HomeMy WebLinkAboutCT 13-02; COASTAL 10; HYDROMODIFICATION SCREENING; 2014-08-29cT
HYDROMODIFICATION S C R E E N I N
FOR
COASTAL 10
(C.T. 13-02, PUD 13-04, DRAWING NO. 480-5A)
August 29, 2014
Wayne W. Chang, MS, PE 46548
chaNO&MM[mm
Civil Engineering o Hydrology o Hydraulics o Sedimentation
P.O. Box 9496
Rancho Santa Fe, CA 92067
(858) 692-0760
FOR REVIEW ONLY
-TABLE OF CONTENTS -
Introduction...................................................................................................................................
Domainof Analysis ......................................................................................................................2
InitialDesktop Analysis................................................................................................................4
FieldScreening .............................................................................................................................5
Conclusion....................................................................................................................................9
Figures.......................................................................................................................................... 11
APPENDICES
SCCWRP Initial Desktop Analysis
SCCWRP Field Screening Data
FOR REVIEW ONLY
INTRODUCTION
The City of Carlshads January 14. 2011, Standard Urban Storm Water Management Plan
(SUSMP) outlines low how thresholds for hydromodification analyses. The thresholds are based
on a percentage of the pre-project 2-year flow (Q2), i.e., O.1Q2 (low flow threshold and high
susceptibility to erosion), 0.3Q2 (medium flow threshold and medium susceptibility to erosion),
or 0.5Q2 (high flow threshold and low susceptibility to erosion). A threshold of 0.1Q2 represents
a downstream receiving conveyance system with a high susceptibility to erosion. This is the
default value used for hydromodification analyses and will result in the most conservative
(greatest) on-site facility suing. A threshold of 0.3Q2 or 0.5Q2 represents downstream receiving
conveyance systems with a medium or low susceptibility to erosion, respectively. In order to
qualify for a medium or low susceptibility rating, a project must perform a channel screening
analysis based on a "hydromodification screening tool" procedure developed by the Southern
California Coastal Water Research Project (SCCWRP). The SCCWRI' results are compared with
the critical shear stress calculator results from the County of San Diego's I3MP Sizing Calculator
to establish the appropriate susceptibility threshold of low, medium, or high.
CITY OF OCEANSIDE
jWAY7
DR
This report provides hydromodification screening analyses for the Coastal 10 residential project
being designed by O'Day Consultants. The project will be located on the south side of Navarra
Drive approximately 400 feet west of Viejo Castilla Way in the city of Carlsbad (see the Vicinity.
Map above and the Study Area Exhibit in Appendix A). The project proposes 10 residential units
and a recreation area on an approximately Y2-acre lot. A proposed private drive from Navarra
Drive will bisect the property and serve the residences.
The site is currently undeveloped, but mass-graded and surrounded by existing multi-family
residential development. The Omni La • Costa Resort and Spa golf course surrounds the site and
the adjacent developments. Under pre- and post-project conditions, storm runoff from the site
will flow onto Navarra Drive. There is negligible off-site run-on to the site. The site runoff is and
will continue to be conveyed easterly approximately 1,000 feet within Navarra Drive to an
existing curb inlet in the cul-de-sac at the easterly end of the street (see the Study Area Exhibit).
The curb inlet connects to an 18-inch RCP storm drain that extends approximately 70 feet
northeast to an existing concrete drainage channel (see Figures 1 and 2). The concrete drainage
channel continues northeast over 330 feet and discharges through a dissipater structure into the
natural drainage course within the golf course (see Figure 3). The natural drainage course
corresponds to the flowline of San Marcos Creek. The natural drainage course flows through the
golf course westerly approximately 1.2 miles and outlets into Batiquitos Lagoon just past El
Camino Real.
The SCCWRP screening tool requires both office and field work to establish the vertical and
lateral susceptibility of a natural downstream receiving channel to erosion. In this case, the
natural channel is the drainage course within the golf course. The vertical and lateral assessments
are performed independently of each other although the lateral results can be affected by the
vertical rating. A screening analysis was performed to assess the low flow threshold for the
project's single point of compliance, which is at the outlet of the concrete channel into the
natural drainage course.
The initial step in performing the SCCWRP screening analysis is to establish the domain of
analysis and the study reaches within the domain. This is followed by office and field
components of the screening tool along with the associated analyses and results. The following
sections cover these procedures in sequence.
DOMAIN OF ANALYSIS
SCCWRP defines an upstream and downstream domain of analysis, which establish the study
limits. The County of San Diego's HMP specifies the downstream domain of analysis based on
the SCCWRP criteria. The HMP indicates that the downstream domain is the first point where.
one of these is reached: . . .
• • at least one reach downstream of the first grade control point(preferably second •
downstream grade control location)
tidal backwater/lentic waterbody
equal order tributary
2
accumulation of 50 percent drainage area for stream systems or 100 percent drainage area
for urban conveyance systems (storm drains, hardened channels, etc.)
