Beach Morphology and Sediment Budget Variability Based on High Quality Digital Elevation Models Derived from Field Data Sets

doi:10.4236/ijg.2011.22012

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International Journal of Geosciences, 2011, 2, 111-119

doi:10.4236/ijg.2011.22012 Published Online May 2011 (http://www.SciRP.org/journal/ijg)

Beach Morphology and Sediment Budget Variability Based

on High Quality Digital Elevation Models Derived from

Field Data Sets

Mohammed Taaouati*, Abdelmounim El Mrini, Driss Nachite

Department of Geology, Abdelmalek Essaâdi University, Tétouan, Morocco

E-mail: [email protected]

Received January 11, 2011; revised February 17, 2011; accepted March 22, 2011

Abstract

The morphological and volumetric changes of a sandy beach were investigated through a series of two-

monthly filed surveys carried out over a 2-year period from April 2005 to January 2007. This paper discusses

the ability of 3-D digital elevation models (DEMs) derived from high accurate data to assess and quantify

beach morphodynamics in relation with wave forcing. The methodology and data acquisition are described

and consist mainly in the production of interpolated DEMs from which a variety of representations can be

made, including as elevation change maps, two-dimensional cross-sections of the beach, and calculation of

net volume. The results of the analysis highlight seasonal changes in beach morphology due to variations in

wave energy. This behavior is characterized by beach erosion and bar decay under high-energy waves and

net accretion and bar formation during relatively fair weather conditions. The sand budgets adjustments show

that the loss of volume in the winter months is compensated for by accumulation to the beach during summer.

This trend suggests that waves are the main forcing which controls the beach evolution. The correlation be-

tween beach changes and wave energy variations highlights a strong relationship between them and supports

the suggestion previously made. The results from this investigation state the value of DEMs utilized and

demonstrate the efficiency of the 3-D approach employed here to assess the erosion and accretion patterns

which would not be visualized using 2-D profiles.

Keywords: Moroccan Coast, Beach Change, Wave Forcing, Digital Elevation Model, 3-D Approach

1. Introduction

Sandy beaches are difficult landforms to study because

of the complex interaction between morphology, sedi-

ment and hydrodynamic forcing, which makes these en-

vironments very dynamic and moving systems.

Many authors have worked on the spatial and temporal

evolution of beach morphology using several models,

classification and indices [1]. Unfortunately, little suc-

cess has been obtained when using such models because

of the complexity of beach systems [2]. An alternative

approach is to repeatedly measure the beach topography

over time to evaluate the changes that have taken place

[2].

Nowadays, high-resolution techniques (satellites, ae-

rial photographs, LIDAR, etc.) allow for improved

monitoring and provide coverage of large sections of

coastal systems. Although the excellent resolution of the

data obtained, some limitations (cost, enormous data sets)

restrict the use of such techniques. Field surveys remain

the best way to frequently collect topographic data. The

most common technique used is beach topographic pro-

filing or 3D surveying [3], which is the case of the pre-

sent work.

On the other hand, the data collected must be repre-

sented appropriately in order to keep as much as possible

the field reality. The digital elevation model (DEM) is a

numerical representation of beach topography, which is

widely used throughout geomorphologic and environ-

mental studies [4,5]. In coastal areas, DEMs have been

utilized in many analyses such as topographic distribu-

tion, slope variations, estimation of scour and/or fill

volumes.

The present work aims to apply DEMs, derived from

accurate topographic data, to identify seasonal morpho-

logic change in a mesotidal high-energy beach, located

M. TAAOUATI ET AL.

112

on the northwestern coastline of Morocco in order to

derive patterns of erosion and deposition within the sys-

tem. The specific objectives of this paper are: 1) to gen-

erate Digi- tal Elevation Models from high-quality field

data, 2) to produce elevation contour maps and determine

areas of accretion and erosion, 3) to calculate cut and fill

volumes and 4) establish the relationship between beach

changes and hydrodynamic forcing. This approach al-

lows us to assess and predict the morphodynamic behav-

ior of the studied beach over time and space.

