INTRODUCTION - a123.g.akamai.neta123.g.akamai.net/7/123/11558/abc123/forestservic.download.akam… · LWM structures in Stormy A following the guidelines provided in the USBR’s

TECHNICAL MEMORANDUM

To: Steve Kolk, PE, Reclamation Matt Wilberding, Yakama Nation

From: Leif Embertson, PE – Natural Systems Design John Soden, PWS – Natural Systems Design Deb Stewart, PE – Natural Systems Design Steve Seville, PE – ICF International

Date: December 30, 2015

Re: Response to 11/25/15 USFS Letter re: Entiat River Stormy A Project Area LWM Structure Design and Post-Wolverine Fire Conditions

INTRODUCTION On October 28, 2015, USFS staff participated in a site visit with Yakama Nation Fisheries to review the proposed habitat enhancement projects that include placement of large woody material (LWM) structures in the mainstem of the Entiat River at the Stillwater Reach (RM 27) and Stormy Reach Area A (RM 20.7). Yakama Nation Fisheries are working with two separate design consultant teams, Inter-Fluve for the Stillwater Reach, and ICF/Natural Systems Design (NSD)/CH2M (through a contract with US Bureau of Reclamation) for the Stormy Reach Area A. Following the site visit on November 25, 2015, the Okanogan – Wenatchee National Forest issued a letter (Whitehall 2015) to Yakama Nation Fisheries that:

• Summarized the extent of burn severity in the upper watershed, • Provided estimates of expected peak runoff flows at three locations in the upper watershed, • Provided recommendations for increased flow estimates that could be applied to the Stillwater Reach and

Stormy Reach A LWM designs Within the letter, the USFS requested that the following questions be addressed:

• How does the design of the wood structures accommodate potentially higher volume of flows and debris? • What is the capacity (the amount of sediment and wood that would be collected) of each wood structure? • At what point will the design capacity of the wood structures become exceeded? • How does the design standards for the wood structures change given the changed condition post-fire?

NSD is currently leading the design and engineering for large woody material (LWM) structures as part of the larger ICF team that is contracted with the Bureau of Reclamation (Reclamation) for the Middle Entiat project. The NSD/ICF team worked with Inter-Fluve to develop this memorandum to summarize the impacts of post fire hydrologic conditions as described in the USFS on the current LWM design in the Stormy A project area. A draft version of this analysis was presented to the USFS on December 17, 2015. During that meeting the USFS requested additional information regarding the magnitude and extent of the 1972 Preston debris flow which have been included in this version of the memorandum. In preparation for responding the questions presented in the letter, Inter-Fluve and the ICF team reviewed the following information:

• Letter from USFS to Yakama Nation Fisheries dated November 25, 2015 (Whitehall 2015) • Wolverine Fire: FS-2500-8 Burned Area Report Summary (USFS 2015) • The Rule of Thumb runoff discharge estimation equation (Kuyumjian 2007) • Appendix B – Hydrology Data and Geographic Information Systems from the Entiat River Tributary Assessment

(USBR 2009)

RECLAMATIONENTIAT – RESPONSE TO 11/25/15 USFS LETTER - STORMY PROJECT AREA A

Response to 11/25/15 USFS Letter – Stormy Project Area A 2 12/30/2015

HYDROLOGY ANALYSIS

Pre Fire Hydrology A comprehensive pre 2015 fire hydrologic evaluation of the Entiat River basin was completed by the USBR and is described in detail in Appendix B of the Entiat River Tributary Assessment (USBR 2009). The evaluation provides return period peak flows for the 2- to 100-year events for each river mile from the mouth at the Columbia River to River Mile 32. These peak discharges have been used by Inter-Fluve and the ICF team to help evaluate habitat enhancement structure designs. For comparative purposes the following Table summarizes the pre 2015 fire hydrology as prescribed by the Entiat River Tributary Assessment at Entiat Falls, River Mile 27 (Stillwaters), and River Mile 20.7 (Stormy A).

TABLE 1 – PRE FIRE PEAK FLOWS LOCATION 2-YEAR PEAK (CFS) 25-YEAR PEAK (CFS) 100-YEAR PEAK (CFS)

Entiat Falls 2,330 4,450 5,480

River Mile 27 2,530 4,840 5,970

River Mile 20.7 2,630 5,040 6,210

River Mile 18 Ardenvoir Gage (USGS 12452800)1

2,680 5,130 6,330

1 Flows from the USGS Gage at Ardenvoir were used for hydraulic modeling within the Stormy A project reach by Reclamation’s Technical Service Center.

