City of Alexandria Storm Sewer Capacity Analysis

The City of Alexandria, Virginia, has experienced repeated and increasingly frequent flooding from storm events which lead to the development of the 2016 "City of Alexandria Storm Sewer Capacity Analysis" or "CASSCA", for short.

Page updated on Aug 28, 2020 at 12:54 PM

2016 City of Alexandria Storm Sewer Capacity Analysis 

Purpose

The City of Alexandria, Virginia, has experienced repeated and increasingly frequent flooding from storm events which lead to the procurement of the "City of Alexandria Storm Sewer Capacity Analysis", or "CASSCA", for short. Finalized in 2016, CASSCA analyzed the City's current storm sewer system and provided the following major outcomes: (1) the identification of problem flooding areas and (2) the development and prioritization of solutions for these areas. 

Methods 

CASSCA provided a thorough assessment of both the stormwater runoff and hydraulics of the storm sewer system for each individual watershed within the City of Alexandria. Models were utilized to generate data for a 10-year, 24-hour storm event (the City’s recommended design event) using an industry standard approach. Results from this model were used to identify and prioritize flooding areas. Stormwater management solutions, including green infrastructure (GI), storage, and conveyance improvements, were identified, evaluated and prioritized using a cost benefit analysis. Three watershed-wide alternatives for managing stormwater were developed and presented as possible solutions for each watershed.

The City of Alexandria covers approximately 10,000 acres (15.3 square miles) and the drainage is subdivided into eight separate watersheds that eventually drain to the Potomac River and the Chesapeake Bay as well as three combined sewersheds that are served by Alexandria Renew Enterprises. The watersheds are Backlick Run; Cameron Run; Four Mile Run; Holmes Run; Hooffs Run; Potomac River; Strawberry Run; and Taylor Run.

Stormwater runoff is generated when precipitation flows over land or impervious surfaces. The City’s storm sewersheds are approximately 43 percent impervious, the majority of which is buildings, roads, and parking. Stormwater runoff in the separate stormwater system is conveyed to the receiving streams through a network of over 1,077,000 linear feet (LF) of pipe (200 miles) and 130,960 LF of streams (25 miles).

Results

Approximately 22 percent of the system may experience flooding and 16 percent may have a water level within 2 feet below the surface, referred to as insufficient freeboard, at some point during the storm. Also 16 percent of the system may be surcharged such that the water surface rises above the crown of the pipe and cause the system to backup. Overall, 90 problem areas were identified across the City with an estimated cost of $61 million (in 2016 dollars); note that the estimated cost focused solely on construction cost and was used for planning purposes only. Actual implementation costs were not included in the scope of CASSCA and include costs associated with feasibility study, design, utility coordination, property acquisition, construction management and inspection, etc. Solutions were developed for each of the problem areas, with the exception of the seven in the Potomac River watershed, where solutions were deferred until after completion of substantial ongoing development in the watershed. Three watershed-wide scenarios were assessed to address flooding in a cost effective and beneficial manner. A preferred alternative was selected for each watershed based on engineering judgement. For each of the 83 remaining problem areas, up to five solutions were identified and modeled:  three levels of GI implementation (high, medium, and low percent impervious area managed), conveyance improvements, and storage construction.

Final Report

The CASSCA final report consists of a summary report  and includes an executive summary as well as summary of the evaluation and results of the analyses of the project. Technical memos and reports are included as separate attachments under "Tasks" and are available below.

CASSCA Summary Report

Task 1:  Review rainfall data and the City’s stormwater design criteria, develop projections for rainfall and tidal boundary conditions based on climate change, and propose potential revisions as appropriate

Technical Memos

1.1 Comparison of Alexandria's Storm Design Criteria to Neighboring Jurisdictions
1.2 Updated Precipitation Frequency Results and Synthesis of New IDF Curves for the City of Alexandria, Virginia
1.3 Rainfall Frequency and Global Change Model Options for the City of Alexandria, Virginia
1.4 Sea Level Rise Potential for the City of Alexandria, Virginia
1.5 Consideration of Design Guidelines for Flow-through BMPs and use of Unit Hydrographs

