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Cape Fear River Estuary: Temporal Influences on Salinity and Circulation presented by May Ling Becker July 27, 2006 Wilmington Navassa Cape Fear R. DRAFT.

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Presentation on theme: "Cape Fear River Estuary: Temporal Influences on Salinity and Circulation presented by May Ling Becker July 27, 2006 Wilmington Navassa Cape Fear R. DRAFT."— Presentation transcript:

1 Cape Fear River Estuary: Temporal Influences on Salinity and Circulation presented by May Ling Becker July 27, 2006 Wilmington Navassa Cape Fear R. DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

2 Outline Research synopsis –River influences on salinity intrusion –Tidal influences on salinity stratification Field study analysis of tidal influences (low flow) –Description of data collection –Findings DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

3 USGS Stream-flow Gauges Wilmington Tide Gauge USACOE Tide Gauges USACOE Velocity Meters LCFRP Sampling Stations Wilmington Wind Gauge Fort Caswell (S1.2/1.3) Southpor t Navassa (S6.9/7.0) Lock and Dam 1 Wilmington Fear River Cape Black River Northeast Cape Fear River S3.9/4.0 S5.9/6.0 M23 M35 S2.0 M42 M54 M61 HB IC AC DP S7.9/8.0 NC11 M18 NCF 6 NCF117 S9.0 S9.9/10.0 N 25 km S11.0 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

4 Purpose Goal: To develop an increased understanding of estuarine hydrodynamics including temporally varying mechanisms that impact changes in stratification, mixing, and circulation –Influence of river inflow on salinity intrusion –Effect of tidal variations on changes in stratification with time including ebb-flood and tidal-range differences DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

5 River Environment for Study Period DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

6 River Influences: Salinity Intrusion A) January 7, 1999 B) July 15, 1999 NAVM18 NAV M18 Upstream DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

7 Salinity Intrusion Discharge Relationships Y = e Q p R 2 = 0.92 Q p =Max[Q(t o -1):Q(t o -11)] where t o = day of salinity intrusion sampling, Q = mean daily discharge DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

8 Tidal Influences and Stratification: Tidal Range Variations S m/s tau ~ DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

9 Tidal Influences and Stratification: Intra-tidal Variations S 3.9 and S 4.0 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

10 Research Summary of Historical Observational Data The salinity and circulation structure are impacted by: 1) Sub-tidal scale variations in freshwater input which influence the location of the salinity intrusion. 2) Tidal range differences based on an approximately 29-day periodicity 3) Intra-tidal variations associated with tidal straining DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

11 2. Differences in stratification occur during the flood-ebb and 29-day tidal range cycles. Analysis suggests these differences impact circulation (velocities). 1. A correlation between peak in river discharge (hydrologic flood) and the salinity intrusion location is observable. Research Summary of Historical Observational Data DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

12 Scientific Questions: Tidal and Hydrologic Influences 2. How does stratification vary with tidal-range (29-day periodicity) and ebb- versus flood-tide conditions? 1.How does the estuary salinity intrusion respond to flood- versus base-flow conditions? How may the previous discharge history influence the location of the salinity intrusion? Proposed data collection for stratified section of estuary: Change in stratification w.r.t time Along-channel and vertical velocities Differences in salinity (in the vertical and horizontal) DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

13 Field Study: Velocity, Salinity and Elevation Measurements CTD salinity sampling Minimum of three stations M42, M44, and M46 along transect 1-m increments July 12, 2005 July 26, 2005 (DWQ) ADCP velocity transects M42 to M46: mid-estuary ~ 3 km per transect ~ 14 hours: flood, ebb, slack 1-m increments July 12, 2005 (Apogee) July 26, 2005 (Perigee) DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

14 Field Study: Velocity, Salinity and Elevation Measurements Coordinated salinity sampling with DWQ at stations M42, M44, and M46 on July 26, 2005 Tidal elevations S 4.0 near mid-estuary June 28-August 8, 2005 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

