SomersetMarshPostTidal_2011.pdf

June 2011

Geosyntec Consultants 289 Great Road, Suite 105 Acton, MA 01720

Prepared for:

MA Dept. of Fish & Game Division of Ecological Restoration 251 Causeway Street, Suite 400 Boston, MA 02114

Prepared by:

Somerset Marsh Tidal Assessment Post‐Construction

Somerset, Massachusetts

Project Background

The Somerset Marsh Site (Site) is located in Somerset, Massachusetts adjacent to Dublin Street. The Site consists of a gravel parking lot where an existing single 24‐inch corrugated metal culvert, approximately 113 feet in length, conveys flood tide flows from the Taunton River to the southernmost extent of the Somerset Marsh. An unnamed stream conveys freshwater to the northernmost extent of the Somerset Marsh. The marsh side of the culvert had a weir structure with a gate, which during winter months dropped to impound water in the Marsh, which would freeze and be used for a public skating rink. A culvert rehabilitation and watershed assessment feasibility analysis (Feasibility Study) was conducted in May through July of 2005, by The Horsley Witten Group, Inc. (Horsley Witten, 2005) to determine the performance of the single 24‐inch diameter metal culvert. The Feasibility Study also assessed the culvert restrictions downstream of the Somerset Marsh at the Dublin Street and South Street crossings. The Feasibility Study proved that under the existing conditions, sequential tidal dampening between the Taunton River and the Somerset Marsh was observed.

In 2010, the 24‐inch pipe was replaced with a 4‐foot by 4‐foot precast concrete box culvert to allow for more tidal flushing into the marsh and allow for restoration of salt marsh habitat which has previously been dominated by freshwater marsh species. According to the New York State Salt Marsh Restoration and Monitoring Guidelines, the most basic means of restoring salt marshes is reintroduction of tidal regime (Niedowski, 2000). To assess the post‐construction restoration, a tidal assessment was conducted and used for comparison to the pre‐construction tidal assessment data.

Photo 1. Downstream tide‐gauge location.

Tidal Survey

To assess the post‐construction restoration at the Site, two tide‐gauges were installed on April 7, 2011. Tide‐gauges consisted of Onset® HOBO U20 Water Level Data Loggers that have operational ranger from 0 to 30 feet with ±0.02 ft accuracy. One tide‐ gauge was installed in the marsh (S2) located upstream of the gravel parking lot on Marsh Street and one downstream (S1)of the box culvert (Photo 1), location shown in Attachment A. The tide‐gauges were deployed to collect continuous water level data at 6‐minute intervals for 28 days, spanning the monthly spring tide. The tide‐ gauges were assigned elevations using a known benchmark at the Site, which was a nail set in a utility pole (elevation 10.31 feet NGVD 29) found at the corner of Dublin Street and the Marsh Street parking lot.

Pre‐construction data at Somerset Marsh was collected by Horsley Whitten Group for 30‐days from May 3 to June 2 of 2005, at 15‐minute intervals (data was provided by Massachusetts Division of Ecological Restoration). As stated in the Feasibility Study, the Dublin Street gauge malfunctioned for the first part of the study (approximately 9 days) and from May 12 of 2005 through June 3 of 2005 collected water level readings. Figure 1 is a plot of 20‐days of pre‐construction data for the Site, which includes the spring tide for both upstream and downstream of the gravel parking lot.

Geosyntec analyzed the data, presented in Figure 1, and determined that from May 12 to May 20, the mean tide range was less than 0.10 feet, making it difficult to determine the tide cycle (high verses low) in the Somerset Marsh. As presented in Figure 1, from May 20 through June 2 of 2005, during the spring tide cycle, the tidal influence in the marsh is more pronounced and therefore, this data was analyzed for tidal metrics. Also, further inspection by MA DER of the pre‐construction tidal dataset revealed a survey error that placed the elevation of the downstream gauge (Dublin Street) 0.52 feet below its actual elevation (Figure 2). A corrected hydrograph is presented in Figure 3, and the assessment of pre‐construction tidal conditions is based upon this corrected dataset.

