CA FLAP LA CR2N40(1)_Site 2 Drainage Memo.pdf

Jacobs Engineering Group 707 17th Street Suite 2300 24th Floor Denver, CO 80202

Memorandum

Jacobs Engineering Group Inc.

Date March 2, 2016

Subject CA FLAP LA CR 2N40(1) –Santa Anita Canyon Road

Site 2: Debris and Sediment Basin Clarification

The Santa Anita Canyon Road (SACR) project included improvements to five sites along within the roadway. Site No. 2 included construction of a culvert and a debris basin in the canyon on the inboard side of the road, located approximately 2,370' to 2,470' south of CM 0.00, to keep debris off the pavement during storms.

During a major downpour in 2005, this site was overwhelmed with runoff and debris, clogging the existing pipe and thence overtopping the roadway. The roadway failed. At that time, there was a 24-inch pipe culvert draining this area under the roadway, and into the downstream ravine. To restore the roadway, a gabion retaining wall was installed. The plans specifically addressed removing the existing 24-inch CMP pipe, and not installing any formal drain in its place. All runoff now enters the roadway when the shallow basin is overtopped. Based on field observations of the site, it appears that additional paved berms were added downstream of the gabion wall, guiding the runoff from this small basin down the roadway to the next ravine, and having it discharge into the existing swale.

It should be noted that the current outfall at the “Downstream Basin Outfall” (CFL Sta 62+45 ) is severely eroded as well. No recommendations have been made to mitigate or repair this downstream site.

March 2, 2016

Enter Document No. via Document Properties 2

The Los Angeles County Department of Public Works (DPW) is requesting that a debris control facility following the guidance of the County’s Design Manual for Debris Dams and Basins be used. As the

County has detailed hydrology information for the site, all of their standards / procedures for generating flows have been used.

The basis for this memo is to address the fact that it appears from initial calculations that we cannot fit a basin that meets the objectives of the County’s standards within the available easement without building a basin with perhaps retaining walls to gain the desired capacity. Our approach to determine the requirements and our proposed solutions follow.

Hydrology:

Using the DPW Hydrology Manual (January 2006), the following input parameters were used:

Basin Area: 3.89

Acres

High Point: 1598

Low Point: 1385

Longest drain path

(red line in the adjacent photo: 525-ft

Slope: 0.4095 ‘/ft.

Densely forested ground with no visible structures

Percent Impervious area: 0.01 (none)

DPW Soil

Classification: 088

(See following figures for DPW ISOHYET figure)

Enter Document No. via Document Properties 3

From the DPW manual, the following figure shows provides the site information used in many of the following computations:

From the map, the following information about the site was obtained:

Debris Potential Zone: DPA-1

Soil Classification Area: 088

50-Yr, 24-Hr Rainfall: 10.4 inches

Inputting this information into the DPW

“HydroCALC” software, the following information was provided:

Basin 2

Santa Anita Canyon

Road

Enter Document No. via Document Properties 4

From the DPW Sedimentation Manual, Second Edition, March 2006, Chapter 3 defines how much sediment debris is generated from each site.

From above, the site area is 3.89 Acres, or

0.006 sq miles. (Per the text to the left, use

0.1 square miles).

From Appendix B-1, Debris Volume = 240,000 CY/ Sq. Mile = 0.0061*240,000 = 1,464 CY

From Chapter 4 of the Sedimentation Manual, Table 4.1.1, below, with DPA Zone 1, a Debris Basin or Elevated Inlet is recommended. However, the “*” in the text denotes: The use of elevated or desilting inlets and bulked flow drains in DPA zones 1 through 4 will only be approved by Public Works in special circumstances. The steepness of the watershed, presence of boulders, and higher sediment and mudflow potential in these DPA zones results in a greater risk of plugging the storm drain and damaging the desilting wall.

Basin 2 = 1,464

Enter Document No. via Document Properties 5

Continuing to use the Sedimentation Manual, we attempted to fit a solution that approximates the elevated inlet design in Figure 4.3.2, shown below.

