1adian Journal of Marine 5cienee5 Vol. 11, DeceMber 1982, pp. 327-332

Beach Stability along the Coast from UUal to Thannirbha vi near

M P M REDDY, V HARlHARAN &: N P KURIAN College of Fi5heries, University of Agricultural Sciences, Mangalore 575002 Received 30 November 1981, revised received 19 April 1982

Dluing the 3 yr study period, beach was generally stable except neai' the estuarine mouth where a depletion of beach material was recorded. Bengre beach was noticed to be gradually eroding. A slight building up of the beach at Thannirbhavi was observed in the 2 yr period. Grain size analysis carried out for UUal and BeJlgeebeackes £orroborated the seasonal features of erosion and accretion of these beaches.

Along the coast, several areas are under 75°E and lat. 12°to 16°N for 1974to 1978.Direction of constant threat of erosion. A preliminary study littoral currents or longshore currents for the region conducted for I yr at Bengre near Mangalore has under study was obtained from the wave refraction revealed considerable erosion 1. Since Mangalore is an diagrams drawn for the area for the predominant wave important region along the Karnataka coast from directions and the most frequent wave period using the industrial, recreational and fisheries point of view, a method of Arthur et al.4 detailed study on the beach characteristics, waves, littoral currents and other related phenomena at Ullal, Results llIId Discussion Bengre and Thannirbhavi beaches has been carried Winds play a significant role, both directly and out. indirectly, in influencing beach changes 5 • However,

Materials and Methods The stretch of the coast under investigation (Fig. 1)is low and comprises sandy sediments. The river Gurpur •I tuns in a southerly direction and joins the river 1253N Netravathi at its mouth near Mangalore. A narrow sand spit separates the Gurpur river from the Arabian sea. Bengre beach is situated on this sand spit just on \ o 0,5 1KlII the northern side of the Netra vathi-Gurpur estuarine mouth. Ullal beach is located on the southern side of the estuarine mouth. Seven stations w~re fixed along the coast between UUal and Thannirbhavi near Mangalore to study the variations in beach configuration (Fig. I). Beach profiles were measured monthly at low tide using a graduated pole and a measuring tape at all stations 12·S{ from Oct. 1975 to Sept. 1978. However, beach profile measurments at Thannirbhavi were carried out only from Oct. 1976 to Sept. 1978. Sand samples, collected from the foreshore region during Jan. 1976to Dec. 1976.near the low water mark at sts US, UM, UN, BS and BN (Fig. I), were subjected to granulometric analysis by sieving. Median size and sorting coefficient for each sample were estimated2• Wind data for Mangalore was obtained from the . , climatic table (1881-1955)published in the West coast 12 49 of Pilot3. Wave climate was evaluated from the data (ship's visual observations) from Indian Daily Weather Reports for the area between long. 710 to Fig. 1--Location map with bea.h prolile statiolls

327 INDIAN J. MAR. SCI., VOL. II, DECEMBER 1982

the indirect effects of winds are most important been obtained for all stations from the wave refraction because they generate ocean waves, which cause diagrams prepared for deep water wave period of 6 sec considerable modification themselves and also and for different deep water wave directions (Table 3). produce longshore currents. Thus, knowledge of For 6 sec period waves approaching from 2200 and statistical wind data will enable to understand the 2400, the littoral current is directed towards north probable directions from which wind generated waves can approach the coastline under study. Mean monthly winds revealed that the predominant winds Table 3-Direction Functions (0) for 6 sec Period Waves for Different Stations over the year are from east (18%), southwest (10%), Stations +23°+14°+30°+12°+31°+28°+22°240°265°_10°_10°_4°_4°+6°290°_9°_8°+27°_9°_6°_8°_7°_3°+24°-23°-18°_-21°_27°-24° 13° Directions west (13%) and northwestUSUMBNTNUNBSTS (31%). Annual wave direction percentage frequencies and annual wave220° period percentage frequencies are presented in Tables 1 and 2 respectively. Deep water waves approach this coast from directions of 2200, 2400 and 2650 during the southwest monsoon (June to September). Waves approach this region during the remaining part of the year mainly from 2900 and 3100• The 5 and 6 see period waves are most frequent in the year. The direction function (0) which provides + = Southward movement information about the littoral current direction has - = Northward movement