Entiat Falls is downstream of the confluence with the North Fork Entiat River and downstream of the 2015 severely and moderately burned area. River Mile 27 is at the upstream extent of the Stillwaters Reach habitat enhancement design projects. River Mile 20.7 is at the Stormy Reach Area A habitat enhancement project site. For design purposes the peak discharges reported for River Mile 20.7 are assumed to be equivalent to those reported for River Mile 21.

As part of the Entiat River Tributary Assessment hydrologic evaluation a basin specific basin area peak discharge scaling equation coefficient was developed (Equation 1). This equation relates the discharge at one location (gaged) to another location (ungagged).

Equation 1

Qu: Ungaged location peak flow Qg: Gage location peak flow

Ag: Area of gaged watershed AU: Area of ungaged watershed

This equation can be used within the basin to translate peak flows from one location to another and was used to estimate the peak flows at Entiat Falls for later comparison to post fire peak flow estimates at that location.

Post Fire Hydrology A post fire hydrologic evaluation is presented in November USFS letter (Whitehall 2015) and includes a number of estimates for peak flows at various burned locations in the Entiat River basin. In relation to post fire hydrology at the habitat enhancement project sites the most relevant estimates provided in the letter are the Rule of Thumb equation (Kuyumjian 2007) peak flow values for the Entiat River at Entiat Falls, reproduced in Table 2.

𝑄𝑄𝑢𝑢 = 𝑄𝑄𝑔𝑔 �𝐴𝐴𝑢𝑢𝐴𝐴𝑔𝑔�0.21



TABLE 2 – BURNED AREA PEAK FLOW ESTIMATESIMATES LOCATION RULE OF THUMB (CFS) BULKED RULE OF THUMB

(CFS, INCL. DEBRIS)

Entiat Falls1 6,232 7,790

1Hydrology per 11/25/15 USFS Letter.

To translate these burned area peak flow estimates downstream to the project areas a simple scenario was assumed. The burned area rule of thumb estimated peak flows were assumed to occur late in the spring snow melt period when the burned area slopes would be snow free but the higher elevation headwaters areas would still be contributing to the regular spring freshet. This scenario is similar to the June 1972 post-fire flood event that caused flooding and debris flows in Preston Creek. It was assumed that regular spring freshet flows could be represented by the May through June average daily discharge from at Ardenvoir gage, basin area scaled to the location of interest. It was also assumed that the burned area rule of thumb estimated peak flow would not attenuate downstream. These assumptions represent a conservative estimate for a post 2015 fire peak flow with a chance of occurring prior to burned area recovery. These post 2015 fire peak flow values are provided in Table 3 for the locations of interest.

TABLE 3 – POST FIRE PEAK FLOWS LOCATION POST FIRE PEAK (CFS) POST FIRE BULKED PEAK (CFS, INCL.

DEBRIS)

Entiat Falls 7,320 8,880

River Mile 27 7,410 8,970

River Mile 20.7 (Stormy A) 7,460 9,020

In comparison to recorded peak flows in the Entiat River these discharges are similar to the flood of record that occurred in May of 1948. Using the basin area scaling equation (Equation 1) the peak flow recorded at the Entiat gage can be translated upstream to the locations of interest. At River Mile 27 the 1948 flood of record would be estimated at 8,750 cfs and 9,120 cfs at River Mile 20.7. Considering the long period of record between all the Entiat River gages, the flood of record represents an event less frequent than the 100-year. The similarity in peak discharges estimated under post 2015 fire conditions and the translated flood of record peak indicates that these discharges are likely to represent an extreme upper limit for peak discharges at the habitat enhancement projects. Based on the proximity of Entiat Falls to the source area, multiple highly confined channel reaches, potential upstream racking and deposition locations, upstream gradient transitions, and the overall distance from the project site it is our opinion that a post-fire bulked peak flow is not applicable to the Stormy A project site and we recommend utilizing the lower post-fire peak flow of 6,850-cfs for future stability analyses.

Hydraulic Modeling for the Stormy A Project Site In preparation of the Stormy A 60% designs the NSD/ICF Team worked closely with Reclamation’s Denver Technical Service Center (TSC) to develop project scale 2D models, utilizing SRH-2D. SRH-2D is two-dimensional fixed-bed hydraulics model focused on the flow hydraulics of river systems developed by Reclamation at the Technical Service Center. The existing condition hydraulic conditions were modeled early in the design process (December 2015). Since that date the model detail has been advanced to include the design features presented in the 60% Design plans. The model domain spans roughly 5 river miles (RM 16.1-20.9) on the Entiat River, encompassing what has been deemed as the Stormy and Gray Reaches and including the area of interest at Stormy A (RM 20.7). For the purposes of modeling this 4 mile length of river Reclamation used hydrology based on the USGS gage at Ardenvoir (USGS 12452800) located at RM 18. Given this approach a 100-year flow event of 6,330-cfs was used to assess structure stability following for all



LWM structures in Stormy A following the guidelines provided in the USBR’s Large Woody Material – Risk Based Design Guidelines (RBDG) (Knutson et. al. 2014). As part of the future design process, we recommend a model run be performed representative of the post-fire peak flow of 6,850-cfs. For the purposes of this stability analysis, results are representative of 6,330-cfs with the difference in stability results between the current 100-yr flow and post-fire peak flow (8% difference in peak flows) considered negligible.

LWM STRUCTURE ANALYSIS Based on property boundaries, NSD assumes that the USFS questions are directed at the LWM structures on USFS managed-lands. NSD also assumes that the USFS is directing the questions within the November 25th letter at the larger structures that are intended to induce a geomorphic and hydraulic response and are more likely to rack sediment and debris. Based on these assumptions, NSD has focused our analysis on structures A0.5, A0.6, and A2 as shown in Figure 1 below. This area includes two deflector type 2 structures and a single apex type 1 structure.

Figure 1. Primary area of interest within USFS-managed lands in Stormy Reach Project Area A.

Description of the Apex Structure The apex type 1 structure is intended to create hard points to split flow as mid-channel jams or at the head of gravel bars. Hydraulically, the structure raises the local water surface elevation and encourages or induces scour to accentuate bend hydraulics and to promote lateral channel migration. Two-dimensional (2d) hydraulic model results indicate structure A0.5 will raise 2-year flow depths and velocities 0.5- to 1-feet and 2,-feet/second, respectively, adjacent to the structure. Biologically, the structure creates cover and adequate depths at all flows that meet juvenile (fry and parr) rearing requirements. Structurally, it is pile supported and comprised of interlaced individual logs ranging 30 to 50 feet in length. Structure A0.5 is approximately 45 feet long and 50 feet wide in plan-view.



Figure 2. Perspective view of the apex type 1 structure as represented in the 60% plans.

Description of the Deflector Structure The deflector structure is intended to create hard points along the river bank to mimic the stability of mature riparian stands and natural LWM loading. Hydraulically, the structure obstructs 25 – 50% of the channel to induce local scour and encourage lateral channel migration. 2d hydraulic model results indicate structure A0.6 and A2 will raise 2-year flow depths and velocities 1- to 2-feet and 2,-feet/second, respectively, adjacent to each structure. Biologically, the structure creates cover and adequate depths at all flows that meet juvenile (fry and parr) rearing requirements. Structurally, it is pile supported and comprised of interlaced individual logs ranging 30 to 50 feet in length. Structures A0.6 and A2 are approximately 40 feet long and 70 feet wide in plan-view.

Figure 3. Perspective view of the deflector type 2 structures as represented in the 60% plans.

The attached Figure shows valley-wide cross sections at each of the three LWM structures with water surfaces from the hydraulic model for the 10- and 100-year peak flows (from the Ardenvoir gage). Structures are represented as scaled



blocks within the channel to conservatively estimate the area occupied by the front face of each jam type. Structures A0.6, A0.5, and A2 obstruct approximately 44%, 43%, and 41%, respectively, of the 100-year flow area at their respective cross sections.

USFS QUESTIONS FROM 11/25/15

Question 1: How does the design of the wood structures accommodate potentially higher volume of flows and debris?