Enter Document No. via Document Properties 6

The challenges:

We attempted to fit a basin design within the available Drainage Easement for this site. That site is shown below:

There used to be a 24-inch pipe draining this basin downstream, across the road and directly into the ravine. It was removed with the installation of the gabion wall. Therefore, the homes downstream have not experienced any runoff from this basin for some time. During the field review, there were concerns expressed with returning water to this ravine after the water was removed.

In addition, if a pipe were sized to convey the 50-yr event through a culvert, the anticipated flow is nearly 21 cfs, and would require a 30-inch pipe (larger than the original 24-inch that was overwhelmed in 2005).To add this pipe under the road would allow more runoff to go down this ravine than was ever conveyed before.

To mitigate larger flows, we propose to put a restrictor pipe in the outfall pipe to only allow say 1-2 cfs to discharge into this ravine. Would this approach be acceptable to DPW?

The “standard” on the previous page has the basin and the elevated inlet ultimately draining to the same pipe outfall. In our concept shown above, we propose to have the spillway of the basin discharge as it does today, onto the roadway ditch. Only the restricted inlet water would discharge to the immediate ravine. While not shown on our sketch, we would include a trash rack in advance of the top of the spillway as shown in the standard.

Enter Document No. via Document Properties 7

The “standard” on the previous page has a 3:1 paved slope to the inlet. We don’t have that much room, so we are proposing 1:1 slopes on the paved (reinforced shotcrete) slope, and 2:1 slopes matching the adjacent terrain for the balance of the basin slopes. In fact, the basin volume we have with our proposed concept (roughly 1,075 CY) is not enough for the debris loading proposed (1,475 CY). To provide for more volume, we would need to exceed the easement, or use retaining walls to build a deeper, larger basin. With such a tight, constrained ravine to build this basin, we don’t know if DPW desires the most we can achieve within the available ROW, or do we excavate and expand the site to provide the volume desired?

Based on the peakflow hydrograph for this site, the total runoff anticipated is 27,923 cu-ft, or 1,035 cu-yds of storage.

Our current basin design does provide that volume, thus restricting the discharge through the elevated inlet would work. Our question to DPW is do we need to maintain both the Debris and the containment volumes concurrently? Our understanding is that today, all of the runoff heads down SACR. With this design, we could provide a basin for potential debris and detain the runoff while the basin is drained down the original ravine with little downstream impacts. Should the basin fill with debris and experience the full design event, the runoff would exceed the capacity of the basin and overtop the spillway, discharging to the roadway, which is what occurs today.

If no detention is allowed, and the discharge pipe must be sized to convey the upstream 50-year event, then a larger 30-inch culvert would need to be installed, and more runoff than what is currently being released from the shallow detention basin would result. Electing to implement this option would further damage / erode the downstream outfall that is currently being used from the roadway. In any event, should the downstream outfall of this routed runoff be improved?

Enter Document No. via Document Properties 8

The standard concept has very gentle approach slopes. We had to use much steeper slopes between the basin and the roadway. We also used a narrower top at the crest of the berm, all to fit within the available easement. The 1:1 slope would be protected by the requested shotcrete lining. The taller and steeper foreslope from the roadway would eliminate the need for guardrail in front of the basin. This overall basin geometry appears similar to many of the debris basins we observed on the Angeles Forest Highway. Can these exceptions be implemented on this site?

Speaking of the Angeles Forest Highway, we observed several elevated basin inlets on the highway, but few looked exactly like the standard in the DPW Design Manual for Debris Dams and Basins. Do you have any updated details or as-built plans for these structures, as we would like to use the CMP inlet structure that appears to be used in most of these locations, shown below?

Date July 20, 2016

The following is an update to the original site memo distributed March 2, 2016.

roadway. Site No. 2 included construction of a culvert and a debris basin in the canyon on the inboard side of the road, located approximately 2,370' to 2,470' south of CM 0.00, to keep debris off the pavement during storms.

