Section 104 authorizes and directs the development of guidelines which include criteria for determining whether work carried out by local interests is compatible with a project for flood control. Compatible work which was carried out prior to project authorization, before 17 November 1986 but after 17 November 1981, may be considered part of the project and credited against the non-Federal share of the cost of project, if the local sponsor applied for consideration of such work not later than 31 March 1987. Local work to be carried out after 17 November 1986 must receive Army approval prior to construction to be eligible for credit, taking into account the economic and environmental feasibility of the project. (Such approval can only be given on the basis of the guidelines required to be issued pursuant to section 104(a); hence, the law is silent with respect to work performed between 17 November 1986 and the effective date of the guidelines.) The credit will not relieve the non-Federal sponsor of the requirement to pay 5 percent of the project costs in cash during construction of the remainder of the project. This legislative authority also provides that benefits and costs of compatible work will be considered in the economic evaluation of the Federal project. This includes the costs and benefits of compatible local work which was carried out after 17 November 1981 or within the 5 years prior to the initial obligation of reconnaissance study funds if that should establish a later date.