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HomeMy WebLinkAboutGATEWAY APARTMENTS - PDP230001 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORT (2) PRELIMINARY DRAINAGE REPORT GATEWAY APARTMENTS FORT COLLINS, COLORADO JANUARY 4, 2023 NORTHERNENGINEERING.COM 970.221.4158 FORT COLLINS GREELEY This Drainage Report is consciously provided as a PDF. Please consider the environment before printing this document in its entirety. When a hard copy is necessary, we recommend double-sided printing. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY COVER LETTER January 4, 2023 City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, CO 80521 RE: FINAL DRAINAGE REPORT FOR GATEWAY APARTMENTS Dear Staff: Northern Engineering is pleased to submit this Preliminary Drainage Report for your review. This report accompanies the Preliminary submittal for the proposed Gateway Apartments. This report has been prepared in accordance with the Fort Collins Stormwater Criteria Manual (FCSCM) and serves to document the stormwater impacts associated with the project. We understand that review by the City of Fort Collins is to assure general compliance with standardized criteria contained in the FCSCM. If you should have any questions as you review this report, please feel free to contact us. Sincerely, NORTHERN ENGINEERING SERVICES, INC. ADAM BOESE, PE Project Engineer Compliance Statement I hereby attest that this report for the preliminary drainage design for the Gateway Apartments was prepared by me or under my direct supervision, in accordance with the provisions of the Fort Collins Stormwater Criteria Manual. I understand that the City of Fort Collins does not and will not assume liability for drainage facilities designed by others. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY TABLE OF CONTENTS TABLE OF CONTENTS GENERAL LOCATION AND DESCRIPTION .......................................................... 1 DRAIN BASINS AND SUB-BASINS ..................................................................... 3 DRAINAGE DESIGN CRITERIA .......................................................................... 4 DRAINAGE FACILITY DESIGN ........................................................................... 7 CONCLUSIONS ............................................................................................ 10 REFERENCES .............................................................................................. 11 TABLES AND FIGURES FIGURE 1 – VICINITY MAP .................................................................................................1 FIGURE 2 – AERIAL PHOTOGRAPH ...................................................................................2 FIGURE 3 – EXISTING FEMA FLOODPLAINS (PRIOR TO CLOMR/LOMR) ..........................3 FIGURE 4 - EXISTING CITY FLOODPLAINS ........................................................................3 TABLE 1 - FULL SPECTRUM DETENTION SUMMARY ..................................................... 10 APPENDICES APPENDIX A – HYDROLOGIC COMPUTATIONS APPENDIX B – HYDRAULIC CALCULATIONS APPENDIX C –WATER QUALITY/LID COMPUTATIONS APPENDIX D – EROSION CONTROL REPORT APPENDIX E – USDA SOILS REPORT MAP POCKET DR1 – DRAINAGE EXHIBIT NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 1 | 11 GENERAL LOCATION AND DESCRIPTION A. LOCATION Vicinity Map The Gateway Apartments project site is located in the southeast quarter of Section 16, Township 7 North, Range 68 West of the 6th Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado. The project site (refer to Figure 1) is bordered to the north by future single-family residential tract within the Gateway at Prospect development area; to the south by Prospect Road; to the east by Boxelder Creek; and to the west by an existing single-family residential area. Boxelder Creek and the Boxelder Creek 100-yr overflow channel run north to south along the eastern boundary adjacent to the project site. B. DESCRIPTION OF PROPERTY The Gateway Apartments are comprised of ±12.49 acres. The site is currently vacant irrigated agricultural land with future proposed backbone infrastructure being designed with the Gateway at Prospect BDR Project. Backbone infrastructure Figure 1 – Vicinity Map NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 2 | 11 will be installed prior to or concurrent with this project includes surrounding connector roads, Frontage Road improvements, utilities and detention ponds. The existing groundcover