S12865.Hydrologic and Hydraulic Report FINAL.BMM.081407.pdf

HYDROLOGIC AND HYDRAULIC REPORT

SITUK RIVER

WATCHABLE WILDLIFE AREA IMPROVEMENTS

YAKUTAT, ALASKA

HYDROLOGIC AND HYDRAULIC REPORT

SITUK RIVER

WATCHABLE WILDLIFE AREA IMPROVEMENTS

YAKUTAT, ALASKA

Prepared for:

Atelier Landscape Architects, Inc.

120 Belmont Avenue East

Seattle, Washington, 98102

Prepared by:

DOWL Engineers 4041 B Street

Anchorage, Alaska 99503

(907) 562-2000

W.O. S12865

August 2007

Hydrologic and Hydraulic Report August 2007 Situk River Watchable Wildlife Area Improvements Yakutat, Alaska

Page i

TABLE OF CONTENTS

Page

1.0 INTRODUCTION

2.0 HYDROLOGICAL ANALYSIS

2.1 Methodology

2.2 Summary of Results

3.0 HYDRAULIC ANALYSIS

3.1 Objective

3.2 Methodology and Input Data

3.3 Cross-Section Data

3.4 Boundary Conditions

3.4.1 Water Surface Elevations at the Most Downstream Cross-Section of Situk River

3.4.2 Discharges at the Most Upstream Cross-Section of the Situk River

3.5 Manning’s Roughness Coefficients

3.6 Energy Loss Coefficients

3.7 Structure Modeling

3.7.1 General Description

3.8 Embankment Protection

3.9 Hydraulic Analysis Result and Conclusion

4.0 REFERENCES

FIGURES

Figure 1: Location Map

TABLES

Table 1: Exceedence Probability Table 2: Frequency-Peak Discharge Relationships Table 3: Flow Data

PHOTOGRAPHS

Photograph 1: Looking Upstream from Forest Highway 10 Photograph 2: Looking Downstream from Forest Highway 10 Photograph 3: Looking Upstream from Existing Boat Launch Photograph 4: United States Geological Survey River Gage Photograph 5: Looking Downstream from Existing Boat Launch Photograph 6: Situk River Downstream of Bridge (looking upstream) Photograph 7: Typical Bank Vegetation Photograph 8: Situk River Upstream of Bridge (looking downstream) Photograph 9: Existing Boat Launch Photograph 10: Existing Gravel Parking Area

TABLE OF CONTENTS (cont’d)

Page ii

APPENDICES

Appendix A......................................................................................HEC-RAS Standard Tables Appendix B ..................................................................................................................... Figures Appendix C ..........................................................................................................Cross-Sections Appendix D.......................................................................................... Cross-Section Locations

1.0 INTRODUCTION

The United States Department of Agriculture Forest Service has proposed improvements to the day use areas near the Situk River Bridge in Yakutat, Alaska The site of the proposed improvements is located along Forest Highway 10 (FH 10) (Figure 1). The purpose of this study is to compute the water surface elevations needed for design of these facilities and determine if the proposed improvements to the visitor facilities at the crossing of FH 10 over the Situk River will impact the 2-, 50-, and 100-year water surface elevations.

Figure 1: Location Map

Photograph 1: Looking Upstream from Forest Highway 10

Photograph 2: Looking Downstream from Forest Highway 10

Photograph 3: Looking Upstream from Existing Boat Launch

Photograph 4: USGS River Gage

Photograph 5: Looking Downstream from Existing Boat Launch

Photograph 6: Situk River Downstream of Bridge (looking upstream)

Photograph 7: Typical Bank Vegetation

Photograph 8: Situk River Upstream of Bridge (looking downstream)

Photograph 9: Existing Boat Launch

Photograph 10: Existing Gravel Parking Area

2.0 HYDROLOGICAL ANALYSIS

2.1 Methodology

Since the United States Geological Survey (USGS) has a gage located just southeast of the bridge, discharges were able to be obtained from Water-Resources Investigations

Report 03-4188 Estimating the Magnitude and Frequency of Peak Streamflows for Ungaged

Sites on Streams in Alaska and Conterminous Basins in Canada. Mean daily data from the

USGS website was sorted to determine the exceedence probabilities and used in the design of the boat launch.

