### Bridge no. 539

### Knik River, Old Glenn Highway

### Step-Backwater Model and 

### Bridge Scour Analysis

Station 15281000

U.S. Geological Survey Water Resources Division

4230 University Drive, Suite 201

Anchorage, Alaska 99508-4664

(907)-786-7041

Alaska District

## Location

6820100N, 391900 E, UTM zone 6, latitude 61 30' 18", longitude 149 01'
48", NE ¼ , SE ¼ of section 2, T 16 N, R 2 E, Anchorage C-6 quadrangle,
Matnuska Susitna Borough, at the Knik River bridge, mile point 8.9 Old
Glenn Highway.

## Type of Measurement

Six-section step-backwater measurement and HEC-18 (Richardson and Davis,
1995) bridge scour analysis.

## Survey 

D. F. Meyer and E. G. Neal surveyed the site July 13, 1999. A Wild T1600
(F. Nr. 362423) total station with a Wild DI 1600 Distomat was used, and
data were recorded by a Wild GRE4 data logger and transferred to a
computer. The survey was run using an arbitrary datum of 100 ft with
arbitrary coordinates of 10,000 ft north and 10,000 ft east with north
aligned approximately upstream. Four instrument setups were needed to
complete the survey (figure 1). The survey was tied to the bridge datum
by surveying the low steel elevation of the bridge and a Corps of
Engineers benchmark on the upstream side of the left abutment.
Coordinates were not transferred to the as-builts. To correct survey
datum to the bridge datum adjust by --24.33 ft.

A gage (station 15281000) was operational at this site from 1958-1988
and from 1991-1992. Gage datum is tied to a Corps of Engineers benchmark
(elevation 62.67 ft above MSL) on the upstream side of the left abutment
of the old bridge. Elevation to gage datum for this point is 32.50 ft.
To correct elevations to gage datum adjust by 30.17 ft.

A discharge measurement was taken at the APPR3 cross section using a
boat and a 50 lb suspended weight with a AA current meter. Fathometer
data were gathered for all of the cross sections.

Six cross sections were surveyed at the site, three downstream and three
upstream of the bridge (figure 1). The survey data from the banks of
section APPR1 were projected upstream and added to the fathometer data
from APPR2 because the bank data were lost for this section. A bridge
section was templated from sections APPR1 ( left bank and channel data),
EXIT1 (embankment points), as-built data (bridge geometry), and survey
points along the road on the right bank. This section was added because
when a bridge is input RAS interpolates bridge sections from the nearest
upstream and downstream cross sections. This composite section was input
as the upstream and downstream bridge section. Adding this templated
section immediately upstream and downstream of the bridge cross section
is a more accurate representation of the area at the bridge than having
RAS interpolate between EXIT1 and APPR1.

![](media/image1.jpeg){width="5.995138888888889in"
height="7.845833333333333in"}

**Figure1:** Schematic of survey data for bridge 539 at Knik River near
Palmer. Coordinate system is arbitrary.

The original survey points were projected in ArcView to determine
accurate distances between sections. This plot file was exported as a
bitmap to HECRAS to create the geometric plan. Survey data are presented
in Appendix H.

## Bridge Geometry

Two bridges are included in the model, bridge 539 and an older structure
approximately 100 ft upstream. Bridge geometry was taken from the
as-built plans for bridge 539 and interpreted from the survey data for
the older structure. The bridge and piers are aligned relatively
perpendicular to flow for both bridges.

## Model Parameters

The model was run as subcritical using the standard step energy method.
Initial boundary condition for the present water surface (PWS) was a
known water surface elevation of 39.92 ft. at the location of the EXIT3
cross-section, while the initial condition for the Q100 and Q500 events
was a normal depth with a slope of 0.0005. This is the slope of the
energy gradient at the downstream cross section for the calibration
discharge. Discharge estimates for the 100- and 500-year events were
taken from the Phase 1 analysis that determined the magnitude using
methods outlined by Jones and Fahl (1994). High volume (up to 359,000
ft^3^s^-1^ ) glacial outburst floods occurred annually on the Knik River
up until 1966. Due to recession of the Knik glacier these flows no
longer occur and were therefore not included in the calculations of the
Q100 and Q500 discharges. Levees were input in the model on the tops of
the spur dikes to avoid split flow calculations in those sections
(sections APPR1, APPR2, APPR3, and EXIT1). Model variables are
summarized in Table 1.

