Levee Breaches

Portions of the existing levees were removed to increase the number of points of connectivity with

the floodplain bounded by Scotts and Queseria Creeks, increase the number and frequency of refuge

habitat units, and create access to floodplain refuge and foraging habitat (Recovery Actions 2.1.1.1

and 2.1.1.2, NMFS 2012). Five levee breaches were distributed along the reach spanning from the

Queseria Creek confluence to the drainage ditch at 3000, at approximate stations 1900, 2000, 2650,

2750, and 2850. This reach was chosen because the wide vegetated riparian buffer and low

floodplain gradient (<0.3%) are expected to minimize risk of flood damage to crop fields by

slowing floodflows, and because observations during overbank events suggest that flood flow

velocities are buffered by backwater effects from the Queseria confluence. Breaches were cited

along sub-reaches where they would likely activate the floodplain at the bankfull stage. The

likelihood of activation was determined by comparing the elevation of bankfull indicators in the

main channel to adjacent floodplain elevations. Two sub-reaches were identified where these

elevations were approximately equal (1800-2100 and 2500-3000) whereas the channel was more

incised from 2100-2500 and was avoided for that reason. Within the favorable sub-reaches, the

approximate locations of the levee breaches were determined by selecting gaps in vegetation, and

by attempting to evenly distribute the breaches throughout the sub-reach.

The sizes of the breaches were minimized to reduce the volume of excavation and the area of

disturbance. Breaches were laid out perpendicular to the main channel and levee berm to minimize

the length required to connect the main channel to the floodplain. The breach bottom widths were

minimized and the side slopes were optimized to minimize volume yet remain stable. While the

breach openings were conceptually regarded as pilot channels with the potential to adjust in size

during a very large flood event, it was necessary to determine a stable geometry to prevent mass-

wasting and sediment delivery to the main channel, and promote regeneration of native riparian

an erosive threshold was chosen, because it resulted in a stable configuration and minimal

disturbance (see hydraulic calculations in Phase I Engineering Docket, Appendix C).

Levee breaches were expected to result in an array of hydrologic, hydraulic, and habitat-related

effects. The breach openings were expected to form five additional refuge habitat features at the

margins of the stream channel during high flows. Increased floodplain connectivity was expected

to improve access to floodplain refuge and foraging habitat, primarily for spawning adults, and

reduce the potential for stranding by providing five additional points of egress/ingress.

Additionally, increased floodplain connectivity could deliver additional floodplain-derived

nutrients to the main channel which would improve instream foraging opportunities.

In addition to increasing floodplain refuge and foraging opportunities for salmonids, levee breaches

were intended to improve floodplain function and increase riparian habitat and species diversity by

restoring the historical frequency and duration of floodplain inundation along lower Scotts Creek.

High productivity, biodiversity and species richness of natural floodplains, and the importance of

the flood pulse to floodplain function, has been documented (Junk et al. 1989, Gregory et al. 1991,

Naiman et al. 2000, Tockner et al. 2000). Intentional levee breaches were found to influence

floodplain topography through depositional sand-splay formation and floodplain scour, and

delivery of large wood to the floodplain which further facilitates scour (Florsheim and Mount

2002). Restoring the flood pulse along lower Scotts Creek may increase variation in floodplain

topography, and as a consequence, increase habitat and species diversity.

Hydrologically, the levee breaches were designed to increase the frequency (spatial) and number

of points of connectivity with the floodplain, the frequency (temporal), duration, and area of

floodplain activation, and increase groundwater recharge. Hydraulically, levee breaches were

expected to reduce main channel discharge, velocities and shear stresses above the bankfull stage

by taking advantage of floodplain storage and conveyance. Levee breaches have the potential to

mitigated by the constructed instream features (see Large Wood Complexes below) located to

increase flow competence in the vicinity of the breach openings, and by a reduction in sediment

loads in the main channel due to conveyance and storage of sediment on the floodplain. Multi-year

repeat longitudinal profiles or topo surveys should be done to track these potential effects (see

Chapter 6. Monitoring).

HEC RAS 4.1.0 was used to check whether the levee breaches, and other features, would activate

at the bankfull stage as designed. To accomplish this, AutoCAD Civil 3D 2014 Imperial was first

used to create a DEM of a design iteration which included five levee breaches, 12 instream large

wood complexes (modeled as prism-shaped obstructions in select main channel cross-sections),

connections with two off-channel ponds, connections with two drainage ditches, and the

enhancement of the Archibald Creek confluence. HEC RAS was used to model the bankfull

discharge through the main channel on the design DEM. The results were analyzed with the HEC

GeoRAS plugin for ArcMap 10.2 (Figure 10). While the one-dimensional HEC RAS model cannot

accurately model overbank flow (Roni and Beechie 2012), the results were useful for comparing

Figure 10. HEC RAS model results for the bankfull event showing potential inundation (shaded blue) through the proposed levee breaches.