Assessment of gravel transport characteristics of the upper Santa Ana River

Scott Wright and Toby Minear

USGS California Water Science CenterSacramento, CA

Information presented in this presentation is draft, subject to revision, and not citable

A publication for submittal to peer-review is in preparation

Objectives/Questions

What is the gravel transport capacity of mainstem reaches? What are the effects of Seven Oaks Dam?

Which tributaries are important sources of gravel to the mainstem?

What is the fate of gravel in the reach currently occupied by sucker (below Rialto drain)?

This study is funded by SBVMWD, with ~30% match from USGS

Methods Review of previous studies and collection of existing data

related to gravel transport

Field surveys of channel cross-sections, slopes, and bed sediment sizes conducted by USGS in 2013

Calculation of bed shear stresses for a range of flows and comparison with threshold values for gravel transport

Calculation of relative gravel transport capacity for a range of flows

Calculation of approximate gravel budgets (in vs out) for various reaches of the Santa Ana

Reac

h 1

Reach 2

Reach 3

Reach 4

City C

r Plunge Cr

Mill Cr

San Timoteo Cr

War

m/T

win

Cr

Lytle Cr

Reche Cr

Seven Oaks Dam

Rialto drain

Mainstem cross-sections and slopes

Reach 1 – Seven Oaks Dam to Mill Creek confluence

64 mmcoarse gravel

Shear stresses exceed threshold values for gravel transport

Seven Oaks dam dramatically reduces flows and bed shear stresses

Reach 1 – Seven Oaks Dam to Mill Creek confluence

Reach 2 – Mill Creek to City Creek

Shear stresses mostly exceed threshold values for gravel transport

Impacts of Seven Oaks dam are less due to inflows from Mill Creek

Reach 2 – Mill Creek to City Creek

Reach 3 – City Creek to Lytle Creek

Shear stresses are declining downstream but still generally capable of gravel transport

Impact of Seven Oaks continues to decrease due to tributary inflows

Reach 3 – City Creek to Lytle Creek

Reach 4 – Downstream from Lytle Creek

Shear stresses continue to decrease downstream and the impact of Seven Oaks becomes even less due to tributary inflows

Smaller floods unlikely to transport much gravel in this reach but larger floods could

Reach 4 – Downstream from Lytle Creek

Mainstem summary

with Seven Oaks

without Seven Oaks

Without Seven Oaks, stresses exhibit classical downstream decrease characteristics of depositional basins, driven by slope changes

with Seven Oaks

Seven Oaks has disrupted this pattern, particularly in the most upstream reach, but stresses are still mostly high enough to move gravel

Reac

h 1

Reach 2

Reach 3

Reach 4

City C

r Plunge Cr

Mill Cr

San Timoteo Cr

War

m/T

win

Cr

Lytle Cr

Reche Cr

Seven Oaks Dam

Rialto drain

Mill CreekSlope ~ 3.6%, width ~ 50 m

Shear stresses are above gravel transport thresholds, indicating potential for large gravel supply to the mainstem

Results are similar to reach 1, both are very steep with very coarse bed sediment

Mill Creek

City CreekSlope ~ 0.7%, width ~ 80 m

Computed stresses are much lower than Mill Creek (lower slope and smaller discharge)

City Creek likely delivers significant gravel to the mainstem only during very large, infrequent floods

City Creek

Lytle CreekSlope ~ 0.6%, width ~ 250 m

Shear stresses are lower than Mill Creek but higher than City Creek

Could be a substantial source of gravel to the mainstem due to large channel size and discharge

Lytle Creek

Other tributaries evaluated

Warm/Twin Creek: Low slope, vegetated sandy channel with no evidence of gravel. Not likely to supply gravel to the mainstem

San Timoteo Creek: Slope and drainage area suggest potential to supply gravel, but settling basins upstream from concrete channel likely trap most gravel

Reche Creek: Visual evidence in the field of gravel supply to the mainstem, but small drainage area likely limits gravel supply in comparison to other larger tributaries