consists of cultivated corn field, grasses, and gravel. The existing on-site runoff generally drains from the north-to-south across flat grades (e.g., <2.00%) into Boxelder Creek and the 100-yr Boxelder Creek overflow channel. From there, the drainage continues through the channel, and ultimately discharging to the Cache La Poudre River. According to the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Soil Survey, a majority of the project site consists of Nunn clay loam and Satanta Variant clay loam which fall into Hydrologic Soil Groups C-D. Boxelder Creek is the only major drainageway within or adjacent to the project site with the exception of remnants of the historic Cooper Slough which exists just off-site to the west. The proposed development will consist of nine multi-family buildings containing a total of 276 units. Other proposed improvements include new asphalt drive aisles, new sidewalks and new landscaping, a clubhouse with pool and a dog park. The proposed land use is Medium Density Residential. This is a permitted use in the Medium- Density Mixed-Use Neighborhood District (MMN). Figure 2 – Aerial Photograph NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 3 | 11 C. FLOODPLAIN The subject property is located in a FEMA floodplain but not a City regulatory floodplain. The Gateway at Prospect backbone infrastructure project will be pursuing a CLOMR/LOMR prior to the construction of this project. Figure 3 – Existing FEMA Floodplains (Prior to CLOMR/LOMR) Figure 4 - Existing City Floodplains NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 4 | 11 DRAIN BASINS AND SUB-BASINS A. MAJOR BASIN DESCRIPTION 1. The Gateway Apartments project is located within the Boxelder Creek/Cooper Slough Basin. Boxelder Creek/Cooper slough Basin encompasses 265 square miles beginning north of the Colorado/Wyoming border and extending southward into east Fort Collins, ending at the Cache La Poudre River. 2. Improvements along the Boxelder Creek were recently constructed and detailed in the LOMR case number 17-08-1354P. A 100-year overflow channel and box culverts under Prospect Road were constructed as a part of that project. 3. Backbone Infrastructure improvements along Boxelder Creek including regional trail grading, a bridge crossing of Boxelder Creek and detention pond grading and outfalls needed to detain and treat runoff from the Gateway Apartments are currently under review. Jacobs prepared a CLOMR for “Boxelder Creek at Gateway at Prospect” which is currently under review and will need approval from FEMA and City of Fort Collins prior to approval of Gateway Apartments. B. SUB-BASIN DESCRIPTION The project site has two outfall locations. The northern half of the site will outfall to Boxelder Creek and the southern half will out fall to the Boxelder Creek Overflow Channel. The existing subject site drainage has been master-planned by the Gateway at Prospect backbone infrastructure project. The existing condition shall hence be referred to as the proposed master planned condition as described in the Final Drainage Report for Gateway at Prospect dated August 24th, 2022. The Gateway Apartments project resides in two master planned drainage basins, Basin E and Basin F. These basins are further divided into seven sub-basins. The master planned Basins E1-E2 generally flow south towards Detention Pond E. Existing Basins F1-F5 generally drain east towards Detention Basin F. The project site does not receive notable runoff from contiguous off-site properties. DRAINAGE DESIGN CRITERIA A. OPTIONAL PROVISIONS Gateway Apartments will show compliance with the previous report which utilized full spectrum for detention and traditional 40-hour water quality treatment for the site. The use of full spectrum detention is a variance from the City stormwater criteria, but was approved for use with the Gateway backbone infrastructure project. B. STORMWATER MANAGEMENT STRATEGY The overall stormwater management strategy employed with Gateway Apartments utilizes the “Four Step Process” to minimize adverse impacts of urbanization on receiving waters. The following is a description of how the proposed development has incorporated each step. Step 1 – Employ Runoff Reduction Practices. Gateway Apartments will be reducing runoff by 10% of the historic value via full spectrum. Full spectrum detention is designed to address two limitations of traditional detention. First, it is focused on controlling peak discharges over the full spectrum