During a major downpour in 2005, this site was overwhelmed with runoff and debris, clogging the existing pipe and thence overtopping the roadway. The roadway failed. At that time, there was a 24-inch pipe culvert draining this area under the roadway, and into the downstream ravine. To restore the roadway, a gabion retaining wall was installed. The plans specifically addressed removing the existing 24-inch CMP pipe, and not installing any formal drain in its place. All runoff now enters the roadway when the shallow basin is overtopped. Based on field observations of the site, it appears that additional paved berms were added downstream of the gabion wall, guiding the runoff from this small basin down the roadway to the

July 20, 2016

It should be noted that the current outfall at the “Downstream Basin Outfall” (CFL Sta 62+45 ) is severely eroded as well. In the original scoping assessment, no recommendations were made to mitigate or repair this downstream site. This site will now require mitigation, and is addressed further below.

The Los Angeles County Department of Public Works (DPW) is requesting that a debris control facility following the guidance of the County’s Design Manual for Debris Dams and Basins be used. As the

County has detailed hydrology information for the site, all of their standards / procedures for generating flows have been used.

ORIGINALLY, the basis for the original site assessment memo was to request variances to the standards and procedures referenced above, as the debris from a full debris flow event could not fit without building a basin with perhaps retaining walls to gain the desired capacity. Our approach to determine the requirements and our proposed solutions follow. Based on responses to the original memo received from the County on 3/17/2016, it was clear the County desires the basin to meet the requirements without any design exceptions. Most of the requested changes were responded to with “We desire a basin providing a debris storage capacity for one Design Debris Event (DDE) and meeting the basin design requirements in our sedimentation manual, and Debris Dams & Basins Design Manual.”

CFL / County Review Meeting; May 10, 2016:

The design team met with the County and discussed possible alternatives and ways to keep the basin improvements within the existing ROW and established drainage easements. Attendees included Kirk

Torossian, Amir Zandieh, John Ng, and Susan Zarei from LA County. From the meeting on May 10, 2016 at the County, County staff reviewed the site information, pictures of the site, and our challenges with finding a means of storing 1,500 cy of debris material. The DPA Zone 1 curve was generating 3-inches of sediment for every square foot of the basin. The tree cover and amount of exposed rock does not support that volume of material being generated.

Upon review of the site images and ground cover, Amir and John suggested we evaluate what changes could be gained from using the next lower curve (DPA Zone 2) on the sediment chart instead of Zone 1. It was noted Zone 1 is the most aggressive for the County, and this small site in this particular location would not likely generate the volumes projected. They also suggested the design team move forward implementing a Desilting Inlet as the type of structure. These provisions were justified based on our contributing area being a very small drainage basin that is currently well established with what was perceived as a lower potential for sediment and mudflows.

After the meeting, as noted in the following pages, changing to sediment curve for DPA Zone 2 projected a sediment loading of 885 cy. The change also made the volume fit the requirements for a Desilting Inlet.

By incorporating the above modifications, the design team is proposing a basin that requires additional excavation and retaining walls to contain the projected sediment loading, but a basin was designed that fit within the site and the existing easement.

The balance of this memo documents the volumes and basin dimensions based on this discussion.

Hydrology:

Using the DPW Hydrology Manual (January 2006), the following input parameters were used:

Basin Area: 3.89

Acres

High Point: 1598

Low Point: 1385

Longest drain path

(red line in the adjacent photo: 525-ft

Slope: 0.4095 ‘/ft.

Densely forested ground with no visible structures

Percent Impervious area: 0.01 (none)

DPW Soil

Classification: 088

(See following figures obtained:

Debris Potential Zone: DPA-1

(From the above meeting, we are projecting the DDE using DPA-2 curves for the LA Basin sediment production).

defines how much sediment debris is generated from each site.

From above, the site area is 3.89 Acres, or

From Appendix B-1, using the production curve for DPA Zone 2, the anticipated sediment production is

145,000 cy/Sq. Mile. For the site, the projected debris volume = 0.0061*145,000 = 885 CY

Elevated Inlet is recommended. However, the “*” in the text denotes: The use of elevated or desilting inlets and bulked flow drains in DPA zones 1 through 4 can be approved by Public Works in special circumstances. This is the case for this task, as PW has approved the use per the above meeting to use the County approved Desilting Inlet for this project application.

Basin 2 = 885 CY

Continuing to use the Sedimentation Manual, we are implementing a Desilting Inlet in accordance with Figure 4.3.3, shown below.

