CA FS 1038(1) Salt Gulch - Final Hydraulics Memo - 04.16.2020.pdf

Subject: Salt Gulch Hydraulics Recommendations Date: April 16, 2020

Project Code: CA FS 1038(1)

From: Cassidy B. Cote, Hydraulics Engineer

FHWA Central Federal Lands Lakewood, CO

To: Justin Henwood, P.E., P.M.P.

FHWA Central Federal Lands Lakewood, CO

Summary:

Hydraulic and hydrologic analyses have been conducted for the CA FS SALT GULCH project located in Klamath National Forest, Siskiyou County, CA. The Federal Highway Administration, Central Federal Lands Highway Division (CFLHD), in partnership with the United States Forest Service (USFS) are performing engineering and construction services to repair the Salt Gulch Bridge and 12 culvert crossings. The culvert crossings have been identified by the USFS as being sub-standard and needing review or replacement. The following memo summarizes hydraulic design criteria, computational methods and proposed recommendations for the project corridor.

A previous option for this project considered an additional 18 culverts. It has been determined that the scope of the project no longer includes services for these structures. The proposed crossings reside along Forest Routes 46N50 (Horse Creek Road), 47N70 and 47N72. These routes are primarily used for logging operations and are located along California State Route 96, approximately 45 miles northwest of Yreka, CA (N 41.872680°, W 123.085316°). Figure 1 provides a location map for reference.

The Klamath National Forest was impacted by the Abney Fire in Fall 2017. A site visit was conducted by several of the cross-functional team members in August 2019 to evaluate the crossings. On site, team members identified that all 12 of the culvert basins included within the scope of this project fell within the footprint of the fire. The USFS requested that the 100-year event be used for hydraulic capacity design. The 25-year event has been selected for stability design. Design discharges include an increase to the base flow due to the hydrologic impact of the fire. This increased flow, herein termed the post-fire flow or the post-fire condition, has been utilized for the hydraulic design of all 12 culvert crossings.

Hydraulic models were generated for the sites identified as major drainage and sites with unique geometric constraints.

The remaining crossings were designed using computed hydrology and a generalized inlet control hydraulic model. It is proposed that ten culverts be replaced along the project corridor. Two existing structures are recommended to be left in place, modified and equipped with relief structures. Proposed actions and replacement recommendations for the Salt Gulch Bridge and each of the 12 culvert crossings along the project corridor are summarized in this memo.

Memorandum https://www.google.com/maps/place/41%C2%B052'21.7%22N+123%C2%B005'07.1%22W/@41.872681,-123.0858632,208m/data=!3m2!1e3!4b1!4m14!1m7!3m6!1s0x0:0x0!2zNDHCsDUyJzIwLjgiTiAxMjPCsDA1JzA2LjciVw!3b1!8m2!3d41.872452!4d-123.085201!3m5!1s0x0:0x0!7e2!8m2!3d41.8726796!4d-123.0853164

Design Methodology:

The Federal Lands Highway Project Development and Design Manual (PDDM) is used to establish the hydraulic design criteria for Federal Lands projects (Federal Lands Highway 2012). The Scoping Trip Report indicates the project corridor is not defined as a critical access road, has a designated speed of 25 mph, and an average daily traffic (ADT) of less than 100 vehicles (FHWA 2019). Based on these parameters, the roadway is classified as a low-standard road for hydraulic design purposes. At the request of the USFS, the hydraulic capacity design storm has been increased to the 100-year storm for the basins identified as within the footprint of the Abney Fire. Consistent with the PDDM low-standard hydraulic design criteria shown in Table 1, and in an effort to reduce the budgetary and environmental footprint of the project, the 25-year storm has been selected as the stability design flood. Hydraulic adequacy for roadway culverts is determined by conformance to all applicable design criterion including allowable headwater (AHW) and the respective headwater to depth ratio (HW/D).

Table 1: PDDM Hydraulic Design Criteria

Road Classification:

Low-Standard Roadway Hydraulic Capacity Design Criteria

Roadway Culverts

Design Flood:

25-year1

Check Flood:

Overtopping

New: AHW ≤ bottom of aggregate base layer Existing: AHW ≤ shoulder hinge point Where D ≤ 48", HW/D ≤ 1.5 & Where D > 48", HW/D ≤ 1.2 Heavy debris or sediment load concerns: 0.8 ≤ HW/D ≤ 1.0 Crossing is stable at the design flood

Temporary Culverts 2-year Seasonal construction may justify a lower design flood WSE ≤ shoulder hinge point

The site visit suggested significant amounts of sediment and debris would be released during the runoff events in the recovery window following the fire. Sediment and debris bulking was not considered in hydrologic computations. In an effort to accommodate the increase in material, a minimum 36-inch culvert diameter has been recommended at each location. Additionally, due to the increased sediment load and risk of aggradation, it was determined that a more stringent hydraulic criteria be applied to the crossings. Culverts with basins subject to the post-fire hydrologic condition have been designed to operate with a HW/D approaching 1.2 for pipes with diameters 48 inches or below, and a HW/D approaching

1.0 for diameters greater than 48 inches.

