Alaganik Slough Boat Ramp Hydrodynamic Report 2010-0210.pdf

Alaganik Slough Boat Ramp Replacement

HYDRODYNAMIC MODEL STUDY

ALAGANIK SLOUGH, ALASKA

Submitted with 60% Working Design Documents

February 10, 2010

Prepared for:

Chugach National Forest US Forest Service 3301 C Street, Suite 300 Anchorage, AK 99503

Prepared by:

Tetra Tech EC, Inc.

19803 North Creek Parkway Bothel, WA 98011 Task Order No. AG-0120-D-09-0020

US Forest Service, Chugach National Park Alaganik Slough Boat Ramp Design

Alaganik Slough Boat Ramp Hydrodynamic Model Study

Page i

TABLE OF CONTENTS

1.0 INTRODUCTION

1.1 Project Description

1.2 Project Location

2.0 ALAGANIK SLOUGH SITE CONDITIONS

2.1 Site Survey

2.2 Water Surface Elevations

2.3 Current Velocity Data

4.0 ANALYSIS APPROACH

4.1 Model System

4.2 Modeling Approach

4.2.1 Model Mesh

4.2.2 Model Parameters and Boundary Conditions

4.3.3 Verification

4.3.4 Shoaling/Erosion Characteristics

5.0 RESULTS

5.1 Existing Conditions

5.2 Proposed Boat Ramp Location

5.3 Water Elevation Estimates

5.3.1 50-Year High Water Elevation

5.3.2 6-Month Low Water Elevation

5.4 Shear Stress Comparison

6.0 CONCLUSIONS

7.0 REFERENCES

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LIST OF FIGURES

Figure 1 – Project Location Maps Figure 2 – Alaganik Slough Site Survey Figure 3 – Water Elevation and Velocity Sample Locations Figure 4 – Alaganik Slough Model Domain Figure 5 – Finite Element Mesh near Alaganik Slough Boat Ramp Figure 6 – Tidal Station TWC1867, Peter Dahl Slough Figure 7 – Alaganik Slough Model Tidal Boundary Conditions Figure 8 – 50-year High Water Elevation Boundary Condition Figure 9 – 6-month Low Water Elevation Boundary Condition Figure 10 – Predicted vs. Measured Water Surface Elevations Figure 11 – Predicted vs. Measured Water Velocities Figure 12 – Flow Distribution at High Water Elevation = 51.4 ft Figure 13 – Flow Distribution at Low Water Elevation = 47.7 ft Figure 14 – Flow Distribution at High Water Elevation = 50.3 ft Figure 15 – Flow Distribution at Low Water Elevation = 47.7 ft Figure 16 – Flow Conditions at 50-year High Tide (Water Elevation = 56.4 ft) Figure 17 – Flow Conditions at 50-year Flow Condition (Water Elevation = 56.3 ft) Figure 18 – Copper River Drainage Basin (NOAA) Figure 19 – Alaganik Slough Connection with the Copper River (USFS, 2009).. 19 Figure 20 – Annual Peak Flows (USGS, 2009) Figure 21 – Frequency of Occurrence of Copper River Flows Figure 22 – Flow Conditions at 6-Month Low Tide (Water Elevation = 47.5 ft) ... 21 Figure 23 – Existing Conditions Shear Stress Figure 24 – Post Project Shear Stress

1.0 INTRODUCTION

1.1 Project Description

The US Forest Service contracted Tetra Tech EC, Inc. to analyze various options for improving the existing boat ramp facility located at the Alaganik Slough. The Alaganik Slough boat ramp is an approximately 150-foot long single lane ramp that is currently difficult to use during low tide conditions due to insufficient water depths located at the toe of the ramp. The purpose and scope of this hydraulic analysis is as follows.

(i) Produce a hydraulic model of the site using HECRAS (or other approved modeling software) from at least 500 feet downstream to 500 feet upstream of the proposed location of the boat ramp. A model for existing conditions and proposed development will be developed. The model will be calibrated with estimated discharge information obtained from regional curves and/or existing USGS data.

