# 9.8.1: Closure of a part of the tidal basin

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The closure of a part of the tidal basin will result in a reduction of the channel volume, \(V_c\), and of the tidal prism \(P\). The basins will adapt to the new situation, see Fig. 9.35. In this example we will neglect the role of the flats and assume that they are already more or less in equilibrium immediately after the closure.

The two lines in Fig. 9.35 represent the relationship of the channel volume and the tidal prism with \(C_V\) is \(65 \times 10^{-6} m^{-3/2}\) and the relationship of the sand volume of the outer delta and the tidal prism with \(C_{\text{od}}\) is \(65.7 \times 10^{-4}\ m^{-3/2}\). Assume an equilibrium situation with tidal prism \(P\) and corresponding channel volume \(V_c\) and sand volume of the outer delta \(V_{\text{od}}\). At a certain moment in time, a part of the tidal basin is closed off, which results in a reduction of the channel volume \(\Delta V_c\) and a reduction of the tidal prism \(\Delta P\). A new equilibrium will arise at the equilibrium lines for tidal prism \(P - \Delta P\). This means that the channel volume is \(a\ m^3\) too big and the sand volume of the outer delta is \(b\ m^3\) too big. The sand of the outer delta is available for the adaptation of the channels (to an amount of \(b\ m^3\)). The rest, if \(a - b > 0\), has to be supplied from outside (resulting in erosion of the downdrift coast). Closure 1 (Fig. 9.35, left) and closure 2 (Fig. 9.35, right) differ in the magnitude of \(\Delta V_c\). As a consequence \(a - b > 0\) for closure 1 and \(a - b < 0\) closure 2 (see Table 9.6).

closure 1 | closure 2 | accretion | |

Prism before | 600 | 600 | 300 |

Prism after | 300 | 300 | 225 |

\(\Delta V_c\) | 300 | 470 | 0 |

\(V_{c, \text{before}}\) | 955 | 955 | 338 |

\(V_{c, \text{after}}\) | 338 | 338 | 219 |

\(a\) | 318 | 148 | 118 |

\(V_{od, \text{before}}\) | 412 | 412 | 176 |

\(V_{od, \text{after}}\) | 176 | 176 | 123 |

\(b\) | 236 | 236 | 52 |

\(a- b\) | 82 | -88 | 66 |

An example of the above situation is the closure of the Lauwerszee in 1969 (see Fig. 9.2) as described in Wang et al. (2009) and reprinted in App. E. Due to the decrease of the tidal basin area, the tidal prism and thereby the magnitude of the flow velocity decreased significantly. The tidal asymmetry changed such that it became more flood-dominant favouring sediment input. The effect of the closure therefore was a sediment deficit that needed to be supplied from outside. Since the closure, the basin has been accumulating sediment and the ebb-tidal delta has been eroding. The sedimentation in the basin and the erosion of the ebb-tidal delta are more or less in balance. As a consequence the closure has not caused erosion of the adjacent coasts.