The design vehicle for this project is a tractor with a 40-foot logging trailer (FHWA 2019). In an effort to accommodate the increased loading of the design vehicle, it was determined that the design of each crossing would utilize a minimum culvert cover depth of 2-feet. However, many crossings are constrained by the low height from the streambed to the road. In an effort to limit project cost and duration of construction, it has been determined that raising the profile of the roadway and use of concrete box culverts are not within the scope of this project. Corrugated metal pipe (CMP) has been recommended where practical throughout the project. Installation of recommended structures, designed to convey the 100-year storm, may require channel excavation to meet the 2-foot culvert cover criteria. It is recommended that the structure and surrounding channel be constructed on a grade that matches the local channel grade, in order to limit maintenance and mitigate sediment deposition issues at and around the vicinity of the crossing.

1 The hydraulic capacity design flood has been increased to a 100-year event for this project.

Hydrology:

A hydrologic model was developed based on fire condition for each of the 30 culvert crossings within the project area.

The final scope of the project has reduced the number of crossings that will receive services to 12 culverts and the Salt Gulch Bridge. Each of the 12 culverts included in the final scope have drainage basins falling within the footprint of the Abney Fire. The USFS requested an analysis of the 100-year rainfall event and provided GPS locations for each culvert. An event of this magnitude has a 1% Annual Exceedance Probability (AEP). The 1% AEP event has a 1% chance of being exceeded each year. The 25-year event (4% AEP) was also evaluated in this analysis. The Watershed Modeling System was used to delineate drainage areas for each crossing. Resulting drainage basins are shown in Figure 2 (Aquaveo 2016).

Post-fire scenarios create complex hydrologic conditions for many years after the fire event. The general outcome is well understood; for any rainfall event, post-fire conditions yield higher runoff rates than pre-fire conditions. However, the runoff response of any given watershed in a post-fire scenario is difficult to quantify and requires a more detailed modeling approach. To analyze the post-fire condition of the Abney Fire, a hydrologic model was developed in HEC-HMS (USACE 2018). Table 2 summarizes the input parameters used for the HEC-HMS model.

Table 2: HEC-HMS Model Inputs

Storm Type 24-hour storm, SCS type IA distribution was used.

Precipitation NOAA Atlas 14, 25-year, and 100-year depths were analyzed.

Soil Type Data was obtained from STATSGO. Majority are type C.

Loss CN method used. No additional losses assumed.

Pre-Fire CN value

The majority of the area was identified as NRCS wooded area in good condition with average antecedent soil moisture conditions. This corresponds to a CN value of 73.

Time of Concentration NRCS Lag Method was used for each basin.

Post-Fire CN Value

Some reports suggest using a CN value as high as 95 in high burn severity areas for post-fire condition runoff modeling. Higher CN values will result in a higher runoff. Maps show the area is considered low to moderate burn severity with canopy cover removed. A CN value of 80 was used for areas in the burn footprint.

Salt Gulch Watershed

Yreka, CA

Figure 1: Location Map Figure 2: Drainage Basins

Table 3 summarizes the drainage basin sizes and the recommended design flows for each of the 12 culvert basins. The discharges provided below reflect the results of the HEC-HMS model considering the post-fire hydrologic condition.

Table 3: Recommended Design Discharges

Crossing Name Hydrologic Method

Drainage Area

(sq. mi.)