The upstream limit is defined as::.
proceed upstream for 20 channel top widths or to the first grade control point, whichever
comes first. Identify hard points that can check headward migration and evidence of
- active headcutting.
SCCWRP defines the maximum spatial unit, or reach (a reach is circa 20 channel widths), for
assigning a susceptibility rating within the domain of analysis to be 200 meters (656 feet). If the
domain of analysis is greater than 200 meters, the study area should be subdivided into smaller
reaches of less than 200 meters for analysis. Most of the:units in the HMP's SCCWRP analysis
are metric. Metric units are used in this report only where given so in the HMP. Otherwise
English units are used.••
Downstream Domain ofAnalysis
The downstream domain of analysis for a study area is determined by assessing and comparing
the four bullet items above. As discussed in the Introduction, the project has a single point of
compliance (POC) where the concrete channel outlets into the receiving natural drainage course
within the La Costa golf course. The POC is the first location where the downstream drainage
conveyance is natural and potentially erodible. The downstream domain of analysis location will.
be selected below the POC.
Per the first bullet item, the first permanent grade control point was located below the POC
through a site investigation. The first permanent grade control occurs at a golf cart crossing of
the natural drainage course (see the Study Area Exhibit). The crossing is approximately 742 feet.•
downstream of the POC. The channel bed and banks at the crossing are lined with riprap (see
Figures 12 and 13), probably to protect the golf cart bridge from channel flows. •
: The second bullet item is the tidal backwater or lentic (standing or still water such as pond s,
pools, marshes, lakes, etc.) waterbody location. The nearest significant tidal backwater or lentic
waterbody is Batiquitos Lagoon, which is over 1.2 miles west of the POC. Since the lagoon is
several thousand feet downstream of the grade control, the lagoon will not govern for
establishing the downstream domain of analysis location. •
The final two bullet items are based on 50 and 100 percent tributary drainage areas. From the
Watershed Exhibit in Appendix A, the drainage area tributary to the POC covers over 33 square
miles. A review of the USGS topographic mapping on the Watershed Exhibit reveals that the •
drainage area clearly does not increase by 50 to 100 percent between the POC and the permanent
grade control. Therefore, none of the tributary drainage area criteria will govern in establishing .
• the downstream domain of analysis location. • . • • •
Based on the above information, the downstream domain of analysis is established by the •
permanent grade control criteria because this is the first point reached among the various criteria.
Per the first bullet item, the downstream domain of analysis is one reach (or 20 channel widths)
3
• • .
below the grade control point. The channel width in the vicinity of the grade control is
approximately 20 feet, so a reach is 400 feet long. Therefore, the downstream domain of analysis
location was selected to be 400 feet below the permanent grade control.
Upstream Domain ofAnalysis
An upstream domain of analysis must be established for the natural drainage course within San
Marcos Creek. The drainage course continues upstream of the POC within the golf course, so the
upstream domain of analysis location will be along the continuation. A site visit revealed that a
riprap grade control exists at the pedestrian bridge approximately 380 feet upstream of the POC.
Therefore, the upstream domain of analysis location was selected at this grade control.
Study Reaches within Domain ofAnalysis
The total domain of analysis (or overall study reach) extends from the upstream grade control to
400 feet below the downstream grade control. The total domain of analysis covers approximately
1,522 feet. The domain of analysis was subdivided into three natural study reaches with similar
characteristics (see the Study Area Exhibit in Appendix A). Reach I extends 380 feet from the
upstream domain of analysis location down to the POC. Reach 2 extends 742 feet from the POC
to the downstream grade control. Reach 3 extends 400 feet from the downstream grade control to
the downstream domain of analysis location.
Reach 2 is longer than the 656 feet (200 meters) maximum reach length specified by SCCWRP.
Review of topographic mapping, aerial photographs, and field conditions reveals that the
physical (channel geometry and longitudinal slope), vegetative, hydraulic, and soil conditions
within Reach 2 are relatively uniform. Subdividing the reach into smaller subreaches of less than
656 feet will not yield significantly varying results within the reach. Although the screening tool
was applied across the entire length of Reach 2, the results will be similar for shorter subreaches
within the reach.
After the domain of analysis is established, SCCWRP requires an "initial desktop analysis" that
involves office work. The initial desktop analysis establishes the watershed area, mean annual
precipitation, valley slope, and valley width. These terms are defined in Form 1, which is
included in Appendix A. SCCWRP recommends the use of National Elevation Data (NED) to
determine the watershed area, valley slope, and valley width. The NED data is similar to USGS
mapping. For this report, USGS quadrangle mapping was used to determine the maximum
watershed area tributary to the study reaches (see the Watershed Exhibit in Appendix A). The
Watershed Exhibit shows that the maximum area is 33.46 square miles. The watershed areas
tributary to each study reach will be nearly the same because the reaches are relatively in the
same location with the overall watershed. Therefore, the maximum area of 33.46 square miles
was used for Reach 1, 2, and I
The mean annual precipitation is provided by the County of San Diego's BMP Sizing Calculator
(see Appendix A) and is 13.3 inches.