of runoff events from small, frequent storms up to the 100-year storm event. Second, full spectrum detention facilities produce outflow hydrographs that, other than a small release rate of the excess NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 5 | 11 urban runoff volume (EURV), replicate the shape of predevelopment hydrographs. Full spectrum detention modeling shows a reduction of urban runoff peaks to levels similar to predevelopment conditions over an entire watershed, even multiple independent detention facilities. For a thorough, in-depth explanation of full spectrum, please refer to Volume 2, Chapter 12 of the Urban Drainage Stormwater Criteria Manual published by the Mile High Flood District. Gateway Apartments aims to reduce runoff peaks, volumes and pollutant loads from frequently occurring storm events (i.e., water quality (i.e., 80th percentile) and 2-year storm events) by implementing Low Impact Development (LID) strategies. Wherever practical, runoff will be routed across landscaped areas or through an infiltration gallery. These LID practices reduce the overall amount of impervious area, while at the same time Minimizing Directly Connected Impervious Areas (MDCIA). The combined LID/MDCIA techniques will be implemented, where practical, throughout the development, thereby slowing runoff and increasing opportunities for infiltration. Step 2 – Implement BMPs that Provide a Water Quality Capture Volume (WQCV) with Slow Release. The efforts taken in Step 1 will help to minimize excess runoff from frequently occurring storm events; however, urban development of this intensity will still have stormwater runoff leaving the site. The primary water quality treatment will occur in the underground vaults located near the center and towards the east end of the project. Rain Gardens will be utilized as a second water quality treatment BMP located towards the south end of the site. In addition to LID treatment, traditional water quality treatment will be provided within the lower stages of the detention ponds. Step 3 – Stabilize Drainageways. As stated in Section I.B.5, above, the Boxelder Creek drainage is adjacent to the subject site and Gateway Apartments proposes to limit the amount of disturbance to Boxelder Creek. With the previously mentioned backbone infrastructure project doing most of the work required on the streambank for the future City of Fort Collins Regional Trail, storm outfalls and detention areas, Gateway Apartments will have minimal to the streambank. The Gateway Apartments project aims to protect the existing Boxelder Creek by limiting any increases beyond historical flow rates within the creek. By utilizing a combination of full spectrum detention and the resulting “beat the peak” model, the peak discharge from the Gateway Apartments project will be passed before the peak of the creek. As such, the peak discharge from the Gateway Apartments project will not be compounded with peak flow in the creek. Furthermore, this project will pay a one-time stormwater development fee, as well as ongoing monthly utility fess, both of which help achieve citywide drainage stability. Step 4 – Implement Site Specific and Other Source Control BMPs. This step typically applies to industrial and commercial developments. C. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS The subject property is part of the master-planned drainage plan set by the Gateway at Prospect backbone infrastructure project. The site plan is constrained on the north and west by connector streets, on the east side by Detention Pond F and future City of Fort Collins Regional Trail and on to the south by Detention Pond E all as designed with the Gateway at Prospect project. D. HYDROLOGICAL CRITERIA The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in Figure 3.4-1 of the FCSCM, serve as the source for all hydrologic computations associated with the Gateway NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 6 | 11 Apartments development. Tabulated data contained in Table 3.4-1 has been utilized for Rational Method runoff calculations. The Rational Method has been employed to compute stormwater runoff utilizing coefficients contained in Tables 4.1-2 and 4.1-3 of the FCSCM. Full spectrum detention has been utilized for detention storage calculations. Two separate design storms have been utilized to address distinct drainage scenarios. The first event analyzed is the “Minor,” or “Initial” Storm, which has a 2-year recurrence