Ronald Reagan Washington National Airport

Final Environmental Assessment G-1 March 2012Runway 4-22 and Runway 15-33 RSA Enhancements Appendix G. Sedimentation Study for the Proposed Runway 15-33 and North Boat House

Appendix G. Sedimentation Study for the Proposed Runway 15-33 Safety Area Extension and Relocated North Boat House

Ronald Reagan Washington National Airport

Final Environmental Assessment G-2 March 2012 Runway 4-22 and Runway 15-33 RSA Enhancements Appendix G. Sedimentation Study for the Proposed Runway 15-33 and North Boat House

THIS PAGE LEFT INTENTIONALLY BLANK

Prepared for: Metropolitan Washington Airports Authority One Aviation Circle Washington, D.C., 20001 Prepared by: EA Engineering, Science, and Technology, Inc. 15 Loveton Circle Sparks, Maryland 21152 August 2010

RONALD REAGAN WASHINGTON NATIONAL AIRPORT (DCA)

SEDIMENTATION STUDY FOR THE PROPOSED RUNWAY 15-33 SAFETY AREA EXTENSION AND RELOCATED NORTH BOAT HOUSE

EA Project 13840.10

EA Engineering, Science, and Technology, Inc.

i

TABLE OF CONTENTS

Page LIST OF FIGURES ........................................................................................................................ ii LIST OF TABLES .......................................................................................................................... ii 1. INTRODUCTON .....................................................................................................................1 2. MODEL DESCRIPTION ........................................................................................................1 3. MODEL VELOCITY SCENARIOS .......................................................................................2 4. SEDIMENTATION ANALYSIS ............................................................................................4 5. CONCLUSIONS......................................................................................................................7

ii

LIST OF FIGURES No. TITLE Figure 1 RMA2 Model Grid for the Runway 15-33 Safety Area Extension and Relocated

Boat House Study Figure 2 RMA2 Model Grid Along the DCA Shoreline Figure 3 Bathymetric Survey Along the DCA Shoreline Figure 4 Modified Record to Contain Spring, Mean and Neap Tide, August 2008 Figure 5 Comparison of Potomac River Velocity at the North Boat House and the Proposed

Location for a Spring, Mean, and Neap Tide at a 2,100 cfs River Flow Figure 6 Comparison of Potomac River Velocity at the North Boat House and the Proposed

Location for a Spring, Mean, and Neap Tide at a 16,700 cfs River Flow Figure 7 Comparison of Potomac River Velocity at the Proposed Location With and

Without the Runway Extension for a for a Spring, Mean, and Neap Tide at a 16,700 cfs River Flow

Figure 8 Average Velocity Difference (ft/sec) Over a Tidal Cycle Between the With and

Without Runway Safety Area Extension Scenarios, 16,700 cfs, Mean Tide Figure 9 Predicted High Flow Scenario Bed Change (m) for Silt in the Vicinity of the

Proposed Boat House Location (Bed change is representative of a high deposition event and not typical daily occurrence)

Figure 10 Predicted High Flow Scenario Bed Change (m) for Silt in the Vicinity of the

North and Proposed Boat House Locations (Bed change is representative of a high deposition event and not typical daily occurrence)

LIST OF TABLES

No. TITLE

Table 1 Potomac River Velocity at the Existing North Boat House and the Proposed

Location for a Spring, Mean, and Neap Tide, Without Runway Extension Table 2 Potomac River Velocity at the Proposed Boat House with the Runway Extension

for a Spring, Mean, and Neap Tide at Three River Flows

1

1. INTRODUCTION In February 2009, EA performed a preliminary assessment of the sedimentation issues associated with relocating the boat house and the 15-33 RSA extension. The additional analysis presented in this technical memorandum incorporates into the model a more detailed design configuration for the 15-33 RSA extension and refinement of the model grid in the vicinity of the proposed project to provide higher resolution for the velocity regime. The proposed boathouse will be a pile supported structure that is open on the bottom and will include an upstream baffle intended to divert floating debris. This task is designed to address the following questions that have been raised by MWAA/PMC:

• What is the likelihood that the proposed location will experience siltation requiring regular maintenance dredging?

