When a river is blocked it bursts its banks — the classical sense of this character.
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壅 is not used alone in modern Chinese; it is bound in literary 壅塞 (to clog up, also figurative for stifled communication) and the farming term 壅土 (to bank soil around roots). It is the earth radical 土 under the phonetic 雍.
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Literary and bound only; everyday Chinese uses 堵塞 for blocking.
