Appendix_M_(Part_1).pdf

APPENDIX M

BASE STORM DRAINAGE STUDY - 2001

85100086-02-02

INVESTIGATION REPORT

BASE STORM DRAINAGE STUDY

REPORT AND EXHIBITS

TYPE 1-A 100% SUBMITTAL

DAVIS MONTHAN AIR FORCE BASE, ARIZONA

Project Number: FBNV 990216 Facility Number: 87110

29 January, 2001

85100086-02-02 FBNV 99216 29 January, 2001

CONTENTS

SECTION 1 – GENERAL INFORMATION/DATA COLLECTION

Introduction

Data Collection and Field Reconnaissance

SECTION 2 – HYDROLOGIC ANALYSIS

Method/Approach of Analysis

SECTION 3 – HYDRAULIC ANALYSIS

Method/Approach of Analysis

Existing Hydraulic Constraints

SECTION 4 – SUMMARY OF DRAINAGE CONCEPT ALTERNATIVES

EXHIBITS

FIGURE 1 Study Area FIGURE 2 Field Survey Exhibit FIGURE 3 Existing Conditions and Proposed Concepts Exhibit

APPENDICES

(SEPARATE VOLUME)

APPENDIX A Hydrologic Calculations APPENDIX B Hydraulic Calculations B-1 Cross-Sections B-2 Existing Culvert and Storm Drain Analyses B-3 Proposed Culvert Analyses B-4 Detention Basins Analyses APPENDIX C Annotated 65% and 90% Review Comments APPENDIX D Building 4844 Analyses and Proposed Concept APPENDIX E Preliminary Concept Cost Estimates

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SECTION 1 – GENERAL INFORMATION/DATA COLLECTION

Introduction

This hydrologic/hydraulic report has been prepared for the purpose of mapping/defining significant watersheds within the boundaries of the Davis-Monthan Air Force Base (AFB) study area, determining the quantity of runoff generated during the 2-, 10-, 50- and 100-year storm events at key concentration points, identifying existing and/or future drainage constraints and concerns, concept recommendation for critical drainage improvements associated with current conditions and proposing concept design alternatives associated with the Davis-Monthan AFB Five Year Capital Improvement Plan.

The study area for the Davis Monthan Master Drainage Study lies west of Kolb Rd. and north of the runway (see Figure 1). The Davis-Monthan AFB generally consists of relatively flat topography draining form southeast to northwest. Major drainage improvements throughout the AFB are minimal.

However, existing improvements include the AMARC (MASDC) Flood Control Detention Reservoir south of Picacho Street, an improved earthen channel from the detention reservoir to the Tucson Diversion channel located along the north side of Golf Links Road (main channel), earthen drainageway along the north side of Quijota Boulevard from approximately Wilmot Road to the Tucson Diversion channel located along the south side of Golf Links Road, and limited storm drain networks located within both the industrial and air field areas of the AFB. In addition, existing roadways include drainage crossings consisting of corrugated metal pipes (CMP’s), reinforced concrete pipes (RCP’s), and concrete box culverts (CBC’s).

Annotated review comments for the 65% and 90% submittals of the Davis Monthan Master Drainage Study are included in Appendix C.

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Data Collection and Field Reconnaissance

Existing drainage information was gathered and reviewed, including available storm drainage plans and existing drainage reports for the AFB. The AFB five-year capital improvement plan (M-tabs dated May 28, 1993, revised October 30, 1998) and storm drainage system (G-tabs) were also utilized.

Aerial photography and topography dated 1997 was utilized for existing and proposed analyses. Additional field survey was performed along the main channel from the detention basin to Golf Links Rd. Site specific survey was also obtained at a hanger (building 4844) adjacent to the airfield, which has experienced flooding. See Figure 2 for resulting survey data. Field reconnaissance was also performed pertaining to area specific drainage patterns and drainage structure verification/ dimension measurement. In additon, a meeting with maintenance staff was attended to facilitate identification of problem areas.