2. Field Site Description

Charf el Akab is a sandy beach located on the Atlantic

side of Tangier Peninsula in the northwestern of Mo-

rocco (Figure 1). It is an interesting sector showing a

progressive increase in human activities during the last

few years, and well-known for high energetic waves

during winter months [6,7].

Due to coastline orientation, the beach is mainly af-

fected by waves (both sea and swell wave conditions),

approaching from western directions especially the ones

that approach the coast from west-northwest quadrant.

Significant wave height is usually within 1 to 3 m and

can reach 7 to 9 m during large storm events [8]. Then,

this area can be classified as a high-energy environment

[9]. Not surprisingly, the impact of these high energetic

events on flat sandy beaches in the region is very impor-

tant.

The coastline, NNE-SSW aligned, is dominated by

long and wide sandy beaches, such as Charf el Akab

(studied beach), Haouara and Briech; it is characterized

by vegetated dunes which protect humid (marshy) zones.

Last, the coastal zone between Cape Spartel and Tahad-

Figure 1. Atlantic coast of Tangier peninsula showing study area.

M. TAAOUATI ET AL.

113

dart is drained by rivers, notably Tahaddart, which carry

terrigenous sediments towards the continental shelf [6].

Charf el Akab is an approximately 2 km long and

200 m wide beach. Throughout this beach is present a

wide vegetated foredune and rocky outcrops in many

places along the backbeach (Figure 2(a)). The study area

(200 × 250 m) is characterized by many features formed

by wind, tide and wave processes. Sand shadows are

usually found on the backshore of Charf el Akab beach,

especially when strong offshore winds prevail in the

studied area. They are associated with pebbles, shells and

considerable heavy minerals fractions (Figure 2(b)). The

foreshore zone exhibits a wide variety of beach features

such as swash marks, ripples, etc. The most usual are the

rill marks which are formed by water drainage in the rip

channels. One of the main features of Charf el Akab site

is an intertidal bar which is provisionally present at the

mid- and upper beach (Figure 2(c)).

3. Data and Methods

Below, we present the data analysis methods used herein

in order to characterize the hydrodynamic (wave) condi-

tions, beach morphology and sediment budget variability.

The data analysis involved mainly 1) seasonal charac-

terization of beach morphology and offshore wave forc-

ing, and 2) exploring for evident correlation between

them.

3.1. Wave Data

Wave data provided by Puertos del Estado (Ministry of

Public Works, Spain).were utilized to characterize the

wave conditions during survey period. Data are collected

from an offshore point “WANA43” (35°45' N, 6° W)

located near the study beach at approximately 70 m

depth (Figure 1). These data are daily wave forecast

output from the WAM wave model [10] and do not come

from direct measurements. However, the data are useful

and reliable since the objective is to analyze the wave

climate at seasonal time scale. The wave data sets ana-

lyzed cover a two-year period from 2005 to 2007 and

consist of sig- nificant wave height (Hs), wave period (Tp)

and wave direction (θ). The WANA data analysis was de-

Figure 2. Ground photographs of beach morphologies in Charf el Akab site.

M. TAAOUATI ET AL.

114

signated to characterize the general offshore wave condi-

tions in the study area throughout the period of beach

monitoring. The wave data are represented by frequency

histograms for the 2-year period.

Additionally, the significant wave height and the cor-

responding period were identified and compared with the

beach changes for correspondence analysis. The under-

lying question was what value should be used to properly

represent the wave forcing as it pertains to morphologi-

cal evolution. Thus, considering that the morphology

responds more rapidly to the more energetic events, the

significant wave height and the corresponding wave pe-

riod were chosen. These values were then used to calcu-

late the wave energy flux or wave power “P” using the

following expression [11]:

32 π





with, ρ is the sea water density 1025 kg/m3 and H and T

are the offshore wave height and period, respectively.

3.2. Topographic Data

Almost two years of beach surveying was conducted on

Charf el Akab site. A 3D topographic mapping was made

using GTS-TOPCON 225, a high resolution laser total

station, at two-monthly and/or seasonal intervals through-

out the study period. In this study, we obtained highly

accurate vertical (±2 cm) as well as horizontal (±5 cm)

position estimates, which allow us to recommend GTS-

TOPCON 225 as a highly effective tool for beach moni-

toring. Previously, many authors [12,13] stated that

ground survey, using GPS and Total Station techniques

give the best accuracy in elevation measurements. Then,

the DEMs established from such data sets seem to be

more representative of the field reality and the calcula-

tion of sediment budget is reliable [2].