Drainage Area (acres)

25-Year Flow (cfs)

100-Year Flow (cfs)

46N50_7.33 HEC-HMS 0.03 21 18 24

46N50_7.38 HEC-HMS 0.03 19 16 22

46N50_7.48 HEC-HMS 0.04 25 22 29

46N50_7.63 HEC-HMS 0.02 13 11 15

46N50_7.68 Proportional Area 0.01 6 5 7

46N50_7.77 HEC-HMS 0.04 28 25 33

46N50_8.28 HEC-HMS 0.05 32 28 38

46N50_8.40 HEC-HMS 0.43 272 210 284

46N50_9.62 HEC-HMS 0.03 22 19 26

46N50_9.73 HEC-HMS 0.04 24 21 29

47N70_1.12 HEC-HMS 0.77 491 389 526

47N72_1.73 HEC-HMS 0.06 39 35 46

Hydraulic Analysis:

Hydraulic performance was evaluated using HY-8 hydraulic modeling software (FHWA 2017). Site specific hydraulic models were generated for proposed crossings exceeding the 36-inch diameter threshold, as determined by the inlet control analysis described in this section. HY-8 models were also generated for sites identified as having complex geometric constraints, such as crossings not applicable to the Standard Detail C251-50 outlet apron dimensions due to a pipe slope in excess of 10% (CFLHD 2015). Table 4 summarizes proposed recommendations for each of the 12 culvert crossings.

Table 4: Recommendation Matrix

Crossing Name

Proposed Size1 (in.)

Structure Type

End Treatment Debris Rack Elbow and

Rundown Outlet

Protection Riprap Class

46N50_7.33 36 CMP FES Yes No Yes III 46N50_7.38 36 CMP FES Yes No Yes III2 46N50_7.48 36 CMP FES Yes Yes Yes III2

46N50_7.63 36 CMP FES Yes No Yes III2 46N50_7.68 36 CMP FES Yes No Yes III2 46N50_7.77 36 CMP FES Yes No Yes III2 46N50_8.28 42 CMP FES Yes No Yes IV 46N50_8.40 103 x 71 CMPA HW & WW No No Yes VI

46N50_9.62 Existing 18 in. CMP equipped with one 36 in. relief CMP FES Yes Yes Yes III

46N50_9.73 36 CMP FES No No Yes III

47N70_1.12 Existing 66 in. CMP equipped with two 48 in. relief CMPs FES Yes Yes No N/A

47N72_1.73 42 3 CMP FES No Yes3 Yes3 IV3

1 Structure size is reported as the opening diameter or as width x depth, in inches.

2 Sized in accordance with Standard Detail C-251-50 (CFLHD 2015).

3 It may be desirable to upsize the pipe to 48”. Elbow and apron may have constructability issues. See Site-Specific Culvert Details.

As a consequence of being within the footprint of a fire, subject basins will observe an increased sediment load. In the interest of mitigating deposition, it is desirable for any new structure to be properly aligned with the channel and constructed on a grade that matches the channel. Many existing crossings are undersized and have observed upstream sedimentation. In an effort to convey the 100-year runoff, provide a minimum of 2-feet of cover, and avoid raising the profile of the road, a number of proposed inlet invert elevations are below those of the existing configuration. These sites will require grading of ditches and upstream channels. Culverts with basins identified as within the footprint of the Abney Fire should be monitored and cleaned as needed as to prevent issues associated with plugging or reductions in capacity due to deposition.

Multiple structures adjacent to one another can convey runoff comparable to one larger structure. However, this can lead to issues with sediment and debris blockage, particularly for basins subject to the post-fire condition. Given the increased need to pass sediment and debris in the post-fire response period, adjacent structures have not been recommended for this project, with the exception of the 47N70_1.12 hydraulic relief structures, which are not intended to convey sediment.

A generalized inlet control analysis was conducted using HY-8 to size drainage crossings that were not selected for a full scale model. The results of the generalized model, detailed in Table 3, were used to determine the discharge that would require upsizing to the next incremental size for the respective HW/D ratio. This model employs a culvert slope of 2% and a projected structure length of 50 feet.

Table 5: Generalized Inlet Control Analysis

Pipe Diameter (in.)

Waterway Area

(sq. ft.)

Post-Fire Condition

HW/D

Maximum Flow (cfs) Flared End Section1

18 1.8 1.2 7 24 3.1 1.2 15 30 4.9 1.2 26 36 7.1 1.2 41 42 9.6 1.2 61

The outcomes of the generalized inlet control analysis have been applied with respect to the site-specific design flows.

Table 6 summarizes the results of this analysis for the crossings that are proposed to be replaced with the minimum 36-inch diameter pipe and were not selected for a site-specific model due to complex geometric constraints. Hydraulic capacity results for the structures selected for site-specific hydraulic models are detailed in Appendix A.

Table 6: Minor Drainage Hydraulic Summary

Station

100-Year Flow (cfs)

Existing Structure Opening Minimum Hydraulic Opening2 Minimum Replacement Opening3

Diameter (in.)

Waterway Area (sq. ft.)

Diameter (in.)

Waterway Area (sq. ft.)

Diameter (in.)

Waterway Area (sq. ft.)

