2 Observations

The wave periods are observed at the shore line. Waves of shorter period succeed the main waves. The onsets of the inflows and outflows in a particular harbour in Flaskebekk, on the eastern side of the Oslofjord, were recorded by the author on 17 February 2016 using a stop watch. The wave periods due to the two ships, DFDS Pearl Seaways and Color Fantasy cruising by Flaskebekk, observation position 1, marked by an asterisk in figure 2(c), the first right after 9 a.m., at a speed of 8.6  $\text{m}~\text{s}^{-1}$ , the second right before 9:30 a.m., at a speed of 10.3  $\text{m}~\text{s}^{-1}$ , were approximately 34 s for both ships, see table 1. A corresponding wavelength of approximately 830 m can be estimated by multiplying the period by the shallow water speed in the fjord. Note that the first inflow-to-inflow period of 26 s due the first slower and somewhat smaller ship may not be accurate, since the wave amplitude was rather small. A third wave period of 13 s due to the second, faster and larger ship was not generated by the slower one. A typical wave height of 1 m is documented in this harbour and a record height of 1.4 m has been measured by the owner, see the further documentation in appendix A.1.

A photograph of the outflow in the harbour due to the dominant depression wave is shown in figure 1. The ship Color Magic causing the harbour wave is seen to the right in the picture. The wave is generated at a position in the fjord located right behind Ildjernsflu lighthouse, which is seen in the picture above the pier, to the right of the white bath house. The harbour shown in figure 1 (position 1) and Ildjernsflu lighthouse are indicated on the sea chart shown in figure 2(a), available at www.oslofjorden.com/map/.

Figure 1.

Outflow of tsunami in harbour at Flaskebekk, generated by the ship Color Magic seen in the picture. The generation site in the fjord is next to the lighthouse seen above the pier and to the right of the white bathhouse. The distance to the ship is approximately 1.2 km. Photo. T. H. Larsen.

Figure 2.

(a) Section of sea chart of the Oslofjord at Ildjernsflu, with marked position of the Ildjernsflu lighthouse ( $\leftarrow$ ) and the harbour at Flaskebekk, imaged in figure 1 ( $\downarrow$ ). The ship track is to the left of the violet dashed line. (b) Sea chart at Askholmene with position of video recording marked by $X$ . The ship track crosses over depths of 81 m, 39 m, 27 m. (c) Overview with positions of Ildjernsflu/Flaskebekk (*), Askholmene ( $\leftarrow$ ) and Snarøya ( $+$ ) indicated. Panels (a,b) are minor sections of charts available at www.oslofjorden.com/map/. The Oslofjord is 109 km long.

The upstream ship waves extending across the 2–3 km wide fjord are observed along both shores. In a small bay at Snarøya on the north-western shore of the fjord, referred to as observation position 2, the wave periods recorded by the author were close to a minute, where the observed ship, Color Magic, was cruising at a speed of $7.8~\text{m}~\text{s}^{-1}$ , see table 1. The first period of $T=57$  s corresponds to a wavelength of approximately $900$  m. (On the day of observation, 7 January 2016, the Norwegian Coastal Administration performed blasting of 24 shallow rocks in the Inner Oslofjord, and the ship travelled at a reduced speed.)

At Askholmene islands, located midway in the fjord north of the village of Drøbak, observation position 3, the southward track is located west of the islands, see the sea chart in figure 2(b). A video recording of the upstream waves at this position due to Color Magic is available at https://www.youtube.com/watch?v=42Ctdk9kpyg. The video shows that the very first motion in the bay is a weak outflow. This means that the leading upstream wave caused by the ship moving along a depth that is reduced is a weak wave of depression. Periods of the dominant wave train obtained from this video show $T=32\pm 3$  s for the leading two waves (table 1) and correspond to a wavelength of approximately 550 m.

The waves are generated when the ships travel across pronounced depth changes. Let $h_{A}$ denote the water depth along the ship’s track before the depth change, $h_{B}$ the typical depth in the shallow region and $h_{C}$ depth after the shallow region. The wave generation is observed for a depth change $\unicode[STIX]{x0394}h=h_{A}-h_{B}$ that is comparable to the average depth $h=(h_{A}+h_{B})/2$ , i.e. $\unicode[STIX]{x0394}h/h\simeq 1$ . The ratio $2(h_{C}-h_{B})/(h_{C}+h_{B})$ determines the relative depth change from shallow to deep water.

The ship track at Ildjernsflu lighthouse is north/north-eastward, to the left of the violet dashed line in figure 2(a). A ridge extends south/south-westward of the lighthouse where depths in metres are marked by 11, 16 and 19 m. From the figure we estimate: $h_{A}\approx 46$  m and $h_{B}\approx 14$  m, giving $h=30$  m and $\unicode[STIX]{x0394}h/h\simeq 1.067$ . Further, an extension of 700 m ${\approx}23h$ of the shallow region along the ship’s track is estimated. The average depth of the fjord further north is approximately 60 m on the eastern side and approximately 25 m on the western side.

The south/south-eastward ship track at Askholmene, position 3, passes over the depths marked by 81, 39, 27 m in figure 2(b). Note that the depth is shallower than 20 m next to the position marked by 39 m. The water is only a few metres deep at each side of the track. The position of the video recording is marked by $X$ . From the figure we obtain: $h_{A}\approx 80$  m and $h_{B}\approx 30$  m giving $h=55$  m and $\unicode[STIX]{x0394}h/h=0.909$ . The distances between observation position and generation site, and the ship’s track, are indicated in table 2. Figure 2(c) shows the positions in the Oslofjord.

The displaced volumes of Pearl Seaways and Crown Seaways are 69 % and 55 %, respectively, of those of Color Line, see table 4 in appendix A.1 where also further descriptions of the observations are given.

Table 2.

Distance ( $D_{source}$ ) between the generation site and the observation position, measured along the ship’s track, distance ( $D_{track}$ ) from observation position to track, depth before depth change ( $h_{A}$ ), depth of shallow region ( $h_{B}$ ), upstream depth ( $h_{C}$ ) and depth change ratio $\unicode[STIX]{x0394}h/h=2(h_{A}-h_{B})/(h_{A}+h_{B})$ at observation positions 1–3.

3 Mathematical formulation

The analysis is three-dimensional. We introduce horizontal coordinates $\boldsymbol{x}=(x_{1},x_{2})$ , vertical coordinate $y$ , with $y=0$ at the free surface at rest, and time $t$ . The generation by the ship is modelled by a moving pressure distribution (hovercraft) of similar length, width, draught and displacement. The surface pressure is moving with constant speed $U$ along the $x_{1}$ -direction where a gentle ramp up of the motion is applied. The linear potential theory accounts for the effect of the bottom variation. The waves are obtained by integrating in time the linear kinematic and dynamic boundary conditions at the free surface:

(3.1a,b ) $$\begin{eqnarray}\unicode[STIX]{x2202}\unicode[STIX]{x1D702}/\unicode[STIX]{x2202}t=\unicode[STIX]{x2202}\unicode[STIX]{x1D719}/\unicode[STIX]{x2202}y,\quad \unicode[STIX]{x2202}\unicode[STIX]{x1D719}/\unicode[STIX]{x2202}t+g\unicode[STIX]{x1D702}=-p/\unicode[STIX]{x1D70C},\quad y=0,\end{eqnarray}$$

where $\unicode[STIX]{x1D702}$ denotes the elevation, $\unicode[STIX]{x1D719}$ velocity potential, $p$ pressure distribution and $\unicode[STIX]{x1D70C}$ density. The bottom variation is given by $y_{B}=-h+\unicode[STIX]{x1D6FD}(\boldsymbol{x})$ where $h$ is an average depth and $\unicode[STIX]{x1D6FD}$ the depth variation. We introduce

(3.2a-c ) $$\begin{eqnarray}\unicode[STIX]{x1D719}_{F}(\boldsymbol{x})=\unicode[STIX]{x1D719}(\boldsymbol{x},y=0),\quad \unicode[STIX]{x1D719}_{B}(\boldsymbol{x})=\unicode[STIX]{x1D719}(\boldsymbol{x},y=y_{B}),\quad V=\unicode[STIX]{x2202}\unicode[STIX]{x1D719}/\unicode[STIX]{x2202}y|_{y=0},\end{eqnarray}$$

where $\unicode[STIX]{x1D719}_{F}$ denotes the potential evaluated at the free surface, $\unicode[STIX]{x1D719}_{B}$ potential along the bottom and $V$ the normal velocity at the free surface. The normal velocity at the bottom is zero. The functions $\unicode[STIX]{x1D719}_{F}$ , $\unicode[STIX]{x1D719}_{B}$ and $V$ are connected through solution of the Laplace equation and are expressed in the form of a set of integral equations. The solution method is an extension of Clamond & Grue (Reference Clamond and Grue2001, § 6), Grue (Reference Grue2002, § 6), Fructus & Grue (Reference Fructus and Grue2007, § 3) and Grue (Reference Grue2015). An evaluation point on the free surface $(y^{\prime }=0)$ gives