The LWM design standards used for this project are guided by Reclamation’s 2014 RBDG. The project team followed each of the design steps within the RBDG manual which evaluated watershed-level, reach-level, and site-level characteristics that informed structure design and placement. This assessment also evaluated the public use, public safety risk, and site specific hydraulics to determine the minimum design criteria for ensuring stability of the proposed LWM structures. Through this design process, the RBDG recommended a stability design flow at the 25-year event. Although the RBDG recommended the 25-year flow as the minimum design flow criteria, the 100-year design flow was applied in order to meet recent legislation in Washington exempting landowners from liability for the LWM structures on their property if the structures are designed for the 100-year event. The use of the 100-year event resulted in increased Factors of Safety criteria for the sliding and buoyancy (the primary design criteria that are required to be met by the RBDG) as shown in Table 4.

TABLE 4 – RISK BASED DESIGN CRITERIA SUMMARY FOR THE MIDDLE ENTIAT PROJECT – AS APPLIED.

STABILITY DESIGN FLOW CRITERIA

FOS_SLIDING FOS_BUOYANCY FOS_ROTATION FOS_OVERTURNING

100-year (6,330 cfs) 1.5 1.75 N/A1 1 Structure failure by rotation or overturning are not applicable to the LWM structures designed for the Middle Entiat because they are:

(1) mid-channel structures, and/or (2) structures not embedded in the bank.

Flow Discussion

Based on the hydrology analysis discussed above an estimated peak flow during a storm event on the Entiat River at the Stormy A site (RM 20.7) ranges between 6,850 (clear) to 8,410 (bulked) cfs. The current design applies a 100-year flow event of 6,330 cfs based on the USGS gage near Ardenvoir (USGS 12452800) at RM 18. As discussed above, the peak discharges estimated under post 2015 fire conditions are likely to represent an extreme upper limit for peak discharges at the habitat enhancement projects and a less frequent event than the 100-year flow. During design process planned for 2016, post-fire hydrology can be explicitly incorporated into the design process to ensure proposed structures are able to withstand the expected increase in peak flows. At this point, we do not expect an 8% increase in peak flows to significantly affect the conclusions presented in this analysis.

Debris Discussion

The assessment of potential debris loading considers the proximity of the input to the project site and the likely type of material that will be ultimately mobilized to the project site. The Wolverine Fire occurred approximately 12 miles upstream of the Stormy A Project site. We assume that the locations of potential debris flows are therefore upstream of Entiat Falls and upstream of multiple reaches that include severe channel constriction. Debris flows that enter the Entiat River from the adjacent valley tributaries will likely consist of water, angular gravels, silts, and sands, and large wood. The mobilization of angular gravels once they enter the Entiat River is driven by the scale of the event, channel slope, channel capacity, and event hydrology.



The 1972 Preston Creek slide provides a reference event for the behavior of potential debris flows that could occur as the result of the Wolverine Fire. Preston Creek is a tributary to the Entiat River located at RM 23.2, downstream of the Wolverine Fire but upstream of Stormy Project Area A. In 1970 25% of the Entiat watershed burned killing “most vegetation, including large trees” which included the majority of the watershed contributing to Preston Creek (Figure 4) (USFS 1972).

In the spring of 1972 the Entiat basin had one of the three highest recorded April snowpacks (as of 2004), and on June 9 – 10, 1972 heavy rain resulted in severe flooding and erosion (Chelan County Conservation District 2004). During the most intense rainfall on the early morning of June 10th, 2.7 inches of rain was recorded an elevation 4,400 feet over a 2 hour period (USFS 1972). This resulted in mud and debris avalanches in Preston Creek, Brennegan Creek, McCree Creek, Fox Creek, and Crow Creek. The Preston Creek debris flow killed four people and destroyed five residences near the town of Brief. The slide “carried boulders, logs, and debris into the Entiat River, causing a temporary blockage to stream flow, which formed an impoundment… the blockage failed gradually and flat bottomland immediately downstream took the brunt of the surge. This minimized the crest farther downstream decreasing damage to private property” (USFS 1972). In 2009 a geomorphic assessment was completed for the Tyee Restoration Project which included a ground reconnaissance of the Entiat River immediately downstream of the Preston Creek confluence (upstream of the Tyee Project site). This reconnaissance noted large gravel bars consisting of angular material similar to the materials described in the USFS Flood Damage Report extending downstream from Preston Creek to immediately above the wide valley bottom that describes the Tyee Project site (Figure 5).

Figure 4. Entiat Fire History Map.



