How to compute an old style shore line drawing? I wish to represent a shore line, old-style, as in the following image (city on the left, water on the right):



UPDATE: new image for another example of what I'm after, namely the thin lines (given the thick lines). [Credit: https://www.atlasobscura.com/articles/early-map-of-chicago ]



My input is an ordered list of XY points that represent the shore line. I have no clue whatsoever how to compute the additional lines. I have trouble finding resource on the subject through search engines since I'm quite unsure of the correct English terms to describe such an operation.

I've thought of applying a simple dilatation, but this is not the way to go here (in particular, it does not work for concave parts of the shore line).

Any ideas?
(and any ideas how to tag this question?)
Define a level function curve (L) like this:

$$ L(x,y) = C_0 \cdot x + C_1 \cdot y + C_2 \cdot x^2 + C_3 \cdot xy + C_4 \cdot y^2 .... $$

It has no constanst term.  Now, find the best fit to the level curve $L(x,y)=1$.  This will yield the values for the coefficients.

Normalize the coefficients so the gradient is of a magnitude you like.

Find the level curves of $L(x,y)=t$ where $t$ is near one.

Finding the best fit is done with Linear Algebra.  Finding the level curves is an interative search algorithm.