Roughness values of 0.08 for the overbanks and 0.03 for the channel were
initially selected and the channel values were adjusted to as low as
.027 and as high as 0.037 to calibrate the modeled water surface
elevation to the observed. Using these values the modeled water surface
elevations are equal to the observed for sections EXIT2, EXIT1, and
APPR1. Section APPR2 has a modeled elevation of 41.3 ft and an observed
of 41.57 ft and section APPR3 has a modeled elevation of 42.0 ft with an
observed water surface at 42.53 ft. The discrepancy between modeled and
observed water surfaces for the approach sections could be attributed to
templating survey data upstream. This may present a situation in which
the templated elevations are lower than actual elevations. This would
result in modeled water surfaces that are lower than observed. These
data are presented in Appendix B.

The Q100 and Q500 discharges have two hydraulic jumps through the bridge
sections. These jumps are the result of the contraction in the channel
through this section of the reach. A plot of the water surface profiles
is presented in Appendix C.

The model errors indicate the need for more cross sections to reduce
velocity head drops and conveyance ratios between sections. Addition of
interpolated cross sections would eliminate these errors, but not
significantly affect the water surface profiles or the scour
computations.

## Reach Conditions

The step-backwater reach is 4,000 ft long and fell 2.61 ft at 23,000
ft^3^s^-1^. The overall slope of the water surface between the uppermost
section and the lowermost section was 0.0007 at 23,000 ft^3^s^-1^,
0.0058 at 79,400 ft^3^s^-1^, and 0.0062 at 104,000 ft^3^s^-1^.

The channel is fairly uniform throughout the reach. It does
significantly contract through the bridge sections and expand downstream
of the bridges. At low flows a sand bar is exposed at the downstream
cross section. Spur dikes extend from the left and right banks of the
channel creating a "hydraulic pillow" that directs the flow under the
bridges.

The bridge approach has a low point on the right bank that would allow
overflow during extreme (\>Q500) discharges. These discharges were
common during annual outburst floods from the glacially dammed Lake
George. The glacier has receded and outburst floods have not occurred
since 1966.

**Table 1:** Select values used in model

  -------------------- ------------------------- -------------------------
  **Variable**         **Value**                 **Notes**

  Manning's roughness  .027-.037 (channel), 0.08 Calibrated to observed
                       (overbank)                water surface elevation

  PWS discharge        23,000 ft^3^s^-1^         

  Q100                 79,400 ft^3^s^-1^         

  Q500                 104,000 ft^3^s^-1^        

  Elevation of PWS     39.92 ft                  At downstream cross
                                                 section

  Slope of water       0.0007                    Determined from surveyed
  surface                                        WS

  Slope of energy      0.0005                    At downstream cross
  gradient                                       section for calibration
                                                 discharge

  D50                  1.08 mm or 0.0035 ft.     Leveen scour report

  Water temperature    45 Fahrenheit             Estimated

  Pier dimensions for  4.3 ft wide, 26 ft long   
  scour calculation                              

  Pier Shape           Sharp nosed               

  Bed condition        Medium dunes (K3=1.1)     Leveen scour report
  -------------------- ------------------------- -------------------------

## Scour Calculations

Using the methodology from HEC-18 (Richardson and Davis, 1995)
contraction (live-bed) and pier scour were calculated using HEC-RAS for
bridge 539. Leveen (unpublished U.S. Geological Survey administrative
report, 1967) determined a D50 of 1.08 mm and measured dunes up to 4 ft
in height and 10 ft in wavelength at the Knik River crossing on the new
Glen Highway. Although these data were downstream of bridge 539 they are
thought to be representative and were used in the scour calculations.

Pier scour was computed using the CSU equation. Water temperature used
for the calculations was 45 Fahrenheit. Angle of attack was left at zero
because the piers are aligned to the direction of flow.

