Local hydrologist: burn severity can raise runoff peaks and overwhelm culverts

Marker Platt presented a hydrology-focused assessment of the three watersheds that feed Kanosh and said the recent fire likely changed runoff characteristics across the entire burned area. Using standard methods (curve-number, Type II storm, 100-year event), he said loss of canopy and increased hydrophobic soils can increase peak runoff and shorten hydrograph times to peak.

Platt emphasized five questions his team would answer quickly: peak runoff increase relative to pre-fire conditions; debris-basin impacts; culvert flows at the spillway; town drainage system capacity; and interactions of multiple tributary peaks. He flagged the culvert at the spillway as a possible choke point (an engineering estimate of about 64 cfs appeared in the discussion) and recommended immediate modeling and field surveys to refine cross sections and elevations. "You have potential for a quicker, peak flow, a higher peak flow, and a higher volume of water," he summarized when showing comparative hydrographs.

He recommended sampling and an expedited burn-severity map (the BAER/bear team was already sampling, presenters confirmed) and said the model outputs would guide short-term excavation, swale elevations and low-water crossing designs before longer-term measures are adopted.