4
The valley slope of each study reach was determined from the City of Carlsbad's 2-foot contour
interval topographic mapping. The valley slope is the longitudinal slope of the channel bed along
the flow line, so it is determined by dividing the elevation difference within a reach by the flow
path length. The 2-foot contour mapping was used because it will provide more precise results
than NED data.
The valley width is the bottom width of the main creek channel. The average valley width within
each reach was estimated from the 2-foot contour interval topographic mapping. The valley slope
and valley width for each reach are summarized in Table 1.
These values were input to a spreadsheet to calculate the simulated peak flow, screening index,
and valley width index outlined in Form 1. The input data and results are tabulated in Appendix
A. This completes the initial desktop analysis.
,F-Reach t j Tributary Area, sq. mi. Valley Slope, rn/rn Valley Width, m
:1 1 33.46 I 0.0071 jI
2 f 33.46 F 0.0043 ii 6.1'
3 33.46 0.0018 6.1
Table 1. Summary of Valley Slope and Valley Width
FIELD SCREENING
After the initial desktop analysis is complete, a field assessment must be performed. The field
assessment is used to establish a natural channel's vertical and lateral susceptibility to erosion.
SCCWRP states that although they are admittedly linked, vertical and lateral susceptibility are
assessed separately for several reasons. First, vertical and lateral responses are primarily
controlled by different types of resistance, which, when assessed separately, may improve ease
of use and lead to increased repeatability compared to an integrated, cross-dimensional
assessment. Second, the mechanistic differences between vertical and lateral responses point to
different modeling tools and potentially different management strategies. Having separate
screening ratings may better direct users and managers to the most appropriate tools for
subsequent analyses.
The field screening tool uses combinations of decision trees and checklists. Decision trees are
typically used when a question can be answered fairly definitively and/or quantitatively (e.g., d50
< 16 mm). Checklists are used where answers are relatively qualitative (e.g., the condition of a
grade control). Low, medium, high, and very high ratings are applied separately to the vertical
and lateral analyses. When the vertical and lateral analyses return divergent values, the most
conservative value shall be selected as the flow threshold for the hydromodification analyses.
Visual observation reveals that all three study reaches contain a channel bed with relatively large
grain sizes (in the cobble range) and channel banks with a fairly uniform cover of tall grasses and
small brush (see the figures following the report text). The overbank areas are within the golf
5
course, so primarily contain a uniform cover of grass that is regularly maintained. Due to the
large grain size, uniform vegetative cover, and lack of significant erosion noted during the site
investigation, the vertical and lateral stability was anticipated to have a limited susceptibility to
erosion.
Vertical Stab ilitv
The purpose of the vertical stability decision tree (Figure 6-4 in the County of San Diego HMP)
is to assess the state of the channel bed with a particular focus on the risk of incision (i.e., down
cutting). The decision tree is included in Figure 19. The first step is to assess the channel bed
resistance. There are three categories defined as follows:
Labile Bed - sand-dominated bed, little resistant substrate.
Transitional/Intermediate Bed - bed typically characterized by gravel/small cobble,
Intermediate level of resistance of the substrate and uncertain potential for armoring.
Threshold Bed (Coarse/Armored Bed) - armored with large cobbles or larger bed
material or highly-resistant bed substrate (i.e., bedrock).
There are several factors that establish the erodibility of a channel such as the flow rate (i.e., size
of the tributary area), grade controls, channel slope, vegetative cover, channel planform, bed
material, etc. The Introduction of the SCCWRP Hydromodfication Screening Tools: Field
Manual identifies several of these factors. The figures after this report text contain photographs
of the natural drainage course in each study reach where some of the factors are visible. For
instance, the vegetative cover throughout the natural drainage course within Reaches 1 through 3
consists of mature, dense, and uniform (see Figures 5 through 15) grasses and brush on the
channel banks. In addition, the channel bed contains large cobbles and grade controls (see
Figures 4, 12, 13, 16 through 18). .
When multiple, factors influence erodibility, it is appropriate to perform the more detailed
SCCWRP analysis, which is to analyze a channel according to SCCWRP's •
transitional/intermediate bed procedure. This requires the most rigorous steps and will generate
the appropriate results given the range of factors that define erodibility. The transitional/
intermediate bed procedure takes into account that bed material may fall within the labile
category (the bed material size is used in SCCWRP's Form 3 Figure 4), but other factors may
trend towards a less erodible condition. Dr. Eric Stein from SCCWRP, who co-authored the
Hydromod?JIcation Screening Tools: Field Manual in the Final HydromodUlcation Management
Plan (HMP), indicated that it would be appropriate to analyze channels with multiple factors that
impact erodibility using the transitional/intermediate bed procedure. Consequently, this
procedure was used to produce more accurate results for each study reach.
Transitional/intermediate beds cover a wide susceptibility/potential response range and need to
be assessed in greater detail to develop a weight of evidence for the appropriate screening rating.
