Gulf of Thailand Tropical Storm Deposits

• Tropical storm washover deposits from 2007-2011were analyzed along the southern Gulf of Thailand coast, related to geomorphic conditions such as perched fans, washover terraces, and sheetwash lineations.
• Washover terraces and sheetwash lineations occurred where the beach configuration was uniform, creating unconfined flow.
• Non-uniform configurations led to confined flow and perched fan deposits.
• Sediments are categorized into thick-bedded sand with multiple reverse grading layers and medium-bedded sand with multiple normal grading layers.
• Thick-bedded deposits show sub-horizontal bedding, reverse grading, lamination, foreset bedding, and wavy bedding.
• Medium-bedded deposits are characterized by horizontal bedding, normal grading, and dunes.
• Rip-up clasts are rare, recognized in the bottom unit where mud supply was available.
• All washover successions have landward inclined-bedding with a sharp basal contact.
• Thick-bedded washovers are associated with high elevated beach ridges and large swales, while medium-bedded ones have small beach ridges with uniformly flat backshore topography.

Introduction

• Washover deposits result from high-energy seawater flooding across beaches or dunes from tsunamis and storms.
• Rapid flooding from coastal storms and tsunamis has caused damage to coastal communities and infrastructure.
• High-energy flows move sediments from the seaward side to landward areas beyond the beach zone.
• Analysis of storm-induced washover deposits has been conducted since the 1960s.
• Storm incidence leads to changes in beach morphology and coastal morphology which is subsequently studied after storm events.
• Stratigraphy of storm washover deposits is commonly a horizontal stratification of laminated sand, sometimes with foreset laminae in distal parts.
• Changes in coastal landform are classified into erosional features (dune erosion, channel incision, washout) and depositional features (perched fan, washover terrace, sheetwash lineations).
• Subfacies in storm deposits represent variations in flow conditions, position relative to sea level during overwash.
• Erosional and depositional characteristics of washover sediments vary based on barrier-island morphologies, vegetation, and sediment properties.
• Sedimentary characteristics and bedform surfaces include normal grading, reverse grading, laminae, sub-horizontal bedding, foreset bedding/laminae, antidunes, rhomboid bedforms, and current ripples.
• Key criteria are needed to differentiate tsunami from storm deposits due to similar sedimentary deposits.
• Factors such as topographical control, local sediment sources, and event intensity affect deposition.
• Thailand's coasts have experienced storm surges from tropical storms, but limited reports exist on storm deposits.
• Storm deposits have been reported along the Gulf of Thailand (GOT) coast.
• Detailed studies on sedimentary characteristics, topography, and flow conditions of storm-induced washover deposits are needed.
• Sedimentary characteristics of storm washover deposits are described from different geomorphic conditions associated with storm events in Thailand during 2007-2011.
• Distinctive sedimentary features of washover deposits from three geomorphic settings along the GOT coast were identified for comparison of topographical and flow conditions.

Setting and Method

• Thailand's climate is influenced by the southwest (SW) monsoon (May-October) and northeast (NE) monsoon.
• NE monsoon (October-February) brings cold, dry air from China, causing mild weather and heavy rain along the GOT coast.
• The sea level in the GOT is normally raised higher during the NE monsoon season due to seawater from the South China Sea, with the average change of sea level being 0.4 m.
• Eastern side of southern Thailand is affected by depressions, tropical storms, and typhoons during November-December, can cause storm surges and overwash, with temporary increases in monsoonal wind velocity also causing storm surges.
• Washover deposits from temporary strong NE winds are more frequent than those from tropical storms/typhoons.
• Strong NE winds induce storm surges being most common from November to January.
• Three sites were studied: Ban Takrop (BT) in Surat Thani, Laem Talumphuk (LT) in Nakhon Si Thammarat, and Khao Mai Ruak (MR) in Prachuap Khiri Khan, all affected by storm surges during 2007-2011.
• Five storm surge events were induced by seasonal sea-level rise with strong NE winds (2007-2010) and a low-pressure system (2011), with maximum wind speeds of 20-22 knots.
• Storm tide heights were 2.30-2.96 m above MSL with storm surges causing beach erosion and expanded inlet/outlet channels.
• Damage to roads and houses along the shoreline were recorded.
• Beach morphology changes, erosion, and deposition features, washover sediments were measured and photographed.
• Sampling and detailed coastal topographical profiles were made, and grain size analysis performed using a Camsizer with sediment compositions identified under a microscope.
• Washover deposit types ("perched fan", "washover terrace", "sheetwash") are classified based on the work of "Morton and Sallenger"
• Perched fan is a small lobate feature oriented perpendicular to shore.
• Washover terrace is elongate, oriented parallel to the shore.
• Sheetwash shows elongated erosion/deposition zones parallel to flow.
• Flood regime (overwash/inundation) follows Sallenger's storm impact model.
• Terminology for bed/laminae thickness follows Campbell.