Basin Design:

Based on previous discussions with the DPW and the study team, the following constraints are being used on the basin design:

• The improvements must fit within the existing roadway easement and existing drainage easement for the basin. Attempting to gain more ROW or a wider easement will likely exceed the design schedule, require significantly more environmental investigation on lands not previously studied, and be difficult to obtain from the underlying property owners (based on City input received on how difficult obtaining the original easement was).

• The outfall must be directed as it does today, headed down the roadway prism in the roadside ditch. The team considered various outfalls directed under the road and projecting in the immediate downhill canyon where this drainage originally outfalled. However, since this runoff has been diverted with the construction of several facilities that specifically routed this runoff onto the roadway prism, and discharging to the ravine located 500-ft down station along Santa Anita Canyon Road, there was no desire to introduce a change to the outfall conditions downstream.

• The outfall of the basin must match the elevation of the existing outfall today, as there is no room to widen the roadside ditch to accommodate a deeper or wider outfall.

• Slopes must be maintained within the existing drainage easement

• An access road (max grade = 25%) to the basin must be retained to remove deposited debris after runoff events.

The existing site appears as shown to the right:

The design team started with a basin design that had a 5-ft depth following the Desilting Inlet design.

The outfall of the basin (the pipe had to outfall to the existing roadway ditch elevation. The concept looked like the figure below:

As the slopes around the perimeter of the basin floor were nearly 2:1 or steeper, we couldn’t excavate into the basin too much without dramatically extending the slopes outside of the easement. The above attempt had a level capacity of approximately 400 cy. The cone capacity of this approach was only around 135 cy… therefore our capacity with this approach was only 535 cy, short of the 885 desired.

To gain more volume, we are excavating more of the basin floor to provide a wider, flatter basin floor. To contain the slopes, we are proposing to build vertical walls around the western perimeter.

This solution resulted in a level basin capacity of 740 cy. The anticipated cone volume will be nearly 150 cy, with a combined capacity of 890 cy, exceeding the capacity desired.

Enter Document No. via Document Properties 9

The profile of the basin floor (the outfall channel), and the profile around the containment walls (Wall Profile) are shown below:

This new approach will require that our containment walls be constructed around the western permitter of the basin. These sections of the wall need to have geotechnical analysis to confirm the foundation materials and wall design parameters.

Enter Document No. via Document Properties 10

Outfall Protection:

If no detention is allowed, and the discharge pipe must be sized to convey the upstream 50-year event, then a larger 30-inch culvert would need to be installed, and more runoff than what is currently being released from the shallow detention basin would result. Electing to implement this option would further damage / erode the downstream outfall that is currently being used from the roadway. In any event, should the downstream outfall of this routed runoff be improved?

Enter Document No. via Document Properties 11

The standard concept has very gentle approach slopes. We had to use much steeper slopes between the basin and the roadway. We also used a narrower top at the crest of the berm, all to fit within the available easement. The 1:1 slope would be protected by the requested shotcrete lining. The taller and steeper foreslope from the roadway would eliminate the need for guardrail in front of the basin. This overall basin geometry appears similar to many of the debris basins we observed on the Angeles Forest Highway. Can these exceptions be implemented on this site?

Speaking of the Angeles Forest Highway, we observed several elevated basin inlets on the highway, but few looked exactly like the standard in the DPW Design Manual for Debris Dams and Basins. Do you have any updated details or as-built plans for these structures, as we would like to use the CMP inlet structure that appears to be used in most of these locations, shown below?

Date July 20, 2016

The following is an update to the original site memo distributed March 2, 2016.

roadway. Site No. 2 included construction of a culvert and a debris basin in the canyon on the inboard side of the road, located approximately 2,370' to 2,470' south of CM 0.00, to keep debris off the pavement during storms.

During a major downpour in 2005, this site was overwhelmed with runoff and debris, clogging the existing pipe and thence overtopping the roadway. The roadway failed. At that time, there was a 24-inch pipe culvert draining this area under the roadway, and into the downstream ravine. To restore the roadway, a gabion retaining wall was installed. The plans specifically addressed removing the existing 24-inch CMP pipe, and not installing any formal drain in its place. All runoff now enters the roadway when the shallow basin is overtopped. Based on field observations of the site, it appears that additional paved berms were added downstream of the gabion wall, guiding the runoff from this small basin down the roadway to the

It should be noted that the current outfall at the “Downstream Basin Outfall” (CFL Sta 62+45 ) is severely eroded as well. In the original scoping assessment, no recommendations were made to mitigate or repair this downstream site. This site will now require mitigation, and is addressed further below.