. The three primary risk factors used to assess vertical susceptibility for channels with
transitional/intermediate bed materials are:
6
Armoring potential - three states (Checklist 1)
Grade control - three states (Checklist 2)
Proximity to regionally-calibrated incision/braiding threshold (Mobility Index Threshold
- Probability Diagram)
These three risk factors are assessed using checklists and a diagram (see Appendix B), and the
results of each are combined to provide a final vertical susceptibility rating for the
intermediate/transitional bed-material group. Each checklist and diagram contains a Category A,
B, or C rating. Category A is the most resistant to vertical changes while Category C is the most
susceptible.
Checklist 1 determines the armoring potential of the channel bed. The channel bed along each of
the three reaches is within category B, which represents intermediate bed material • within
unknown armoring potential due to a surface veneer and/or dense vegetation. The soil was
probed and penetration was relatively difficult through the underlying layer of each reach. Due to
the cobbles in each reach, the armoring potential could have been rated higher in those reaches,
but Category B was, conservatively (i.e., more potential for channel incision) chosen.
Checklist 2 determines grade control characteristics of the channel bed. Checklist 2 classifies
grade controls according to the channel's 2/Sw and 4/Sw values, where S., is the longitudinal
channel slope. Table 2 provides these values as well as each reach length. As discussed above, a
riprap grade control exists at the downstream end of Reach 2. The grade control is 1,122 feet
(380+742==1,122) maximum distance from Reach 1. This distance is greater than the 2/Sw, but
less than the 4/Sw value for Reach 1. Therefore, Reach 1 is within Category B on Checklist 2. On
the other hand, the grade control is 742 feet maximum distance from Reach .2. This distance is.
less than both the 2/Sw and 4/Sw values for Reach 2, so Reach 2 is within Category A On Checklist
2. Furthermore, the 400 foot length of Reach 3 is less than both of its 2/Sw and 4/Sw values,, so
Reach 3 is within category A on Checklist 2.
. .1 Reach Length, feet 2/Sw, feet 4/Sw, feet
.1 .'I,3P •923 - 1,847
1. . 71 .742 ' 1,521 3,043
400 3,750 .1 7,499
Table 2. Summary of Valley Slope and Valley Width
The Screening Index Threshold is a probability diagram that depicts the risk of incising or
braiding based on the potential stream power of the valley relative to the median particle
diameter. The threshold is based on regional data from Dr. Howard Chang of Chang Consultants
and others The probability diagram is based on d50 as well as the Screening Index determined in
the initial desktop analysis (see Appendix A). Appendix A shows that the Screening Index values
for Reach 1, 2, and 3 are 0.064, 0.039, and 0.016, respectively. The probability diagram shows .
that the 50% Risk values closest to, but just greater than these Screening Index values are 0.070, '
7
0.049, and 0.0 18, respectively, which correspond to a d50 of 32, 16, and 1 millimeter (1.26, 0.63,
and 0.04 inches). Figures 16, 17, and 18 show that the d50 of the channel bed material is much
larger than 32, 16, and 1 millimeters. Since each reach's channel bed material is larger than the
d50 size associated with its 50 percent value, Reaches 1 through 3 fall within Category A, i.e., the
channel bed material is larger than the size at which the risk of incising or braiding occurs.
The overall vertical rating is determined from the Checklist 1, Checklist 2, and Mobility Index
Threshold results. The scoring is based on the following values:
Category A = 3, Category B = 6, Category C = 9
The vertical rating score for each of the four reaches is based on these values and the equation:
Vertical Rating = [(armoring x grade control)1/2 x screening index score] 1/2
Table 3 summarizes the checklist 1, 2, and 3 values for each reach as well as their vertical rating.
Reach I Checklist! 9 Checklist 2 Checklist 3 J Vertical Rating
1 1 6 1 6 1.. 3 4.2
1 2 - 6 I 3 1 3.6
3 6 ;I 3 1 3 3.6
Table 3. Overall Vertical Rating
Since the vertical rating is less than 4.5, each reach has a low threshold for vertical susceptibility.
Lateral Stability
The purpose of the lateral decision tree (Figure 6-5 from County of San Diego HMP included in
Figure 20) is to assess the state of the channel banks with a focus on the risk of widening.
Channels can widen from either bank failure or through fluvial processes such as chute cutoffs,
avulsions, and braiding. Widening through fluvial avulsions/active braiding is a relatively
straightforward observation. If braiding is not already occurring, the next logical step is to assess
the condition of the banks. Banks fail through a variety of mechanisms; however, one of the most
important distinctions is whether they fail in mass (as many particles) or by fluvial detachment of
individual particles. Although much research is dedicated to the combined effects of weakening,
fluvial erosion, and mass failure, SCCWRP found it valuable to segregate bank types based on
the inference of the dominant failure mechanism (as the management approach may vary based
on the dominant failure mechanism). A decision tree (Form 4 in Appendix B) is used in
conducting the lateral susceptibility assessment. Definitions and photographic examples are also
provided below for terms used in the lateral susceptibility assessment.