• How will the Runway 15-33 safety area extension into the Potomac River affect the siltation rate at the proposed boat house location?

• How will alternative design configurations for the Runway 15-33 Runway Safety Area affect the siltation rate and maintenance dredging requirements at the Alternative 3 location?

• What portion of the Potomac River would experience a velocity decrease? • Is the magnitude enough to cause siltation? • Will there be velocity increases elsewhere and, if so, how big? • Would the RSA impacts on hydrology cause erosion and, if so, how much? • If the RSA extension is a pile supported deck instead of a fill, how would the impacts

change? 2. MODEL DESCRIPTION For the current task, EA developed a RMA2 model of the Potomac River in the vicinity of Ronald Reagan Washington National Airport (DCA) based on refinements to the model used in 2009. In 2009, an existing RMA2 Potomac River model was modified by simplifying the model grid upstream of Roosevelt Island. This was done primarily to decrease the number of model cells, and thus decrease execution time. The upstream grid modification was done in a manner to preserve the river’s surface area upstream to the head of tide just below Little Falls. Maintaining the surface area preserves the upstream intertidal volume such that the correct amount of water flows in and out of this upstream region during each tidal cycle. For the 2010 task, the model was further modified along the DCA shoreline by refining the model grid and by employing updated bathymetric data. The original model grid along the DCA shoreline employed cells with approximate dimensions of 200 x 280 ft. For an approximate 8,000 ft distance extending downstream from the North Boat House and for a 600 ft distance offshore, each model cell was subdivided into 4 cells. In the vicinity of the runway safety area extension and the proposed boat house, each of these cells was further subdivided into 4 cells. The entire RMA2 model domain used for the 2010 study is illustrated in Figure 1. The model grid in the vicinity of DCA is illustrated with greater detail in Figure 2. The entire model grid contains 4,283 cells and 12,451 nodes. A model node is located at the corner and mid side of each cell. On 5 April 2010 a bathymetric survey was performed along the DCA shoreline. Depths were measured along 26 transects uniformly spaced between the North and South Boat houses. Along

2

each transect, depths were measured approximately 50 ft, 100 ft, 200 ft, 350 ft, 500 ft and 650 ft from shore. In the vicinity of the runway safety area extension and proposed boat house, additional depths were taken 800 ft from shore. The location of each depth measurement was recorded using GPS. The depth measurements were corrected to a mean lower low water (MLLW) datum using the 6-minute NOAA Washington DC tide data. These bathymetry data were used to update the RMA2 model depths along the 600 ft shore zone in the vicinity of the DCA. The 5 April 2010 bathymetric survey data are illustrated in Figure 3. A comparison of elevation datums for Washington DC is provided in the following table.

Datum Elevation (ft) MHW 2.94 MSL 1.55

NAVD88 1.37 MLW 0.15

MLLW 0.0 The model was executed by applying a tidal condition at the downstream boundary and a uniform flow at the upstream Potomac River and the Anacostia River boundaries. The same tidal boundary conditions were used as in the 2009 assessment. A time-series tidal boundary was constructed based on the observed 6-minute NOAA tide data for August 2008. During the month of August a representative 2 tidal cycle sequence was identified for a typical spring, mean, and neap tide. Intervening tidal cycles were deleted while maintaining a 2 tidal cycle transition between each of the selected tide series. A 2 tidal cycle period was also maintained prior to the spring tide sequence to allow for model initiation. The entire tide sequence was 160-hrs (6.6 days) in length and is illustrated in Figure 4. 3. MODEL VELOCITY SCENARIOS The RMA2 model was executed for a low and high flow scenario. Historical USGS flows for the Potomac River at Little Falls were examined using a 20-year data set. The lowest median (50-percentile) monthly flow was 2,100 cfs in October. The 80-percentile (upper 20-percentile) annual flow was 16,700 cfs. These two flows were used for the low and high flow condition. The corresponding Anacostia River flows were 46 cfs and 216 cfs. A higher high 95-percentile flow scenario was also performed with the runway safety area extension to examine conditions that may have a higher scour potential.

