coastal inlets and development of fishing...

COASTAL INLETS AND DEVELOPMENT OF FISHING HARBOURS

Prabhat Chandra Chief Research Officer

Central Water and Power Research Station, Pune 1.0 INTRODUCTION Coastal inlets are very common features on most of coast lines and are very vital for many coastal developments. These coastal inlets may connect lagoons connected to sea or may form part entrances of small rivers. Such coastal inlets on sandy beaches are characterized by deep inlet gut and formation of offshore and bay shoals or bars. These are formed due to the interaction of the tidal flows during flood and ebb tides to transport sand to and fro vis-a vis action of waves and associated littoral currents. The littoral currents and the drift due to the waves contribute in the formation of shallow bars which in turn result in transfer of sand from updrift and downdrift sides of the inlet whereas the tendency of the tidal flows is in increasing the depths over the bar or in the inlet gut. Naturally the behaviour of the coastal inlet depends on the relative influences of tidal flow volumes, littoral drift, tidal ranges etc. The inlet is always in dynamic state. Since the west and east coasts of India have considerable differences in respect of seasonality of the wave climate and availability of sufficient supply along the coast lines and beach slopes, there is marked difference in the behaviour. Many of the rivers on the west coast in the coastal regions are marked with the presence of inlets having large fresh water discharges during three to four months which help in maintaining the stability of the coastal inlets even though some seasonal changes occur. The east coast of India is characterized by very large littoral drifts during south and north west monsoon seasons, associated with small river inlets and limited fresh water discharges. As a result, many of the inlets on east coast show tendency for migration and shallowing. Littoral drift on east coast of India is about ten times more in comparison to that on the west coast and the predominant direction of littoral drift is from south to north particularly on the north of east coast of India. The coastal inlets and the backwaters are very vital especially for development of fishery harbours in tranquil backwaters even during the monsoon season but they also require proper safe assess to the inlet and safe conditions at the shallow entrance bar. Frequent dredging to provide adequate depths is seldom possible and an attempt is made to provide some training works like breakwaters, flow training jetties etc. to achieve adequate wave tranquility at the entrance and to maintain proper depths. Since physical features at each coastal inlet are different, hence evolving a layout for coastal developments requires very extensive studies on coastal processes like littoral drift, behaviour of beaches and tide related phenomenon.

Training Course on Coastal Engineering & Coastal Zone Management

“Coastal Inlets and Development of Fishing Harbours” by Prabhat Chandra, CRO, CWPRS 2

2.0 STABILITY CONCEPTS AND ROLE OF TRAINING JETTIES The cross section of the coastal inlet is function of parameters like tidal prism, net yearly littoral drift, maximum spring tidal discharge, fresh water discharges etc. The main mechanism which tends to create and maintain the inlet gorge is the flood and ebb flow due to tides. Fresh water discharge passing through the inlet gorge would also assist in maintaining the inlet. On the other hand, the longshore littoral drift approaching the inlet could cause reduction in the inlet area due to accumulation of sand from the adjacent beaches. These mechanisms were identified by O’Brien (1931 and 1969) on the basis of statistical analysis of coastal inlets of Europe and USA. The following relationship was identified by O’Brien

nPCA= where,

A = Area of cross section of the Inlet P = the tidal prism , volume of water between low and high water in a bay C and n = are constants which are functions of various complex meteorological and coastal parameters

Subsequent to O’Brien, various investigators Nayak (1971), Moore (1972), Jarrent (1976), Krishnamurthy (1977) aimed at arriving at magnitudes of coefficients of C and n , by classifying on the type of training works at the coastal inlets. Srinivassn and Kapileshwar (1985) found values of C as 18.14 x 10-6 and n as 1.09 for coastal inlets India.

These relationships are, however, approximate and considerable scatter is noticed in the data while this type of analysis relating the cross sectional area to tidal prism gives some idea about the potential of the inlet gorge to maintain. It does not help either in predicting the likely velocities in the inlet gorge or in predicting rates of siltation.

