ATTACHMENT E: Hydrodynamics and Hydraulic Analysis

ATTACHMENT E: Hydrodynamics and Hydraulic Analysis

This attachment describes the environmental and engineering design considerations for the formulation and design of the alternatives. Environmental considerations were provided by the environmental team to guide the formulation of the engineering alternatives and designs. Environmental Design Considerations

Tidal Wetlands: To restore historical conditions, dredged material should be placed at the location of previously existing tidal wetlands. Appropriate sites include the former islands just inside the mouth of the creek, a band along the shoreline (particularly the southern shoreline), and a broader shoreline area in the cove inside the creek immediately to the north of the inlet. Created tidal wetlands would be dependent upon good tidal exchange to ensure use by Bay fish and wildlife, as well as promote wetland surface accretion to grow upwards with rising sea level. Northwest Creek Interior Shoreline: The length of existing armored shoreline that could be retrofitted with living shorelines as possible, subject to landowner acceptability. Inlet: Inlet dimensions should be adequate to promote tidal exchange in Northwest Creek and maintain Bay water quality conditions (salinity, etc.). The inlet channel should connect with water of equal depth in the bay. Open Shallow Water of Northwest Creek: Water depths should increase bayward to meet ambient depth in Bay (or inlet) to promote tidal exchange and good water quality in Northwest Creek. This is similar to water quality constraints for navigation and marina projects (MDE, 2016), but differs in that depths would be selected based upon promoting adequate tidal exchange only, not navigability. Defined channels that cut through Northwest Creek are not required, unless necessary to ensure tidal exchange. To optimize for SAV growth in Northwest Creek, it is best to have as much water as possible within photic zone, such that SAV could grow in years when Bay water quality conditions are conducive. This zone generally ranges from about 1/2 to 3 feet in depth, although in high water clarity years could extend a foot or two deeper. Bay Shoreline: This shoreline is already in an optimal environmental condition. Alternatives should ideally create or induce formation of new shoreline comparable to the existing shoreline. Jetties could cause loss of natural shoreline condition and associated habitats along Bay shoreline. At a minimum, loss of habitat features would occur at the inlet mouth. Future maintenance dredging should be placed on created tidal wetlands to maintain surface elevation as sea-level rises, or placed on the Bay beach to maintain natural shoreline habitats. Engineering Design Guidelines & Constraints Preliminary data gathering and initial analyses reveal a number of design constraints given the setting of Northwest Creek. Firstly, bathymetry indicates the limited depth offshore from Northwest Creek (see Figure 2 in Appendix E). From Maryland Department of Natural Resources (MDDNR) surveys of the area, shallow areas of -2 to -2.5 feet Mean Low Water (MLW) are indicated from the shoreline to approximately 300 feet into the Bay. Based on other data, this

depth may continue up to 600 feet from the shoreline. In addition, according to the MDDNR survey, it is noted that much of the depth within the Northwest Creek lagoon is above MLW and the lowest depth in the lagoon is -0.56 feet MLW. Furthermore, the low elevation of this area is apparent in the MDDNR topography when looking at the highest points in the area (5 to 6 feet in elevation) at the beach berm/dune established between the Northwest Creek lagoon and the Bay. Due to the shallow offshore bathymetry, any inlet opened to improve connection between Northwest Creek and the Bay would be limited by the depth of approximately -2.5 feet MLW, or -2 feet referenced to NAVD88, and the associated cost of creating a channel that would extend offshore to deeper water. Design is also constrained by the tides over the course of a year. The tides in the area can be heavily influenced by local and seasonal winds. A review of the most recent 18 months (January 2015 to June 2016) reveals that there are about 20 tidal cycles during which the Annapolis gauge (closest tidal gauge to the project site; https://tidesandcurrents.noaa.gov/waterlevels.html?id=8575512) shows water levels below -2 feet NAVD88, (see Attachment E, Figure 3 for monthly tidal charts). This occurs predominantly over the winter months moving into the spring. For project considerations, this means a creek with a bed elevation dredged to -2 feet NAVD88 could have periods where water levels will be low or the creek could potentially go dry for a tidal cycle. Therefore, as there is a limit to the depth of the inlet that can be created based on the shallow offshore bathymetry, it may be desirable to have a portion of the Northwest Creek interior dredged to a depth lower than the creek inlet to allow the creek to run dry, but to maintain an area within the creek lagoon where water will still be present in these situations. Thirdly, the Chesapeake Bay’s wave climate and water levels for different return periods, as well as the accretionary nature of this stretch of shoreline, constrain the project design. For an initial review of wave and water levels in the area, information from Poplar Island, a project in the vicinity, were reviewed. The western portion of Poplar Island is approximately seven miles south of Northwest Creek in the Chesapeake Bay. It has similar fetch distances and bearings from the western shore of the Bay (Figure 5). Table 2 shows the wave height, wave periods, and water levels determined in the Life-Cycle Analysis of the Mid Bay and Poplar Island Projects, Chesapeake Bay, Maryland, in September 2005. Specifically, design analysis point number 8 was used for the Northwest Creek information. From the wave data shown in Table 2 and the nature of the topography in the Bay adjacent to Northwest Creek, it was determined that the 50-year wave height of 7 feet (which is consistent with information provided to USACE by ARNC), corresponding to a water level over 5 feet (MLW), is the maximum design return period for the upper bound of this project. This was determined because the elevation of the shoreline at the beach berm/dune is a maximum of 5 to 6 feet. The elevation of any shoreline stabilization would need to take into account the above return period design criteria. Shoreline longshore transport and accretion also need to be fully realized. The accretionary tendencies of the area are evident when observing the rapid nature of the shoreline changes when the inlet closes after a breach. The rapid closure of the inlet is due to the longshore transport of sand along this natural non-armored area of the Kent Island shoreline. From Figure 2 it can be seen that since 1849 until the present the opened area, or connection to the Bay

from Northwest Creek has filled in approximately 300 feet at its central point from the 1849 shoreline.