46N50_7.38 22 18 1.8 30 4.9 36 7.1 46N50_7.48 29 18 1.8 36 7.1 36 7.1 46N50_7.63 15 18 1.8 30 4.9 36 7.1 46N50_7.68 7 18 1.8 24 3.1 36 7.1 46N50_7.77 33 18 1.8 36 7.1 36 7.1

1 A mitered inlet configuration has been used in the model as a substitute for a flared end section, which is not an option in HY-8.

2 Based on the results of the generalized inlet control model for the 100-year capacity design flow.

3 Upsizing the results of the inlet control hydraulic model to meet the minimum 36-inch diameter.

It was identified that some sites would benefit from additional retrofits such as debris racks and elbow rundowns. A headwall entrance treatment is recommended to offset buoyant forces for structures exceeding a 48-inch diameter (Federal Lands Highway 2012). Headwalls and cutoff walls also serve to mitigate undermining (piping), which is known to lead to embankment failure. It is desirable for structures in excess of 48-inches to be equipped with flared wingwalls, matched into the channel banks at the inlet and outlet, to smoothly transition flow and protect the structure and roadway approaches from erosion. Beveled edges should be used on all headwalls (Federal Lands Highway 2012).

For structures with site specific HY-8 models, Appendix A summarizes capacity design results for both existing and proposed configurations, and Appendix B summarizes hydraulic stability design results, including proposed outlet protection and culvert geometry. Recommendations specific to each of the 12 culvert crossings along the project corridor are detailed in Appendix C.

Outlet Protection:

Structures that have not been identified as having outlets far enough from the roadway prism to mitigate risk of eroding the toe of slope include recommendations for an outlet riprap apron. Culverts with pipe slopes less than 10% may be sized in accordance to Standard Detail C-251-50, Placed Riprap at Culverts (CFLHD 2015). Proposed crossings exceeding a 36-inch diameter or a 10% slope were evaluated using the Hydraulic Toolbox (FHWA 2018) in conjunction with the site specific hydraulic model to determine adequate riprap class and outlet apron dimensions. Results of this analysis were then compared to Standard Detail C-251-50 to ensure stability of the outlet apron at the 25-year stability design flow. For sites where the standard apron was not sufficient, site-specific outlet protection recommendations have been provided in Appendix B.

A number of proposed culverts have been identified as suitable for run down elbow retrofits, such as those shown in Standard Detail C602-50, 24-inch Run Down and Pipe Anchor Assembly (CFLHD 2015). These retrofits are beneficial in locations where a standard apron is not possible due to stability concerns, or where it is desirable to limit fill excavation.

Riprap apron stability concerns exist where there is a tailwater slope in excess of 1:1.5 (V:H). Due to a measure of energy dissipation provided by the elbow, it was determined that riprap aprons for structures using an elbow and rundown may be sized in accordance with Standard Detail C-251-50 (CFLHD 2015).

It has been identified that select crossings outlet into confined channel settings. In these locations, recommended riprap limits (particularly the end width) can result in aprons that extend well beyond the extents of the channel. In an effort to limit the elevation that riprap is placed for any particular apron, it is proposed that the outlet apron extend the recommended depth, length and end width, up to an elevation that is at least one foot above the top of the opening, adjusted along the channel profile. For the purpose of stability, riprap should tie into the existing ground at a slope not exceeding 1:1.5 (V:H).

Site-Specific Culvert Details:

Due to the large magnitude of fill over the existing 47N70_1.12 and 46N50_9.62 crossings, replacement has been deemed outside the scope of this project. Hydraulic relief structures have been proposed for both sites. The respective hydraulic models indicate that the crossings will convey 100-year runoff beneath the surface of the roadway and that headwater depths will be reduced by the addition of these structures. These crossings will not meet the hydraulic standard established in this report, but are considered acceptable improvements given the budgetary and environmental constraints of the project.

The 47N70_1.12 basin is within the footprint of the Abney Fire and has a large sediment load. The hydraulic model indicates that this structure would need to be replaced with an approximate 102-inch CMP with a headwall to be hydraulically adequate (100-year HW/D = 0.96). The improved inlet is intended to improve sediment continuity of the crossing. Equipped with relief structures and an improved inlet, it is anticipated that the site will still continue to observe upstream deposition at and around the structure inlet. The USFS should continue to monitor this crossing. It is recommended that the structure inlet be kept clear of sediment and debris in order to maximize available conveyance.