The Los Angeles County Department of Public Works (DPW) is requesting that a debris control facility following the guidance of the County’s Design Manual for Debris Dams and Basins be used. As the

County has detailed hydrology information for the site, all of their standards / procedures for generating flows have been used.

ORIGINALLY, the basis for the original site assessment memo was to request variances to the standards and procedures referenced above, as the debris from a full debris flow event could not fit without building a basin with perhaps retaining walls to gain the desired capacity. Our approach to determine the requirements and our proposed solutions follow. Based on responses to the original memo received from the County on 3/17/2016, it was clear the County desires the basin to meet the requirements without any design exceptions. Most of the requested changes were responded to with “We desire a basin providing a debris storage capacity for one Design Debris Event (DDE) and meeting the basin design requirements in our sedimentation manual, and Debris Dams & Basins Design Manual.”

CFL / County Review Meeting; May 10, 2016:

The design team met with the County and discussed possible alternatives and ways to keep the basin improvements within the existing ROW and established drainage easements. Attendees included Kirk

Torossian, Amir Zandieh, John Ng, and Susan Zarei from LA County. From the meeting on May 10, 2016 at the County, County staff reviewed the site information, pictures of the site, and our challenges with finding a means of storing 1,500 cy of debris material (the volume generated using the logarithmically increased valued for DPA Zone 1 charts for such a small area). Using this basis, the DPA Zone 1 curve was generating 3-inches of sediment for every square foot of the basin, which all agreed was excessive for this small contributing area. The tree cover and amount of exposed rock does not support that volume of material being generated.

Upon review of the site images and ground cover, Amir and John suggested we evaluate what changes could be gained from using the next lower curves or moving down the DPA Zone 1 line. We chose to use the base sediment rate documented in the Sediment Manual for DPA Zone 1, which is 120,000 cy/Sq

Mile (Paragraph 2, Page 14 of the Sedimentation Manual) It was noted Zone 1 is the most aggressive for the County, and this small site in this particular location would not likely generate the volumes projected.

They also suggested the design team move forward implementing a Desilting Inlet as the type of structure. These provisions were justified based on our contributing area being a very small drainage basin that is currently well established with what was perceived as a lower potential for sediment and mudflows.

After the meeting, as noted in the following pages, changing to sediment production rate to the base 120,000 cy/Sq Mile for DPA Zone 1 projected a sediment loading of 720 cy. The change also made the volume fit the requirements for a Desilting Inlet.

By incorporating the above modifications, the design team is proposing a basin that requires additional excavation and retaining walls to contain the projected sediment loading, but a basin was designed that fit within the site and the existing easement.

The balance of this memo documents the volumes and basin dimensions based on this discussion.

Hydrology:

Using the DPW Hydrology Manual (January 2006), the following input parameters were used:

Basin Area: 3.89

Acres

High Point: 1598

Low Point: 1385

Longest drain path

(red line in the adjacent photo: 525-ft

Slope: 0.4095 ‘/ft.

Densely forested ground with no visible structures

Percent Impervious area: 0.01 (none)

DPW Soil

Classification: 088

(See following figures obtained:

Debris Potential Zone: DPA-1

(From the above meeting, we are projecting the DDE using DPA-2 curves for the LA Basin sediment production).

defines how much sediment debris is generated from each site.

From above, the site area is 3.89 Acres, or

From the Sedimentation Manual and discussions with the DPW staff, the base sedimentation rate for

DPA Zone 1 is 120,000 cy/Sq Mile, which is sufficient for the debris loading for this project site. Therefore, for the site, the projected debris volume equals: 0.0061 (Sq Miles) *120,000 (cy/Sq Mile) = 720 CY

Elevated Inlet is recommended. However, the “*” in the text denotes: The use of elevated or desilting inlets and bulked flow drains in DPA zones 1 through 4 can be approved by Public Works in special circumstances. This is the case for this task, as DPW has approved the use of a Desilting Inlet per the May 10 meeting to be used for this project application.

