1. (a) Unless otherwise specified, all design criteria in this section shall apply to new and replacement tier 2, tier 3, and tier 4 stream crossings.
2. (b) Tier 2 and tier 3 stream crossings shall be designed in accordance with the NH stream crossing guidelines, available as noted in Appendix B;

3. (c) Tier 2, tier 3, and tier 4 stream crossings shall be designed:

   1. (1) To meet the general design considerations specified in Env-Wt 904.01;

   2. (2) Of sufficient size to accommodate the greater of:

      1. a. The 100-year 24-hour design storm;

      2. b. Flows sufficient to:

         1. 1. Prevent an increase in flooding on upstream and downstream properties; and
         2. 2. Not affect flows and sediment transport characteristics in a way that could adversely affect channel stability; or
      3. c. Applicable federal, state, or local requirements;
   3. (3) With the bed forms and streambed characteristics necessary to cause water depths and velocities within the crossing structure at a variety of flows to be comparable to those found in the natural channel upstream and downstream of the stream crossing;
   4. (4) To provide a vegetated bank on both sides of the watercourse or to provide a wildlife shelf of suitable substrate and access to allow for wildlife passage;
   5. (5) To preserve the natural alignment and gradient of the stream channel, so as to accommodate natural flow regimes and the functioning of the natural floodplain;
   6. (6) To simulate a natural stream channel;
   7. (7) So as not to alter sediment transport competence; and
   8. (8) To avoid and minimize impacts to the stream in accordance with Env-Wt 313.03.

4. (d) In addition to meeting the criteria specified in (c), above, new, repaired, rehabilitated, or replaced tier 4 stream crossing shall be designed:

   1. (1) Based on a hydraulic analysis that accounts for daily fluctuating tides, bidirectional flows, tidal inundation, and coastal storm surge;
   2. (2) To prevent creating a restriction on tidal flows; and
   3. (3) To account for tidal channel morphology and potential impacts due to sea level rise.

Source. #12809, eff 12-15-19