To: Honorable Mayor and Members of Town Council
From: Mark Marlowe, Director of Castle Rock Water
David Van Dellen, Stormwater Manager
Title
Department Updates: Fluvial Hazard Zone Mapping Pilot Study of East Plum Creek by the Colorado Water Conservation Board
Body
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Executive Summary
This memo is to provide an update and to seek Council input prior to entering a Memorandum of Understanding (MOU) with the Colorado Water Conservation Board (CWCB) to provide technical resources and support for the study. After extensive flooding in the Colorado Northern Front Range in September 2013, the Colorado legislature passed Senate Bill 15-245, which supports Natural Hazard Mapping efforts. Through this Bill, the Colorado Hazard Mapping Program was established, which works to promote long-term planning and resiliency efforts, and provides a framework for land use and other decision-making in areas likely to be affected by future flooding, erosion, and debris flow events. The CWCB is the lead agency coordinating the river erosion or fluvial hazard zone (FHZ) mapping component of this project. East Plum Creek in the Town of Castle Rock was selected as part of the FHZ pilot study being conducted by the CWCB which is scheduled to be completed in 2018.
The Town has adopted a flood insurance study in cooperation with Federal Emergency Management Agency (FEMA) and CWCB to identify flood hazard zones in accordance with the National Flood Insurance Program (NFIP). These limits are established based on design flow rates routed through a hydraulic model using the best available survey data on the stream corridor. The FHZ study will not affect regulatory limits of E. Plum Creek. Rather, the FHZ study will provide limits of potential risk based on the area the stream has occupied in recent history, could occupy, or could physically influence as it stores and transports sediment and debris during flood events. The objective of the mapped FHZ is to identify lands most vulnerable to fluvial hazards in the near term. CWCB is requesting that the Town make good faith efforts to use the fluvial hazard zone information in land use decision making. It will be up to the Town how we determine to best use the data and whether to adopt an overlay zone district based on the study.
Castle Rock Water recognizes the inherent risks on East Plum Creek given the flooding that occurred in 1965. On the afternoon of June 16, over 12 inches of rainfall occurred south of Town near Larkspur producing an estimated flow rate of 126,000 cubic feet per second (cfs) on E. Plum Creek. The flood destroyed four major transportation bridges over multiple railroad crossings through Town. This was estimated to be over a 500-year event which caused widespread damage far beyond the normal 100-year floodplain limits at the time. One of the guiding principles of the Stormwater Enterprise Program is to protect people and property by managing stormwater through minimizing potentially hazardous conditions associated with stormwater runoff, flooding and erosion. One of the key ways to accomplish this charge is to establish reasonable buffers between development and the floodplain. Castle Rock Water supports the FHZ study as a tool for communicating real potential risk that exists adjacent to East Plum Creek.
By signing the MOU with CWCB, the Town is indicating our support of the project in providing data, resources and cooperation in communicating with landowners where the study will be conducted (see Attachment A). The Town is not obligated to adopt any land use zoning and flood insurance rate maps will not be impacted by the new mapping. Property owners will be informed of the new mapping once available. Should residents outside the 100-year floodplain wish to pursue a flood insurance policy on a voluntary basis, residents will be subject to the qualifications and rates as established in the NFIP.
Attachments
Attachment A: Map