Results

2007-2008 storm deposits at Ban Takrop (BT), Surat Thani

• BT area has a prograded shoreline with relict strand lines and a swale that narrows northward, with an outer beach ridge at 2 m above mean sea level (MSL).
• The storm tide high was at least 2.96 m above MSL, with a maximum inundation distance of 100-300 m from the shoreline.
• Washover deposit at BT exhibited a narrow band of sand oriented parallel to the shore, termed "washover terrace", being 30 m in width and 600 m in length, which splits at the distal part.
• Over ten lobes existed, approximately 10 m in width each.
• Washover sediment thickness reached 80 cm, terminating with a steeply avalanche face, burying a palm tree and showing a sharp contact with mud in the swale.
• The washover deposit dips landward showing eleven layers of coarse to very coarse-grained sand and laminae of medium to coarse-grained sand with reverse grading.
• Thicknesses range from 2-6 cm in the lower unit vs 4-7 cm in the bottom one.
• Foreset bedding up to 35° in distal margin.
• Grain size distribution in the coarse/very coarse sand layer and medium sand laminae shows unimodal and bimodal distribution, respectively, with bimodal distribution showing two peaks of sediment size, from potential contamination of layers from top/bottom mixing.
• Reconstructed surface from aeolian processes are similar to Australia's, having two layers split by thin reworked sand.
• Recent storm surge event of 2.56 m on 29th November 2007 caused overwash, as evidenced by fresh garbage and burial of Nipa palm.
• Sedimentary structures in washover deposits show lamination, foreset bedding, wavy bedding, and reverse grading.
• Horizontal bedding at the proximal part, whereas foreset bedding in the distal.
• Grain composition includes quartz, shell fragments, feldspar, and rock fragments which are moderately well sorted.

2009 storm deposits at Laem Talumphuk (LT), Nakhon Si Thammarat

• LT is an active sand spit, 6 km long and 500-700 m wide, corresponding to the major longshore current.
• The spit recurves west and consists of a small modern beach ridge, along with subaqueous sandbars.
• Potential storm tide (height of 2.3m) caused severe flooding for the LT sand.
• Reflecting strong wave action, the erosional features are well preserved- pine trees were torn apart
• Washed over sediments appear as a band parallel to the shores- similar in classification to terraces- of Sallenger
• Deposits behind beaches (shrimp farms, roads) appeared as small features- classification: perched
• Physical traits: basal sharp contact; buried grass (recent timings); 25 x 8 meter wash; thin
• Fine sand grain- bottom; course sand grain- top
• Eroded base existed in force area
• Normal erosion of soil was revealed. A thin organic layer was found. Volume limited-8cm (1st); 2cm (2nd)
• Normal bedding was found- from small-course to fine
• Higher sorting in 1st unit

2010 and 2011 storm deposits at Khao Mai Ruak (MR), Prachuap Khiri Khan

• Area displays a sand barrier at the front of a tidal space
• Barrier had steep slope (14 degrees) at the front and low elevation (2.5 meter) at the back
• High/low tides were 1.12/2.08 meters
• Storm was from December 2010. 2.58m surge generated- along with another storm on March 2011.
• Eroded sand surfaces (500 meters) + Scars (40-50 cm)
• Roots of coconut trees shown (sand was previously present)
• Due to high tide data- storms likely caused these changes due to 2011 level being elevated

Discussion

• Storm types: Terraces and "thick beds" are located at BT. Perched Fans (LT and MR). Sheetwash Lines (MR)- distances from 100 meters
• Sedimentary qualities: topography and the types of layers corresponded. Thick bedded is 50-80cm . Medium is 15-20 cm
• Several storm qualities contributed- height difference, coastal profile- to the size of the flow. For instance, terrace yields a similar sediment each time due to topography
• Erosion (Sand/Bedding)
• Tidal barriers lead to less flow qualities- it makes sediment deposition and other geologic processes simpler due to predictable weather patterns
• Normal/Reverse grading is an observed pattern in some sediments, for example BT