Escoffier (1940) postulated that the inlet when subjected to large rates of littoral drift will respond in such a way that due to reduced area of cross section due to siltation, the velocities in the inlet gorge will be increased and this mechanism will cause consequent flushing of the sediment from the inlet and ensure the stability. He, however, indicated that for an inlet of a given tidal prism there will be some critical area upto which this self improving mechanism can be sustained and if the area of cross section reduces below the critical areas there will be progressive reduction in the tidal prism and the inlet will tend to choke, a condition which is not desirable from the point of view of development. The concept of Escoffier is very useful method for checking whether a particular site can sustain under different values of the inlet cross section. It may, however, be noted that the concept of Escoffier does not incorporate the effect of littoral drift on the siltation process in either an empirical way or deterministic way.



Bruun and Gerritsen (1958) analysed a large number of inlets on various coastlines in the world, examined the different types of sand bypassing mechanisms prevailing at these inlets and classified the coastal inlets into two categories:

(I) Bar Bypassing types (II) Tidal flow Bypassing types

When the annual littoral drift (M) is very high and the tidal discharge (Q) is very small, the natural sand bypassing near the inlet is on an offshore bar. Such types of inlets are bar bypassing inlets and occur when the ratio of M/Q exceeds 200. On the other hand when the tidal flow is very high compared to the littoral drift (M/Q less than 7), the drift would bypass the inlet by the tidal flow. In the former case, the inlet would be characterized by a prominent bar with shallow water depths at the entrance where waves would break and bypass the sand due to wave induced currents. On the other hand, in case of tidal flow bypassing inlets, deep depths would be available for navigation. Based on further analysis, Bruun and Gerritsen (1959) indicated that the stability of the inlet is related to tidal prism (P) and mean drift rate (M)

Good if P/M > 150 Fair if 100 < P/M < 150 Fair to Poor if 50 < P/M < 100 Poor if P/M, 50

Bruun further included a parameter area of cross section of the inlet (A) and classified the stability of inlet as follows:

Poor if 2A/3M < 0.0045 Fair if 2A/3M between 0.0045 and 0.0090 Good if 2A/3M > 0.009

Bruun also indicated that for stable coastal inlets, the magnitudes for the maximum tidal velocity would be 1.0 m/s. The above criteria are useful for understanding the stability of the inlets and the types of sand bypassing prevailing at the inlets, mainly for those not affected by fresh water discharges. Even though some attempts have been made further by Bruun (1978) to include the affects of fresh water discharges on the stability analysis, these are not adequate. Empirical relations would help in getting some qualitative information about an inlet. These criteria alone would, however, not be sufficient for planning the development of a coastal inlet. The Training jetties are useful due to the following reasons:

i) They provide channelised flow in the approaches to inlets at the entrance and facilitate flushing of sediments.



ii) Avoid frequent changes in inlet gut cross section. iii) Provide “buffer volumes” to accommodate littoral drifts o either side of

the inlet in the event of excessive littoral drift. iv) Provide adequate wave tranquility in inner bay areas to facilitate

development. v) Provide adequate wave tranquility to the dredger during the capital and

maintenance dredging. vi) The depths in the approaches to the inlet are enhanced.

3.0 CASE STUDY OF MUNAMBAM INLET

Munambam inlet is located on the west coast of India, 40 km north of Cochin in Kerala state. The northern branch of river Periyar after joining the Chalakudi and Pullut rivers, meets the Arabian Sea at Munambam (Fig.1). Azhikode port is located on the northern bank of the river at about 1.2 km upstream from the `inlet’. Munambam fisheries harbour is located on the southern bank of the river at about 400m upstream of the inlet. A sandspit extended from the north and encroached the river mouth as shown in fig.2.

Fig. 1 Index plan Fig. 2 Sand spit extending from gut

This had resulted in the formation of a natural channel along the southern bank in the direction of south-south-west on the seaside. The main problems faced by fishing crafts in absence of suitable training works, had been; (i) waves were broadside on the vessels plying in the natural channel and thus boats getting capsized, (ii) shifting nature of natural channel and inadequate depths for navigation, (iii) heavy breakers occurring at shallow bar at the entrance creating additional navigational hazards even during nonmonsoon season.

