BYPASS IN GROYNE FIELDS: CASE STUDY ALONG THE LOBITO SPIT

Sten Esbjørn Kristensen, DHI Denmark, [email protected] Nils Drønen, DHI Denmark, [email protected] Rolf Deigaard, DHI Denmark, [email protected] Berry Elfrink, DHI Denmark, [email protected]

ABSTRACT sand from south. The stability of the spit can be A simple box-type model is applied for the morphological compromised by large-scale interventions such as ports or description of a long groyne field located along the Lobito other coastal structures and by permanent loss of sand to spit in Angola, West Coast of Africa. The box-type model the offshore during storms. quantifies bypass transport for groyne fields with a simple relation developed in Kristensen et al. (2016) which is The offshore loss of sand is caused by avalanching along based on the average annual littoral drift distribution and certain sections of the coastal profile which are located the dimensions of the individual groynes. below the depth of closure. At these sections a local bed slope up to 22 deg. is observed thus indicating that sand The box-type morphological model is first setup to may be deposited in this area during storms and reproduce observed gradual filling of the groyne field subsequent permanently lost during an avalanche when shown in Figure 1. The model is then used to discuss the slope exceeds the critical value. long-term evolution of the groyne field and the impact of the groynes on the equilibrium littoral drift for the site. This A MODEL FOR BYPASS IN GROYNE FIELDS includes the effect of offshore loss of sand during storms, A development version of DHIs new model MIKE 21 which is significant for this site. It is also shown that the Shoreline Morphology was used in Kristensen et al. offshore loss of sand, be it continuous or event based 1) (2016) to simulate morphological development and the reduces the rate of filling of the groyne field thereby resulting equilibrium littoral drift in groyne fields. The reducing overall beach width in the downstream groyne simulations covered a range of different groyne compartments and 2) modifies the equilibrium littoral drift dimensions and wave conditions and the results were for the area thereby requiring downstream groyne compiled into a functional formulation based on the compartments to be half filled. groyne field geometry and the littoral drift distribution for the undisturbed case. The bypass description is closely THE LOBITO SPIT related to simpler geometric considerations often seen in The Lobito spit is a sandy spit with a length of about 8 km. traditional 1-line models but includes also effects of The spit was created by the large angle of wave streamline contractions, thus providing a better incidence, which causes instabilities to grow in time. The representation of the bypass. Figure 2 shows the relative dominant wave direction is from SW, while the coastline bypass as function of the relative potential shoreline is generally oriented NW. The spit is currently protected advance, Lmorph. by a large number of groynes having a length of approximately 100 m and spacing along the coast of 300 m.

Figure 1 - Northern tip of the Lobito spit, Angola West coast Figure 1 – Relative bypass as function of rel. potential of Africa. Historic shoreline are extracted from Google Earth. shoreline advance and rel. geometric bypass (in colours).

Sand is transported from the mouth of the river Catumbela, located approximately 10 km south of the spit. REFERENCES The sand is transported passed the groyne field to the Kristensen, Drønen, Deigaard and Fredsoe (2016). northern end of the spit where it accumulates thus causing Impact of groyne fields on the littoral drift: A hybrid gradual migration of the spit towards north. morphological modelling study, Coastal Engineering, 111:13-22. The stability of the spit depends on the steady supply of