Tributary summary

Mill, City, and Lytle have the greatest potential to supply gravel to the mainstem, particularly Mill (due to steep slope) and Lytle (due to large drainage area)

Mill

City

Lytle

Lytle

Mill

Upper SA

Plunge

San Timoteo

Reche

CityTwin

Tributary summaryTributaries with high gravel supply potential have high elevation headwaters and large drainage areas

Gravel transport capacity

Shear stress indicates the likelihood of gravel transport, but not rates

Rates can be estimated based on the “excess shear stress”, i.e. the amount of stress above the threshold for movement

We used the shear stress calculations to compute a relative measure of gravel transport rates, for comparison of mainstem reaches and tributaries, and construction of gravel budgets (in vs out)

Rates were computed for each flow recurrence interval and integrated to estimate a long-term rate

Gravel transport index - mainstem

8 mm gravel 32 mm gravel

Pre Seven Oaks, gravel transport capacity decreased downstream with decreasing slope

Seven Oaks has reduced gravel transport capacity in all reaches, with the greatest reductions in the upstream reaches. This is typical of the downstream effects of dams on sediment transport

Gravel transport index - tributaries

8 mm gravel 32 mm gravel

Mill Creek has by far the greatest potential to supply gravel to the mainstem

Lytle Creek also has relatively large transport potential

City Creek likely does not supply much gravel to the mainstem due to relatively low slope and small drainage area

Mill

City

Lytle

Mill

City Ly

tle

Reach 1 gravel budgets

with

Sev

en O

aks

0

40

with

out S

even

Oak

s

unknown

300

8 mm

with

Sev

en O

aks

0

2

with

out S

even

Oak

s

unknown

30

32 mm

Because the gravel supply has been shut off by Seven Oaks, this reach should erode and coarsen (armoring)

Pre-Seven Oaks, it was likely depositional

Reach 2 gravel budgets

with

Sev

en O

aks

150

70

with

out S

even

Oak

s

400

200

8 mm

with

Sev

en O

aks

10

5

with

out S

even

Oak

s

40

20

32 mm

Flows and gravel transport are reduced by Seven Oaks

The reach remains depositional with Seven Oaks because of the large gravel supply from Mill Creek. Deposition rate is less post-dam

Inputs: Reach 1 + Mill Cr

Out: Reach 2 transport

Reach 3 gravel budgets

with

Sev

en O

aks

80

20

with

out S

even

Oak

s

200

50

8 mm

with

Sev

en O

aks

5

1

with

out S

even

Oak

s

20

3

32 mm

Flows and gravel transport are reduced by Seven Oaks

The reach remains depositional with Seven Oaks because of the decrease in slope

Inputs: Reach 2 + City Cr

Out: Reach 3 transport

Reach 4 gravel budgets

with

Sev

en O

aks

60

30

with

out S

even

Oak

s

90

40

8 mm

with

Sev

en O

aks

2

1

with

out S

even

Oak

s

5

2

32 mm

Because of the distance downstream from Seven Oaks and inputs from tributaries (Mill and Lytle), the effect of the dam is apparent but not as substantial for this reach

Inputs: Reach 3 + Lytle Cr

Out: Reach 4 transport

Summary of findings

Flow regulation and sediment trapping by Seven Oaks dam is expected to reduce gravel supply and transport rates in all downstream reaches, with the effects decreasing downstream as tributaries “resupply” the mainstem with water and gravel

Post Seven Oaks, the four mainstem reaches studied should still have the capacity to transport a range of gravel sizes, particularly during infrequent floods

Mill Creek and Lytle Creek have the greatest potential to supply gravel to the mainstem reaches

Gravel supply to reach 4 (Santa Ana sucker habitat), along with ample in-channel gravel storage, suggest that the inset channel below Rialto drain can maintain a partial gravel bed, so long as sufficient wastewater discharge levels are maintained