Sediment Erosion Characteristics for the Potomac River and Is there a critical bed shear stress for deposition?

1. Sediment Erosion Characteristics for the Potomac River andIs there a critical bed shear stress for deposition? Jerome P.-Y. Maa Virginia Institute of Marine Science College of William and Mary

2. Experimental sites for erosion characteristics Gunston Cove Douglas Pt Maryland Pt

3. VIMS sea Carousel

4. -3 10 Douglas Pt and Maryland Pt (05) -4 10 Gunston Cove (MD side, Aug. 04) -5 10 Gunston Cove (MD side, Sept. 05) EROSION RATE (g/cm2/s) -6 10 Gunston Cove (VA side, Sept. 05) -7 10 -2 -1 0 10 10 10 APPRXIMABED EXCESS BED SHEAR STRESS (Pa) -1 10 -2  (s-1) 10 Erosion is always near equilibrium during tidal acceleration phases and ceased for other times -3 10 0 0.2 0.4 0.6 0.8 1 BED SHEAR STRESS (Pa)

5. A sediment sample from Gunston Cove site on Virginia side

6. Q: Is there a critical bed shear stress for deposition ? Conclusions from previous studies were all based on the interpretation of the Suspended Sediment Concentration (SSC) time series, either in laboratories or fields, with conflicting results. Laboratory experiments from Mehta and others suggested that there is a cd and when the bed shear stress, b > cd , there is no deposition. Early field experiments (e.g. Sanford and Halka, 1993), However, cannot confirm the above statement, and that is the problem. Is it possible to observe deposition directly? We did in lab and working on alternatives at field.

7. Tidal level other phases Acceleration phase other phases Isovel (cm/s) TSS (mg/L) 30 cm An example of field measurements given by Sanford and Halka (1993)

8. Sanford and Halka’s (1993) interpretation Sanford and Halka’s (1993) interpretation b b cr cr cd cd Time Time E D C C Expected if E and D are mutully exclusive Expected if E and D are mutully exclusive Time Time E D Time Time Observed The concerns are (1) the interpretation is based on SSC at 30 cm above bed (2) D is downward flux, not necessary deposition, (3) New finding: E ~ 0 after the tidal acceleration phases

9. New interpretation b b cr cr cd cd Time Time E D SSC Upward flux Downward flux Time Time Because the downward flux, not necessary deposition, is always there, but upward flux only available when E > 0.

10. Mehta (1991) suggested the possible existence of a stirred layer right above the bed, but so far, there is no proof. • If we cannot observe it, for example, we cannot measure the “stirred layer” even in laboratory, then let’s provide an experiment environment that a stirred layer cannot be developed. A relatively strong “Secondary circulation” in the VIMS lab carousel is a good candidate. • Direct observation on “when” deposition occur

11. Cross section of the VIMS Lab Carousel 0.4 Average b = 0.37 Pa b (dyne/cm2) bdistribution 0.2 ring speed=8 rpm 0 1 1.05 1.1 1.15

12. A general Pattern of the bed shear stress distribution for the VIMS Carousel b distribution for different ring speeds b cd Deposition length 0 1 1.05 1.1 1.15 Outer wall R (m) Inner wall If cd does not exist, then we should observe deposition at anyplace on the bottom. However, we only see deposition at the corners where b is small, especially at the inner corner.

13. Initial SSC 3.5 g/L b (pa) SSC(g/L) Deposition length(cm) Time (min) Experiment Results using VIMS Lab Carousel

14. At a high bed shear stress (0.9 Pa)

15. Almost clear water Inner wall Outer wall At a low bed shear stress (0.042 Pa)

16. Conclusions: 1. Clear different erosion rates were observed in the Potomac River: a spatial gradient with high erosion rate near the turbidity maximum and low rate at other areas. More experiments (in-situ and lab) are necessary to address the temporal gradient and cover the entire river. 2. Direct observation of deposition shows that cd exists clearly in a laboratory experiment. Field observation is difficult because of the small deposition depth. Observation of the stirred layer is also not easy, but we are working on this subject. 3. Downward flux in the water column  deposition on bed 4. If cd exist, then erosion and deposition are mutually exclusive, for cohesive sediment.