Geosyntec Consultants, Inc. (Geosyntec) collected post‐construction restoration data at the Site for 31‐days from April 7 to May 8 of 2011. Figure 4 is a plot of water elevation data over the complete tidal assessment period, Figure 5 is a plot of 10‐days of water elevation data from April 8 to April 18 of 2011 and Figure 6 is a single day hydrograph. To determine the differences between pre‐ construction and post‐construction and evaluate restoration, tidal metrics were evaluated on the tidal assessment data sets. The tidal metrics include:

• height of each tidal days Mean Higher‐High Water – the average of the higher‐high water within the survey period;

• els; Mean High Water – the average of all observed high water lev

• Mean Tide Level – mean of high water and low water levels;

• Mean Low Water – average of all observed low water levels;

• ter height of each tidal day within Mean Lower‐Low Water – average of the lower‐low wa the survey period;

• Tidal Maximum – highest tide during the survey period;

• Tidal Minimum – lowest tide during the survey period;

• Mean Tide Range – difference in height between Mean High Water and Mean Low Water;

• Tidal Range Ratio – ratio of upstream (restricted) mean tide range over downstream mean tide range; and

• Maximal Tidal Dampening and Associated Phase Delay – largest disparity between up and downstream gauges and associated time delay between peak water elevations.

Tidal Assessment

Prior to deployment of the tide‐gauges at the Site, the gauge calibration, completed by the manufacturer, were verified by Geosyntec to ensure that the loggers were accurately recording water depth. The tide‐gauges were verified by resting the gauges in a bucket of fresh water for approximately 15‐minutes and recording water level during this time. The downstream and upstream gauges appeared to be accurately recording water level. At the end of the deployment period, the depth of water was measured prior to removing the gauges and used as verification that the gauges were accurately recording water depth during the deployment period. Since the gauges appeared to be accurately recording water level, no correction factors needed to be applied to the data.

On‐site elevation surveys of the tide‐gauges were conducted by Geosyntec on April 26 of 2011.

Each of the tide gauges were assigned an elevation using the existing Site benchmark (nail in utility pole at El. = 10.31 ft NGVD 29). Based on the benchmark elevation it was determined that the upstream tide‐gauge was installed at an elevation of 1.43 ft NGVD 29 and the downstream gauge was installed at an elevation of 1.67 ft NGVD 29.

Table 1. Somerset Marsh Upstream and Downstream Tidal Metrics

Tidal Metrics

Upstream (S2) Condition Downstream (S1) Condition

Pre‐Construction1

Water Elevation (ft)

Post‐Construction2

Water Elevation (ft)

Pre‐ Construction1

Water Elevation (ft)

Post‐Construction2 Water Elevation (ft)

Mean Higher‐High Water 3.59 3.41 3.95 3.54 Mean High Water 3.41 3.22 3.74 3.34 Mean Tide Level 3.26 2.75 2.36 2.55 Mean Low Water 3.11 2.27 0.98 1.75 Mean Lower‐Low Water 3.02 2.26 0.93 1.73 Tidal Maximum 4.03 4.76 4.36 5.12 Tidal Minimum 2.77 2.19 0.32 1.67 Mean Tide Range 0.31 0.95 2.77 1.59 Notes:

1. Analysis based on 13 days of pre‐construction data that occurred during the spring tide, at 15‐minute collection interval

2. Analysis based on 30 days of post‐construction data, at 6‐minute collection interval

Table 2. Somerset Marsh PreConstruction and PostConstruction Tidal Metrics

Tidal Metrics1 Pre‐Construction Post‐Construction

Tidal Range Ratio 11% 60% Maximal Tidal Dampening (feet) 0.99 0.38 Phase Delay (minutes) 45 36

Notes:

1. Analysis based on 13 days of pre‐construction data that occurred during the spring tide, at 15‐minute collection interval and 30‐days of post‐construction data at 6‐minute intervals

Figure 3 shows that the post‐construction tidal cycles for the upstream (marsh) and downstream tide‐gauges and indicates that the tidal regime has been increased in the Somerset Marsh when compared to the pre‐construction condition. It appears that rain events on April 13 (2.55 inches), April 23 (0.61 inches) and April 28 (0.46 inches) of 2011, caused a rise in the low tide elevations.

On April 17 of 2011, approximately 0.80 inches of rain fell; however, during this event winds ranged from 20‐45 miles per hour (mph) which may have caused a slightly different shaped precipitatihydrograph that the other on events.

To determine differences between pre‐construction and post‐construction for the upstream and downstream systems, the tide metrics were calculated and are presented in Table 1. Table 2 presents the tidal metrics to assess the overall pre‐construction and post‐construction restriction severity.

