DEEP: Coastal Jurisdiction Line Technical Report

Coastal Jurisdiction Line - Calculations
 
 
 
What is the Coastal Jurisdiction Line in Connecticut?
 
As of October 1, 2012, the jurisdiction line for activities requiring permits under CGS Section 22a-361 has been changed from the "high tide line" to the "coastal jurisdiction line" (CJL).  In response to the legislative change, the Connecticut Association of Land Surveyors (CALS) and the Department of Energy & Environmental Protection (DEEP) have examined the tidal data and have determined elevations acceptable for each municipality which comport with the new statutory language for CJL.  While the law does not require that the CJL elevations be used, DEEP strongly encourages all coastal applicants to use them. 
 
For those individuals who elect not to use the pre-determined CJL elevations, DEEP has provided the acceptable methodology for independently calculating the CJL below.  How is the CJL elevation computed, and how does it differ from the former tidal regulatory line?  Basically, CJL is a series of elevations that are computed from the highest predicted tides found in Long Island Sound, and the Connecticut, Housatonic and Thames Rivers up to their respective heads of tide.  The CJL elevation is placed on the ground, where it is also represented on maps.
 
Why was a predicted tide chosen?
 
Predicted tide removes the effects of weather, leaving only the orbits of the sun and moon as factors in predicting the tide.  The orbits of the sun and moon can be modeled and computed and the highest predicted tide can be computed at a date certain.  Towns have been assigned a CJL elevation that reflects the long-term elevation of the highest predicted tide, fairly distributed along the shoreline and tidal rivers based on scientific data.  The older methodology was based upon observation of the evidence of annual high tides, including weather events, which could vary from one shoreline property to another.  The new CJL elevation cannot be determined by field investigation as there is no actual evidence of the CJL other than an elevation.  This is also true of the owners’ shoreline boundary, mean high water (MHW).  Both CJL and MHW elevations are determined from data collected and analyzed by the National Ocean and Atmospheric Administration (NOAA).
 
What is an elevation and why is it important?
 
NOAA is responsible for establishing mean high water elevations and computing predicted tides for the entire country.  This data is contained on NOAA's tides and currents website.  Both MHW and predicted tides are available for most of the established NOAA tide stations.  These elevations are based on a datum (where the data is 0.0 feet) called the mean low low water (MLLW) datum.  MLLW is the average of the lowest daily tides.  The values are averaged over the complete lunar cycle of about 19 years.  When we say MHW, we mean that it is the average of the two daily high tides over 19 years.  This 19-year cycle is called a "tidal epoch" and the predicted tide data used is in Epoch 83-01, or water height data from 1983-2001. 
 
By utilizing NOAA data for both MHW and CJL elevations, there is a single source for authoritative data and the data is in one system of elevations based on the MLLW datum.  Both elevations can be computed and placed on a site map in a short period of time, thereby speeding up the regulatory process for shoreline developments.  Unfortunately, this MLLW datum is water-based, and unique to each NOAA tide station.  Since the projects are sited on land, we must convert the water based datum to a land based datum, called the North American Vertical Datum of 1988 (NAVD88), which is also created, adjusted, and maintained by NOAA.  The land-based datum relates all ground elevations in North America to a single datum (where the data is 0.0’) which is at an arbitrary point that approximates mean sea level, but is not exactly mean sea level.  Many, but not all, NOAA tide stations contain this conversion factor.  In Connecticut, the Connecticut Department of Transportation computed NAVD88 conversions for all NOAA tide stations that lacked them.
 
In order for a property owner to prepare a map for coastal permit application, he or she can request a land surveyor to make a map that shows the shoreline boundary (MHW) elevation and the Coastal Jurisdiction Line (CJL) elevation, to help them plan, design, and evaluate where to site their shoreline improvement.  The land surveyor will find the nearest benchmark that contains an authoritative elevation in the NAVD88 datum.  He or she will then use a leveling instrument and rods to transfer the elevation to the site and certify the accuracy of the elevation placed on the site.  With a NAVD88 elevation on the shoreline owner's site, ground elevations can easily be collected to make a contour map that shows where MHW and CJL is on the ground.  Further, the land surveyor can place stakes at the MHW and CJL elevations so that the property owner can easily see the limits of his or her shoreline ownership and OLISP regulation.  This work can be accomplished in a matter of days. 
 
