4.2.4
Natural drainage patterns shall be maintained, and discharges from the project site shall occur at the natural location, to the maximum extent practicable. The manner by which runoff is discharged from the project site must not cause a significant adverse impact to downstream receiving waters and downgradient properties. All outfalls require energy dissipation (see Vol II–4.2 Conveyance System Design Flow).
Creating new drainage patterns results in more site disturbance and more potential for erosion and sedimentation during and after construction. Creating new discharge locations can create significant stream channel erosion problems as the receiving water body typically must adjust to the new flows. Diversions can cause greater impacts than would otherwise occur by discharging runoff at the natural location.
Where no conveyance system exists at the adjacent downgradient property line and the discharge was previously unconcentrated flow or significantly lower concentrated flow, then measures must be taken to prevent downgradient impacts. Drainage easements from downstream property owners may be needed and shall be obtained prior to approval of engineering plans.
Volume II, Chapter 4 provides detailed design requirements for conveyance systems and outfall systems to protect against adverse impacts from concentrated stormwater runoff. Volume II, Chapter 4 also provides standards for downstream analyses, including when a downstream analysis is required, the level of analysis that must be performed, and documentation requirements.