EW-2: Runoff Flow Control

Goal

Mimic predevelopment hydrological conditions in the right of way (ROW) and minimize offsite stormwater controls.

Requirements

1. Develop a stormwater management plan for the site using stormwater best management practices (BMPs) for flow control. Explicitly state the goals of this plan and how performance will be measured.
2. Use low-impact development (LID) BMPs to the maximum extent feasible as determined in Project Requirement PR-8 by a licensed professional.
3. Compute the 90th percentile average annual rainfall event values for the following predevelopment and post-construction conditions:
   • VpreROW = inside ROW predevelopment volume of runoff
   • Vpre-out = outside ROW predevelopment volume of run-on
   • Vtotal-pre = VpreROW + Vpre-out = total predevelopment volume
   • Qpre = predevelopment flow rate measured at water body receiving effluent from the site, based on Vtotal-pre
   • VpostROW = inside ROW post-construction volume of runoff 
   • Vpost-out = outside ROW post-construction volume of run-on
   • Vtotal-post = VpostROW + Vpost-out = total post-construction volume
   • Qpost = post-construction flow rate measured at the same location as Qpre, based on Vtotal-post
4. Provide BMPs for stormwater flow control. List the types, manufacturers, total volumes and flow rates controlled by BMPs within the ROW or outside of the ROW.
5. Demonstrate that the planned BMPs meet the following flow control criteria: 
   • BMPs conform to all applicable minimum flow control standards for all effluent leaving the ROW set by the governing jurisdiction for volume, flow control and time of concentration. State the minimum requirements, including critical erosive flow criteria, and provide referenced document or policy.
   • RV = Ratio of Volume Achieved = Vtotal-post/Vtotal-pre ≤ 1.20
   • RF = Ratio of Flow Rate Achieved = Qpost/Qpre ≤ 1.20
   • Points are awarded per Table EW-2.1 based on type of alignment, location of BMPs and level of control achieved. Use RV and RF, whichever is higher, to determine if points have been earned.

Documentation

A copy of the executive summary of the project drainage design report that includes, at minimum, the following:

• Short narrative that addresses each of the credit requirements.

As needed, supporting calculations for runoff areas, runoff volume (output from any rainfall modeling software used is adequate), and treatment levels may be requested.

Public commentary on credit EW-2