Consideration and Approval of Grant Award: FY18 Cooperating Technical Partners Grant Agreement.

The City of Flagstaff entered into a Cooperative Technical Partners (CTP) agreement with FEMA on April 11, 2011. This agreement allowed the City of Flagstaff to receive grant funding from FEMA to complete technical studies or other work associated with the National Flood Insurance Program (NFIP). The Rio de Flag Hydrology Study will be the most comprehensive analysis ever undertaken to determine the 100-year flood of the Rio de Flag in downtown Flagstaff and to determine the 100-flood for major tributaries of the Rio de Flag including Clay Ave Wash and Sinclair Wash. This effort was born from wide variability in calculated 100-year flood flows from FEMA, the US Army Corps of Engineers, and private consulting firms. There remains large uncertainty in how the Rio de Flag watershed responds to storms. This hydrology study will primarily focus on data collection for calibration of our existing Rio de Flag watershed computer model.

The City of Flagstaff will receive $200,000. This grant award does not require matching funds from the City of Flagstaff, however, the City has contributed $110,000 of matching funds for other work that has already been completed or will be completed this fiscal year by the Stormwater Section. This work, for which FEMA has accepted towards our required matching funds, includes the expansion of the City of Flagstaff's rain and stream gage network and the hiring of a meteorological consultant to determine the appropriate 100-year design storm for the Rio de Flag. Funds to cover the portion of this grant match-work that will occur this Fiscal Year have been authorized by Council through the adoption of this year's Fiscal Budget and allocated in account 206-08-385-3236-0.

This study could lead to an increase or decrease in the FEMA regulated floodplain areas.

TRANSPORTATION & PUBLIC INFRASTRUCTURE: Deliver quality community assets and continue to advocate and implement a highly performing multi-model transportation system.
Goal T.1. Improve mobility and access throughout the region.
Goal T.2. Improve transportation safety and efficiency for all modes.

No. However the City received $200,000 for a related effort from another award in 2014 under this same CTP agreement to create a Rio de Flag watershed-wide computer model.

The City can continue to size City-wide drainage infrastructure using flood flows calculated by FEMA, private consultants or the US Army Corps of Engineers.

The Rio de Flag, Clay Ave Wash, and Sinclair Wash experience flooding from large, regional winter storms that behave differently than summer convective monsoons. There remains great uncertainty around the flooding from these storm events.  Since the original hydrology study was adopted in 1983, there has been only one other known hydrologic study completed for the Rio de Flag in 2000 by the United States Army Corps of Engineers. The study discusses the lack of data available for calibration, the many assumptions made to create a rainfall distribution, and the high uncertainty in how the watershed responds.

The City of Flagstaff now has a 2-D hydrologic model created for the watershed under a previous FEMA CTP grant and can model highly heterogeneous and variable watershed conditions if adequate data is collected. The model also has the ability to model highly variable temporal and spatial distributions of rainfall.

Since 2000, there has been more data collected that can be applied to a hydrology study. Advancements in weather radar combined with precipitation and stream gauges installed have helped meteorologists more accurately define extreme weather patterns and events. Under a separate contract using City of Flagstaff funds, the City of Flagstaff has hired a meteorological consultant to prepare a design storm for Flagstaff applicable for determining the 100-year and 500-year flood for the Rio de Flag. The City of Flagstaff intends to incorporate this design storm into its 2-D model for determining the 100-year and 500-year flood for the Rio de Flag and its major tributaries.

More data is needed to perform an improved hydrologic analysis. While the City of Flagstaff has installed several rain and stream gauges in the Rio de Flag watershed over the last decade, this new data alone is not enough for meaningfully reduced model uncertainty and improved model calibration. By far the most sensitive parameter of watershed hydrologic response is the infiltration. Soil sampling at key locations will verify model parameters for infiltration, and these site visits can also help verify forest canopy and understory that can have impacts to watershed storage capture.  Previously unused, non-traditional forms of data can be used to help calibrate and verify the model if they are examined carefully for their applicability. These include historic photos, fluvial geomorphology measurements and statistical analysis of measured stream gauge readings.

Improved understanding of how larger watersheds in Flagstaff respond to different storm events can provide direction on how we can correct flooding and appropriately size stormwater infrastructure for the Rio de Flag and major tributaries.

As a condition of this award, the city is required to contribute a cost match in the amount of $110,000.00 of non-Federal funds, or 35 percent of the total approved project costs of $310,000.00.

Improved understanding of how larger watersheds in Flagstaff respond to different storm events can provide direction on how we can correct flooding and appropriately size stormwater infrastructure for the Rio de Flag and major tributaries.

1. Inform. This study will lead to improved floodplain mapping that will guide the public for development activities and flood insurance.