Swamp Stomp

Volume 14, Issue 16

Last month the US Army Corps of Engineers (Corps) published several documents about how wetland vegetation is determined. They are reexamining the calculations and methods that we use to make wetland determinations. Some of the factors that the Corps considered for these studies include the importance of dominance (or not), plot sizes, the impact of stratum, and plot shape. All of these observations and conclusions are intended to be included in a new Corps wetland delineation manual.

This week we will discuss the document that focuses on the comparison between the formulas that are used to determine the presence or absence of a wetland plant community. The document is entitled,” Examining Discrepancies Among Three Methods Used to Make Hydrophytic Vegetation Determinations for Wetland Delineation Purposes,” (Lichvar and Gilrich, 2014).

The purpose and need statement for this document is quite clear. In the past there have been discrepancies between the use of the dominance test (DR) and the newer prevalence index (PI). The dominance test dates back to the original Corps Wetland Delineation Manual (1987). The prevalence index is now used in all of the regional supplements; however it first made its appearance to the regulatory scene in the 1989 Federal Manual. It was buried deep in the Federal Manual appendix as an advanced wetland delineation method.

There is a new method under consideration for replacement of both of these calculations. It is called Hydrophytic Cover Index (HCI). At present this method remains unpublished and is not to be used with any current wetland delineations. It will most likely be published in the Federal Register as part of the new Corps Wetland Delineation Manual. However, it is an interesting approach that appears to have much promise. Chief among its features is the elimination of the 50/20 rule for determining dominance. Have you ever tried to explain to a non-wetland person at a cocktail party what the 50/20 rule is? Well I have and it is not fun. For some reason, I do not get invited to many cocktail parties anymore.

There are several variables that were discussed in the Corps report that are interesting. Factors like number or strata, odd or even numbers of species, and plot size all had impacts on the traditional methods of determining a wetland plant community. It seems that the more strata you have the more likely you are going to find a wetland plant community. It also appears that plant communities that have odd number of species were more likely to be a wetland community than even numbered populations. In theory, these factors should have no bearing on the presence or absence of a wetland plant community. However, it appears that it does, hence the need to fix the problem.

The Hydrophytic Cover Index (HCI) method was compared to both of the existing in use methods. There were three tests. The first one compared bottom line results. In this test each method was run on data collected by the Corps in the field. The first test showed that the use of the HCI resulted in more areas being called hydrophytic. The PI (69%) and the DR (76%) produced significantly smaller percentages of hydrophytic vegetation determinations than the Hydrophytic Cover Index (HCI) (80%) produced.

The second test compared all three tests with the total number of all species found in the field. In situations where greater than 50% of the species were identified as Obligate (OBL), Facultative Wet (FACW) or Facultative (FAC) the HCI tested positive for hydrophytes 100% of the time. When the numbers of species with OBL. FACW and FAC indicators were less than 50% the HCI tested negative for hydrophytes 100% of the time.

The third test looked at the even versus odd number of species that were calculated using all three methods. The DR test seems to show that if you have an even number of species examined, there is a 10% decrease the area passing the DR test. The HCI and the PI tests did not seem to show this bias.

The actual procedure for using the HCI test was not described in the report. The formula was provided and is:

HCI = (Sobl + Sfacw + Sfac)/(Sobl + Sfacw + Sfac + Sfacu + Supl) × 100

All of the species identified in the study area are to be counted and run though this equation. It does not appear that the vegetation is separated into stratum (tree, shrub, herb, & woody vine). Dominance is also not calculated.

You should be able to use the data that you currently collect for The PI test to run this equation. It is fairly simple and very quick.

If the Corps and perhaps EPA decide to use this equation, it will be part of a new manual and put out for public comment. Until then the HCI method is not to be used for jurisdictional determinations. However, it might be worth your while to try it out on a few projects to see if there is any significant difference in the results. It would also be interesting to correlate the presence or absence of hydric soils and hydrology indicators with the HCI result. This would make comments to the new method a lot more meaningful and scientific when the time comes for the public to respond.

Have a great week!