For the improvement of the inlet for fisheries development, studies were conducted at CWPRS by using wave and tidal models (both physical and mathematical) and the recommendations included construction of a south breakwater of 360m length and a north breakwater of 625m at the entrance (Purandare et al 1997) as shown in fig.3. The construction of breakwaters was completed in the year 1995-96. The comparison of surveys of the years 1991 and 1997, before and after construction of breakwaters is shown in fig.4.



For the sustainability of the present training works for long term use and for proposed expansion for port development at inlet in the future, there is need to monitor the behaviour of inlet after construction of training works, their effectiveness in improving navigational conditions in respect of depth and tranquility, morphological changes, shoreline behaviour etc. At the request of CWPRS, the necessary prototype data were collected by Harbour Engineering Department (HED), Kerala state.

Fig. 3 Proposal as recommended Fig. 4 Post construction & Comparison of by CWPRS surveys of years 1991 and 1997

Post construction behaviour is analyzed as per the available prototype data and remedial measures for sustainability of the inlet in future, have been highlighted and generalized solutions for similar types of inlets on the west coast of India have been provided as well.

3.1 Brief Background of Previous Studies

The reference for the studies to CWPRS was made in 1976 and further in 1989. The physical tidal model (scales H-1/350, V-1/60) studies were conducted in 1976 and south breakwater of 360m length for channelizing the flow and a channel of 60m width dredged to –4m with a bearing of 230 deg. north was recommended. The project construction could not be taken up and in 1989 again, the studies were referred and physical wave model (GS 1:120) studies and 2-D mathematical model studies for flow conditions were taken up. The studies confirmed the suitability of earlier physical tidal model studies. With the help of physical wave model studies, a north breakwater of 625m length was evolved for additional tranquility and arrest of littoral drift, in addition to 360m long south breakwater.

3.2 Analysis of Hydraulic Prototype Data Prototype data in respect of river discharges, wave, tidal, float track, current observations etc, as provided by HED were analyzed as below:

River Discharge Analysis

Due to bifurcation of Kalady, joining of rivers Chalakudi and Pullut, proper assessment of discharge passing the inlet at Munumbam was complex. On the basis of analysis of river discharge data made available by HED, it was established



that the river discharge at Munambam is contributed by 35 % of Periyar discharge (bifurcation ratio for northern and southern branch of Periyar river is about 35% and 65% respectively), total flow from Chalakudi river and 35% of Karvanur river and discharge through Pullut river. The percentage exceedance of river discharge at Munambam inlet for data during the period 1975 to 1988 is shown in fig.5.

Fig. 5 Percentage exceedance of river discharge at Munambam inlet

Wave data analysis

The Munambam inlet is subjected to waves incidence from the quadrant bounded by North-north-west to south. On the basis of wave data from 1949 to 1965 collected by ships plying in the region and reported in the Indian daily weather charts, it was found that the waves from the west and southwest directions occur mainly during the southwest monsoon (June to September) and waves from northwest direction are prominent during non-monsoon (October to May) season. Northwesterly waves of height more than 2m occurred for about 5 days in year and westerly waves of height more than 2.4m exceeded for about 11 days in a year. The detailed wave refraction analysis indicated that the wave conditions for westerly directions would be more critical compared to southwest direction and moreover, the frequency of occurrence of waves is about three times for the westerly waves compared to southwesterly waves.

Tidal data

The tides are of `Mixed type’ with significant variation in tidal heights in different successive flood and ebb ranges. The maximum spring tidal ranges for the flood and ebb tides were observed to 1.2m and 0.9m respectively. As tides propagate from Munambam towards Mangalapuzha and Kalady, the tidal ranges go on reducing and at Kalady, they become significantly less.