Can a shoreline owner perform this work by themselves?  Legally, only a land surveyor can map and stake the shoreline boundary (MHW).  An unlicensed individual with knowledge of leveling can bring in and mark the CJL elevation, however the liability for errors in that work then rests with the property owner rather than the land surveyor. 

How was the CJL Computed?
 
NOAA Highest Predicted Tides
 
The idea for using the highest predicted tide as a regulatory line for DEEP started with Bill Giel, PLS (deceased).  Mr. Giel wrote his own software for computing predicted tides, and compared his results to NOAA which were identical.  Data was generated for the tidal Epoch 83-01 and the highest predicted tide at the five closest NOAA primary tide stations was identified.  The high water results for four of the five stations occurred on the same date, October 16, 1993.  Then predicted high tides were generated back to 1907 and forward to 2040 and it was found that the predicted tide of October 16, 1993 was the highest tide during this time period.  Therefore, October 16, 1993 is taken as the highest predicted tide of the Epoch 83-01 – and incidentally is also the highest predicted tide for a period extending over 125 years.  These highest predicted tide elevations form the basis of the CJL.
 
Once the date for the highest predicted tide was established, values needed to be retrieved for these elevations in NOAA’s MLLW datum.  As noted above, there are five NOAA primary tide stations in and around Connecticut as shown in the table below:
 
 Town, State   
NOAA ID Elev., MLLW (ft) NAVD88 Conversion (ft) Elev., NAVD88 (ft) Conversion Reference
 Newport, RI  8452660
 5.25
 -2.04
 3.2
 NOAA
 New London, CT  8461490
 3.88
 -1.84
 2.0
 NOAA
 New Haven, CT  8465705
 8.14
 -3.56
 4.6
 CT DOT
 Bridgeport, CT  8467150
 8.82
 -3.84
 5.0
 NOAA
 Kings Point, NY  8516945
 9.65
 -4.12
 5.5
 NOAA
 
Table 1:  NOAA primary tide stations in and around Connecticut, showing the highest predicted tide (CJL) in Epoch 83-01.  Elevations are shown in water- and land-based datums.
 
Why are the primary tide stations so important?  Primary tide stations collect and analyze water elevations and weather every six minutes and the data have been collected over a span of decades.  All of the data can be accessed online at NOAA's tides and currents website
 
Anyone can look in the website at the highest predicted tide for October 16, 1993 and find the values shown in MLLW column of Table 1, in meters.  The conversion is 1 meter = 3.28083333 ft.  Primary tide stations are used as control to establish mean high water and other tidal information at numerous NOAA secondary and tertiary tide stations in Connecticut.  Secondary and tertiary tide stations have data that was collected for shorter time periods, and are not actively collecting tide heights on an ongoing basis. In our work, four of the five tide stations were utilized.  The tide station in Newport, RI was not utilized due to its long distance from Long Island Sound.  An additional resource for finding the highest predicted tide on October 16, 1993 lies with NOAA secondary tide stations.  These tide stations do not collect data in real time.  These tide stations collected at least one year of water elevations, and NOAA determined the 37 harmonic constituents necessary for computing predicted tide in their computer software.  NOAA has provided the highest predicted tides for October 16, 1993 for the following secondary tide stations:
 

Town, State 
NOAA ID Elevation MLLW (ft) NAVD88 Conversion (ft) Elevation NAVD88 (ft) Conversion Reference
Clinton, CT
8463701 
6.2
-2.8
3.4
NOAA
Black Rock, CT
8467373 
8.9
-3.8
5.1
NOAA
Southport, CT
8467726
9.1
-3.8
5.3
NOAA
S. Norwalk, CT
8468448
9.4
-4.0
5.4
CT DOT 
Long Neck, NY
8468799
9.5
-4.0
5.5
CT DOT

 

Table 2:  NOAA secondary tide stations in and around Connecticut, showing the highest predicted tide (CJL) in Epoch 83-01.  Elevations are shown in water and land based datums.
 