Incorporate tide level fluctuations into the model, including extreme high and low tide events. Cross sections will be a maximum of 1 channel width apart as required for modeling.

(ii) Provide the model and a synopsis of hydrologic and hydraulic conditions for both the existing conditions and the proposed site development. This report will include the following:

a) Evaluation of the effects of high and low streamflows and tide levels on the integrity and functionality of the site.

b) Evaluation of the sediment transport regime, and evaluation of deposition and/or scour at the site.

c) Evaluation of winter ice conditions and the impacts of ice at the site.

d) Evaluation of local channel and bank conditions at the site, including bank erosion from human uses.

1.2 Project Location

Alaganik Slough boat ramp is located approximately 20 miles southeast of Cordova, AK (Figure 1), within the Copper River Delta. The area contains numerous inter-tidal sloughs and braided glacial rivers and streams which drain runoff from nearby mountains and glaciers. The surrounding environment consists of low-lying marshlands and shallow ponds with stands of alder and spruce trees in higher areas.

Figure 1 – Project Location Maps

2.0 ALAGANIK SLOUGH SITE CONDITIONS

Site investigation activities were conducted from September 22nd – 25th, 2009. These activities included a pre-construction survey of the Alaganik Slough Boat Ramp area and collection of water elevation velocity data. The following sections detail the data collected during these investigations.

2.1 Site Survey

The survey results are shown below in Figure 2. All elevations are based on an ellipsoidal height of 28.42 meters at PID UW 8134 located at the Merle K Smith Airport.

The survey found that this portion of the Alaganik Slough has an approximate bank elevation of +56 feet. It also shows that the existing boat ramp becomes unusable at water elevations of less than +48 feet (ramp’s toe location).

Figure 2 – Alaganik Slough Site Survey

2.2 Water Surface Elevations

The water surface elevation at the Alaganik Slough Boat Ramp is influenced by both tidal fluctuations and river flow conditions. Tides propagate upstream from the Bay of Alaska, while flow conditions are influenced by glacial melt, rain and snowfall. Data, however, is limited within the project vicinity. Therefore it was imperative to collect both water elevation and velocity data during the site investigation studies to allow comparison with the model’s results. Table 1 shows the snapshot of water elevations collected. Figure 3 shows the water elevation sample location relative to the project area.

Table 1 – Water Elevation Data

Water Elevation Data Local Monument CORD UW8134

Date Time Water Elevation

Sept 23, 2009 10:12 AM 47.75 Sept 23, 2009 11:11 AM 47.60 Sept 23, 2009 01:27 PM 47.44 Sept 23, 2009 03:20 PM 48.00 Sept 23, 2009 05:22 PM 50.60 Sept 24, 2009 08:30 AM 47.05 Sept 24, 2009 12:22 PM 46.25 Sept 24, 2009 01:17 PM 46.20 Sept 24, 2009 02:33 PM 46.20 Sept 24, 2009 04:30 PM 47.10 Sept 24, 2009 05:44 PM 48.60 Sept 25, 2009 09:15 AM 48.10 Sept 25, 2009 02:00 PM 47.45

2.3 Current Velocity Data

Current velocity data were also measured at the Alaganik Slough boat ramp concurrently with the site survey. Velocity data were measured at approximately 60% of the water column depth using a Marsh McBirney Flo-Mate Model 2000. The data are shown below in Table 2. All station locations are shown in Figure 3

Table 2 – Current Velocity Data Current Velocity Data

NAD27, UTM Zone 06, Feet

Pt Name X Y Velocity (ft/sec) Time

Velocity 1 1947499.419 21981979.03 3.5 9/25/2009 9:11 Velocity 2 1947664.841 21982049.4 2.4 9/25/2009 9:36 Velocity 3 1947658.351 21982070.44 2.5 9/25/2009 9:39 Velocity 4 1947637.886 21982110.19 3.4 9/25/2009 9:45 Velocity 5 1947860.458 21982400.57 3.5 9/25/2009 10:14 Velocity 6 1947818.745 21982424.6 2.3 9/25/2009 10:18 Velocity 7 1947779.245 21982441.86 2.5 9/25/2009 10:22 Velocity 8 1947918.236 21982584.23 1.6 9/25/2009 10:26 Velocity 9 1947950.249 21982558.65 1.7 9/25/2009 10:28