Marc

Swamp Stomp

Volume 14, Issue 14

The Swamp School Guide to Using a Laser Level for Ecological Studies

The use of a survey grade level is critical for obtaining accurate measurements of various biological features, biological benchmarks, etc.  This information is used for many purposes including stream restoration, coastal restoration, wetland restoration, and other design purposes.

There are two types of levels used for construction and design.   The older of the two is known as the Dumpy level.  This level is like a spotting scope with cross hairs.  It is highly accurate (despite its name) and also has an added advantage of being able to measure distance.  However, it does require much more work to operate and is limited to a range of about 30 feet.  It also requires two people to operate.

Laser levels are the other commonly used measuring tool and are a great improvement over the Dumby levels.  The major benefit is that the distance away from the level is pretty much as far as you can see.  This reduces the number of station moves and speeds the process along.  You also only need to have one person to operate the level.

There is a third option which is to use a surveyor total station.  This however, is a very complicated process and usually beyond the level of detail needed for most biological assessments.  A corollary to this is the use of GPS.  GPS is great for x and y coordinates, but it is often meters off on the elevation (z).

Laser Levels

You do not need to spend a lot of money to purchase a quality laser level.  You can often find these for sale in big box home improvements stores and hardware stores.  They are around a couple of hundred dollars.  You can also rent one from a survey supply shop for about $20-$30 per day.  They will also be happy to sell you one if you need to use it more than just a few times.  Survey grade levels are usually in the $500 to $1000 range.  This is worth the investment if this type of work is a regular thing for you.  Also, do not cheap out on the box.  The level will get bounced around so you will need a quality instrument case.  This is sometimes a problem with the home center levels.

The laser receiver is usually included with the laser level.  This is a little box that attaches to the survey rod with thumb screws.  It takes batteries and makes a tone when the laser beam from the level hits it.

You also need legs.  The level should be placed upon a quality tripod.  This is not the same type as you use for a camera.  Survey grade tripods are usually made of wood or aluminum and have steel spikes to set it into the ground.  The legs are adjustable so that you can level your level.  The level should be about chest high when mounted so the tripod needs to be 4-5 feet high when set up.

Next on your shopping list is a survey rod.  You want to get the smallest rod that will serve the purposes of the site work you are doing.  A 12 foot rod is much better than a 25 foot rod if you only need to go up a few feet.  The bigger the rod the more sway you have and the measurements will be less accurate.  However, if you have a steep slope on you site a bigger rod may be necessary.

You will also need a measuring tape or carpenters rule.  It is better to get one that is calibrated to 1/10’s of a foot rather than inches.  The survey rod is almost always in 1/10’s of a foot, however make sure you are not using some sort of metric rod.  That really clean rod in the back of the tool closet that nobody uses might be there for a reason.

Set Up

The first thing that you want to do is take a walk around the area that you need to survey.  You want to find the best place to set up the level so that you do not need to move it more than necessary.  Keep an eye on slope, trees and other obstructions.  The level needs a clear line of sight.  You can clear some of the vegetation away, but it is usually easier to find a spot that would require the least amount of work to get your shots.  The tripod should be set up above the highest point you are going to survey.  You need to include the height of the level receiver on the rod when you are making this estimate.  This translates to about 5 feet above your highest point.  The level needs to be able to “see” the receiver.  If the level is set is too low it will shoot below the receiver mounted on the rod.  You can move the receiver down, but that would require that you recalculate for those shots.

Keep the level in the box until you are ready to place it on the tripod.  Do not attach it to the tripod and then walk around with it.  It should be boxed when moving it around the site.

Set the legs up on as level a surface as you can.  Adjust the legs so that the level mounting plate is fairly level.  You can use a hand level to do this or the bubble level on the laser level itself.

The laser level attaches to the tripod by way of a large screw below the mounting plate.  Do not tighten this too much until you have leveled the level.  There are three or four leveling screws on the level.  There is also a glass bubble level on the mounting plate.  Adjust the leveling screws so that the level is dead on level in all directions.  This will require that you spin the level around and make adjustments.  If you have attempted to level the tripod before you mounted the level, this will go fairly quickly.  It is critical that the level be mounted level.  Otherwise your data is junk.