The results of the scour computations are presented in Appendix D and
Table 2. Plots of scour at the bridge for all three discharges are
presented in Appendix E.

**Table 2**: Summary of scour results for bridge 539. All scour values
are in feet.

+----------------+----------------+------------------+---+---------------+---+
| ** **          | **PWS Q=23,000 | **Q100=79,400    |   | *             |   |
|                | ft^3^s^-1^**   | ft^3^s^-1^**     |   | *Q500=104,000 |   |
|                |                |                  |   | ft^3^s^-1^**  |   |
+----------------+----------------+------------------+---+---------------+---+
| *Bridge 539*   | channel        | channel          | > |               |   |
|                |                |                  |   |               |   |
|                |                |                  | c |               |   |
|                |                |                  | h |               |   |
|                |                |                  | a |               |   |
|                |                |                  | n |               |   |
|                |                |                  | n |               |   |
|                |                |                  | e |               |   |
|                |                |                  | l |               |   |
+----------------+----------------+------------------+---+---------------+---+
| Contraction    | 0.26           | 0.55             | > |               |   |
|                |                |                  |   |               |   |
|                |                |                  | 0 |               |   |
|                |                |                  | . |               |   |
|                |                |                  | 7 |               |   |
|                |                |                  | 7 |               |   |
+----------------+----------------+------------------+---+---------------+---+
| Pier 1 (left   | 7.61           | 11.65            |   | 12.91         |   |
| bank)          |                |                  |   |               |   |
+----------------+----------------+------------------+---+---------------+---+
| Pier 2 (right  | 7.61           | 11.65            |   | 12.91         |   |
| bank)          |                |                  |   |               |   |
+----------------+----------------+------------------+---+---------------+---+
| Total scour    | 7.87           | 12.20            |   | 13.69         |   |
+----------------+----------------+------------------+---+---------------+---+

## References Cited

Jones, S.H., and Fahl, C.B., 1994, Magnitude and frequency of floods in
Alaska and conterminous basins of Canada: U.S. Geological Survey
Water-Resources Investigations Report 93-4179, 122 p.

Richardson, E.V., and Davis, S.R., 1995, Evaluating scour at bridges (3d
ed.): U.S. Federal Highway Administration, FHWA-IP-90-017 HEC-18, 204 p.

## Appendix A

# Description of files

+---------------------+------------------------------------------------+
| **File name**       | **File description and software**              |
+---------------------+------------------------------------------------+
| 539_knik_ics.txt    | Raw data files from the data logger in         |
|                     | Northing, Easting, Elevation (ics) and full    |
| 539_knik_printed    | information formats.                           |
+---------------------+------------------------------------------------+
| 539_knik_survey.xls | Excel spreadsheet containing transformation of |
|                     | points, surveyed cross sections, interpolated  |
|                     | cross sections, and data exported to HEC-RAS   |
+---------------------+------------------------------------------------+
| 5                   | This document                                  |
| 39_knik_writeup.doc |                                                |
+---------------------+------------------------------------------------+
| 539_knik.g02        | Final HEC-RAS geometry file                    |
+---------------------+------------------------------------------------+
| 539_knik.h01        | Final HEC-RAS hydraulic design file            |
+---------------------+------------------------------------------------+
| 539_knik.f02        | Final HEC-RAS flow file                        |
+---------------------+------------------------------------------------+
| 539_knik.p02        | Final HEC-RAS plan file                        |
+---------------------+------------------------------------------------+
| 539_knik.prj        | Final HEC-RAS project file (details of files   |
|                     | used, units, default parameters, etc.)         |
+---------------------+------------------------------------------------+
| 539_knik.r02        | Final HEC-RAS run file                         |
+---------------------+------------------------------------------------+

***Appendix B:*** Flow data

![](media/image2.wmf){width="8.503472222222221in"
height="5.726388888888889in"}

***Appendix C:** Plot of water surface profiles*

![](media/image3.wmf){width="9.020833333333334in" height="5.0in"}

***Appendix D:*** Scour calculations

***^[Scour\ calculations\ for\ Qmmt\ where\ Q\ =\ 23,000\ ft3s]{.underline}^***-1

  --------------- ------------------------- ----------- ----------- -----------
  **Contraction                                                     
  Scour**                                                           