The first step in the decision tree is to determine if lateral adjustments are occurring. The
adjustments can take the form of extensive mass wasting (greater than 50 percent of the banks
are exhibiting planar, slab, or rotational failures and/or scalloping, undermining, and/or tension
cracks). The. adjustments can also involve extensive fluvial erosion (significant and frequent
8
bank cuts on over 50 percent of the banks). Neither mass wasting nor extensive fluvial erosion
was evident within any of the three reaches during a field investigation The banks are intact in
the photographs included in the figures and support mature vegetation. The adjacent golf course
has not sustained damage associated with mass wasting or fluvial erosion.
The next step in the Form 4 decision tree is to assess the consolidation of the bank material. The
banks were moderate to well-consolidated. This determination was made because the banks were
difficult to penetrate with a probe In addition, the banks showed limited evidence of crumbling
and were, composed of well-packed particles.
Form 6 (see Appendix B) is used to assess the probability of mass wasting. Form 6 identifies a
10, 50, and 90 percent probability based on the bank angle and bank height. The 2-foot contour
interval topographic mapping and site investigation indicates that the average bank height in
each reach is in the range of 2 to 4 feet (0.6 to 1.4 meters). Form 6 shows that a bank height of
1.4 meters can support a bank angle as steep as 51.3 degrees (0.8 to 1, horizontal to vertical) and
still maintain a probability of mass wasting and bank failure of less than 10 percent. The
topographic mapping and site investigation shows that the average bank angles are flatter than".
0.8:1, so the probability of Reach 1, 2, and 3 are less than 10 percent.
The final two steps in the Form 4 decision tree are based on the braiding risk determined from
the vertical rating as well as the Valley Width Index (VWI) calculated in Appendix A. If the
vertical rating is high, the braiding risk is considered to be greater than 50 percent. Excessive
braiding can lead to lateral bank failure. For all three study reaches, the vertical rating is low, so
the braiding risk is less than 50 percent. Furthermore, a VWI greater than 2 represents channels'
unconfined by bedrock or hillslope and, hence, subject to lateral migration. The VWI
calculations in the spreadsheet in Appendix A show that the VWI for each reach is much less
than 2.
From the above steps, the lateral susceptibility rating is low for each of the three study reaches
(red circles are included on the Form 4: Lateral Susceptibility Field Sheet decision tree in
Appendix B showing the decision path). A review of aerial photographs and site visit confirms a
lack of braiding or lateral migration throughout the natural drainage course
CONCLUSION
The SCCWRP channel screening tools were used to assess the downstream channel
susceptibility for the Coastal 10 residential project being designed by O'Day Consultants. The
project's storm runoff will be conveyed away from the site in a street, storm drain pipe, and
concrete channel before ultimately discharging into the San Marcos Creek natural drainage
.course within the La Costa golf course. The discharge into the natural drainage course is the',.
project's point of compliance.. The natural drainage course was assessed both upstream and
downstream of the point of compliance. The assessment was performed based on office analyses
and field work. The results indicate a low susceptibility for vertical and lateral channel erosion
for the entire study area.
The 14MP requires that these results be compared with the critical stress calculator results
incorporated in the County of San Diego's BMP Sizing Calculator. The BMP Sizing Calculator
critical stress results are included in Appendix B for all three reaches. Based on these values, the
critical stress results returned a low susceptibility to erosion. Therefore, the SCCWRP analyses
and critical stress calculator demonstrate that the project can be designed assuming a low
susceptibility, i.e., 0.5Q2.
I
Figure 1. Upper End of Concrete Channel below outlet of 18" RCP
Figure 2. Looking Downstream along Concrete Channel towards Natural Drainage Course
11
Figure 3.
Figure 4. Riprap Grade Control at Upstream End of Reach 1
12
Figure 5. Looking Downstream at Reach 1 from Upper End
Figure 6. Middle of Reach 1
13
Figure 7. Looking Upstream at Reach 1 from Lower End
-.:..._,.--.. ~~
~ -..1..,Y ...
Figure 8. Looking Downstream at Reach 2 from Upper End
14
Figure 9. Looking Upstream from Middle of Reach 2
Figure 10. Looking Downstream from Middle of Reach 2
15
Figure 11. Looking Upstream at Reach 2 from Lower End
Figure 12.
16
Figure 13. Riprap Grade Control under Cart Crossing between Reach 2 and 3
Figure 14. Looking Downstream at Reach 3 from Upper End
17
Figure 15. Looking at Middle to Lower End of Reach 3
Figure 16.
Figure 18. Large Rock in Drainage Course in Reach 2 and 3
19
Figure
LABILE BED INTERMEDIATE BED COARSEIARMCRED BED
a Snaied a Moderalely iaIoe1y- d3a > 128 mm
am1c 16mm packed cbbIigr& a Boulder Harecobble
a % surfacean4 '25%. a Hardpan of iwcartaln i tightly-packed
a Looely-pad depiti ,xtmt aroMflilly '6% sand
. bedrock
I
a Conbfliiou9 concrete
HIGH EXAMINE RISK FACTORS LOW .gdeconttl
I .1 armoring potential
.ximyICinOflliId .
