Coastal Banks: Definition and Delineation Criteria for Coastal Bank (DWW Policy 92-1) Issued: March 3, 1992
Purpose The purpose of this policy is to clarify the definition of coastal bank contained in the Wetlands Regulations, 310 CMR 10.00, by providing guidance for identifying 'top of coastal bank'. Regulatory Standards Coastal wetlands are defined in the Wetlands Protection Act (MGL c. 131, s.40) as:
"any bank, marsh, swamp, meadow, flat or other lowland subject to tidal action or coastal storm flowage".
Coastal banks are defined at 310 CMR 10.30(2) as:
"the seaward face or side of any elevated landform, other than a coastal dune, which lies at the landward edge of a coastal beach, land subject to tidal action, or other wetland".
When these two definitions are read together, coastal banks can be inferred to be associated with lowlands subject to tidal action or subject to coastal storm flowage. Coastal banks, therefore, can occur around non-tidal ponds, lakes and streams provided that these elevated landforms confine water associated with coastal storm events, up to the 100-year storm elevation or storm of record. Land Subject to Coastal Storm Flowage, in turn, is defined at 310 CMR 10.04 as:
"land subject to any inundation caused by coastal storms up to and including that caused by the 100-year storm, surge of record or storm of record, whichever is greater".
The Department uses the 100-year coastal flooding event as defined and mapped by the Federal Emergency Management Agency (FEMA) per the National Flood Insurance Program, as the maximum flood elevation associated with land subject to coastal storm flowage, unless recorded storm data reveals a higher flood elevation (which is the storm of record). Analysis Top of Coastal Bank Delineation The phrase "top of coastal bank" is used to establish the landward edge of the coastal bank (310 CMR 10.30). There is no definition for "top of coastal bank" provided in the Act or the Regulations. A Guide to the Coastal Wetlands Regulations, prepared by the Massachusetts Coastal Zone Management Office, upon which Conservation Commissions and the Department have relied for guidance, states that the landward boundary of a coastal bank is "the top of, or first major break in, the face of the coastal bank", and implies that it is easily identified using United States Geologic Survey topographic quadrangles. However, the scale of topographic quadrangle maps generally do not allow for parcel specific analysis. No further definition of "top of" and "major break" is provided. The following standards should be used to delineate the "top of coastal bank" [refer to figures 1-7 for a graphic presentation of the information below]:
A) The slope of a coastal bank must be greater than or equal to 10:1 (see Figure 1).
B) For a coastal bank with a slope greater than or equal to 4:1 the "top of coastal bank" is that point above the 100-year flood elevation where the slope becomes less than 4:1. (see Figure 2).
C) For a coastal bank with a slope greater than or equal to 10:1 but less than 4:1, the top of coastal bank is the 100-year flood elevation. (see Figure 3).
D) A "top of coastal bank" will fall below the 100-year flood elevation and is the point where the slope ceases to be greater than or equal to 10:1. (see Figure 4).
E) There can be multiple coastal banks within the same site. This can occur where the coastal banks are separated by land subject to coastal storm flowage [an area less than 10:1]. (See Figures 5 and 6).
When a landform, other than a coastal dune, has a slope that is so gentle and continuous that it does not act as a vertical buffer and confine elevated storm waters, that landform does not qualify as a coastal bank. Rather, gently sloping landforms at or below the 100-year flood elevation which have a slope less than 10:1 shall be regulated as "land subject to coastal storm flowage" and not as coastal bank (see Figure 7). Land subject to coastal storm flowage may overlap other wetland resource areas such as coastal beaches and dunes. Information Requirements for Project Review Due to the complex topography associated with coastal banks, the following requirements are intended to promote consistent delineations. In order to accurately delineate a coastal bank, the following information should be submitted, at a minimum,, to the Conservation Commission and the Department of Environmental Protection: the coastal bank should be delineated and mapped on a plan(s) to a scale of not greater than 1 inch = 50 feet, including a plan view and a cross section(s) of the area being delineated showing the slope profile, the linear distance used to calculate the slope profile, and the location of this linear distance. In addition, there must be an indication which of the five diagrams mentioned above is (are) representative of the site. Averaging and/or interpolating contours on plans can result in inaccurate delineations. Therefore, it is strongly recommended that follow-up field observations be made to verify delineations made from engineering plan data and as shown on the submitted plans. The final approval of resource boundary delineations rests with the issuing authority (Conservation Commission or Department of Environmental Protection).