Each of the 9 surveys covered most of the beach from

the dune to just below the waterline. The elevation points

measured are referenced to a benchmark installed at the

foot of the dune. The collected data were interpolated

using triangulation with linear interpolation (TLI) and

converted into a grid with 2 m cell spacing. The grids are

used to produce the DEMs for which the quality (preci-

sion) is assessed. Precision is usually evaluated by indi-

ces with no spatial dimension such as the mean error or

the root mean square error [4]. In this study, In this study,

estimated height from the selected interpolation tech-

nique was compared at each point to observed height

using a cross-validate option in a basic mapping software

program SURFER® (Golden Software, Golden, CO). An

empirically derived error margin of 5 cm, covering both

field measurement and data interpolation, was applied to

the raw data.

4. Results

4.1. Hydrodynamic Characterization

Wave climate for the period 2005 – 2007 was character-

rized by conducting a statistical analysis of the data sets

corresponding to the “wana43” point. Results shown in

Figure 3 indicate that the most likely waves come from

the W quadrant, with almost 90% of the incident angle of

the incoming waves range between SW and NW. The

most frequent wave directions are WNW and W, which

represent approximately 60% and 24%, respectively.

With respect to the wave energy, the most frequent (45%)

significant wave height is between 0.5 m and 1 m, and

93.5% of the time the wave height is below 2 m. During

the study period the significant wave energy exceeded

rarely (6.5%) 2 m, but the occurrence of such “storm”

events can affect strongly the beach morphology. The

most likely wave periods are within 8 to 12 s and 65% of

the time the wave period is above 8 s.

In order to characterize energy conditions just before

the surveying times, significant events were identified

and the wave height and period were chosen to determine

the corresponding energy level (Table 1). The results

show that the offshore waves are characterized by a

mean significant wave height and period of 2 m and 9 s,

respectively. However, considerable annual variation in

the wave conditions is experienced (Table 1). Mean sig-

nificant wave conditions in winter (January - March) are

characterized by a wave height of 2.85 m and a period of

13.4 s. Mean significant wave conditions in summer

(June - September) are characterized by a wave height of

1.6 m and a period of 6.7 s, which remain, however, rela-

tively energetic conditions.

Table 1. Offshore wave energy conditions.

Survey times Hs (m) Tp (s) P (kgms−3)

23/04/2005 1.8 13.4 42 618

22/06/2005 2 9.4 36 915

24/09/2005 1.4 6.3 12 124

07/01/2006 2.5 12.3 75 472

04/03/2006 3.2 14.5 145 767

29/04/2006 1.2 7 9 896

27/06/2006 1.3 5.3 8 792

09/09/2006 2 6.5 25 526

20/01/2007 2.5 7.5 46 019

Mean 1.99 9.13 44 792

M. TAAOUATI ET AL.

115

Figure 3. Histograms of wave direction, significant wave height and peak period for the 2-year period.

In summary, results from wave climate analysis prove

that Charf el Akab beach was subjected to seasonal

variation in incident wave conditions during the survey

period. This seasonality is characterized by more ener-

getic waves in winter than in summer.

4.2. Beach Morphology

The beach morphology is described herein by represent-

ing the main changes occurred the survey period. The

DEMs illustrated in Figure 4; show two contrasting pro-

files where the morphology is affected by the accretion

and/or decay of an intertidal bar. The profile during

summer “accreted profile” was characterized by a non-

permanent feature (swash bar) aligned in the longshore

direction (Figure 4(a)). The formation and onshore mi-

gration of the swash bar is induced by low-energy condi-

tions prevailing during the summer season (Table 1).