River Flow (cfs) Potomac River Anacostia River

Lowest 50th-% Monthly 2,100 46 80th – Percentile Annual 16,700 216 95th – Percentile Annual 41,500 537

The RMA2 model was executed for the low and high flow scenarios using the 160-hour tidal time-series. Thus, each model run also contained a spring, mean, and neap tidal condition. The model was executed with a 0.25-hr time step and velocity data was output at a 0.5-hr interval. The low and high flow scenarios were executed with and without the Runway 15-33 RSA

3

footprint extended into the Potomac River and the higher high flow scenario was only executed with the RSA footprint. The runway safety area extension was based on a CAD drawing that was geo-referenced into the model’s coordinate system. The results of the model runs are provided in Table 1 for the North and Proposed Boat House locations for the low and high flow scenarios, without the runway safety area extension. Predicted velocities for the proposed boat house location with the runway safety area extension are provided in Table 2 for the low, high (80%), and higher high (95%) scenarios. The tables contain the average and upper 90-percentile velocity over the two tidal cycles representing the spring, mean and neap tide phases for each flow scenario. For each boat house location, the table contains results at model nodes corresponding to approximately 50 ft intervals from 110 ft to 370 ft from shore. A comparison of predicted velocities at the North and proposed boat house locations is provided in Figure 5 for the 2,100 cfs and in Figure 6 for the 16,700 cfs flow scenarios. Each figure contains the average spring, mean, and neap tide velocity at increasing distance from shore. In Figure 5 (2,100 cfs scenario), the average velocity at the existing North Boat House location increased from 0.07 ft/sec to 0.13 ft/sec (Table 1) as the offshore distance increased to 368 ft. For the same 2,100 cfs scenario, velocities at the proposed location were over 2.5 times larger than at the North Boat House, increasing from 0.24 ft/sec to 0.35 ft/sec as the offshore distance increased to 370 ft. Figure 5 indicates that at each offshore distance, average velocities were slightly higher during a spring tide than a mean tide and significantly lower during a neap tide. A similar relationship is illustrated in Figure 6 for the 16,700 cfs scenario with velocities for the proposed location being over 2.5 times larger than at the North Boat House. At the existing North Boat House location, the average tidal velocity increased from 0.11 ft/sec to 0.21 ft/sec and at the proposed location from 0.34 ft/sec to 0.52 ft/sec with increasing distance from shore (Table 1). A velocity comparison at the proposed location with and without the runway safety area extension is illustrated in Figure 7 for the 16,700 cfs flow scenario. The runway safety area extension reduced the average tidal velocity at the proposed location by 0.01 to 0.02 ft/sec, a less than 5 percent change. For the 16,700 cfs/mean tide scenario, the difference between the average velocity over a tidal cycle for the with and without runway safety area extension scenarios is illustrated in Figure 8. The figure indicates that with the runway safety area extension, there was a 0.05 ft/sec velocity decrease in a zone on the upstream side of the RSA extension, and a 0.1-0.2 ft/sec velocity decrease in a zone on the downstream side. At the offshore corner of the runway safety area extension, average velocities over a tidal cycle increased by 0.2 ft/sec. The proposed boat house location is between the 0.00 ft/sec and -0.05 ft/sec velocity difference contours, consistent with the 0.01-0.02 ft/sec velocity difference indicated between Tables 1 and 2 and Figure 7. Velocity Conclusions The factor of 2.5-3 increase in modeled velocities at the proposed location relative to the existing North Boat House is indicative that the propensity for deposition at the proposed location will be