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SECTION 2 - HYDROLOGIC ANALYSIS

Method/Approach of Analysis

The quantity of runoff impacting the project area at key concentrations points for the 2-, 10-, 50- and 100-year storm events were determined utilizing the City of Tucson peak discharge method. Key concentration points include major channels, drainage structures, and areas of identified drainage problems (refer to Figure 3, for concentration point locations). Future developed condition discharges were based upon projected development within the air force base. The hydrologic analyses are contained in Appendix A, and the discharges for the range of storm events are summarized in a table in Appendix A.

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SECTION 3 - HYDRAULIC ANALYSIS

Method/Approach of Analysis

Capacity of existing major drainage structures; channels, culverts, storm drains, and detention basin; were determined and existing flood prone limits were delineated for areas with 100-year peak discharges in excess of 100 cfs.

Manning’s equation was utilized for open-channel and floodplain analyses. A Dodson program was utilized for culvert analyses and in-house computer programs were utilized for storm drain analyses and detention basin evaluation (Modified Puls routing). Resulting Manning’s ratings of cross-sections, drainage structure capacities and detention basin routing results are contained in Appendix B. Cross-section locations and resulting floodplains are shown on Figure 3.

Existing Hydraulic Constraints

The existing detention basin and associated outlet structure attenuates the existing 100-year peak discharge concentrating in the main channel at Picacho St. by approximately 250 cfs. Downstream of Picacho St., an under-capacity channel and pedestrian bridge structures results in approximately 50% of the 100-year peak discharge (approximately 1100 cfs) breaking out of the channel and draining west across the golf course as sheet flow. Additional flow (approximately 900 cfs) also leaves the channel further downstream (see Figure 2 for sheet flow depths). The remaining 100-year peak discharge within the channel is generally contained within the existing channel and associated drainage structures from Ironwood St. to the outlet at Golf Links Rd. The existing main channel system has capacity ranging from the 2-year to 10-year flow event. Recent improvements to the channel have been taken into consideration within the analyses. The limiting factor within the conveyance system is existing, under-capacity structures (culverts, pedestrian bridges).

Due to insufficient conveyance, flooding is also evident during the 100-year event in the residential area in the northeast corner of the study area. In the AMARC area the channel along Irvington Rd. has capacity for the peak

85100086-02-02 FBNV 99216-5 29 January, 2001 discharge associated with approximately the 2-year event, while the drainage structure under Safford Ave. has capacity for the peak discharge associated with approximately the 10-year event. The 5th St. storm drain, in the vicinity of Arizola has capacity for approximately 150 to175 cfs, equating to approximately the 2- year event for runoff generated within the local watershed.

The localized watershed for runoff concentrating at Building 4844 is shown in Appendix D. The existing grate inlets have capacity for less than the 2-year storm event (see Appendix D for hydrologic and hydraulic calculations).

Existing grades in the asphalt parking/drive areas direct excess runoff to the office doorways of the building.

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SECTION 4 - SUMMARY OF DRAINAGE CONCEPT ALTERNATIVES

Proposed drainage concepts are illustrated on the attached Figure 3.

The following is a summary of the targeted areas of concern.

The main channel downstream of Picacho St., and adjacent undeveloped floodplain areas are under-capacity for the 100-year flow. In addition, existing pedestrian bridges and culverts are inadequate to convey the 100-year flow.

Increasing the detention basin volume by modifying the outlet structure will reduce flow in the main channel and reduce downstream breakout flow.

Currently, only 39 acre-ft of the available 89 acre-ft within the Picacho St.

detention basin is being utilized for flow attenuation. Increasing the effective basin volume to 50 acre-feet (state standard, see below) will decrease the 100-year discharge by an additional 350 cfs (approximation, see Appendix B).

