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
Rea
ch 1
Reach 2
Reach 3
Reach 4
City C
rPlunge Cr
Mill Cr
San Timoteo Cr
Warm
/Tw
in C
r
Lytle C
r
Reche Cr
Seven Oaks Dam
Rialto drain
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
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
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
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
Rea
ch 1
Reach 2
Reach 3
Reach 4
City C
rPlunge Cr
Mill Cr
San Timoteo Cr
Warm
/Tw
in C
r
Lytle C
r
Reche Cr
Seven Oaks Dam
Rialto drain
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
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
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 summary
Tributaries 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
Cit
y
Lytl
e
Mill
Cit
y Lytl
e
Reach 1 gravel budgets
wit
h S
even O
aks
0
40
wit
hout
Seven O
aks
unknown
300
8 mm
wit
h S
even O
aks
0
2
wit
hout
Seven O
aks
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
wit
h S
even O
aks
150
70
wit
hout
Seven O
aks
400
200
8 mm
wit
h S
even O
aks
10
5
wit
hout
Seven O
aks
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
wit
h S
even O
aks
80
20
wit
hout
Seven O
aks
200
50
8 mm
wit
h S
even O
aks
5
1
wit
hout
Seven O
aks
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
wit
h S
even O
aks
60
30
wit
hout
Seven O
aks
90
40
8 mm
wit
h S
even O
aks
2
1
wit
hout
Seven O
aks
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