4

significantly reduced. At the proposed boat house location, velocities averaged over a tidal cycle decreased by only 0.01-0.02 ft/sec (less than 5 percent) with the addition of the runway safety area extension. This change in velocity would not be considered significant relative to deposition rates at the proposed boat house location. The model predicted average velocity decreases of 0.05 ft/sec on the upstream side and 0.2 ft/sec on the downstream side of the runway safety area extension. The higher 0.2 ft/sec velocity decrease on the downstream side of the runway safety area extension does raise a concern for increased local siltation. 4. SEDIMENTATION ANALYSIS Based on the scenarios developed for the velocity comparison between the existing North Boat House and proposed location, the SED2D model was used to examine sedimentation. SED2D is the companion sediment transport model to RMA2, which uses the hydrodynamic output file from RAM2 as one of its input files. To help identify suspended sediment concentrations and grain sizes appropriate for the analysis, available historical data was examined and three sediment cores were collected at the site. Suspended sediment data were available from the USGS for the Potomac River at Chain Bridge, Washington DC Station (01646580), just below Little Falls. Weekly suspended solids concentrations were available from 1973 to 2009. The average suspended solids concentration was 40.5 mg/L, while the median was 15 mg/L. Suspended particle diameter data were also available typically 4-10 times a year, reported as the percent of sediment less than 0.0625 mm. A 0.0625 mm particle diameter is the boundary between fine sand and silt. The average solids fraction below 0.0625 mm was 89.8 percent and the median value was 92.5 percent. This indicates that suspended sediment at Chain Bridge was primarily silt and clay. Three sediment samples were collected on 5 April 2010 during the bathymetry survey. The sample locations are shown in Figure 9. The particle size distribution reports for the 3 samples are provided in Appendix A. The percent contribution of the primary particle size classes are provided in the following table. The table also includes the 50-percentile grain size (D50).

Particle Size Distribution (%) Sample Fine Sand Silt Clay

D50 (mm) > 0.074 mm 0.002 - .074 mm <0.002 mm SED-01 10.9 54.3 34.6 0.0066 SED-02 72.7 11.3 5.3 0.2796 SED-03 46.9 25.0 19.5 0.1608 Average 43.5 30.2 19.8

The above table indicates that sample SED-01 was primarily silt and clay, while sample SED-02 was primarily sand. This is further indicated by the D50 grain size, which for SED-01 was 0.0066 mm (fine silt), and for SED-02 and -03 (>= 0.16 mm) representing sand. It is noted that a slightly higher grain size boundary between silt and sand was used in the particle size reports (0.074 mm) than the 0.0625 mm size in the American Geophysical Union Sediment Classification System.

5

SED2D uses a critical shear stress for deposition and erosion when simulating silt and clay. A value for the critical shear stress below which deposition takes place is commonly 0.06 Newton/m2. To help interpret the SED2D results, shear stresses were calculated using the formulation within SED2D for the average velocity over a mean tide (Table 1 and 2) and a mean sea level depth. The resulting shear stress values for increasing distance from shore are provided in the following table.

Shear Stress (N/m2) Distance from

Shore (ft) North Boat House Proposed Location

Low Flow High Flow Low Flow High Flow 160 0.003 0.008 0.026 0.056 270 0.005 0.012 0.036 0.077 370 0.007 0.019 0.040 0.087

Reflecting the higher velocity regime, shear stresses for the proposed location are greater than at the existing North Boat House. For the low flow scenario, shear stresses at the North Boat House are < 0.01 N/m2, while at the proposed location they are 0.02-0.04 N/m2. For the high flow scenario, shear stresses at the North Boat House are < 0.02 N/m2, while the proposed location has values of 0.05-0.09 N/m2. Within the model at specific time steps for the low flow scenario, shear stresses were observed to range up to approximately 0.03 N/m2 in the vicinity of the North Boat House and up to 0.05 N/m2 at the proposed location. The calculated shear stresses at the North Boat House are significantly below 0.06 N/m2, indicating a deposition prone area. At the proposed location and as the velocity increases towards the high flow scenario, deposition of silt and clay would be expected to be significantly reduced. SED2D was executed using the flow/tide scenarios previously discussed. For an upstream river flow, each scenario contained a spring, mean, and neap tide within a 160 hr time-series. The model was executed separately for a sand, silt, and clay. A grain size of 0.001 mm was assumed for clay, 0.01 mm for silt, and 0.08 mm for sand. The initial suspended sediment concentration in the model was set at 40 mg/L for each individual size class, the average total concentration observed at Chain Bridge for all size classes. By modeling each class as the observed average total concentration, there is a greater total amount of sediment in the model than in the natural system. Using 40 mg/L for both silt and clay equals a total concentration that is 5.3 times greater than the observed median concentration at Chain Bridge of 15 mg/L. Sedimentation prone events are often associated with occasional times of higher sediment load than with more typical conditions. For the sand scenarios, virtually all the sand dropped out of the water column during the first several hours of the low and high flow runs. At Chain Bridge the sand fraction was typically less than 10 percent. The sand fraction is a small component of the sediment load and the model indicates that if present at the upstream boundary, it would most likely be deposited prior to reaching the vicinity of DCA. Sand is not considered to play a significant role in sedimentation dynamics in the vicinity of the DCA. The SED2D model was executed for silt and clay for the 160 hour flow/tide time-series. The predicted change in bed thickness during the approximately 1 week model run was converted to