These nine NOAA tide stations provide the fixed elevations for the highest predicted tides or CJL in their respective locations. These values should be used to a precision of 0.1’.  The highest predicted tide elevation for each shoreline and tidal river town needs to be adjusted in a rational way between these fixed points. 
 
Computing the CJL Along the Shoreline of Long Island Sound
 
Once the fixed CJL elevations were established, the remaining towns along the shoreline and at the entrance of the three tidal rivers were computed.  There are many mean high water elevations established by NOAA along the Long Island Sound shoreline and the three tidal rivers. The grade of elevation rise or fall between tide stations for MHW followed the change of grade between highest predicted tides in many respects.  Since these changes of grade were well established for MHW, the CJL followed these changes of grade and the CJL elevation was prorated to follow the changes in the MHW elevation between the primary stations. 
 
After plotting the locations of MHW elevations on a map of the Connecticut shore, it could be seen that were two general areas of little change.  The first area was located from Niantic east to the Rhode Island border, and the second from Westport westerly to the New York border.  Therefore, the CJL elevation in New London was generally held from Niantic Bay easterly to Rhode Island.  The area west of Westport in the western sound contained many fixed CJL elevations from NOAA that reinforced the fact that there is little change in elevation of the CJL elevation in the western sound.
 
The CJL elevation falls at a relatively gentle rate from Westport easterly to the border between Branford and Guilford, a drop of 1.0’, through a number of fixed CJL elevations from NOAA.  The fall in elevation of the CJL is greatest from Branford to Niantic, approximately 2.2’.  CJL elevations were prorated between Branford and Clinton (fixed tide station CJL elevation) using mean high water determinations as a guide.  CJL elevations were prorated from Clinton to Niantic by the same method.  A CJL elevation was computed for the geographic center of each town along the shoreline, as well as the mouths of the tidal rivers at the shoreline.  Because the change in elevation from one town to another does not exceed 0.3’, it was felt that the entire town could be assigned a CJL elevation without negatively impacting fairness to the shoreline property owner.  In the unique case of Branford, a second drop of 0.1’ was placed to insure a maximum 0.3’ change from one town to the next. 
 
Protocol for Determining the Coastal Jurisdiction Line
 
Individuals who wish to determine an individual CJL for a given property must use the following formula:
1. Using data from the NOAA tides and currents website, find "Products" and open the product listing for “Benchmarks (Present Epoch).”  Every tide station in the United States is listed by town and state, for ease of finding tide stations that bracket the site owner’s property. 
 
2. Identify and record the mean high water elevation and NAVD88 conversion elevation (note that the values shown are in meters and need first to be converted to feet).  The land-based elevation of MHW = (MHW value) – (NAVD88 value).   
 
3. Using Google Earth or another on-line mapping tool, plot the nearest fixed point CJL tide station from Table 1 and/or 2.
 
4. Measure and record the distance in miles between the two fixed tide stations.
 
5. Measure and record the distance from one of the fixed tide stations to the subject property.
 
6. Prorate the difference in elevation using the ratio between the two fixed tide stations and the distance between one fixed tide station and the subject property.
 
7. Use this elevation as the CJL for your project.
The computation process is shown in the following example: 
 
Example 1
 
A site owner lives in Branford, CT and wishes to compute the CJL elevation for her property. There is a CJL elevation in the NAVD88 datum at the primary tide station in New Haven (4.6’) and at the secondary tide station in Clinton, CT (3.4’), both shown in Tables 1 and 2.  Using a mapping tool, the owner can measure between New Haven Harbor and Clinton Harbor (20.5 miles).  She can then measure from New Haven Harbor to her parcel in Branford, CT (6.5 miles).  What would be the CJL elevation for her property in Branford?
 
This is a rather simple calculation. The difference in CJL elevation between New Haven and Clinton is 4.6’ - 3.4’ = 1.2’.  The difference in elevation for the CJL in Branford is prorated by the distance 6.5/20.5 or 0.31 or 31%.  The elevation of the CJL in Branford is computed as 4.6’ – 0.31(1.2’) = 4.2’.   The published table of CJL elevations for Branford shows a CJL elevation of 4.3’, which is 0.1’ higher than our calculation.  Why is there a difference?  One reason there is a difference is because there is a major change in slope of the CJL elevation at the Branford/Guilford border and east to Niantic.  In spite of this, the simple calculation is within 0.1’ of a more complex evaluation and will return results within 0.1’ of the published CJL elevation in all cases.
 
