I'm struggling to find a community focused around stromwater. Is there one out there?
There is a Water/Wastewater subcategory within this space. See here:
Another good space here on GeoNet is https://community.esri.com/groups/a4w . The ArcGIS for Water team at Esri hosts a number of meetups/webinars that are publicized there and are a great way to help you connect with others in stormwater.