Beach Recovery Processes in Urban Beach Systems : A Study in Hong Kong after an Extreme Storm Event LIFE+ FLANDRE international workshop "Management of coastal dunes and sandy beaches" Session 1. Climate change & coastal dynamics H. Chiu, T.E. Chan, K. Lee, S. Leung and Y. Leung Department of Geography, Hong Kong Baptist University 1
Typhoon Hato (August 2017) Rundown Natural beach recovery processes in Hong Kong Observations of different recovery processes observed after Typhoon Hato in Big Wave Bay, Ma On Shan and Golden Beach 2
Typhoon Hato Maximum wind speeds 130 km/h (mean hourly) 193 km/h (gusts) Effects 129 hospitalised widespread landslide, flooding... some property damage and fallen trees... 3 Hong Kong Observatory
Regional Maximum Storm Surge : 2.79 m (Zhuhai) Regional Maximum sea level : 6.14 m (Zhuhai) Hong Kong water levels : Maximum : 3.57 m (2nd highest on record) Storm surge ~ 1 m + during astronomical high tide ~ 2.4 m 4 Hong Kong Observatory
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Study area : 3 types of urban coasts Ma On Shan Golden Beach South China Tai Long Wan (Big Wave Bay) 10km Hong Kong 6 Stamen
Big wave bay Popular swimming and windsurfing spot for tourist and locals Vegetated backshore dune since at least 1949 Strong wave input from prevalent wind from east Photo ID 6765 (2700' 1963-02-01) 7
Big Wave Bay 8
T3 T2 9
Vegetated dunes Backshore 10
August 2017 (Typhoon Hato) 11
Beach Recovery in 2008-2009 (Typhoon Hagupit) 12 Yu et al. 2013
Hong Kong beach characteristics Some observations :- Vertical accretion of about 1 m in Tai Long Wan from October 2008 to January 2009 Tai Long Wan sand replenishment occurs in winter time In contrast to textbook theory of sand replenishment occurring in summer. 13
Sept Oct Nov Dec Jan Feb This study (Typhoon Hato 2017) Dune loss coincided with beach recovery, but much less than in 2013. Further erosion due to seasonal rain and (mild) tropical storm. 14
Sept Oct Nov Dec Jan Feb Very low replenishment due to loss of dune vegetation no more source of sand Further erosion due to seasonal rain and (mild) tropical storm. 15
Observations Vegetation significantly control sand replenishment abilities Whether removal of remaining healthy trees is necessary or redundant a management decision? 16
Ma On Shan
T1 19
Sept Oct Nov Dec Jan Feb Apparent landward dune migration
Small dune formation along urban coastline Funnel shaped artificial coastline promote deposition (rather than erosion) of sand Pond of sediment created by artificial structure 22
Golden Beach 23
T3 T2 24
Sept Oct Nov Dec Jan Feb Artificial sand nourishment by redistribution of sand Formation of a sizeable sand dune after erosion from storm 25
Sept Oct Nov Dec Jan Feb Dune formation Removal of sand for artificial nourishment 26
Artificial sand nourishment Results show beach capable of natural replenishment Sand nourishment is to provide a flat terrain for recreation use Disregard natural coastal processes 27
Some observations Winter replenishment is observed in all types of beaches after typhoon impact However, removal of vegetation (by typhoon damage and by management) on the dune surface significantly changed styles of natural sand replenishment. Dune formation in MOS and Golden Beach were obvious - the mostly artificial coastlines retained some natural ability to recover. 28
More Climate change means increasing intensity and frequency of extreme storm events Sea level rise also raises the base water level storm surges will get worse (More) permanent destruction of (more) dune systems that were originally untouched by storms and waves?
Conclusions Urban coasts are also capable of replenishment through natural recovery processes Urban coastal management strategies require consideration of natural processes Climate change definitely a serious threat to all dune systems 30