Velocity 10 1948004.618 21982498.92 2.3 9/25/2009 10:35 Velocity 11 1948154.5 21982566.44 2.5 9/25/2009 10:39 Velocity 12 1948133.142 21982596.21 2.2 9/25/2009 10:42

Figure 3 – Water Elevation and Velocity Sample Locations

4.0 ANALYSIS APPROACH

4.1 Model System

This study is applying the Surfacewater Modeling System (SMS), developed by the Environmental Modeling Research Center (ERML, 2005). SMS is a system of pre- and post processors that support data entry, editing, mesh generation and visualization of numerical simulation results in an efficient and convenient Windows-based environment.

SMS includes a range of numerical engines for the simulation of hydrodynamic, water quality, sediment transport, and wave transformation. This study specifically is using the two-dimensional finite element hydrodynamic (RMA2 WES) model developed by the Corps of Engineers (Donnell et al., 2005a & b). This model in various versions has been widely applied by the Corps and other modeling groups for nearly 30 years.

RMA2 WES is a two-dimensional, horizontal flow, depth-averaged, free surface, finite element program for solving hydrodynamic problems. RMA2 can perform both steady state and transient solutions for water surface elevations and flow velocities at nodal points in a finite element mesh representing water bodies such as rivers, harbors, and estuaries.

4.2 Modeling Approach

The model simulation of the hydrodynamic and sediment transport characteristics of the boat ramp and adjacent waters consists of the following steps:

1. Develop a model mesh that represents the three-dimensional characteristics of the Alaganik Slough from the Gulf of Alaska to a point sufficiently upstream of the site to avoid boundary condition influences.

2. Introduce the tidal boundary conditions and dynamically run the hydrodynamic model. Progressively adjust the model parameters to verify the model results.

Water level variations and currents recorded during the September 2009 field survey are being used in this model adjustment step.

3. Develop the set of tide and stream flow boundary conditions appropriate for the analysis.

4. Run the existing conditions model simulations for high and low tides and peak stream flows.

5. Review the model flow patterns and current magnitudes in the vicinity of the boat ramp and develop options for the new boat ramp location and alignment.

6. Run the developed conditions model simulations for the ramp design options and evaluate current patterns and magnitudes, as well as bed shear stress conditions (principal driver of sediment transport) to determine the best option and refine if appropriate.

4.2.1 Model Mesh

The model domain covers an approximately 33,500 ft segment of the lower Alaganik Slough between the boat ramp site and the Gulf of Alaska. The mesh extends upstream about 14,500 ft upstream of the ramp site. USGS Electronic Quadrangle Charts were pieced together to form the background image seen in Figure 4. This composite base map was imported into SMS and its coordinates were converted to NAD 27 UTM Zone 6 coordinate system used in the site survey. A scatter set of elevations from the site survey was created and was used to develop the detailed stream bottom detail on the ramp area. Figure 5 shows the finite element mesh near the boat ramp site.

NORTHEAST MESH

BOUNDARY

BOAT RAMP

LOCATION

SOUTHEAST MESH

BOUNDARY

Figure 4 – Alaganik Slough Model Domain

Figure 5 – Finite Element Mesh near Alaganik Slough Boat Ramp

The finite element system calculates the hydrodynamic parameters at the nodes or corners of triangular and quadrilateral units called elements. The finite element mesh is therefore the backbone of the computational scheme. The finite element scheme allows a great deal of flexibility in setting up the model mesh. It can easily represent even very complex water body configurations. Small elements are used where solution accuracy is important and/or a high degree of spatial resolution is required. The vicinity of the boat ramp therefore had a high density of small elements. Larger elements are used in channel segments that are separated by relatively large distances from the boat ramp.

This helps reduce the size of the model simulations and increase the speed of the model runs. The Alaganik Slough model mesh contains 922 elements.

