How does the author in the citation spell their name? If it's with a capital "D" then that's the way it should appear in the bibliography. However, you need to use a little trick to get bibtex to treat the "De" part as a prefix:
\documentclass{article}
\usepackage{natbib}
\bibliographystyle{plainnat}
\begin{filecontents*}{test.bib}
@inproceedings{17Degroot,
title={Critical scour: new bed protection design method},
author={{\uppercase{d}e} Groot, M.B.},
booktitle={Journal of Hydraulic Engineering},
volume={114},
pages={1227--1240},
year={1988},
publisher={ASCE, New York, USA}
}
\end{filecontents*}
\begin{document}
Text cite: \citet{17Degroot}
Parenthetical cite: \citep{17Degroot}.
\bibliography{test}
\end{document}
If the author spells their name with a lower case "d" then that's the way it should appear in the text, unless the citation starts a sentence.
\documentclass{article}
\usepackage{natbib}
\bibliographystyle{plainnat}
\begin{filecontents*}{test.bib}
@inproceedings{17Degroot,
title={Critical scour: new bed protection design method},
author={de Groot, M.B.},
booktitle={Journal of Hydraulic Engineering},
volume={114},
pages={1227--1240},
year={1988},
publisher={ASCE, New York, USA}
}
\end{filecontents*}
\begin{document}
Text cite: \citet{17Degroot}
Parenthetical cite: \citep{17Degroot}.
\Citet{17Degroot} blah blah.
\bibliography{test}
\end{document}
Edit: Here's how to sort on "G" rather than "d":
\documentclass{article}
\usepackage{natbib}
\bibliographystyle{plainnat}
\newcommand*{\swap}[2]{#2#1}
\begin{filecontents*}{test.bib}
@inproceedings{17Degroot,
title={Critical scour: new bed protection design method},
author={{\swap{Groot}{de }}, M.B.},
booktitle={Journal of Hydraulic Engineering},
volume={114},
pages={1227--1240},
year={1988},
publisher={ASCE, New York, USA}
}
@inproceedings{Gadzooks,
title={Sample},
author={A. Gadzooks},
booktitle={Blah},
year=2013
}
@inproceedings{Grunt,
title={Sample},
author={A. Grunt},
booktitle={Blah},
year=2013
}
@inproceedings{Datone,
title={Sample},
author={A. Dat-one},
booktitle={Blah some more},
year=2013
}
@inproceedings{Disone,
title={Sample},
author={A. Dis-one},
booktitle={Blah some more},
year=2013
}
\end{filecontents*}
\begin{document}
\Citet{17Degroot} blah blah.
\Citep{17Degroot}.
\cite{*}
\bibliography{test}
\end{document}
Result:
