Jared WilkersonProject Manager - Stormwater

September 2021

DPW’s Revised Stormwater Standards

This Photo by Unknown Author is licensed under CC BY-

Agenda• Intent of the current revisions• Process• Revisions of Note (Sections 100-300)• Variance/Design Exception Requests• Drainage Design Submittals

Intent of Current Revisions• Reformat and Reorganize• Incorporate Memorandum of Clarification• Update/Remove reference, figures, and

tables.• More consistency with INDOT Standards

– Standards consistently applied on private projects. Public project tend to combined INDOT and DPW standards.

Intent of Current Revisions

• Remove exceptions, inconsistent language, textbook explanations (Tc). – Deleted statements of exception. “Do this, except for here”.

“Can be approved on a case-by-case basis”– Brevity, state the standard clearly. Any exception is a

variance (process defined in City Code).– It is assumed Engineers using the Manual have experience

in H&H analysis (Section 101.01). Textbook material removed.

• More frequent updates– 1995, 2001, current 2011 (rev 2019)

Intent of Current Revisions• “Big Ticket” items intentionally not

addressed (yet…)– Significant changes to water quality and

detention standards– Updates to green infrastructure

standards– Standard Details– Detention and WQ for public projects

Process• Revisions began in 2019• Internal DPW review 2019• BNS and ACEC review Spring 2020• Stakeholder engagement, Summer 2020

• DMD’s regular petition filers• ASCE• Land Surveyors • ACPA • APAI • BAGI (Builders Assoc of Greater Indianapolis) • BOMA (Building Owners and Managers) • NAIOP (National Association for Industrial and Office Parks)

• DPW Media Release and two webinars September 16 and 17, 2020• Council and DPW Board this fall, 2021• Website: https://www.indy.gov./activity/public-works-specifications-and-

manuals

Revisions of Note• Redevelopment

– Definition revised to include alteration of existing development that currently does not meet standards.

CurrentWhen properties redevelop they are subject to the same standards applied to a new development project.

UpdateThree (3) scenarios that accommodate specific variances currently issued, generally focused on detention requirements. 1. Regional Center = match existing 2. Alterations less than 0.5 half acre = match existing3. Alterations greater than 0.5 acres = mitigate 2X disturbed area, current

standard

Revisions of Note• Chapter 200 - Hydrology

– Limitations on the use of rational method and were hydrographs are required.

– Tc references TR-55, textbook equations removed– Rainfall references NOAA Atlas 14 (current rainfall table

had no reference)– NRCS method is baseline for hydrographs. Other methods

must be approved prior to use.– Overall lengthy explanations were removed and instead

appropriate methods are referenced (example Tc method references TR-55).

Revisions of Note• Chapter 300 – Hydraulics

– Downstream Analysis– Detention– Open Channels– Culverts– Storm Sewers and Inlets

• Section 301.02 Downstream Analysis– Required unless ex. downstream conveyance will

not accept runoff.– Minimum Req. – First downstream conveyance

(pipe, channel, pond)– Difficult to strictly define limits. Ultimately the

engineer must determine what is appropriate and address this in the drainage report.

Revisions of Note

Revisions of Note

• Section 302 Detention– No change to release rates– Modified Rational Method not allowed. All

detention must be designed by routing runoff hydrographs with modeling software.

– Design components (beyond hydraulic calcs) from Chapters 400 and 500 were moved to Chapter 300.

Revisions of Note• Section 303 Open Channels

– Divided into categories• Artificial open channel and natural channels (no structures in 1%

AEP area, drains area beyond R/W and rear/side yards)• Roadside ditch (10% AEP, drains R/W only)• Minor collector swales (rear and side yards, slope and length)• Hydraulic Analysis - Step-backwater methods (HEC-RAS or similar)

must be used where uniform flow is not maintained and for natural channels.

• Channel lining requirements. Engineer must document permissible shear stress for selected lining and shear stress calculations.

Revisions of Note• Section 304 Culverts and Bridges

• Culverts on artificial open channel within private developments

– 4% AEP (no overtopping) and maximum 7” overtopping for 1% AEP (current standard)

• Driveway culverts (Roadside ditches 10% AEP, drains R/W only)

– 10% AEP and no increase in water elevation nor flooding on adjacent properties or public right of way.

• Bridges and culverts under public roadways or draining natural channels.

– Allowable headwater based on Indiana Design Manual (INDOT)

Revisions of Note• Culverts that pose a threat of damage to

property due to backwater and/or road overflow….

• Require step backwater modeling (HEC-RAS or similar)• Summarize existing and proposed conditions including

water surface elevations and velocities upstream and downstream for the 2-year through 100-year storms (50% - 1% AEP)

• The intent is to understand upstream and downstream conditions when there will be obvious impacts after culvert replacement.

Revisions of Note• Storm Sewers

• Manning’s Eq./Rational Method – 10% AEP at 90% capacity or less.

• HGL – 2% AEP below casting• Drainage System Overflow Design (private)

– Ponding and overflow path shall be identified, including spill over elevations. Illustrating stormwater is contained within the property boundary and routed to detention basin or acceptable outlet.

Variances and Design Exceptions• Private Projects

– Submit Variance Request Letter to BNS through DRN review process

– BNS will forward Variance Request to DPW for review. BNS will notify applicant of DPW’s determination.

• Public Projects– Include a “Design Exception” section in their

drainage report.– Work with DPW PM through typical plan review

process.

Variances and Design Exceptions • Include engineering analysis and documentation that

supports the feasibility of meeting the design standards• Written narrative stating:

– Specific design standard– Reason for the request– How the intent of the current standards is being

addressed. Provide something the City can say “yes” to.

• Engineering Analysis– Exhibits and/or calculations that illustrate what’s

required to meet the standard and what is proposed.– Document details such as size, type, location of conflicts

on plans and in narrative.

Drainage Design Submittals• Hydrologic Calculations at 30%

• Drainage Areas, peak flows• Overall and detailed drainage area map• Present alternatives at this stage

• Full Drainage Report due at 60%• Narrative should include a discussion of the scoping

memo/report (Stormwater Projects).• Storm sewer calculations/results must be in a tabular

format. • Operations and Maintenance• DDSS

Questions