erosion and scour depth

huangyhg

erosion and scour depth
info on scour depth for structures by the sea. what is required for footing and pile by the sea shores?
where?  what are you planning to build?  do you have marine experience?
they are condos on the west coast florida 3 stories and really not much marine experience.
you didn't specifically say where in florida; but the local building officials ought to be able to provide some guidance.  fema may have data, too.  and the florida universities (with schools of engineering) should be able to help point you in the right direction.  others may have additional suggestions -
i'm aware of a building in pensacola that had about 11 feet (3.4 m) of scour, so the issue is clearly relevant.
is the condo on a barrier island?  if so, you should advise the developer to acquire any aerial photos taken immediately after hurricanes that made landfall "in the area" to look for beach erosion or old channel cuts through the island that are near the planned site.  (i'd get the photos for mainland sites, too, frankly.)
the first need for all pile footing work is the regime of the tide and current shift of the bed, or you will achieve scour by adding to the current flow deflection and wave vortices, lateral stress and scour hollows around the piers
secondly it is imperative to ensure there are no lense, or cells of soft material, especially organic plant waste in the pile zone or compression will occur and additional stress such that you piles will sink differentially (see mit library building)this can be acheived by seismic and bore studies, use an expert on soft ground, not one used to hard rock and clay
thirdly it is likely the material will be semi-mobile, especially in storm events and thus you have to calculate the load from the wave, the sediment lateral drift, the subsidence twisting of the base and possible lift is load is relieved on the basal materials and any core you make can cause erosion of the friction pile edge as it takes place
heavy engineering piles and possibly a large system of base protection
the work obviously needs to be conducted during calm weather so the effects during rough weather will not be apparent unless theory studies are enacted and then after the construction of the supports. it can be advisable to anchor to known rigid near shore so the system fails to drift around and obviously the tactic of oil rig can be used to assemble a sound unit that retains its form even if the base drifts off the site during a storm event. old pier forms used to be individual and the invention of oil rig towable units put that problem out of order but it does add cost and weight.
all beach systems have a stable, or balanced steep zone limit and a recognised scour pattern, including the search for rip current activity which a geomorphologist with local knowledge could advise upon. if the balance of erosion, restoration is altered as you piles will so do then new vortex and backwash parameters come inot effect and it is imperative to know how much the balance can be assumed to be retained and how far the pile can go in altering the base flow states. swash may even erode if the piles alter the incoming wave pattern substantially, although swash is normally a building form, it may remove material from you pile site up the beach. all piling delfects therefore the least the better, but to anchor and ensure structural strength in some manner you need to accommodate a high load of strength on as few piles as can be safely anchored. it might be best to get in touch with somebody alike hydraulics rsearch ltd wallingford england to do the scour studies if this is seen as a potential problem.marine piling as with many other forms is an individual site nightmare but unlike other forms where one can just deal with the local soils and possibly a river or some erosion and years of knowledge exist for living there by somebody, marine sites need erosion studies and are subject to the lackof data on wave sea events so the individual design has to be worked for the sands and silt,clay, rock ledges and organic waste and for wave incidence, regimes, unusual events, deflections caused by the design and heavy engineering installation ... mostly to do with backwash and impact due tot tide regimes. you have to assume that there are several beach level processes that may occur due to tides and storm raising and lowering the effective level. one large scour and the lateral stress fromsediment may be removed but a large hollow exists around your piles into which allthe wave energy is crashed.
mike     
an interesting post, mikehydrophysfff">fff">, but a little hard to follow.  might i suggest that you use the preview capabilities of the eng-tipsfff">fff"> system before you press the submit button?
i don't believe that scour studies are warranted - we aren't talking about a north sea winter, rather a gulf of mexico hurricane.  the hurricane's effects are much shorter in duration, and the direction of approach is a major unknown.  the bulk of the damage occurs during the surge tide, not simply waves pounding at the beach.  prior historical performance is a better tool for this particular problem, particularly for smaller projects.
the use of precast driven piles is quite common in florida.  i would expect the piles to be 24 inch (610 mm) square, and on the order of 60 feet (18.3 m) to 80 feet (24.4 m) long.  the pleistocene/holocene contact is at about -80 feet (-24.4 m) msl, so it's pretty hard to get much pile penetration below that depth.
properly designed, a building can easily withstand 15 feet of scour with no foundation damage.  of course, every window may be gone at that point...