                                            Left        Channel     Right

  Input Data                                                        

                  Average Depth (ft):                   13.57       

                  Approach Velocity (ft/s):             4.21        

                  Br Average Depth (ft):                13.56       

                  BR Opening Flow (cfs):                23000       

                  BR Top WD (ft):                       390.89      

                  Grain Size D50 (ft):                  0.008       

                  Approach Flow (cfs):                  23000       

                  Approach Top WD (ft):                 402.24      

                  K1 Coefficient:                       0.64        

  Results                                                           

                  Scour Depth Ys (ft):                  **0.26**    

                  Critical Velocity (ft/s):                         

                  Equation:                             Live        

                                                                    

  **Pier Scour**                                                    

                  All piers have the same                           
                  scour depth                                       

  Input Data                                                        

                  Pier Shape:               Sharp nose              

                  Pier Width (ft):          4.33                    

                  Grain Size D50 (ft):      0.008                   

                  Depth Upstream (ft):      21.34                   

                  Velocity Upstream (ft/s): 5.38                    

                  K1 Nose Shape:            0.9                     

                  Pier Angle:               0                       

                  Pier Length (ft):         26                      

                  K2 Angle Coef:            1                       

                  K3 Bed Cond Coef:         1.1                     

                  Grain Size D90 (ft):                              

                  K4 Armouring Coef:        1                       

  Results                                                           

                  Scour Depth Ys (ft):      **7.58**                

                  Froude #:                 0.21                    

                  Equation:                 CSU                     
                                            equation                

                                                                    

                                                                    

  **Combined                                                        
  Scour Depths**                                                    

                                                                    

                  Pier Scour + Contraction                          
                  Scour (ft):                                       

                                            Channel:    **7.84**    
  --------------- ------------------------- ----------- ----------- -----------

***^[Scour\ calculations\ for\ Q100\ where\ Q\ =\ 74,900\ ft3s]{.underline}^***-1

  --------------- ------------------------- ----------- ----------- -----------
  **Contraction                                                     
  Scour**                                                           

                                            Left        Channel     Right

  Input Data                                                        

                  Average Depth (ft):                   17.58       

                  Approach Velocity (ft/s):             10.78       

                  Br Average Depth (ft):                17.28       

                  BR Opening Flow (cfs):                79400       

                  BR Top WD (ft):                       409.97      

                  Grain Size D50 (ft):                  0.008       

                  Approach Flow (cfs):                  79400       

                  Approach Top WD (ft):                 418.76      

                  K1 Coefficient:                       0.64        

  Results                                                           

                  Scour Depth Ys (ft):                  **0.54**    

                  Critical Velocity (ft/s):                         

                  Equation:                             Live        

                                                                    

  **Pier Scour**                                                    

                  All piers have the same                           
                  scour depth                                       

  Input Data                                                        

                  Pier Shape:               Sharp nose              

                  Pier Width (ft):          4.33                    

                  Grain Size D50 (ft):      0.008                   

                  Depth Upstream (ft):      25.94                   

                  Velocity Upstream (ft/s): 13.37                   

                  K1 Nose Shape:            0.9                     

                  Pier Angle:               0                       

                  Pier Length (ft):         26                      

                  K2 Angle Coef:            1                       

                  K3 Bed Cond Coef:         1.1                     

                  Grain Size D90 (ft):                              

                  K4 Armouring Coef:        1                       

  Results                                                           

                  Scour Depth Ys (ft):      **11.51**               

                  Froude #:                 0.46                    

                  Equation:                 CSU                     
                                            equation                

                                                                    

                                                                    

  **Combined                                                        
  Scour Depths**                                                    

                                                                    

                  Pier Scour + Contraction                          
                  Scour (ft):                                       

                                            Channel:    **12.05**   

                                                                    
  --------------- ------------------------- ----------- ----------- -----------

***[Scour calculations for Q500 where Q = 104,000
ft^3^s^-1^]{.underline}***

  --------------- ------------------------- ----------- ----------- -----------
  **Contraction                                                     
  Scour**                                                           