(3.3) $$\begin{eqnarray}\int _{F}\left(\frac{1}{r}+\frac{1}{r_{1}}\right)\frac{\unicode[STIX]{x2202}\unicode[STIX]{x1D719}}{\unicode[STIX]{x2202}n}\,\text{d}S=2\unicode[STIX]{x03C0}\unicode[STIX]{x1D719}_{F}^{\prime }(\boldsymbol{x}^{\prime })+\int _{F+B}\unicode[STIX]{x1D719}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}n}\left(\frac{1}{r}+\frac{1}{r_{1}}\right)\,\text{d}S,\end{eqnarray}$$

where $1/r=(R^{2}+(y^{\prime }-y)^{2})^{-1/2}$ denotes a source function, $R=|\boldsymbol{x}^{\prime }-\boldsymbol{x}|$ horizontal distance, $1/r_{1}=(R^{2}+(y^{\prime }+y+2h)^{2})^{-1/2}$ the image with respect to $y^{\prime }=-h$ , $F$ the free surface, $B$ bottom of the fluid layer and $\text{d}S$ integration element. The normal, $n$ , points out of the fluid. A prime indicates evaluation variable where $\unicode[STIX]{x1D719}^{\prime }=\unicode[STIX]{x1D719}(\boldsymbol{x}^{\prime },y^{\prime })$ etc. For an evaluation point at the bottom ( $y^{\prime }=-h+\unicode[STIX]{x1D6FD}^{\prime }$ ) we obtain

(3.4) $$\begin{eqnarray}2\unicode[STIX]{x03C0}\unicode[STIX]{x1D719}_{B}^{\prime }(\boldsymbol{x}^{\prime })=\int _{F}\left(\frac{1}{r}+\frac{1}{r_{1B}}\right)\frac{\unicode[STIX]{x2202}\unicode[STIX]{x1D719}}{\unicode[STIX]{x2202}y}\,\text{d}S-\int _{B}\unicode[STIX]{x1D719}_{B}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}n}\left(\frac{1}{r}+\frac{1}{r_{1B}}\right)\,\text{d}S,\end{eqnarray}$$

where $1/r_{1B}=(R^{2}+(y^{\prime }+y)^{2})^{-1/2}$ . Solution of (3.3) and (3.4) is obtained expanding $1/r$ and its images in powers of the bottom excursion $\unicode[STIX]{x1D6FD}$ . Consider the integral over $F$ in (3.4) where the function $1/r+1/r_{1B}$ is expressed by

(3.5a,b ) $$\begin{eqnarray}\displaystyle \frac{1}{r}+\frac{1}{r_{1B}}=2\left(1-\unicode[STIX]{x1D6FD}^{\prime }\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}h}+\frac{1}{2!}{\unicode[STIX]{x1D6FD}^{\prime }}^{2}\frac{\unicode[STIX]{x2202}^{2}}{\unicode[STIX]{x2202}h^{2}}-\frac{1}{3!}{\unicode[STIX]{x1D6FD}^{\prime }}^{3}\frac{\unicode[STIX]{x2202}^{3}}{\unicode[STIX]{x2202}h^{3}}+\cdots \,\right)\frac{1}{R_{0}},\quad R_{0}^{2}=R^{2}+h^{2}. & & \displaystyle \nonumber\\ \displaystyle & & \displaystyle\end{eqnarray}$$

Note that $1/R_{0}={\mathcal{F}}^{-1}[(2\unicode[STIX]{x03C0}/k)\text{e}^{-\text{i}\boldsymbol{k}\cdot \boldsymbol{x}-kh}]$ where ${\mathcal{F}}$ denotes Fourier transform, ${\mathcal{F}}^{-1}$ inverse transform, $\boldsymbol{k}=(k_{1},k_{2})$ wavenumber vector in Fourier space and $k=|\boldsymbol{k}|$ . This gives for the integral over $F$ in (3.4)

(3.6) $$\begin{eqnarray}\displaystyle \frac{1}{2\unicode[STIX]{x03C0}}\int _{F}\left(\frac{1}{r}+\frac{1}{r_{1B}}\right)\frac{\unicode[STIX]{x2202}\unicode[STIX]{x1D719}}{\unicode[STIX]{x2202}y}\,\text{d}S & = & \displaystyle {\mathcal{F}}^{-1}\left[\frac{2\hat{V}e_{1}}{k}\right]+\frac{1}{1!}\unicode[STIX]{x1D6FD}^{\prime }{\mathcal{F}}^{-1}(2\hat{V}e_{1})+\frac{1}{2!}{\unicode[STIX]{x1D6FD}^{\prime }}^{2}{\mathcal{F}}^{-1}(2k\hat{V}e_{1})\nonumber\\ \displaystyle & & \displaystyle +\,\frac{1}{3!}{\unicode[STIX]{x1D6FD}^{\prime }}^{3}{\mathcal{F}}^{-1}(2k^{2}\hat{V}e_{1})+\cdots \,,\end{eqnarray}$$

where $e_{1}=\text{e}^{-kh}$ . Similarly the integrals over $B$ in (3.4) are expressed by:

(3.7) $$\begin{eqnarray}\displaystyle & & \displaystyle \displaystyle -\int _{B}\unicode[STIX]{x1D719}_{B}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}n}\frac{1}{r}\,\text{d}S=\int _{B}\unicode[STIX]{x1D6FB}_{1}\unicode[STIX]{x1D719}_{B}\cdot \left(\left[(\unicode[STIX]{x1D6FD}-\unicode[STIX]{x1D6FD}^{\prime })-\frac{1}{3!}(\unicode[STIX]{x1D6FD}-\unicode[STIX]{x1D6FD}^{\prime })^{3}\unicode[STIX]{x1D6FB}_{1}^{2}+\frac{1}{5!}(\unicode[STIX]{x1D6FD}-\unicode[STIX]{x1D6FD}^{\prime })^{5}\unicode[STIX]{x1D6FB}_{1}^{4}+\cdots \,\right]\right)\nonumber\\ \displaystyle & & \displaystyle \displaystyle \qquad \times \,\unicode[STIX]{x1D6FB}_{1}\frac{1}{R}\,\text{d}\boldsymbol{x},\end{eqnarray}$$

(3.8) $$\begin{eqnarray}\displaystyle & & \displaystyle \displaystyle -\int _{B}\unicode[STIX]{x1D719}_{B}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}n}\frac{1}{r_{1B}}\,\text{d}S=-\int _{B}\unicode[STIX]{x1D719}_{B}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}(2h)}\frac{1}{R_{1}}\,\text{d}\boldsymbol{x}\nonumber\\ \displaystyle & & \displaystyle \qquad +\,\int _{B}\unicode[STIX]{x1D6FB}_{1}\unicode[STIX]{x1D719}_{B}\cdot \left(\frac{1}{1!}(\unicode[STIX]{x1D6FD}+\unicode[STIX]{x1D6FD}^{\prime })-\frac{1}{2!}(\unicode[STIX]{x1D6FD}+\unicode[STIX]{x1D6FD}^{\prime })^{2}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}(2h)}+\frac{1}{3!}(\unicode[STIX]{x1D6FD}+\unicode[STIX]{x1D6FD}^{\prime })^{3}\frac{\unicode[STIX]{x2202}^{2}}{\unicode[STIX]{x2202}(2h)^{2}}-\,\cdots \right)\nonumber\\ \displaystyle & & \displaystyle \displaystyle \qquad \times \,\unicode[STIX]{x1D6FB}_{1}\frac{1}{R_{1}}\,\text{d}\boldsymbol{x},\end{eqnarray}$$

where in the latter $R_{1}^{2}=R^{2}+4h^{2}$ and $\unicode[STIX]{x1D6FB}_{1}^{\prime }=(\unicode[STIX]{x2202}/\unicode[STIX]{x2202}x_{1}^{\prime },\unicode[STIX]{x2202}/\unicode[STIX]{x2202}x_{2}^{\prime })$ denotes the horizontal gradient. Using that $1/R={\mathcal{F}}^{-1}[(2\unicode[STIX]{x03C0}/k)\text{e}^{-\text{i}\boldsymbol{k}\cdot \boldsymbol{x}}]$ and $1/R_{1}={\mathcal{F}}^{-1}[(2\unicode[STIX]{x03C0}/k)\text{e}^{-\text{i}\boldsymbol{k}\cdot \boldsymbol{x}-2kh}]$ the resulting set of equations for $\unicode[STIX]{x1D719}_{B}$ is obtained by (A 4), (A 5) in appendix A.3. By similar procedures, the integral over $B$ in the last term in (3.3) becomes