Basin 2 = 720 CY

Continuing to use the Sedimentation Manual, we are implementing a Desilting Inlet in accordance with Figure 4.3.3, shown below.

Based on previous discussions with the DPW and the study team, the following constraints are being used on the basin design:

• The improvements must fit within the existing roadway easement and existing drainage easement for the basin. Attempting to gain more ROW or a wider easement will likely exceed the design schedule, require significantly more environmental investigation on lands not previously studied, and be difficult to obtain from the underlying property owners (based on City input received on how difficult obtaining the original easement was).

• The outfall must be directed as it does today, headed down the roadway prism in the roadside ditch. The team considered various outfall solutions directed under the road and projecting in the immediate downhill canyon where this drainage originally discharged. However, since this runoff has been diverted with the construction of several facilities that specifically routed this runoff onto the roadway prism, and discharging to the ravine located 500-ft down station along Santa Anita Canyon Road, there was no desire to introduce a change to the outfall conditions downstream.

• The outfall of the basin must match the elevation of the existing outfall today, as there is no room to widen the roadside ditch to accommodate a deeper or wider outfall.

• Slopes must be maintained within the existing drainage easement

• An access road (max grade = 25%) to the basin must be retained to remove deposited debris after runoff events.

The existing site appears as shown to the right:

The design team started with a basin design that had a 5-ft depth following the Desilting Inlet design.

The outfall of the basin (the pipe had to outfall to the existing roadway ditch elevation. The concept looked like the figure below:

As the slopes around the perimeter of the basin floor were nearly 2:1 or steeper, we couldn’t excavate into the basin too much without dramatically extending the slopes outside of the easement. The above attempt had a level capacity of approximately 400 cy. The cone capacity of this approach was only around 110 cy… therefore our capacity with this approach was only 510 cy, short of the 720 desired.

To gain more volume, we are excavating more of the basin floor to provide a wider, flatter basin floor. To contain the slopes, we are proposing to build vertical concrete retaining walls around the western perimeter.

This solution resulted in a level basin capacity of 585 cy (a base surface of 2000 sq ft, a top level surface of 4300 sq ft, and an average depth of 5-ft). The anticipated cone volume will be nearly 160 cy [4300 level surface x 4-ft (slope depth) / 4 ] / 27), with a combined capacity of 745 cy, exceeding the capacity desired.

Enter Document No. via Document Properties 9

The profile of the basin floor (the outfall channel), and the profile around the containment walls (Wall Profile) are shown below:

This new approach will require that our containment walls be constructed around the western perimeter of the basin. When we scoped the project, there was no estimation that the basin would need to be so large that walls were required, so no geotechnical investigation of these areas were performed. These new wall sections need to have geotechnical analysis to confirm the foundation materials and wall design parameters.

Per our analysis, we believe the above is in accordance with the requirements of the LADPW Sedimentation Manual, and provides an adequate solution for this site.

Enter Document No. via Document Properties 10

Outfall Protection:

Per discussions with the DPW both at scoping, and at the subsequent May 10 staff meeting, the discharge from the basin will continue to drain to the Santa Anita Canyon road prism. Runoff that outfalls from the previously discussed sediment basin will outfall into a roadside ditch on the left side of the roadway, as it does today. The existing superelevation will immediately take the runoff across the roadway and back again as the superelevation rotates around the roadway curves. The runoff will eventually outfall the roadway, behind the guardrail at an eroding outfall scupper located near Station 62+00.

The discharge from Basin 2, using the County’s HydroCalc software documented earlier, will produce a 50-year discharge of 20.6 cfs onto the roadway. This is not a change, but the condition that is being maintained today as all of that flow goes directly onto the roadway. The 50-yr Q of 20.6 cfs is rather large to be contained by the road prism alone. The calculated Q using the standard Rational Equation for this

3.89 acre area is Q CFL version of the Rational Formula, = C i A = (0.4) [ C value for densely landscaped, undeveloped land] x (6.21 in/hr from LADPW rainfall curves) x 3.89 acres (basin area) = 9.66 cfs (or nearly half that proposed by using the HyrdoCalc software. The real solution is likely in between.