15 Summer 2005 Study Area Fear Cape Salinity Stations Southport Wilmington River S4.0 M42 M46 N NAVASSA Tide Gauge M44 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

16 River and Wind Environment Summer 2005 Study Period 51 m 3 sec m 3 sec -1 7/12/05 Average daily wind speed: 2.9 m/s generally from S or SW (from 8 am to 11 pm) Maximum daily wind speed: 6.2 m/s 7/26/05 Average daily wind speed: 2.1 m/s generally from N, NW, calm or var. (from 9 am to 11 pm) Maximum daily wind speed: 3.6 m/s DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

17 Tidal Environment for Summer 2005 Study Period Wilmington Measured Tides S4.0 Measured Tides June 28-Aug. 7, 2005 July 12: LOW July 26: HIGH S S S 29 days DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

18 Along-channel Velocities Sampling Tidal Range ~ 1.0 m Sampling Tidal Range ~ 1.4 m DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

19 Apogee Transect Average (7/12/05) DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

20 Perigee Transect Average (7/26/05) DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

21 Stratification DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

22 Scientific Questions 2. How does up-estuary transport differ? How may in (1) be related to (impact) up-estuary transport? 3. What differences are observed in the density, shear, and mixing characteristics? How do these differences influence the evolution of the bottom boundary layer (BBL)? 4. How well can observed differences be explained by classical estuarine analysis (analytical solutions, physical scaling)? 1.What determines differences in patterns of stratification observed during the flood vs. ebb tide and high vs. low tidal ranges? DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

23 Stratification Frequency N 2 = N = Stratification Frequency DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

24 M44 Density Distribution (7/12/05) Depth (m) Transect No. Kg m -3 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

25 M44 Density Distribution (7/26/05) Transect No. Depth (m) Kg m -3 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

26 Vertical Salinity Sections (7/12/05) Transect 1 Near slack Transect 4 Strong Flood Transect 6 Weaker Flood Transect 10 Strong Ebb Transect 12 Near slack Transect 11 Weaker Ebb 19 ~“saltiest” apogee profile M46 M42 Depth below surface (m) DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

27 Vertical Salinity Sections (7/26/05) Transect 1 Start of Flood Transect 4 Strong Flood Transect 8 Weaker Flood Transect 16 Strong Ebb Transect 18 Near slack Transect 17 Weaker Ebb 17 ~“saltiest” perigee profile M46 M42 Depth below surface (m) DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

28 M44 Density Stratification Structure and Bottom Salinity: 7/12/05 F (“convective”) F (“constrained mixed”) E E S F = Flood E = Ebb S = Slack Kg m -4 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

29 M44 Density Stratification Structure and Bottom Salinity: 7/26/05 F = Flood E = Ebb S = Slack E F (“convective”) F (“partially constrained mixed”) E S Kg m -4 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

30 Strong “internally produced” shear on max. and falling ebb tide Transect Average Shear Structure: 7/12/05 and 7/26/05 s -1 “internally produced” shear on rising and near max. flood tide Strengthening of ebb near surface DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

31 Horizontal Richardson Number Defines “threshold condition” for onset of stratification (Stacey et al., 2001, Stacey and Ralston, 2005, Simpson et al, 2005) Assumes advection and vertical turbulent diffusion balance to determine degree of stratification (evolution of stratification equation) Defined by: Ri x > threshold value (order 1)water column stratifies Ri x < threshold valuetidal mixing sufficient to break down stratification β = saline expansivity u * = friction velocity (“scale of turbulent velocity fluctuations”) H = depth, g = gravitational acceleration, Γ ~ DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

