§ 5.2.0. DESIGN GUIDELINES
The following rules shall be observed in the design of storm drain systems located in public right of way or public drainage easements to promote proper operation of these systems and to minimize maintenance requirements:
A.
Select pipe size and slope so that the velocity of flow will increase progressively or at least will not appreciably decrease at inlets, bends or other changes in geometry or configuration.
B.
Do not discharge the contents of a larger pipe into a smaller one even though the capacity of the smaller pipe may be greater due to a steeper slope. For projects where a proposed storm drain system associated with the project has greater capacity than an in-place receiving system, and an upgrade to the receiving system is not imminent, a temporary exception may be allowed. For this exception to be allowed, the design of the system must incorporate temporary features as needed to prevent any increase in flooding due to the improved conveyance of the proposed upstream system. The proposed system with interim features shall prevent (a) the capacity of the existing in-place system from being exceeded during the design storm at any location or (b) the exacerbation of existing flooding during the design storm at all points along the receiving system, including the tie-in point. The more stringent of these two requirements shall apply. Supporting calculations, signed and sealed, shall be submitted by the design engineer to demonstrate that there will be no adverse flooding impacts due to the proposed storm drain improvements during the interim condition while the receiving system remains undersized and proposed temporary features are in place to prevent any increase in flooding.
C.
Where there is a connection of different conduit sizes on the trunk line, the soffit elevations, rather than the flow line elevations, shall be approximately the same. The soffit elevation of the incoming pipe may be offset (increased) by an amount equal to the headloss at the structure where the conduits meet.
D.
For design purposes, wherever two or more incoming conduits intersect at a single location, the incoming conduit having the greatest cross-sectional area shall be considered to be the "trunk line." If more than one incoming conduit has the same cross-sectional area, then the conduit having both the greatest cross-sectional area and the highest peak flow rate for the design storm shall be the "trunk line." All other incoming conduits at this location shall be considered to be "lateral conduits" or "laterals."
E.
Where a lateral conduit intersects the trunk line at a manhole, the soffit elevation of the incoming lateral conduit shall be at approximately the same elevation as the soffit of the trunk line conduit. Exceptions may be made when the lateral does not need to be constructed to the depth of the trunk line or when the presence of existing utilities prevents the soffit elevation of the lateral from matching the soffit elevation of the trunk line. Where the soffit elevations of the trunk line and lateral are nearly the same, an offset in the soffit elevations may be provided in an amount equal to the headloss at the junction.
F.
Where a lateral conduit intersects the trunk line at a wye junction, the soffit elevation of the incoming lateral conduit shall match the approximate soffit elevation of the trunk line conduit. If the soffit elevations are not proposed to be approximately the same, a request for a waiver will be required. Special consideration may be given in cases where there are design constraints such as the presence of existing major utilities which prevent the matching of soffit elevations without extensive utility relocations. In the case of a lateral pipe intersecting a box culvert, allowances shall be made to allow the wall of the pipe to penetrate the wall of the box without having to notch either the pipe wall or the top slab of the box.
G.
For all pipe junctions other than a manhole, the angle of intersection between any two flow paths shall not be greater than 45 degrees. This includes discharges into box culverts and channels.
H.
No storm drain system shall discharge into or through an inlet box. Instead the inlet shall discharge to the trunk line through a lateral line. A single connection from a one-lot or two-lot commercial subdivision or an irrigation system may discharge to an existing inlet if it does not impede the function of the inlet, and if the receiving storm drain system has the capacity to convey the additional flows. The inlet shall then be considered a junction box.
I.
No proposed conduit having a diameter or height greater than 50 percent of the minimum dimension of an existing box culvert shall be allowed to discharge into that box culvert. Exceptions must be justified by structural engineering analysis demonstrating a sound structural design.
J.
Conduit shall be reinforced concrete. Conduit shall be manufactured and installed in compliance with the City's standard specifications.
K.
Plastic pipe (schedule 40 PVC or greater strength, 6″ minimum diameter) shall be used inside water quality ponds (where the size of pipe required dictates its use) and for retention/re-irrigation systems and may be used within 50 feet of a water quality pond filtration bed (if the pipe is not subject to any type of vehicular loading). End treatment is required for outfall pipe in accordance with City standard specifications. Threaded cleanouts are required within 50 feet of every portion of lateral and collector drain lines and at every bend. Junctions between PVC and RCP shall occur at a manhole or cleanout, as determined by the City.
L.
The 25-year hydraulic grade line shall remain a minimum of six (6) inches below the theoretical gutter flow line of inlets.