                                            Left        Channel     Right

  Input Data                                                        

                  Average Depth (ft):                   18.21       

                  Approach Velocity (ft/s):             13.56       

                  Br Average Depth (ft):                17.72       

                  BR Opening Flow (cfs):                104000      

                  BR Top WD (ft):                       412.24      

                  Grain Size D50 (ft):                  0.008       

                  Approach Flow (cfs):                  104000      

                  Approach Top WD (ft):                 421.19      

                  K1 Coefficient:                       0.64        

  Results                                                           

                  Scour Depth Ys (ft):                  **0.74**    

                  Critical Velocity (ft/s):                         

                  Equation:                             Live        

                                                                    

  **Pier Scour**                                                    

                  All piers have the same                           
                  scour depth                                       

  Input Data                                                        

                  Pier Shape:               Sharp nose              

                  Pier Width (ft):          4.33                    

                  Grain Size D50 (ft):      0.008                   

                  Depth Upstream (ft):      26.62                   

                  Velocity Upstream (ft/s): 16.79                   

                  K1 Nose Shape:            0.9                     

                  Pier Angle:               0                       

                  Pier Length (ft):         26                      

                  K2 Angle Coef:            1                       

                  K3 Bed Cond Coef:         1.1                     

                  Grain Size D90 (ft):                              

                  K4 Armouring Coef:        1                       

  Results                                                           

                  Scour Depth Ys (ft):      **12.75**               

                  Froude #:                 0.57                    

                  Equation:                 CSU                     
                                            equation                

                                                                    

                                                                    

  **Combined                                                        
  Scour Depths**                                                    

                                                                    

                  Pier Scour + Contraction                          
                  Scour (ft):                                       

                                            Channel:    **13.49**   
  --------------- ------------------------- ----------- ----------- -----------

**Appendix E**: Plots of Scour

***[Scour plot for Qmmt where Q = 23,000 ft^3^s^-1^]{.underline}***

![](media/image4.wmf){width="6.999305555555556in"
height="5.429166666666666in"}

***[Scour plot for Q100 where Q = 79,400 ft^3^s^-1^]{.underline}***

![](media/image5.wmf){width="7.373611111111111in"
height="5.726388888888889in"}

***[Scour plot for Q500 where Q = 104,000 ft^3^s^-1^]{.underline}***

![](media/image6.wmf){width="7.373611111111111in"
height="5.726388888888889in"}

***Appendix F***: Cross section plots with modeled water surfaces.

APPR3

![](media/image7.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

APPR2

![](media/image8.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

Old Bridge Up

![](media/image9.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

Old Bridge down

![](media/image10.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

APPR1

![](media/image11.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

USBRIDGE

![](media/image12.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

New BRIDGE up

![](media/image13.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

New Bridge down

![](media/image14.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

New DSBRIDGE

![](media/image15.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

EXIT1

![](media/image16.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

EXIT2

![](media/image17.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

EXIT3

![](media/image18.wmf){width="5.996527777777778in"
height="4.165277777777778in"}

***Appendix G:*** Select site photographs

View upstream from bridge.

![](media/image19.jpeg){width="5.0in" height="3.7430555555555554in"}

View upstream to bridge.

![](media/image20.jpeg){width="5.0in" height="3.7430555555555554in"}

Knik River right bank downstream of the bridge from the left bank.

![](media/image21.jpeg){width="5.256944444444445in"
height="3.4631944444444445in"}

Looking downstream at pier on Bridge 539.

![](media/image22.jpeg){width="3.2333333333333334in"
height="4.743055555555555in"}

***Appendix H***: Cross section data

![](media/image23.png){width="5.995833333333334in"
height="8.261111111111111in"}

![](media/image24.png){width="5.990972222222222in" height="8.40625in"}

![](media/image25.png){width="5.995138888888889in"
height="8.36736111111111in"}

![](media/image26.png){width="5.909722222222222in"
height="8.988888888888889in"}

![](media/image27.png){width="5.995833333333334in" height="7.975in"}

![](media/image28.png){width="6.036111111111111in"
height="6.8381944444444445in"}

![](media/image29.png){width="5.4631944444444445in"
height="8.989583333333334in"}