Numerical Modeling

“The floods caused over $20,000,000 in property damage and took two lives.”

“The Office of the Governor issued a Declaration of Emergency for the area and requested assistance from the Federal Emergency Management Agency on 15 July. President Clinton declared the city a disaster area on 19 July.” –taken from Haro, Daley, and Runk, November 1999

Following is a presentation given at the southern Nevada Chapter of the American Public Works Association (APWA) Spring Conference, in Mesquite, Nevada, May 7, 1999, culminating with the July 8, 1999 storm event in Las Vegas. The presentation summarized two years of work from 1997 to 1999 that doubled the peak flowrate in the Las Vegas Wash from approximately 6,000 cfs to 12,000 cfs. As a result, a peaking detention basin was built in North Las Vegas along the Las Vegas Wash.

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See NBC’s  Today Show broadcast for July 9, 1999:

 

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Although controversial, this work adequately assessed the risk of major flood occurrence, borne out by the July 1999 storm event. The 1999 storm event did not center directly over the modeled area, however, the intensity and proximity of the 1999 storm compared to the revised, modeled storm center clearly validated the need to double the flow rate and construct a future peaking detention facility.

As typical, the technical details were known to myself and perhaps a few others. Because the project was technical by nature– yet controversial because it doubled the flowrate– significant outreach was performed to municipal stakeholders such as the Clark County Regional Flood Control District (CCRFCD), the City of North Las Vegas (CNLV), the City of Las Vegas (CLV), the City of Henderson (COH), and Clark County. The outreach was necessary because the increased flowrate impacted the Las Vegas Wash from North Las Vegas to Henderson, until discharge into Lake Mead. Not only were bridge crossings under capacity, other significant issues arose along the Las Vegas Wash south of the Flamingo Wash/Las Vegas Wash confluence. A perchlorate plume discharged into the Las Vegas Wash in Henderson,  and  head cut (erosion) advanced up the Las Vegas Wash fed by “hungry,” treated waste water discharged by Clark County, the CLV, and the COH. Flood flows exasperated the erosion by scouring the head-cut channel deeper and wider, thereby lowering the bed elevation of Las Vegas Wash over time.

Ultimately the project became and ethics dilemma, as public safety risk was weighed against programmatic risk. A 54-inch high density polyethelyne (HDPE) sewer trunk line was installed in the Las Vegas Wash, north of the Pecos/Lake Mead intersection; the wash was to be immediately lined with concrete to stabilize erosion and protect the sewer in place. The revised hydrology postponed lining of the Las Vegas Wash, as contract change orders were processed to address the change in scope of work. One option to consider was merely accepting an existing, albeit flawed, storm centering and recommending a smaller flowrate, keeping the project on track. However, I determined that risk to public safety far outweighed the programmatic risk of a delayed project, and recommended doubling the flowrate–a decision based on ethics given the potential for severe downstream consequences, including the loss of both life and property.

I presented the two years of work at the southern Nevada Chapter of APWA, in May 1999 before hundreds of peers and colleagues. Two months later the July 8, 1999  storm event clobbered Las Vegas, causing significant property damage and loss of life, including overtopping of the Charleston Avenue bridge. Although the July 1999 storm was not centered over the modeled watershed, it clearly validated the decision to double the flowrate in the Las Vegas Wash– a decision based on accurate risk assessment and ethics. The outcome resulted in the construction of the Cheyenne Peaking Basin in North Las Vegas (see map below).

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References (click on the hyperlink to access the reference)

1. Black & Veatch (1997). Technical Memorandum on Hydrology for the ‘A’ Channel Lining Project, for City of North Las Vegas.
2. Black & Veatch (1998). Addendum No. 1 to the Technical Memorandum on Hydrology for the ‘A’ Channel Lining Project, for City of North Las Vegas.
3. Black & Veatch (1998). Existing Conditions Hydrologic Model for the Las Vegas Wash, Flamingo Wash Confluence to Alexander Road, for the Clark County Regional Flood Control District.
4. Haro, J.A., Daley, H.R., Runk, K.J. (1999). The Las Vegas Flash Floods of 8 July 1999: A Post Event Summary. Western Regional Technical Attachment No 99-26
5. Li, J., Maddox, R.A., Gao, X., Sorooshian, S., & Hsu, K. (2003). A numerical investigation of Storm Structure and Evolution during the July 1999 Las Vegas Wash Flash Flood. Monthly Weather Review, 131, 2038-2059 
6. Stachelski, C., & Pierce, B. (2009). Record flash flood of July 8, 1999: Ten year anniversary. National Oceanic and Atmospheric Administration
7. Sutko, T.E. (1999). Rainfall Event Report. Clark County Regional Flood Control District

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Click the link below to access a map to the Cheyenne Peaking Basin (constructed by others):

https://mapsengine.google.com/map/edit?mid=zQQqU_auD468.k2F0GOtcXXww