10.5010.9025.002.442.307.804.603.606.602.503.103.30------1.601.501.152.00 12.5010.82.6212.0017.2015.014.3014.839.5215.4018.0010.2011.1015.2018.9020.012.8212.609.006.6011.601.750.87-10.81.11MAMAS2.442.501.60F12.72.007.103.1020.4041.4614.6148.2043.3036.006.003.1134.40-4.082.306.129.301.601.00J17.3027.5022.7015.7014.823.00ND3.5410.019.3060.301.009.0012.007.705.46.873.54--1.155.357.4014.101.501.81.0018.609.751.601.503.61.370.24-2.55.020.028.011.205.308.7030.01.75J5.41.82.118.3712.92.004.9012.7012.519.301.006.408.101.806.60-5.35 in0--- yr Table I-Annual.~ Wave Direction PercentageDirectionMonths Frequencies for the Region around Mangalore 3WO 240°220°200°130°lWO335°175°150°265°40°290°60°85°20° J total Direction 360° REDDY et 01.: BEACH STABILITY AT MANGALORE COAST while waves from 2900, the littoral flow is towards The Gurpur and Netravathi rivers originate in the south for all stations. western ghats where the average annual rainfall is Coastal current measurements by drag and float about 330cm (280cmbetween June and September). using double theodolite method showed that the The maximum discharge of the 2 rivers is 1700 and currents in the sea near Thannirbhavi, Bengre and 7100m3jsec respectively8. The Netravathi is estimated Ullal during the fair weather season are directed to carry 6,4S,700m3 bed loadjyr as against 62,320m3 towards north while the currents during the southwest estimated for the Gurpur. The estimated suspended monsoon season off the estuarine mouth are found to loads of the Netravathi and Gurpur rivers are be directed towards south6•7• Tides at Mangalore are 19,12,000 and 3,4S,SOOm3jyrrespectively9. of semiftiurnal mixed type with springs 1.7m and Beach profiles at st US on UIlal beach (Fig. 2) neaps 1.2m during the study period. showed erosion from June to September and gradual a---t.OCT.'75-SEP.'76;. __ OCT.'76- SEP.'77; cr--O OCT'77-SEP"78 STATION- UN

4]a iOC~~~~ T ...... c-'\ !X,7~._-..-,- :: ..·w·L·W, ]~ NOV. ~\~

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] '20 40 80 lIe o 80 1(0 200 o 40 80 110 DISTANCE FROM REFERENCE POINT (m) Fig.2-Beach profiles for sts US, UM and UN

329 INDIAN 1. MAR. SCt. VOL. II, DECEMBER 1982

deposition from December to May. At sts UM and was a loss of material from year to year at these UN, the opposite trend was noticed with general stations. This feature was observed throughout the erosion of the beach during November to May and year both at the time of erosion and building up of th~ building during June to September (Fig. 2). However, beach. this feature was more pronounced at st UN than at st At sts TS and TN on Thannirbhavi beach, the beach UM. No definite trend ofloss or gain of beach material levels showed similar trend with gradual cut in the was observed from year to year at st UM, while at UN, southwest monsoon months of June to September and a gradual depletion of material was noticed during gradual fill during the fair weather months of November to May especially in the foreshore from December to May (Fig. 3). During the years a slight 1975 tQ# 1978. building up of the beach was found at these stations, Sts BS and BN on Bengre beach showed marked Seasonal and annual variations of beach levels in the erosion during June to September and gradual study area appear to be primarily influenced by waves deposition during November to April (Fig. 3). There and littoral currents. In addition, river discharge seems STATION-BN tr---f). ocr:75 - SE P.'76; .-. OCT.'76- 5 EP.' 77; 0--0 OCT.'77- S EP.'78 , 0' • , 1 ..

o •• SEP. J --, ~ . 1 ••• HO~. SE P. HOV • 2- ~ 41 2 r_.~ 2~ !. ' u'J , •.. ~, sTATION-Bs, , '. ' 'J0;I ~'~1 '. \ 'J' " OCT. STATION-TS,MAY., ,,,-~ "JUN APR M- .••..•• ___ '"W, ): '~ ']SEp. . Jue ,'J ~h.. \ ... , . \ '- J )(. "].~~ 'j HO~'MAR., , 'J ~MAR.", '1 " STATION-TNMAR."., J.~ '-• AUG :]' '. ~ ,J: ~•\ ~ Jue '''. ~'J, ' ~~ ~'\.."N· 'J~''J, ~ >AN. ~' ,].J'J --~\-~~'--c~ _' .]'J 'J.J ~ DEe . .. '" ','it~]I ,j ~ MAY. ~v.,~APR,: ,,- ''J'; ••js~]~MAY]_ ~ ~'" ~ JUNJ~ ']J\ ~ . .. \ ,.1.]::x:J, ~ ~ '="....J.. x;>]~'"JF , ~ ~. ,\..\ 'J''J ~ ~'t'--~6'] ' '~+J J 'J] MAV} ~FEB ~"'- -5] ] ;1 1..FEB·, , . HOVn,, DEe ~o"- , ,'~~'" ~T

'0 u 40 80 '10 o 80 110 DISTANCE FROM REFERENCE POINT (m) Fig.3-Beach profiles for sts BS, BN, TS and TN