4.2.2 Model Parameters and Boundary Conditions

Boundary conditions are constraints applied along the flow boundaries that are necessary to drive the model throughout the simulation. Boundary conditions are typically physical processes such as the water level changes of the tide or current velocities at the edges of the model that drive the circulation and water quality processes within the model domain.

The boundary conditions needed to drive the model are tidal elevations located at the mouth of Alaganik Slough at the Gulf of Alaska and an upstream flow condition located as shown in Figure 4.

4.2.2.1 Tidal Boundary Condition

The National Oceanic and Atmospheric Administration (NOAA) provides tidal predictions at numerous tide stations throughout the US. A limited number of stations exist within the vicinity of the project area. Subordinate tidal station TWC1867, located at the mouth of the Peter Dahl Slough, is the closest and is shown in Figure 6. The Pete Dahl Slough tide station predictions for September 20th – October 3rd, 2009 are shown in Figure 7.

Calculation 3954-01 documents the model input parameters in more detail.

Figure 6 – Tidal Station TWC1867, Peter Dahl Slough

Pete Dahl Slough Tide Prediction

September 20th - October 3rd, 2009

-4

-2

0 50 100 150 200 250 300 350

Time (Hrs)

W at er E le va tio n

(ft , M

LL

W

Figure 7 – Alaganik Slough Model Tidal Boundary Conditions

4.2.2.2 Upstream Flow Boundary Condition

Again, the USGS provides stream flow data for numerous rivers throughout the US.

However, no data is available for the Alaganik Slough. Therefore, the upstream flow boundary condition was determined by comparing the model’s results with the snapshot of current velocity and water elevation data that was obtained during the site investigations on September 23rd-25th, 2009. Calculation 3954-04 determined that an upstream flow of 3,000 ft3/s correlates reasonably well with the site data.

4.2.2.3 Water Elevation Estimates

The design requirements call for estimation of the high water elevation associated with an event that has a Mean Recurrence Interval (MRI) of 50 years (average annual frequency of occurrence of 0.02). The requirements also call for the estimation of the low water elevation associated with a low tide with a 6-month MRI. The high water value has the potential to be caused by either extreme tidal conditions or fluvial flooding conditions on the Copper River. Low water conditions are specified in the design requirements as tidally controlled. Figure 8 and Figure 9 depict the tidal boundary conditions utilized to estimate the 50-year high water elevation and 6-month low water elevations at the Alaganik Slough boat ramp. Calculation 3954-03 details the methodology, assumptions and procedures used.

-4

-2

0 50 100 150 200 250 300 350

Time (Hrs)

W at er E le va tio n

(ft , M

LL

W

Pete Dahl Slough Tide Prediction 50-Year Tide Estimate

Figure 8 – 50-year High Water Elevation Boundary Condition

-6

-4

-2

0 50 100 150 200 250 300 350

Time (Hrs)

W at er E le va tio n

(ft , M

LL

W

Pete Dahl Slough Tide Prediction 6-Month Low Tide Estimate

Figure 9 – 6-month Low Water Elevation Boundary Condition

4.3.3 Verification

Verification of a model is a multi-step process of model adjustments and comparisons, influenced by careful consideration of both the model and the data. For this model, the verification steps included:

1. Continuity Checks - Sharp angle geometry conditions on the model boundary may result in boundary flows that are not exactly parallel to the boundaries. This results in a numerical leakage of water from the model. The objective when simulating is to retain the correct amount of fluid flow from one point to the next, within a tolerance of about plus or minus 3% (Donnell et al., 2005b). Continuity checks along the length of the model showed excellent continuity with net flow differences less than or equal to 1%.

2. Effects of Initial Conditions - The initial conditions can adversely affect the results during the first part of a dynamic simulation. The model simulation provided an 80 hr period to spin-up the model. Review of the model results verified that startup transients have decayed within this period and have no significant effect on the retained portion of the hydrodynamic simulation.

3. Reasonable Model Behavior - Review of transient model results show flow patterns and currents that are consistent with expected conditions in the slough.