A corrugated metal pipe arch (CMPA) has been proposed for the replacement of crossing 46N50_8.40. A hydraulically adequate CMP at this location would require lowering of the inlet invert to accommodate the proposed pipe depth and cover requirements, resulting in the removal of a substantial amount of material to regrade the upstream channel. The recommended CMPA provides a unique opportunity to convey the 100-year capacity design event while minimizing reductions to inlet invert elevations, reducing the extent of channel grading and mitigating the risk of deposition.

Stability concerns have been identified with the installation of an outlet apron at crossing 47N72_1.73. Tailwater slopes at this crossing exceed 1:1.5 (V:H) and there are constructability concerns associated with the installation of an elbow and riprap apron on such a steep slope. If it is indeed determined that an elbow and riprap apron are not constructible at this location, it is recommended that a rundown assembly outlet water reasonably far from the predominant roadway prism to mitigate risk of eroding the slope. It may be desirable to upsize this structure to a 48-inch CMP in order to lump the rundown assembly and anchor system with two other 48-inch assemblies proposed on this project.

Culvert Discussion:

Several design methodologies are in place to accommodate the post-fire condition of the culvert basins on this project. In an effort to compensate for the increased sediment load, more stringent HW/D ratios have been selected. A post-fire hydrologic condition has also been applied to applicable basins, yielding higher runoff rates than pre-fire conditions. Post-fire design methodologies supplemented with an increased capacity design event may compound factors of safety.

At the request of the USFS, the 100-year storm has been employed for hydraulic capacity design. This measure is an increase from the 25-year capacity design event detailed in the PDDM for a low-standard roadway (Federal Lands Highway 2012). It is understood that supplementing post-fire design methodologies with the 100-year storm introduces a level of conservatism to recommendations which may impact the overall cost and constructability of the project.

Salt Gulch Bridge Discussion:

The Salt Gulch Bridge was designed in 1958 by FWHA and was constructed using FHWA drawings RG-1397-A. The bridge is 120 feet in total length with three spans (35’-50’-35’). The deck is approximately 16 feet wide. The deck is approximately 40 feet above the stream channel at the highest point. The bridge is composed of a cast in place (CIP) concrete deck supported by two CIP concrete girders. The superstructure was supported at abutment 1 (later reconstructed), Pier 2 and Pier 3 by CIP concrete piers supported by spread footings. The piers are constructed with two columns each. Abutment 4 is composed of a CIP wall abutment with wing walls, all supported on spread footings.

The bridge was repaired in 1975 after damage was incurred when the foundation material moved due to an active landslide. At that time, Abutment 1 was replaced with a concrete pile cap supported by steel piles. Cracking in Pier 2 was also repaired. The Forest Service designed and constructed the repair with Forest Service Drawing R-1136.

It appears the foundation material continues to move, as the pile cap constructed in 1975/76 is cracked and the crack size is increasing. Differential movement has been observed. The Forest Service requested that the CFLHD perform engineering and construction services to repair the existing structure.

A hydraulic capacity analysis was conducted at the bridge location to determine the water surface elevations and velocities at the upstream face of the bridge for the bridge repair plans. The existing conditions were modeled using Aquaveo’s Surface-water Modeling Software (SMS-SRH2D) version 13.0.9. The peak flows at the bridge were determined using regional regression equations. Table 7 summarizes the information provided for the bridge plans.

Table 7: Hydraulic Data at Salt Gulch Bridge

Q (cfs) Velocity (ft/s) WS El. (ft.)

Q2 225 9.1 2,132.0 Q50 1,310 11.7 2,136.4 Q100 1,630 12.5 2,137.2 Q200 2,070 13.2 2,138.3

A scour analysis of the bridge foundation elements was not performed. Continued regular bridge inspection and inspection after high flow events is recommended to monitor the stability of the existing foundations. According to the as‐built plans, the pier footings are not founded in rock and are located within the flow area of the creek. As such, the existing piers are likely vulnerable to scour. Protection of the existing piers from scour was not included in the scope of the project. For the repairs at the south abutment, the scour potential was deemed to be low given the setback of the abutment from the main channel. Contraction scour and local abutment scour were not taken into consideration in the foundation design of the repairs at the south abutment.

One point of consideration in future inspections is the landslide immediately upstream of the structure on the south channel bank, which has choked the flow in the area. In higher flows, the result of choked flow can include significant expansion turbulence and scour. This would likely occur at the toe of the embankment below the south pier of the bridge.

This type of erosion could be a contributing factor to the embankment slumping on the south bank and material loss at the toe, which could destabilize the pier foundation and/or the embankment slope from the main channel to the south abutment.

References:

Aquaveo. 2016. Watershed Modeling System (WMS). Software Version 10.1.11.

