Board of Adjustment Agenda and Packet 2003 02 19

Home Browse Search

not increase the water surface elevation of the base flood more than one foot at any point. Certification by a registered professional engineer stating that any encroachment shall not result in such an increase in flood levels during the occurrence of a 100-year flood shall be required." As presented in the application information, the Boulder County Transportation Department Water Resources Engineer states the following: ".... I used the full flow conditions on the existing channel, which includes a flow of approximately 310 cfs, (water flow in drain - CFS = cubic feet per second of water run during a flood in drain), and a depth of 3.5 feet.", (the depth of the existing channel). The Water Resources Engineer continues by stating, "Although the overall flow area is reduced slightly from that of the existing channel, using a flow of 310 cfs on the new channel section results in a depth of 3.33', which is less that the existing channel flow depth by 2 inches. As a result, the flow characteristics of the proposed channel appear to be an improvement over existing conditions and will not increase the flood elevation in the Coal Creek floodway." The Engineer further states that, "It is important to note, as part of the realigned channel, the new channel will essentially restore the stream banks. The existing conditions indicate a severe erosion problem at the bend. Realigning the channel and armoring the banks will further enhance the hydraulic conditions along this reach of the ditch. In addition, the channel design appears adequate in preventing scour at the concrete abutments." It is therefore understood by Planning Staff that by permitting the work to occur with the issuance of a Floodplain Development Permit, the water flow in the drain during a 100-year flood will actually be lowered with the bank work being proposed. The banks will allow the water to flow quicker and in turn, lower the flow of any water in the drain thus reducing the possibility of overflow in the drainage ditch. As listed on the FEMA Elevation Certificate application, the project is proposed at the 100-year flood elevation of 5,424 feet, and will not exceed this height when constructed. (Please note: At this time, the Coal Creek drain is designed to accept any flow capacity from the drain, improved or not improved). The requirement of certification by a registered professional engineer is included in the application information found in the Memorandum dated January 23, 2003. • 17.56.160 B. Structures: 1. "Structures will have low flood damage" 2. "The structure will be constructed and placed on the building site so as to offer the minimum obstruction to the flow of floodwaters...." 3. "Structures will be firmly anchored as required to prevent the structure from floating away...." 3