This mapping shows that angular gravels were mobilized approximately 3,400 linear feet downstream of the event (CH2M HILL 2010). This indicates that the debris consisting of wood and gravels were moved through the reach at the Preston Creek confluence (1.1% slope, geologically confined) and generally settled in Reach 3A which is characterized as an unconfined wide valley with a slope of 0.3% (Bountry et al 2009). Based on the magnitude and resulting limited mobilization of angular gravels downstream of the Preston Creek debris event, we assume that little to no angular gravels or a debris torrent will be mobilized from the Wolverine Fire tributaries to the project site (a distance of approximately 12 miles). It is likely that silts and sands will be mobilized to the project site through suspension in the water column. In addition, based on the proximity of Entiat Falls to the source area, multiple highly confined and unconfined channel reaches, potential upstream wood racking locations, upstream gradient transitions, and the overall distance from the project site it is also unlikely that significant amounts of intact large wood will be mobilized from the event location to the project site. As wood is mobilized it will likely be broken into smaller pieces and ultimately arrive at the project site as small 1- to 6-ft to medium sized 6- to 12-ft pieces. This is similar to the size of material that is currently mobilized into the project site from upstream reaches. The only large wood (>20’) observed within the project reach are those pieces that are recruited within the reach itself. We therefore assume that assume that debris that would be transported from the Wolverine Fire to the project site would consist of smaller woody material, flotsam, and suspended sediments. The design of both the deflector type 2 and apex type 1 structures is intended to allow the racking and shedding of woody material that is transported through the reach. The structures themselves are not intended to create channel spanning conditions, and given the smaller size of woody material likely transported to the project site at any given event, it is unlikely that material will accumulate in quantities to create a channel spanning debris jam; however, this worst case scenario is examined below in response to question 2.

Figure 5. Location and extend of the 1972 Preston Creek debris event (image Google Earth 2015).



Question 2: What is the capacity (the amount of sediment and wood that would be collected) on each wood structure?

Question 3: At what point will the design capacity of the wood structures become exceeded?

The amount of sediment and wood that could accumulate on structures A0.5, A0.6, and A2 from the Wolverine Fire will vary based on conditions at the time of a debris flow. The likely type of material associated with a debris flow is discussed above. For the purposes of this analysis, we assumed worst case scenario for structure stability would entail accumulation of enough debris to create channel spanning conditions and complete blockage of the main channel. However, it is our opinion that that the likelihood of a channel spanning condition to be created and persist in response to a post-fire debris event is extremely low given the proximity of Entiat Falls to the source area, multiple highly confined channel reaches, potential upstream racking locations, upstream gradient transitions, and the overall distance from source location to the project site. To answer questions 2 and 3, two stability analysis scenarios were completed to determine the point at which the design capacity of structures A0.5, A0.6, and A2 may become exceeded as a result of additional debris accumulation. The attached Figure provides a cross-section view at each of the three structure locations. Each structure is represented by a solid block that represents the dimensions of the front face of the structure. In scenario 1, the LWM stability calculations were modified to assume that the width of the structure along the upstream face will increase as a result of debris accumulation in 10- to 25-ft increments. This increase in width corresponds to an increase in cross-sectional area that hydraulic forces will act on the structure. The maximum width evaluated was set to the current channel width, thus assuming full channel blockage as a result of additional debris accumulation. In scenario 2, it was assumed that as the structure cross-sectional area increases, the flow area decreases commensurately and all flow for the 100-year event will pass through the constricted channel section. This increase in constriction increases drag forces on the structure resulting in increased lateral sliding forces and subsequent decreased in structure stability. Scenario 2 represents a “worst-case” scenario with a low probability of occurring because channel blocking event, significant portions of the total flow will be routed through adjacent side channels and overland on the floodplain, scour will occur within the channel, and erosion/channel avulsion around the structure could occur, all of which, will increase the total flow area and reduce the velocity in the channel acting on the structure. The two scenarios were developed for the purposes of analyzing the effect of debris accumulation on structure stability and buoyance and sliding factors of safety (FOS). The first scenario calculates FOS values based on an incremental increase in structure width until the entire channel is blocked by the structure and accumulated debris. The drag coefficient in the drag force equation was also maximized as a worst case scenario. Table 5 provides the FOS results of this scenario.



TABLE 5 – SCENARIO 1 BUOYANCY AND SLIDING FACTOR OF SAFETY (FOS) FOR 3 UPSTREAM STRUCTURES IN STORMY A REACH

ASSUMING VARYING EXPANSION OF STRUCTURE WIDTH AS A RESULT OF DEBRIS ACCUMULATION.