4. Critical Check Points – Current velocity and water elevation data were obtained concurrently with the site survey completed during September 23rd - 25th, 2009.

Completing model simulations for these specific dates allow comparison of computed model results at the Alaganik Slough boat ramp and the snapshot of water velocity and elevation data provided by the site investigation activities.

Calculation 3954-04 details the model calibration process while Figure 10 and Figure 11 show the results of a comparison of the predicted and measured tides and velocities at the boat ramp.

Calibration Run #3 Results

0 20 40 60 80 100 120 140 160 180

Time (Hrs)

W at er E le va tio n

(ft

El lip

Predicted Elevations Measured Elevations

Figure 10 – Predicted vs. Measured Water Surface Elevations

Calibration Run #3

0.5

1.5

2.5

3.5

126 126.2 126.4 126.6 126.8 127 127.2 127.4 127.6 127.8 128

Time (Hrs)

Ve lo ci ty

(f t/s ec

Predicted Velocity Measured Velocity

Figure 11 – Predicted vs. Measured Water Velocities

4.3.4 Shoaling/Erosion Characteristics

Hydrodynamic models can be used to determine the shoaling/erosion characteristics of a particular area. It is often useful to compare these characteristics for existing conditions and post-project conditions to determine if any major changes occur. Shear stress at the bed is the primary sediment transport mechanism. Large magnitudes of shear stress change can indicate possible areas of sediment deposition and erosion.

SMS 9.0 uses a data calculator to obtain the shear stress at each node in the mesh.

The equation below is used as input into the data calculator:

R ngVρτ = where τ is the shear stress at the bed, ρ is the density of water, g is gravity, V is the depth-averaged velocity, is Manning’s n, andn R is the hydraulic radius (assumed to be the water depth).

5.0 RESULTS

5.1 Existing Conditions

The initial model runs evaluate the existing flow behavior in the slough near the boat ramp and can be seen in Figure 12 and Figure 13. The dark blue areas within the model contain peak flow velocities, while the red areas have the weakest. The model also depicts the wetting and drying conditions typical of tidally influenced areas. Only the blank mesh elements are considered ‘dry’ and completely exposed at the model’s predicted water elevation.

The model simulations were based upon the flow conditions observed during the September 20th – October 3rd, 2009 field survey program and are considered to represent normal ramp operating conditions. The high and low water elevations predicted by the model for this time period were 52.1 ft and 47.5 ft respectively. Peak current velocities located within the thalweg adjacent to the existing ramp range from 4.5 ft/sec adjacent to the existing ramp to nearly 6.0 ft/sec just to the north.

The site survey determined that the existing boat ramp’s toe is located at approximately +48 feet. The model results indicate that water elevations drop below this threshold on a frequent basis. Figure 13 depicts typical low water conditions at the existing boat ramp site and clearly shows that the existing ramp would be unusable during these conditions.

Figure 12 – Flow Distribution at High Water Elevation = 51.4 ft

Figure 13 – Flow Distribution at Low Water Elevation = 47.7 ft

5.2 Proposed Boat Ramp Location

Model simulations found that the existing boat ramp’s toe is routinely exposed during low water conditions; rendering it unusable on a frequent basis. Model simulations also found that a channel constriction located upstream of the boat ramp area has a significant influence on the flow conditions in the vicinity of the ramp. Typically, the stronger current velocities, and therefore increased water depth, is located on the outside of a meander bend in a stream. The channel constriction appears to focus the current toward the bank area just upstream of the existing ramp resulting in increased flow and water depth at this location.

Therefore, the proposed boat ramp was relocated upstream to take advantage of the deeper water depths. The ramp location was iteratively adjusted to minimize the hydraulic footprint within the Slough and was also oriented parallel to the existing ramp to minimize cross-currents during navigation on and off of it.

Figure 15 depicts typical low water conditions at the proposed boat ramp. The existing boat ramp location is also shown for comparison. The model shows that while the existing ramp would be unusable, the proposed ramp maintains enough water to facilitate the launching and loading of boats. The model predicts that peak current velocities will approach 2.5 ft/sec at the toe of the ramp.

