1. Laguna Creek Watershed Management Plan Hydrology and Geomorphology
Watershed Assessment
Hydrology and Geomorphology
Gary E. Palhegyi, P.E., D.WRE
Hydrology & River Mechanics
Geosyntec Consultants

2. Presentation Outline Overview Watershed Assessment Definitions
Fundamentals
Watershed Assessment
Hydrology
Geomorphology
Conclusions
Geosyntec Consultants
Hydrology & Geomorphology

3. Definitions Hydro-Geomorphology Channel Stability
the study of how water forms the landscape through the process of erosion, transport and deposition
Channel Stability
requires a balance among flow energy, sediment supply and channel resilience
Hydrologic Conditions of Concern
change in watershed runoff characteristics due to land use changes (primarily urbanization)
change in sediment supply delivered to the stream

4. Hydrologic & Geomorphic Process
Hydrology
Geomorphology
Climate Physiography, Geology
Climate
Physiography, Geology
Hill slope erosion (landslides, debris flows)
Precipitation
Soil Type
Land Cover
Surface Runoff
Infiltration (Interflow, baseflow)
Sediment Supply
Boundary Material (valley fill)
Stream Flow Hydrographs
Sediment Transport
Channel Resilience
Stream Channel Morphology
Ecological Health

5. Lanes Balance Illustrates Stability
Watercourses reach a balance: Qs d50 a Qw S
General Changes/Responses:
Increase flow: Qw+ Potential response: Qs+ d50+ a Qw+ S-
Decrease Sediment Supply: Qs- Potential response: Qs- d50+ a Qw S-

6. Disruption of the Balance leads to Unstable Conditions
Increased runoff and stream flow intensifies sediment transport and erosion processes.
Observed as excessive erosion, incision and widening

7. Increased flows Channelization
Channel Evolution Model (CEM)
Evolutionary phases of an incised channel
“Dt at a Station” or “Moving Downstream” perspective
Similar model for braided channels

8. Hydro-Geomorphic Assessment
Purpose
Characterize existing conditions
Identify dominant physical processes controlling form and stability
Develop model parameters
Question
How sensitive is Laguna Creek to the effects of urbanization and hydromodification?
How do we expect the channel to respond?

9. Hydrologic Assessment
Ephemeral to perennial
Nuisance flows, irrigation return flows and possibly seepage
Low relief, hummocky landscape
Low permeable soils, hardpan
Water migrates as overland flow and as shallow sub-surface flow (interflow)
Naturally has a high amount of runoff
Only about 4 to 5 inches of rainfall produces runoff

10. Historically Multi-channel High flow secondary Drainage swale
Seasonal wetlands
Vernal pools

11. Geomorphic Assessment
Location of Data Collection
William-Lettis sections
Geosyntec sections

12. Geomorphic Assessment Data Types
Geometry and longitudinal slope
Bed and bank material properties
Sediment supply & transport
Vegetation type and density
Evidence of instability
Excessive bank and bed erosion
Mechanism of failure

13. Stable in Upper Reaches
Unstable below Blodgett Reservoir

14. Upstream of Grant Line Road
High width/depth ratios
Frequent access to floodplain
Firm bed, little loose sand and gravel
Scour pool at meander bend
Surrounding land: Pasture
Vegetation growing on bed and banks

15. Downstream from Grant Line Road
Thin cobble layer on bed
Drop
Scour pool
Natural Erosion

16. Unstable Channel Downstream from Blodgett Reservoir
Headcuts
Bank failures

17. Stable Upstream from Excelsior Road to above Eagles Nest

18. Upstream of Eagles Nest Road
Densely vegetated banks
Stable, no observed failures
Large woody debris
Hardpan under bed
Quiescent flow
Upstream of Eagles Nest Road

19. Upstream from Excelsior Road
Shallow bank height & slope
No bank failures observed
No exposed soil
Frequent access to floodplain
High width/depth ratios
Surrounding land: Pasture
Vegetation: grasses growing on bed
Mucky bed w/ cobble layer
Upstream from Excelsior Road

20. Unstable Upstream from Vineyard Road

21. Exposed hardpan
Nick Points
Provides grade control
If lost, channel incision will migrate upstream
Pools will be lost

22. Unstable channel migrating from downstream channelized segment
Incising, head cutting
alignment change?

23. Elevated
Outfalls
Incising channel

24. Headcut Forming Secondary Channel through Floodplain
Flood Flows Spill Over
Scour Pool
Cobble
Headcut Forming Secondary Channel through Floodplain

25. Unstable Downstream from Vineyard Road

26. Downstream from Bradshaw Road
W/D = 12
W/D = 6

27. Unstable Channel Downstream from Vineyard Road
Scarp
Channelized h>hc
Slump
Scarp
Over steep banks, slumping, incision, infrequent access to floodplain
Slump
Scour along toe

28. New Channel Forming in Larger Channel
Vertical banks
Scour along toes
New Channel Forming in Larger Channel

29. Bank Erosion and Fallen Trees

30. Unstable Channel Upstream of Calvine
Slumps
Bank slumping
Mid-channel bar

31. Channel Change near Calvine
Deep channel, h>hc
Infrequent floodplain access
Shallower
More frequent floodplain access

32. Stable Downstream from Sheldon Road

33. Downstream from Sheldon Road
Return to higher W/D ratio
Shallower bank heights
Access to floodplain

34. Downstream of Waterman and Bond
High width/depth ratios
Levee
Surrounding land: Residential & commercial
Vegetation: Some trees, mostly shrubs, grasses, forbs

35. Camden Lake
Backwater Influence

36. Observed Channel Conditions
DRAFT
Stable, Meta-Stable
Modified but stable
Unstable

37. Conclusions
Laguna Creek is dynamic and shows clear signs of geomorphic activity
i.e., it’s not static
Laguna Creek would be susceptible to the effects of development and hydromodification
Increased channel width, scour pools at meander bends and meander enlargement
Increased bank slumping and bank toe erosion
Increased bed incision and head cutting

38. Questions? Laguna Creek, Sacramento For more information:
Gary E. Palhegyi, P.E., D.WRE
Hydrology & River Mechanics
Geosyntec Consultants