Proposed Sun Valley FEMA 100-year Flood Map
The old FEMA 100-year flood map was outdated, many of the gravel pits that absorbed the floods have been filled for a variety of purposes.
A new FEMA 100-year flood map was commissioned.
There seems to be two technical omissions to this map that would impact downstream flow estimations.
Strathern Pit
Strathern pit was in the flood zone of the current map and it is being removed. It flooded several years ago and it is likely to flood again. This will be discussed with photos and videos. The omission of Strathern pit makes the estimated flood levels downstream to be too high.
Strathern Pit lies north of Strathern Ave and east of Tujunga Ave. The main flood channel during storms is down Tujanga Ave. In 2005, the storm waters overran the berm along the pit and undercut the road, swallowing the stream. After several years the sinkhole in the road was repaired, and a concrete berm was added between the road and Strathern Pit. Following is a photo description of the current conditions along Tujunga Ave.
The sinkhole was at the beginning of the green fencing.
The area behind the driveway has a concrete berm of equal height the the berm under the green fencing.
The high portion of this berm includes the sinkhole and a little beyond it. Where the sinkhole entered the pit had been reinforced with concrete coating to reduce the chance of undercutting the road in the future. However, beyond the former sinkhole where the concrete berm is a lower height, undercutting into the pit is likely.
At the end of the newly reinforced section the concrete berm drops in height and scouring is visible along the road. The scouring may undercut the berm, and any overflow, such as a wave from a stuck car or truck over the berm would quickly undercut into the road, capturing the stream on the road.
Any water jumping the berm would quickly undercut the berm and road. Strathern pit stills has significant volume capacity.
As the stream approaches Strathern Ave, there is a driveway gap in the berm which may allow water to find an entrance to the pit. And around the corner on Strathern, there is only a dirt berm of varying heights which also allow the potential of cutting into the pit.
Once the pit is breached, the water on the streets below this point will be significantly reduced.
Video Tujunga Ave Sinkhole area.
Video Strathern side Strathern Pit
Sun Valley Stormwater system
The proposed map does not show current flow of storm water through Strathern Park and out to Vineland at Lorne Ave. Therefore it's impact on the reduction of flooding on Fair Ave was omitted.