Head branches of this river flows to Caspian beach plain (called: Zavat River) after passing Kelardasht plain and mount Chahar Saman. Zavat River in initial of beach plain divided to several branches again that its western branch called Hejirud and its eastern branch called Sardabrud. This stream cuts Tonekabon-Chalus coastal road in km: 55+200.
Area of drainage basin for Sardabrud (Zavat River) is estimated more than 150 km2. Gradient of Sardabrud is 5/1000 between Zavat village and Gileh Kola and it is 7/1000 between Gileh Kola and Caspian Sea. Local base level (LBL) of this river is -25 m.
Sardabrud is a river valley that there are several meander in its flow path. Index of meander for this stream is low to mean. Width of stream-bed is 15-40 m.
There are 2 groups of river terraces in beach plain for Sardabrud:
- H1 (River terraces with down surface level): 10 to 20-year boundary of flood plain (Height: 2-3 m)
- H2 (River terraces with high surface level): 50 to 100-year boundary of flood plain (Height: 4-6 m)
Materials for terrace of most outcrops are coarse-grained and very coarse-grained alluviums that are without cement and slumping and erodible completely.
Stream flood of Sardabrud happens approximately yearly (Flood risk: 2). Floodwater of 1342 was the most devastating floods of Sardabrud. Floodwaters in recent years (1370s & 1380s) have been caused to economic damages too but there have not been had any victims.
Vulnerable area of Sardabrud lies between 1.9 km of southern Sardabrud city and 100 m of northern coastal road. Width of flood plain for Sardabrud is 100-150 m. Width of active flood plain is 40-60 m and width of alternative flood plain is 60-80 m. Active flood plain expansion is observed at eastern bank of river.
Vegetation for flood plain of Sardabrud determinates theirs position in floodwaters. Alternative flood plain is determinated by grassland cover and scattered shrubs and 50 to 100-year flood plain determinated by compact vegetation (mainly tree).
Human interference in active flood plain:
- Extraction from sand and gravel of stream-bed
- Construction of wall in order to protection of walls from erosion
- Strict interdict of construction in active flood plain and in concave parts of bights of river
- Strict recommendation in order to protection of terraces wall in urban area (Because of high rate of terraces recession)
- Construction of wall in parts of bights for river (Protection of walls from erosion and under cutting of stream walls)
- Correct and systematic management for extraction from sand and gravel resources of stream
- Requisiteness of exact recognition for active, alternative and destructive flood plains in Sardabrud area