az275(1)_FinalHydraulicsdocx.pdf

Subject: AZ FTFS 275(1), Stone Creek AOP Improvements

From: Megan Frye Hydraulic Engineer

Lakewood, CO

To: Dustin Robbins Project Manager Lakewood, CO

This memorandum presents the field site observations, design analyses, and hydraulic recommendations for the proposed project repairs at Bob Thomas Creek and Stone Creek both located on the Apache Sitgreaves National Forest. Figure 1 provides a project location map for reference.

Figure 1. AZ FTFS 275(1), AOP Improvements - Project Location Map

The project site lies within a recent burn scar and as such, these watersheds have been transporting an increased amount of sediment. Both crossings performed well prior to the 2011 Wallow fire; however, since the fire and increased sediment load, the structures have required more frequent maintenance.

The Bob Thomas crossing is currently an at grade low water crossing. The USFS completed downstream repairs (large riprap and cutoff walls) to the crossing in 2016 to arrest down cutting concerns. While the crossing is performing well, the USFS is interested in hydraulic improvements to the crossing. The Stone Creek crossing consists of two CMP arch culverts, which periodically plug with debris. The USFS is interested in increasing the efficiency of the crossing while reducing the maintenance requirements. In addition, the USFS would like to accommodate aquatic organism passage at the Stone Creek crossing.

Figure 2 provides an aerial image of the site.

Memorandum

Date: May 7, 2020

Figure 2. Proposed Site Improvements

This memo addresses the hydraulic recommendations associated with the proposed project. The following items will be addressed:

Hydrologic and hydraulic criteria and computational methods

Hydraulic analysis and recommendations for major drainage structures.

DESIGN CRITERIA

Based on anecdotal information from the Forest Service, the currently and seasonal ADT on NFSR 275 is approximately 20. The road is also closed during the winter month (generally December to March). The hydraulic design criteria are based on the criteria defined for a low standard road by the Project Development and Delivery Manual. As such, the following design criteria apply:

Culvert Design

Capacity/Stability: 25-year flood

Headwater/Depth Ratio: 0.8 for heavy debris or sediment laden streams

AOP Design: Based on HEC-26 design methodology

Low Water Crossing Design

Stability Design: 25-year

EXISTING CONDITIONS

Bob Thomas Creek The Bob Thomas watershed was severely burned in the 2011 Wallow Fire. The basin is still recovering and large debris flows continue during storm runoff events. Burned drainage basins yield higher runoff rates for normal, reoccurring storm events. This exposes the receiving channel to a change in flow condition and can destabilize the channel. The Forest Service has taken proactive measures to protect

Bob Thomas

Stone Creek the current low water crossing by installing a series of three cut off walls (made of jersey barrier) downstream of the low water crossing. The existing low water crossing goes through the stream bed.

Figure 3. Upstream end of LWC Figure 4. Downstream end of LWC

Figure 5. Upstream Channel Figure 6. Downstream Channel

Stone Creek The existing drainage at the Stone Creek crossing is two, 6’ by 8’ arch corrugated metal pipes. The inlet and outlet are projecting without headwalls or toewalls. Upstream, Stone Creek makes and abrupt turn to the inlets of the structure. This causes sediment to build up on the inside of the curve. The structures plug regularly, approximately 3-4 times a year. Debris as large as 36” diameter trees have obstructed the entrance causing sediment built up and a dam obstruction.

Figure 7. Existing Inlet Configuration Figure 8. Existing Outlet Configuration

Figure 9. Upstream Channel Figure 10. Downstream Channel

PROPOSED CONDITIONS

HYDROLOGIC ANALYSIS

The Watershed Modeling System (WMS v. 11.0) in conjunction with the Natural Resource and Conservation Service TR-55 methodology was used to estimate peak flow discharges for the 2-, 5-, 10-, 25-, 50-, 100-year events for both Bob Thomas Creek and Stone Creek. As a check, the United States Geologic Survey regional regression equations for computing peak flow in ungaged watersheds were used to estimate peak flow discharges.

2.1.1 Watershed Delineation

The watersheds were delineated using WMS A watershed was delineated using a roughly ten-meter resolution digital elevation model (DEM). The drainage areas were compared to delineations completed using the USGS web application, StreamStats. The drainage areas are shown in Table 1.

Drainage Area (sq. miles)

WMS StreamStats

Bob Thomas Creek 1.63 1.65

Stone Creek 6.35 6.38

Table 1: Drainage Areas

A detailed analysis of the hydrology throughout the project was completed. The Watershed Modeling System (WMS v. 11.0) was used to delineate drainage basins at the proposed aquatic organism crossings. The drainage basin delineations are shown in Figure 11.

Figure 11. Stone Creek and Bob Thomas Drainage Basins

The peak discharges for each drainage basin were calculated using the NRCS TR-55 methodology. The 2-, 5-, 10-, 25-, 50- and 100-year frequency storm events were developed using the NOAA Atlas 14 depth-duration-frequency values. These values are summarized in Table 2.

Duration Average Recurrence Interval (years)

2-yr. 5-yr 10-yr 25-yr 50-yr 100-yr

5 min 0.320 0.414 0.482 0.573 0.642 0.712

15 min. 0.605 0.781 0.910 1.08 1.21 1.34

1 hr. 1.01 1.30 1.52 1.80 2.02 2.24

2 hr. 1.11 1.42 1.67 2.01 2.28 2.56

3 hr. 1.19 1.50 1.76 2.12 2.41 2.71

6 hr. 1.47 1.83 2.14 2.58 2.93 3.30

12 hr. 1.78 2.21 2.56 3.07 3.46 3.89

24 hr. 1.93 2.37 2.72 3.20 3.57 3.95

Table 2. NOAA Atlas 14 – Depth-Duration-Frequency Data

The composite curve numbers for each drainage area were determined using WMS along with land use data and NRCS soil type data. The composite curve number for each basin is presented in Table 3.

Basin Composite Curve Number

Bob Thomas 61

Stone Creek 60

Table 3. Composite Curve Number

The peak discharges calculated using the TR-55 methodology are presented in Table 4.

Basin Drainage Area (sq.

miles)

Peak Discharge (cfs)

2-yr 5-yr 10-yr 25-yr 50-yr 100-yr

Bob Thomas

1.63 8 35 75 160 245 340

Stone Creek

6.35 20 80 175 370 565 795

Table 4. TR-55 Peak Discharge

Each drainage was also evaluated using the USGS regional regression equations for computing peak flow in ungagged watersheds. In development of the regional regression equations, five hydrologic flood regions in Arizona were defined based on mean basin elevation, mean annual precipitation, and soil permeability. Regression equations were developed for each of these regions based on one or more of the statistically significant variables: drainage area, mean basin elevation, and mean annual precipitation.

Both drainages fall within the Region 1 where the statistically significant variables were determined to be drainage area and mean basin elevation. The USGS also reports average standard errors of prediction for each recurrence interval in the regression regions. The upper and lower limits for each peak discharge were estimated using USGS website StreamStats and the results were compared to the peak discharge values calculated using the TR-55 methodologies. Tables 5 and 6 compare both method for determining peak discharge.

Basin Peak Discharge (cfs)

2-yr 5-yr 10-yr 25-yr 50-yr 100-yr

StreamStats 5-45 25-95 40-140 65-215 85-285 105-365

TR-55 8 35 75 160 245 340

Table 5. Bob Thomas Peak Discharge Comparison

Basin Peak Discharge (cfs)

2-yr 5-yr 10-yr 25-yr 50-yr 100-yr

StreamStats 10-110 5-220 90-322 145-490 190-645 235-820

TR-55 20 80 175 370 565 795

Table 6. Stone Creek Peak Discharge Comparison

The peak discharges calculated using the TR-55 methodologies fall within the limits of the regional regression equations. These discharges were used in the hydraulic analysis of the low water crossing at Bob Thomas Creek and the aquatic organism passage structure at Stone Creek.

HYDRAULICS ANALYSIS

BOB THOMAS CREEK LOW WATER CROSSING

As part of the proposed project, improvements to the existing low water crossing at Bob Thomas Creek are proposed. The low water crossing will be formalized with a concrete surface, cutoff walls to prevent undermining, and riprap protection to mitigate erosion upstream and downstream of the crossing.

This project will install an unvented low water crossing, for which there are no design standards as all the water must pass over the roadway. The depth of flow over the roadway at the crossing was determined for the 25-year event and riprap was designed to be stable for the 25-year event. The design considered two parameters 1) the width of the crossing measured from survey data and 2) the depth of overtopping during the 25-year flood event.

To determine the depth of overtopping, a broad crested weir equation is assumed, which implies that the water approaches critical depth as it passes over the roadway.

Q = C1LH

3/2

The parameters used for the calculation are shown below in Table 7.

Variable Name Assumed Value

25-year Flow (Q) 160

Weir Coeff. (C) 2.62

Length of weir (L) 128

Table 7. Weir equation variables

At this length, the depth of overtopping is 0.61 feet for the 25-yr event. It needs to be noted that to function as a LWC a structure should have less than 1 feet of water overtopping during the design event.

Riprap Stabilization requires riprap protection on the inlet and outlet for a low water crossing. Riprap was sized using the Hydraulic Toolbox (FHWA 2014) using the Embankment Overtopping method (see Appendix B). Riprap should be Class 3 (D50 = 12.5 inches) and be installed at a thickness of 2 feet.

STONE CREEK AQUATIC ORGANISM PASSAGE STRUCTURE

Before the analysis and design of the AOP structures, field observations at each crossing were gathered.

The observations were used as the basis of the initial sizing of the proposed structures. The observations are detailed in Table 8.

Crossing Upstream

Bankfull (in.)

Downstream Bankfull (in.)

Bed Observations

Stone Creek 6-9’ 5-8’ Sandy gravel with 8- 12” occasional rocks

Table 8. Field Observations

The aquatic organism passage design was completed using the method defined in Hydraulic Engineering Circular 26 – Culvert Design for Aquatic Organism Passage and the AOP design process in the FHWA HY-8 culvert design program.

The initial step in working through the design procedure for aquatic organism passage is to determine the peak design flood, the high passage design flow, and the low passage design flow. The peak design flow was identified as the 25-year discharge according the PDDM for low standard roads. The recommended headwater to depth ratio for a 25-year event is 0.8 for heavy debris or sediment laden streams. The high passage design flow, or the maximum discharge used for aquatic organism passage design, was identified as the 2-year discharge. The low passage design flow was identified as half of the 2-year discharge.

The proposed crossing at Stone Creek was designed such that peak discharge capacity requirements were met with a structure that allowed for placement of bed material in the culvert. The proposed embedment is 30 inches. A low flow channel is also proposed in the structure with a width of 5 feet, a depth of 6 inches, and side slopes no steeper than one vertical to four horizontal.

Table 9 represents the proposed structure size and the corresponding headwater to depth ratio for the 10-, 25-, 50-, and 100-year storms.

Crossing Proposed Culvert HW/D Ratio

10-yr 25-yr 50-yr 100-yr

Stone Creek 14’-2” (span) x 9’-10” (rise) Structural Plate

Pipe arch

0.4 0.7 0.8 1.2

Table 9. Proposed Condition Headwater to Depth Ratio

As shown in the table above, the capacity requirements for the project are exceeded with the proposed AOP structures. The proposed structures were also checked against field observations of bankfull width to ensure the width of the culvert provided at least the bankfull width.

The proposed embedment depth for the culverts was checked with the minimum embedment requirements using a standard of 20% of the culvert rise for the pipe arch and concrete box culverts. The minimum embedment depth for the structure is 24 inches.

The next step was to determine the bed material gradation both upstream and downstream of the structure. Several photos of representative bed material were taken. Those photos were used in the FHWA Hydraulic Toolbox to determine the sediment gradation upstream and downstream of the proposed crossing. The representative gradation plots are shown in Figure 12.

Figure 12. Grain Size Distribution

The next step was to determine if the proposed bed material in the structure performs similarly to the bed material in the channel cross sections. A bed material gradation was selected based on the channel bed material gradations plotted using the Hydraulic Toolbox. The gradation is shown in Table 10. The intent of the design is to conserve the bed material excavated to install the structure and supplement that material as necessary.

Percent Passing

Grain Size (in.)

D5 0.3

D16 0.5

D50 1.1

D84 1.9

D100 2.9

Table 10. Bed Material Gradation

The stability of the bed material in the proposed structure was analyzed for the designated high flow and the peak flow conditions. The shear stress on the culvert bed and in the reach cross section either need to both be above the permissible shear stress meaning the bed is mobile through all of the sections or less than the permissible shear stress meaning the bed is not mobile through any section.

The results of this analysis are shown in Table 11.

Crossing High Passage Flow (2-year) Peak Design Flow (25-year)

Permissible Shear Stress

Shear on Culvert Bed

Shear in Reach Cross Sections

Permissible Shear Stress

Shear on Culvert Bed

Shear in Reach Cross Sections

Stone Creek

0.5 0.8 1.1 0.5 3.2 3.6

Table 11. Bed Material Stability and Mobility

The final steps are to ensure the average velocity at the high passage discharge and the depth of flow at the low passage discharge are acceptable in comparison to the channel cross sections. These values are shown in Table 12.

Crossing

High Passage Flow (2-year) Low Passage Flow (25-year)

Max. Culvert Velocity (ft/s)

Max. Cross Section Velocity

(ft/s)

Max. Culvert Depth (ft)

Max. Cross Section Depth

(ft)

Stone Creek 2.3 4.3 0.8 0.4

Table 12. Velocity and Depth Comparison

The AOP analysis results can be found in Appendix A.

REFERENCES

FHWA (2007) “Project Development and Design Manual for Central Federal Lands Highway Division” Chp. 7. Federal Highway Administration (FHWA).

FHWA (2010) “HEC-26: Culvert Design for Aquatic Organism Passage.” Federal Highway Administration (FHWA).

FHWA HY-8 v. 7.5 (October 2017).

FHWA Hyrdraulic Toolbox v. 4.3 (December 2015).

APPENDIX A – HY-8 PROPOSED CONDITIONS

CULVERT SUMMARY TABLE

HIGH PASSAGE FLOW STABILITY ANALYSIS

PEAK FLOW STABILITY ANALYSIS

APPENDIX B – HYDRAULIC TOOLBOX, RIPRAP