6

a deposition rate (m/day). A deposition rate for silt and clay at model cells in the vicinity of the existing North and Proposed Boat House locations are summarized in the following table.

Deposition Rate (m/day) Scenario North Boat House Proposed Location

Silt – Low Flow 7.24x10-4 7.50x10-4 Silt – High Flow 1.50x10-3 1.16x10-3

Silt – Very High Flow 3.43x10-4 2.62x10-4 Clay – Low Flow 2.67x10-5 2.04x10-5 Clay – High Flow 1.96x10-5 1.24x10-5

Clay – Very High Flow 3.27x10-6 2.43x10-6 Except for the silt low flow scenario, the deposition rate at the proposed location was always smaller than at the existing North Boat House. At both locations, the deposition rate generally decreases with increasing flow. The lower deposition rates for clay reflect a smaller fall velocity; 0.23 ft/day for 0.001 mm clay and 23.3 ft/day for 0.01 mm silt. The low flow silt scenario behaved differently than the others. During the model run, the water column sediment concentrations were significantly reduced, and for the last several days there was a net flux along the concentration gradient from the sediment bed back to the water column. For the high flow silt scenario, the higher sediment mass flux (8 times the flow at the same concentration) at the model boundaries maintained higher water column concentrations throughout the model. The modeled change in sediment bed thickness for silt is provided in Figure 9 and 10 for the high flow scenario. Figure 9 illustrates the Potomac River in the vicinity of the proposed boat house and the runway safety area extension. Figure 10 extends farther north to include the existing North Boat House. Both figures also show the sediment sample locations. Figure 9 indicates a higher deposition region along the downstream (south) side of the runway safety area extension. This region coincides with the 0.2 ft/sec decrease in average velocity illustrated in Figure 8. During the model run, the deposition rate downstream of the runway safety area was typically 0.0025 m/day compared to 0.00088 m/day for pre-existing conditions. In reference to the deposition rates, it should be remembered that the modeled sediment loadings were significantly higher than the median observed concentrations at Chain Bridge. The higher deposition region extends approximately 400 ft downstream of the safety area extension. At the existing North Boat House, a higher deposition region is present, similar to the downstream side of the runway safety area extension (Figure 10). The model predicted sedimentation rates at the proposed boat house location were typically 25-percent less than at the existing North location. This reduction was less than what was initially visualized based on the differences between the two locations in shear stress. The reported deposition rates reflect comparative differences between the locations and not true daily values. The deposition rates were calculated assuming an initial water column concentration of 40 mg/L for each size class, while the median observed concentration at Chain Bridge was 15 mg/L for total suspended solids. The deposition rates in the above table represent a high deposition event

7

and not a typical daily occurrence. Deposition rates can be very site specific, dependent on water column concentrations, salinity, and chemical and physical conditions. 5. CONCLUSIONS

• There is a factor of 2.5-3 increase in average tidal velocity between existing North Boat House and proposed location.

• The 15-33 runway safety area extension has a very small (< 0.02 ft/sec) effect on velocities at the proposed boat house location.

• The average tidal condition shear stress at the proposed location approaches the upper expected limit for deposition of silt/clay for the 80-percentile (16,700 cfs) flow scenario.

• There is a predicted 25-pecent decrease in deposition rate for silt/clay at Proposed Boat House location relative to the deposition rate at the existing North Boat House.

• All velocity scenarios exhibited deposition in the vicinity of the proposed boat house and 15-33 runway safety area extension, but with decreasing deposition with increasing flow

• There is deposition similar to the existing North Boat House location on the downstream side of 15-33 Runway safety area extension.

Figu

re 1

RM

A2 M

odel

Grid

Use

d fo

r the

Run

way

15-

33 S

afet

y Ar

ea E

xten

sion

and

Relo

cate

d bo

at H

ouse

Stu

dy

Figu

re 2

RM

A2 M

odel

Grid

Alo

ng th

e DC

A Sh

orel

ine

5

4

3

6

21

7

9

0

8

10

1112

6

11

8

9

2

8

106

7

10

11

T-7

T-9

T-4

T-3

T-2

T-1

T-5

T-6

T-8

T-15

T-16

T-17

T-11

T-22

T-21

T-14

T-13

T-12

T-18

T-19

T-20

T-10

T-25

T-23

T-24

Figure 3. Bathymetric Survey Along the DCA Shoreline

Q:\P

RO

JEC

TS\G

IS\1

3840

10\D

CA

\BO

ATH

OU

SE\B

ATH

Y_U

PD

ATE

.MXD

0 500 1,000250Feet

These data are provided for this studyand not intended for design purposes.

Reference Datum: MLLWBathymetry Interval: 1 Foot

-0.50.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Elevation (ft , MLW)

Day

Figu

re 4

Mod

ified

Rec

ord

to C

onta

in S

prin

g, M

ean,

and

Nea

p Ti

de, A

ugus

t 200

8

Sprin

gM

ean

Nea

p

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

5010

015

020

025

030

035

040

0

Velocity (ft/sec)

Dist

ance

from

Sho

re (f

t)

Figu

re 5

Com

paris

on o

f Pot

omac

Riv

er V

eloc

ity a

t the

Nor

th B

oat H

ouse

and

the

Prop

osed

Lo

catio

n fo

r a S

prin

g, M

ean,

and

Nea

p Ti

de a

t a 2

,100

cfs

Riv

er F

low

Sprin

g -N

orth

Mea

n -N

orth

Nea

p -N

orth

Tren

d -N

orth

Sprin

g -P

ropo

sed

Mea

n -P

ropo

sed

Nea

p -P

ropo

sed

Tren

d -P

ropo

sed

Nor

th B

oat H

ouse

Prop

osed

Loca

tion

0.00

0.10

0.20

0.30

0.40

0.50

0.60

5010

015

020

025

030

035

040

0

Velocity (ft/sec)

Dist

ance

from

Sho

re (f

t)

Figu

re 6

Com

paris

on o

f Pot

omac

Riv

er V

eloc

ity a

t the

Nor

th B

oat H

ouse

and

the

Prop

osed

Lo

catio

n fo

r a S

prin

g, M

ean,

and

Nea

p Ti

de a

t a 1

6,70

0 cf

s Riv

er F

low

Sprin

g -N

orth

Mea

n -N

orth

Nea

p -N

orth

Tren

d -N

orth

Sprin

g -P

ropo

sed

Mea

n -P

ropo

sed

Nea

p -P

ropo

sed

Tren

d -P

ropo

sed N

orth

Boa

t Hou

se

Prop

osed

Loca

tion

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

5010

015

020

025

030

035

040

0

Velocity (ft/sec)

Dist

ance

from

Sho

re (f

t)

Figu

re 7

Com

paris

on o

f Pot

omac

Riv

er V

eloc

ity a

t the

Pro

pose

d Lo

catio

n W

ith a

nd W

ithou

t th

e Ru

nway

Ext

ensi

on fo

r a S

prin

g, M

ean,

and

Nea

p Ti

de a

t a 1

6,70

0 cf

s Riv

er F

low

Sprin

g -W

ithou

tM

ean

-With

out

Nea

p -W

ithou

tTr

end

-With

out

Sprin

g -W

ithM

ean

-With

Nea

p -W

ithTr

end

-With

Figure 8 Average Velocity Difference (ft/sec) Over a Tidal Cycle Between the With and Without Runway Safety Area Extension Scenarios, 16,700 cfs, Mean Tide

Figure 9 Predicted High Flow Scenario Bed Change (m) for Silt in the Vicinity of the Proposed Boat House Location (Bed change is representative of a high deposition event and not typical daily occurrence)