Water surface elevations for the boat launch needed to be determined. The mean daily gage data (1996 through 2006) was analyzed to determine the exceedence probability for the various water surface elevations obtained from the USGS website. Data entries missing elevations, those with four-digit elevations, and data from October through March were deleted. As discussed with Bob Gubernick of the United States Forest Service, only the data from April through September was included in this analysis, as this is the period of highest use of the launch facilities. Using the data from this period also eliminates any data that may have been affected by ice during the winter months. The remaining data was sorted using a spreadsheet from highest at the beginning to lowest at the end. The exceedence probability of the data was then determined using:

[M/N] * 100% = % Exceedence Where

N = # of Flows M = Rank of Flow

% Exceedence = equivalent discharge equaled or exceeded a certain percentage of the time

Table 1: Exceedence Probability

Stage

Exceedence Probability 64 100 65 98.9 66 32 67 6.3 68 1.9 69 0.8 70 0.3

Using the elevations obtained in this analysis and approximating that the boat launch to require 3 feet of water, an elevation of 62 was determined as the maximum elevation for boat launch. Two boat launch options were analyzed. Option 1 was located approximately

200 feet downstream of the roadway. Option 2 was located just downstream of a large scour hole. Because the existing elevation of the streambed was approximately elevation 64 for

Option 1, this option would not provide adequate depth for the boat launch without dredging and/or maintenance. Option 2 was chosen as a better alternative due to available depth. New options were then developed to locate the boat launch at the tail end of the scour pool below the bridge.

2.2 Summary of Results

The relationship between the flood frequency and the peak discharge derived from frequency analysis is shown in Table 2.

Table 2: Frequency-Peak Discharge Relationships

Return Period (Years)

Peak Discharge for Situk River

(CFS)

2 1940 10 2840 25 3290 50 3630 100 3960

3.0 HYDRAULIC ANALYSIS

3.1 Objective

The objective of the analyses is to compute the water surface profiles of the Situk River for the existing condition and the proposed condition with the proposed site improvements.

3.2 Methodology and Input Data

The water surface profiles for 2-, 50-, and 100-year floods were computed using the

Hydrologic Engineering Center’s River Analysis System computer program (HEC-RAS 4.0).

The HEC-RAS program is designed to perform one-dimensional hydraulic analysis, and it has the ability to perform sub-critical, supercritical, and mixed flow regime calculations.

The basic input data for the HEC-RAS modeling include cross-sections, Manning’s n values, loss coefficients, and boundary conditions. Prior to the running the computations, a selection about which flow regime to use needed to be made since the boundary conditions to be imposed are related the flow regime. A preliminary computation of mixed flow regime was conducted to check if supercritical flow existed. The result showed that no supercritical flow existed, therefore, sub-critical flow computations were conducted in this study.

3.3 Cross-Section Data

The cross-section data used in the HEC-RAS model was based on field survey for the channel and part of the floodplain and coded from left to right facing downstream. The boundary cross-sections are selected sufficiently away from the bridge so that the normal water depth assumption at the downstream boundary is valid. Cross-sections 575 and 524 are located immediately upstream and downstream of the proposed bridge, respectively. In the computation, the HEC-RAS program automatically interpolates two additional cross-sections for the bridge hydraulic calculation.

Nineteen cross-sections were used to model the existing condition of the Situk River as shown on the topographical map. The distances between the cross-sections vary from 17 to

95 feet. For both the existing and proposed conditions of the Situk River, nineteen cross-sections were used in the model. Except for the cross-sections near the improvements at the proposed boat launch facility and parking improvements for the campground, the existing cross-sections of Situk River are much the same as those for the proposed condition. The locations of cross-sections are shown in Appendix D.