Figure 14 – Flow Distribution at High Water Elevation = 50.3 ft

Figure 15 – Flow Distribution at Low Water Elevation = 47.7 ft

5.3 Water Elevation Estimates

The design requirements call for estimation of the high water elevation associated with an event that has a Mean Recurrence Interval (MRI) of 50 years (average annual frequency of occurrence of 0.02). The requirements also call for the estimation of the low water elevation associated with a low tide with a 6-month MRI. The high water value has the potential to be caused by either extreme tidal conditions or fluvial flooding conditions. Low water conditions are specified in the design requirements as tidally controlled. The following sections summarize the methodology and assumptions required to obtain the resulting elevations. Calculation 3954-03 provides a more detailed evaluation of their determination.

5.3.1 50-Year High Water Elevation The 50-year high water elevation was estimated for both extreme tidal conditions and fluvial flooding conditions. The 50-year high water elevations for both are shown below in Table 3.

Table 3 – 50-Year High Water Elevation Estimates

50-Year High Water Elevation Estimate Alaganik Slough Boat Ramp

Condition Elevation

50-Year Tide 56.4 ft, Ellip 50-Year Flow 56.3 ft, Ellip

Figure 16 and Figure 17 show the flow distribution at the 50-year high water elevations for both the extreme tidal and fluvial flood conditions. There is a noticeably large velocity magnitude difference between the 2 cases. The tidally influenced high water elevation has velocities of less than 1 ft/sec, while the flood condition velocities approach 4.7 ft/sec.

Figure 16 – Flow Conditions at 50-year High Tide (Water Elevation = 56.4 ft)

Figure 17 – Flow Conditions at 50-year Flow Condition (Water Elevation = 56.3 ft)

5.3.1.1 50-Year High Tide

Solar and lunar conditions drive a progressive wave through the Gulf of Alaska. The interaction of that wave with the water depths and shoreline configurations result in modifications of the progressive wave that results in variations in tidal conditions around the perimeter of the Gulf. The nearest tide prediction station to Alaganik Slough is the Pete Dahl Slough station approximately 2.75 miles to the southeast of the mouth of Alaganik Slough. Table 4 provides the details of the station.

Table 4 – Pete Dahl Slough Tide Station (NOAA, 2009)

Station ID TWC1867 Latitude 60.3833 N

Longitude 145.4000 W Station Type Subordinate

Reference Station Cordova (Station ID 9454050) Height offset – high -2.4 ft Height offset – low 0.0 ft Time offset – high 6 min Time offset – low 38 min

The Pete Dahl Slough tide station is a subordinate station. Tide values are developed by applying differences or ratios to the full harmonic constant predictions developed at a specific harmonic station. NOAA developed the height and time offsets of Table 4 through a comparison of measurements at Pete Dahl Slough and at the Cordova reference station.

The Cordova tide station (NOAA, 2009) on Orca Inlet in Prince William Sound to the west of Alaganik Slough was initially established in 1949. It is the nearest station with long term measurements that may be used to estimate extreme water levels.

Attachment A (NOAA, 2009) provides a listing of the monthly high and low tide values over the period of generally continuous records (1982 – present). We used direct frequency analysis to estimate the 50-year high tide level at Cordova. Direct frequency analysis selects from the total data only that information that is required for the design, i.e. the maximum tide level in each year (annual series). We used the method of plotting position as the mean (Kite, 1977) as the approach to estimate the probability of occurrence of the events in the annual series. First the 50-year high tide at Cordova was estimated. Then it was transferred to Pete Dahl Slough by application of the NOAA developed high water offset (-2.4 ft), Figure 8. Finally, the tidal wave was propagated upstream to the site using the hydrodynamic model.

5.3.1.2 50-Year Flow Conditions

With a drainage basin area of about 24,000 square miles, the Copper River is the largest drainage basin in the Gulf of Alaska region and the sixth largest in the state of Alaska.