32 CFRE Low Flow Horizontal Richardson Number Goal: To explain the onset of prolonged stratification in the CFRE (during apogee) Means: Compare Ri x during 1) perigee flood, ebb and 2) apogee flood, ebb where β = saline expansivity, A)Parameterize u * 2 = C d U 2 where U is boundary layer averaged mean velocity (Stacey and Ralston, 2005) H = depth, u*= friction velocity, g = gravitational acceleration, Γ ~ from field data where u * = friction velocity = (τ o / ρ) 1/2 where τ o = bed stress B) Determine the height of the bottom boundary layer (BBL) (Stacey and Ralston, 2005) 1)Corresponds to height of maximum tidal velocity on flood 2)Determined by interaction of stratification and shear on ebb C) Calculate: DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

33 CFRE Low Flow Horizontal Richardson Number Sampling Cycle Ri x C d =0.001 Ri x C d = /12/05 Apogee Strong Flood (Transects 3,4) /12/05 Apogee Strong Ebb (Transects 10,11) /26/05 Perigee Strong Flood (Transects 4,5) /26/05 Perigee Strong Ebb (Transects 15,16) DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

34 Stratification Stratification maintained Ri x > ~1 water column stratifies water column stratified For max. flood, ebb and For C d = DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

35 Salinity Structure 7/12/05 Flood-tide Near-bottom salinity: 18.0 to 21.0 ppt Ebb-Slack tide Near-bottom salinity: 20.9 to 19.2 ppt DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

36 Salinity structure 7/26/05 Flood-tide Near-bottom salinity: 16.6 to 21.4 ppt Ebb-Slack tide Near-bottom salinity: 21.3 ppt to 16.4 DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

37 Low Flow Findings: Summer Field Study The low-flow salinity and circulation structure is impacted by: 1) tidal range differences based on an approximately 29-day periodicity and 2) intra-tidal variations associated with tidal straining 1. Analysis of field data (summer 2005) supports the hypothesis that salinity, circulation and transport characteristics are critically linked to ebb-flood and tidal-range variability. During periods of reduced friction (e.g. apogee), stratification is high (approaches state of more “permanent stratification”) and up-estuary salt transport increases. 2. Analysis suggests the vertical differences (e.g. in stratification, shear) influence the evolution of the bottom boundary layer. Observations suggest the “salty bottom water” is generally constrained to a bottom boundary layer capped by a sharp density gradient during the low tidal range conditions (but are able to reach near-surface levels during higher tidal range, e.g perigee). 3. Initial computations suggest differences in stratification patterns may be parameterized based on the relative strength of stabilizing vs. destabilizing influences (e.g. Ri x ). DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker

38 References Chatwin, P.C., 1976, “Some Remarks on the Maintenance of the Salinity Distribution in Estuaries,” Estuarine and Coastal Marine Science, 4, p Hetland, R.D., Geyer, W.R.,2004, “An Idealized study of the Structure of Long, Partially Mixed Estuaries,” Journal of Physical Oceanography, 34, p MacCready, P., 2004, “Toward a Unified Theory of Tidally-Averaged Estuarine Salinity Structure,” Estuaries, 27, (4), p Monismith, S.G., Kimmerer, W., Burau, J.R., and Stacey, M.T., 2002, “Structure and Flow- induced Variability of the Subtidal Salinity Field in Northern San Francisco Bay,” Journal of Physical Oceanography, 32, p Simpson, J.H., Williams, E., Brasseur, L.H., Brubaker, J.M., 2005, “The Impact of tidal straining on the cycle of turbulence in a partially stratified estuary,” Continental Shelf Research, 25, p Stacey, M.T., Burau J.R., Monismith, S.G., 2001, “Creation of Residual flows in a partially stratified estuary,” Journal of Geophysical Research, Vol. 106, No. C8, p Stacey, M.T. and Ralston, D.K., 2005, “The Scaling and Structure of the Estuarine Boundary Layer,” Journal of Physical Oceanography, 35, p DRAFT NC Division of Water Quality (DWQ) Meeting 7/27/06 presented by May Ling Becker


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