2019-10-28-minutes-public-works-study-session

Home Browse Search

nhc During the modeled proposed conditions (Figure A-5 in Appendix A), the backwater (due to the existing two NF Teanaway Road culverts) is eliminated and thus no overtopping of the forest service road occurs. This is due to the increased conveyance from the proposed 40 -foot bridge, which will meet WDFW water crossing design guidelines to allow for natural stream processes and unimpeded fish passage. While the proposed design restores the natural drainage path of Lick Creek, it increases the flow ("'44 cfs during the estimated USGS 100 -year flow and unobstructed culverts) downstream compared to existing conditions due to flow no longer overtopping the forest service road and flowing down the west NF Teanaway Road ditch. This increase in flow has some effect on the water surface elevations through the private properties. The highest increase in water surface elevation is approximately 0.4 feet nearthe inlets of the private property culverts. This suggests that the private driveway culverts may have not been designed to convey the 100 -year flow without overtopping, which is not uncommon for low volume roads or privately owned crossings. It is unlikely that any of the culverts in the model domain were designed based on the attenuation that upstream culverts may have, as engineering standard practice is to design water crossings for steady-state, peak flow conditions. With this said, the preliminary hydraulic model results provide graphics as a way to continue discussions with the property owners and to discuss potential changes to Lick Creek due to restoring the natural drainage path, stream processes and unimpeded fish passage through the NF Teanaway Road crossing. Figure A-6 in Appendix A shows a reduction in water surface elevations upstream of the existing NF Teanaway Road culverts due to the additional conveyance provided by the proposed 40 -foot bridge. Similar to the 2 -year results, immediately downstream and through the realigned portion (approximately 325 feet downstream of the proposed crossing) of Lick Creek, there is an increase in water surface elevations (shown as red in Figure A-6) due to the proposed alignment currently being dry in the existing conditions. This area, based on historic aerial photos, is one of the historic alignments of Lick Creek. The other red areas throughout Lick Creek are also representative of locations where the 100 -year estimated flow did not inundate (was dry) during existing conditions. As mentioned above, the highest increases in water surface elevations (approximately 0.4 feet) compared to existing conditions are near the private driveways, likely due to the culverts not being designed to convey the 100 -year flow without backwater. In general there is no increase in water surface elevations downstream of culvert 3 (shown in Figure 2-3). This is due to all of the flow in existing conditions being in Lick Creek, as the flow in the west NF Teanaway Road ditch overtops and re-enters Lick Creek upstream of this location. 5 CONCLUSION This technical memorandum summarizes the hydraulics assessment that was completed to assist WDFW in assessing whether they will take actions to support the County's original proposed project to afford protections and streamlined permitting for fish barrier removal projects through RCW 77.55.181, The hydraulic model developed, as part of this flood rise analysis, is a tool which can be utilized to assess changes in hydraulic characteristics due to changes in hydrology or assumptions of existing or proposed infrastructure. This assessment provides the requested information from WDFW, allowing discussions with stakeholders to assess project goals and communicate potential changes due to the NF Teanaway Road fish enhancement project. North Fork Teanaway Road - Lick Creek Fish Passage Enhancement Project Flood Rise Analysis — Maintaining Creek Flows Along Teanaway Road 13