Richmond, 

 California. 



(a) 1982 Selected Plan proposed by Contra Costa County and U.S. Army 

 Corps of Engineers. 





(b) 1984 Modified Plan proposed by a coalition of North Richmond 

 community organizations. 



trail 



i n 



m tn » 



(c) 1986 Consensus Plan developed by a design team of community, 

 county, and federal representatives. 



SOURCES U S Army Corps of Engineers. General Design Memorandum and Basis of Design tor 

 Reach r. Wildest and San Paolo Creeks (Sacramenio. Calif : US ACE, Sacramento District. October 

 1985). Poster of the Modified Plan published by a coalition of North Richmond community organiza- 

 tions including the East Bay Regional Park District, and U S Army Corps of Engineers. Supplement 

 No 3 to Design Memorandum t. Wildcat and San Pablo Creeks Environmental Mitigation Project 

 (Sacramento. Calif US ACE. Sacramento District. August 1988) 



