Delineation of Flood-Inundation Areas in Grapevine Canyon near Scotty’s Castle, Death Valley National Park, California

Science Center Objects

On October 18, 2015 an intense, nearly stationary, thunderstorm triggered flooding in Grapevine Canyon. Grapevine Canyon Road, power and water infrastructure, and several buildings at Scotty’s Castle were damaged by the flood water, necessitating closure of the area to the public. In response to the flood event, the National Park Service requested the USGS Nevada Water Science Center perform a channel survey and hydraulic computation of peak flow for the event and update the information on current channel conditions to delineate the flood-inundation area of Grapevine Canyon and Tie Canyon near Scotty’s Castle.

Scotty’s Castle, in the northeast part of Death Valley National Park, lies within the flood-inundation area of Grapevine Canyon near the confluence with Tie Canyon. Grapevine Canyon drains the steep, western slope of the Grapevine Mountains near the Nevada and California state line with a drainage area of approximately 47.9 mi2. Tie Canyon makes up the northwestern part of the Grapevine Canyon basin and has a drainage area of approximately 14.5 mi2. Scotty’s Castle is a popular tourist attraction on the National Register of Historic Places that includes several historically and architecturally significant buildings, in addition to park administration buildings and public water supply and utility infrastructure. Scotty’s Castle is accessible via Grapevine Canyon Road that passes along the entire length of Grapevine Canyon and, in some locations, takes up the entire width of the canyon floor. Several springs are located in Grapevine Canyon that provide surface flow over short distances, but flow in most of the canyon is driven by episodic precipitation. 

Thunderstorm over Scotty's Castle taken from Owens Valley, CA
Thunderstorm over Scotty's Castle from Owens Valley during October 18, 2015, storm. (Credit: A. and C. Grams. A. and C. Grams)

The thunderstorm on October 18, 2015, triggered flooding in Grapevine Canyon.

A rain gage at Scotty’s Castle recorded 2.72 in of precipitation over a five-hour period.

Weather radar suggested 3-4 in of precipitation elsewhere in the lower part of the drainage basin during the same event, which the National Weather Service estimated to be a “1,000-year rainfall event”

The most recent flood-inundation map for the area was published in 1989 and delineated the 4, 2, and 1-percent annual exceedance probability (AEP) flood areas (25, 50, and 100-year return intervals, respectively) for lower Grapevine Canyon and Tie Canyon based on discharge computed using regional flood regression equations from Waananen and Crippen. The water level for the October 18, 2015, flood rose to the 1-percent AEP flood elevation at Scotty’s Castle, with a computed flow of 12,000 cubic feet per second (cfs), as defined by previous work by Bowers. After channel surveys and hydraulic computation, the NVWSC determined a peak flow for the event of 3,200 cfs. The computed discharge corresponded to the 4 to 2-percent AEP based on updated regional flood frequency regressions in Gotvald and others. These most recent regression equations compute significantly less flow (35-40%) than the previous equations that were used by Bowers for the 4, 2, and 1-percent AEP.

Debris at Scotty's Castle from the October 2015 flood
Debris at Scotty's Castle from the October 2015 flood. (Public domain.)

The October 18, 2015, indirect measurement and the 1989 mapping determined the channel to be in a super-critical flow regime. The combination of super-critical flow, shallow depths, a very steep channel, and erodible channel bed material, results in an unstable channel with significant scour and fill during floods. Channel instability has been demonstrated by scour at the entrance of the Castle Gate bridge during the 2015 flood, significant fill deposited by an unnamed tributary wash between channel cross-sections X2 and X5 during the 2015 flood, and overflow scour at the parking lot observed by NPS personnel in July 1976.

Road damage from October 2015 flood at Scotty's Castle, Death Valley, CA
Road damage from the October 2015 flood (Public domain.)

Significant channel changes have reduced the applicability of the 1989 map to current channel conditions. In order to update the information on current channel conditions, the NVWSC is updating the flood–inundation area map for the 4, 2, 1, 0.5, 0.2-percent AEP floods in Grapevine and Tie Canyons in response to current (2016) channel conditions and the updated regional flood frequency regression equations by

  • Collecting high resolution elevation data using a terrestrial laser scanner (TLS) and deriving a high resolution digital terrain model (DTM) of the area. 
  • Using the U.S. Army Corp of Engineers (USACE) HEC-GeoRAS (Hydrologic Engineering Centers River Analysis System) application to derive cross sections of the flood areas from the TLS DTMs.
  • Develop a one dimensional hydraulic model using the cross sections and USACE HEC-RAS.
  • Performing simulations of the 4, 2, 1, 0.5, 0.2-percent AEP flood for the reach.
  • Determining the roughness coefficients for the simulations by comparison with channels with quantified roughness, field observations, and other methods given in Arcement and Schneider.
  • Using the simulated water surfaces computed by HEC-RAS along with the DTM to construct datasets of the predicted flood-inundation areas for the 4, 2, 1, 0.5, 0.2-percent AEP floods and then comparing with those predicted in Bowers to identify locations of aggradation or degradation.
  • Comparing the simulated water surfaces to those surveyed from the October 2015 flood, allowing the indirect measurement of that event to be used as an independent verification of the Grapevine Canyon model. Because there are no quantitative data associated with specific floods available at Tie Canyon, its model cannot be independently verified.

Crest Stage Gages

NVWSC installed crest-stage gages in Grapevine and Tie Canyons to collect peak flow data. Scour chains have been installed near both CSGs to determine if significant scouring and filling is occurring during peaks.