For the example of Branford above, the more rigorous calculation takes into account the difference in elevation of MHW between New Haven and Branford.  Primary tide station 8465710 in New Haven has a CJL elevation of 4.58’ and MHW elevation of 2.83’. The difference between MHW and CJL is 1.75’  The secondary tide station 8465233 in Branford has a MHW elevation of 2.67’.  Using the direct method, the height difference (CJL - MHW) from New Haven is applied to the MHW elevation in Branford.  2.67’ + (2.93-2.67) = 4.4’.  In this singular instance, after evaluating the CJL elevations in East Haven (4.5’) and Guilford (4.0’), the decision was made to drop the CJL elevation for Branford to 4.3’ so that a maximum difference in elevation between towns of 0.3’ was maintained to account for the slope.  The example calculation for the CJL elevation in Branford was 4.2’, which was within 0.1’ of the published elevation of 4.3’. 
 
Computing the CJL Along the Connecticut, Housatonic and Thames Rivers
 
The CJL elevations for towns along the Connecticut, Housatonic and Thames Rivers were computed starting at a point where each river met Long Island Sound.  In the case of the Thames River the NOAA primary tide station in New London Harbor served as the starting point.
 
There was a tertiary tide station at the mouth of the Connecticut River, Saybrook Jetty which was utilized as the starting point for the Connecticut River. The starting point for the Housatonic River was computed using the direct method from the primary tide station in Bridgeport.  The values for the starting point for each river are shown below:
 
Mouth of River Tide Station CJL Elevation (ft) MHW Elevation (ft)
CJL -MHW
 Connecticut
Saybrook Jetty      
NOAA Table 2
 2.9
1.4
1.5
 Housatonic
Direct method
from 8467150
3.0
4.8
1.8
 Thames
8461490
New London 
2.0
0.9
1.1
 
Table 3:  CJL elevations at the mouth of 3 tidal rivers in Connecticut
 
For each of the three rivers noted in Table 3 the direct method was used to compute the CJL elevation.  There are no NOAA tide stations on the three rivers having sufficient water elevation data to compute a predicted tide, but all of the tide stations on these rivers have a MHW elevation.  The direct method takes the difference between the CJL and MHW elevations at the mouth of the river and applies it to the NOAA published MHW elevations along these rivers.  Although the methodology here is very simple, the problem lies with finding NOAA tide stations that have a NAVD88 conversion number; a number of tide stations on the Connecticut River have no published NAVD88 conversion number.  CT DOT has provided unpublished NAVD88 conversions for all the tidal river tide stations, and these values were used in computing CJL elevations on the tidal rivers.  A simple example of this computation process is shown in the following example:
 
Example 2
 
A site owner in Portland, Connecticut wants to know the CJL elevation for a property along the Connecticut River.  The mouth of the Connecticut River has a CJL-MHW of 1.5’.  Therefore, if the MHW elevation is known in Portland, then one can add 1.5’ to that value and arrive at the CJL elevation.  On the NOAA tides and currents website under “Products,” find “Bench Marks (Present Epoch).”  Find tide station 8464336 in Middletown, Connecticut.  It has an NAVD88 conversion and the MHW elevation is 1.67’.  Middletown is across the river from Portland, so its MHW elevation would presumably be about the same elevation.  The site owner can compute the CJL as 1.7’+1.5’ = 3.2’.  Why is there a difference between the published CJL elevation of 3.3’ and the computed one at 3.2’?  Because the geographic center of Middletown is 28 miles above the mouth of the Connecticut River and Portland’s geographic center is 35 miles above the river’s mouth.  The problem with the tidal rivers is relating the tide station MHW elevations to the geographic center of the tidal towns along the rivers, rather than prorating the CJL elevations which were performed along the shoreline.
 
If an individual has questions regarding the use, determination, or calculation of the CJL, please contact the DEEP Office of Long Island Sound Programs Permitting and Enforcement Section at (860) 424-3034 for assistance.
 
 

Content Last Updated on September 27, 2012