Sediment concentration and salinity data

At the gut portion, the average salinity was about 30 to 35 ppt during nonmonsoon season and during monsoon season, it was observed to reduce to 14-15 ppt with no significant difference in gradient over vertical. River discharge has significant effect and the salinities reduce to even zero value during the ebb tides. The sediment concentration at the gut portion was about 0.26 ppt during monsoon season and it was about 1.18 ppt in the sea portion.

Bed samples

The bed samples collected along the shore and in the channel portion in the year 1991 indicated presence of fine to medium sandy material with mean dia. of about 0.35 mm. 3.3 Analysis of Post Construction Surveys and Data The construction of south and north breakwaters as proposed by CWPRS of 360m and 625 m length respectively was completed in the year 1995–96. For long term use and for future development at the inlet, the effectiveness of breakwaters in providing stability and adequate depths in the channel, arrest of littoral drift, stability of inlet and wave tranquility near the gut etc, was required to be assessed. Accordingly, the CWPRS requested HED, Kerala state to collect the hydrographic surveys, beach cross sections on the north of north breakwater (northern shoreline) and south of south breakwater (southern shoreline) periodically every year. The requisite data were supplied to CWPRS and the findings are summarized as below:

Hydrographic surveys, Shoreline profiles and Beach cross sections

i) Along centerline of channel

The natural channel is located at a bearing of 230 deg.N and no capital dredging was required to be carried out. The hydrographic surveys were analysed to monitor the navigational depths along the channel after the construction of breakwaters at the inlet. The comparison of the hydrographic surveys collected in October- November (post-monsoon) during the period of 1991 and 2000 at the center line of the channel, was observed as shown in fig.6.

Fig. 6 Depths along centerline of Channel during different years



As seen from the figure, there has been significant increase in the depth at the gut portion and within the breakwaters upto 5-6m from earlier depths of 2-3m when breakwaters were not constructed. This is basically due to channelising of the flow due to south breakwater. On the seaward side ahead of breakwaters in the channel, the depths were observed to be more or equal to 3.0m, the required draft for fishing crafts.

The breakwaters have been very effective in creating suitable depths along the channel for navigation of fisheries traffic during any stage of tide without carrying out any capital dredging. In the upstream portion in the river, practically no change in depths has been observed after the construction of breakwaters.

ii) Shoreline profiles of Northern shoreline

The shoreline surveys of northern shoreline upto chainage of 1500 m from the root of breakwater were analysed for the period 1991 to 2001. The position of the shoreline has been measured from the existing sea wall and the shoreline profiles have been shown in fig.7.

Fig. 7 Shoreline survey of northern shoreline during different years

At the root of north breakwater at 0.0m chainage, the shoreline was at a distance of 50m in the year 1993 and with the construction of breakwaters, it started progressing towards sea side and in the year 1998, the shoreline advanced to 150m. Subsequently during 1998 and 2001, there was marginal advancement in the position of shoreline upto 165m. Along north breakwater also, the shoreline was at a distance of 200m from the root in 1993 but subsequently the same advanced to 510m in 1998 and more or less stabilized thereafter. At the chainages 300m, 600m, 900m, 1200m from the root of breakwater, the shoreline advancement was observed but it also showed decreasing trend in advancement and beyond 1500m, there was no change in the position of shoreline either. The maximum accumulation or advancement in the shoreline due to construction of breakwater / arrest of drift was observed within the area along the north breakwater and 0.0m chainage.



On the basis of beach cross sections, it was observed that the position of shoreline was observed to be more or less stabilized during the last three years but thickness of the arrested deposits was increasing during the period. In the year 1998, the thickness of deposit was observed as 1.65m in the first 70m length and 1.2m in next 95m length along the shoreline at 0.0m chainage and the same was increased to about 1.8m in the year 2001. At the chainages 500m, 600 m, 900m, 1200m also, the same trend was observed (sections shown in fig.8). Despite the shoreline position being almost same for the last three years, the thickness of the accumulation was observed to increase.

