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Measuring Pond and Tidal Elevations at Heron’s Head Salt Marsh

Wayne

This is the other side of Cris’s previous post. My job on that outing, and on a subsequent outing, was to measure the elevation of the Heron’s Head Park salt marsh ponds and of the tides that wash those ponds. Here is the most recent version of our instrument:

Measuring Pond NS-1


Our new system consists of a STRAIT-LINE Laser Level 120 mounted on a photographer’s tripod, a Rod, and a Stanley 8-meter steel tape.

The Rod was made from a 1-inch X 96-inch aluminum tube, fitted with an HDPE plastic round “Target” attached to the tube by an HDPE plastic block drilled for a tight-fit on the tube.

The block allows the Target to slide up and down the rod. A similar HDPE block was fitted below the target, also made for sliding up and down. Both blocks have one beveled edge that faces out, presenting a sharp edge.

On the Target block, the spot where the laser light strikes the tube can be set to the sharp beveled edge by sliding the Target up and down. On the bottom block, upside down with respect to the Target block, the beveled edge can be set exactly on the water level while the end to the Rod rests on the bottom of the pond. Once a measurement is made, the metric steel tape is used to determine the distance in meters between the bottom block and the target block.

Bottom block

Our previous attempt at pond elevations looked like this:

First try pond elevations

This elevation plot was made before the Rod was built. Then, we used the Stanley tape by itself, focusing the laser spot on the tape while it was held above the land or water.

The land elevations were not very accurate, but the water elevations left a lot to be desired, as can be seen by the numbers under the green lines. The wind flapped the tape when it was held just to the water line. It was also very difficult to find the laser light spot with the narrow tape. Accurate water measurements were necessary, because we do not have a survey from a datum “bench mark”, but must determine the datum from the tide tables and the Bay water level at known tide times. This prompted invention of the Target and Rod.

Three separate measurements of the 3 Rocks elevation were made with the new Rod and Target, each at a different time. Subtracting tide levels, we got the following determinations of 3 Rocks elevation above the datum: 1.848-m, 1.863-m, and 1.844-m. Averaging and rounding off gave an elevation at 3 Rocks = 1.85-meters. This is the lowest point in the Pond Complex.

At the time of the measurements, water from the marsh and ponds was spilling over the 3 Rocks low point, and draining back into India Basin. The “3 Rocks” name comes from three large flat rocks placed on the path to provide stepping stones over this drainage. It was difficult to measure the elevation of water in this drainage point, but an accurate measurement of the water level in the pond at Site NS-1 gave a connected pond water level = 1.87-meters, rounded off from 1.865-m. In other words, the 3 Rocks low point was about 2-cm below the pond water level, permitting slow drainage from the Pond Complex through the marshy area and back into India Basin and the Bay.

The elevation of the bank between the pond and trail at NS-1 = 2.14-m; the elevation of the bank between the pond and the trail at OS-1 = 2.27-m; and the elevation of the shell mound bank at Big Pond = 2.67-m. These banks provide an effective barrier between the tide washing over the trail and the Pond Complex. Not shown on the elevations plot is that the bank from the Shell Mound east of Big Pond, and curving around and along the north side is also higher, well above the 2-meter mark.

On the elevation plot, this is what it looked like:

Heron's Head Elevations August drawing

Here you can see from the diagram that pond water easily drains out of the complex over the 3 Rocks low point. During rising tides, any tide over 1.85-meters above the datum will wash the pond at NS-1 by flowing through the marsh area and up into the pond. Tidal washing of the NS-1 pond does occur now and then, when the tide is 2.14-meters above datum. Equally, tidal washing of the pond at OS-1 results from Bay water flowing over the 2 Rocks low point, up through the NS-1 pond, and into the OS-1 pond. A more rare tide higher than 2.27-meters can flow over the bank between OS-1 and the trail.

So, how frequently are the Heron’s Head ponds in the Pond Complex washed by tides? If we examine a typical 2-month tide plot, we see:

Long tide plot showing rare washing of Big Pond, more frequent of other ponds.

What jumps out immediately is that Big Pond is only going to be tidally washed, resetting the salinity to the Bay average, only about once every month. In between those tides exceeding 2-meters [enough to flood over the “Connector”, the lowest point into Big Pond], only ponds at NS-1 and OS-1 are going to be tidally washed. Even then, complete washing of the lower Pond Complex will occur only every 15-days, or so.

This explains why the salinity excursions of Big Pond substantially exceed those of OS-1 and NS-1. Big Pond is reset to the Bay average, in summer around 33-PPT, and evaporation for almost 30-days drives the salinity up. The other two ponds are reset every 15-days, and have a shorter time for evaporation.

Now it is useful to look down on the Pond Complex and see it clearly from an aerial photograph.

The aerial photograph below was created from a number of photographs that Cris Benton took during trips to Heron’s Head. He has “stitched” those photos together into a montage that shows the “Pond Complex”. I supplied the heading, arrows, and general cutting and sizing to fit this page.

HERON'S HEAD AERIAL VIEW POND COMPLEX

What this aerial view shows is that the several ponds we first thought to be individual ponds are actually part of a complex of ponds connected by marsh and drainage. At the top, or eastern edge of the complex is “Big Pond”. Big Pond is protected from direct tidal action on the south shore line of Heron’s Head by a shell mound and ridge more than 2.5-meters above the datum. The high ground circles Big Pond on the east and north, completing the barrier to tidal flooding.

Only the neck of slightly lower ground, at 2.27-meters elevation and identified as the “Connector”, links Big Pond to the two ponds and marsh to the west. We see clearly from the aerial photograph, that the only path to Big Pond is through 3-Rocks, filling the lower ponds, then over the “Connector” into Big Pond.

As it turns out, the marshy ground between the pond from OS-1 to 3-Rocks is actually slightly sloping, so the pond at OS-1 is washed slightly less frequently than the pond at NS-1.

To be continued…

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