The Swamp Stomp

Volume 14, Issue 12

A biological benchmark is a concept I first ran into while working in the Chesapeake Bay region.  They are used in shoreline restoration using native grasses.  These are often called living shorelines or vegetative erosion control.

The concept of biological benchmarks is based upon empirical data and direct observation of natural plant communities.  The issue relates to specific hydroperiods that the native plants can tolerate.  This results in an establishment of a given plant community based upon a frequency and duration of inundation by water.  Many plant species have highly specific hydroperiod tolerances that can be measured in the field and extrapolated elsewhere.

The best example of plants with highly specific hydroperiods  are represented by the coastal Spartina genus.  Two of the Spartina species in particular include Spartina alterniflora (cordgrass) and Sparitna patens (salt hay).  Both of these species occur in the intertidal zone and are found all along the east coast and the Chesapeake Bay.  They are salt marsh grasses and also tolerate some salt water.

What makes the two species of Spartina unique is that they only grow in two very distinct regions of the intertidal zone.  S. alteniflora is found between mid tide (MT) and extends up to mean high water (MHW).  S. patens picks up from there and occurs between mean high water (MHW) and mean high high water (MHHW) or spring tide.  What is amazing about this is that both of these species do not vary more than 0.1 feet in elevation from these tidal zones.  They are very precise about where they will live.

To establish our biological benchmarks we need to make sure that our study area is not under any major stress.  This mostly comes in the form of bank erosion and herbivory.  If either of these is excessive the area may not yield accurate results.  I also use a fetch rule of thumb.  Fetch is the distance from the shoreline across open water.  This is measured perpendicular to the shore line.   If the fetch is more than one (1) mile, I do not consider the site suitable for further study.  The wave action is just too great.  High boat traffic can also be a problem.  What usually happens in this circumstance is that my mid tide elevation is missing.

Once we have satisfied the disturbance issue we can start measuring.  We need to take a number of elevation shots using a level of the extreme limits of both the S. alterniflora and S. patens.  This is usually done using a laser level.  First establish and back site a site benchmark.  Get your instrument height and then you are ready to start measuring  fore site elevations.  Each fore site shot should be corrected and converted based upon the site benchmark.  I usually set this at elevation 100.  I will cover how to do this in next week’s newsletter.

You should see a consistent range of elevations that can be extrapolated to MT, MHW and MHHW.  This can be cross checked against the published tide tables for your region.   If you have a two (2) foot tide range then the elevation change from MT to MHW should be one (1) foot in height.  The cordgrass should confirm this.

You may ask why all the bother if we have the tide tables?  The answer relates to tidal restrictions and local variations.  The tide tables will tell you what the exact elevation is at a given tide gauge.    However, in survey terms you would need to drag that control across the water to your site.  If there are no restrictions and you are relatively close to the gauge you may be able to do this.   However, one bridge or culvert between you and the gauge can have a dramatic effect on the tidal exchange.

I had a project in upstate New York on the Hudson River that had a major problem with tidal restrictions.  It was a freshwater tidal marsh and had about a 1.5 foot tidal exchange at the gauge near West Point, NY.  The gauge was across the river but close by.  The marsh restoration designers had based there plant species selection and placement based upon the use of the gauge.  Unfortunately, they missed the fact that there was a railroad crossing bridge between the marsh and the river.  The bridge impinged the tidal flow into and out of the marsh by close to a foot.  The result was that the tidal exchange inside the marsh was about 0.75 feet rather than the calculated 1.5 feet.   Now this may not sound like a big difference, but it would result in the marsh planting being placed about a foot above the waterline.  This would be bad as the plats were all emergent species and required frequent inundation.  That is hard to do if you are a foot above the waterline.

Biological benchmarks saved the day.  We were able to establish the proper elevation for the new marsh based upon the observed limits of a few selected freshwater tidal species.   There is a species of Typha that is unique to the region that served as a great biological benchmark indicator species.  The end result was the design was lowered by about a foot and all the plants are happy.

The trick to all of this is the need to understand what the plants require.  Once you understand what the plants need the rest falls into place.

Have a great week!

Marc