Figure 18 shows the Copper River drainage basin. Despite ranking sixth in basin area, the average discharge of 57,000 cubic feet per second (cfs) ranks second behind only the Yukon (Bowersox, 2002). The Copper River flows into a large, relatively flat alluvial plain near its mouth. Alaganik Slough is the westernmost distributory channel of this alluvial plain.

Figure 18 – Copper River Drainage Basin (NOAA)

In 2007, approximately 18% of the Copper River basin was covered by glaciers which significantly influence the hydrology of the Copper River. During the winter months (November through April), the river is ice covered and flow averages 11,700 cfs near the mouth. During the open-water months (May through October), however, glaciers contribute significant flow to the Copper River, and average flow increases by an order of magnitude – to 113,300 cfs near the mouth. Additionally the lower Copper River is subject to rapid increases in flow on a roughly 6-7 year period when the glacier dammed Van Cleve Lake breaks out (Brabets and Conaway, 2009).

Ice-dam breakouts, also know as jökulhlaups, are common throughout glaciated Alaska.

The Miles Glacier flows toward the Copper River passing the Van Cleve Valley. As the glacier passes the valley outlet, Van Cleve Lake is formed behind the glacier from glacier/snow melt and precipitation. Jökulhlaups occur when the ice dam floats and the seal is broken. Records of ice-dam breakouts of Van Cleve Lake have taken place since 1909 when a river stage of 19 ft was recorded. The only consistently recorded water stage level during a Van Cleve Lake jökulhlaup were documented in the 1992 occurrence. The Copper River discharge was about 150,000 cfs on August 4th, peaked to 300,000, cfs during the jökulhlaup on August 9th, and returned to the 150,000 cfs by August 12th (De Paoli, 2002).

Magnitudes and frequencies of future floods from storms and snow melt can be predicted from long-term stream flow records. Future floods from most glacier dammed lakes, however, cannot be estimated reliably using these standard statistical procedures, because the hydrologic characteristics of the drainage basin may change suddenly and discontinuously (Post and Mayo, 1971). However, the discharge associated with a Van Cleve Lake jökulhlaup is estimated to be equivalent to a 1.5 to 2 year flood event (De Paoli, 2002).

Alaganik Slough branches off of the main braided channel complex of the Copper River delta about 1.5 miles south of Flag Point (Mile 27 on the Copper River Highway – Route 10). Figure 19 shows the Copper River branching to form Alaganik Slough just below the “Insert 4” annotation on the map. Starting at Flag Point, the Copper River Highway begins a 13 mile crossing of the alluvial plain of the Lower Copper River.

Figure 19 – Alaganik Slough Connection with the Copper River (USFS, 2009)

Studies in support of stabilizing the highway crossings found that there has been an eastward shift of the Copper River channels in the mid-to-late 1980’s to early 1990’s.

Analysis of flow measurements were made at bridges along the Copper River Highway indicate that the total flow of the Copper River has shifted from approximately 50% passing through the bridges at Mile 27 (Flag Point) and 50% through the bridges at Mile 36-37 to approximately 5% passing through the bridges at Mile 27 and 95% through the bridges 36-37 during average flow periods (Brabets and Conaway, 2009). It therefore appears that significantly less river flow finds its way down the Alaganik Slough compared to pre-1980’s conditions.

Discharge events with a 2-year recurrence interval (50% probability of occurrence in any given year) are roughly equal to a water level near bankfull (Bowersox, 2002). On the Copper River at the Million Dollar Bridge gage station (USGS Station 15214000), the 2-year instantaneous discharge is about 287,000 cfs. The formative discharge or one that is capable of mobilizing sufficient sediment to alter stream channels is estimated to be a flow event with a 1.5 to 2 year recurrence interval (Wooster, 2002).

A storm in August 2006 produced a peak discharge of about 311,000 cfs with about a 4-year recurrence interval (25% chance of annual occurrence). A subsequent October 2006 storm provided a peak discharge of 444,000 cfs that was estimated to have only a 1 to 1.5% chance of occurrence (66.7 to 100 year recurrence interval) (Brabets and Conaway, 2009). Figure 20 summarizes the annual peak flow occurrences during the recent years when the gage station was in operation. It also captures some of the extreme events discussed above.