Fig. 8 Beach cross sections along northern shoreline during different years

The accumulation on the north of north breakwater was observed to be about 1,46,000 cum during 1993-98 was and between 1993-2001, it was about 2,36,800 cum on the basis of shoreline surveys and beach cross sections available. If all the accumulation is attributed to alongshore littoral drift, the rate would be about 30,000 cum/year covering upto contour at –3m.

iii) Shoreline profiles of Southern shoreline The shoreline surveys and beach cross sections on the Munambam side i.e. south of south breakwater have been analysed for the period 1993 to 2001. The accumulation has been observed after the construction of breakwaters and along south breakwater, the shoreline advanced from 20m to about 260m from the root of the south breakwater. At other sections beyond 400m from the root of the breakwater along the southern shore line, the shoreline was observed to be receding very marginally during the period of time as shown in beach cross sections in fig.9. The accumulation in the fillet within the root of south breakwater and upto a chainage of 300m, was about 24,250 cum in the period 1991-2001 but in the southern reaches beyond, no accumulation trend was observed but rather it was marginally reverse. The thickness of the deposit was not observed to be increasing but it was stabilized immediately after construction of breakwaters. The accumulation may be attributed to minor alongshore drift from north to south. The position of shorelines has been confirmed with satellite imageries also, of different years.



Fig. 9 Shoreline profile of southern shoreline during different years

Fig. 10 Beach cross sections along southern shoreline during different years

3.4 Longshore Transport

Longshore sediment transport along Kerala coast has been examined by researchers. Sanjeev R. et al (1997) estimated the annual southward and northward littoral drifts of 0.4 and 0.2 million cum respectively with a net drift of 0.2 million cum towards south at Andhekavanazhi which is 20 km south of Cochin.

Chandramohan P, Nayak B.U. and Raju V.S. (1990), computed long shore transport along south Indian coast using energy flux method. At Cochin, they predicted southward and northward long shore transport of 0.98 and 0.69 million.cum respectively. It may be noted that many researchers have indicated that the energy flux method gives higher values of long shore transport. Patil, B.M.(2000) estimated the net annual littoral drift as 0.30 Million cum southward ( 0.65 Million cum southward and 0.35 Million cum northward) at Cochin (40 km south of Munambam) by using the available prototype wave data and model LITPACK. The distribution of the littoral drift has been assumed to be over a distance of 4.5 km from the shore extending upto 8m depth contour. If the distribution is restricted to 3m depth contour, then the net annual littoral drift rate is about 30,000 cum which matches with the accumulated quantity observed as about 30,000 cum per year along north breakwater at Munambam.



The theoretical values of overall long shore drift are suitable if sandy beds prevail totally upto the surf zone or even more. If due to less supply of sand in the region, the width of sand bed is limited, the theoretical values do not hold good and actual drift would be less by that magnitude.

3.5 Discussions and Remedial Measures

Considering the dominant effect of ebb flow in maintaining depths, seaward of the entrance, the flow fields were examined by using shallow water broad jet theory and the work as done by Ozsoy(1977) and Purandare(1984). It was noticed that even during nonmonsoon season, velocities more than 0.3m/s prevailed across the width of navigation channel for almost 1000m distance seaward of the breakwater entrance and they were more than 0.5m/s for average fresh water discharges and more than 1.0m/s, for peak river discharges. This explained the major cause of absence of offshore bar in addition to prevention of lateral entry of the littoral drift. The north and south breakwaters constructed about 5 to 6 years back have been serving their purposes well and almost all the long shore drift within a zone upto –3m contour is being arrested on the north of north breakwater and no bypassing across the channel is observed as no shoaling has been observed and the depths are maintaining well. The wave tranquility at the berth and the wave approach angles have been suitable for navigation and berthing after the construction of breakwaters. During the flood flow and wave action combined, the sediments pushed inside the channel, tend to get accumulated on the north side of channel whereas, the south side of the channel is flushed dominantly during ebb flow thus increasing depths on the south side. It is desirable that the minor trimming is done across the channel to make uniform depth across the channel. After stabilization of shoreline advancement and thickness of littoral deposits on the north of north breakwater, the bypassing across the channel likely to place predominantly in a period of few years. For the present fishing traffic, minor extension of north breakwater would serve the purpose for few more years but keeping in view the long term sustainability and future development at the inlet, the detailed studies are required. The natural bypassing across the channel would result in shoaling of the channel, instability of inlet, siltation etc. For avoiding the adverse effects of such bypassing one or more of the following methods may be adopted; (i) extension of north breakwater (ii) nourishment of southern coastline by suitable bypassing methods (iii) maintain the channel by dredging or,(iv) constructing additional groyne on the northern shoreline for containing littoral drift. For evolving any of the above remedial measures, the model and desk studies, field data etc, would be required for detailed studies and suitable recommendations. 3.6 Concluding Remarks 1.0 The north and south breakwaters constructed 5 to 6 years back at