These conditions favored onshore sediment transport by

swash processes; the sand is then accumulated in the

upper beach (Figure 4(a)). Following this calm period,

significant increase in wave energy (Hs > 2.5 m) re-

corded in winter (Table 1 ) caused a total decay of the bar

and yielded an “eroded profile” (Figure 4(b)). Such

evolution permitted us to suggest that Charf el Akab

beach present a cyclic behavior mainly modeled by wave

energy variations during the study period. It should be

stated; however, that beach slope remained gentle with a

mean gradient of tanβ = 0.02, and exhibited low variabi-

lity throughout the different surveys.

4.3. Net Beach Change and Sand Budget

Variability

Quantifying the net beach change between successive

surveys permitted us to assess both visual and quantita-

tive patterns of erosion and deposition within the system.

In this way, we used different grid commands of a soft-

ware program (SURFER®), and subsequent grid was

subtracted from previous grid to produce altimetry change

M. TAAOUATI ET AL.

116

(a) (b)

Figure 4. Examples of beach profile during the survey period. Elevations are in meters relative to the Moroccan General

Leveling (N.G.M).

map and calculate sand budget adjustments. The esti-

mated error of about ±5 cm on the data utilized corre-

sponds to an uncertainty of ±2000 m3 in the computed

volumes.

Results shown in Figure 5 provide information on

portions of the beach that had eroded, remained un-

changed or accreted between survey times and show the

net volume corresponding to each period. The main

morphologic changes observed and the volumes involved

are described briefly below.

1) Jun 22nd to Sep 24th, 2005

Low-energy conditions prevailed during this period

(Table 1) and beach accretion is more pronounced than

erosion yielding a significant deposition, the highest re-

corded over the 2-year survey period (Figure 5(a)). The

accretion was mostly observed in the intertidal zone,

whereas the erosion predominated on the backbeach.

This accretion phase is associated with a build up and

onshore migration of the swash bar. The sediment vol-

ume deposited was larger than that eroded, and the net

beach gain was of +8809 m3 of sediments.

2) Sep 24th, 2005 to Jan 7th, 2006

Relative increase in wave energy was observed

throughout this period and several high events (Hs > 2 m)

were recorded. This time was marked by net erosion,

especially in the lower and middle intertidal zones where

the bar was eroded causing an important beach flattening

(Figure 5(b)). In addition, minor spatially areas of net

deposition were observed on the backbeach. As a result,

the eroded volume was very important and a net beach

loss of -6593 m3 was recorded.

3) Jan 7th to Mar 4th, 2006

During this period, many high-energy events (Hs > 2 m,

Tp > 10 s) were recorded (Table 1). Accordingly, the

beach underwent the major erosional episode which

cause the bar destruction and/or offshore bar migration

resulting in a flatter beach profile (Figure 5(c)), whereas

a weak sediment deposition occurred on the backshore.

As a result, the beach developed a relatively flat and

featureless profile. Since the beach underwent dramatic

erosion, lost volume was the greatest and reached

–14302 m3.

4) Mar 4th to Jun 27th, 2006

This was a period when significant net deposition oc-

curred (+6746 m3). There was 0.25 - 0.75 m of accretion

that extended from 80 to 180 m cross-shore distance

(Figure 5(d)). The most significant occurrence during

this time was the formation of a swash bar in a parallel

direction of the shoreline. This was a relatively low-en-

ergy period; a maximum wave height of only 2 m was

recorded once. There was a long-duration event when Hs

< 1.25 m. Additionally, there was a moderate and spa-

tially minor area of erosion, notably on the backbeach.

This erosion can be ascribed to the dry eastern winds

which moved sediment in offshore direction.

5) Jun 27th to Sep 9th, 2006

Persistence of relative calm conditions during this pe-

riod (Table 1) induced swash bar growth and onshore

migration. The beach was significantly accreted com-

pared to the eroded backbeach (Figure 5(e)), always pro-

bably due to the eastern winds. This summer period was

characterized by a small sediment gain (+3666 m3) com-

paratively to the previous summer. This can be ascribed

to few separate energetic events (Hs > 1.8 m) which seem

moved offshore some sediment accumulated in the inter-

tidal zone.

6) Sep 9th, 2006 to Jan 20th, 2007

Almost minor net changes were recorded during this

period due to alternation of high- and low-energy events.

As a result, the swash bar moved till the backbeach and

formed a planar berm; whereas spatially minor zones of

erosion were located on the lower beach (Figure 5(f)).

