Poomacha Fire

Geology/Hydrology

 

Summary of Potential Areas and Assets at Risk

1. Palomar Mountain Uplands. Steep debris-flow areas on Cleveland National Forest lands (5,226 acres) on the southwest side of Palomar uplands were underlain with metasedimentary rocks south and west of Boucher Hill Lookout and by granitic rocks southeast of the lookout. The rocks in this area have weathered to well drained cobbly fine sandy loams, coarse sandy loams, and loamy coarse sands. Low-cohesion soils in this area tend to fail in shallow-seated debris slides where sufficient water is present on steep slopes. (Fire intensity ranged from unburned to High, being largely Moderate south and northwest of Boucher Hill Lookout and mostly Moderate or High southeast of the lookout.) It was concluded by the BAER team that debris-slide / flow initiation rates may increase in the short term given sufficient winter storms, subjecting downslope and downstream areas to short-term increased risk. Possible significant debris-flow and flooding hazards were focused on the La Jolla Campground at the confluence of Cedar Creek and San Luis Rey River in the La Jolla Indian Reservation.


2. Lake Sutherland Aqueduct. The aqueduct, located on northand northwest-facing slopes along Santa Ysabel Creek, generally follows Black Canyon Road west and southwest from Lake Sutherland towards Ramona. Slopes above the aqueduct were underlain by granitic rocks that weather to thin coarse sandy loams and to rocky sandy loams and loams with clay-loam subsoils. Runoff typically is rapid to very rapid and erosion hazard is high to very high. The Cleveland National Forest Geomorphology inventory showed the degree of instability on the slope crossed by the aqueduct to be Moderate. Slopes range from 30 to 60 percent. Burn severity was Low on the upper slopes where debris flows tend to originate and generally Moderate in the lower half of the slopes. Field observations found no evidence of recent debris flows. The lack of evidence of recent debris flows and the Low burn severity of the upper part of the slopes suggested any increase in debris-flow activity on these slopes as a result of the 2007 fires would be minor.


3. Residences on the Mesa Grande Indian Reservation. Located downslope of Cleveland National Forest land about 1.5 miles north of Lake Sutherland, a subdivision of 21 new residences was recently constructed on engineered pads on both sides of Scholder Creek. Of these houses, 8 were on the west side of the creek at the foot of what appeared to be a large deep seated landslide complex. This complex appeared to be very old, showing no significant movement for thousands of years. However, houses in the 8-house complex west of the creek may be at risk from debris flows delivered to their street by the two parallel east-flowing drainages.

4. Pauma Residential areas. High steep mountain side above one house creating moderate risk of debris flow and rock fall. At a different location a trailer and storage area at high risk of debris flow and rock fall. At another location several homes situated on a debris fan at moderate risk. Culvert under Adams Drive and another location, and a drainage ditch (see photos below) full of sediment  and debris.

5. Cuca Ranch Subdivision. Moderate risk to roads, slops and structures due to mud slide and flooding. Moderate potential of landslide to one house located in this area (preexisitng).

6. Lilac Lane off Highway 76. High to moderate potential damage to road due to debris flow, mud slide, rock fall, flooding, erosion, and washout. One house in flood plain (preexisitng).


7. Dirt road at 42.5 mile post (N32.3900, W116.8934). Moderate risk to road due to debris flow, rock and tree fall, and flooding. Moderate risk to life and property due to rock fall (pre-existing condition).

8. Church Road off Highway 76. Roads, slopes and structures at risk. High to moderate potential damage to road due to debris flow, mud slide, rock
fall, flooding, erosion and washout. Moderate potential damage to slopes due to erosion. Moderate to low potential harm to structures due to mud slide and flooding

9. South Grade Road. Moderate risk to culvert and road  due to debris flow, rock and tree fall, and flooding. Modeate to low risk to houses in area due to mud sliding and flooding.

10. Various buildings, including occupied homes. Moderate to low risk to occupants and properties due to debris flow, and flooding.

11. Culvert and Roads throughout burn area. Moderate to low risk to transportation routes due to debris flow and flooding.

12. Utilities throughout burn area. Moderate to low risk to utility infrastructure, including power and telephone lines due to debris flow and flooding.

 

   

Interagency State Burned Area Response (BAER) Report (2007, November 17)

Top Left: Palomar Mountain potential debris/flooding site

Bottom Left: Steep slop area in Pauma Valley

Top Right/Bottom Right: Debris accumulation in drainage channels in Pauma Valley after the fire

Source/Photo Credits:

Interagency State Burned Area Response (BAER) Report. (2007, November 17). The Poomacha Fire