The URL can be used to link to this page

Home My WebLink AboutNORTHERN INTEGRATED SUPPLY PROJECT - SPA210001 - SUBMITTAL DOCUMENTS - ROUND 1 - STORMWATER-RELATED DOCUMENTS To: Christie Coleman Northern Water Conservancy District 220 Water Avenue Berthoud, CO 80513 AECOM 804 Colorado Avenue Glenwood Springs, CO 81601 aecom.com Project name: Poudre River Intake Structure Project ref: 60649542 From: Andy Steininger, P.E. John Sikora, P.E. Date: May 14, 2021 Memo Introduction This memo describes preliminary floodplain modeling for the proposed river diversion and intake structure (Poudre River Intake Structure) on the Cache La Poudre River (Poudre River) just upstream of the Mulberry Street Bridge in Fort Collins, Colorado for the Northern Integrated Supply Project (NISP). Background The purpose for the Poudre River Intake Structure is to divert water that is released from Glade Reservoir to the Poudre River. Releases would range from 18 cfs during the winter months to 25 cfs during the summer months, or 14,350 acre-feet annually. This water is left in the Poudre River for 12 miles through the reach of the Poudre River as shown in the figure below. The Poudre River Intake is a riffle grade structure and concrete intake structure that diverts that water into a pipeline. The diversion and intake structure are shown in the SPAR submittal plan set. The proposed diversion location for the Poudre River Intake was mainly selected by the legal description for this point of diversion in the associated water rights filings. The point of diversion is also upstream of the Mulberry Water Reclamation Facility. Pathogens and emerging contaminants can be a concern with effluent water. The current takeout location is just upstream of the Mulberry Water Reclamation Facility outfall to maintain water quality standards. There are several existing geographic constraints and considerations that are being considered for the proposed Poudre River Intake. The proposed project encompasses an area in the active river channel and the associated riparian zone on the Poudre River in the Homestead Natural Area. This area is a successful restoration area wherein the natural function of riparian and upland areas has been restored and enhanced. It is recognized that this area is sensitive to disturbance and is an important public use area with established access paths. As such, the project is being designed to minimize the temporary and long-term footprint of the diversion structure. Mulberry Diversion Structure Preliminary Floodplain Modelling Figure 1 – Location Map Diversion Structure The proposed diversion structure consists of the installation of a channel spanning Riffle-Crest Weir structure protruding approximately one foot above the existing bed grade to create necessary headwater conditions for the diversion intake. It would convey up to 25 cfs through the intake structure at all river flows. This structure would be comprised of a boulder grade control at the headward end with a sheet pile and concrete cap hydraulic cutoff placed within the structure to be blended with the natural visual aesthetics of the riverbed. The crest of the boulder grade control structure would have varied elevations of up to approximately 0.75-feet to create hydraulic diversity and shadows for fish passage and to present visually as a natural riffle head. Additionally, the structure would ramp up slightly towards the banks to guide flows toward the center preventing bank erosion and confining flows for low flow fish passage and boater safety. Downstream of the grade control would be an alluvial riffle to tie the grade control structure invert to existing grade at approximately the same bed slope as the existing downstream riffle. The geometry, gradation and material type would be designed to increase fish passage and boater passage stability metrics. The intent of the in-channel structure would be to integrate into the natural hydraulics and aesthetics of the reach while providing the needed hydraulic head for the diversion structure. Mulberry Diversion Structure Preliminary Floodplain Modelling Figure 2 – Example of a Riffle Grade Structure Intake Structure The concrete intake structure consists of a concrete sill and guide walls, hinge crest gate, and sluice return flow gate to allow for flows from the diversion structure to enter into a vault and a pipeline. The intent of the sluice gate would be to clear deposited sediment from in front of the intake structure. By utilizing a hinge crest gate to divert flow into the intake structure, bedload will be maintained in the river to minimize impacts to the Poudre River. This structure is designed to minimize impacts to the natural aesthetic of the reach. Natural alluvial materials and vegetation would be used to obscure the structure. Flood Model Hydrology The preliminary floodplain modeling hydrology is based on the Cache la Poudre Watershed Risk Mapping, Analysis and Planning (Risk MAP) Study (CLP Risk MAP Study). Flow data was retained from CLP Risk MAP Study model for the 100-yr and 500-yr return period flows. These flow rates are 13,501 and 18,960 cubic feet per second (cfs) respectively. Manning’s roughness coefficients form nearby sections of the RiskMap model were used to characterize the roughness for the new and updated