Enter Document No. via Document Properties 11

While the basin 2 runoff is being routed through the sediment basin, the runoff itself is not detained. It would flow through the sediment containment features, and flow onto the roadway, as it has since the ravine culvert was removed following the 2005 event.

In addition to the flows from Basin 2, the runoff from the adjacent mountains south of Basin 2 will also be discharging onto the highway. The area south of Basin 2 discharging to Santa Anita Canyon Road is approximately 4 acres in size (nearly the same as Basin 2). Both drainage areas discharge the flow to the same discharge point at nearly the same time, so the combined flow for the 50-year event is well over 20 cfs.

Today, this discharge point is a grouted scupper behind the guardrail. Beyond the grouted materials, the steep slope is eroding as these heavy flows are discharging down the canyon.

While the improvement application did not address any upgrades or mitigation at this discharge location, the design team is recommending that some form of outfall protection be added to this discharge point to protect the outfall and hopefully prevent an undermining of the mainline roadway that happened in the upstream canyon in 2005.

Enter Document No. via Document Properties 12

As the available ROW is limited, and the adjacent outfall slope is very steep. Currently, there is no formal outfall protection or energy dissipation. At the end of the paved scupper, the outfall channel is eroding and has become incised into the embankment. Beyond the scupper, the slope of the outfall approaches 1:1.5 (Rise:Run).

We propose to install a reinforced outfall section, replacing the current paved section with a concrete outfall scupper leading to a gabion outfall.

From the gabions, dump riprap in the existing channel outfall to the limits of the ROW to mitigate for erosion.

EXISTING GUARDRAIL

TYPICAL GABION SIZE: 3-FT X 3-FT

FACE BATTER: 1:1

EXISTING OUTFALL

EXISTING ROW

REPLACED REINFORCED

OUTFALL SCUPPER

Enter Document No. via Document Properties 13

The above proposed improvements are proposed at this site.

We are requesting that LADPW review this concept so that the improvements can be added to the design.

For the design to be completed, additional geotechnical information will need to be acquired for the design of the retaining walls proposed in the Site 2 basin. (While the sediment basin barrier adjacent to the roadway has already been bored during the original geotech efforts, this additional work is for the walls into the basin itself, where the depth of the soil / rock is unknown.

707 17th Street Suite 2400 24th Floor

Date May 5, 2017

The following memorandum documents progress and design development on the Site 2 sediment and drainage improvements. It is primarily in response to comments received from the County of Los Angeles

Department of Public Works (DPW) regarding previous drainage concepts. These previous memos

(March 2, 2016 and July 20, 2016) are hereby referenced as DPW accepted the basin design volumes and sizes proposed in this design. The comments were provided in response to the above memos and during subsequent meetings with DPW staff.

Site 2 of this Santa Anita Canyon Road (SACR) project today encompasses two basins; one upstream of the Site 2 sedimentation basin, and one downstream of Site 2 sedimentation. As noted in previous emails, the runoff from these combined areas today exits the roadway at the single downstream outfall under the guardrail via a scupper. This outfall is experiencing heavy erosion.

May 5, 2017

The following comments were received from DPW. This memo and the 70% design is intended to meet the obligations of the improvement application and these comments:

1. We have reviewed the memo from Jacobs regarding the drainage basin at Site 2. We agree with the consultant’s projected basin sediment loading of 720 CY.

Response: The project design is proceeding with this basin design and layout.

2. The downstream gabion outfall is extremely vulnerable to undermining and prone to potential failure. Revise and resubmit.

Response: Based on discussions with DPW, the outfall for the sedimentation basin will piped under the SACR roadway and outfall directly to the downstream canyon as this drainage basin did prior to the 2005 flooding event. The size of the outfall pipe will be increased to a 30-inch diameter reinforced concrete pipe.

The fact that a sizeable amount of runoff generated above the limits of the sedimentation basin will no longer reach this existing outfall, the fact remains that the existing scupper is eroding the earthen roadway embankment, making the roadway stability at risk over time. Since the roadway rehabilitation of Site 5 encompasses this site replacing the pavement and barrier in this location, it seems prudent to address the erosion of this site.

