104 West 40Th Street 14Th Floor

104 West 40th Street – 14th Floor New York, NY 10018 Phone (212) 768-7454  Fax (212) 768-7936 MEMORANDUM

To: Chris Webb, Weixa Jin From: Tucker Mahoney Date: March 9, 2009 Subject: Numerical Modeling Results M&N Job No.: 6266 Parsons Slough

The following memorandum is presented as an addendum to the previous memorandum submitted: ‘6266 Parsons Modeling Results 7-22-08.doc’. All background material and prior results are included in this memorandum.

The model being used for these presented tests to modify water levels in Parsons Slough and test the impact on velocities in Parsons Slough and the main chancel of Elkhorn Slough was developed by Phillip Williams & Assoc. (PWA). It has been calibrated and verified by PWA and has been used in previous studies. For the purposes of this study, the December 2005 verification period was used as it contains a full spring-neap cycle and it is the longest period available for simulation. The PWA model setup was not modified other than to incorporate the proposed alternatives.

1. Additional Simulations – Winter 2009 The client wished to model the impact of narrowing the entrance to Parsons Slough to widths of 25 ft and 50 ft and installing a weir at the entrance at a height of -2 ft or -5 ft. Due to limitations in model resolution, this could not be modeled explicitly and funds were not available to modify the model.

The client accepted that a box culvert could be used as a proxy for the weir scenario described above because the culvert could be implemented in the existing model without a major modification to the existing model geometry. The following two scenarios have been simulated and results are presented in this addendum: 1. One inflow/outflow culvert with a bottom elevation of -5 ft NAVD88 and a width of 25 ft. 2. One inflow/outflow culvert with a bottom elevation of -2 ft NAVD88 and a width of 50 ft.

Each culvert was chosen to have a height of 15 ft so that it would never be full and a length of 1 ft, to more closely approximate a weir. Flow is allowed both into and out of the culvert, depending upon tidal stage in Elkhorn Slough vs. Parsons Slough.

For both of these simulations, the following information is presented:  Velocity and water levels at Lower Parsons, South Marsh, Outside, Main Channel 1, Main Channel 2, Main Channel 3 as well as two additional points: Upstream 500 ft in

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the Main Channel (Upstream 500) and inside the culvert (Culvert). All points are shown in Figure 1-1on top of the model bathymetry.  An estimate of tidal prism is presented for each scenario and compared to existing conditions.

1.1 Modeled Water Levels Water levels at these locations are presented in Figure 1-2 through Figure 1-13 for both culvert configurations. Existing conditions are also shown for reference. The first figure for each location (e.g. Figure 1-2) presents the modeled water level over the duration of the simulation, while the second figure for each location (e.g. Figure 1-3) presents a shorter snapshot of water levels from February 13, 2005 through February 17, 2005). This second period encompasses a spring tide peak.

1.2 Modeled Velocity Modeled velocity results at each location are given in the attached Excel file: ‘Mach 2009 Addl Simulations - Velocity.xls’. The velocity is specified by its east and north components in ft/s.

1.3 Estimated Residence Time for Culvert Configuration 1 In this addendum, the residence time is estimated in the same manner as presented in ‘6266 Parsons Modeling Results 7-22-08.doc’. Please reference this memorandum for background and methodology.

Under Culvert Configuration 1, the volume exchanged through the culverts over this typical tidal cycle is 2,498,000 cy. Comparing this to the volume of Parsons Slough (approx. 2,783,000 cy) and the modeled existing conditions, it can be determined that the residence time has increased to be 1.4 times that of existing conditions, or approximately 2.2 tidal cycles.

1.4 Estimated Residence Time for Culvert Configuration 2 Under Culvert Configuration 2, the volume exchanged through the culverts over this typical tidal cycle is 3,232,000 cy. Comparing this to the volume of Parsons Slough and the modeled existing conditions, it can be determined that the residence time has increased to be 1.1 times that of existing conditions, or approximately 1.7 tidal cycles.

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Figure 1-1. Location of Water Level and Velocity Points (over Model Bathymetry – ft NAVD88)

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7 No Project Culvert Configuration 1 Culvert Configuration 2

4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/5/2005 12/10/2005 12/15/2005 12/20/2005 12/25/2005 12/30/2005 Figure 1-2. Water Level in Lower Parsons

4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/13/05 0:00 12/13/05 12:00 12/14/05 0:00 12/14/05 12:00 12/15/05 0:00 12/15/05 12:00 12/16/05 0:00 12/16/05 12:00 12/17/05 0:00 Figure 1-3. Water Level in Lower Parsons

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4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/5/2005 12/10/2005 12/15/2005 12/20/2005 12/25/2005 12/30/2005 Figure 1-4. Water Level in South Marsh

4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/13/05 0:00 12/13/05 12:00 12/14/05 0:00 12/14/05 12:00 12/15/05 0:00 12/15/05 12:00 12/16/05 0:00 12/16/05 12:00 12/17/05 0:00 Figure 1-5. Water Level in South Marsh

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4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/5/2005 12/10/2005 12/15/2005 12/20/2005 12/25/2005 12/30/2005 Figure 1-6. Water Level Outside of Culvert

4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/13/05 0:00 12/13/05 12:00 12/14/05 0:00 12/14/05 12:00 12/15/05 0:00 12/15/05 12:00 12/16/05 0:00 12/16/05 12:00 12/17/05 0:00 Figure 1-7. Water Level Outside of Culvert

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4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/5/2005 12/10/2005 12/15/2005 12/20/2005 12/25/2005 12/30/2005 Figure 1-8. Water Level at Main Channel 1

4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/13/05 0:00 12/13/05 12:00 12/14/05 0:00 12/14/05 12:00 12/15/05 0:00 12/15/05 12:00 12/16/05 0:00 12/16/05 12:00 12/17/05 0:00 Figure 1-9. Water Level at Main Channel 1

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4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/5/2005 12/10/2005 12/15/2005 12/20/2005 12/25/2005 12/30/2005 Figure 1-10. Water Level at Main Channel 2

4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/13/05 0:00 12/13/05 12:00 12/14/05 0:00 12/14/05 12:00 12/15/05 0:00 12/15/05 12:00 12/16/05 0:00 12/16/05 12:00 12/17/05 0:00 Figure 1-11. Water Level at Main Channel 2

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4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/5/2005 12/10/2005 12/15/2005 12/20/2005 12/25/2005 12/30/2005 Figure 1-12. Water Level at Main Channel – Upstream 500ft

4 ) 8 8 D V

A 3 N t f ( l e v e

L 2 r e t a W 1

-2 12/13/05 0:00 12/13/05 12:00 12/14/05 0:00 12/14/05 12:00 12/15/05 0:00 12/15/05 12:00 12/16/05 0:00 12/16/05 12:00 12/17/05 0:00 Figure 1-13. Water Level at Main Channel – Upstream 500ft

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