When a property goes over 24% impervious surface, a post construction stormwater device is required to be built to control runoff, flooding, and to improve water quality. Landscape and aesthetic value is important to a stormwater device, but it is secondary to the other objectives listed previously. Impervious surface is manly artificial structures-such as pavements (roads, sidewalks, driveways, and parking lots) that are covered by water-resistant materials such as asphalt, concrete, brick, stone, and rooftops. Gravel is considered impervious if used for vehicle traffic.
Below we will go over some of the stormwater devices allowed in Trinity.
- Very Common Design
- Low maintenance needs
- Can be an attractive addition to landscape
- 'Weak Link' is the outlet - Drawdown orifice easily clogs
- Allows for sediment to deposit to bottom before water enters creek/stream.
- Long-term maintenance is cleaning of forebays
- WHY? May have a long, depressed, wetland-area already on property. Use what you have.
- FOREBAYS & DEEP POOLS
- Removes most nutrients
- Encourages biodiversity
- Aquatic life refuge during drought and winter
- SHALLOW WATER AREAS
- Facilitates sheet flow of water
- Allows dense vegetation to grow
- Facilitates filtration, nutrient removal, biological processes
- WHY?
- Site is dry - no shallow water table, no running water
- Watershed is stable - low probability of dediment deposition
- 'Look' of a landscape bed is desired (shrubs, trees, or grass)
- Small piece of property that needs a small solution. Example, residential property on less than 1 acre or commercial property.
- Does not hold water for more than 96 hours at a time. Also known as a 'Dry Pond'.
- Rain Gardens are very popular for residential properties that would like to build over 24% of the property.
- 30-ft riparian buffer required on all perennial streams.
- No permanent structure can be built withing the buffer without Army Corp of Engineer approval.
- During construction, you can grade within 10-ft of the stream, but the graded area must be stabilized within 10 days.
- Level Spreader: A level spreader is an erosion control device desigend to reduce water pollution by mitigating the impact of high-velocity stormwater.
- An example of such a device is a pool plunge. See the diagram to the left.
- Good idea to have with subdivisions that are being built above other occupied properties, but the impervious % is less than 24%.