Tuolumne River Report - U.S. Fish and Wildlife Service

RESTORING THE TUOLUMNE RIVER CORRIDORFloodway width of at least 500 ft in the gravelbeddedzone is recommended for the followingreasons:• In reaches with minor or no channel damageduring the January 1997 flood, the minimumfloodway width was 500 ft.• Most bridge crossings and hard infrastructureconstraints can be avoided by a 500 ft widefloodway.• A 500 ft wide floodway will increase floodplainstorage of high flows, helping attenuate shortduration peak flows.• A riparian corridor can have several maturecottonwoods or valley oaks on each side of a200 ft wide bankfull channel, capable ofcreating a continuous corridor on both sides ofthe river with a closed canopy and associatedterrestrial habitat.• A 500 ft wide floodway provides room for thechannel to migrate in the future (migratethrough a floodplain or terrace rather thanthrough dikes).A 500 ft wide floodway also conveys a maximumdesired flood control release of 15,000 cfs. TheNDPP outlet works capacity is 14,500 cfs, andadded to 500 cfs of downstream flow accretionduring a storm event, results in a desired floodwaycapacity of 15,000 cfs upstream of DryCreek. Flow capacity was computed in a 500 ftwide channel (Figure 4-5), using the followingassumptions:1. Energy slope = 0.0015 based on 5400 cfswater surface slope measured at manylocations in the gravel-bedded zone in thespring of 1996;2. Manning’s n = 0.028 over entire channelimmediately after construction (smoothestchannel conditions) based on HEC-RASback-calculations of the 5,400 cfs flowmeasured in the spring of 1996;3. Manning’s n = 0.035 within the bankfullchannel and n = 0.075 on floodplain/terracesurfaces after vegetation has established andbegun to mature (roughest channel conditions);4. At least 3 feet of freeboard above watersurface required for dike stability.Results suggest that a 20,000 cfs flood could beconveyed with 6 feet of freeboard immediatelyafter construction, and 4 to 5 feet of freeboardafter riparian vegetation establishment roughenedthe channel (Figure 4-5). The 20,000 cfs valuewas used as a conservative discharge. Bothroughness scenarios pass the “safe” test with thesame safety factor (2 to 3 feet). For the rougherchannel condition, average channel velocitieswere 5.5 to 6.0 ft/sec, floodplain velocities wereapproximately 2 ft/sec, and main channel velocitieswere 7.5 to 8.0 ft/sec. This simple calculation,however, does not account for reaches with lowerslope, non-uniform flow, and increased roughnessfrom bars and channel curvature. Each channelrestoration project should evaluate floodway widthneeds using an appropriate hydraulic model (e.g.,HEC-RAS).4.3.3. Riparian restoration strategiesRiparian revegetation must be compatible withrestored hydrologic and fluvial processes. Asshown in Chapter 3, riparian plant species haveadapted to specific geomorphic surfaces that areinundated by specific flood recurrence intervals.General riparian restoration strategies shouldtarget Fremont cottonwood and valley oak as thehighest priority species due to their historic roleas dominant canopy structure and their subsequentdemise over time. Riparian restorationstrategies include:• plant cottonwoods on floodplain surfaces;• plant valley oaks on floodplain and terracesurfaces;• integrate revegetation with channel restorationwherever possible (e.g., use narrow-leaf willowbioengineering on outsides of meanders whereconstraints prohibit migration);• construct topographic depressions on floodplainsurfaces to create moist seedbeds that willencourage natural riparian regeneration;• revegetate restored floodplains and terraces inthe gravel-bedded zones in patches rather thanrigid grid spacing, preferably followingconstructed topographical depressions (seeFigures 2-19 and 2-38). This mimics thenatural patchiness of historic riparian forests inthe gravel-bedded zone, creating interiorhabitat for wildlife, creating perching androosting canopy structure, and providing aclosed canopy;CHAPTER 4151