190 



ment load from ending up in the maish 

 or significantly decreasing the chan- 

 nels' capacity. 



Another difficult design issue to re- 

 solve was how to make up for the loss 

 of 24 acres of riparian vegetation. The 

 county's 1982 proposal called for plant- 

 ing trees on some acreage north of 

 Wildcat Creek. In the Consensus Plan, 

 trees planted along the two creeks' low- 

 flow channels would help guide chan- 

 nel formation and shade the bank to 

 prevent it from clogging with rushes, 

 reeds, and sediment. However, county 

 engineers did not want vegetation near 

 the channels because they felt this 

 would make channel maintenance dif- 

 ficult for them. Thus, choosing rough- 

 ness values that would determine how 

 much vegetation could be allowed 

 without reducing the needed channel 

 capacity became a critical aspect in the 

 design of the Consensus Plan. 



Roughness values are calculated by 

 using the Manning Equation to de- 

 scribe the flow resistance caused by the 

 texture of the surface over which the 

 water must flow. But the assignment of 

 roughness values is a very subjective 

 process. The corps originally consid- 

 ered using the values 0.100 for the ri- 

 parian areas south of the low-flow 

 channels and 0.045 for the north flood- 

 plains. (Lower roughness values mean 

 more vegetation is allowable.) The de- 

 sign team finally decided that a com- 

 posite value for the low-flow channels 

 and souih bank riparian forests would 

 be 0.050 (conditional upon maintain- 

 ing clear low-flow channels), and a 

 roughness value of 0.035 was assigned 

 to the north bank floodplains for low 

 shrubs and grasses. 



Once roughness values had been 

 chosen, the design team had to agree 

 upon a maintenance plan for keeping 

 the low-flow channels cleared of vege- 

 tation until a riparian canopy could 

 grow to shade out the unwanted, clog- 

 ging reed growth expected in exposed, 

 low-flow channels. The agreement ne- 

 gotiated between the county supervisor 

 and the corps' project manager pro- 

 vides for inexpensive hand labor by 

 conservation crews to clear the unwant- 

 ed vegetation. Potential maintenance 



crews include the State of California 

 Conservation Corps and a local East 

 Uay Conservation Corps as well as la- 

 bor from the state's new workforce 

 program. It was also agreed that the 

 standard, annual maintenance routines 

 for removing sediment or clearing veg- 

 etation would be substituted with a 

 maintenance schedule based on actual 

 need. Thus, maintenance activities, 

 costs, and negative environmental im- 

 pacts resulting from channel mainte- 

 nance should be reduced. 



Maintenance 



The consensus maintenance plan is 

 one of the most important innovations 

 of this project. Federal government 

 policy mandates that local project 

 sponsors must accept long-term re- 

 sponsibility for the maintenance of any 

 project. But corps officials readily ad- 

 mit that such maintenance costs have 

 been grossly underestimated over the 

 years. These costs may have been un- 

 derestimated simply because they fall 

 on the costs side of the cost-benefit 

 analyses, but another likely reason for 

 the misjudgment is that the corps' 

 channelization projects have not per- 

 formed as the engineers expected. Many 

 flood-control channels quickly re-es- 

 tablish their original grades when sedi- 

 ment fills in the project's designed 

 grade, thus greatly reducing the chan- 

 nel capacities. Lowered capacity results 

 in more frequent and more expensive 

 maintenance bills. 



Because the design team also had to 

 face the reality of the project's limited 

 maintenance budget, a critical need of 

 the Consensus Plan was to provide a 

 channel design that would reflect the 

 equilibrium in a natural system and 

 that would assume a certain amount of 

 sediment deposition in the calculation 

 of channel capacities. The Wildcat-San 

 Pablo Creek Maintenance Master Plan 

 was as much a negotiated part of the 

 design team's Consensus Plan as the 

 project features. It requires an annual 

 field inspection of the project by inter- 

 ested agencies and community organi- 

 zations. The Hydraulic Engineering 

 Center-2 water surface profile model 



will be used to estimate channel capac- 

 ity at cross sections selected for moni- 

 toring. When vegetative growth and 

 sediment deposition reduce the two 

 creeks' freeboards by 50 percent, par- 

 ticipants in the maintenance planning 

 will prescribe how to thin the vegeta- 

 tion and/or remove sediment to re-es- 

 tablish the channels' capacity while 

 minimizing maintenance activity im- 

 pacts on the environment. 



To design a revegetation plan that 

 would reflect the needs of the U.S. Fish 

 and Wildlife Service, the California 

 State Lands Commission, and other 

 members of the design team, the coun- 

 ty asked the corps of engineers to con- 

 tract with the Soil Conservation Serv- 

 ice, which has experience with the re- 

 vegetation and restoration of streams. 

 In September 1988, the Soil Conserva- 

 tion Service and the corps issued a rec- 

 reation and revegetation supplement to 

 the corps' design memorandum about 

 the Consensus Plan. 15 Their revegeta- 

 tion design objective is not to landscape 

 a flood-control project but to restore a 

 riparian environment along the low- 

 flow channels. Revegetation will be 

 done with cuttings from nearby plants, 

 seeds from California species native to 

 the locale, and some container slock. 

 Because of the competence demon- 

 strated by the landscape architects in 

 the design process, the design team 

 asked the corps to retain the Soil Con- 

 servation Service staff for the actual 

 plant installation. 



The most significant test of this inno- 

 vative project remains, however: to 

 complete construction according to the 

 design team's plans and specifications. 

 The Army Corps of Engineers estimates 

 that construction should be completed 

 in 1990. 



The Funding Strategy 



The coalition's Modified Plan and 

 the county's Selected Plan had very 

 similar cost estimates. The Consensus 

 Plan's costs were higher because the 

 sediment basin was redesigned and re- 

 located. The transition of this project 

 from a single-objective flood-control 

 (continued on page 29) 



December 1989 



