Tide restrictions usually result from the construction of a travel route over a salt marsh, particularly where a bridge or culvert is installed on the tidal creek. Tidal crossings are restrictive if they block or inhibit water from flowing freely from one side of the marsh to the other, resulting in a reduction of tidal influence on the landward, or restricted side of the estuary. The seaward, or unrestricted, side of the estuary is a good indication of what the restricted side would resemble in the absence of the tide restriction. In tidal influence studies, the unrestricted marsh is usually the reference marsh and the restricted marsh is usually the study marsh. A comparison of tidal ranges between the reference site and study site provides a good indication of the effect of the tide restriction on tidal hydrology.
There are two types of restrictive tidal crossings. One occurs when the opening of the culvert or bridge is too small or has started collapsing and does not allow natural amounts of water to pass through during each tidal cycle. The most common effect of this type of restriction is a decrease in salinity and especially flooding at high tide in the restricted marsh. The second type of restrictive crossing occurs where a culvert is elevated too high in relation to the creek bed. In this case, sufficient amounts of water may enter the restricted marsh during an incoming tide, but with a delayed effect since the tidal level in the unrestricted side must reach the height of the culvert before passing through it. Elevated culverts may prevent complete drainage of the restricted side because water cannot leave once water levels drop below the culvert, and even during low tide, there is standing water in the restricted marsh. Bank erosion may be evident on either side of the culvert with both types of tidal restrictions. Bank erosion resulting from tide restrictions is often described as "round-shaped pools," which form on either side and directly next to the culvert.
Why Should Tidal Flow Be Monitored?
A reduction in tidal flow can have numerous adverse effects on salt marshes, the most important of which is a change in natural salinity regimes. Many plants and animals that exist in salt marshes are adapted to a specific range of physical and chemical conditions, and large-scale alterations such as tide restrictions can cause intolerant species to perish. When salinity levels fall below 20 ppt, the invasion of opportunistic brackish plants such as Phragmites australis becomes a problem. Tide restrictions may also block the passage of estuarine invertebrates and fish into the upper estuary, thereby reducing the export of organic matter from the salt marshes. A reduction in tidal flushing may result in the accumulation of detritus, nutrients, and pollutants in the restricted marsh.
How Should Tidal Flow Be Monitored?
Automatic water level recorders will be installed and operated by project coordinators for a minimum of two weeks, i.e., one lunar cycle of spring and neap tides (one month, or two lunar cycles is better) near the source of tidal influx. For tidally restricted marshes, recorders will be installed both upstream and downstream of the tidal restriction. When automatic data collection gauges are unavailable, volunteers can be used to collect 10-minute measurements over 13-hour periods using a simple tide staff (a vertical ruler fixed in the tidal channel) for three spring and three neap tides would provide adequate information. Using either method, the elevations of the upstream and downstream devices are required, preferably referenced to NGVD (National Geodetic Vertical Datum).
Knowledge of the tidal current in the main channel can be useful when designating the tidal conduit for a tidal restoration and to assess the function of the current structure. Tidal current should be assessed over several tidal cycles and can be measured with a recording current meter.
Measuring the surface elevation of the marsh may assess net balances in critical soil processes that allow salt marshes to persist over time. Loss in elevation indicates peat degradation, whereas gains may be due to accretion at the surface or peat development below the surface. Standard survey techniques are unable to measure short-term changes in the sediment elevation (2 to 3 years), but are adequate for documenting long-term change (10 years or greater). Short-term changes in the sediment elevation of salt marshes around the world are being monitored using Sediment Elevation Tables (SET). Installation of the SETs is difficult and requires professionals. In New England, more than 30 stations exist in several salt marshes. Data are collected to provide baseline information, assess projects to restore hydrology, and assess sea level rise.