ELJ A0.5 – APEX 1 ELJ A0.6 – DEFLECTOR 2 ELJ A2 – DEFLECTOR 2

Structure Width

(ft.)

FOS Buoyancy

FOS Sliding

Structure Width

(ft.)

FOS Buoyancy

FOS Sliding

Structure Width

(ft.)

FOS Buoyancy

FOS Sliding

501 4.1 3.2 70 3.1 2.7 70 3.2 2.8

75 4.0 2.3 100 3.0 2.1 100 3.1 2.2

100 3.9 1.9 125 2.9 1.9 125 3.1 1.9

1152 3.8 1.7 150 2.8 1.7 150 3.0 1.7

1602 2.8 1.6 1702 3.0 1.5 1 First row provides FOS values for the original structure width without debris accumulation. Widths are increased incrementally until

debris accumulation creates a complete channel spanning structure. 2Width for full channel spanning debris blockage at the structure location. The results in Table 5 show a decrease in the buoyancy and sliding FOS values as structure width increases with increased debris accumulation. At the worst case full channel spanning condition, the risk-based design criteria FOS for buoyance and sliding are not exceeded suggesting that the structures as designed are adequate to withstand debris flow accumulations at current streamflow velocities from the Wolverine Fire for these criteria. The second scenario calculates FOS values the same as the first scenario but also assumes that velocities will increase as the structure width increases and the flow area is constricted. Increased velocity adjacent to the structure further increases the drag force acting on the structure as compared with scenario 1. The continuity equation was used to calculate increases in velocity as flow area decreases. A maximum velocity passing through the constriction of 15 ft. /sec was assumed for this scenario. This is based on engineer’s judgement as the maximum instantaneous velocity that might occur within a constriction as any velocity in excess of this amount would cause scour and/or bank erosion increasing the flow area and decreasing velocities. If the continuity equation resulted in a velocity greater than 15 ft. /sec as the constricted area decreases to zero, 15 ft. /sec was used. Table 3 provides the FOS results of this scenario.

TABLE 6 – SCENARIO 2 BUOYANCY AND SLIDING FACTOR OF SAFETY (FOS) FOR 3 UPSTREAM STRUCTURES IN STORMY A REACH ASSUMING VARYING EXPANSION OF STRUCTURE WIDTH AS A RESULT OF DEBRIS ACCUMULATION.

ELJ A0.5 – APEX 1 ELJ A0.6 – DEFLECTOR 2 ELJ A2 – DEFLECTOR 2

Structure Width

(ft.)

FOS Buoyancy

FOS Sliding

Structure Width

(ft.)

FOS Buoyancy

FOS Sliding

Structure Width

(ft.)

FOS Buoyancy

FOS Sliding

501 4.1 3.2 70 3.1 2.7 70 3.2 2.8

75 4.0 1.4 100 3.0 1.6 100 3.1 1.5

100 3.9 0.9 125 2.9 0.7 125 3.1 0.9

1152 3.8 0.9 150 2.8 0.7 150 3.0 0.6

1602 2.8 0.6 1702 3.0 0.6 1 First row provides FOS values for the original structure width without debris accumulation. Widths are increased incrementally until

debris accumulation creates a complete channel spanning structure. 2Width for full channel spanning debris blockage at the structure location. The results in Table 6 show a decrease in the buoyancy and sliding FOS values as structure width and velocity increase with increased debris accumulation. For this scenario, the FOS criteria of 1.25 for sliding is exceeded for structure A0.5



as the structure width increases from 75ft to 100ft (65% and 87% constriction, respectively). For structure A0.6, the sliding FOS is exceeded as the structure width increases from 100ft to 125ft (63% and 78%, respectively). For structure A2, it is also exceeded as the structure width increases from 100ft to 125ft (59% and 74%, respectively). These results suggest that stability may become comprised when debris accumulation approaches 2-3 time constructed structure width and represent a worst case scenario of hydraulics acting on the structures assuming that all flow passes through the constriction adjacent to the structure. This may represent a condition before flow backwaters on the structures and finds alternate paths over the floodplain and in adjacent side channels. Related to increases in post-fire peak flows the 8% increase flow is not expected to significantly affect structure stability or capacity as indicated by the high FOS for buoyancy and sliding during the as-built condition (row 1 of tables 5 and 6)

Question 4: How does the design standards for the wood structures change given the changed condition post-fire?

