ELEVATION-DERIVED HYDROGRAPHY DEVELOPMENT IN NEW HAMPSHIRE

High-resolution elevation-derived hydrography (EDH) has become a high priority as an input for hydraulic modeling in New Hampshire due to growing population, demand for natural resources, and flood hazard mitigation. As stewards of the National Hydrography Dataset (NHD) in New Hampshire, the New Hampshire Geological Survey (NHGS), within the New Hampshire Department of Environmental Services, is committed to developing accurate hydrography for both public and private use. In 2013, NHGS developed BotHat scripts that have allowed EDH to be extracted from the statewide 1-2-meter LiDAR. With the publication of the EDH specifications for the NHD in 2020 by USGS, we have begun implementing a work plan.

NHGS began collaborating with the White Mountain National Forest (WMNF) to develop EDH across the forest to aid ecological management and timber sales. The roadless areas there have provided unimpeded access to streams, which has enabled field verification of stream permanence for every flowline in a watershed. Once field-verified, the digital dataset has to be processed according to the EDH specifications, among which is attributing each vertex with a monotonic elevation value with respect to the entire network. This step has been challenging, and has required Lidar-processing software. With the successful pilot here in the WMNF, we hope to eventually bring this workflow statewide, where NH has a culvert database known as SADES, the interagency Statewide Asset Data Exchange System. The SADES database has over 9,000 culverts, and field interns are working to assess them for hydraulic vulnerability. Under the new EDH specifications, these culverts will all eventually be coded as such in the NHD, allowing users to combine both digital datasets—NHD with accurate length and location in a traceable geographic network and SADES with its site-specific hydraulic vulnerability attributes.