(3.9) $$\begin{eqnarray}\frac{1}{2\unicode[STIX]{x03C0}}{\mathcal{F}}\left(\int _{B}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}n}\left(\frac{1}{r}+\frac{1}{r_{1}}\right)\unicode[STIX]{x1D719}_{B}\,\text{d}S\right)=\frac{2\text{e}^{-kh}\text{i}\boldsymbol{k}}{k}\cdot \left[{\mathcal{F}}(\unicode[STIX]{x1D6FD}\unicode[STIX]{x1D6FB}_{1}\unicode[STIX]{x1D719}_{B})+\frac{k^{2}}{3!}{\mathcal{F}}(\unicode[STIX]{x1D6FD}^{3}\unicode[STIX]{x1D6FB}_{1}\unicode[STIX]{x1D719}_{B})+\cdots \,\right],\end{eqnarray}$$

while the surface integrals on the left-hand side and right-hand side of (3.3) become $2\unicode[STIX]{x03C0}\hat{V}(1+e_{1}^{2})/k$ and $-2\unicode[STIX]{x03C0}\hat{\unicode[STIX]{x1D719}}_{F}e_{1}^{2}$ , respectively, giving

(3.10) $$\begin{eqnarray}\hat{V}=kT_{1}\hat{\unicode[STIX]{x1D719}}_{F}+\left(\frac{1}{1!}\hat{A}_{1}+\frac{k^{2}}{3!}\hat{A}_{3}+\cdots \,\right)\frac{1}{C_{1}},\end{eqnarray}$$

where $T_{1}=\tanh kh$ , $C_{1}=\cosh kh$ and $\hat{A}_{n}=\text{i}\boldsymbol{k}\boldsymbol{\cdot }{\mathcal{F}}(\unicode[STIX]{x1D6FD}^{n}\unicode[STIX]{x1D6FB}_{1}\unicode[STIX]{x1D719}_{B})$ . The resulting set of equations expresses $(V,\unicode[STIX]{x1D719}_{B})$ in terms of the surface potential $\unicode[STIX]{x1D719}_{F}$ and expansions of the bottom variation ordered according to $\unicode[STIX]{x1D6FD}^{n}$ ( $n=0,1,2,\ldots$ ) where the smallness parameter is $\text{max}(\unicode[STIX]{x1D6FD}/h)=O((\unicode[STIX]{x0394}h/h)/2)$ . The set of variables of the leading two orders become

(3.11a,b ) $$\begin{eqnarray}(k\hat{\unicode[STIX]{x1D719}}_{B}^{(0)},\hat{V}^{(0)})=(k\hat{\unicode[STIX]{x1D719}}_{F}/C_{1},kT_{1}\hat{\unicode[STIX]{x1D719}}_{F}),\quad (k\hat{\unicode[STIX]{x1D719}}_{B}^{(1)},\hat{V}^{(1)})=(-T_{1}\hat{A}_{1},\hat{A}_{1}/C_{1}).\end{eqnarray}$$

Regarding the vertical velocity we obtain that $\hat{V}^{(2)}=0$ and $\hat{V}^{(3)}=k^{2}\hat{A}_{3}/(6C_{1})$ where $T_{1},C_{1},\hat{A}_{n}$ are defined below (3.10). The representations $\unicode[STIX]{x1D719}_{B}^{(0)}+\unicode[STIX]{x1D719}_{B}^{(1)}+\unicode[STIX]{x1D719}_{B}^{(2)}+\unicode[STIX]{x1D719}_{B}^{(3)}$ and $\hat{V}^{(0)}+\hat{V}^{(1)}+\hat{V}^{(3)}$ are obtained iteratively from (3.11) and (A 4)–(A 5) in appendix A.3. We define the truncation level of the approximations of $(\unicode[STIX]{x1D719}_{B},V)$ by

(3.12) $$\begin{eqnarray}\displaystyle & \displaystyle s^{(1)}=(\unicode[STIX]{x1D719}_{B}^{(0)}+\unicode[STIX]{x1D719}_{B}^{(1)},V^{(0)}+V^{(1)}), & \displaystyle\end{eqnarray}$$

(3.13) $$\begin{eqnarray}\displaystyle & \displaystyle s^{(2)}=(\unicode[STIX]{x1D719}_{B}^{(0)}+\unicode[STIX]{x1D719}_{B}^{(1)}+\unicode[STIX]{x1D719}_{B}^{(2)},V^{(0)}+V^{(1)}), & \displaystyle\end{eqnarray}$$

(3.14) $$\begin{eqnarray}\displaystyle & \displaystyle s^{(3)}=(\unicode[STIX]{x1D719}_{B}^{(0)}+\unicode[STIX]{x1D719}_{B}^{(1)}+\unicode[STIX]{x1D719}_{B}^{(2)}+\unicode[STIX]{x1D719}_{B}^{(3)},V^{(0)}+V^{(1)}+V^{(3)}), & \displaystyle\end{eqnarray}$$

where convergence of the computations is illustrated using the approximations $s^{(2)}$ and $s^{(3)}$ , see § 5 below. The approximation $s^{(2)}$ is computationally less expensive than $s^{(3)}$ .

The resulting equation system for the free surface variables $\unicode[STIX]{x1D702}$ and $\unicode[STIX]{x1D719}_{F}$ is expressed by Fourier transform

(3.15a,b ) $$\begin{eqnarray}\frac{\unicode[STIX]{x2202}\hat{\unicode[STIX]{x1D702}}}{\unicode[STIX]{x2202}t}=kT_{1}\hat{\unicode[STIX]{x1D719}}_{F}+{\hat{h}}_{1},\quad \frac{\unicode[STIX]{x2202}\hat{\unicode[STIX]{x1D719}}_{F}}{\unicode[STIX]{x2202}t}+g\hat{\unicode[STIX]{x1D702}}={\hat{h}}_{2},\end{eqnarray}$$

where ${\hat{h}}_{1}=\hat{A}_{1}/C_{1}+k^{2}\hat{A}_{3}/(6C_{1})+\cdots \,$ is obtained from (3.10) and ${\hat{h}}_{2}=-\hat{p}/\unicode[STIX]{x1D70C}$ is given by the moving pressure distribution. Time integration of (3.15) is expressed in matrix form: ${\hat{Y}}_{t}+\unicode[STIX]{x1D63C}{\hat{Y}}={\hat{H}}$ where ${\hat{Y}}=[\hat{\unicode[STIX]{x1D702}},(\unicode[STIX]{x1D714}/g)\hat{\unicode[STIX]{x1D719}}_{F}]^{T}$ , ${\hat{H}}=[{\hat{h}}_{1},\unicode[STIX]{x1D714}{\hat{h}}_{2}/g]^{T}$ , $\unicode[STIX]{x1D714}^{2}=gk\tanh kh$ and $[\,]^{T}$ denotes transpose. The matrix $\unicode[STIX]{x1D63C}$ and its variant $\text{e}^{As}$ are obtained by

(3.16a,b ) $$\begin{eqnarray}\unicode[STIX]{x1D63C}=\left[\begin{array}{@{}cc@{}}0 & -\unicode[STIX]{x1D714}\\ \unicode[STIX]{x1D714} & 0\end{array}\right],\quad \text{e}^{As}=\left[\begin{array}{@{}cc@{}}\cos \unicode[STIX]{x1D714}s & -\sin \,\unicode[STIX]{x1D714}s\\ \sin \unicode[STIX]{x1D714}s & \cos \unicode[STIX]{x1D714}s\end{array}\right].\end{eqnarray}$$

Time integration of $(\text{e}^{At}{\hat{Y}})_{t}=\text{e}^{At}{\hat{H}}$ gives ${\hat{Y}}=\int _{t_{0}}^{t}\text{e}^{A(s-t)}{\hat{H}}\,\text{d}s$ where no elevation, motion or pressure impulse for $t_{0}<0$ are assumed. The elevation is obtained by

(3.17) $$\begin{eqnarray}\hat{\unicode[STIX]{x1D702}}=\int _{t_{0}}^{t}\cos \unicode[STIX]{x1D714}(s-t)\,{\hat{h}}_{1}(s)\,\text{d}s+\int _{t_{0}}^{t}\sin \unicode[STIX]{x1D714}(s-t)\,\frac{\unicode[STIX]{x1D714}\,\hat{p}}{\unicode[STIX]{x1D70C}g}\,\text{d}s,\end{eqnarray}$$

where the first term accounts for the effect of the bottom variation and the second term the effect of the pressure distribution moving along a fluid layer of constant water depth $h$ . Equation (3.17) is used in the asymptotic analysis in § 4.