interval. The second event considered is the “Major Storm,” which has a 100-year recurrence interval. E. HYDRAULIC CRITERIA The drainage facilities proposed with the Gateway Apartments project are designed in accordance with criteria outlined in the Mile High Flood District’s “Urban Storm Drainage Criteria Manual” (UDFCD) and Fort Collins Stormwater Criteria Manual (FCSCM). As stated in Section I.C.1, above, the subject property is located next to a FEMA designated floodplain. F. FLOODPLAIN REGULATIONS COMPLIANCE As previously mentioned, this project is adjacent to a FEMA regulated floodplain with pending CLOMR/LOMR application. No occupied structures are being proposed within the revised floodplain however, Detention Pond F will remain in the revised floodplain. Storm outfalls which are proposed with the Gateway at Prospect project will be located within the floodplain and floodway, and that work will be subject to the applicable floodplain regulations. Construction of these outfalls are not a part of the Gateway Apartments project. Any grading within the floodplain will be subject to applicable floodplain regulations. Despite most of the project not being located within the floodplain, consideration has been given to the floodplain elevations as they relate to the proposed buildings and the finished floors, garages, and HVAC have been elevated accordingly. G. MODIFICATIONS OF CRITERIA Gateway Apartments will be following the requirements as set forth in the Final Drainage Report for Gateway at Prospect and will be utilizing full spectrum detention as laid out by the Mile High Flood Control District as a variance from for the Fort Collins Stormwater Criteria detention requirements. H. CONFORMANCE WITH WATER QUALITY TREATMENT CRITERIA City Code requires that 100% of runoff from a project site receive some sort of water quality treatment. This project proposes to provide water quality treatment through the use of Bioretention (aka - Rain Garden located within Basin E) and two separate underground infiltration galleries (aka – underground chambers located under the drive aisles within Basin F). Both Detention Ponds E and F utilize full Spectrum Detention which will provide standard water quality treatment within the lower stages. Through these methods 100% of the on-site area will be treated for water quality. I. CONFORMANCE WITH LOW IMPACT DEVELOPMENT (LID) The project site will conform with the requirement to treat a minimum of 75% of the proposed impervious surfaces using a LID technique. Please see Appendix C for LID design information, table, and exhibit(s). As shown in the LID table provided in the appendix, 77% of the proposed site impervious area will receive LID treatment, which exceeds the minimum required. On-site LID NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 7 | 11 treatment does include treatment for the adjacent roads to the project as required by the Final Drainage Report for Gateway at Prospect. J. SIZING OF LID FACILITIES Infiltration Gallery The Infiltration Galleries were sized by first determining the required water quality capture volume (WQCV) for the contributing Basins. A 12-hour drain time was used in this calculation. Once the WQCV was identified, the minimum number of vaults needed to achieve the minimum WQCV was calculated. This volume includes the adjacent aggregates As a result of all underground vaults being wrapped in a geofabric and the potential constriction of flows that could result from sedimentation in the fabric, the total release rate through the fabric was calculated. This rate was determined by multiplying the vault bottom square footage x 0.35 gpm (manufacturer’s estimated infiltration rate of fabric). A volume calculation utilizing the WQ flow rate into the chamber and the calculated release rate through the fabric was completed. The number of chambers has been increased as needed to confirm that the resulting volume is provided within the empty volume of the underground chambers. This is intended to ensure that the chambers do not become overwhelmed in the water quality storm event before “discharging” flows into the surrounding aggregates. DRAINAGE FACILITY DESIGN A. GENERAL CONCEPT The main objective of the Gateway Apartments drainage design is to maintain existing drainage patterns, while not adversely impacting adjacent properties. There are small offsite flows that pass through the site originating from the three adjacent streets proposed with the Gateway