Preliminary Coastal Analysis of Escalera Nautica
at Bahia Santa Rosaliita
By Seamus Ian Innes, P.E.
Tuesday, April 1, 2003
Oceanographic Conditions
The predominant waves in this region of the coast are derived from extratropical storms yielding west and northwest swell. Local sea comes mainly from the northwest and is due to the dominant north wind. Hurricane swell from the south and southern hemisphere swell are mainly blocked by the southern headland of Bahia Sebastian Vizcaino and Isla Cedros. Quantitative wave heights throughout the region site can be determined through numerical wave modeling and interviews with locals.
Tides in the region are a mixed diurnal and semi-diurnal. Conditions are similar to those found in Southern California, with an approximately 20 minute time lag. The typical spring tide range is approximately 2 meters.
Littoral Environment
The new marina is located in the northwest somewhat sheltered corner of Bahia Santa Rosaliita (also spelled Rosalillita, Rosalilita, Rosalia). This bay is part of a larger littoral cell extending from at least (but not limited to) Punta Rocosa in the north to Punta Rosarito in the south.
The net littoral drift in this region is from northwest to southeast. This is evidenced by the numerous crenulate, or hook shaped bays in the region with the rocky headland located to the northwest and the wider end of the logarithmic spiral extending to the southeast. In addition, the local net sand transport at the marina is to the east as evidenced by the wide beach buildup on the west side of the marina and the associated shoreline recession on the east side.
The primary littoral (sand) sources for this cell are the Arroyo (River bed) San Andreas, located between Punta Santa Rosaliita and Punta Rocosa, and two smaller arroyos emptying to the east of the marina. These arroyos deliver sand to the coastline during the infrequent rains. The Arroyo San Andreas is the larger source, creating a beach north of Punta Santa Rosaliita. Aeolian transport carries this beach sand to the dune system and to a small extent bypassing the point with deposition within Bahia Santa Rosaliita. The dunes covering a large portion of Punta Santa Rosaliita are somewhat stabilized with vegetation. The majority of the sand coming into the bay comes from around the point. While there is no beach on the west side of the point, there is evidence of a sandy bottom within a few meters of the water surface and there are small sandy pocket beaches at select locations. Sand is carried around the point with the predominant westerly wave action and is deposited just to the east of the point. This well known sand bar to the east of the point extends hundreds of meters offshore during low tide and provides well shaped breaking waves for surfing and windsurfing.
A small amount of sand is derived from the eroding volcanic rocks of Punta Santa Rosaliita.Marina Structures
The marina at Bahia Santa Rosaliita is the first in a series planned by Fonatur, the Mexican tourist bureau. The overall project is termed Escalera Nautica, with the goal to improve tourist revenue in the coastal regions through increased recreational boating facilities. This first marina is relatively small, with a planned 40-slip capability. To date, the rock jetties have been completed and some dredging has been performed.
The jetties extend ___ meters from the original shore, terminating in shallow water. After approximately one year from initial construction, the beach on the west side of the marina has widened extensively and the beach on the east side has eroded to the vegetation line. Survey data is available from Algalita Marine Research Foundation. The jetties are too short to effectively prohibit sand transport into the mouth and interior of the marina. They currently terminate in approximately 1 meter of water, relative to Mean Lower Low Water (MLLW). The current opening depth is approximately 3 meters below MLLW, and the rest of the marina basin is shallower.
The north revetment of the marina is exposed directly to open ocean swell penetrating through the marina entrance. It is expected that storm waves passing around the point will pass through the entrance creating uncomfortable if not damaging berthing conditions on this north face.
The jetty and revetment structures are of adequate rock material and size. The largest armor stone was placed on exterior faces, with successively smaller stones underneath. There is evidence of filter fabric between the armor stone and the core material. Common practice of using at least two layers of armor stone for each stone size was not followed. The current placement has only one layer of the exterior larger armor stone. This will result in a rapid deterioration of the jetties and high maintenance costs. In addition, a flat road exists on the top of the jetties, supposedly for maintenance purposes. This road itself will be a source of damage as the cobble and fine materials comprising the road are not covered with armor stone and are therefore susceptible to rapid erosion during high wave events.
A new concrete wall (less than one year old) was observed to be heavily damaged, with extensive cracking, spalling, and exposed rebar. Likely reasons for this rapid deterioration are poor materials and construction methods.
Due to the short jetty length and ample availability of sediment in the area, it is expected to be difficult to maintain the entrance depth required for a navigable entrance. A typical minimum entrance depth for this type of marina is 5 meters relative to MLLW and a typical minimum basin depth is 3 meters MLLW. Waves will likely break across the marina entrance during high wave events. Continuous dredging will be required to maintain the requisite entrance depth and sufficient basin depth.
Currently, the east jetty extends landward to approximately 10 meters landward of the vegetation line. It is reasonable to expect the beach east of the marina to continue to recede landward, likely resulting in erosion behind the landward end of the east jetty.
Extensive downcoast erosion has been measured during the first year after jetty construction. It is expected that some structures will be lost to the sea within another year. The historic sandy beach that did exist on the east of the marina has been replaced by a steep cobble and stone berm with some exposed bedrock and vertical sandstone beach scarping. The downcoast erosion will likely reach a dynamic equilibrium within a few years.
Recommendations
The first marina of the Escalera Nautica system is deeply troubled. The lack of planning and poor selection of location has resulted in a marina that will be very expensive to maintain. If it is determined that the marina is essential, we recommend some improvements that will make the marina useful some of the time.
- Hire a qualified coastal engineer to perform littoral studies and make recommendations on standard construction practice and structure dimensions. Request quantity and cost estimates for the amount of dredging that will be required as well as the required dredging cycle. This will be useful in determining the economic viability of the project.
- Extend the west jetty to reduce sand transport into the marina entrance and berthing basin. This will also reduce wave penetration into the marina. The jetty should extend out to the –3 or –4 meter bottom contour.
- Consider purchasing a small dredge to continuously maintain the basin and entrance depth. Dredge spoils should be placed east of the marina to offset erosion on that shoreline.
- The east jetty of the marina should either be lengthened landward or a shore parallel revetment should be constructed to limit landward erosion of the beach east of the marina. This would reduce the likelihood of wraparound erosion behind the marina.
It is likely that the marina will not be economically justified. In this case, it should be removed returning the beach to its natural planform and profile. If abandoned, the marina will provide a nice boat launch and swimming area and ultimately a good flat soccer field. This of course would be at the cost of the downcoast property lost from beach erosion.
Future Escalera Nautica projects should consult qualified consultants and perform adequate studies prior to construction.
Posted: 4/11/03