Hydro-Geomorphic Failure Profile of Reach 2 Alignment
1. Topographic Gradient Mapping
High-density barometric and spatial telemetry split the project wash path into three distinct geometric sections based on elevation changes over a 2,847-foot continuous watershed run:
- Reach 1 (The Mountain Funnel): Station 0+00 to 7+52. Features a sharp vertical drop calculating out to a steep 15.27% slope gradient.
- Reach 2 (The Flat Shelf Bottleneck): Station 7+52 to 18+15. Sudden terrain leveling directly behind private property lines, dropping flatly to a 3.70% slope gradient.
- Reach 3 (The Outflow Chute): Station 18+15 to 28+47. Continues away from the neighborhood at a 3.50% slope.
2. The Hydraulic Velocity Crash
Stormwater behavior is governed by the laws of fluid mechanics. Utilizing standard open-channel flow parameters, water descending the steep 15.27% slope of Reach 1 achieves a roaring velocity of 19.86 feet per second.
The exact instant that volume hits the flat 3.70% shelf of Reach 2 directly behind our homes, gravity-driven acceleration stops. The flow velocity experiences an immediate 50.7% drop, crashing down to 9.78 feet per second.
3. The Bedrock “Silt Trap” Mechanism
Because the channel construction encountered a hard Precambrian bedrock shelf next to residential foundations, a wide, shallow channel design was implemented instead of a deep cut.
- Sediment Drop: Rushing mountain water carries heavy gravel and rocks. When velocity drops by half in Reach 2, the water instantly loses its carrying power. Heavy debris drops straight to the bottom of the concrete channel.
- Surface Friction: Spreading water wide and shallow across a flat floor dramatically maximizes surface boundary friction, slowing the water even further.
- Capacity Block: Within minutes of a heavy monsoon storm, accumulated mountain gravel forms a rapid silt dam that chokes the shallow concrete pan, triggering a calculated 93 CFS capacity deficit that forces muddy water over the banks, introducing a foreseeable overtopping hazard to previously secure residential properties.
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