Higgins Beach Management Plan
Section 3.3


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3.3 The Higgins Beach Sand Budget System

The overall morphological changes undergone by the Higgins Beach barrier spit over the duration of its recent history is typical of similar geological forms along the East coast - landward migration of the frontal beach shoreline and spit extension towards and into an adjacent inlet. This shift in shoreline environment shape is indicative of the long-term trend of sediment transport associated with the system.

The overall sediment transport is from west to east. Beach and dune sand is transported from the western end of the beach, east to the spit end by shoaling waves as they break on the beach. This transport is the long-term net littoral drift direction of the Higgins Beach system. Sediment is transported from a source to a "sink." The sediment source, the transport pathway or pathways, and sediment sinks constituted a sand budget system.

The Higgins Beach sand budget system, as deduced from geologic observations and investigation of the historical data, consists of a source of sediment, transport and temporary storage elements, and sediment sinks.

3.3.1 Source of Sediment for the Higgins Beach System

The sediment for most beaches along the southwestern Maine coast is derived from wave-reworked glacial river deposits. Submergence of these deposits during the Holocene sea-level rise has allowed local wave energy to rework these sandy deposits into beach and coastal dune systems (Timson, 1978). These sandy glacial deposits occur adjacent to existing beach systems or seaward of their respective beach systems as shallow submerged deposits veneering the bedrock.

Most evidence to date points to a genesis of beaches along the southwestern Maine coast approximately 3,000 years ago. This genesis was coincident with a general coastwide deceleration of sea level rise - a slowing down of sea level rise which allowed sufficient time for shoreline marine processes to rework glacial deposits into shoreline deposits.

Higgins Beach most likely originated from similar circumstances - its sediment originally derived from glacial deposits which mantle the bedrock or fill low valleys in what now constitutes the northeastern margin of Saco Bay or from the deposits which were a part of the ancestral Spurwink River.

The recent historical data, however, indicates that Higgins Beach is now erosional. It exhibits characteristics of a beach system which is losing more sediment than is being supplied to it. There are no extensive glacial sand deposits exposed along the shoreline either to the west or east of the beach system, and it is unlikely that extensive submerged deposits are supplying more or similar amounts to the beach as is being lost. For the most part, and certainly for the past forty years, Higgins Beach has been a beach system which has been consistently losing sediment. The source of sediment for Higgins Beach has been those beach and dune deposits formerly deposited during its early stages of formation.

3.3.2 The Transport Pathway of the Beach System

With the sediment source for the Higgins Beach system largely being the beach and dune itself, the transport of this sediment to a sediment sink must be closely linked with the beach as well. As has been discussed before, the net long-term sediment transport is eastward, along the beach to the spit end. Further, inspection of the historical photography indicates that some sand is also transported into the inlet mouth of the Spurwink to be added to sand bodies associated with the flood tidal delta, lower estuarine point and lateral sand bars, and a relict beach attached to the Cape Elizabeth headland (Figure 4). This is but one transport pathway of three through which sand is transported. It is probably the dominant transport pathway.

Another transport pathway of sand is that of a sub-system which originates from the Spurwink River estuary and transports sand to the beach system during periods of ebbing flood waters. Sand is eroded from estuarine sand storage bodies and transported seaward through the mouth of the inlet. Here, especially during rising tide stages, incoming waves transport sand over the surface of the ebb tidal delta associated with the inlet to the spit end of the beach system. This is a small transport sub-system which operates infrequently, but is nonetheless important in that it recycles sand temporarily stored in the inlet back to the spit end of the Higgins Beach system.

The third and final transport pathway sub-system operates during and after storm events. Storm events erode beach sediment and transport this sediment seaward and towards the west. Some sediment, higher on the beach, is transported eastward toward the inlet. The majority of sediment, however, is pulled offshore and westward to reside in the shallow nearshore until gentler waves generated by prevailing winds can transport this material back onto the beach (Figure 5).

3.3.3 Sediment Sinks in the Higgins Beach System

Some beach and dune sediment eroded from their respective environments is removed from the beach system entirely or for such long periods that it can be considered to be lost to the system over a time frame of up to 100 years.

Two sediment "sinks" are associated with the Higgins Beach system. One sink includes those sand bars and relict beaches occurring in the mouth of the Spurwink River estuary. Sediment is transported around the spit end on the intertidal reach and into the inlet channel. Some of this sediment is then transported by flood tide currents on subsequent tides to bar surfaces. Storage of sand on these bars is geologically temporary. Future storm ebb tides may re-transport this material out of the Spurwink inlet for reintroduction to the beach system. Other sand bar material may eventually be incorporated into the beach system as the beach system environments migrate further inland. This latter process of sediment reintroduction to the beach system would take place over a time frame of at least a thousand years.

The second sediment sink of the beach system is the shallow offshore area. Beach sediment is transported offshore during storm events. Much of this sediment lies just offshore from the beach immediately following a storm, and is subsequently transported onshore during periods of calmer wave activity. Some sediment, however, is transported offshore to areas of greater depth where subsequent wave activity is incapable of moving the material onshore again. This material is permanently lost to the beach system.

3.3.4 The Generalized Sand Budget System

From the foregoing discussion of sand budget elements, a generalized sand budget system can be described. The Higgins Beach barrier spit system consists of the beach elements (beach and dune), the Spurwink River estuary inlet elements (inlet channel, ebb-tidal delta, flood-tidal delta, and relict spit), the estuarine bars, and the shallow offshore bottom, as depicted in the previous figures.