Figure 10 Predicted High Flow Scenario Bed Change (m) for Silt in the Vicinity of the North and Proposed Boat House Locations (Bed change is representative of a high deposition event and not typical daily occurrence)

Table 1 Potomac River Velocity at the Exisitng North Boat House and the ProposedLocation for a Spring, Mean, and Neap Tide, Without Runway Extension

Velocity (ft/sec) at Distance from Shore (ft) Tide Range 120 170 220 271 322 368

North Boat House, Low Flow (2,100 cfs)Spring Mean 0.07 0.09 0.10 0.11 0.12 0.14

90-% 0.14 0.17 0.20 0.22 0.26 0.30

Mean Mean 0.07 0.08 0.09 0.10 0.11 0.1390-% 0.12 0.15 0.17 0.20 0.23 0.26

Neap Mean 0.05 0.06 0.06 0.07 0.08 0.0990-% 0.09 0.10 0.11 0.12 0.14 0.16

North Boat House, High Flow (16,700 cfs)Spring Mean 0.12 0.14 0.16 0.18 0.20 0.22

90-% 0.19 0.22 0.25 0.28 0.30 0.34

Mean Mean 0.11 0.13 0.15 0.17 0.19 0.2190-% 0.18 0.21 0.24 0.26 0.29 0.33

Neap Mean 0.10 0.11 0.13 0.15 0.17 0.1990-% 0.16 0.18 0.21 0.23 0.25 0.28

Velocity (ft/sec) at Distance from Shore (ft) Tide Range 106 159 211 264 317 370

Proposed Location, Low Flow (2,100 cfs)Spring Mean 0.26 0.30 0.33 0.36 0.37 0.38

90-% 0.41 0.46 0.54 0.58 0.62 0.64

Mean Mean 0.24 0.28 0.31 0.33 0.35 0.3590-% 0.38 0.44 0.50 0.52 0.55 0.56

Neap Mean 0.16 0.18 0.21 0.22 0.23 0.2390-% 0.27 0.31 0.34 0.36 0.37 0.38

Proposed Location, High Flow (16,700 cfs)Spring Mean 0.36 0.43 0.48 0.52 0.54 0.55

90-% 0.55 0.63 0.70 0.76 0.79 0.80

Mean Mean 0.34 0.41 0.46 0.49 0.51 0.5290-% 0.52 0.62 0.69 0.74 0.78 0.79

Neap Mean 0.28 0.33 0.38 0.41 0.43 0.4390-% 0.45 0.54 0.61 0.65 0.68 0.69

Table 2 Potomac River Velocity at the Proposed Boat House Location with the RunwayExtension for a Spring, Mean, and Neap Tide at Three River Flows

Velocity (ft/sec) at Distance from Shore (ft) Tide Range 106 159 211 264 317 370

Low Flow (2,100 cfs)Spring Mean 0.25 0.29 0.32 0.35 0.36 0.37

90-% 0.40 0.45 0.52 0.56 0.60 0.62

Mean Mean 0.23 0.27 0.30 0.32 0.34 0.3590-% 0.38 0.42 0.48 0.50 0.53 0.55

Neap Mean 0.16 0.18 0.20 0.21 0.22 0.2390-% 0.26 0.30 0.33 0.35 0.36 0.37

High Flow (16,700 cfs)Spring Mean 0.35 0.42 0.47 0.50 0.53 0.54

90-% 0.53 0.61 0.68 0.74 0.77 0.79

Mean Mean 0.33 0.39 0.44 0.48 0.50 0.5190-% 0.50 0.60 0.67 0.73 0.76 0.78

Neap Mean 0.27 0.33 0.37 0.40 0.42 0.4390-% 0.44 0.52 0.59 0.64 0.67 0.68

Very High Flow (41,500 cfs)Spring Mean 0.59 0.73 0.84 0.92 0.98 0.99

90-% 0.85 1.03 1.19 1.31 1.38 1.39

Mean Mean 0.60 0.74 0.86 0.95 1.00 1.0290-% 0.83 1.02 1.18 1.30 1.36 1.37

Neap Mean 0.62 0.78 0.90 0.99 1.05 1.0690-% 0.77 0.95 1.10 1.21 1.28 1.29

Appendix A

Particle Size Distribution Reports