Task 2:  Analyze the capacity of the City’s existing stormwater collection system

Technical Memos

2.1 Stormwater Capacity Analysis for Hooffs Run, City of Alexandria, Virginia
2.2 Stormwater Capacity Analysis for Four Mile Run, City of Alexandria, Virginia
2.3 Stormwater Capacity Analysis for Holmes Run, City of Alexandria, Virginia
2.4 Stormwater Capacity Analysis for Taylor Run, City of Alexandria, Virginia
2.5 Stormwater Capacity Analysis for Cameron Run, City of Alexandria, Virginia
2.6 Stormwater Capacity Analysis for Backlick Run, City of Alexandria, Virginia
2.7 Stormwater Capacity Analysis for Strawberry Run, City of Alexandria, Virginia
2.8 Stormwater Capacity Analysis for Potomac River, City of Alexandria, Virginia

Task 2, Supplemental Documents

1 Comparison of Model Hyetograph Generation and Hydrologic Computation Methods
2 Inlet Capacity Analysis for City of Alexandria Storm Sewer Capacity Analysis
3 Summary of Data Gaps and Assumptions in the Hooffs Run Watershed

Task 3:  Perform field verification of selected drainage facilities through survey, manhole inspections, and closed-circuit television (CCTV) inspection 

Technical Memos

3.1 Hooffs Run Task 3.1 – Pilot Study Area Field Verification – Survey and Inspection
3.2 City of Alexandria Storm Sewer Capacity Analysis (CASSCA) Four Mile Run Condition Assessment
3.3 City of Alexandria Storm Sewer Capacity Analysis (CASSCA) Holmes Run Condition Assessment
3.4 City of Alexandria Storm Sewer Capacity Analysis (CASSCA) Taylor Run Condition Assessment
3.5 City of Alexandria Storm Sewer Capacity Analysis (CASSCA) Cameron Run Condition Assessment
3.6 City of Alexandria Storm Sewer Capacity Analysis (CASSCA) Backlick Run Condition Assessment
3.7 City of Alexandria Storm Sewer Capacity Analysis (CASSCA) Strawberry Run Condition Assessment
3.8 City of Alexandria Storm Sewer Capacity Analysis (CASSCA) Potomac River Condition Assessment

Task 3, Supplemental Documents

1 City of Alexandria Storm Sewer Capacity Analysis (CASSCA) Corrugated Metal Pipe Condition Assessment
2 City of Alexandria Storm Sewer Capacity Analysis Task Order 2A.04D – Cleaning & CCTV (Business Center Drive & Colvin St)
3 City of Alexandria Storm Sewer Capacity Analysis Task Order 2B – Cleaning & CCTV (DASH Facility)
4 City of Alexandria Storm Sewer Capacity Analysis Task Order 2B – Cleaning & CCTV (King St, Beauregard St, and Walter Reed Dr)
5 City of Alexandria Storm Sewer Capacity Analysis Task Order 2D – Heavy Cleaning & CCTV (N. Beauregard St) Storm Pipe Segment: 001316STMP

Task 4:  Identify and prioritize problem areas (capacity limitations), and develop and prioritize solutions to high-priority problems 

Reports

4.1 Problem and Solution Identification and Prioritization for Hooffs Run, Alexandria, Virginia
4.2 Problem and Solution Identification and Prioritization for Four Mile Run, Alexandria, Virginia
4.3 Problem and Solution Identification and Prioritization for Holmes Run, Alexandria, Virginia
4.4 Problem and Solution Identification and Prioritization for Taylor Run, Alexandria, Virginia
4.5 Problem and Solution Identification and Prioritization for Cameron Run, Alexandria, Virginia
4.6 Problem and Solution Identification and Prioritization for Backlick Run, Alexandria, Virginia
4.7 Problem and Solution Identification and Prioritization for Strawberry Run, Alexandria, Virginia
4.8 Problem Area Identification Results for Potomac

Task 4, Supplemental Tech Memos

1 Planning Level Cost Information
2 Task 4 Evaluation Criteria Scoring Systems






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