cross sections. Hydraulic Modeling and Floodplain Compliance Preliminary hydraulic modeling of the project reach has been performed for existing conditions using the Hydrologic Engineering Center River Analysis System (HEC-RAS) (ver. 5.0.3). The Risk MAP project model was used as a base condition for the hydraulic modeling. This model is currently under review to be adopted as the Federal Emergency Management Agency (FEMA) effective hydraulic model. This model represents the best available information and regulatory model based on the anticipated construction schedule for this project. The with-project model is based on the existing Risk MAP model wherein the Risk MAP geometry was updated with 7 new cross sections and one existing section revised with new spatial data. For all 8 cross sections, elevation data was used from two hydrographic surveys. The location of the additional cross sections is shown on Figure 3. Mulberry Diversion Structure Preliminary Floodplain Modelling The with-project conditions model represents the proposed diversion structure and intake structure as described above. This geometry includes modified cross section data from the existing conditions model representing the proposed structures. A blocked obstruction was added to the with-project model to reflect the riffle structure and intake structure for the with-project model. Refinement of the hydraulic analysis is anticipated based on final design of this diversion and intake structure. Current modeling has shown no rise for the proposed conditions for the 100-year and 500- year hydrologic events. “No Rise” criteria are defined for the Poudre River floodplain and FEMA basin floodplains a calculated rise in flood depth of 0.00 feet as defined in the hydraulic models for said floodplains. Figure 3 - HEC RAS Cross Section Locations Added HEC RAS Cross Sections Figure 4- HEC RAS Riffle Crest Diversion Structure Invert Cross Section 0 200 400 600 800 1000 1200 1400 1600 1800 4925 4930 4935 4940 4945 4950 4955 4960 4965 CLPR_FtCollins Plan: 1) PG Floodway Updated2 2) EG Floodway RS = 226245 Station (f t)Elevation (ft)Legend WS 1% - EG Floodway WS 1% - PG Floodway Updated2 - EG Floodway Ground - EG Floodway Ineff - EG Floodway Bank Sta - EG Floodway - PG Floodway Updated2 - PG Floodway Updated2 Ground - PG Floodway Updated2 Ineff - PG Floodway Updated2 Bank Sta - PG Floodway Updated2 Mulberry Diversion Structure Preliminary Floodplain Modelling Figure 5 - Poudre River Intake Structure HEC RAS Hydraulic Profile 7000 8000 9000 10000 4925 4930 4935 4940 4945 CLPR_FtCollins Plan: 1) PG Floodway Updated2 5/14/2021 2) EG Floodway 5/14/2021 Main Channel Distance (ft)Elevation (ft)Legend WS 1% - PG Floodway Updated2 WS 1% - EG Floodway Lat Struct Ground - PG Floodway Updated2 Ground - EG Floodway 22401...224468224668224710224750224796224911224956225228225525225677225727226034226101226162226193226245226288226341226395226806227283CLPR Lincoln Mulberry Diversion Structure Preliminary Floodplain Modelling Table – 1 Poudre River Intake Structure HEC RAS Results Reach River Sta Profile Plan Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl (cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft) Lincoln 227283 1.0% PG Floodway Updated2 13501.12 4931.02 4941.145 4941.5 0.002808 5.65 2940.55 790.62 0.38 Lincoln 227283 1.0% EG Floodway 13501.12 4931.02 4941.149 4941.51 0.002799 5.65 2944.02 790.86 0.38 Lincoln 226806 1.0% PG Floodway Updated2 13501.12 4930.94 4940.354 4940.62 0.001214 4.24 3432.13 1208.98 0.26 Lincoln 226806 1.0% EG Floodway 13501.12 4930.94 4940.361 4940.62 0.001209 4.24 3438.36 1210.22 0.26 Lincoln 226395 1.0% PG Floodway Updated2 13501.12 4928.99 4939.792 4940.06 0.00135 5.95 3637.88 1024.64 0.34 Lincoln 226395 1.0% EG Floodway 13501.12 4928.99 4939.819 4940.05 0.001357 4.88 3655.27 1026.01 0.28 Lincoln 226341 1.0% PG Floodway Updated2 13501.12 4928.97 4939.768 4939.98 0.001044 5.23 3998.82 1080.17 0.3 Lincoln 226341 1.0% EG Floodway 13501.12 4928.97 4939.767 4939.98 0.001044 5.23 3998.17 1080.08 0.3 Lincoln 226288 1.0% PG Floodway Updated2 13501.12 4929.12 4939.727 4939.92 0.000889 4.76 3994.63 1196.9 0.28 Lincoln 226288 1.0% EG Floodway 13501.12 4929.12 4939.725 4939.92 0.00089 4.76 3993.5 1196.52 0.28 Lincoln 226245 1.0% PG Floodway Updated2 13501.12 4929.5 4939.667 4939.88 0.000991 5.05 3849.28 1227.26 0.29 Lincoln 226245 1.0% EG Floodway 13501.12 4928.84 4939.667 4939.88 0.000966 5.06 3871.53 1227.33 0.29 Lincoln 226193 1.0% PG Floodway Updated2 13501.12 4929.15 4939.6 4939.82 0.00112 5.25 3773.35 1277.4 0.31 Lincoln 226193 1.0% EG Floodway 13501.12 4928.95 4939.599 4939.82 0.00111 5.23 3780.05 1277.4 0.31 Lincoln 226162 1.0% PG Floodway Updated2 13501.12 4928.65 4939.565 4939.79 0.001039 5.34 3793.23 1200.84 0.3 Lincoln 226162 1.0% EG Floodway 13501.12 4928.47 4939.565 4939.79 0.001038 5.37 3794.78 1200.69 0.3 Lincoln 226101 1.0% PG Floodway Updated2 13501.12 4927.36 4939.357 4939.71 0.00118 6.1 3441.33 1097.69 0.33 Lincoln 226101 1.0% EG Floodway 13501.12 4927.36 4939.357 4939.71 0.00118 6.1 3441.33 1097.69 0.33 Lincoln 226034 1.0% PG Floodway Updated2 13501.12 4926.73 4939.313 4935.79 4939.65 0.001105 4.83 3148.42 1153.97 0.26 Lincoln 226034 1.0% EG Floodway 13501.12 4926.73 4939.313 4935.79 4939.65 0.001105 4.83 3148.42 1153.97 0.26