CFLHD, Central Federal Lands Highway Division. 2015. "24‐inch Run Down and Pipe Anchor Assembly (C602‐50)." Detail Drawings: FP‐14.

CFLHD, Central Federal Lands Highway Division. 2015. "Placed Riprap at Culverts (C‐251‐50)." Detail Drawings: FP‐14.

Federal Lands Highway. 2012. "Project Development and Design Manual, Chapter 7 Hydrology and Hydraulics."

https://flh.fhwa.dot.gov/resources/design/pddm/.

FHWA, Federal Highway Administation. 2019. "CA FS Salt Gulch Bridge & Legacy Sites – Culvert Replacements." Inspection Report.

FHWA, Federal Highway Administation. 2017. HY‐8 Culvert Analysis Program. Software Version 7.50, Aquaveo.

FHWA, Federal Highway Administation. 2018. Hydraulic Toolbox. Software Version 4.4, Aquaveo.

USACE, U.S. Army Corps of Engineers. 2018. Hydraulic Engineering Center Hydrologic Modeling System (HEC‐HMS).

Software Version 10.1.11.

Attachments:

Appendix Title Description

A Capacity Design Hydraulic capacity (100‐year) results for crossings with a site specific hydraulic model.

B Stability Design Outlet protection and proposed geometric configurations for crossings with site specific hydraulic models.

C Recommendation Summary Detailed recommendations for each of the 30 drainage crossings.

Appendix A – Capacity Design

CA FS SALT GULCH – Hydraulics Memo

Appendix A – Capacity Design CA FS SALT GULCH – Hydraulics Memo

Crossing Name 100-Year

Capacity Design Flow (cfs)

Proposed or Existing

Structure Size Structure Type Model Inlet

Configuration1

Headwater Elevation

(100-Year, ft.)

Allowable Headwater

Elevation2 (ft.)

Target HW/D Ratio HW/D Ratio Overtopping

Flow (cfs) Meets Capacity Design Criteria?

Span (in.) Depth (in.)

46N50_7.33 24 Existing 18 18 CMP Thin Edge 1000.3 999.8 1.2 3.31 15 No

Proposed 36 36 CMP Mitered 998.7 1000.0 1.2 0.91 44 Yes

46N50_8.28 38 Existing 18 18 CMP Thin Edge 1000.0 999.9 1.2 3.80 16 No

Proposed 42 42 CMP Mitered 998.4 999.6 1.2 0.99 62 Yes

46N50_8.40 284 Existing 48 48 CMP Thin Edge 1000.4 999.7 1.2 1.66 96 No

Proposed 103 71 CMPA Headwall 996.8 999.8 1.0 0.96 426 Yes

46N50_9.62 26 Existing 18 18 CMP Thin Edge 994.7 999.9 1.2 8.49 29 No

Proposed Existing with one 36 in. relief pipe CMP Mitered 986.8 999.4 1.2 1.60 139 No3

46N50_9.73 29 Existing 18 18 CMP Thin Edge 999.5 999.7 1.2 2.83 14 No

Proposed 36 36 CMP Mitered 998.3 999.5 1.2 1.01 43 Yes

47N70_1.12 526 Existing 66 66 CMP Thin Edge 1001.3 1000.3 1.0 3.85 426 No

Proposed Existing with two 48 in. relief pipes CMP Mitered 992.9 1000.1 1.0 3.20 797 No4

47N72_1.73 46 Existing 24 24 CMP Thin Edge 1001.9 1002.0 1.2 1.57 16 No

Proposed 42 42 CMP Mitered 999.7 1001.0 1.2 0.99 74 Yes Alternate5 48 48 CMP Mitered 999.0 1001.0 1.2 0.82 98 Yes

1 – A mitered inlet configuration has been used in the model as a substitute for a flared end section, which is not an option in HY-8.

2 – Allowable headwater elevation (AHW) criteria is established by the PDDM (Federal Lands Highway 2012). New structure: AHW ≤ bottom of aggregate base layer; Existing structure: AHW ≤ shoulder hinge point.

3 – This is considered an acceptable improvement from the existing configuration. In the event that the existing structure becomes blocked by sediment or debris, the model indicates that the 36 in. relief structure would operate with an approximate 100-year headwater elevation of 987.8 ft., HW/D = 0.87, and an overtopping flow of 109 cfs.

4 – This is considered an acceptable improvement from the existing configuration. The model indicates that the relief pipes will operate with a HW/D = 0.87 at the 25-year event and a HW/D = 1.48 at the 100-year event.