C30 h) go to bad widiblRy
1
FlIgure
163
Figure chckhstsand incision
diagram check list
Fill out SCCiNRPg
crilerla ID dolormho if ff
recel ng
MEDUM r LOW auscapfibirdy
HIGH MEDIUM 1 LOW
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Figure
6-5
Figure 54. SCCWRP Vertical Sascept!blliy .
Figure 19 SCCWRP Vertical Channel Susceptibility Matrix
20
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Figure 20. SCCWRP Lateral Channel Susceptibility Matrix
21
APPENDIX A
SCCWRP INITIAL DESKTOP ANALYSIS
FORM 1: INITIAL DESKTOP ANALYSIS
Complete all shaded sections.
IF required at multiple locations, circle one of the following site types:
Applicant Site / Upstream Extent / Downstream Extent
Location Latitude I 330901 Longitude I -117.2485
Description (river name, crossing streets, etc.): ICoastal 10 1
GIS Parameters: The International System of Units (SI) is used throughout the assessment as the field
standard and for consistency with the broader scientific community. However, as the singular exception, US
Customary units are used for contributing drainage area (A) and mean annual precipitation (P) to apply regional flow
equations after the USGS. See SCCWRP Technical Report 607 for example measurements and 'Screening Tool
Data Entry xIs for automated calculations
Form I Table 1. Initial desktop analysis in GIS.
Symbol Variable Description and Source Value
A Area Contributing drainage area to screening location via published
(mi) Hydrologic Unit Codes (HUCs) and/or :5 30 m National Elevation Data
(NED), USGS seamless server
P Mean annual Area-weighted annual precipitation via USGS delineated polygons using
w precipitation records from 1900 to 1960 (which was more significant in hydrologic See at
(in) models than polygons delineated from shorter record lengths) Form 1
S, Valley slope Valley slope at site via NED, measured over a relatively homogenous on ne
(rn/rn) valley segment as dictated by hilislope configuration, tributary for cak
confluences, etc., over a distance of up to -500 m or 10% of the main- value f channel length from site to drainage divide .each Cl
W Valley width Valley bottom width at site between natural valley walls as dictated by
(m) clear breaks in hillslope on NED raster, irrespective of potential
(1) armoring from floodplain encroachment, levees, etc. (imprecise
measurements have negligible effect on rating in wide valleys where
V'INI is >> 2, as defined in lateral decision tree)
ached
table
page
ulated
for study
Form I Tabl e 2. Simplif ied peak fib w, screening index, and valley width index. Values for this
table should be calculated in the sequence shown in this table, using values from Form I Table 1.
Symbol Dependent Variable Equation Required Units Value
Qlocfs 10-yr peak flow (ft3/s) Q10 = 18.2 * A°87 * 0.77 A(rni2)
See attached
Qio 10-yr peak flow (m3/s) Q10 = 0.0283 * Qiocts Qio (ft3/s) Form 1 table
INDEX 10-yr screening index (m151s°5) INDEX = S*Qio 0.5 Sv on next pag
10
• for calculated
Wref Reference width (rn) Wref = 6.99 * 0.438 Qio (m3/s) values for study
VW Valley width index (rn/rn) VWI = Wv/Wref WV (m)) reach.
(Sheet I of 1)
B-3
SCCWRP FORM 1 ANALYSES
Area Mean Annual Precip. Valley Slope Valley Width 10-Year Flow 10-Year Flow
Reach A, sq. mi. P, inches Sv, rn/rn Wv, m Qlocfs, cfs Q10, cms
1 33.46 13.3 0.0071 7.3 2830 80.1
2 33.46 13.3 0.0043 6.1 2830 80.1
3 33.46 13.3 0.0018 6.1 2830 80.1
10-Year Screening Index Reference Width Valley Width Index
Reach INDEX Wref, m VWI, rn/rn
1 0.0636 47.7 0.15
2 0.0386 47.7 0.13
3 0.0157 47.7 0.13
GRAPHIC SCALE
0 8000
I I
1 INCH -8,000 FEET
COASTAL 10
WATERSHED EXHIBIT
Map Details
Result View
Define Drainage Basins Basin San Marcas Creek Watershed Project Coastal 10
_i L1 Basin LI LI I
Manage Your Basins Name
'treate.4,hq%i Basin by clicking the Niew button and scroll down to viéw San Marcos Creek Watershed
entry. AIternatvely, select an existing Basrifrom table and view.-.-
properties below: CIiôk Edit button to change Basin properties then
presSave tocommit changes.