3.4 Boundary Conditions

3.4.1 Water Surface Elevations at the Most Downstream Cross-Section of Situk River

Boundary conditions need to be imposed properly for the HEC-RAS model. Since sub-critical flow regime was computed in this study, starting water surface elevations need to be specified for the most downstream cross-section of the Situk River. In this study, the starting water surface elevations at the most downstream cross-section (Cross-section 000) were obtained using the slope-area method, using a channel slope of 0.003 feet/feet. The defined slope was calculated by averaging the slope of the stream thalweg for the first six cross-sections at the downstream end.

To assess the sensitivity of the tailwater elevation at the bridge to the energy slope at the downstream boundary, two cases were computed with the tailwater elevations increased or decreased by 1 foot. With the increase or decrease of the starting water surface elevations by

1 foot, the water surface elevations at the downstream face of the bridge varied by less than

0.05 foot.

3.4.2 Discharges at the Most Upstream Cross-Section of the Situk River

Discharges were specified at the most upstream cross-section of the Situk River.

Flow data with elevation information was obtained from the USGS. This data was utilized to

“calibrate” this model with the actual data through the determination of the appropriate

Manning’s ‘n’ coefficients for the channel and overbanks. The data is listed in the table below.

Table 3: Flow Data

Stage Discharge

71.91 3090

71.37 2560

69.22 1390

3.5 Manning’s Roughness Coefficients

Manning’s ‘n’ values were estimated during several field visits along with the recommendations in the HEC-RAS Hydraulic Reference Manual. The ‘n’ values are described as follows:

Channels: (1) 0.035 - 0.050 Clean, winding, some pools, shoals, weeds, and stones.

(2) 0.045 - 0.060 Same as above, but more stones

Mountain Streams: (1) 0.030 - 0.050 Bottom: gravels, cobbles, and few boulders

(2) 0.040 - 0.070 Bottom: cobbles and large bounders

Floodplains: (1) 0.040 - 0.080 Light brush and trees, in summer

(2) 0.070 - 0.160 Medium to dense brush, in summer

The channel typically consists of a gravel and cobble bed load material with a moderate degree of irregularity and variations of cross-section geometry. The Manning’s ‘n’ value used for channel bottom of the Situk River is 0.045. The floodplain ‘n’ value is 0.08. This

‘n’ value was seen appropriate for the areas along the edge of the river, but the overbanks were considered dense with willows and brush. When calibrating the model, much of this area was considered part of the ineffective area. The same Manning’s ‘n’ values were used for both the existing and the proposed conditions.

3.6 Energy Loss Coefficients

Several loss coefficients are used to evaluate energy losses in the HEC-RAS model. They are as follows:

• Manning’s ‘n’ value for friction loss;

• Contraction and expansion coefficients; and

• Bridge loss coefficients to evaluate losses related to weir shape, pier configuration, and pressure flow.

Friction losses are calculated using Manning’s equation. Losses due to the contraction and expansion of flow between cross-sections are determined according to in which, C = contraction or expansion loss coefficient; V1,V2 = average velocities; α1,α2 = velocity weighting coefficients. Contraction and expansion coefficients of 0.1 and 0.3 were used where the change in river cross-section is insignificant. At the bridges, coefficients of

0.3 and 0.5 were used to account for the large contraction and expansion losses.

3.7 Structure Modeling

3.7.1 General Description

The HEC-RAS model computes energy losses caused by structures in three parts, losses that occur in the reach immediately upstream and downstream of the structure, and losses at the structure itself. The HEC-RAS bridge routines utilize four cross-sections in the g V g V

Ch ce αα computations of energy losses due to the structure. During the hydraulic computations, the program automatically formulates two additional cross-sections inside the bridge. The geometry inside the bridge is a combination of the bounding cross-sections and the bridge geometry. The bridge data such as bridge deck, pier, abutment, and roadway can be entered in the Bridge/Culvert Data Editor.