Instrument Height

There is usually a marking on the level where you should measure downwards to the ground.  We are also going to determine height of instrument using the back site, but you should always measure the distance from the instrument to the ground.  We don’t really use this data, but it seems that it is always done.  It is sort of a cross check.

Backsight

You should place a project benchmark somewhere near the level set up.  This serves as your project control and can be surveyed for real later if you need to derive actual elevation points from your level runs.  This control should be set using a pin, rod, pipe or other relatively permanent maker.  Wooden stakes do not work as they can be easily removed or damaged.

The backsight elevation is any number that makes sense.  The convention is to set it at 100.  You may come back later in drag control onto the site to determine the actual elevation, but that is not necessary for this type of work.

The laser level indicator should be attached to the rod, usually at the tip.  Note the location of the indicator.  For example it is on the rod at 4.5 feet.  Most telescopic rods have a height indicator on the back of the rod.  As you raise the rod the height indicator numbers will change.  Be careful to raise the rod in the proper order.  This varies with some rod types so be sure to check with the manufacturer on the use of the rod.

A Direct Elevation Rod or a “ Lenker Rod ” is the most common type and has numbers in reverse order on an  graduated strip that revolves around the rod on rollers. Figures run down the rod and can be brought to a desired reading—for example the elevation of a point or benchmark. Rod readings are preset for the backsight, and then, due to the reverse order of numbers, foresight readings give elevations directly without calculating backsights and foresights.

Turn the laser on and position the rod on the benchmark and raise the rod until you hear a steady tone.  You will usually hear a slow chip when you are just below the laser bean and a fast chirp when you go past it.  Note the rod reading.  That is your backsight reading.  Add the elevation from the benchmark rod reading and you have your height of instrument (HI).

For example.  Your benchmark is elevation 100.  Your rod reading on that benchmark is 4.06.  Therefore your Height of Instrument (HI) is 104.06’.  Your benchmark elevation should be lower than the instrument ground location.  Otherwise you need to adjust the detector and do some more math.   It can be done, but it takes more time.

Now you are ready to go to work on the foresights.

Position the rod directly on the ground at each feature shot.  The rod should be straight up and down.  There are plumb levels you can attach to the rod to help you.  Some laser detectors will also beep at you if you are too far out of plumb.

Raise the rod until you hear that steady tone.  Note the rod reading.  Make sure that you raised the rod in the right order and that the numbers are being read correctly.  Usually, this is a matter checking reality.  If your rod reading suddenly jumps by 5 feet from the last point you may have raised the rod sections in the wrong order.

Record each feature and provide some notes.  A level book works great for this.  This is an example of a level book set up.

Set  Up  1

At each feature subtract the foresight (FS) from the Height of Instrument (HI) to derive the elevation.

In this example we have a 1.28’ difference from mid tide (MT) to Mean High Water (MHW).  We can therefore assume that our total tidal exchange would be 2.56’ from Mean Low Water (MLW) to MHW.

We would need to check many other points.  Usually for a biological benchmark survey we would to stationing along a cross section.  Each feature would be relative to the feature type and its location on the cross section.  In our example above the distance from the MT to MHW is 25’.  This is measured by setting a fixed staring point at 0+00 and measuring along that line.

If you need to move the level you will need to calculate a new HI.  Make sure to reference the setup with the data.  Start a new table for a new set up.

Finally, if you need to determine the real elevations of your features survey the benchmark.  This will require a surveyor to locate nearby elevation control and drag that onto the site using traverse lines.  You can also use high end GPS for this.  In our example above the real elevation for the benchmark is 456’ NAVD 88.  Our new HI is 460.06.  We need to make sure we cite the vertical data source.  In this case it is North American Vertical Datum of 1988 (NAVD 88).  Therefore our MT is 453.43’ NAVD 88 and the MHW is 454.71’ NAVD 88.  You can do this for all of the data associated with the benchmark.

 One final note of caution

If you are doing level runs for design purposes you may or may not need a licensed land surveyor to sign off on them.  However, if you are doing any floodplain calculation work you will most likely need the help of a licensed surveyor.    Some jurisdictions allow licensed professional engineers to do this as well.  This is a matter of state and federal law so be careful and ask questions.

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