The design standards for the wood structures will continue to follow the standards set in Reclamation’s RBDG. Given the concern related to post-fire conditions (increased flow and debris), the increased forces from higher flows and debris racking can be explicitly accounted for in the design process for structure A0.6, A0.5, and A2. If during the design process, explicit analysis of post-fire condition identifies a particular risk to failure (safety factor < 1.0) additional measures can be incorporated into the design of these structures. Possible modifications to the structure designs could include increasing pile diameter, increasing the number of piles, adding alluvium or boulder ballast, or reducing the amount of obstruction each structure is causing. All modifications to the structure designs will be coordinated with Reclamation, USFS and Yakima Nation. At this point, the analysis performed as part of this assessment suggests no modifications are required for the proposed wood structures given slight increase in post-fire peak flows and low likelihood of a channel blocking debris jam to occur.



REFERENCES Bountry, J., Godaire, J., and K. Russell. 2009. Entiat Tributary Assessment. Chelan County, Washington. U.S. Department

of Interior, Bureau of Reclamation, Technical Service Center, Denver, CO. CH2M HILL. 2010. Geomorphic Assessment: Reach 3A (Preston and Tyee Reaches), Entiat River, Washington. Technical

Memorandum. Prepared for the US Bureau of Reclamation. April 1o. 69 p. Chelan Country Conservation District. 2004. Entiat Water Resource Inventory Area (WRIA) 46 Management Plan:

Prepared for the Entiat WRIA Planning Unit by the Chelan County Conservation District; 318 pp. plus appendices. Submitted pursuant to Washington Watershed Planning Act, Chapter 173-546. www.cascadiacd.org

M. Knutson, J. Fealko. 2014. Large Woody Material - Risk Based Design Guidelines. U.S. Department of the Interior Bureau of Reclamation Pacific Northwest Region & Technical Services. Boise, Idaho. September 2014. Online: http://www.usbr.gov/pn//fcrps/documents/lwm.pdf

Kuyumjian, Greg. 2007. Rule of Thumb by Kuyumjian. U.S. Department of Agriculture Forest Service. Online: http://forest.moscowfsl.wsu.edu/BAERTOOLS/ROADTRT/Peakflow/Rule_Thumb/ Accessed December 11, 2015.

United States Bureau of Reclamation (USBR). 2009. Entiat Tributary Assessment: Chelan County, Washington. Bureau of Reclamation Technical Services Center. Denver, CO., Online: http://www.usbr.gov/pn/fcrps/habitat/projects/uppercolumbia/reports/entiat/tribassmt/entiattribassmt.pdf Accessed December 11, 2015.

United States Forest Service (USFS). 2015. WOLVERINE FIRE: FS-2500-8 Burned Area Report Summary. U.S. Department of Agriculture Forest Service. Online: http://centralwashingtonfirerecovery.info/wp-content/uploads/2015/10/WolverineFire2500-8Summary.pdf Accessed December 11, 2015.

USFS. 1972. Flood Damage Report. Entiat Ranger District, Wenatchee National Forest. June 9-10, 1972. 88p. Whitehall, Randy. 2015. Letter to Chris Clemons and Matt Wilberding of the Yakima Nation Fisheries, Yakama Tribe. Dated

November 25, 2015. File Code 2670. Okanogan – Wenatchee National Forest, U.S. Department of Agriculture Forest Service.

http://www.usbr.gov/pn/fcrps/documents/lwm.pdf

http://forest.moscowfsl.wsu.edu/BAERTOOLS/ROADTRT/Peakflow/Rule_Thumb/

http://www.usbr.gov/pn/fcrps/habitat/projects/uppercolumbia/reports/entiat/tribassmt/entiattribassmt.pdf

http://centralwashingtonfirerecovery.info/wp-content/uploads/2015/10/WolverineFire2500-8Summary.pdf

http://centralwashingtonfirerecovery.info/wp-content/uploads/2015/10/WolverineFire2500-8Summary.pdf

Stillwaters Reach - Entiat River, WA - Yakama Tribe 12/15/2015 5:43 PM

Entiat River, WA ‐ Historical and Post Fire Peak Flow Hydrology Summary

Historical Events ‐ Basin Area Scaling Peak Flow Transfer

Peak Flow Transfer Equation per USBR 2009, Appendix B, Equation B‐2, page B‐10

Event Date and Gage

Ungaged

Drainage Area,

A u (sq. mi)