Observations by beach residents and a study of beach profile changes at three locations between 1976 and 1980 indicate that the present beach system consists of three transport patterns which can be combined into one transport budget system.

The three patterns which constitute the overall budget system are:

  1. the storm budget,
  2. the post-storm budget; and
  3. the normal budget.

1. The Storm Budget

The most frequent storms to effect Higgins Beach are northeast storms. Waves from these storms approach from the east-northeast (Farrell, 1972). Higgins Beach, however, is directly protected from the east-northeast by Cape Elizabeth and Richmond Island. Wave patterns visible on aerial photographs indicate that northeast storm waves most probably approach the beach from the south-southeast and southwest after being refracted around Richmond Island. This wave approach direction, coupled with the storm waves capability of eroding sand from the beach and transporting it offshore, creates two transport pathways for sand eroded from the beach. These two transport pathways affect different areas of the beach.

Figure 5 indicates oncoming northeast storm waves to Higgins Beach. Because of the different configurations which the low-tide and high-tide beach present to these waves, sand eroded from the lower beach along the entire width of the beach and sand eroded from the upper beach on areas southwesterly of Vesper Street is transported both offshore and in a southwesterly direction. Sand eroded from the upper portions of the beach northeasterly of Vesper Street is transported along the beach into the Spurwink River. The 1978 northeast storm was characterized by a wave approach angle which generated northeasterly transport along the entire length of the beach at high tide.

It is estimated that over 109,000 m3 of sand were eroded from Higgins Beach during the February, 1978 storm. Most of this sand was deposited offshore, but some was probably lost into the Spurwink River.

2. The Post-Storm Budget (Recovery)

Figure 6 depicts the probable sand transport pathways during a recovery period which follows a storm. Following small magnitude or moderate storms, much of the sand lost from the beach probably returns within several months into the spring season. Observations of beach profiles after the large storm of 1978 indicate, however, that sand removed from the beach then may still be in the process of returning to the beach.

Wave approach to Higgins Beach during periods of prevailing onshore winds is from the south-southwest. These waves transport sand from the shallow offshore onto the lower intertidal beach to form low ridges in the low-tide terrace. These ridges eventually migrate to the upper beach to widen the beach berm. In the two years since the 1978 storm, approximately 15,000 m3 of sand have been returned to the Higgins Beach system.

3. The Normal Budget

The normal beach budget, as described herein, refers to the sand budget system well after storm recovery of the beach has taken place and most of the sand, which is going to return, has returned to the beach. Transport during this phase of the budget is entirely in a northeasterly direction along the intertidal beach. Sand is transferred from the western end of the beach to the eastern end of the beach by littoral drift. Thus, the eastern spit-end of the beach progrades and accretes at the expense of the western end of the beach. This was well documented historically by the spit progradation from 1952 to 1969 when the beach in front of Ocean Drive receded.

Also during this transport phase, sand is transported from the beach into the Spurwink River. Some of this sand is transported back to the beach via the inlet channel and ridge, migrating across the ebb tidal delta, but some is also lost to the beach system by becoming permanently stabilized as sandbars well up into the river channel (Figure 7).

The summation of these three budget phases is a continual loss of sand to the beach as a whole, although each portion of the beach may appear to have sand added to it from season to season or, even, year to year.

Sand is lost offshore from storms, or transported far enough into the Spurwink River so as to be lost from the year to year budget system.

3.3.5 A Semi-Quantified Higgins Beach Sand Budget

An established method for calculating how much sand is lost from a receding shoreline is an assumption, established by the U.S. Army Corps of Engineers (1972), that for each foot of recession of the shoreline, a cubic yard of sand is lost for every linear foot of beach length. This relationship does not hold up for all beaches, and, therefore, must be applied with caution.

Fortunately, three beach profiles established in 1976 by Prof. L. Kenneth Fink could be reoccupied in 1980, measured, and the observed volume changes quantified. The three profiles indicate general beach changes for three beach sections:

  1. Section 1: Houghton to Morning Street;
  2. Section 2: Morning to Kent Street; and
  3. Section 3: Kent Street to the Spurwink River inlet.

Volume changes as measured from the beach profiles (Figure 8) indicate that Section 1 loses 861 m3 of sand annually, Section 2 remains relatively stable, while Section 3 loses in the order of 2,200 m3 each year. These measured volumes compare favorably with calculations for the areas based on the U.S. Army Corps of Engineers relationship.

              Corps Method                       Profile Method
              ------------                       --------------

Section 1: 857 m3/yr. loss                       861 m3/yr. loss
            (Recession Rate of
             1.2 feet/yr.)

Section 2: stable                                stable

Section 3: 1,800 m3/yr. loss                     2,200 m3/yr. loss
            (Recession Rate of
             2.3 feet/yr.)

The Higgins Beach beach and dune system loses anywhere from 2,657 m3 to 3,061 m3 of sand annually. How much is lost offshore during storms versus how much is lost into the Spurwink River cannot be determined from the available data.


References:

(1) Farrell, S.C., 1972. Coastal processes, historical changes, and the post-Pleistocene geologic record of Saco Bay, Maine: unpub. PhD dissertation, Coastal Research Center, Univ. of Massachusetts.

(2) Timson, B.S., 1978. A handbook of recent marine coastal geologic environments: unpub. report for the Maine State Planning Office, Augusta, Maine. 167 pp.

(3) U.S. Army Corps of Engineers, 1972. Shore protection manual: Vol. 1-3, Coastal Engineering Research Center, Wash., D.C.


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Creation Date: October 30, 1997
Reproduced with Permission of Barry Timson and Arthur Lerman