at Prospect backbone infrastructure. Generally, these flows consist of half- right-of-ways of connector streets. Gateway Apartments will be responsible for the LID treatment and water quality of these offsite basins in conformance with the Gateway at Prospect drainage report. A list of tables and figures used within this report can be found in the Table of Contents at the front of the document. The tables and figures are located within the sections to which the content best applies. Drainage for the project site has been analyzed using seventeen (17) drainage sub-basins, designated as sub-basins E1.1-E1.4, E2, F1.1-F1.8, F2, F3, F4 and F5. The drainage patterns anticipated for the basins are further described below. Sub-Basin E1.1 Sub-basin E1.1 is comprised of a section of Street B and portions of the clubhouse and multi- family development. Drainage is routed to Pond E via storm drains and an inlet in Street B. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 8 | 11 Sub-Basin E1.2 Sub-basin E1.2 is comprised of portions of multifamily development north of the clubhouse including parking and drive isles. Drainage is routed to Pond E via sidewalk chase which flows via drainage pan to Rain Garden 1 where it will be treated for LID. Sub-Basin E1.3 Sub-Basin E1.3 is comprised of portions of the clubhouse, open area and a dog park. Stormwater runoff for this basin will be conveyed via drainage pan to Rain Garden 1 where it will be treated for LID prior to infiltration and release via subdrain to Pond E. Sub-Basin E1.4 Sub-Basin E1.4 is comprised of Detention Pond E. Sub-Basin E2 Sub-Basin E2 is comprised of a section of Street B and open space. Drainage is routed to Pond E via storm drains and an inlet in Street B. Sub-Basin F1.1 Sub-Basin F1.1 is comprised of portions of the multi-family development. Stormwater runoff for this basin will be routed to Stormtech Chambers 1 via storm inlet within the private drive aisles where it will be treated for LID prior to release to Pond F. Sub-Basin F1.2 Sub-Basin F1.2 is comprised of portions of the multi-family development. Stormwater runoff for this basin will be routed to Stormtech Chambers 1 via storm inlet within the private drive aisles where it will be treated for LID prior to release to Pond F. Sub-Basin F1.3 Sub-Basin F1.3 is comprised of portions of the multi-family development. Stormwater runoff for this basin will be routed to Stormtech Chambers 1 via storm inlet within the private drive aisles where it will be treated for LID prior to release to Pond F. Sub-Basin F1.4 Sub-Basin F1.4 is comprised of portions of the multi-family development. Stormwater runoff for this basin will be routed to Pond F via a storm inlet in private drive aisle. Pond F will provide traditional water quality and detention for Basin F1.4. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 9 | 11 Sub-Basin F1.5 Sub-Basin F1.5 is comprised of portions of the multi-family development. Stormwater runoff for this basin will be routed to Stormtech Chambers 2 via storm inlet within the private drive isles where it will be treated for LID prior to release to Pond F. Sub-Basin F1.6 Sub-Basin F1.6 is comprised of portions of the multi-family development. Stormwater runoff for this basin will be routed to Stormtech Chambers 2 via storm inlet within the private drive isles where it will be treated for LID prior to release to Pond F. Sub-Basin F1.7 Sub-Basin F1.7 is comprised of portions of the multi-family development. Stormwater runoff for this basin will be routed to Underground Chambers 2 via storm inlet within the private drive isles where it will be treated for LID prior to release to Pond F. Sub-Basin F1.8 Sub-Basin F1.5 is comprised of portions of the multi-family development, future City of Fort Collins Regional Trail, and Detention Pond F. Stormwater runoff will generally sheet flow into a swale where it will continue through Detention Pond F. Pond F will provide Standard Water Quality Treatment and Detention for Basin F1.7. Sub-Basin F2 Sub-Basin F2 is comprised of a section of Street D and portions of the multi-family development. Stormwater runoff is captured in an inlet in Street D and conveyed through a storm drain and into Stormtech Chambers 2 where it will be treated for LID prior to release into Detention Basin F. Sub-Basin F3 Sub-Basin F3 is comprised of half the right of way for Street E and half of the bridge section spanning Boxelder Creek. Runoff is conveyed through a sidewalk