Munambam inlet as per the recommendations of CWPRS have been very effective by maintaining adequate depths in the navigational channel without any capital dredging, arresting of littoral drift, stabilizing the channel



and inlet and providing wave tranquility with suitable approach angles in the channel and at berth.

2.0 The accumulation of north of north breakwater indicate the net littoral drift

rate of about 30,000 cum per year southward in a zone upto –3m contour against the theoretical computation of 3 lakhs cum per year overall. The difference in the actual and theoretical quantity may be attributed to less width of sandy bed.

3.0 Detailed studies for evolving suitable remedial measures to avoid sand

bypassing across the channel are required to be carried out in the light of long term use and future development.

4.0 For the improvement of coastal inlet, detailed model studies are necessary

as to evolve the correct flow patterns, wave tranquility effects for different layouts, wave incidence and river discharge conditions.

5.0 The state of coastal inlet is dynamic due to many bathymetry changes

occurring as a result of river discharges, tidal discharge and littoral drift etc, prototype data greatly help in for morphological aspects, stability of inlets and for proving of models etc.

REFERENCES Bruun P. and Gerritsen F. (1958). Stability of Coastal Inlets. Proc. of ASCE , Journal of WW and H Division, Vol. 8 Bruun P. (1978). Stability of Tidal Inlets. Theory and Engineering , Elsevier Scientific Publishing Company, Amsterdam Chandramohan P., Nayak B.U. and Raju V.S. (1990), `Longshore transport model for south Indian and Sri Lankan Coasts’, J. of Waterway, Port, Coastal and Ocean Engineering, 116, (4), pp. 408-424. Escoffier F. F. (1940). The Stability of Tidal Inlets. Shore and Beach, Vol. 8, No. 4 Jarret J.T. (1976). Tidal Prism – Inlet Area Relatioships”. GITI Report 3, U.S.Army Corps of Engrs., Coastal Engg. Research Centre Krishnamurthy M. (1977). Tidal Prism of Equilibrium Inlets. Proc. Of ASCE, Journal of WW and H Division, Vol. No. 103. Ozsoy, E. (1977), ‘Flow and Mass transport in the vicinity of Tidal inlets’ Technical Report No. TR- 036, Coastal and Oceanographic Engineering Laboratory, University of Florida Patil, B.M., Atkekar, N.D., Kanetkar, C.N. (2000), ‘Mathematical Modelling for estimation of Littoral Drift and Shoreline evolution for development of Cochin Port’



Proc. Recent advances in Hydraulics & Water Resourses Engineering HYDRO 2000 at Kurukshetra, pp 368- 374 Purandare, U.V.(1984), ‘ Laboratory study of Tidal inlet ebb flow jets’, Master’s project report, Coastal and Oceanographic Engineering Laboratory, University of Florida Purandare U.V., Prabhat Chandra and Nayak B.U. (1997) ` Improvement of coastal inlet at Munambam for fisheries development’, Proc. 2nd INCHOE-97 at Thiruavnanthapuram, Kerala. Sanjeev R., Chandramohan P., Josanto V. and Sankaranarayanan V.N. (1997), `Studies on sediment transport along Kerala coast, south-west coast of India’, Indian Journal of marine Sciences, 26, pp. 11-15.

coastal inlets and development of fishing...

Documents