330

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REDDY eta/.: BEACH STABILITY AT MANGALORE COAST to play a major role particularly in the southwest northern side of the estuary, supply of material from monsoon period. According to King10, high, steep south to these 2 stations may be very less from June to waves tend to remove material from beaches, whereas September. In addition, the occurrence of a nodal zone low, flat waves tend to move it shoreward. High wave of diverging flow near these stations can lead to more energy conditions prevailing during southwest erosion in this area. Since the erosion during the monsoon in the region cause considerable change in southwest monsoon is severe, even the building up of the beach and surf zone in a short period of time. the beach in the fair weather season which is relatively During such period of high wave energy there will be much less than the loss from June to September, is considocable sediment in suspension landward of the unable to prevent the gradual erosion of Bengre beach breakers where the waves collapse 11.12. The most from Oct. 1975to Sept. 1978. Slight building up of the significant role played by southwest monsoon waves beach at sts TS and TN from year to year is perhaps probably is to provide great quantities of suspended due to the accretion of greater amount of eroded sediment in the surf zone. During the fair weather 620 season from December to May, the beach and )t " UN. 0----0 UM. nearshore zone is subjected to only moderate to low 580- ii:\Ji=ou.~:zzIf)WI.'<:N\!lQ:0IUE 340f. 3 wave energy conditions. This situation is commonly I-'-~ii5380,z1.'1 420 1·6 I." us. - - , as. associated with small locally generated waves or with,aD,.....2.604605005401.1.0220 1300 --- breaking swell generated some distance away. Such h • • BN. waves produce a net transport of sediment in a landward direction giving rise to the building up of the beach. '·'1 Movement of sediment along the coast is the result of many individual littoral drifts acting on discrete coastal segments. Whenever, either a longshore ••.....•.••/\1\ '\ current is interrupted or the waves that provide the energy to maintain the longshore current are interrupted, the competency of flow is decreased and sediment deposition occurs. The Netravathi-Gurpur estuary with its concentrated flow (strong tidal currents and monsoon flow) seems to interfere with the normal longshore current. The building up of the beach at st UN during June to September is perhaps due to the interruption of longshore transport by Netravathi-Gurpur estuary. As st UN is located just on the southern side of the estuarine mouth, the littoral current during the southwest monsoon season being towards north at this station, the entire sediment b coming from south of this point is likely to be deposited from June to September near this place. Another fact.orleading to deposition at this station and to a lesser extent at st UM may be the southerly component of the flow of rive, water discharged during this period. The gradual erosion during December to May at sts UN and UM can be due to the discontinuity of the normal southerly littoral flow at the estuarine mouth. Since the supply from north is interrupted just north of these stations, depletion of beach material occurs at sts UN and UM from December to May. The severe erosion at sts BS and BN during June to September can be due to the interruption of normal northerly flow at the estuarine mouth and the J FMAM J J. A SON 0 occurrence of divergence of littoral currents between MONTHS sts BN and BS at this time for waves approaching from Fig. 4-MonthIy variations of (a) median grain size and (b) sorting 2650• Since sts BS and BN are located just on the coefficient at different stations

331 INDIAN J. MAR. SCI., VOL. II, DECEMBER 1982 material brought from Bengre beach from st BN as a Acknowledgement result of northerly littoral drift from June to The authors wish to express their grateful thanks to September to Thannirbhavi beach area. Prof H P C Shetty, Director of Instruction (Fisheries) Monthly variations in median grain size and sorting for encouragement and facilities. coefficients of the beach sediments at sts UN, UM and US on the southern side and at BN and BS on the References northern side of the estuarine mouth (Fig. 4) reveal marked seasonal variations. Relatively higher grain I Reddy M P M, Hariharan V & Kurian N P, Mahasagar-Bull size values of beach sand at sts UN and UM from natn Inst Oceanogr, 10 (1977) 97. 2 Trask P 0, Econ Geol, 25 (1930) 581. November to May and rather lower values from June 3 West coast of India Pilot (Published for the Hydrographic Dept to September show erosion during the former and Admirality, London), 1961,393. building up tendency during the latter period. This is 4 Arthur R S, Munk W H & IsaccsJ D, Trans Am Geophys Union, also associated with comparatively lower values of 33 (1952) 855. sorting coefficient during the erosion period and 5 Svasck J N & Terwindt J H T, Sedimentology, 21 (1974) 311. 6 Reddy M P M, Hariharan V & Kurian N P, J Inst Engineers higher sorting coeffteient during the accretion period. (India), 58 (1978) 161. On the northern side, sts BN and BS, as also the 7 Reddy M P M, Hariharan V & Kurian N P, Proc Indian Acad Sci, southern side, st US, similar association of higher 88 (1979) 121. grain size and lower sorting coefficients with the 8 Specific Note No. 1184 (Central Water and Power Research erosion periods, in this case between June to Station, Poona) 1971. September, is in confirmity with earlier observations in 9 Intermediate Ports Development Committee Report (Ministry of Transport and Communications, New Delhi) 1960, 175. Calangute beach 13 wherein coarser mean grain size 10 King C A M, Beaches and coasts (Edward Arnold, London) 1959, was found to be associated with the higher wave energy 250. and steep waves of the southwest monsoon season. II King C A M, Beaches and coasts (Edward Arnold, London) 1972, Relatively wider variation of sorting coefficient at st 315. UN could be due to the wide fluctuations in grain size 12 Brenninkmeyer B, in Beach and nea;shore sedimentation, edited which may be reflecting on the source of the sediment, by R A Davis & R Ethington (Tulsa, Sepm Spec Publi No. viz. the Netravathi-Gurpur rivers during the various 24) 1976,69. 13 Veerayya M, Indian J mar Eci, 1 (1972) 28. times of the year.

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