culvert and a swale which will convey the into Pond F. Pond F will provide Standard Water Quality Treatment and Detention for Basin F3. Sub-Basin F4 Sub-Basin F4 is comprised of half the right of way for Street B and portions of the multi-family development. Drainage is routed to Stormtech Chambers 2 via an inlet within Street B and a storm drain where it will be treated for LID prior to release to Detention Basin F. Sub-Basin F5 Sub-Basin F5 is comprised of half the right of way for Street B and open space. Drainage is routed to an existing Rain Garden (LID 3) west of Street B via curb cut and sidewalk chase within Street B. The existing Rain Garden will treat the runoff for LID prior to infiltration or release via subdrain to Detention Pond F. A full-size copy of the Drainage Exhibit can be found in the Map Pocket at the end of this report. NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 10 | 11 B. SPECIFIC DETAILS Gateway Apartments is located near the end of the Boxelder Creek/Cooper Slough Basin and will be discharging into Boxelder Creek. In order to “beat the peak”, full spectrum detention was utilized to size the final detention ponds. Gateway Apartments proposed providing 100-year detention, detaining the EURV, and utilizing extended detention for water quality in conformance with the final drainage report for Gateway at Prospect. Existing Storm line F which runs from Basin F5 to Detention Pond F was designed to convey the 100-yr storm flows from sub-basins F5 and F4. A new manhole installed near Stormtech Chambers 1 will divert the water quality storm to the underground chambers and include a 100-yr overflow weir for the 100-yr event. The proposed combination inlet within Basin F2 on Street D has been sized for the 100-yr storm. A manhole will be provided along the proposed storm line near Stormtech Chambers 2 which will divert the water quality storm to the underground chambers and include a 100-yr overflow weir for the 100-yr event. Landscape drains located adjacent to Building 5 have also been sized for the 100-yr event. Detention Pond Calculations 1. Detention Pond Calculations were done via full spectrum method as described in Volume 2, Chapter 12, Section 3 in the Urban Storm Drainage Criteria Manual. Pond ID Total Storage Volume (ac-ft) WQCV (ac-ft) EURV (ac-ft) Final Release Rate (cfs) Pond E 0.482 0.074 0.134 7.2 Pond F 1.680 0.267 0.526 14.6 Table 1 - Full Spectrum Detention Summary CONCLUSIONS A. COMPLIANCE WITH STANDARDS The detention design proposed with the Gateway Apartments project does not comply with the City of Fort Collins’ Stormwater Criteria Manual but does meet the Mile High Flood District Criteria Manual. However, the City of Fort Collins has approved this variance from the detention NORTHERNENGINEERING.COM | 970.221.4158 FINAL DRAINAGE REPORT: GATEWAY APARTMENTS FORT COLLINS | GREELEY 11 | 11 standards set in the FCSCM. The proposed drainage design does comply with the requirements set forth by the Final Drainage Report for Gateway at Prospect BDR. The drainage design proposed with Gateway Apartments complies with the City of Fort Collins Master Drainage Plan for the Boxelder Basin. There are regulatory floodplains associated with the Gateway Apartments development. The drainage plan and stormwater management measures proposed with the Gateway Apartments project are compliant with all applicable State and Federal regulations governing stormwater discharge. B. DRAINAGE CONCEPT The drainage design proposed with this project will effectively limit potential damage associated with its stormwater runoff. Gateway Apartments utilized full spectrum detention as a mechanism to “beat the peak” of Boxelder Creek. The proposed Gateway Apartments development will not impact the Master Drainage Plan recommendations for the Boxelder major drainage basin. REFERENCES 1. City of Fort Collins Landscape Design Guidelines for Stormwater and Detention Facilities, November 5, 2009, BHA Design, Inc. with City of Fort Collins Utility Services. 2. Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by Ordinance No. 159, 2018, and referenced in Section 26-500 of the City of Fort Collins Municipal Code. 3. Soils Resource Report for Larimer County Area, Colorado, Natural Resources Conservation Service, United States Department of Agriculture. 4. Urban Storm Drainage Criteria Manual, Volumes 1-3, Urban Drainage and Flood Control District, Wright-McLaughlin Engineers, Denver, Colorado, Revised April 2008.