FEDERAL-AID POLICY GUIDE
September 30, 1992, Transmittal 5
NS 23 CFR 650A
OPI: HNG-31
(1) Where facilities are in existence which convey storm water to the highway either by surface drainage or by existing storm drains, the highway agency is believed responsible for providing drainage on and across the right of way. The facilities for this drainage should avoid diversion and concentrations of water as much as possible.
(2) Drainage designs should be adequate and provide for increased runoff from developments within the drainage areas for 20 years hence.
(1) If the municipality requests additional main drains or outlet sewers beyond the needs and responsibilities of the highway project, then the cost of providing this betterment should be borne by the municipality.
(2) In locations where municipalities wish to drain additional areas into the highway system to avoid constructing a separate system or for some other reason costs should be shared on an equitable basis. Such cooperative projects are believed desirable, but an agreement should be drawn upstating clearly the responsibilities if each agency involved.
(a) One method to prorate cost is in proportion to the storm water supplied to the proposed facility. This should be done using the same frequency of runoff for all areas considered, unless a control for regulating is possible.
(b) In cooperating with local municipalities, the highway agencies should guard against permitting overloading of the drainage system on or near the highway with the resulting hazard of damage by floods.
The analysis of design alternatives required by 23 CFR 650.115(a) for encroachments should not include consideration of non-reinforced cast-in-place pipe (CIPP) under the pavement or under high fills. Non-reinforced CIPP is considered acceptable on Federal-aid highway projects at locations such as storm drain and irrigation systems which are not subject to highway live loads or under high fills.
The intent of the statement, "as appropriate, a risk analysis or assessment," in section 23 CFR 650.115(a)(1) is to allow judgment as to the detail of design studies. Where site conditions or structural requirements substantially limit practicable design alternatives, the conventional hydraulic analysis coupled with a risk assessment should meet the requirements of the design standards. Where site conditions permit a range of design alternatives and flood losses are anticipated, an abbreviated or partial risk analysis may be appropriate. We would anticipate that use of the full scale detailed economic (risk) analysis as described in Hydraulic Engineering Circular number 17 would not be necessary for normal stream crossings, but would apply to unusual, complex or high cost encroachments involving flood losses.
The procedures contained in Attachment 2 shall be followed to meet FHWA's responsibility to be consistent with the standards of the National Flood Insurance Program (NFIP) as set forth in 23 CFR 650, Subpart A, Location and Hydraulic Design of Encroachments on Flood Plains. The procedures establish some flexibility for achieving cost-effective encroachments on floodplains within communities that are in the NFIP. For all locations outside of NFIP communities or NFIP identified flood hazard areas, 23 CFR 650, Subpart A shall be followed for encroachment design.