§ 4.1.0. GENERAL

The primary purpose of storm drain inlets is to intercept surface runoff so that flows will be conveyed safely in an engineered storm drain system.

The most common location for inlets is at the curb line of streets where surface flow collects. Inlets must be designed to avoid conflicting with the movement of vehicular traffic and to provide travel space free of surface flow in accordance with Section 3 of this manual.

The following guidelines shall be used in the design of inlets to be located in streets:

A.

Curb inlets shall be used to intercept surface runoff for curb and gutter streets unless the use of an alternative type inlet has been approved in writing by the Director of the Watershed Protection Department. The use of an alternative type inlet also needs to be approved in writing by the Director of Public Works for any effect that the layout of the inlet might have on the remainder of the right of way.

B.

All curb inlets, whether in a sump or on grade, shall be a minimum of five (5) feet in length and be designed and constructed in accordance with the City's standard details for curb inlets. In all cases, the size and layout of inlets shall be such that all applicable requirements of the City street drainage criteria will be met.

C.

Curb inlets in excess of ten feet in length shall be designed and constructed with a modified top in accordance with the City's standard details for curb inlets.

D.

The standard curb inlet transition length is 9′-8″ as shown in the standard details. Transition lengths may be reduced to a minimum of 7′-6″ for curb inlets having a standard depth of depression only when the inlet is located on the outside lane of a street where the outside lane is clearly designated for parking only.

E.

The use of a reduced throat inlet may be considered on travel lanes where a proposed inlet is less than ten feet from an existing driveway or street corner. Hydraulic requirements must be satisfied. All reduced throat inlets must be designed and constructed in accordance with the City's standard details. Approval in writing by the Director of the Watershed Protection Department is required in all cases where a reduced throat inlet is proposed.

F.

The use of recessed inlets is encouraged on any street where space is available and subsurface utility lines can be accommodated. The advantage of recessed inlets is that they minimize the dip in the travel lane; the disadvantage is that they can make subsurface utility placement more difficult due to the increased potential for location conflicts. The width of the sidewalk shall not be decreased whenever a recessed inlet is used.

G.

Grate inlets, combination inlets and slotted drains are discouraged from use due to the tendency of grates and slotted drains to clog, the increased maintenance needed to keep them free of debris, the hazards caused by broken or missing grates, and the difficulties associated with replacing broken or missing grates. When the use of a grate inlet, combination inlet or slotted drain is being considered, a written request to allow the proposed grate inlet, combination inlet or slotted drain may be made when accompanied by supporting information that clearly demonstrates that the proposed grate inlet, combination inlet or slotted drain is the only feasible solution. If approved in writing by the Director of the Watershed Protection Department, the selection and use of a grate inlet, combination inlet or slotted drain shall be in strict accordance with the manufacturer's design guidelines.

H.

Trench drains with grates, when approved for use in writing by the Director of the Watershed Protection Department, shall be designed and constructed in accordance with the City's standard details for trench drains with grates. All trench drains with grates shall be designed for HS-20 loading. For all trench drains with grates located within City right-of-way, two additional sets of the full grate length shall be provided at the time of construction for City maintenance crews to have on hand for emergency replacement of the grate.

I.

Inlet design and location must be in accordance with the criteria established in Section 3 of this manual.

J.

The selection, use and maintenance of water treatment systems at inlets to improve water quality shall be in accordance with the manufacturer's recommendations. Inlet structures shall be sized so that the peak rate of flow entering the inlet from the surface will pass over or around the treatment device when the device is fully clogged. Flow that enters the inlet from the surface and flow in the storm drain system that is passing through the inlet structure shall not be hindered by the presence of a treatment device. The designer of the storm drain system must account for all increased headlosses due to the presence of a fully-clogged treatment device.