Description 0öasta1 10 1 i Point of Compliance OutfaII Into Natural DranagJ
Design GaI Fireatnient + FIo Contkt
-
Poje i ri, Area (ic) i412 00 j
RâinfáJI Basin: Qcpsjde rM7annnuälPreeipition(ifl): 3:3
MEAN ANNUAL PRECIPITATION FROM COUNTY BMP SIZING CALCULATOR
GRAPHIC SCALE
0 100
I I I
1 INCH = 100 FEET
COASTAL 10
STUDY AREA EXHIBIT
APPENDIX B
D
X
D
Form 3 Support Materials
Form 3 Checklists 1 and 2, along with information recording in Form 3 Table 1,
are intended to support the decisions pathways illustrated in
Form 3 Overall Vertical Rating for Intermediate/Transitional Bed.
A
B
C
Form 3 Checklist 1: Armoring Potential
A mix of coarse gravels and cobbles that are tightly packed with <5%
surface material of diameter <2 mm
Intermediate to A and C or hardpan of unknown resistance, spatial extent
(longitudinal and depth), or unknown armoring potential due to surface
veneer covering gravel or coarser layer encountered with probe
Gravels/cobbles that are loosely packed or >25% surface material of
diameter <2 mm
Form 3 Figure 2. Armoring potential photographic supplement for assessing intermediate beds
(16 < d50 < 128 mm) to be used in conjunction with Form 3 Checklist 1.
(Sheet 2 of 4)
REACH 1, 2, AND 3 RESULTS
8 -7
:x
D
Form 3 Checklist 2: Grade Control
A Grade control is present with spacing <50 m or 2/Sv m
B
C
• No evidence of failure/ineffectiveness, e.g., no headcutting {>30 cm), no
active mass wasting (analyst cannot say grade control sufficient if mass-
wasting checklist indicates presence of bank failure), no exposed bridge
pilings, no culverts/structures undermined
• Hard points in serviceable condition at decadal time scale, e.g., no apparent
undermining, flanking, failing grout
• If geologic grade control, rock should be resistant igneous and/or
metamorphic; For sedimentary/hardpan to be classified as 'grade control', it
should be of demonstrable strength as indicated by field testing such as
hammer test/borings and/or inspected by appropriate stakeholder
Intermediate to A and C -artificial or geologic grade control present but
spaced 2/Sv m to 4/Sv m or potential evidence of failure or hardpan of
uncertain resistance
Grade control absent, spaced >100 m or >4/Sv m, or clear evidence
of ineffectiveness
Form 3 Figure 3. Grade-control (condition) photographic supplement for assessing intermediate
beds (16 < dso < 128 mm) to be used in conjunction with Form 3 Checklist 2.
(Sheet 3 of 4)
REACH 1 RESULTS
REACH 2 AND 3 RESULTS
B -8
Regionally-Calibrated Screening Index Threshold for Incising/Braiding
For transitional bed channels (d50 between 16 and 128 mm) or labile beds (channel not incised
past critical bank height), use Form 3 Figure 3 to determine Screening Index Score and complete
Form 3 Table 1.
-"' d -!!?
1
~ ~§. 0.1
Z "' -o 0
~0.01
0.1 1 dso (mm) 10
• stable x Braided
10% risk --50% risk
100
+ hcising
9(1% risk
GIS-derived: 10-yr flow & valley slope
Field-derived: d50 ( l 00-pebble count)
C 0 -~ E ~ E Cl (t) ~ .... o N
il J ·a,
0 -'
Cl E Q) E 0:::
0 (t)
.:I ....
j ' I
128
96
80
64
48
32
16
8
4
2
1
0.5
0.145
0.125
0.114
0.101
0.087
0.070
0.049
0.031
0.026
0.022
0.018
0.015
50% Risk
for:
Reach 1
Reach 2
Reach 3
Form 3 Figure 4. Probability of incising/braiding based on logistic regression of Screening Index
and d50 to be used in conjunction with Form 3 Table 1.
Form 3 Table 1. Values for Screening Index Threshold (probability of incising/braiding) to be used
in conjunction with Form 3 Figure 4 (above) to complete Form 3 Overall Vertical Rating for
Intermediate/Transitional Bed (below) .. Screening Index Score: A = <50% probability of incision
for current 010, valley slope, and d50; B = Hardpan/d50 indeterminate; and C = ~50% probability of
incising/braiding for current 0 10, valley slope, and d5o.
d50 (mm)
From Form 2
Sv*010 o.5 (m 1.5,so.s)
From Form 1
5/01/·5 (m 1.5,so.5)
50% risk of incising/braiding
from table in Form 3 Figure 3 above
Screening Index Score
(A, B, C)
Overall Vertical Rating for Intermediate/Transitional Bed
Calculate the overall Vertical Rating for Transitional Bed channels using the formula below.
Numeric values for responses to Form 3 Checklists and Table 1 as follows: A = 3, 8 = 6, C = 9.
Vertical Rating = { (~armoring * grade control ) * screening index score}
Vertical Susceptibility based on Vertical Rating: <4.5 = LOW; 4.5 to 7 = MEDIUM; and >7 = HIGH.
(Sheet 4 of 4)
REACH 1, 2, AND 3 RESULTS
B -9
FORM 4: LATERAL SUSCEPTIBIL TY FIELD SHEET
Circle appropriate nodes/pathway for proposed site
OR use sequence of questions provided in Form 5.
