CHOH 206(1) 913(1) Hydraulics Report-2.pdf

Dargan Bridge over the C&O Canal

DATE: January 19, 2022

PROJECT NAME: CHESAPEAKE AND OHIO CANAL NATIONAL HISTORICAL PARK

FHWA PROJECT NUMBER: NP CHOH 206(1) 913(1) ETC

PURPOSE: Provide hydrologic and hydraulic (H&H) justification for not performing H&H analysis on Dargan Bridge, as requested by MDE.

Dargan Bridge Project Background

• Structure number 3100-005P

• Span length = 40.3 feet

• Span width = 12.1 feet

• Skew angle = 0

• Perpendicular orientation relative to the Potomac River floodplain

• Steel beam construction with wood plank deck and concrete abutments

• Year constructed: 1969. Superstructure reconstructed in 1985, new timber deck planks 1992, painted 1994, 2006 deck replaced and repainted.

• The bridge provides pedestrian and vehicle access to the Dargan Bend boat ramp from a parking lot, across a dry section of the C&O canal, to the ramp.

• Per a bridge inspection performed in 2019, the multi-beam bridge has been rated in poor to fair condition overall, with some severe corrosion at the beam ends, peeling paint and sporadic corrosion throughout beam webs and flanges, wingwall damage due to vehicle collision, section loss of some of the beams, sheared anchor bolts at bearing plates, and other miscellaneous deficiencies.

• The project involves replacing the bridge, abutments, and wingwalls, micro-pile foundation and reconstructing the approaches and the boat ramp.

• The proposed bridge construction will maintain the existing deck elevation and thickness and replace the existing 18” deep steel beams with 24” deep beams, effectively reducing the low steal elevation by 6”, which equates to decrease in conveyance area of (0.5 x 40) 20 sf.

• The bridge is located in Zone A of the regulatory FEMA floodplain, Washington County, MD, Map Number 24043C0455D, effective August 15, 2017. Zone A is a plan view delineation of the floodplain based on approximate methods. Since there no detailed hydraulic studies for Zone A floodplains, there are no base flood elevations (BFE).

Bridge Hydraulic Analysis Warrant Computations and Discussion

• The canal does not convey flow, so local hydraulic analysis of the canal and bridge is not applicable nor warranted.

• USGS bathymetric and topological mapping of the Potomac River floodplain in the project vicinity, generated in 2019, in conjunction with FEMA-based approximate floodplain elevations extrapolated to the NPS mapping, were used to generate a floodplain typical section in the vicinity of the bridge. The floodplain cross section was scanned and scaled into CAD (Bentley ORD ® ) and the 100-year flow area was computed to be approximately 38764 sf with a wetted perimeter of approximately 1546 ft, and a flow top width of 1533 ft.

• The net decrease in conveyance area of 20 sf associated with the proposed bridge’s larger depth beams results in a net decrease in flow area of (20)/(38764)*100%) = 0.0515%.

• An energy gradient of 0.00148436 ft/ft was computed in the nearby Zone AE floodplain, from Sandy Hook (CWSE = 278.0) to approximately 9431 ft east to a cross section with a CWSE = 264.0.

• The digitized project area floodplain cross section was visually separated into channel and overbank areas, and a wetted perimeter-weighted, composite Manning’s n value of 0.0715 was computed based on a 0.045 channel and 0.110 overbank Manning’s n values. [(464+167) *0.110] +[(1546-464-167)*0.045]/[1546] = 0.0715.

• With approximations of s (0.00148436), n (0.0715), Rh (38764/1546), Manning’s equation yields an average velocity of V = 1.49*(38764/1546) 2/3 * 0.00148436 ½] / 0.0715 = 6.89 fps.

• A continuity check shows that Q = AV = 6.89*38764 = 266,613 cfs. The FEMA 100-year discharge computed at the gage located upstream at Hancock MD is 244,400 cfs (FIS -24043CV001A, Effective August 15, 2017, page 15). Consequently, the approximation of the average velocity is reasonably consistence with the FEMA documented in the nearby areas that have detailed study data.

• The bottom of the channel at the project FEMA cross section is 263.55 per the previously described NPS cross section.

• Assuming that the energy loss between the existing and proposed conditions caused by the increased beam depth is insignificant, it can be neglected from both sides of the energy balance.

Thus, the combined potential and kinetic energy at the project FEMA cross section is the depth + velocity head. For the pre-project condition, E1 = Y1 + V1

2/2g = (302.2-263.55) + 6.892 / (2*32.2) = 38.65 + 0.737 = 39.387 ft.

• Thus, the combined potential and kinetic energy for both conditions remains constant at 39.387 ft (E1 = E2).

• The reduction of conveyance area caused by the increased beam depth = 20 sf, thus the total flow area of the post-project condition, A2, is reduced to 38764 – 20 = 38744 sf. Continuity shows that the post-project velocity, V2 = 266,613/38744 = 6.881 fps.

• As previously mentioned, assuming E1 = E2, rearranging the energy equation and by substitution, Y2 = E1-V2

2/ (2*32.2) = 39.387 – 6.8812/2*32.2 = 39.387 - 0.735 = 38.652.

• The increase in water surface elevation caused by the increased beam depth = Y2 - Y1 = 38.652 -

38.65 = 0.002 ft.

• The increase in the CWSE caused by the project (0.002 ft) falls below the standard accepted

FEMA threshold of significance typically of 0.01 ft typically required by FEMA in detailed studies.