The Tidal Metrics indicate the following tidal assessment results:

• Post‐construction Mean High Water decreased on the upstream side when compared to the pre‐construction by 0.20 feet. This is an artifact of the pre‐restoration condition being biased towards spring tide conditions, not encompassing the full 28‐day cycle, and water m g t eelevations within the upstream arsh bein perched above he h ight of neap tides;

• Upstream Mean Tide Range increased by more than 0.61 feet from pre‐ to post‐ construction, showing a change in the tidal regime in the marsh due to replacement the 24‐ inch culvert with the 4’ by 4’ box culvert. The increase in Mean Tide Range is largely due to improved drainage and the resulting decrease in Mean Low Water post‐construction;

• Post‐construction Low Water elevations (MLW and MLLW) are lower than pre‐construction conditions and illustrate that the marsh drains more completely post‐construction;

• Maximal Tidal Dampening for the system decreased by 62% from 0.99 feet pre‐construction to 0.38 feet post‐construction; and

• The Phase Delay for pre‐construction was estimated to be 45 minutes. The Phase Delay for the post‐construction is approximately 36 minutes, showing a decrease of approximately 9 minutes. Note, the recording interval for the pre‐construction data loggers was 15 minutes and the post‐construction was 6 minutes; therefore, the actual Phase Delay for pre‐ construction is likely between 30 and 45 minutes, indicating that there may not be a significant difference in the phase delay.

One indicator of the severity of a tidal restriction is the Tidal Range Ratio, which represents the ratio of the upstream and downstream tidal range (Carlisle, et. al., 2002). While several factors can influence the Tidal Range Ratio (e.g. differences in channel elevations and logger location, as well as physical hydrologic restrictions), comparison of pre and post‐restoration Tidal Range Ratios offers another useful means to assess the achievement of tidal restoration goals.

Under the pre‐construction condition the Tidal Range Ratio was 11%, indicating that the upstream side receives only 11% of the tidal range. Under the post‐construction, the Tidal Range Ratio increased to 60%. While the upstream marsh receives approximately 50% more of the available tidal range post‐construction, this comparison is spurious because downstream tidal hydrology was assessed in different locations before and after construction. Regardless, much of the post‐ construction increase in Tidal Range Ratio is the result of having alleviated the impounded conditions within the upstream marsh. However, examination of tidal metrics (MHW, MLW, phase delays, and tidal dampening) illustrate that the upstream site remains tidally restricted post‐ construction.

In summary, while there has been a marked hydrodynamic improvement (62% reduction in tidal dampening) in the Somerset Marsh, post‐construction conditions upstream of the new culvert remain tidal restricted, by as much as 0.37 ft during spring tide events. The potential impact of the survey error on the final project design has not been investigated. However, the inaccurate assessment of pre‐construction tidal conditions may have influenced culvert sizing and ultimately restoration outcome at the Somerset Marsh.

References

Carlisle, B.K., A.M. Donovan, A.L. Hicks, V.S. Kooken, J.P. Smith, and A.R. Wilbur. 2002. A Volunteer’s Handbook for Monitoring New England Salt Marshes. Massachusetts Office of Coastal Zone Management, Boston, MA.

Horsley Whitten Group, Inc., 2005. Labor In Vain Brook Culvert Rehabilitation and Watershed achusetts Riverways Program, Somerset, MA. Assessment Feasibility Analysis. Mass y Department, 1996. The Commonwealth of Massachusetts Survey MMassachusetts Highwa anual.

Niedowski, N.L. 2000 New York State Salt Marsh Restoration and Monitoring Guidelines. New York State Department of State Division of Coastal Resources and Department of Environmental Conservation, Albany, NY.

Figure 1. Somerset Marsh Pre‐construction data from May 13 to June 2 of 2005. Note response of the upstream hydrograph in the absence of forcing from downstream. This asynchrony is indicative of a survey error. In this case, the downstream gauge was surveyed

0.52 feet below its true elevation.

5/20 5/21 5/22

W at er E le va tio n (ft , N G

VD

9)

Date

Dublin St (downstream) Somerset Marsh (upstream) Dublin St. (uncorrected)

Figure 2. Pre‐construction tidal hydrograph from May 20‐23, 2005 illustrating relationship between the elevation corrected and uncorrected hydrographs for the downstream gauge (Dublin St.).

Figure 3. Corrected hydrograph for Somerset Marsh. Pre‐construction data from May 13 to June 2 of 2005.

Figure 4. Somerset Marsh Post‐construction data from April 7 to May 7 of 2011

Figure 5. Somerset Marsh Post‐construction data from April 8 to April 18 of 2011.

Figure 6. Somerset Marsh Post‐construction data for April 17 of 2011.

Figure 7: Spot elevations and water level logger location