(Sheet 1 of 1)
REACH 1, 2, AND 3 RESULTS
B -10
FORM 6: PROBABILITY OF MASS WASTING BANK FAILURE
If mass wasting is not currently extensive and the banks are moderately-to well-consolidated, measure
bank height and angle at several locations (i.e., at least three locations that capture the range of
conditions present in the study reach) to estimate representative values for the reach. Use Form 6 Figure
1 below to determine if risk of bank failure is >10% and complete Form 6 Table 1. Support your results
with photographs that include a protractor/rod/tape/person for scale.
Bank Angle
(degrees)
(from Field)
Bank Height
(m)
(from Field)
_L_e_tt_B_an_k __ < __ 5 __ 1 __ . 3......._(0 __ . 8: 11 --=9 .4
Right Bank <51.3 (0.8:1) <1.4
Corresponding Bank Height for
10% Risk of Mass Wasting (m)
(from Form 6 Figure 1 below)
> 1.4
> 1.4
probability of mass wasting 0
in moderately /well consolidated banks "
0 Stable 35
4 40
X
45 0 X 0 X 50
3 \ X 55 E .x X ~ .l: 0 \ 60
tl.O 2 6) g ·X X 'ai ' 65
:I:
,:,,: o 00 0 xx X 70 C
c2! 1o~O~ O
X 1 X 80
~ ~Co cJ> 0 oo 0
0
30 40 50 60 70 80
Bank Failure Risk
(<10% Risk)
(>10% Risk)
<10%
<10%
B e h
I o , •o
• a
4:7
3.7
2.1
1.5
1.1
0.85
0.66
0.52
0.34
Form 6 Figure 1. Probability Mass Wasting diagram, Bank Angle:Height/% Risk table, and
Band Height:Angle schematic.
(Sheet 1 of 1)
REACH 1, 2, AND 3 RESULTS
B -12
Map data provided by OpenStreetMap
Map Details
Result View CRITICAL STRESS CALCULATOR RESULTS FOR REACH I
Define Drainage Basins Basin: San Marcos Creek Watershed Project: Coastal 10
LiFOCI I
Manage Your Point of Compliance (POC)
Analyze the receiving water at the 'Point of Conipliance by completing
this form. Click Edit and enter the appropriate fields, then click the I Channel Susceptibility: FLOW ]
Update button to calcu'aLe the critical flow and low flow threshold
condition. Finally, click Save to commit the hanges. Low Flow Threshotd: I0.5Q2
Channel Assessed: JYes1 Vertical Susceptibility: ILowcVercaI) ______
Watershed Area (ac): 121412.00
-
Lateral Suseeptibility:.Low(LateraI)-- -
Large Vw
Material: 1yegietation -
Roughress: O100J
Channel Top Width (ft) 136.0
Channel Bottom Width (ft)
Channel Height(ft): 4.0
Channel Slope: 10.0071
_____
- Map data provided by Open StreetMap
Map Details
Result View CRITICAL STRESS CALCULATOR RESULTS FOR REACH 2
I—, .: /../ Define Drainage Basins Basin: San Marcos Creek Watershed Project, Coastal 10
_l LI u •_Li
Manage Your Point of Z.omplianq.e (POC)
Analyze the receiving water at the Point of Compliance by completing
this form. Click Edit and enter the appropriate fields, then click the Channel Susceptibility: IL0W
Update buttbn to calculate the critical flow and low-flow threshold
condition Finally click Save to commit the changes Low Flow Threshold EOiQ2_
- --
lflils(I• !JI
Channel Assessed: JYes Vertical Susceptibility: LowWeica! __LJ
Watershed Area(ác): 121412.00. I Lateral Susceptibility: I Lo- w (Lateral) _H
Large View
Material: Vegetation [j
Roughness: PO-11 00
Channel Top Width ft) 132.0
- - -
Channel Bottom Width(ft):
Channel Height ft): 04.0 -.
Channel Slope: OM43 ,
Map Details
Result View CRITICAL STRESS CALCULATOR RESULTS FOR REACH 3
Define Drainage Basins Basin San Marcos Creek Watershed Project Coastal 10
ULI [j(
Manage Your Point of Compliance (POC)
Analyze the receiving water at the Point of Compliance by completing
this fomi Click Edit and enter the appropriate fields then dick the Channel Suscepttbility [Low
Udate button to calculate the cntical flow and low-flow threshold
condition.. FihaUy;dickSave tbcbmmit &.
,
6b he thane Low Flow Threshold: [ö.5Q2
flfp _________
Channel Assessed DYes [j Vertical Susceptibility J Low (Vertical)
-
Latërál Susceptibility: Low(LaeraI) - []
Large View
Material
-.LVLJ -
Roughness: jO 100
Channel TO Width (ft) 32.0,
Channel Bottom Width. (ft): H20.O
Channel Height (ft) 140
-
Channel S160. jq.pom