Figure 20 – Annual Peak Flows (USGS, 2009)

Figure 21 shows the frequency of occurrence plot of the Copper River flow data. Based upon these reported values, the estimated value of the 50-year peak discharge on the Copper River at the Million Dollar Bridge is about 423,000 cfs.

Copper River Extreme Flows

2.00%, 423000 y = -43110Ln(x) + 254483

200,000

250,000

300,000

350,000

400,000

450,000

500,000

1.00%10.00%100.00%

Frequency of Occurrence

Pe ak

F lo w s

(c fs

USGS Gage Data 50-Year Flow Estimate Log. (USGS Gage Data)

Figure 21 – Frequency of Occurrence of Copper River Flows

The procedure to estimate the 50% flow in the Alaganik Slough is based upon the following assumptions:

• Alaganik Slough is directly connected to the Lower Copper River and its mean daily flows and extreme flows are proportional to the flows measured at the USGS gage at the Million Dollar Bridge

• Flows in the Copper River and Alaganik Slough were nominally the median (50th percentile) daily flows (7-11 year period of record) during the days of the field survey (USGS gage was not operational during the Sept 23-25, 2009 field survey period)

• The median Copper River flow over September 23-25, 2009 was about 84,500 cfs

• The measured flow in the Alaganik Slough over Sept 23-25, 2009 was about 2,900 cfs

• The Copper River: Alaganik Slough ratio is 29.14:1

• The estimated Alaganik Slough 50-year fluvial flow is 14,500 cfs (423,000 cfs/

29.14)

5.3.2 6-Month Low Water Elevation The 6-month low tide elevation is shown in Table 5. The flow conditions shown in Figure 22 are more similar in magnitude to the typical conditions experienced on the Alaganik Slough. Peak velocities ranged from 4.4 ft/sec adjacent to the existing boat ramp to approximately 5.8 ft/sec just north of the ramp.

Table 5 – 6-Month Low Tide Elevation Estimate

6-Month Low Tide Elevation Estimate

47.5 ft

Figure 22 – Flow Conditions at 6-Month Low Tide (Water Elevation = 47.5 ft)

5.4 Shear Stress Comparison

Figure 23 and Figure 24 show a comparison between the existing and post-project bed shear stresses in the vicinity of the Alaganik Slough boat ramp. The model simulations show no significant changes between the post-project and existing conditions bed shear stresses. The model results indicate that the proposed boat ramp location and orientation minimizes shoaling/erosion effects.

Figure 23 – Existing Conditions Shear Stress

Figure 24 – Post Project Shear Stress

6.0 CONCLUSIONS

The study results allow us to draw the following conclusions:

1. The verification process indicates that the simulations during this study provide a good representation of the hydrodynamic processes in the Alaganik Slough.

2. The simulations predict that the boat ramp will experience a 50-year high water elevation of approximately 56.4 ft, Ellip and 6-month low water elevation of 47.5 ft, Ellip.

3. Model simulations found that the existing boat ramp’s toe is routinely exposed during low water conditions; rendering it unusable on a frequent basis.

Conversely, the proposed boat ramp location maintains enough water to facilitate the launching and loading of vessels.

4. The model results indicate that the proposed boat ramp location and orientation minimizes shoaling/erosion effects.

7.0 REFERENCES

Bowersox, R. 2002. “Hydrology of a Glacial Dominated System, Copper River, Alaska” Glacial and Periclacial Processes as Hydrogeomorphic and Ecological Drivers in High- Latitude Watersheds. Editors, J Mount, P. Moyle and S. Yarnell, University of California, Davis, Davis, CA.

Brabets, T.P. and Conaway, J. S., 2009. Geomorphological and river dynamics of the lower Copper River, Alaska, U.S. Geological Survey, Scientific Investigations Report 2009-5257, 42 p. URL http://pubs.er.usgs.gov/usgspubs/sir/sir20095257, accessed 13 Jan 2010.

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5.4 Shear Stress Comparison