Increasing the effective basin volume further would allow for additional attenuation of peak discharges; with associated modifications to the outlet structure. State (Department of Water Resources) dam standards must be taken into account when increasing the size of the detention basin. For example, a basin which stores greater than 50 acre-ft., must be less than six

ft. in height to be non-jurisdictional. Increasing channel capacity in break-out areas is not recommended as downstream channel and structure capacity is limited and associated flooding will translate downstream (unless the structures are also up-sized).

A storm drain system and channel improvements are recommended to facilitate conveyance of flow within the northeast residential area. These improvements are based upon existing physical constraints, and not a specified design flow. The proposed conveyance system may improve existing drainage conditions during the 2-year or 10-year rainfall event, but may not contain the 100-year peak flow of runoff in these areas. The proposed concept includes a storm drain system within Mustang Dr. which would outlet into an improved earthen channel for ultimate discharge into an existing channel which parallels Golf Links Rd . An improved earthen channel is also proposed to drain flow from Saratoga Ave. A catch basin in conjunction with an open channel will facilitate conveyance of flow off the street and between the existing homes.

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Maintenance staff identified concerns with flows from the AMARC area which concentrate at Wilmot Rd. An improved channel network and an approximately 8 acre detention basin are recommended to reduce flows in this area (refer to Figure 3 for channel sections). Additional channels and drainage structure improvements are also proposed in the AMARC area to improve existing flooding/ponding conditions identified in that area and account for potential future development as identified in the previously identified AFB 5-year Capital Improvement Plan, specifically buildings no. 3, 4 (aircraft processing ramp), 17, 29, 40, 49, 44, 45, 35, and 46.

Recommended improvements at the hanger (building 4844) include increasing the capacity of the existing inlet grate, and raising or berming (asphalt berm) the area in front of the doorway to prevent flow from entering the building. A canopy over the doorway is recommended to be constructed in conjunction with these improvements. See attached concept, Appendix D.

More costly alternatives include raising the door threshold, or increasing the catch basin capacity at the storm drain inlet (essentially creating a small underground vault for stormwater storage).

Schematics/concept drawings of the recommended improvements are included on Figure 3 and in Appendix D for building 4844. Preliminary concept cost estimates are included in Appendix E. The recommended improvements to the existing detention basin and improvements in the northeast residential area will result in improving existing potential flooding conditions in existing residential areas. Therefore, these improvements, and building 4844 improvements are recommended to be implemented as a first priority.

The proposed drainage improvements will mitigate existing flooding on the base in addition to increases in runoff generated by development identified in the AFB 5-year Capital Improvement Plan. However, as future development occurs within the base, site specific analyses should be performed to address finished floor elevations based upon proposed future structures and associated flooding elevations. Detail hydrologic/hydraulic analysis and design is required in order to satisfactorily implement any of the recommended concept drainage solutions. Increases in runoff caused by increased development may be mitigated, where possible, with water harvesting in landscaped areas and improved localized conveyance systems.

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The enclosed information provides drainage-related calculations, estimates, analyses and designs for estimated flooding events up to and including the

100-year frequency flood based on specific engineering methodologies, ordinances, regulations, policies, etc. in effect and applicable at this time.

This study was performed in accordance with, and to the level of, current engineering standards as established by the regulatory agency having jurisdiction of the work. A flood event of a magnitude exceeding the 100-year event as currently defined may cause or create the risk of greater flood damage than is or can be anticipated or presented in this assessment.

Current regulatory agency requirements do not require that this assessment address flood events greater than the 100-year. In addition, our contract did not include, nor did our client request, that flood events greater than the 100-year event be addressed in this study. Stantec assumes no responsibility for actual flood damage, increased risks of flood damage, or increased construction or development cost resulting from or related to any such events, nor shall Stantec be responsible for any changes in or additions to regulatory requirements which may result from or be related to any such events.

FIGURE 1 – Study Area

FIGURE 2 – Field Survey Exhibit

FIGURE 3 – Existing Conditions and Proposed Concepts Exhibit