4 Asymptotic analysis for long waves

The very long upstream ship waves motivate an asymptotic analysis valid for $kh\ll 1$ . The long wave analysis is complemented by fully dispersive calculations in § 5 below. Wave generation due to a depth change at $x_{1}=0$ is considered, where the water depth is assumed to be $h$ for $x_{1}<0$ and $h-\unicode[STIX]{x0394}h$ for $x_{1}>0$ , with $\unicode[STIX]{x0394}h/h\ll 1$ . This means that $\unicode[STIX]{x1D6FD}=0$ for $x_{1}<0$ and $\unicode[STIX]{x1D6FD}=\unicode[STIX]{x0394}h$ for $x_{1}>0$ . For small $kh$ we have from (3.11) $\hat{\unicode[STIX]{x1D719}}_{B}\simeq \hat{\unicode[STIX]{x1D719}}_{B}^{(0)}\simeq \hat{\unicode[STIX]{x1D719}}_{F}$ where $\cosh kh\simeq 1$ is used. Evaluating the Fourier transform we obtain for ${\hat{h}}_{1}$ :

(4.1) $$\begin{eqnarray}{\hat{h}}_{1}\simeq \text{i}k_{1}\unicode[STIX]{x0394}h\int _{0}^{\infty }\int _{-\infty }^{\infty }\frac{\unicode[STIX]{x2202}\unicode[STIX]{x1D719}_{F}}{\unicode[STIX]{x2202}x_{1}}\text{e}^{-\text{i}\boldsymbol{k}\cdot \boldsymbol{x}}\,\text{d}\boldsymbol{x}+O(k^{2})\simeq -\text{i}k_{1}\unicode[STIX]{x0394}h\int _{-\infty }^{\infty }\unicode[STIX]{x1D719}_{F}(x_{1}=0)\,\text{d}x_{2}+O(k^{2}),\end{eqnarray}$$

where partial integration in the $x_{1}$ -direction has been used. This obtains the leading contribution to the long wave formation by the wave potential integrated laterally along the step. The result (4.1) may alternatively be obtained directly from the integral equation formulation, see (A 3) in A.2. Time derivative of (4.1) gives:

(4.2) $$\begin{eqnarray}\frac{\unicode[STIX]{x2202}{\hat{h}}_{1}}{\unicode[STIX]{x2202}t}\simeq -\text{i}k_{1}\unicode[STIX]{x0394}h\int _{-\infty }^{\infty }\frac{\unicode[STIX]{x2202}\unicode[STIX]{x1D719}_{F}(x_{1}=0)}{\unicode[STIX]{x2202}t}\,\text{d}x_{2}\simeq \frac{\text{i}k_{1}\unicode[STIX]{x0394}h}{\unicode[STIX]{x1D70C}}\int _{-\infty }^{\infty }p(x_{1}=0)\,\text{d}x_{2},\end{eqnarray}$$

where the dynamic boundary condition at the free surface is used. By partial integration in time the first term in (3.17), which we denote by $\hat{\unicode[STIX]{x1D702}}_{0}$ , becomes,

(4.3) $$\begin{eqnarray}\hat{\unicode[STIX]{x1D702}}_{0}=-\int _{t_{0}}^{t}\frac{\sin \unicode[STIX]{x1D714}(s-t)}{\unicode[STIX]{x1D714}}\frac{\unicode[STIX]{x2202}{\hat{h}}_{1}}{\unicode[STIX]{x2202}s}\,\text{d}s\simeq -\frac{\text{i}k_{1}\unicode[STIX]{x0394}h}{\unicode[STIX]{x1D70C}}\int _{t_{0}}^{t}\frac{\sin \unicode[STIX]{x1D714}(s-t)}{\unicode[STIX]{x1D714}}\int _{-\infty }^{\infty }p(x_{1}=0)\,\text{d}x_{2}\,\text{d}s,\end{eqnarray}$$

where (4.2) has been used. Equation (4.3) expresses $\hat{\unicode[STIX]{x1D702}}_{0}$ in terms of a pressure impulse acting at the depth change caused by the moving pressure distribution. By assuming that the pressure, moving with constant speed $U$ along the $x_{1}$ -direction, is modelled by the Dirac delta function in the two horizontal directions, i.e. $p(x_{1}-Ut,x_{2},t)=\unicode[STIX]{x1D70C}gV_{0}\unicode[STIX]{x1D6FF}(x_{1}-Ut)\unicode[STIX]{x1D6FF}(x_{2})$ , we obtain

(4.4) $$\begin{eqnarray}\hat{\unicode[STIX]{x1D702}}_{0}=\frac{\text{i}k_{1}\unicode[STIX]{x0394}h\,V_{0}}{U\unicode[STIX]{x1D714}/g}\sin \unicode[STIX]{x1D714}t,\end{eqnarray}$$

where $V_{0}$ is the volume of the pressure distribution. For a distributed pressure, symmetrical in $x_{1}$ and $x_{2}$ , the result (4.4) is multiplied by a function $f(\unicode[STIX]{x1D705})=\int _{-l_{0}/2}^{l_{0}/2}\,\text{d}s^{\ast }\cos (\unicode[STIX]{x1D714}s^{\ast }/U)\times \int _{-w(x_{1})/2}^{-w(x_{1})/2}\,\text{d}x_{2}\tilde{d}(-s^{\ast },x_{2})$ where $\unicode[STIX]{x1D705}=\unicode[STIX]{x1D714}l_{0}/(2U)$ , $l_{0}$ is the length, $w(x_{1})$ local width and $\tilde{d}$ the shape function of the pressure. Assuming, e.g. $\tilde{d}=d_{0}(1-(2x_{1}/l_{0})^{2}-(2x_{2}/w_{0})^{2})$ we obtain $f(\unicode[STIX]{x1D705})=f_{0}(\unicode[STIX]{x1D705})=8J_{2}(\unicode[STIX]{x1D705})/\unicode[STIX]{x1D705}^{2}$ where $J_{2}$ denotes the Bessel function of the first kind of order two, and is used for illustrative purposes in figure 3.

Figure 3.

Asymptotic upstream wave train due to a delta function moving over a bottom step with $\unicode[STIX]{x0394}h>0$ for $Fr=0.5$ and $t^{\ast }=t\sqrt{g/h}=30,90$ . (a,b) Two-dimensional case obtained by (4.6) (——), leading part of elevation obtained by (4.7) (– – –), (4.6) multiplied by $f_{0}(\unicode[STIX]{x1D705}_{0})=8J_{2}(\unicode[STIX]{x1D705}_{0})/\unicode[STIX]{x1D705}_{0}^{2}$ , $\unicode[STIX]{x1D705}_{0}=\unicode[STIX]{x1D714}_{0}l_{0}/(2U)$ , $l_{0}/h=3.82$ , for $Fr=0.5$ ( $\boldsymbol{\cdot }\boldsymbol{\cdot }\boldsymbol{\cdot }\,\boldsymbol{\cdot }$ ) and $Fr=0.4$ (— ⋅ —). (c,d) Three-dimensional case. Cut along the $x_{1}$ -axis with (4.11) (——) and (4.12) (– – –).