Therefore, the beach net gain was only of +3020 m3 of

ediments. s

M. TAAOUATI ET AL.

117

(a) (d)

(b) (e)

Figure 5. Net DEMs beach changes and the sand budget involved for the survey beach. NBV = net beach volume. The plots

size is quite identical for all maps and is about of 40 000 m2. The area between yellow lines represents the elevation trench

considered as a negligible variation.

The overall net volume between the first and last sur-

vey highlighted the negligible changes in sediment

budget of Charf el Akab beach over the study period.

This stability leads to suggest a local distribution of

M. TAAOUATI ET AL.

118

sediments; the sand eroded from beach during winter is

moved off-shore and deposited in the subtidal zone and

following a period of fair weather conditions (summer)

the beach is accreted by sediment supply from the sub-

merged area.

5. Discussion and Conclusions

The use of digital elevation models in beach morpho-

logical analysis has expanded throughout the decade of

the 1990s allowing a comparison between surveys from

different times [2]. However, the DEMs utilized in this

work allowed both visual and quantitative assessment of

beach changes. Accordingly, this study demonstrates the

usefulness of DEMs in the evaluation of morphological

and volumetric beach changes. In fact, the survey area is

highly exposed to swell events and beach morphody-

namics should be controlled by wave forcing. The results

obtained by both wave and morphology analysis support

this statement. It seems, therefore, that the sand removed,

during winter, is transported seaward and accumulates

mainly in the subtidal zone. The loss of volume is par-

tially compensated for by accumulation to the beach face

during fair-weather conditions in summer months.

In addition, a comparison between the beach change

maps and the sand volume adjustments showed that

greatest morphological and volumetric changes were

strongly associated with the accretion and erosion of an

intertidal bar. This result coincides with the findings of

Masselink and Hegge [14] who demonstrated that

changes in Nine Mile Beach morphology, on the Austra-

lian coasts, were mainly associated with the formation

and evolution of secondary morphological features, in

particular, swash bars in the intertidal zone. Therefore, it

appears that onshore and offshore bar migration, oc-

curred in response to low- and high-wave conditions,

respectively.

In order to examine the role of wave forcing on beach

morphodynamics, we correlated net beach volume with

wave power variations during the survey times. The

Figure 6 shows that beach volume changes is inversely

proportional to wave power variations. In other words,

an increase in wave energy leads to a loss sediment stock

whereas a weak energy level permits to gain consider-

able quantities of sediment. This trend is supported by

the well correspondence between volume change and

wave power variations with a coefficient of correlation

of ap- proximately r2 = 0.9. This allows us to infer that

the beach changes occurred in Charf el Akab site during

the survey period were primarily caused and controlled

by wave forcing.

Summarizing, the use of DEMs based on high resolu-

tion filed data to quantify beach changes allow the fol-

lowing conclusions to be drawn from this study:

Figure 6. Beach volume and wave power variations during the survey period.

M. TAAOUATI ET AL.

119

1) Charf el Akab beach presented almost seasonal be-

havior during the study period;

2) this trend consisted in net accretion during low- en-

ergy conditions (summer) and beach erosion and flatten-

ing due to high wave energy in winter months;

3) the sediment budget variations were found to be

strongly influenced by seasonal changes in wave energy;

and

4) the 3-D DEM employed here is valuable tool with

which morphological and volumetric changes can be

clearly visualized and quantified.

Finally, this investigation presents an effective method

of determining beach accretion and/or erosion from

high-resolution spatial data which enables accurate

three-dimensional DEMs production. The results from

the present work support this suggestion since the vol-

ume change calculated from DEMs, were highly corre-

lated to variations in wave forcing. These dynamics

would not be identifiable using two-dimensional profiles

and this em- phasizes the value of the three-dimensional

DEMs uti- lized in this study.

6. Acknowledgements

This work was funded by the Moroccan PROTARS III

D16/07 program research. The principal author was

sponsored by a PhD studentship provided by the Na-

tional Center for Scientific and Technical Research of

Morocco. Authors are grateful to the anonymous re-

viewers for their constructive remarks and suggestions

for improvement.

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