Moved from Winged Insects.

The image is of a male cicada killer. While they are territorial and aggressive, they are anatomically incapable of stinging. They often sit and watch over their territory like this. No harm. Females go out hunting cicadas. Each female has her own nest burrow (the diggings in the sand that you describe). She paralyzes a victim with her sting, then drags it back and stores it in the burrow. A couple more victims and she lays an egg in the cell. Her larval offspring eat the cicadas and emerge the following summer as adults. This whole nesting business will be over in about a month, probably less, and once the burrows are finished you can groom the sand again with no harm. Cicada killers are victims of urban sprawl that is making it hard for them to find suitable nesting sites. Consider yourselves lucky to have them. Invite the neighbors over to watch them. Seeing a female towing a cicada is quite something:-)

Sphecius speciosus - Cicada Killer, info in the guide here