We first investigate the motion along a narrow channel where the far field motion can be considered as two-dimensional. (The two-dimensional analysis involves Fourier transform in the $x_{1}$ -direction only, however, the result (4.4) is unchanged. The volume $V_{0}$ is two-dimensional.) Inverse transform of (4.4) gives

(4.5) $$\begin{eqnarray}\displaystyle \unicode[STIX]{x1D702}_{0}(x_{1},t) & = & \displaystyle -\frac{\unicode[STIX]{x0394}h\,V_{0}}{2\unicode[STIX]{x03C0}\,U/g}\int _{0}^{\infty }\frac{k_{1}}{\unicode[STIX]{x1D714}}\cos (k_{1}x_{1}-\unicode[STIX]{x1D714}t)\,\text{d}k_{1}\nonumber\\ \displaystyle & & \displaystyle +\,\frac{\unicode[STIX]{x0394}h\,V_{0}}{2\unicode[STIX]{x03C0}\,U/g}\int _{0}^{\infty }\frac{k_{1}}{\unicode[STIX]{x1D714}}\cos (k_{1}x_{1}+\unicode[STIX]{x1D714}t)\,\text{d}k_{1}.\end{eqnarray}$$

The upstream waves are obtained by the method of stationary phase for $t\rightarrow \infty$ , giving,

(4.6) $$\begin{eqnarray}\unicode[STIX]{x1D702}_{0}(x_{1},t)=-\frac{k_{1,0}\unicode[STIX]{x0394}h\,V_{0}}{(U\unicode[STIX]{x1D714}_{0}/g)(2\unicode[STIX]{x03C0}|\unicode[STIX]{x1D714}^{\prime \prime }|t)^{1/2}}\cos \left(k_{1,0}x_{1}-\unicode[STIX]{x1D714}_{0}t+\frac{\unicode[STIX]{x03C0}}{4}\right)+O(t^{-1}).\end{eqnarray}$$

Here, $k_{1,0}$ is determined by $\unicode[STIX]{x2202}\unicode[STIX]{x1D714}/\unicode[STIX]{x2202}k_{1}=x_{1}/t<\sqrt{gh}=c_{0}$ , $\unicode[STIX]{x1D714}_{0}=\unicode[STIX]{x1D714}(k_{1,0})$ and a double prime means double derivative. For $x_{1}/t\rightarrow \sqrt{gh}$ both $k_{1,0}\rightarrow 0$ and $\unicode[STIX]{x1D714}_{0}^{\prime \prime }\rightarrow 0$ , and the asymptotic expression (4.6) diverges. For $x_{1}/t>\sim \sqrt{gh}$ the first term in (4.5) is then expressed in terms of the Airy integral where the phase function for small $k_{1}$ is expanded by $k_{1}x_{1}-\unicode[STIX]{x1D714}t\simeq k_{1}(x-c_{0}t)-c_{0}h^{2}k_{1}^{3}t/6+\cdots \,.$ Introducing the variables $Z=[2(x_{1}-c_{0}t)^{3}/c_{0}h^{2}t]^{1/3}$ and $k_{1}(x_{1}-c_{0}t)=Z\unicode[STIX]{x1D6FC}$ , following Mei (Reference Mei1989), we obtain

(4.7) $$\begin{eqnarray}\unicode[STIX]{x1D702}_{0}(x_{1},t)\sim -\frac{\unicode[STIX]{x0394}h\,V_{0}}{(Uc_{0}/g)(4c_{0}h^{2}t)^{1/3}}\,\text{Ai}(Z),\end{eqnarray}$$

where $\text{Ai}(Z)$ denotes the Airy function.

Inverse transform of (4.4) in the three-dimensional case gives:

(4.8) $$\begin{eqnarray}\unicode[STIX]{x1D702}_{0}(x_{1},x_{2},t)=\frac{\unicode[STIX]{x0394}h\,V_{0}}{4\unicode[STIX]{x03C0}^{2}\,U/g}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}x_{1}}\int _{0}^{\infty }\int _{0}^{2\unicode[STIX]{x03C0}}\text{e}^{\text{i}kR\cos (\unicode[STIX]{x1D6FC}-\unicode[STIX]{x1D6FE})}\frac{\sin \unicode[STIX]{x1D714}t}{\unicode[STIX]{x1D714}}k\,\text{d}\unicode[STIX]{x1D6FC}\,\text{d}k,\end{eqnarray}$$

where $(x_{1},x_{2})=R(\cos \unicode[STIX]{x1D6FE},\sin \unicode[STIX]{x1D6FE})$ and $(k_{1},k_{2})=k(\cos \unicode[STIX]{x1D6FC},\sin \unicode[STIX]{x1D6FC})$ are introduced. The inner integral in (4.8) is $J_{0}(kR)$ , the Bessel function of the first kind of order zero, giving

(4.9) $$\begin{eqnarray}\unicode[STIX]{x1D702}_{0}(x_{1},x_{2},t)=\frac{\unicode[STIX]{x0394}h\,V_{0}}{2\unicode[STIX]{x03C0}\,U/g}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}x_{1}}\int _{0}^{\infty }\frac{\sin \unicode[STIX]{x1D714}t}{\unicode[STIX]{x1D714}}J_{0}(kR)k\,\text{d}k.\end{eqnarray}$$

Investigating the contribution for large $kR$ where asymptotically $J_{0}(kR)\sim (2/(\unicode[STIX]{x03C0}kR))^{1/2}\times \cos (kR-\unicode[STIX]{x03C0}/4)$ , we obtain

(4.10) $$\begin{eqnarray}\displaystyle \unicode[STIX]{x1D702}_{0}(R,t) & {\sim} & \displaystyle \frac{\unicode[STIX]{x0394}h\,V_{0}}{4\unicode[STIX]{x03C0}\,U/g}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}x_{1}}\int _{0}^{\infty }\frac{k}{\unicode[STIX]{x1D714}}\left(\frac{2}{\unicode[STIX]{x03C0}kR}\right)^{1/2}\left(-\sin \left(kR-\unicode[STIX]{x1D714}t-\frac{\unicode[STIX]{x03C0}}{4}\right)\right.\nonumber\\ \displaystyle & & \displaystyle \left.+\,\sin \left(kR+\unicode[STIX]{x1D714}t-\frac{\unicode[STIX]{x03C0}}{4}\right)\right)\,\text{d}k.\end{eqnarray}$$

The asymptotic elevation, obtained by the method of stationary phase for large $R$ and $t$ , becomes

(4.11) $$\begin{eqnarray}\unicode[STIX]{x1D702}_{0}(R,t)\sim -\frac{\unicode[STIX]{x0394}h\,V_{0}k_{0}}{2\unicode[STIX]{x03C0}\,U\unicode[STIX]{x1D714}_{0}/g}\,\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}x_{1}}\,\frac{\sin (k_{0}R-\unicode[STIX]{x1D714}_{0}t)}{(k_{0}R|\unicode[STIX]{x1D714}_{0}^{\prime \prime }|t)^{1/2}},\end{eqnarray}$$

where $k_{0}$ is determined by $R/t=\unicode[STIX]{x2202}\unicode[STIX]{x1D714}/\unicode[STIX]{x2202}k<\sqrt{gh}$ , $\unicode[STIX]{x1D714}_{0}=\unicode[STIX]{x1D714}(k_{0})$ and a double prime means double derivative.

Equation (4.11) is not valid ahead of the wave front. Clarisse, Newman & Ursell (Reference Clarisse, Newman and Ursell1995, equation (6.15)) have developed a second-order asymptotic expression for the integral $\int _{0}^{\infty }(1/\unicode[STIX]{x1D714})\sin \unicode[STIX]{x1D714}tJ_{0}(kR)k\,\text{d}k$ in (4.9) valid for $R/t\sim \sqrt{gh}$ , giving for the present wave field

(4.12) $$\begin{eqnarray}\displaystyle \unicode[STIX]{x1D702}_{0}(x_{1},x_{2},t)\sim \frac{\unicode[STIX]{x0394}h\,V_{0}/h^{2}}{2\unicode[STIX]{x03C0}\,U/\sqrt{gh}}\frac{\unicode[STIX]{x2202}}{\unicode[STIX]{x2202}x_{1}}\frac{2^{1/3}\unicode[STIX]{x03C0}}{(R^{\ast }/t^{\ast })^{1/2}t^{\ast 2/3}}[E_{0}\text{Ai}^{2}-t^{\ast -2/3}C_{0}{\text{Ai}^{\prime }}^{2}+2t^{\ast -4/3}A_{1}\text{Ai}\text{Ai}^{\prime }]. & & \displaystyle \nonumber\\ \displaystyle & & \displaystyle\end{eqnarray}$$

The argument of the Airy function $\text{Ai}$ and its derivative $\text{Ai}^{\prime }$ is specified below (Clarisse et al. Reference Clarisse, Newman and Ursell1995, equation (6.15)) and $R^{\ast }=R/h$ and $t^{\ast }=t\sqrt{g/h}$ . The functions $E_{0}$ and $C_{0}$ are obtained from saddle points 1 and 2 of the phase function of their asymptotic integral, see Clarisse et al. (Reference Clarisse, Newman and Ursell1995, equation (5.4)), and are evaluated here.

The asymptotic analysis provides in mathematical terms a qualitative explanation of the upstream wave generation which by (4.3) is expressed in terms of a pressure impulse acting at the bottom step. The leading waves propagate with the shallow water speed $c_{0}=\sqrt{gh}$ where the wave amplitude is proportional to the depth change $\unicode[STIX]{x0394}h$ times the volume $V_{0}$ of the ship. In the three-dimensional case, the wave crests are circular. The amplitude is multiplied by the cosine of the wave angle defined from direction of motion of the pressure and appears because of the derivative $\unicode[STIX]{x2202}/\unicode[STIX]{x2202}x_{1}$ in (4.11). For a positive $\unicode[STIX]{x0394}h$ , where the depth is reduced from $h$ to $h-\unicode[STIX]{x0394}h$ , the leading upstream wave is a depression, while the downstream waves have a positive leading elevation, both in two and three dimensions. A leading upstream wave of depression at a depth change from deep to shallow water is in accordance with the video recording at the observation position 3 at Askholmene (https://www.youtube.com/watch?v=42Ctdk9kpyg). The conclusion is opposite for a negative step with $\unicode[STIX]{x0394}h<0$ . Upstream wave trains obtained by the asymptotics in two and three dimensions are visualized in figure 3 ( $\unicode[STIX]{x0394}h>0$ ).