• Dargan Bridge is located on the fringe of the floodplain, where the partial conveyance is negligible due to shallower average flow depth coupled with much higher Manning’s n values attributed to heavy vegetative and tree growth typical of the floodplain fringe. In addition, NPS have field documented a sediment deposition in the profile of the canal just downstream of the Dargan Bridge. The sediment deposit forms an alluvial fan that originates from an adjacent tributary, which impedes the flow in this area and potentially shifts fringe conveyance into the Potomac River channel.

Conclusion

• Although the method documented above is significantly simplified, continuity and energy checks and comparison to nearby detailed data indicate that the approach is reasonable. The resulting numerically insignificant increase in the estimated water surface elevation caused by the project, coupled with engineering experience indicate a detailed hydraulic study of the project is not warranted, and is tempered by the reality that any hydrologic and hydraulic modeling of the Potomac River requires significant effort. Current FEMA-approved hydraulic modeling software would essentially register no increases in water surface elevations and velocities, given typical model resolution capability. Normally low conveyance in the floodplain fringe and the field documented alluvial fan just downstream of the bridge further reduce the potential for hydraulic impacts of the project.

Appendices snoppenberger Polygonal Line snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Rectangle snoppenberger Callout

PROPOSED BRIDGE WILL SIT APPROX. 6" LOWER THAN EXISTING BRIDGE

snoppenberger Callout

DARGAN EXISTING & PROPOSED CHANGES TO BRIDGE GEOMETRY

(PROPOSED BRIDGE DIMENSIONS OVERLAYED IN RED ON EXISTING 1960S PLANS)

APPROX. AREA UNDER 100‐

YEAR WSE = 38,764 SQ.FT.

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

EL

EV

AT

IO

N (F T N

AV

D8 8)

OFFSET (FT FROM ORIGIN)

SAMPLE CROSS‐SECTION AT POTOMAC RIVER

(SOURCE: USGS 2019 TOPOBATHYMETRIC SUVEY)

POTOMAC RIVER CROSS‐SECTION FEMA 100‐YEAR WSE (302.2')

Chesapeake & Ohio Canal National Historical Park

USGS MARYLAND POTOMAC RIVER TOPOBATHYMETRIC LIDAR - DARGAN BRIDGE

National Park Service U.S. Department of the Interior

PRODUCED BY: ENGINEERING SERVICES 04 NOVEMBER 2021

Data Sources: ESRI Roads, NPS Park Data, MD 2011 Imagery, etc

0 500 1,000250 Feet

I SAMPLE CROSS-SECTION AT POTOMAC RIVER

SOURCE: USGS 2019

OFFSET (FT FROM ORIGIN)

2,0001,8001,6001,4001,2001,0008006004002000

EL

EV

AT

IO

N (F

T

NA

VD

Legend

FEMA 100-YEAR FLOODPLAIN

1FT CONTOURS

USGS 2019 TOPOBATHY

Value

High : 400

Low : 260

SAMPLE CROSS-SECTION

EXISTING DARGAN BRIDGE

Chesapeake & Ohio Canal National Historical Park

USGS MARYLAND POTOMAC RIVER TOPOBATHYMETRIC LIDAR - DARGAN BRIDGE

National Park Service U.S. Department of the Interior

PRODUCED BY: ENGINEERING SERVICES 09 NOVEMBER 2021

Data Sources: ESRI Roads, NPS Park Data, MD 2011 Imagery, etc

0 150 30075 Feet

I

Legend

FEMA 100-YEAR FLOODPLAIN

1FT CONTOURS

USGS 2019 TOPOBATHY

Value

High : 400

Low : 260

SAMPLE PROFILE

EXISTING DARGAN BRIDGEAREA A

AREA B

AREA C

S AM P LE P R O FILE AT C & O C AN AL

SO UR C E: USG S 2019

OFFS E T (FT FR OM OR IG IN )

1,2001,1001,0009008007006005004003002001000E L

E V

A T

IO

N

F

T N

A V

D

286.5

285.5

284.5

283.5

AREA C

EXISTING DARGAN BRIDGE

AREA B

AREA A

abhishek.kapoor Oval

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TABLE 5 –SUMMARY OF DISCHARGES (Continued)

PEAK DISCHARGES (cubic feet per second)

FLOODING

SOURCE

AND LOCATION

DRAINAGE

AREA

(sq. miles)

10- Percent- Annual- Chance

2- Percent- Annual- Chance

1- Percent- Annual- Chance

0.2- Percent- Annual- Chance

MARSH RUN I

At Confluence with

Antietam Creek

31.4 3,035 4,720 5,604 8,027

Approximately 700 feet downstream of

Confluence of West Branch

24.4 2,741 4,148 4,877 6,842

POTOMAC RIVER**

At USGS Gage, Hancock, MD

* 123,000 202,900 244,400 361,500

SEMPLE RUN

At confluence with

Tributary No 33

2.3 462 859 1,090 1,812

TOMS RUN I

Approximately 1,400 feet downstream of Big Pool Road

3.7 518 1,070 1,411 2,550

Approximately 250 feet upstream of North Hawbaker Road

1.6 312 709 970 1,910

TONOLOWAY CREEK

Approximately 0.7 mile upstream of Interstate

113.1 6,758 11,627 14,258 22,243

TRIBUTARY NO 3

At Confluence with Little Tonoloway Creek

0.9 120 216 271 441

TRIBUTARY NO 29

At Confluence with Conococheauge Creek

8.4 1,048 1,917 2,418 3,954

Just upstream of Confluence of

Tributary No 31

4.7 721 1,345 1,711 2,848

*Data not available ** Data taken from Berkley County, WV FIS (Reference 8)

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