Table 1. Wave and water level characteristics in the vicinity of the Northwest Creek shoreline (data from Mid Bay and Poplar Island Life Cycle Analysis, September 2005)

Return Period (years)

Significant Wave Height, HS (feet)

Peak Wave Period, TP (seconds)

Water Level, MLLW (feet)

Water Level, MLW (feet)

5 3.8 5.0 1.5 1.3 10 4.8 5.6 2.3 2.1 25 6.2 5.9 5.0 4.8 50 7.4 6.8 5.5 5.3 100 8.7 7.9 6.9 6.7

Figure 1. Distance and location of the western shore of Poplar Island (south island) and Northwest

Creek (north) with regards to the western shore of the Chesapeake Bay and associated fetch lengths and bearing.

The fourth set of design criteria are the inlet realignment and attributes. A straight channel is necessary to increase flow rates through the inlet and remove sand that enters the channel. Geomorphic Characteristics of Chesapeake Bay Lagoons with Methods for Lagoon and Watershed Assessment (Jenner and Prestegaard, 2012) was reviewed as an initial basis for the realigned channel. This paper indicates that most of the lagoon inlets that have stable openings in the Chesapeake Bay have inlet widths that are greater than 15-20 meters or approximately 50 to 65 feet. Additionally, “stream discharge is the dominant driver of both lagoon surface area and inlet cross section morphology; however, inlet characteristics can be altered by wave sediment transport.” Therefore, any realigned channel connecting the bay should take these observations into account, but also be narrow enough to produce velocities great enough to scour any sand deposits that make it into the channel. Most likely the channel will also need to be protected by

jetties to prevent longshore transport from closing the channel and to keep up the scouring velocities. Finally, Sea level Change (SLC) should also be considered for any coastal project. Figure 6 shows SLC predictions for three scenarios based on USACE possibilities of low, intermediate and high rates of SLC. As shown in Figure 6, SLC was calculated at Annapolis for a project starting in 2017 with a design life of 50 years until 2067. In the case of the Annapolis gage which is the closest long running gauge that the calculator has to Northwest Creek, a SLC of 0.87 feet in 2067 was predicted for low SLC, 1.30 feet in 2067 for the intermediate SLC, and 2.88 feet in 2067 for the high rate of SLC.

Figure 2. Sea Level Change in the Vicinity of North West Creek over 50 years.

References: See references contained in main report

Supplemental Figures

Figure 1: MDNR Survey of North West Creek

Figure 2: Expanded View - MDNR Survey of North West Creek

Figure 3: Predicted/Verified (18 Months) January 0215–June 2016 Tides, Annapolis, Md. Gauge 8575512

Figure 4: Datum Diagram Annapolis, Md. Gauge 8575512

Figure 5: Relative distance and location of the western shore of Poplar Island (south island) and Northwest Creek (north)

Figure 6: Approximate Return Intervals for Significant Wave Height, Peak Wave Period and Water Surface Elevation

Figure 7: Location of Design Analysis Station 8 - Location for the Return Intervals for Significant Wave Height, Peak Wave Period and Water Surface Elevation noted in Figure 6.

Figure 8: Sea Level Change (SLC) – USACE SLC Curves Calculator – http://www.corpsclimate.us/ccaceslcurves.cfm

Figure 3: MDNR Survey of North West Creek (reference map, see Figure 2 for expanded view)

Figure 4: Expanded View - MDNR Survey of North West Creek

-2.41 ft MLW

-0.56 ft MLW

Figure 5: Predicted/Verified (18 Months) January 0215–June 2016 Tides, Annapolis, Md. Gauge 8575512

Figure 6: Datum Diagram Annapolis, Md. Gauge 8575512

Note: Station datum is a fixed base elevation at a given tide station to which all water level measurements are referred. The datum is unique to each station and is established at a lower elevation than the water is ever expected to reach. It is referenced to the primary bench mark at the station and is held constant regardless of changes to the water level gauge or tide staff. The datum of tabulation is most often at the zero of the first tide staff installed

Station website: https://tidesandcurrents.noaa.gov/stationhome.html?id=8575512

Figure 7: Relative distance and location of the western shore of Poplar Island (south island) and Northwest Creek (north)

Life-Cycle Analysis of Mid Bay and Poplar Island Projects, Chesapeake Bay, Maryland September 2005 The Western Shore of Poplar Island Approximate Return Intervals for Significant Wave Height, Peak Wave Period and Water Surface Elevation. This area is approximately 7 miles from Northwest Creek Both areas have similar fetch lengths and bearing.

Return Period, years

Significant Wave Height HS, (ft)

Peak Wave Period TP, sec

Water Level MLLW, (ft)

Water Level MLW, (ft)

5 3.8 5.0 1.5 1.3 10 4.8 5.6 2.3 2.1 25 6.2 5.9 5.0 4.8 50 7.4 6.8 5.5 5.3

100 8.7 7.9 6.9 6.7

Figure 8: Approximate Return Intervals for Significant Wave Height, Peak Wave Period and Water Surface Elevation

Figure 9: Location of Design Analysis Station 8 - Location for the Return Intervals for Significant Wave Height, Peak Wave Period and Water Surface Elevation noted in Figure 6.

Figure 10: Sea Level Change (SLC) – USACE SLC Curves Calculator - http://www.corpsclimate.us/ccaceslcurves.cfm