While (4.4) and the subsequent derivations are based on a localized effect of the pressure impulse in the form of a delta function, a distributed pressure modifies the far field waves. Results multiplying the two-dimensional wave amplitude in (4.6) by the function $f_{0}(\unicode[STIX]{x1D705}_{0})=8J_{2}(\unicode[STIX]{x1D705}_{0})/\unicode[STIX]{x1D705}_{0}^{2}$ , with $\unicode[STIX]{x1D705}_{0}=\unicode[STIX]{x1D714}_{0}l_{0}/(2U)$ , as obtained below (4.4), with $l_{0}/h=3.82$ , a length ratio fitting with the conditions discussed in § 5, are included in figure 3(a,b), for $Fr=0.4$ and 0.5. Apart from the leading trough, the amplitude of the modified far field waves exhibit a weak increase with the Froude number.

5 Fully dispersive calculations

Calculations of fully dispersive wave fields complement the asymptotic analysis in § 4 and are connected to the observations in § 2. The equation system $(\text{e}^{At}{\hat{Y}})_{t}=\text{e}^{At}{\hat{H}}$ defined in the text above and below (3.16a,b ) is integrated forward in time using a fourth-order Runge–Kutta scheme (RK4) (with time step $\unicode[STIX]{x0394}t\sqrt{g/h}=0.025$ ) where the moving pressure distribution $p(x_{1}-x_{1,0}(t),x_{2})$ is specified below. The coordinate $x_{1,0}(t)$ gives the forward motion where $U={\dot{x}}_{1,0}(t)$ . The numerical discretization is obtained over horizontal rectangles of length and width of $L_{1}$ and $L_{2}$ , respectively, with $N_{1}$ by $N_{2}$ computational points in each respective direction and a resolution of $(\unicode[STIX]{x0394}x_{1},\unicode[STIX]{x0394}x_{2})=(L_{1}/N_{1},L_{2}/N_{2})$ . Use of Fourier transform implies that the computational domain is periodic in both directions, where length $L_{1}$ much greater than the width $L_{2}$ in practice models a long channel with straight walls. The finite length $L_{1}$ puts limits on the length of the time simulation where the calculations are stopped before the motion interacts with itself. The wavenumber vectors in spectral space read $(0,\unicode[STIX]{x0394}k_{1},2\unicode[STIX]{x0394}k_{1},\ldots ,(N_{1}/2-1)\unicode[STIX]{x0394}k_{1},-(N_{1}/2)\unicode[STIX]{x0394}k_{1},\ldots ,-\unicode[STIX]{x0394}k_{1})$ and $(0,\unicode[STIX]{x0394}k_{2},2\unicode[STIX]{x0394}k_{2},\ldots ,(N_{2}/2-1)\unicode[STIX]{x0394}k_{2})$ where symmetry in the $k_{2}$ -direction is assumed. The number of operations of the method is governed by the fast Fourier transform (FFT) operations which is $O(N_{0}\log N_{0})$ , where $N_{0}=N_{1}\cdot N_{2}$ , the number of calculation nodes, and is up to $N_{0}=720\times 720\simeq 0.5\times 10^{6}$ using a Lenovo T430 thinkpad.

5.1 Bottom step in a moderately narrow channel

The bottom profile along the $x_{1}$ -direction is assumed to be given by the function

(5.1) $$\begin{eqnarray}\unicode[STIX]{x1D6FD}=-\frac{1}{2}\unicode[STIX]{x0394}h+\frac{1}{2}\unicode[STIX]{x0394}h[\tanh (\unicode[STIX]{x1D6FC}_{0}(x_{1}-x_{1,a}))-\tanh (\unicode[STIX]{x1D6FC}_{0}(x_{1}-x_{1,b}))],\end{eqnarray}$$

where $x_{1,a}$ denotes the centre position of the depth change, $x_{1,b}$ the similar position where the water becomes deep again and $\unicode[STIX]{x1D6FC}_{0}$ is either $0.7$ or $2.0$ , see figure 4. A uniform depth in the lateral direction is assumed. The applied surface pressure, given here by $p(x_{1},x_{2})=p_{0}\exp (-(2x_{1}/l_{0})^{6}-(2x_{2}/w_{0})^{6})$ , where $l_{0}$ and $w_{0}$ denote length and width, respectively, is connected to the ship volume $V_{0}$ by $\int \int p\,\text{d}x_{1}\,\text{d}x_{2}=\unicode[STIX]{x1D70C}gV_{0}$ . The velocity ${\dot{x}}_{1,0}(t)$ of the moving pressure has a gentle ramp up phase generating minimal upstream waves ( ${\dot{x}}_{1,0}(t)=U\sin (t/T_{0}),T_{0}\sqrt{g/h}=150$ , $x_{1,0}(t=0)=6h$ ).

Figure 4.

Bottom geometries, section at $x_{2}=\text{const}.$

With reference to the conditions at the observation position 3 (table 2) a reference depth of $h=55$  m and depth change ratio of $\unicode[STIX]{x0394}h/h=0.909$ are used. The non-dimensional volume and length of the largest among the ships in table 4 in appendix A.1 become $V_{0}^{\prime }=V_{0}/h^{3}\simeq 0.216$ and $l_{0}/h\simeq 3.82$ , respectively. Two different computational rectangles are used, the first, for $\unicode[STIX]{x0394}h>0$ : $(L_{1},L_{2})=(200h,13h)$ , $(N_{1},N_{2})=(1400,200)$ , $x_{1,a}=60$ , $x_{1,b}=180$ and the second, for $\unicode[STIX]{x0394}h<0$ : $(L_{1},L_{2})=(320h,13h)$ , $(N_{1},N_{2})=(1800,200)$ , $x_{1,a}=80$ , $x_{1,b}=300$ . Note that the width of the numerical channel of $L_{2}=13h$ corresponds to twice the distance between the observation position 3 and the ship’s track.

A typical ship speed of ${\dot{x}}_{1,0}=U=10~\text{m}~\text{s}^{-1}$ gives $Fr=U/\sqrt{gh}\simeq 0.43$ where simulations are carried out for $Fr\sim 0.26-0.51$ . The elevation ahead of the ship, comparing the two approximations $s^{(2)}$ and $s^{(3)}$ given in (3.13) and (3.14), is obtained for $\unicode[STIX]{x0394}h>0,Fr=0.43$ and $t^{\ast }=281$ in figure 5(a). Results for the case with $\unicode[STIX]{x0394}h<0$ are shown for $Fr=0.51$ , $t^{\ast }=260$ in figure 5(b). Both sets of results are obtained with a bottom slope of $\unicode[STIX]{x1D6FC}_{0}=0.7$ , where the very small difference between the two approximations illustrates the convergence of the method. Note that the approximations $s^{(2)}$ and $s^{(3)}$ model short wave effects differently, e.g. the bow wave of the pressure distribution, see figure 5(c,d). The entire computational domain is shown in figure 5(e) for $\unicode[STIX]{x0394}h<0$ , $t^{\ast }=293$ . The upstream wave due to a slope factor of $\unicode[STIX]{x1D6FC}_{0}=2.0$ , obtained for $Fr=0.43,t^{\ast }=281$ , is somewhat ahead of the wave produced with a slope factor of $\unicode[STIX]{x1D6FC}_{0}=0.7$ . The wave is also somewhat higher (figure 5 a). Otherwise differences are minor compared to the bottom profile with $\unicode[STIX]{x1D6FC}_{0}=0.7$ .

Figure 5.

