The determination of the feasibility of a 'Sand Breakwater' on the Nigerian coastline at Badagry

Abstract

The coastal waters of Nigeria are a difficult environment to build structures to control the local morphology and hydrodynamic conditions to ones desire. This difficulty is a consequence of the persistent high-energy swell wave climate and Longshore Sediment Transport (LST) in the order of one million cubic metres per
year. Besides these tough conditions, the costs of conventional coastal mitigation solutions are high because of the lack of suitable material (rock) close by. These characteristics of the Nigerian coast led to the interest, whether it is possible to design and construct a ’Sand Breakwater’ for a planned port on the Nigerian coastline at Badagry. A ’Sand Breakwater’ is the complete construction which functions as the protection of the port and creates shelter from incoming waves for ships inside, consisting out of components (partly) made out of sand and hard structures. Especially the very uni-directional and consistent character of the wave climate creates an opportunity to embrace natural processes in the design. In addition, much sand needs to be dredged for the creation of the approach channel along with the port’s basin. This volume of sand might possibly be re-used for a sand breakwater.
This report presents the feasibility of a sand breakwater on the Nigerian coastline at Badagry. This feasibility is determined both morphologically and economically. Long term coastline development was modelled with the one-line coastline evolution model LITPACK. Results show that the persistent uni-directional character of the wave climate prevailing at the coast of Badagry forces an equilibrium coastline orientation of 277.5 degrees with respect to True North (TN). Because of this, all other orientations of coast rather than the equilibrium orientation are vulnerable and will develop to the equilibrium orientation over time. The implementation of this equilibrium orientation in the design along with some hard structures led to multiple long term morphological
stable conceptual variants for a sand breakwater. The coastline impacts of these variants west of the sand breakwater were examined on large scale. In 38 years after construction a maximum of 20 meters coastline retreat occurs, which is considered acceptable. After this period, only shoreline accretion takes place.
To establish complete conceptual designs, the cross-shore profiles are first determined. The submerged part of the profile up to the top of the intertidal profile is constructed by integration of measured data in the design. The emerged part of the profile concerns the cross-section of a dune and was the critical part for the designing objective. To determine these profiles, first the design crest height is determined by applying maxmimum acceptable overtopping volumes (Tilmans, 1983). Knowing the design crest height, the dune width is determined by modelling storm impact. This width is established by examining storm impact and determine what minimal required width needs to be present for the sand breakwater to live up to its design criteria.
Storm impact for the found conceptual variants is examined with the numerical software XBeach to determine cross-shore sediment transport during storm impact. The impact of storms with shorter return periods results to be relatively high in comparison to the storms with longer return periods. This is taken into account by adapting the design conditions. The design storm conditions are based on the impact of a storms with a 1/100 year return period along with an expansion for a consecutive storm with a return period of 1/1 year. With this
storm impact known, the required crest width is determined and with that the minimal required cross-shore profiles for the multiple variants are established. Consequently, besides long term morphologically stable, the conceptual designs are capable to design storm impact, confirming the morphological feasibility.
In order to define the economical feasibility of a sand breakwater, a comparison between the different conceptual variants and a conventional design is made. This comparison is conducted via a rough cost comparison along with examining future prospects. In order to be able to make this cost comparison, rough volume estimations are done for all conceptual variants and conventional design. These volume estimations combined with unit prices lead to a rough cost comparison. From this rough cost comparison appears that a sand breakwater
showed to be in the same price range as a conventional design.
Three future prospects of a sand breakwater are determined. First of all the by-pass of sand is compared for the conceptual designs to the conventional design. The characteristic LST is problematic due to the occurrence of sand by-passing causing sedimentation in the approach channel and port. One of the conceptual variants shows that a sand breakwater is proven to be able to provide a period without by-passing of sand in the same order of magnitude as a conventional design.
Another future prospect is the required maintenance necessary to execute for the sand breakwater. The XBeach results of lower storm conditions show that maintenance costs are high. However, these results are assumed to be heavily overestimated. Results with more specific ’Nigerian conditions’ create reason to believe that maintenance for lower storm conditions will be much lower and in acceptable range.
The last future prospect concerns the accreted land which arises due to the blockage of LST. A sand breakwater creates the possibility of acquiring land for new port space which is not possible with a conventional breakwater. In addition, the value of a Building with Nature component is present in this project and could enhance its economical interest.
Not only the morphological but also the economical feasibility of a sand breakwater is confirmed. The ’Sand Breakwater’ succeeded to convert the drivers of the problem to its solution, leading to an innovative and in all likeliness even more cost-effective solution compared to the traditional approach.