3.8 Embankment Protection

Riprap embankment protection is recommended for the area upstream and downstream of the boat launch and along the embankment where revegetation is not used. A velocity of 5 feet per second (fps) was computed for the area downstream of the bridge and at the boat launch area. This velocity is used to compute the minimum stable weight of riprap to be used.

Determine Minimum Stone Weight for imaging flow.

Equation 1: Computations for riprap protection for boat launch area

V 5.0 := Velocity for impinging flow in (fps)

SGR 2.65:= Specific gravity of riprap r 70 deg:= ° (for randomly placed rubble, a constant)

S 2.0 := Cross slope (e.g. 2 to 1)

Therefore:

Equation #1

W impinging:=

W impinging 3.2= Minimum Stone Weight (lbs)

[0.00002(V.1.33)6 SGR]

(SGR – 1) 3 sin(r – atan) 1)) 3

S

Therefore, using design velocity 5 fps, it is recommended that the riprap protection be a

Class I stone for the area around the boat launch to reduce the risks associated with impinging flow where the Situk River flows against the launch facility.

Equation 2: Computations for riprap protection for embankment area

Assuming that the design velocity is 5 fps, it is recommended that the embankment protection be a Class I stone for areas along the embankment because of the risks associated with parallel flow.

3.9 Hydraulic Analysis Result and Conclusion

The hydraulic analysis results are presented in Appendix A. The water surface elevations downstream of the bridge for the proposed condition remain the same (within 0.01 feet) as that for the existing condition. At the most upstream cross-section (1029), there is no difference of water surface elevations between the existing and the proposed for the 2- and

100-year events. There was a slight difference (0.01 feet) for the 50-year storm event.

It is concluded that the proposed improvements have little effect on the water surface elevations and minimize impacts to the stream channel and floodplain. Class I riprap is recommended for the area around the boat launch pad and for embankment protection, if necessary.

Determine Minimum Stone Weight for parallel flow.

Equation #2

Wparallel:=

Wparallel 0.57= Minimum Stone Weight (lbs)

[0.00002(V.1.33)6 SGR]

4.0 REFERENCES

Curran, Janet H., Meyer, David F., Tasker, Gary D., U.S. Geological Survey, Estimating the

Magnitude and Frequency of Peak Streamflows for Ungaged Sites on Streams in

Alaska and Conterminous Basins in Canada, Water-Resources Investigations

Report 03-4188

U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-RAS River Analysis

System, User’s Manual, Version 4.0 Beta, November 2006

U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC-RAS River Analysis

System, Hydraulic Reference Manual, Version 3.1, November 2002

APPENDIX A

HEC-RAS STANDARD TABLES

Appendix A - Page 1

Appendix A - Page 2

Appendix A - Page 3

Appendix A - Page 4

Appendix A - Page 5

Appendix A - Page 6

Appendix A - Page 7

Appendix A - Page 8

APPENDIX B

FIGURES

Appendix B - Page 1

Appendix B - Page 2

Appendix B - Page 3

APPENDIX C

CROSS-SECTION

Appendix C - Page 1

Appendix C - Page 2

Appendix C - Page 3

Appendix C - Page 4

Appendix C - Page 5

Appendix C - Page 6

Appendix C - Page 7

Appendix C - Page 8

Appendix C - Page 9

Appendix C - Page 10

Appendix C - Page 11

Appendix C - Page 12

Appendix C - Page 13

Appendix C - Page 14

Appendix C - Page 15

Appendix C - Page 16

Appendix C - Page 17

Appendix C - Page 18

Appendix C - Page 19

Appendix C - Page 20

Appendix C - Page 21

APPENDIX D

LOCATION OF CROSS-SECTIONS

Appendix D - Page 1