Elevation $\unicode[STIX]{x1D702}$ obtained by approximations $s^{(2)}$ and $s^{(3)}$ for $\unicode[STIX]{x0394}h/h=0.909$ and $h=55$  m.  $V_{0}/h^{3}=0.216$ . (a) $\unicode[STIX]{x0394}h>0$ . $\unicode[STIX]{x1D702}$ in the middle of the channel, as a function of $x_{1}-x_{1,0}$ , for $t^{\ast }=281$ with $s^{(2)}$ (– ⋅ – ⋅ –), $s^{(3)}$ (——) and $\unicode[STIX]{x1D6FC}_{0}=0.7$ in both cases, and $s^{(2)}$ with $\unicode[STIX]{x1D6FC}_{0}=2.0$ ( $\cdots \cdots$ ). (b) $\unicode[STIX]{x0394}h<0$ . $\unicode[STIX]{x1D702}$ in the middle of the channel, as a function of $x_{1}-x_{1,0}$ , for $t^{\ast }=260$ with $s^{(2)}$ (– ⋅ – ⋅ –), $s^{(3)}$ (——) and $\unicode[STIX]{x1D6FC}_{0}=0.7$ . (c) $\unicode[STIX]{x0394}h>0$ . $\unicode[STIX]{x1D702}$ for $t^{\ast }=281$ by $s^{(3)}$ . (d) Same as (c) but $s^{(2)}$ . (e) $\unicode[STIX]{x0394}h<0$ . Entire computational domain for $t^{\ast }=293$ .

The generation during the interaction with the depth change at $x_{1,a}=60h$ is visualized for $Fr=0.43$ in figure 6 at times $t^{\ast }=211,223,246$ and in figure 5(c,d) for $t^{\ast }=281$ . The main wave quickly extends laterally across the channel. It is basically characterized as a one-dimensional hump in straight forward motion. The propagation speed is $0.994\sqrt{g(h-(\unicode[STIX]{x0394}h/2))}$ , between $t^{\ast }=246$ and $281$ . Succeeding shorter waves along the channel walls are observed (figure 5 c,d). When the depth changes from shallow to deep water, the similar wave is a depression. The speed is $0.96\sqrt{g(h+(\unicode[STIX]{x0394}h/2))}$ , right ahead of the ship, between $t^{\ast }=281$ and $293$ , where the negative elevation ( $-\unicode[STIX]{x1D702}$ ) is plotted in figure 7 for times $t^{\ast }$ between 251 and 293.

Figure 6.

Elevation $\unicode[STIX]{x1D702}$ obtained by $s^{(3)}$ in grey scale. Ship moving from deep to shallow water. $Fr=0.43$ . (a) $t^{\ast }=211$ , (b) 223, (c) 246. Centre position of depth change at $x_{1}-x_{1,a}=0$ . $\unicode[STIX]{x0394}h/h=0.909$ .

Figure 7.

The negative elevation $-\unicode[STIX]{x1D702}$ obtained by $s^{(2)}$ in grey scale. $Fr=0.43$ . Ship moving from shallow to deep water. Times: (a) $t^{\ast }=251$ , (b) 265, (c) 281, (d) 293. Depth change at $x_{1}-x_{1,a}=0$ . $\unicode[STIX]{x0394}h/h=0.909$ .

The main wave is always succeeded by a train of shorter waves of small amplitude. A leading upstream elevation of small distinct amplitude precedes the main depression wave in the case when the depth changes from shallow to deep water ( $\unicode[STIX]{x0394}h<0$ ) (figure 8 c). This is in accordance with the asymptotics derived in § 4. However, for positive $\unicode[STIX]{x0394}h>0$ , a corresponding leading depression is very small, where the computed surface for time $t^{\ast }=281$ in figure 8(a) is only slightly below the level at the earlier times $t^{\ast }=223,246$ .

Figure 8.

Upstream elevation $\unicode[STIX]{x1D702}L_{2}/(\unicode[STIX]{x0394}hV_{0}/h^{2})$ ahead of ship. $\unicode[STIX]{x0394}h/h=0.909$ . (a) Deep to shallow water; $Fr=0.43$ , $t^{\ast }=223,246,281$ calculated by $s^{(3)}$ . (b) $Fr=0.43$ at $t^{\ast }=281$ and $0.51$ at $t^{\ast }=280$ by $s^{(3)}$ (solid lines); $Fr=0.26$ at $t^{\ast }=340$ , $0.34$ at $t^{\ast }=320$ by $s^{(2)}$ (dashed lines). (c) Ship moving from shallow to deep water. $Fr=0.34,0.385,0.43,0.47,0.51$ . Calculations by $s^{(2)}$ .

The main upstream elevation or depression clearly depend on the forward speed (figure 8 b,c). The maximum elevation, $\unicode[STIX]{x1D702}_{max}$ , for $\unicode[STIX]{x0394}h>0$ , and the minimum elevation, $\unicode[STIX]{x1D702}_{min}$ , for $\unicode[STIX]{x0394}h<0$ , both grow according to $Fr^{n}$ , with $n\simeq 3.2$ for $\unicode[STIX]{x1D702}_{max}$ and $n\simeq 4$ for $\unicode[STIX]{x1D702}_{min}$ (figure 9 a). We define the wave width at half-crest height, at $y=(1/2)\unicode[STIX]{x1D702}_{max}$ , by $\unicode[STIX]{x1D706}_{1/2}^{cr}$ , when the bottom changes from deep to shallow water, and similarly, the wave width at half-trough height, at $y=(1/2)\unicode[STIX]{x1D702}_{min}$ , by $\unicode[STIX]{x1D706}_{1/2}^{tr}$ , when the bottom changes from shallow to deep water. The wave widths in the range of $6.5h{-}12h$ decay with increasing $Fr$ (figure 9 b). The symmetry between the dominant upstream wave for positive and negative $\unicode[STIX]{x0394}h$ is illustrated by forming the products $\unicode[STIX]{x1D702}_{max}\unicode[STIX]{x1D706}_{1/2}^{cr}$ and $-\unicode[STIX]{x1D702}_{min}\unicode[STIX]{x1D706}_{1/2}^{tr}$ . These become approximately equal and exhibit a common algebraic growth in the Froude number, in these calculations (figure 9 c).

The calculations are compared to the observations at position 3. A period of approximately 32 s of the leading two main waves can be estimated from the video recording of the flow in the bay at Askholmene. Multiplying by the shallow water speed, this gives a wavelength of approximately 550 m corresponding to ${\sim}10h$ . The calculated wavelength for $Fr\sim 0.43$ at $t^{\ast }=281$ in figure 8(a) corresponds fairly well to the observation. Calculated maximal non-dimensional wave elevations of ${\sim}0.0013h$ ( $Fr=0.43$ ) and ${\sim}0.0021h$ ( $Fr=0.49$ ) in figure 8(a) correspond to amplitudes of 7 cm and 12 cm in the fjord, respectively, of the main wave ahead of the larger among the ships, of displaced volume of $36\,000~\text{m}^{3}$ . The small elevations generate run-ups and run-downs of the order of 1 m along the shores, as evidenced from the movie. The calculations show one dominant elevation and succeeding minor shorter waves along the channel wall, comparing rather well to the short wave train which is typical for the observations at position 3.

The elevation ratio $\unicode[STIX]{x1D702}^{\prime }=\unicode[STIX]{x1D702}L_{2}/(\unicode[STIX]{x0394}hV_{0}/h^{2})$ and wave width $\unicode[STIX]{x1D706}^{1/2}$ are compared to the asymptotics in § 4. The dispersive calculation in figure 8(b) for $Fr=0.51,t^{\ast }=280$ corresponds to a time $t^{\ast }-t_{a}^{\ast }\simeq 88.5$ upstream of the bottom transition at $x_{1,a}$ . Similarly, the calculation in figure 8(c) for $Fr=0.51,t^{\ast }=260$ corresponds to a time $t^{\ast }-t_{a}^{\ast }\simeq 29.3$ upstream of $x_{1,a}$ . Table 3 compares the wave height $\unicode[STIX]{x1D702}_{max}^{\prime }-\unicode[STIX]{x1D702}_{min}^{\prime }$ , measured as the height from the first elevation to the second trough, for $\unicode[STIX]{x0394}h>0$ , and the height from the first trough to the second crest, for $\unicode[STIX]{x0394}h<0$ . The table also compares $\unicode[STIX]{x1D706}^{1/2}$ of the leading main wave in figure 8(b,c) to those obtained by the asymptotics in the two-dimensional case, in figure 3(a,b), where the latter is measured at $y=0$ . Apart from some differences in the wavelength for $\unicode[STIX]{x0394}h<0$ , the comparison is rather good. Both the crest, trough and wave height increase with $Fr$ , where the growth is weak for $\unicode[STIX]{x0394}h/h\ll 1$ (the asymptotics), and very strong for $\unicode[STIX]{x0394}h/h\simeq 1$ . The leading trough (crest) for $\unicode[STIX]{x0394}h>0$ ( $\unicode[STIX]{x0394}h<0$ ), pronounced for $\unicode[STIX]{x0394}h/h\ll 1$ but either weak or almost absent for $\unicode[STIX]{x0394}h/h\simeq 1$ , is another difference.