"Gage" Drainage

Area, A g (sq. mi)

"Gage" Peak Flow

Value, Q g (cfs)

Ungaged Peak

Flow Value, Q u

(cfs)

June '72, 12452800 ARDENVOIR 154 203 6,430 6,070

May '48, 12453000 ENTIAT 154 419 10,800 8,760

June '99, 12452990 ENTIAT 154 419 5,600 4,540

Gaged area from the USGS gage data, ungaged area at River Mile 27

Fire Peak Flow Hydrology ‐ Rule of Thumb Event during Spring Freshet

Post Fire Downstream Peak Flow Equation Developed by Inter‐Fluve for this project analysis

Rule of Thumb, QROT (cfs) 6,232 Ardenvoir gage spring freshet

Bulked Rule of Thumb, QROTB (cfs) 7,790 May ‐ June Avg. 1250 (cfs)

Location

Avg. Spring

Freshet, Qfreshet

(cfs)

Post Fire Peak, QPF

(cfs)

Bulked Post Fire

Peak, QPFB (cfs)

Entiat Falls 1090 6,780 8,340

River Mile 27 1180 6,820 8,380

River Mile 20.7 1230 6,850 8,410

Flows Notes

Monthly Statistics from the USGS 12452800 ENTIAT RIVER NEAR ARDENVOIR, WA

Rule of thumb peak flows from the USFS November 2015 Letter

Values from the analysis equations rounded up to the nearest 10 cfs.

References

Entiat Tributary Assessment

USFS November 2015 Letter

United States Bureau of Reclamation (USBR). 2009. Entiat Tributary Assessment: Chelan County,

Washington. Bureau of Reclamation Technical Services Center. Denver, CO., online at


Whitehall, Randy. 2015. Letter to Chris Clemons and Matt Wilderding of the Yakima Nation Fisheries, Yakama

Tribe. Dated November 25,2015. File Code 2670. Okanogan – Wenatchee National Forest, U.S. Department of

Agriculture Forest Service.

.

\\Wally\Terra\Client Files\U-Z\UpperStillwatersDesign#3SignalPeak_150216\Design & Analysis\Hydrology\Peak_Flow_Analysis_BasinScale.xlsx

Inter-Fluve, Inc. Tab: Hist_Events-FireHydro 1/1

Stillwaters Reach - Entiat River, WA - Yakama Tribe 12/15/2015 5:42 PM

Entiat River, WA ‐ Stillwaters Reach to Entiat Falls ‐ Peak Flow Hydrology Summary

Basin Area Scaling Peak Flow Transfer

Peak Flow Transfer Equation per USBR 2009, Appendix B, Equation B‐2, page B‐10

Return Period (yr)

Ungaged

Drainage Area,

A u (sq. mi)

"Gage" Drainage

Area, A g (sq. mi)

"Gage" Peak Flow

Value, Q g (cfs)

Ungaged Peak

Flow Value, Q u

(cfs)

2 102 154 2,530 2,330

5 102 154 3,460 3,180

10 102 154 4,070 3,740

25 102 154 4,840 4,450

50 102 154 5,400 4,960

100 102 154 5,970 5,480

Input Data

Basin drainage area

From the USFS BAER report (at Entiat Falls) and the USBR 2009 Tributary Assessment (at River Mile 27).

Peak Flows

"Gaged" Peak Flow from USBR 2009, Appendix B, Table B‐7 | River Mile 27

Note "Gage" refers to a location of predetermined hydrology.

Output Data

Peak Flow Values

Value from the analysis equations rounded up to the nearest 10 cfs.

References

Entiat Tributary Assessment

United States Bureau of Reclamation (USBR). 2009. Entiat Tributary Assessment: Chelan County,

Washington. Bureau of Reclamation Technical Services Center. Denver, CO., online at


.

\\Wally\Terra\Client Files\U-Z\UpperStillwatersDesign#3SignalPeak_150216\Design & Analysis\Hydrology\Peak_Flow_Analysis_BasinScale.xlsx

Inter-Fluve, Inc. Tab: RM27-EntiatFalls 1/1

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Documents

INTRODUCTION - a123.g.akamai.neta123.g.akamai.net/7/123/11558/abc123/forestservic.download.akam… · LWM structures in Stormy A following the guidelines provided in the USBR’s