The gabions proposed previously have been used successfully on other canyon embankment failures. Rather than using stacked gabions, we have modified the detail below incorporating gabions with reinforcement tails that provide enhanced anchoring and resistance to lateral movement or bulging. The runoff collected by the roadway will be discharged through an enlarged scupper, and the scupper will outfall over these rock gabions. The combined solution will stabilize the existing outfall, provide energy dissipation of the runoff leaving the scupper before being released on the earthen embankment, and push the erosion away from the current roadway embankment.

3. A concrete gutter across the entire proposed basin walls is needed immediately downstream of the walls.

Response: A concrete gutter has been added to the back of the basin walls where the walls are above the existing ground and drain towards the wall. In areas where the existing ground drains/slopes away from the walls, no swale was provided.

4. Provide a profile view showing the proposed inlet structure and outlet pipe.

Response: The plans include a continuous profile of the basin through the walls, outlet structure, and outfall pipes in Sheet Z1 of the project plans.

5. Provide hydraulic calculation for the existing roadside ditch (the roadway prism) to demonstrate it can convey the clear burned peak flow rate. Also, provide justification to prove that the existing ditch surface can withstand the flow’s erosive force.

Response: As discussed with the County during the previous drainage review meeting, the intent of the repair at this site did not address changing the drainage patterns of the existing roadway. The runoff conveyed on the roadway is an existing condition accepted by the County in advance of this site improvement.

Per the County’s desires, the outfall from this basin is no longer being discharged down the SACR, but rather, conveyed under the roadway in a pipe, and discharged into the original ravine immediately downstream of the basin.

6. Introduce a spillway at the proposed desilting wall (debris wall) and show it in the “Wall Profile” on

Page 9.

Response: The desilting wall elevation view showing inlet slots and spillway dimensions is included in the plans

(Sheet G9). The reinforced concrete floor

(minor concrete pavement) for the spillway area behind the desilting wall is shown in the roadway plans on Sheet

C2.

7. Provide a profile view of the proposed desilting basin similar to the profile view shown in Figure

4.3.3 on Page 42 of our

Sedimentation Manual (2nd Edition, March 2006).

Response: We provided a continuous profile as noted in comment 4 above.

Plan Sheet Z1 provides a profile view of the entire inlet and outlet system. The concept shown in Figure 4.3.3 shows the runoff and debris leaving through a pipe screened by a trash rack.

Our concept, due to the close proximity of the road and profiles of the basin vs. the roadway, a horizontal pipe outfall from the spillway does not fit.

Our concept has the debris basin behind the desilting wall (shown in light red). Beyond the desilting wall, runoff falls into the spillway

/ inlet (shown in green). Rather than a horizontal discharge pipe behind a trashrack, we provide a drop inlet with a grate.

The outfall pipe cannot discharge directly into the ravine as the embankment is behind a gabion faced retaining wall. The pink zone shows the limits of the reinforcement. The concrete pipe under the road discharges into a junction structure where the discharge pipe exits on grade to a location that is below the limits of earthen embankment. This extension of the pipe outfall is to avoid any potential damage to the existing roadway embankment below the embankment reinforcement limits. The pipe outfall at the bottom is a Bureau of Reclamation standard energy dissipater structure, where the water is forced through a series of baffles before exiting the structure.

Basin Access: The entrance ramp into the basin has been adjusted per comment received by the

DPW at the previous review meeting. The initial roadway apron through the access radius returns are paved with the same section as the balance of SACR (shown in dark grey). The balance of the access ramp is simply graded material (shown in brown).

The existing barrier is replaced with a combination of Guardrail and temporary concrete barrier

(shown in bright magenta). The concrete K-rail can be removed when access to the basin is needed

(ie: debris needs to be removed from the basin).

With these changes and additions, we believe the plans meet the intent of the comments received to date, and fulfill the requested improvements shown in the FLAP application for this corridor.

HGL = 1282.79

HGL = 1282.72

HGL = 1282.59

HGL = 1281.19

HGL = 1208.37

90' 0.8111'/ft

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow

Fyie, John Arrow