This plant is a bit like the plant I’ve come to know as Nut Grass (see sighting 4651566). It is growing amidst some runner grasses, but its leaves are thinner, slightly yellower, and have a v-like profile. The runner grasses have flatter, slightly darker green leaves.
The flower head has 3 clusters, not the single cluster of the plant at sighting 4651501 or the 5-cluster of the plant at sighting 4651566.