5 – It may be desirable to upsize this structure to a 48-inch CMP in order to lump the rundown assembly and anchor system with two other 48-inch assemblies proposed on this project. An alternate configuration row has been added for this structure, detailing the capacity design results for this configuration.

Appendix B – Stability Design

Appendix B – Stability Design CA FS SALT GULCH – Hydraulics Memo

Crossing Name 25-Year Stability Design Flow (cfs)

Proposed Structure Size Proposed

Structure Type Inlet Invert

Elevation1 (ft.)

Outlet Invert

Elevation1 (ft.)

Projected Structure Length

(ft.)

Culvert Slope

Outlet Protection

Span (in.) Depth (in.) Stone Class Apron Length (ft.)

Apron Thickness (ft.)

Apron End Width (ft.)

46N50_7.33 18 36 36 CMP 995.94 989.57 35.4 18.0% III 16.03 3.0 14.0

46N50_8.28 28 42 42 CMP 994.89 983.67 46.9 23.9% IV 21 3.0 16.0

46N50_8.40 210 103 71 CMPA 991.12 989.03 40.2 5.2% VI 51.5 4.0 48.5

46N50_9.622 19 Existing 18 in. CMP with one 36 in.

relief pipe CMP (relief) 985.20 981.85 55.9 6.0% III 16.0 3.0 14.0

46N50_9.73 21 36 36 CMP 995.23 988.74 35.2 18.4% III 16.0 3.0 14.0

47N70_1.122 389 Existing 66 in. CMP with two 48 in.

relief pipes CMP (relief) 987.00 981.00 105.1 5.7% It is recommended that this structure outlet far enough away from the roadway prism that an outlet apron is not necessary.

47N72_1.734 35

42 42 CMP 996.22 993.59 32.9 8.0% IV 21 3.5 16.5

48 48 CMP (alternate configuration) 995.73 991.93 38.7 9.8% It is recommended that this structure outlet far enough away from the roadway prism that an outlet apron is not necessary.

1 – Proposed invert elevations reflect the elevation of the invert at the entrance to the closed conduit. For applicable sites this is the juncture between the enclosed pipe and the flared end section.

2 – The recommendations for these crossings involve leaving the existing structure in place and installing relief structure(s). Proposed relief structure geometry is detailed in this row. The values given for the outlet elevation and projected length correspond to the proposed location for the main pipe to transition to the rundown.

3 – It is anticipated that the actual apron length with be closer to 25 ft. in order to tie into the existing ground at a maximum slope of 1:1.5 (V:H).

4 – Tailwater slopes at this crossing exceed 1:1.5 (V:H) and there are constructability concerns associated with the installation of an elbow and riprap apron on such a steep slope. If it is indeed determined that an elbow and riprap apron are not constructible at this location, it is recommended that a rundown assembly outlet water reasonably far from the predominant roadway prism to mitigate risk of eroding the slope. It may be desirable to upsize this structure to a 48-inch CMP in order to lump the rundown assembly and anchor system with two other 48-inch assemblies proposed on this project. An alternate configuration row has been added for this structure, detailing the proposed design of this configuration.

Appendix C – Recommendation Summary

Appendix C - Recommendation Summary CA FS SALT GULCH – Hydraulics Memo

Name

Existing Culvert Size

Proposed Culvert Size

Proposed Structure

Type

Elbow & Run Down Debris Rack End

Treatment Site Specific Recommendations Diameter or width x depth (in.)

46N50_7.33 18 36 CMP No Yes FES

Install 36 in. CMP with flared end sections at inlet and outlet. Grade ditch at inlet to drain and install debris rack over the inlet. Class III armor should be used to protect any disturbed channel banks or roadway slopes at the structure's outlet, up to a height at least one foot above the top of the opening. The outlet apron should extend 3 ft. in depth, sloping 1:1.5 (V:H) longitudinally to tie into existing ground (this occurs approximately 25 ft. from the outlet). The apron should extend to a 14 ft. width at the end of the apron, or to an elevation that is at least one foot above the top of the opening adjusted along the channel profile.

46N50_7.38 18 36 CMP No Yes FES Install 36 in. CMP with flared end sections at inlet and outlet, place outlet riprap apron per standard, grade ditch at inlet to drain, install debris rack over inlet.

46N50_7.48 18 36 CMP Yes Yes FES Install 36 in. CMP with flared end sections at inlet and outlet, add elbow and extend culvert beyond roadway prism at outlet, place outlet riprap apron per standard, install debris rack over inlet.

46N50_7.63 18 36 CMP No Yes FES Install 36 in. CMP with flared end sections at inlet and outlet, place outlet riprap apron per standard, grade ditch at inlet to drain, install debris rack over inlet.

46N50_7.68 18 36 CMP No Yes FES Install 36 in. CMP with flared end sections at inlet and outlet, place outlet riprap apron per standard, improve existing berm on the upstream side, install debris rack over inlet.

46N50_7.77 18 36 CMP No Yes FES Install 36 in. CMP with flared end sections at inlet and outlet, place outlet riprap apron per standard, grade ditch at inlet to drain, install debris rack over inlet.

46N50_8.28 18 42 CMP No Yes FES

Install 42 in. CMP with flared end sections at inlet and outlet, grade ditch at inlet to drain and install debris rack over inlet. Class IV armor should be used to protect any disturbed channel banks or roadway slopes at the structure's outlet, up to a height at least one foot above the top of the opening. The outlet apron should extend 3 ft. in depth and 21 ft. longitudinally to a 16 ft. width at the end of the apron, or to an elevation that is at least one foot above the top of the opening adjusted along the channel profile. The riprap should tie into the existing ground with a slope not exceeding 1:1.5 (V:H).

46N50_8.40 48 103 x 71 CMPA No No Headwalls and wingwalls

Install a 103 in. x 71 in. corrugated metal pipe arch with headwall and wingwalls at the inlet and outlet. Class VI armor should be used to protect any disturbed channel banks or roadway slopes at the structure's outlet, up to a height at least one foot above the top of the opening.

The outlet apron should extend 4 ft. in depth and 51.5 ft. longitudinally to a 48.5 ft. width at the end of the apron, or to an elevation that is at least one foot above the top of the opening adjusted along the channel profile.

46N50_9.62 18

Existing structure supplemented with one 36

in. relief pipe

CMP

(relief) Yes Yes FES

Leave existing culvert in place, add flared end sections at the inlet and outlet, and a debris rack at the inlet. Install one 36 in. CMP with flared end sections at inlet and outlet as a relief structure. Add elbow and extend relief structure beyond roadway prism at outlet. Class III armor should be used to protect any disturbed channel banks or roadway slopes at the structure's outlet, up to a height at least one foot above the top of the opening. The outlet apron should extend 3 ft. in depth and 16 ft. longitudinally to a 14 ft. width at the end of the apron, or to an elevation that is at least one foot above the top of the opening adjusted along the channel profile.

46N50_9.73 18 36 CMP No No FES

Install 36 in. CMP with flared end sections at inlet and outlet and grade ditch at inlet to drain. Class III armor should be used to protect any disturbed channel banks or roadway slopes at the structure's outlet, up to a height at least one foot above the top of the opening. The outlet apron should extend 3 ft. in depth and 16 ft. longitudinally to a 14 ft. width at the end of the apron, or to an elevation that is at least one foot above the top of the opening adjusted along the channel profile.

47N70_1.12 66

Existing structure supplemented with two 48

in. relief pipes

CMPs (relief) Yes Yes FES

Install two 4 ft. CMPs with flared end sections as relief structures. We recommend using 987.0 as the inlet invert elevation and setting the culverts on an approximate 8.7% slope, beginning the run down at the roadway prism slope inflection point (approximate elevation 981.0 ft.).

Equip these structures with an elbow and extend beyond the roadway prism, outlet near the existing primary structure outlet. For the existing primary structure inlet, remove the drop structure and replace with Pipe Riser and Debris Rack improved inlet. Reference "CMP Riser, Type B" from Special 602-A, "Pipe Riser and Debris Rack," project FTNP SEKI 10(12), derived from CalTrans Detail D93C. USFS should continue to monitor this site and may consider debris removal should the primary pipe be inundated with sediment and/or woody debris.

47N72_1.73 24 42 CMP Yes No FES

Install 42 in. CMP with flared end sections at inlet and outlet, grade ditch at inlet to drain. Class IV armor should be used to protect any disturbed channel banks or roadway slopes at the structure's outlet, up to a height at least one foot above the top of the opening. The outlet apron should extend 3.5 ft. in depth and 21 ft. longitudinally to a 16.5 ft. width at the end of the apron, or to an elevation that is at least one foot above the top of the opening adjusted along the channel profile.

If it is determined that an elbow and riprap apron are not constructible at this location, it is recommended that a rundown assembly outlet water reasonably far from the predominant roadway prism to mitigate risk of eroding the slope. It may be desirable to upsize this structure to a 48-inch CMP in order to lump the rundown assembly and anchor system with two other 48-inch assemblies proposed on this project.