Table 3.

Wave height $\unicode[STIX]{x1D702}_{max}^{\prime }-\unicode[STIX]{x1D702}_{min}^{\prime }$ , where $\unicode[STIX]{x1D702}^{\prime }=\unicode[STIX]{x1D702}L_{2}/(\unicode[STIX]{x0394}hV_{0}/h^{2})$ , and wavelength at half-height $\unicode[STIX]{x1D706}^{1/2}$ , of the first main wave, obtained by asymptotic model (in two dimensions) $(\unicode[STIX]{x0394}h/h\ll 1,Fr=0.5)$ and dispersive calculation ( $\unicode[STIX]{x0394}h/h=0.909,Fr=0.51$ ). Time $t^{\ast }-t_{a}^{\ast }$ after passage of the depth change at $x_{1,a}$ .

Figure 9.

(a) Maximum upstream elevation $\unicode[STIX]{x1D702}_{max}$ (●) for ship moving from deep to shallow water, and maximum upstream depression $\unicode[STIX]{x1D702}_{min}$ (▫) for ship moving from shallow to deep water versus $Fr$ . $\unicode[STIX]{x0394}h/h=0.909$ . Fitted curves: dotted and dashed lines. (b) $\unicode[STIX]{x1D706}_{1/2}^{cr,tr}/h$ versus $Fr$ . (c) $\unicode[STIX]{x1D702}_{max}L_{2}\unicode[STIX]{x1D706}_{1/2}^{cr}/V_{0}$ (●) and $-\unicode[STIX]{x1D702}_{min}L_{2}\unicode[STIX]{x1D706}_{1/2}^{tr}/V_{0}$ (▫) versus $Fr$ . Fitted curve (dotted line).

6 Upstream waves in a wide channel

A channel of width of 2.4 km corresponds to twice the distance from the observation position 1 to the ship’s track. The depth changes from moderately deep to shallow to moderately deep water, with $h=(h_{A}+h_{B})/2=30$  m and $\unicode[STIX]{x0394}h/h=1.067$ , where the depth just north of the shallow region is $h+\unicode[STIX]{x0394}h/2$ , while still further north average depths of the fjord are given in table 1. The shallow region extends approximately $23h$ along the track. The positions $x_{1,a}=60h$ and $x_{1,b}=83h$ define its extension in the numerical calculations. The computational domain is $(L_{1},L_{2})=(300h,80h)$ with $(N_{1},N_{2})=(1200,432)$ computational points. A ship speed of $10~\text{m}~\text{s}^{-1}$ corresponds to $Fr=0.58$ where the range $Fr\sim 0.3{-}0.7$ is explored. Note that $U/\sqrt{g(h-\unicode[STIX]{x0394}h/2)}$ becomes critical in the shallow region for the largest speed ( $Fr=0.7$ ). However, nonlinear effects, known to propagate laterally in the wide channel (Pedersen Reference Pedersen1988), are assumed to be small over the relatively short extension of the shallow region (of length $23h$ ).

The ship volume $V_{0}=36\,000~\text{m}^{3}$ is made dimensionless by the reference depth $h=30$ m, giving $V_{0}/h^{3}=1.33$ . This is used in the calculations shown in figures 10 and 11. Note that this non-dimensional ship volume is approximately 6 times larger than the non-dimensional ship volume at Askholmene, where the reference depth is $h=55$  m. The upstream waves calculated using the approximation $s^{(2)}$ in (3.13) exhibit a curved pattern where the dominant feature is a leading, large wave trough, preceded and succeeded by less dominant crests. A train of shorter waves of smaller amplitude follows behind the leading wave motion. The leading waves ahead of the ship in the wide channel are similar to the upstream waves generated at the depth change from shallow to deep water. More precisely, the calculated elevation in the narrow channel is in the range $-0.13<\unicode[STIX]{x1D702}L_{2}/(\unicode[STIX]{x0394}hV_{0}/h^{2})<0.04$ , see figure 8(c). The latter corresponds to a wave height ahead of the ship of $H/h\simeq 0.003$ in the wide channel. A simple doubling gives $2H/h\simeq 0.6\times 10^{-2}$ , fitting fairly well with the results in figure 11(b), for the ship passing over the two depth changes in the wide channel, for the similar $Fr=0.51$ .

Figure 10.

Upstream elevation pattern obtained by $s^{(2)}$ for times (a) $t^{\ast }=261.7$ , (b)  $275.5$ , (c) $311.4$ . Ship with $Fr=0.58$ passing from deep to shallow to deep water with $x_{1,a}=60h$ , $x_{1,b}=83h$ and $\unicode[STIX]{x0394}h/h=1.067$ . Horizontal coordinate $(x_{1}-x_{1,b})/h$ with end of topography located in $x_{1}-x_{1,b}=0$ . $L_{1}=300h,L_{2}=80h,N_{1}=1200,N_{2}=432$ . Depth $h=30$  m.

Figure 11.

(a) Upstream wave elevation along channel wall obtained by $s^{(2)}$ as function of $(x_{1}-x_{1,b})/h$ for $Fr=0.7$ , $t^{\ast }=252.8$ (– ⋅ – ⋅ –), $Fr=0.58$ , $t^{\ast }=287.4$ (——) and $Fr=0.46$ , $t^{\ast }=311$ (– – –). (b) Maximum wave height at channel wall at $x_{2}=0$ , $x_{1}-x_{1,b}=50h$ (●) and ahead of ship at $x_{2}=40h$ , $x_{1}-x_{1,b}\sim 60h$ (♢). Fitted curves (dotted and dashed lines). $\unicode[STIX]{x0394}h/h=1.067$ , $h=30$  m. Same discretization as in figure 10.

Wave reflections at the side walls, where the propagation speed is enhanced (results not shown), produce a complex wave pattern in the channel. The leading curved crests eventually straighten up. Note that the wave interaction with the channel walls is not related to the Mach reflection, since the waves in consideration are linear. Snapshots of the wave elevation at the side wall illustrate further the dominant trough and preceding and succeeding crests (figure 11 a). The dominant wave trough corresponds to the strong outflow observed in the harbour at observation position 1, see figure 1. The observation position 1 is located $50h$ upstream of the generation site ( $x_{1,b}$ ) where the maximum wave height $H$ grows according to $Fr^{n}$ where $n\simeq 3.7$ at the channel wall and $n\simeq 3.5$ ahead of the ship. The height is approximately 45 %–55 % higher at the walls than ahead of the ship, at the similar $x_{1}$ -position (figure 11 b). We obtain $H/h\simeq 0.025$ at the channel wall, for $Fr=0.7$ , giving $H\simeq 0.75$  m (with $h=30$  m) and is in the range of the observations in the harbour.

The calculated leading trough-to-trough period at $x_{1}-x_{1,b}=50h$ becomes $T=21.8\sqrt{h/g}\simeq 38$  s for $Fr=0.58$ , and $T=24.2\sqrt{h/g}\simeq 41$  s for the smaller $Fr=0.52$ . A calculated third crest to crest period becomes 15 s for $Fr=0.58$ . The periods of the idealized calculations are rather close to the observed periods at position 1, see table 2. Similar calculations at the channel wall at an upstream position of $x_{1}-x_{1,b}=83h$ for the still smaller $Fr=0.46$ give a wave period of $T=26.2\sqrt{h/g}\simeq 45$  s. This upstream position corresponds to observation position 2 in table 2, where the numerical period is somewhat smaller compared to the observed period of 57 s, for the similar ship speed, where, however, the fjord is much shallower ( ${\sim}25$  m) than in the calculation. In summary, despite the rather ideal conditions of the numerical channel, with a constant water depth of 46 m outside the shallow region, the calculated wave height and periods agree fairly well to the observations at positions 1 and 2 in the Oslofjord. Note that effects due to local bathymetry, bottom slope or shape of the shoreline, not accounted for in the computations, contribute to the actual run-ups, run-downs as well as the wave period.

The results in figure 11(b) are used to calculate the wave height produced by the two different ships at position 1, cruising with speeds of $8.6~\text{m}~\text{s}^{-1}$ and $10.3~\text{m}~\text{s}^{-1}$ , respectively, see table 1. The corresponding wave heights become $H/h\simeq 0.70\times 10^{-2}$ and $1.36\times 10^{-2}$ . Multiplying the former by the volume of Pearl Seaways, which is 69 % of Magic/Fantasy (table 4), we obtain that the faster and larger ship makes a wave height of approximately three times higher compared to the slower and smaller ship. The wave height ratio becomes four times, for a sailing speed of $11.5~\text{m}~\text{s}^{-1}$ of Fantasy/Magic which has been observed by the people living in Flaskebekk.